Back Side of the Power Curve

Doug and I have been working on some new stuff lately, as you may have read in previous posts. One of the big things has been to change our approach to the 4 cycle setups that we have been using. So far I would say that we have been very successful, and I have developed an entire new philosophy for running these engines (maybe any engine). This is not a new philosophy to many, but it is to me.

Basically, I have been trying to take advantage of the dyno testing performed by Frank Williams. Frank has been saying from the early 1990's that stunt flyers should be flying with their engines "on the back side of the power curve". This has always been a fairly dificult reality for me to grasp, although the theory has always been very simple.

Basically, the HP/torque peak on any engine is fixed. This should especially true for the 4 cycle engines because they are more mechanically limited than the 2 cycle engines. Once that peak RPM is achieved, the power peak now becomes the RPM limiter, not the load on the engine. In theory, one should be able to underload the engine once past the peak, and the RPM would be limited simply due to the descending curve only. After all, theoretically, if the engine tries to increase the RPM past the power peak the power is decreasing.

In theory, the only point of importance should be that the engine should create enough RPM with a given propeller to achieve a working RPM on the back side of the peak. In that respect the load from the propeller is no longer serving as a governor. If the RPM tries to drop (as in the case of loading the prop doing verticals) the point on the curve moves to the left, and the power increases. If the prop tries to increase in RPM (in the case of descending verticals) the power decreases.

As an example, Frank Williams tests showed a power peak for the Saitos to be right at 10,500 RPM. (I would concur with this statement if we assume the nitro to be 10%. If the nitro were assumed to be 20%, I would say the peak is closer to 11,500 RPM). Now lets say we bolt on propeller that spins at 11,000 RPM. Right there you are already working on the descending part of the curve. Particarly if the curve is steep (as in the case of a speed pipe) the power increases as the propeller tries to slow down the , if the prop tries to speed up the power decreases. This peak is independent of any propellers (again, assuming a steep curve). In this case the only function of the load from the prop is to be sufficiently underloaded as to allow the engine to start at an RPM that is on the back side of the peak.

I spoke to Frank again the other night at length. I am very interested as to how to increase the steepness of the power curve, be it either the 4 cycle or the 2 cycle.

It appears that the nitro content serves to move the peak to the right, or to maintain the location of the peak with changing conditions. I see this as no different than what the piped engines guys do too.

I was thinking about this this past week. As to what would happen if we ran our 2c engines "past" the peak, just like a piped engine. I was planning on running my 2c non-piped engine next year like this. Or at least to experiment in that direction.

Crist Rigotti
"A driver trying to be a pilot."

LAST EDITED ON Oct-27-03 AT 01:03 PM (CST)
 
>I was thinking about this this past week. As to what would
>happen if we ran our 2c engines "past" the peak, just like a
>piped engine. I was planning on running my 2c non-piped
>engine next year like this. Or at least to experiment in
>that direction.

That's what we have been doing since 1948, in one form or another. That's what the 4-2 break is about, for instance. Running on the low RPM side of the peak is always unstable. It follows that any stable system has to be past peak.

People only started getting into trouble when the peak was higher than they were trying to run. The "schneurle wars" and associated runaway problems were due to the torque peak on schneurle motors being far higher revs than many were willing to run. A 40FSR will spin a 13-6, and to get that to fly a decent speed, you need about 8500 rpm. Problem is, the engine power peaks at about 11-12000. Hence - unstable.

One of our more infamous threads discussed this at length:

http://www.clstunt.com/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=3841&forum=DCForumID1

Of course, the performance of any particular combination depends a lot on where you run (not just before or past peak). An ST46 will probably spin a 12-4 2-blade fast enough to get a decent level flight speed, but it's gutless in the maneuvers since it's too close to the peak, and thus provides poor control. It's a better prop than a 12-6, but you have to have the engine "live" enough to regulate. Same with a 9-4 on a 45FSR - plenty of engine "liveness" but not enough prop to fly the airplane well overhead.

That's the beauty of piped engines - you have a fair bit of control over the operating point.
Brett

LAST EDITED ON Oct-27-03 AT 02:12 PM (CST)
 
>That's what we have been doing since 1948, in one form or
>another. That's what the 4-2 break is about, for instance.

I am not so sure.

Sure, I do believe that the NET EFFECT of plane, prop, engine run (load, fuel, compression, etc) can result in a SYSTEM that appears to maintain constant speed, but I am not so sure that we have truly been running the engine itself on the "back side of the curve" in the past. I am not saying this from an informed position either, I really do not know.

In fact, Frank's dyno testing pretty much proves that most of the power peaks of even classic stunt engines are way further out on the RPM band than they are run in the classic mode. A Fox 35 might actually dyno to peak way over 10,000 RPM for example.

In running the 4 cycle I was able to achieve pretty constant speeds by matching prop load, pitch, airplane weight and drag, but this is certainly much different than running the engine truly on "the back side of the curve".

> Of course, the performance of any particular combination
>depends a lot on where you run (not just before or past
>peak). An ST46 will probably spin a 12-4 2-blade fast enough
>to get a decent level flight speed, but it's gutless in the
>maneuvers since it's too close to the peak, and thus
>provides poor control. It's a better prop than a 12-6, but
>you have to have the engine "live" enough to regulate.

Exactly. These engines did not have a tall enough peak to allow running on the back side. The slope was too flat near the peak (my theory) so the reaction time of the engine to the demands of the airplane was too slow or non existant.

Todays modern schnuerle engines and 4 cycles have a much more spiked peak and much more power reserve. Dropping the pitch and allowing the engine to run at a higher power just makes the effect more dramatic.

I agree with Brett.

I've been there, done it (I used an OS.32F at 12,000 rpm, 10 X 3 prop, 10% nitro, 23% oil, half castor half synth, Fox Muffler with its butt sawed off to have a BIG hole on it, can't recall the venturi size, PA NVA, Dubro platic tank modified to uniflow and muffler pressure.

It finally worked, I nicknamed it "the poor man's pipe".

Brett helped me a lot!

But...

When I recently put my hands on a REAL tuned pipe, WOW!

That was the real thing, I love it.

I'd pipe my car if I could...

God bless you!

In Christ,

To me, the concept of running on the back side of the power curve is wrong. I see where, in theory, if the engine tries to speed up going down hill that the power will fall off further, limiting the rpm increase, but there are other methodsof doing this (not the least of which is the fact that the tank is above the engine and causes it to go ric). However, with the same set up (running on the back side of the power curve) and going up hill you have a big problem. Yes, when the engine slows down it will gain power, but it IS slowing down, and therein is the problem.

I am not looking for a constant engine rpm run, but a constant airplane speed run. Thus we ideally need the engine to not only slow, slightly, down hill, to help with a braking action, but we need it to increase its power going uphill. A slight increase in power and rpm giong up hill helps to maintain that constant air speed in the maneuvers. This is where the 4-2 run comes in to play, as does the lean 4-cycle that still picks up rpm when the engine is pointed uphill. An increase in rpm in this mode is thus desireable, so long as it is controlled, but it is someting that running on the back side of the power curve will not allow.

In addition, when flying on the back side of the power curve, you will be setting the engine to run at an excessively high rpm. I am not talking here about the high rpm, low pitch run for a Schnurle. I am talking about going over the curve to an excessive high rpm, which, by its very nature, will thus force you to limit the size of prop that you will be able to turn.

There may be three or four ways to set up an engine, some of which work better than others. Some have tried excessively long pipe settings on their pipe engines to achieve the same result, but to me, this is not the way to do it.

Combat flyers, who do not want to slow at all in maneuvers, have been running their engines past peak HP RPM for years. I don't think they care if they speed up in maneuvers.

>To me, the concept of running on the back side of the power
>curve is wrong.

To run it otherwise is unstable - see above. Hard to get around that.

BTW, the thing Brad (and maybe you) may have overlooked is that we are talking about the HP *as run* - not with the mixture optimized at every point as traditional dyno testing has done. Not surprisingly, this makes a *huge, huge* difference in the curve. A Fox, run at a flight mixture setting, has a much lower peak than if you come up with a HP curve where you vary the load and then max if out at every point.

Lest we forget, to add to the equation, the higher rpm lower pitched prop will naturally provide more static thrust than the lower rpm higher pitched prop, notwithstanding position on the curve (about which I agree with Bret and Brad), and that's good! Brad, you proved this to me in spades when we worked on my prop setup. I am still in awe of flying the Legacy through the pattern on a 6 second lap time, when before, a 5.35 was pushing it! I think it is BOTH the position on the power curve AND the additional static thrust. Unless I am misunderstanding things, these are two separate things.

Brad's essay on running the 4 Stroke beyond the power peak sounds very much like Danny Dirt's article in Stunt News. The one that caused so much flak from the East. Throw in a few references to "Puddle Poo" and "Witch Slapping" (I gave Dan flak for the latter), and it'll be "just like Deja Vu all over again".

My question is what happens when you start coughing valvetrain bits out the muffler, and they get stuck in your highly polished model, or some sharp-eyed judge or competitor sees your engine's internal parts scooting across the pavement? Steve

>Brad's essay on running the 4 Stroke beyond the power peak
>sounds very much like Danny Dirt's article in Stunt News.

I am highly offended, I may start a letter writing campaign...

>My question is what happens when you start coughing
>valvetrain bits out the muffler, and they get stuck in your
>highly polished model, or some sharp-eyed judge or
>competitor sees your engine's internal parts scooting across
>the pavement? Steve

There is that, but it appears that the 4 strokes see to be even happier with the more free wheeling high RPM setup than with the brutal loaded setup. The OS Max in particular is very, very, happy at 11,000 RPM.

I don't sweat breaking a Saito anymore than any other mook with his PA or RO Jett, and I have seen a few of them shell some parts (bearings in particular).

My current Saito setup actually runs cooler than it did before. Along with increase in RPM, the throttle has been opened by double. The engine is breathing much better and burning more fuel.

>>To me, the concept of running on the back side of the power
>>curve is wrong.
>
> To run it otherwise is unstable - see above. Hard to get
>around that.
>
> BTW, the thing Brad (and maybe you) may have overlooked is
>that we are talking about the HP *as run* - not with the
>mixture optimized at every point as traditional dyno testing
>has done.

Not true. Frank's testing used a 4 cycle setting on the 2 cycle, not a top RPM setting. Obviously, these curves are far apart.

Len,

I ain't no "rokkit injineer" like Brett, he has all the math, gfraphics, etc on the tip of his tongue.

I'm just like the blind man on The Gospel of John, whr the pharisees were gettin' on his case , asking him if Jesus was the Messiah or not.

He said something like "I don't know about theology, prophets, if he's the Christ or not, etc etc., but one thing I know, I was born blind and now I can see"...

All I know is that at 12.000 rpn with a 10 X 3 prop that little stiker pulled a Brodak Tanager like Superman, and lost no speed on maneuvers.

At first I was using no nitro, i was (and still am) an ignoramus. The ship was slowing a tiny bit on maneuvers, but it could do them.

Then, Brett talked me into using 10%.

He said that the engine at the same 12.000 rpm would have more space to breathe".

It sure worked! Now, I noticed a slight increase in the maneuvers, maybe a couple 100 rpm, but it flw nice and dandy.

My buddy Chicão flew it, and the pattern he did was almost as good as the one he did with his BIG stunter.

I am too dumb to deal with "concepts", all I have to share is my experiece, just the plain facts.

God bless you!

In Christ,

I have read Brad's original post several times I just cannot agree with the load is not the rpm governer concept. I have seen first hand and so has he that the load is the ultimate RPM regulator. We have no ignition system so the only way to regulate rpm is with load. All of our model engines 2s and 4s are ultimately regulated by the LOAD. Without a prop it would spin itslef into oblivion. Pretty simple. 14X5 goes 8800 and a 12X3.9 goes 11200. That would be a load effected rpm right? Steve threw a prop in mid air at the NATS and it went nuts until it starved out for fuel. It spun harder than I have ever heard a motor spin. It was cool.

Also load effects how fast it will fall away from peak power. In the case of the 4s I have witnessed the exact opposite of what happens in a 2s.

Example.

You have 14X5 on there running a loaded setup. Nowhere near max rpm of the valvetrain itself. On the ascending power curve of the motor. Yet when the motor is pointed up it falls on it ass. You can visually see the plane slow down due to power loss. If the power was there it would not slow down. The load really chokes off the motor in a lean situation. Then when pointed down the motor gets a rush of fuel and spins that prop no problem. An descending tip pitch kills the speedups, ala Berringer.

You have a 12X4 on the same motor at 11200 nearing max rpm as specified in the manual. This is theoretically by all you theorists out there, on the mysterious backside of the curve. However when the motor is leaned in a pointed up attitude it has much less trouble keeping the plane on the move. The engine keeps the prop spinning in the lean situation, hence less LOAD on the motor equals less trouble keeping the prop nearer the power range. It falls away from the curve slower than the much more loaded setup stated above. On the way down the rich fuel doesnt create speedups due to the flat pitched prop.

On a 2s running under the peak at say 8800 always has the chance of running hard on you at any point when the nose is pointed up, hence the unstable tag. The 4s just falls apart. It acts just the opposite from what I have seen and experienced.

I have found that the less the load the better and more consistant the run.

The load is the ultimate rpm regulator on the engine. There is no other adjstment that can be made. Shims, pipe length, that is not an option. LOAD determines the style of run not the engine setup. It is opposite in a 2s. I can setup my 2s the way Len runs his and I have to run a certain style of prop. I can reverse it and have to run a totally different prop. However the props are not interchangeable on each setup. On a 4s you just swap props and set the RPM and go. Pretty cool for sure. However the less loaded setup is way more productive IMHO...

Brad said, "In this case the only function of the load from the prop is to be sufficiently underloaded as to allow the engine to start at an RPM that is on the back side of the peak."

Doug, I think you guys are agreeing, just from different points of view. Brad, correct me if I'm wrong, but I think you are saying that ONCE YOU GET SUFFICIENTLY UNLOADED TO GET ON THE BACK SIDE OF THE POWER CURVE, you get the desired effect. There can still be a matter of degree (rpm change?), but the general effect has been obtained. I think I'm reading that the above quote is the heart of his point about prop load. Right? Wrong?

LAST EDITED ON Oct-28-03 AT 00:47 AM (CST)
 
Yes Craig we agree on alot of this. But he did say that the load is no longer the govener. The load is always the ultimate govener on any engine.

In the case of each individual airplane they all need their own loads that work with that plane and engine combo. Like your broken Legacy. Once it is fixed the prop you had been using may no longer be the one. It may under overload the motor in conjunction with with the new weight of the plane. But it probably wont be that much different.

I think we are agreeing for sure. But I simply say throw out all this talk of back side of the curve and theories and stuff and get to talking about what is really happening in the air. The test stand power curves are very enlightening for sure and can give us a real starting point but when it comes down to it it doesnt compute for me.

Take off at 8800 on a large prop. The motor unloads and gains some RPM. Now it is flying along just below the stated "curve" As the motor leans in a positive AoA it should make good power as it approaches the top of the stated curve and pull on through the maneuvers with authority. Yet it doesnt it slows and falls off at a quick rate. By the end of the square eight it is begging for a break. The load has killed the motor. OR and that is a big "OR" the motor is actually losing RPM in the lean state of Positive AoA. However there is no real way to monitor the RPM in flight. I have thought for some time now this could be the true culprit of the power loss when the 4s is running a large prop in a heavily loaded situation. Load up a 2s and it just breaks and goes into high gear. The 4s cant so it just burns down. It leans but actually loses RPM?.?...????

Take off at 11200 with no load and just above the peak unlaod and you are past the "curve" by a good amount. Add in positive AoA and you are way over the curve, unless it actually loses rpm, yet it makes good power and stays in the maneuver with much more authority and the lap times are in much more unison with the maneuver speeds. Showing for sure that the motor is making much more power in an underloaded state. Simply put it wants to spin.

If it is actaully losing RPM in a lean state then this would explain why the less load props are working so much better than the long 14" props. The less load loses less rpm and lets the motor stay near the optimum operating range. Just thoughts....

In the 2s running below the curve will get you a runaway on an FP 40 once it unloads and on some other 2s motors it will get you hard boosts if you run below the curve then lean it with positive AoA. The motor climbs up the curve as it leans. With the 4s motor it always seems to loose power no matter what load you have on it. So the key is to underload it. It seems to me that changing the load changes the behavior of the power curve by quite a bit. I know many wont agree. But come fly the plane in the two setups and it will prove it to you every single time. The 2s proves out exactly as many beleive. The 4s doesnt seem to do so the way I have seen it work first hand in the air.

I wonder if we are mixing metaphors on some of this. I am agreeing with Doug in a lot of what he says (scarry, isn't it?) but am also wondering if people (such as Brett) are speaking of being on the back side of the torque curve, not the power curve. Brett talks a lot of using a high rpm, low pitch run. In doing so, the engine still has not reached the peak of the horsepower curve, but is a lot further up than when running the same engine with higher pitch and lower rpm.

Under those circumstances, some engines will lean out and then go bonkers. When a 2-stroke engine is run rich, it will pick up power (and rpm) when it leans out. How much it does so will depend on a number of factors including timing, compression, and load. That is where we need to control the run. But we are still running the engine on the low side of the peak power curve. And we still want the engine to increase some in rpm when aimed up.

Iskander here mentions the load on the engine as being the prime factor when the engine is aimed up. It is a factor, but I am not convinced it is the primse factor. Sure, if the engine is on a test stand running rich and you grab the spinner (carefully) you will increase the load and it will lean out. That is the effect of load. However, take your 2-cycle engine set for a stunt run and tach it with the plane sitting level. Now aim the plane up (it is not flying, you are merely holding it, so there is no increase in load) and you will get a higher reading on the tach. In most cases you will even hear the difference. This is what I am saying is needed. But this effect comes, in this particular situation, not from increased load, but from attitude with the fuel. It is the fuel draw there here leans it out. So load AND fuel draw work together with the 20stroke engine. And you will not get that effect if you are running the engine beyond the power peak (horsepower).

Still, when you are running closer to the power peak (faster) you are not only delivering more power to the prop, but have less in reserve for runaway if the engine is not controlled in some other way. I am talking of still running below the peak power curve, however. Run past that curve and you limit not only the power, but the amount of prop, thrust, all sorts of things.

Now, this thread was started talking about 4-cycle engines. I don't have any experience here and can only comment on what I have seen or observed. If the 4-cycle does not respond like the 2-cycle in picking up rpm by leaning it out (pointing the nose up), and if you are bogging it down on the low side of the power curve, when you point it up, then you do, indeed, increase the load without getting a corresponding increase in power. This will result in a loss of speed in the maneuvers. This is what I have observed in a lot of 4-cycle runs. If you run the engine faster with a lower pitch prop (but still on the low side of the peak power curve) then you may be experiencing more power and less percentage of slow down when aimed up. That may be what is happening with Brad, and that may, indeed, be very beneficial. I am still not convinced that you are running on the back side of the power curve (haven't witnessed it) or that it would be beneficial to do so. Still seems counterproductive.

Where is the horsepower peak, and what rpms are you truly running?

>I think we are agreeing for sure. But I simply say throw
>out all this talk of back side of the curve and theories and
>stuff and get to talking about what is really happening in
>the air.

I think this is fun stuff!

Load up a 2s
>and it just breaks and goes into high gear. The 4s cant so
>it just burns down. It leans but actually loses
>RPM?.?...????

Ke-rect. That is one way to get the "brake effect" with a 4 cycle. "It just burns down". Absolutely correct. Running slightly leaner always reduces windup, I have always said that. Leaner runs promote the "burn down effect".

Works good at 80 degrees.

I am not into that anymore.

Now I am into using the torque curve. In fact, I plan to reduce the exhaust temperature to try to eliminate the "burn down effect". It simply is not a positve thing, getting the motor so hot it just gives up.

>Yes Craig we agree on alot of this. But he did say that the
>load is no longer the govener. The load is always the
>ultimate govener on any engine.

I do not think this is completely true.

Given that the power peak is sharp and drops off steep enough, the prop could break in half and it still would not go any faster. This has always been the true theory behind using the pipe on a 2 cycle. Unfortunately, this peak is not pronounced with the baffled pipes as it would be with, say, a speed pipe. The effect appears to be way more subtle with the current stunt pipes (very load dependant like you said).

Ion,

Thinking "out the box" leads to progress. Refusing to do so will only ensure that someone else will, and use it to beat you. IMHO.

Brad

LAST EDITED ON Oct-28-03 AT 10:58 AM (CST)
 
Brad, correct me if I'm wrong, but I think
>you are saying that ONCE YOU GET SUFFICIENTLY UNLOADED TO
>GET ON THE BACK SIDE OF THE POWER CURVE, you get the desired
>effect.

Ding! Ding! We have a winner! Indeed, that load is important, but less important once the engine becomes self limiting.

What Doug may be missing is that the curve has to be sufficiently steep to realize an effect. If the curve is too flat, you get the effect that Len described, which is the airplane not reacting fast enough to the increase and decrease in power.

The curve must be steep and sufficient up on the power numbers to get the effect.

Did anyone see Frank William's plane at the Nats? It was literally unaffected by the wind. It was slowing down and governing the speed more than any plane in the competition. Frank has always asserted that the baffled pipes have little power curve effect compared to a speed pipe.

This is why I started the thread in the first place. It appears that we are still governing the engines with load (as Doug said) which is really not that far away from the Fox 35 days.

Maybe it is time we stopped doing that... or not... I can tell you this, I am interested in what Frank is saying, And I think very few have really taken the time to try to understand it, including me. Not true any more.

Doug, You are leaving out one RPM limiter, Valve Float. When I was racing 4 strokes(pylon) this was something we had to figure into out prop choice. There is a point where the valves begin to float and the engine stops putting out HP and almost sounds as if it has gone to a rough idle. In the older 4s that I was racing this was around 12,000 rpm. Mind you this was the older OS 90 but I do not think the springs have been stiffened for any new motor of today as there would be no reason. Just part of the equation.
Richard Oliver

Richard,

We havent gotten into any valve float yet. I had it once on an enya and it wasnt a pretty sound.

>Doug, You are leaving out one RPM limiter, Valve Float.

I do not think that will be necessary, ultimately. Even though I am not sure the valves might not be floating mildly in some of my runs. There seems to be no ill effects. The engine sounds wonderful, and in fact vibrates less the further up the RPM range you go. Kind of like the TP motors, huh?

Doug,

you are kinda oversimpifying the thing...

OK, the LOAD is governer, but what is load? Only a big prop?

On a heavy ship, you try to climb, and the engine is loaded because it is like on a break in stand.

You know that Brett's engine tekesoff at 10.100 rpm, but as son at it gos the engine goes to 10.800 rpm.

SO:

Load=prop + weight of the plane...

NOW,

If the engines has enough ponies to handle the plane, like a piped .60 w/a 53 oz.ship, the ship ceases to be a big part of the load equation.

Bon Hunt has his new Genesis at 47.5 oz, 630 sq. inches..

Will the prop be the sole govenrnor?

That;'s where the tuned pipe on 2s engines coms in.

You start the wingover and the engine pulls the sucker up, no sweat.

On the way down, the pipe won't let the engine go "free", and it brakes the plane.

An interssting test would be to put the piped engine on a stan, and start gradually using smaller props.

The pipe should hold the revs at some point, maybe thesmaller the prop higher revs, but the engine shouldn't (and can't) just scream like a banshee...

Another thing to consider:

Strong engine is less affected by the load.

SO:

The governer is not the load "per se", but the ship - engine power - prop ratios.(and some other things we probably ignore)

Makes sense?

.

God bless you!

In Christ,

Ion,

You are very correct but when working on the plane the prop is the one thing we are changing to change the load. All the other things you mentioned are constant and dont change. Weight of the plane length of the lines these kinds of things stay constant.

>Ion,
>
>You are very correct but when working on the plane the prop
>is the one thing we are changing to change the load. All
>the other things you mentioned are constant and dont change.
>Weight of the plane length of the lines these kinds of
>things stay constant.

So you mean that given a fixed set of factors that will remain constant and the only variable being the prop, the load is the governer. And the load will be affected by the prop used at each situation.

Is that it?

Well, if you put it like this, I gotta agree 100% w/you!

God bless you!

In Christ,

>You have 14X5 on there running a loaded setup. Nowhere near
>max rpm of the valvetrain itself. On the ascending power
>curve of the motor. Yet when the motor is pointed up it
>falls on it ass. You can visually see the plane slow down
>due to power loss. If the power was there it would not slow
>down. The load really chokes off the motor in a lean
>situation. Then when pointed down the motor gets a rush of
>fuel and spins that prop no problem. An descending tip
>pitch kills the speedups, ala Berringer.

The exact same thing should happen on a 2 stroke if you load it down below the power curve peak (or, actually, I think, the torque curve peak). And I don't think it's got anything to do with fuel. It's the change of load on the prop. Going up -> more load, going down -> less load. Since it's on the front part of the curve, a little added load leads to a great deal of slowing down of the prop.

>You have a 12X4 on the same motor at 11200 nearing max rpm
>as specified in the manual. This is theoretically by all
>you theorists out there, on the mysterious backside of the
>curve. However when the motor is leaned in a pointed up
>attitude it has much less trouble keeping the plane on the
>move. The engine keeps the prop spinning in the lean
>situation, hence less LOAD on the motor equals less trouble
>keeping the prop nearer the power range. It falls away from
>the curve slower than the much more loaded setup stated
>above. On the way down the rich fuel doesnt create speedups
>due to the flat pitched prop.

And this is exactly what you'd expect on the back side of the torque curve. Point the plane up, increased load on prop, prop tries to slow down (AND it will slow down a small amount), but the increasing torque keeps it from slowing down anywhere near what you'd experience in the first case.

I've heard it's not just stunt plane motors that operate on the back side of the torque curve. Caterpillar tractors do too, for the same reasons. The more the engine slows down, the higher the torque that resists the slowdown.

>>You have 14X5 on there running a loaded setup. Nowhere near
>>max rpm of the valvetrain itself. On the ascending power
>>curve of the motor. Yet when the motor is pointed up it
>>falls on it ass. You can visually see the plane slow down
>>due to power loss. If the power was there it would not slow
>>down. The load really chokes off the motor in a lean
>>situation. Then when pointed down the motor gets a rush of
>>fuel and spins that prop no problem. An descending tip
>>pitch kills the speedups, ala Berringer.
>
>The exact same thing should happen on a 2 stroke if you load
>it down below the power curve peak (or, actually, I think,
>the torque curve peak). And I don't think it's got anything
>to do with fuel. It's the change of load on the prop. Going
>up -> more load, going down -> less load. Since it's on the
>front part of the curve, a little added load leads to a
>great deal of slowing down of the prop.

Load down and FP 40 take off and it may run rich for a few laps. Do a wingover and lean the motor. It climbs the curve and gets into a 2s and it is off to the races. Now we all know there is alot more going on there but it is just an example. Brett said that it can be unsatble to run under the peak. I was trying to make an example. At least it has played out for me this way on many different motors.

When the load is raised on a 2s it can break from 4 to 2. The 4s cant break so it just peters out until it gets all of its fuel again. The fuel and the load work together in both situations.

>
>>You have a 12X4 on the same motor at 11200 nearing max rpm
>>as specified in the manual. This is theoretically by all
>>you theorists out there, on the mysterious backside of the
>>curve. However when the motor is leaned in a pointed up
>>attitude it has much less trouble keeping the plane on the
>>move. The engine keeps the prop spinning in the lean
>>situation, hence less LOAD on the motor equals less trouble
>>keeping the prop nearer the power range. It falls away from
>>the curve slower than the much more loaded setup stated
>>above. On the way down the rich fuel doesnt create speedups
>>due to the flat pitched prop.
>
>And this is exactly what you'd expect on the back side of
>the torque curve. Point the plane up, increased load on
>prop, prop tries to slow down (AND it will slow down a small
>amount), but the increasing torque keeps it from slowing
>down anywhere near what you'd experience in the first case.
>
>I've heard it's not just stunt plane motors that operate on
>the back side of the torque curve. Caterpillar tractors do
>too, for the same reasons. The more the engine slows down,
>the higher the torque that resists the slowdown.

QUESTION??? LOADS are it not the "DRAG FORCE" (my term) on the plane and engine NO mater what the prop is? Size of prop is just the THING that turns RPM’S into forward motion. LOAD is heat, fuel consumption, timing, (i.e. RPM’S)
A plane that is on the test stand has load, that can’t be duplicated in the air. So when an engine UNLOADS what happens? Engine reaches peek performance and CLCYES in tune. IS this the BACK SIDE of the curve? SURE I think. DID that make sense???

Scott (DYNOS don’t tell the Story) Riese

>>Scott (DYNOS donÂ’t tell the Story) Riese

Not to be argumentative, but if dynos do not tell the story for stunt, it would be the first application of the internal combustion engine where the dyno did not tell the story.

The dyno reveals the true nature. How you choose to use the nature of the engine in its application is a different story...

>>>Scott (DYNOS donÂ’t tell the Story) Riese
>
>Not to be argumentative, but if dynos do not tell the story
>for stunt, it would be the first application of the internal
>combustion engine where the dyno did not tell the story.
>
>The dyno reveals the true nature. How you choose to use the
>nature of the engine in its application is a different
>story...
"The point of what I am saying is take the prop away. the prop is simply the transmission. The engine output is the engine output. First understand the engine output, then match the transmission to the prop. Analyze the source of the power not the system. The same can be said for dragsters, motorcycle engines, road racers, etc. First understand the engine, then design the transmission to the best benefit"

ZILLA....Transmission is a good word to use. THIS is what I was leanning too. "SOURCE OF POWER NOT THE SYSTEM"

Scott(need to express myself better)Riese

LAST EDITED ON Oct-28-03 AT 03:25 AM (CST)
 
Brett's explained this phenomenon before somewhere on this forum (and he's got the graphs and everything to explain it). What happens is that, for that needle setting, there are two stable conditions - one is running slow and rich, the other lean and fast. What bumps it out of one to the other is either actual leaning out, OR it might actually take place on the downward leg of a maneuver. On the downward leg, load decreases, and since you're on the front side of the torque curve, RPM increases a LOT, the engine makes more power, causing even more speed, and then you're over the torque curve hump, and into a region where, when you increase the load on the engine again, it'll resist slowing down and you're stuck with a high RPM run.

OK, I found it..

http://www.clstunt.com/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=3841&forum=DCForumID1#3

>And this is exactly what you'd expect on the back side of
>the torque curve. Point the plane up, increased load on
>prop, prop tries to slow down (AND it will slow down a small
>amount), but the increasing torque keeps it from slowing
>down anywhere near what you'd experience in the first case.

...and the high RPM and low pitch quickens the reaction time. Like low gear.

LAST EDITED ON Oct-28-03 AT 11:46 AM (CST)
 
I think speaking about POWER curve is oversimplification.

1/ Why to speak about Power curve? Isn’t it better to speak about TORQUE curve? The running RPM is stable at point where load of prop (torque necessary to twist the prop at given RPM) equal to torque on engine shaft. If load by prop changes little, the RPM of engine WILL change – does not matter if you are on up side or down side of torque or power curve. The only important is ratio of slope of that engine and that prop curve (either power or torque) – it gives response of RPM on speed difference. What I want to say is, that you need to think also about prop. Pictures provided by Brett in that other thread mentioned by Len shows it clearly.

2/ Why you think the peak of torque curve is something fixed? The richness can alter that point dramatically. And not only statically, it is variable in-flight parameter. If you slow down, the prop makes more thrust – means the fuel pressure at needle is lower, mixture is leaner, and peak is at higher RPM. Result is ACCELERATION on LOAD – exactly what Len calls for.

3/ Why you think slowing down converts to more load of prop? UCT props load MORE if they produce LESS lift - means in level, or even downhill. It means that not only engine torque curve is variable in flight (varies with acceleration – point 2/), but also prop load curve is variable (varies with speed).

Looks like speaking about 4 dimensional charts

>I think speaking about POWER curve is oversimplification.
>
>1/ Why to speak about Power curve? IsnÂ’t it better to speak
>about TORQUE curve?

In speaking with Frank, he said the curves are basically the same. the torque pretty much follows the HP curve. The peaks are very close.

The running RPM is stable at point where
>load of prop (torque necessary to twist the prop at given
>RPM) equal to torque on engine shaft. If load by prop
>changes little, the RPM of engine WILL change – does not
>matter if you are on up side or down side of torque or power
>curve. The only important is ratio of slope of that engine
>and that prop curve (either power or torque) – it gives
>response of RPM on speed difference. What I want to say is,
>that you need to think also about prop. Pictures provided by
>Brett in that other thread mentioned by Len shows it
>clearly.

The point of what I am saying is take the prop away. the prop is simply the transmission. The engine output is the engine output. First understand the engine output, then match the transmission to the prop. Analyze the source of the power not the system. The same can be said for dragsters, motorcycle engines, road racers, etc. First understand the engine, then design the transmission to the best benefit.

>2/ Why you think the peak of torque curve is something
>fixed? The richness can alter that point dramatically.

For a 2 cycle engine, yes, not so much for the 4 cycle. The mixture has much less effect. None positve from my experience.

>3/ Why you think slowing down converts to more load of prop?

When is the engine the most loaded? When the airplane is standing still. The prop is cavitating on itself, right? When the airplane slows, the same must be true to some degree...

LAST EDITED ON Oct-28-03 AT 02:30 PM (CST)
 
>>>In speaking with Frank, he said the curves are basically the same. the torque pretty much follows the HP curve. The peaks are very close. <<<

I do not know what you spoke exactly, but torque curve peak cannot match power peak. The power peak is exactly on one very well specified place of torque curve. This is true because of fact that:

Power = torque * RPM * constant

The constant depends on units you use (I am metric guy, so I do not place exact number) not any property or power train. It means that power peak is at place where torque is decreasing the same way the value (RPM * constant) is increasing. It means the maximal torque is always at lower RPM then maximal power because torque is decreasing at higher RPM.

The same goes on with torque for prop and power for prop. So you can just forget RPM (it is the same for prop and for engine) and you are at different numbers – because peak for torque is at different place than peak for power. I hope now you see that speaking about power or torque peak is not enough, you MUST see also prop.

>>> The point of what I am saying is take the prop away. the prop is simply the transmission.<<<

That is what you cannot. You say that “back side” is stable. It is true and I agree. But I say that it can be “more” or “less” stable. The stability (amount of rpm difference depending on amount of load change) is higher if engine torque curve meets prop torque curve at higher RPM, because descending slope of engine torque and ascending torque necessary to twist the prop has stronger and stronger slope at higher rpm – one goes up, the second goes down. That is all. And it also means that if ascending torque for prop is increasing enough, the stability is still present also if either torque curve of engine or power curve of engine is still ascending. You cannot say “it is good past the peak and wrong before”. You can only say it is better at higher and worse at lower RPM. Past or before peak is not any qualitative difference.

But if we say that higher rpm is better, the final result can be still disappointing. If torque is descending and you want cross it at that place by prop shaft load, the prop must be small. But smaller prop has worse response to speed change – in lift and also in drag. The trick is, that if you slow down by 10%, the smaller prop rotating at the same rpm brings less thrust than bigger one. The same goes on with drag – smaller prop has flatter torque curve and thus also the slope is not so strong at that place. So if you go to too high rpm, small prop can cancel the gain. The same goes on with too large prop – it has good response, but you can kill stability in too low RPM. Something similar goes on with pitch – just opposite. The only way is to find that sweet place where all works well.

>>>For a 2 cycle engine, yes, not so much for the 4 cycle. The mixture has much less effect. None positve from my experience. <<<

You are right, I did not read it well enough

>>>When is the engine the most loaded? When the airplane is standing still. The prop is cavitating on itself, right? When the airplane slows, the same must be true to some degree...<<<

Good question, but answer is not trivial. Our props are at or under ZERO AoA in level flight (unloaded PROP THRUST). If you slow down, it gets higher AoA, means the prop makes higher lift. But shaft load depends on airfoil drag, not on produced lift. Undercambered airfoils has typically high drag at no or little lift, while semi symmetric or symmetric airfoil has typically low drag at 0 AoA. It means that UCT prop can unload engine shaft if you let it produce some moderate lift – UCT airfoil has clear optimum. If you load it lot (on ground), it gets drag; if you unload it (level, downhill), it gets drag again; but it clearly unloads engine uphill. Flat of semi symmetric airfoil is not so resistant.

>When is the engine the most loaded? When the airplane is
>standing still. The prop is cavitating on itself, right?

No- not cavitating. Cavitation involves a state change, and can really only happen in incompressible fluid. Cavitation can happen in a boat prop, in the water.

Separated, stalled flow is what you are talking about, not cavitation.

>The point of what I am saying is take the prop away. the
>prop is simply the transmission. The engine output is the
>engine output. First understand the engine output, then
>match the transmission to the prop. Analyze the source of
>the power not the system. The same can be said for
>dragsters, motorcycle engines, road racers, etc. First
>understand the engine, then design the transmission to the
>best benefit.

That is just it. You cant take the prop away. I have seen it and the motor spins uncontrolably until it shuts itself off and it is way higher than any rpm we have ever tried. It screams loud very loud. You stated earlier that if you broke the prop in half it wouldnt run any faster. Not so. It will run faster. Try it on the stand and you will see. Some run the motor without a prop and you will be amazed. It wont be to good for it though.

Dont get me wrong I undersatand what you are saying. The dyno helps and it helps out alot.

Sure we design the transmission, er Brian does anyway... He seems to have a good idea of the best way to get it working...

After pokin' my nose in this discussion and takin' a good look at the writings I arrived to a conclusion:

We (we = all the SSF gang) have the bad habit of starting threads on a "it's possible", "I think", "IMHO", "this concept" and so on.

Since there's no real meat (facts) to chew on we start getin' carried by the arguing just for arguing's sake.

Sometimes the thing gets ugly and our egos start getin' in the way.

I have a good friend, a psichiatrist, that told me once that if one wants to arrive at anything near truth but at the same time he carries a bag full of ideologies, he'll never be able to see any thruth, the ideology will blind him.

This whole thread reminds me of a heated discussion that took place during one of the first councils of the catholic church:

Them bishops were trying to establish, based on the scriptures, the number of teeth a horse carries on his mouth.

At some point they are calling each other names, bishop A is a heretic, bishop B an infidel, and ..well, very similar to waht takes place very often here at SSF.

At this point a young bishop suggested that instead of fiddlin' with the Bible why not just get a horse and count his teeth?

It's said that the rest of the guys gave him such a dirty look that he wisely figurd that such suggestions might be harmful to his health.

Since this ain't the catholic church and barbequeing people is out of fashion nowadays I can heve the chutzpah to suggest that instead of speculating withot no facts to tell that we do our thing in practice and share the results with the whole gang.

Instead of "I think that" or "IMHO", etc., how about "I did this and I got the folowing results".

BTW, Brett usually posts stuff he found out, no speculation.

And many times with ample documentation.

Am I being a party pooper?

God bless you!

In Christ,

My question was one of semantics, not what works or doesn't work. How do you prove you are on the back side of the power curve? I have heard this said before and I have heard of people who have tried it. But... ??

I am not in any way questioning Brad's conclusions that his engine runs better for him with his present set up of higher rpm's and lower pitch. I am thinking that he is running in the higher portion of the power curve than he was earlier, which is what is giving him his greater power, and not because he is on the back side. That was why I asked the question of at what rpm does the engine reach peak horsepower, and at what rpm is he running the engine? If he is indeed running it faster than the peak, OK, then he is indeed running it on the back side. I am not familiar enough with the 4-stroke to comment one way or another, but with a 2-stroke I definitely do not like the concept of running it on the back side.

Len,

is it possible to know what's goin' on with aan engine when it's airborn?

Brett knew, for instance, that his PA was launched at 10.100 rpm and that it picked up some revs after it ws on the go.

By pure luck he ws flyin' his plane when some guy fired his PA, same pipe, prop, atc.

He heard the sound and ralized that both engines were singing the same "note", like two flutes in unisson.

He screamed "Tach that thing!!!", and the guy did it.

Turned out that it was doin' 10,800 rpm, so he mow knows that after launching his bird at 10,100 the PA unloads to + 700 rpm.

To determine "beyond any reasonable doubt" on what side of which curve the engine is would demand a whole bunch of instruments on board of the plane.

We are discussing a "hipothetical impossibility"...

God bless you!

In Christ,

>Len,
>
>is it possible to know what's goin' on with aan engine when
>it's airborn?
>Brett knew, for instance, that his PA was launched at 10.100
>rpm and that it picked up some revs after it ws on the go.
>By pure luck he ws flyin' his plane when some guy fired his
>PA, same pipe, prop, atc.
>He heard the sound and ralized that both engines were
>singing the same "note", like two flutes in unisson.
>He screamed "Tach that thing!!!", and the guy did it.
>Turned out that it was doin' 10,800 rpm, so he mow knows
>that after launching his bird at 10,100 the PA unloads to +
>700 rpm.

That sounds about right. I always figured maybe up to a thousand rpm increase, but it certainly depends on the prop used, fuel, venturi, day--a whole bunch of things. But this is still considerably below the top of the curve for peak horsepower.

>To determine "beyond any reasonable doubt" on what side of
>which curve the engine is would demand a whole bunch of
>instruments on board of the plane.
>
>We are discussing a "hipothetical impossibility"... (snip)

>Ion

Not really. Most engine manufacturers had a rpm vs. horsepower curve. (This engine developes xx horsepower at xxxx rpm). That's a start. However, I know that when I point the nose up on my engine it picks up rpm. I know that when I lean the needle on the engine it picks up rpm. I know that just before it runs out of fuel it picks up rpm. I know that I am not on the back side of the horsepower curve and that it has more to give if I want it to. That's the way I want to run it.

>My question was one of semantics, not what works or doesn't
>work. How do you prove you are on the back side of the
>power curve?

Dyno the engine, find the power peak, then position the engine RPM past the peak. I have no doubt that I am running deep into the back side of the curve now. I also have no doubt that thrust effect is regulated much more by the regulation of the output of the engine than the load of the prop. From a very few practical tests I found that unloading the engine more results in even more regulation of the RPM in flight. It simply locks into "the curve" and stays there.

Frank said the power peak of the Saito was somewhere around 10,500. My GROUND RPM is greater than that, and the effect is even more drastic at a ground RPM over 11,000 RPM.

>I am not in any way questioning Brad's conclusions that his
>engine runs better for him with his present set up of higher
>rpm's and lower pitch. I am thinking that he is running in
>the higher portion of the power curve than he was earlier,
>which is what is giving him his greater power, and not
>because he is on the back side.

Frank said the power output is dependant on the venturi size. The peak position stays relatively the same, it just jumps up higher and higher (with the same slope?) with larger venturis.

That was why I asked the
>question of at what rpm does the engine reach peak
>horsepower, and at what rpm is he running the engine?

See above.

If he
>is indeed running it faster than the peak, OK, then he is
>indeed running it on the back side. I am not familiar
>enough with the 4-stroke to comment one way or another, but
>with a 2-stroke I definitely do not like the concept of
>running it on the back side.

I think the piped engines are definately set to run on the descending part of the curve (if the 4-2 break mode is not employed). I never said that pipes are not running in this mode. I was simply asking the question how do we increase the slope of the peak to make the effect even more dramatic. Frank's speed pipe simply falls completely off the charts if the engine tries to overspeed, with the baffled pipes the effect is less dramatic so the satandard stunt tunings still come into play a lot (compression, venturi, fuel, etc).

With today's modern engines I really feel there is very little need for "boost", we already have such a power to weight ratio none is needed, what we need is more "brake", especially in the harsh wind conditions. Typically constant speed (boost) is sacrificed in an attempt to get "brake". Using the steepest curve available would allow for both, without having to rob one from another.

I simply find it hard to believe that no one else saw what I saw at this year's Nats. The winds were howling and I don't think Frank even changed props. He certainly did not bias his manuevers any and flew straight down wind even in the worst winds of qualifier. He was closer to the Top 5 than he has ever been because his engine was simply the most regulated thing there (instant advantage). It literally was trying to idle down instead of wind up. It was really amazing. I am just trying to learn from what I saw...

I fly with Frank and we talk about these things and I was there the day he did the charts for the 4s and the RO-Jett 65. In Franks system the key is his exhaust, not his motor. The chart establishes where the drop off is in the curve and he sets his system up to work in that area. The full wave pipe does the work, not the engine. All full wave pipes have different tuning stages. In racing we have worked with at least 3. Stage 4 was too high of an RPM range to be useful. A standard FAI piped 40 pylon racer without changing anything will have 3 points of RPM where it will hit the pipe. This is assuming the prop is for the 3rd stage and can hit the first 2 stages in a rich setting. When looking for 26000 to 28000 rpm for racing the pipe will come on at 19,23, and finally at 26,000. I think what Frank is doing is operating in between these points where the pipe jumps on and then falls off. Can't keep it much more simple than that. I also do not think you can transfer this data to a non-piped set up for purposes of trying to find the back side as it is to shallow a drop of to do any good. Way to many variables to contend with to have a consistent day to day set up. The curve changes enough from day to day to be a 5 flight warm up to find it before you can practice or compete. The dynamics are the same as it is in racing and there the prop of the day became the factor that determined who won, that day. Today the prop might be bitting harder and the back side of the curve can not be reached. Next day it is free-wheeling and is always on the back side(not bitting enough). This operating range is narrow and appears to move but is really the load chaging. This is the nature of working the system in this realm. Just my experience on the subject.
Richard Oliver

>I fly with Frank and we talk about these things and I was
>there the day he did the charts for the 4s and the RO-Jett
>65. In Franks system the key is his exhaust, not his motor.
>The chart establishes where the drop off is in the curve and
>he sets his system up to work in that area. The full wave
>pipe does the work, not the engine. (snip)

>I think what Frank is doing is
>operating in between these points where the pipe jumps on
>and then falls off. Can't keep it much more simple than
>that. I also do not think you can transfer this data to a
>non-piped set up for purposes of trying to find the back
>side as it is to shallow a drop of to do any good. (snip)

>Richard Oliver

From what I remember reading of Franki's stuff earlier, this is probably closer to what he was saying. Although I don't agree with setting it that way exactly (it is choice) Frank was setting the engine to fall off the pipe if it sped up, and thus lose power and speed. But he was not running on the back side of the power curve of the engine, just the pipe. And he was still getting a power gain when climbing. This is something that wouldn't translate to the non-piped 4-stroke.

> But he was not running on
>the back side of the power curve of the engine, just the
>pipe.

This is, I think, the crux of the problem - the engine/prop/ancillary devices are *A SYSTEM*. Trying to make the sort of distinction you make here is completely misunderstanding the situation.

The effect of the pipe on the engine is to reshape the torque curve - to put a descending slope where you want it. On open exhaust, the torque curve is one thing. With a pipe, it's something else.

I would also note that the distinction between "speed pipes" as Doug calls them above (i.e. a pipe with a single reflecting surface - the converging cone - like Frank uses) and "baffled pipes" (pipes with multiple reflecting surfaces - like the Future Force, Smith Werewage, etc) is just that - the number of places the wave can reflect from. Single reflecting surface pipes reflect waves at only one principle frequency (and the harmonics thereof). Other pipes reflect at multiple principle frequencies. The effect on the torque curve is that the single-surface pipe provides only one "hump" (and associated "dip") in the torque curve, and the miltiple-reflecting surface pipe provides many humps and dips (that tend to merge together due to damping). Frank has a torque curve demonstrating this, but it's perfecly obvious that this is what should happen even without seeing the data.

I have seen Frank and his many different pipes for many years now. He has a few things different than the rest of the stunt world. One is the sound. Really different. Second he has a more responsive system then the baffle pipes. It really shuts down the motor quick. When it needs boost it is on and very quick about it too. I have seen his power curves and compared them to the Baffle pipe curves. His have such a more drastic fall off the power once it goes over the top. My question is how come people all use the baffle pipe? The empty pipe seems to be the one that would do more of what we are after "Braking" I want to try one but I dont know where to start.

Any suggestions?

Perhaps because the baffle pipes are easier to deal with. From what Richard was saying, getting the simple pipe "just right" for a given day might be more of a problem than we might think.

>(snip) My question is how come people all use the
>baffle pipe? The empty pipe seems to be the one that would
>do more of what we are after "Braking" I want to try one
>but I dont know where to start.
>
>Any suggestions?

If you want to try one, then why not go to Frank, get one of his, and try one?

However, with the big prop that Matt is using, with the baffle pipe (Smith/Werwage) and the set up that he is now using, the engine "thuds out" on the down hill leg. It is almost like slow motion going down the hourglass. Seems to be braking just fine.

But, it is not just about braking on the downside. I want a set up that will pick up a bit in the overheads. I want a system that will pick up "just a bit" when you point the nose up. I want a controlled run in doing so. And I want a system that is predictable, and where I have adjustabliity for colder or warmer temperatures, and where I don't have to make adjustments with props or venturis or pipe settings every time the weather changes. That is what the baffle system does for you.

As a follow-up, we spent tons of flights trying to find the set-up that matt is currently using. The best set up always became "square one" and from there we tried to make it better. We tried different pipe settings, different pipes, different venturis, different amounts of head shims. We went through over a thousand dollars worth of props experimenting with the small ones, the large ones, underchambered, 2-blades, 3-blades, 4-blades, props that weren't even supposed to work on the 61 (but did--oh, and some that were supposed to work on the 61 but stank). We repitched these, tried less pitch at the tips, even pitch at the tips, more pitch at the tips. Tried every trick that we could find whether we thought it would work or not. The goal has been one to constantly improve the engine run while working to constantly improve Matt's pattern. In the end Matt is happy with what was achieved, but the goal remains to still make it better.

Matt is on his third 61 engine now and that is getting worn out. He just recently got a new 65. Here is the real story. We put a couple of hours on the engine on the test stand (using an old 61 pipe for break in). Matt was already using the 65 pipe on his 61 engine, so when he was ready to put it in the air, he measured the head shims and put in the same amount he had been using in the 61. He used the same pipe, the same prop (which was used for break in) and the same needle setting he had been using on the bench. And the plane flew nice 5.2 second lap times in a deep rich tone with no wind up in maneuvers in 15 mph winds. It seems to be a successful system.

What will it do in 30 or 40 mph winds? I don't care. I wouldn't fly my "real" plane in that wind. I am not going to fly my model either. That is a good day to play chess.

>As a follow-up, we spent tons of flights trying to find the
>set-up that matt is currently using. The best set up always
>became "square one" and from there we tried to make it
>better. We tried different pipe settings, different pipes,
>different venturis, different amounts of head shims. We
>went through over a thousand dollars worth of props
>experimenting with the small ones, the large ones,
>underchambered, 2-blades, 3-blades, 4-blades, props that
>weren't even supposed to work on the 61 (but did--oh, and
>some that were supposed to work on the 61 but stank). We
>repitched these, tried less pitch at the tips, even pitch at
>the tips, more pitch at the tips. Tried every trick that we
>could find whether we thought it would work or not. The
>goal has been one to constantly improve the engine run while
>working to constantly improve Matt's pattern. In the end
>Matt is happy with what was achieved, but the goal remains
>to still make it better.

You spent a thousand on props...Oh my word who has the kind of time to test out $1000 in props. I think I spent 200 on props since 1996. And I only had to spend it then due to flying on a looses gravel field that would chip the prop while waiting for launch. Wow that is alot of props.

>
>Matt is on his third 61 engine now and that is getting worn
>out. He just recently got a new 65. Here is the real
>story. We put a couple of hours on the engine on the test
>stand (using an old 61 pipe for break in). Matt was already
>using the 65 pipe on his 61 engine, so when he was ready to
>put it in the air, he measured the head shims and put in the
>same amount he had been using in the 61. He used the same
>pipe, the same prop (which was used for break in) and the
>same needle setting he had been using on the bench. And the
>plane flew nice 5.2 second lap times in a deep rich tone
>with no wind up in maneuvers in 15 mph winds. It seems to
>be a successful system.

You have just stepped into the nice world of just a bit more excess power. I started in on the 65 and found the very same thing you desribed. It was the same yet smoother and cleaner on the corners and exits. The extra size is a big help. I really like the 65 it is so predictable. I think you will find the very same results. No matter how you set it up. I am glad you like it. I know I love mine.

>
>What will it do in 30 or 40 mph winds? I don't care. I
>wouldn't fly my "real" plane in that wind. I am not going
>to fly my model either. That is a good day to play chess.

>You spent a thousand on props...Oh my word who has the kind
>of time to test out $1000 in props. I think I spent 200 on
>props since 1996. And I only had to spend it then due to
>flying on a looses gravel field that would chip the prop
>while waiting for launch. Wow that is alot of props.

Heh heh heh..

I think I've spent maybe $300 on props... and I've still got a lot of them. Dozens and dozens of Taipan, Tornado, Master Airscrew (Scimitar, APC 5x3, 8x6s, 7x4s, 7x6.. not to mention the few dozen free Top Flite nylons and the Top Flite Pylon Props (7x6N mostly) I bought for 9 cents apiece..

People often buy an engine for a specific airplane they have a kit of. Others build airplanes to use a specific engine they have. I'll end up building airplanes and getting engines just so that I can use up some of the prop stockpile. Yeah, one of those Noblers and a NIB Fox Stunt should work with those wood Top Flite 10x6s...

(And now, ironically, since I want to fly F2D, I'll have to spend another $200-300 in the next few years buying those $4 Russian glass props.)

>I have seen Frank and his many different pipes for many
>years now. He has a few things different than the rest of
>the stunt world. One is the sound. Really different.
>Second he has a more responsive system then the baffle
>pipes. It really shuts down the motor quick. When it needs
>boost it is on and very quick about it too. I have seen his
>power curves and compared them to the Baffle pipe curves.
>His have such a more drastic fall off the power once it goes
>over the top.

Because (with damping) a single reflecting surface is much more "clean" than several closely spaced reflecting surfaces. There's only so much total energy in the exhaust waves to be returned. If you bounce it off a single surface, it all comes back at once in a strong return wave. If you get reflections off 3-4 different surfaces, only a fraction comes back in each wave, and given that you are only tuning it for one of the 3, you get less energy in that one. The fact that some of the others might either waste the return reflection, or even interfere with the one you are using, reduces the effect.

Note also that this is why most people use the first baffle reflection, instead of one of the others, or the tail cone (whether they are aware of it or not...). Tuning it to use the first/quickest reflection ensure that you have to drag the engine all the way past this first stable point, to the second, slower point - as opposed to giving it a *higher* stable point at which it might decide to run if it unloads.

> My question is how come people all use the
>baffle pipe?

Easy answer is that's all they can get. And Frank's early experiments (like the one that David could hear 5 miles away in the Lubbock airline terminal) were unusuably loud, and didn't really win anybody over. They seem to be OK now. More sophisticated answer is that more boost/brake is not necessarily what you want. It's better for regulation, but it's not necessarily ideal to have the power change that abruptly.

>The empty pipe seems to be the one that would
>do more of what we are after "Braking" I want to try one
>but I dont know where to start.
>
>Any suggestions?

Copy Frank's. You have a fantastic resource on which to draw. I suggest that pay rapt attention.

More
>sophisticated answer is that more boost/brake is not
>necessarily what you want. It's better for regulation, but
>it's not necessarily ideal to have the power change that
>abruptly.

It's not?

How do you know?

>More
>>sophisticated answer is that more boost/brake is not
>>necessarily what you want. It's better for regulation, but
>>it's not necessarily ideal to have the power change that
>>abruptly.
>
>It's not?
>
>How do you know?

Because I have had engines with too much boost and brake, and didn't like it.

>>More
>>>sophisticated answer is that more boost/brake is not
>>>necessarily what you want. It's better for regulation, but
>>>it's not necessarily ideal to have the power change that
>>>abruptly.
>>
>>It's not?
>>
>>How do you know?
>
>
> Because I have had engines with too much boost and brake,
>and didn't like it.
>
>
> Brett
'Zilla,

I might add that the four stroke is pretty much the diametric opposite of "on and off" power and has proven a very desireable power delivery for stunt. What I understand you are now doing is trying to find a way to finesse a tweak of boost overhead from an otherwise constant thrust system.

I agree with Brett (quel surprise! as we say in France, huh) that a strong on and off makes flying precisely difficult. The lack of on and off is probably why I just love to fly the Doctor with the four stroke because you always know from second to second what you're going to have...even if it ain't quite enough from time to time.

LAST EDITED ON Oct-29-03 AT 05:51 PM (CST)
 
Tu quoque, Brutus (Ted) ?

I thought you'd know better than get into this thread that might win the Stuka Stunt Award for the hottest thread in 2003, but you posting was excelent and it not only revealed how you grasped the essence of Godzillas posting, that we all missed, but also showed a great quality you have:

the nature of a peacemaker, that shall be called a child of God.

(Matthews,5:9)

Funny thing, Ted, it's the second time I mention you being a child of God!

First time was when your Father went to a better place.

I said that when our father passes away God fills in...

Now I see you fulfilling the "pre-requisites" of that position by your gestures.

That was the most impressive stunt I've seen you perform, Ted!

Flawless shape, bottom height and intersections.

God bless you!

In Christ,

Ion

"Heuereux ces qui procurent la paix, car ils seront apelés fi8ls de Dieux!"

I got this photo today, and I'm a complete sucker for it, can't see enough of those lovely gringos...

>I might add that the four stroke is pretty much the
>diametric opposite of "on and off" power and has proven a
>very desireable power delivery for stunt. What I understand
>you are now doing is trying to find a way to finesse a tweak
>of boost overhead from an otherwise constant thrust system.

No., not really, I have plenty of boost.

What I want is the "brake" effect" to be magnified.

Mights as well wade in too.

For years we have been operating our two stroke engines at less than ideal conditions and expecting steady runs. The classic 4-2-4 depends on a wet needle setting and changes in fuel head pressure at the needle to give us power (increased rpm actually) during nose up maneuvers. Where are we on the “Power Curve”? Below the engines peak torque or HP under these conditions. If we set the needle for optimum fuel to air ratios, then where we are on the engine’s torque curve is a function of load on the engine. (This is always true…. load affects rpm and always determines where we are on the torque curve not needle settings).

To achieve consistent runs we need to adjust the needle until we are slightly rich of peak (near the ideal stoichiometric ratio), then change prop pitch (load) or engine rpm via a throttle device until we are satisfied with the speed of the plane. What I’m really advocating is to optimize the fuel to air ratio, use fuel pressure to deliver steadier fuel supply (making it less susceptible to attitude changes) and a throttle to control the amount of rpm. Use the needle for optimizing the fuel/air ratio not as a “throttle”. We then can operate the engine on top of the torque curve where there is less change in rpm for changes in load from maneuvers, wind etc.. Isn’t that what we all seek? This is the pipe ship methodology sans throttle. The results of all this is higher rpms than we normally see and lower pitch props. Puts us on the engine’s sweet spot. We try to force some engines to run where they aren’t designed to run (usually trying to run wet 4 cycles on a Schnuerle motor) and then are surprised when the engine unloads on a maneuver gains rpms, boost ports start to work, air to fuel ratios approach optimum and we are off to the races. This motor found it’s sweet spot in spite of our best efforts to screw it up.

I can’t agree with the concept of an engine being able to slow down if it’s unloaded. No matter where you are on the torque curve this engine will speed up if you reduce the load and slow down if you increase the load. I think this makes a case for operating at the top of the curve where changes in load will have less impact on rpm.

I think Dick has got it! Run the motor where it is designed to run and this is where you will find the best stunt run "That" engine can give you. Problem is as he states if you are running a motor that does not want to run where you want it to run. This goes back to my last posting about prop choice being the most important factor. Very few people are willing to give up there day testing props and finding the ones that do the job. This is very labor intensive and thank goodness people are willing to pass on their findings.

One of the things that Brad does not know about Franks system is his prop. This is my theory on this and it also comes from racing where the magic prop was gold and to die for. I have one of Franks props and this is what I think gives the wind performance that Brad was seeing, as well as the pipe set up. The prop is thin at the tips and has under-camber. This is what I see as happening. The under-camber helps hold the motor back(Braking) and the thin tips depitch under load(back side of loops at wind up point) and together gives the good wind performance Brad saw at the Nats. The magic prop in racing is the one that loaded up (pitched up) in the straightaways and depitched in the turns. This was the fastest and the hardest to find and duplicate. In stunt what Frank's was doing was pretty much the same. When loaded from the extra RPM on the wind up side of the maneuvers the tips would depitch, slowing down and driving less, and in the up lines would load up the pitch from slowing down with less RPM.
Richard Oliver

>>One of the things that Brad does not know about Franks
>system is his prop.

I told Frank to send me one. He only has a couple. Should work for the Saito too (I hope).

Please don't misconstrue what I am saying about power curves to say that props aren't equally important as the engine. I would just like to isolate one variable at a time. It would be nice to understand the curve of the output of the engine, especially with the 4 stroke, which is mechanically limited as opposed to exhaust limited.

The net effect of "the back side of the curve" is definately there, but we (I in particular) know very little about how to manipulate the engine portion without a lot of trial and error. No matter how unrealistic the idea of eliminating all this trial and error may be, it would still be a nice goal.

I have a lot of ideas about how the 4 cycle should be dyno'd. I believe the peak can be easily manipulated using the fuel and venturi, and the net effect could be mapped. Once the effects are mapped, you might be able to tune the engine (and prop) on any given day and condition to suit the airplane. This is simply the same thing that Ted talked about doing at the Nats using compression, pitch, pipe length, and RPM to make the best of the bad conditions. I think the 4 cycle would be tuned with pitch, nitro, and venturi in the same manner to find the "sweet spot" that would bore through any condition. Once the Saito finds the "sweet spot" it is very sweet indeed.

BTW I saw Frank's speed pipe do the same thing ten years ago in Dallas. It was way more regualted than anything else around. Frankly, that may not be the ideal, and many of the baffled pipes may have other advantages, but that kill switch effect is pretty cool too.

On the few graphs I've seen, the power and torque curves really don't coincide. I'll have to go look at them again, but I think it's usually the torque curve that peaks first. Power continues to go up because the falloff in torque is, for a while, less than the corresponding increase in RPM. How close are the peaks to each other? I don't know, but I suspect they're further apart on racing type engines without tuned pipes (e.g. Pylon Formula 1, Nelson .36 Combat Special) and closer together on Stunt and Pattern engines and especially on piped engines.

And to get a "near constant RPM" run, you'd want to be on the back of the TORQUE curve.

Back to Igor's post - I don't think either curve is constant, either - perhaps changing the needle setting might change them (though I suspect not that much unless you're running grossly rich or grossly lean) but what would definitely change them is throttle setting (i.e. venturi size, in our case).

The peak point can be certainly modified by ventury, pipe, nitro, richness and so. I would really see a dyno test of 4 cycling twocycle. I think it was already here on forum, but I cannot find it now. I think the ventury, richness modify hi-rev torque. The low rev is not touched because diameter does not restrict too much and also burning of rich mixture is not limiting factor at low rev – so it really moves the peak point not the toque value.

The same is happening if you drive car – if you keep gas pedal on some place, the car accelerates very well until you reach speed (the rpm) at or past the peak. If you push it more, the low rev acceleration does not change too much but top speed is higher.

It is also clear that rich engine can run well in 2-cycle mode at low rpm (loaded), but if you unload it, it goes to 4 cycling. I post a picture of ventury diameter effect. It clearly shows the power gain is at peak, not in whole band.

The nitro effect is little different, it adds torque in whole band – just because it adds little amount of oxygen and thus it allows mode energy in every revolution – BUT also allows quicker burning and thus also moves the peak to higher rpm.

The pipe is different story it acts depending on its internal structure, but single chamber resonator with expansion and compression cone can effectively add or limit RPM – I put pictures of several engines in different topic. The first picture shows effect of such pipe on R/C engine – “A” markers show pipe tuned for that rpm.

Remember that when looking at Dyno curves that torque is the measured value and horsepower is a calculated value. Torque x RPM)/5252.1 = Horsepower.

That explains the fact that horsepower curves (in general) continue upward for awhile after the torque starts to fall off. Simple math.

I agree with Igor regarding mechanical configuration affects on the peak torque value but I don't agree with the nitromethane comment

"The nitro effect is little different, it adds torque in whole band – just because it adds little amount of oxygen and thus it allows mode energy in every revolution – BUT also allows quicker burning and thus also moves the peak to higher rpm."

Nitromethane burns much slower than methanol but it's mass flow rate is much higher hence it produces more power. Think of it in the same manner as an "octane booster". The cylinder pressure curve is wider but not as high with nitro additions.

Yes Dick, nitro itself burns slowly, and it also does not produce too much heat, because it absorbs lot of energy to release oxygen, but that oxygen with methanol brings mixture to higher temperature and pressure and thus quicker burning process. It can even lead to slefdetonation.

I have some torque curves in that another thread. First and last shows also curves with and without nitro.

Thanks Igor... I'm a Physicist not a Chemist so I'm not in my area of expertise, but...... my understanding of the effects of nitro in our fuel is that niro requires far less oxygen to "burn" than does methanol. So you can bring additional fuel into the engine without flowing anymore oxygen. To a point.

Chemists... please feel free to jump in.

That is it! Nitromethan brings oxygen in liquid form to engine – as well as methanol – just in higher amount – 2x more for one CH3 cell. You do not bring it in limited amount of air. If you compress and heat the mixture methanol and nitromethan dissociates. If you have stechiometric ratio, you must have in cylinder more oxygen and more CH3 cells than with pure methanol. After ignition, carbon and hydrogen from methane (methane is base for both – nitromethane and methanol) burns to its oxides. Water from methanol and nitrogen from nitromethane just absorbs little amount of thermal (but not chemical) energy. But AMOUNT of burning particles like carbon and hydrogen and thus its oxygen in gas form can be higher thanx released oxygen from nitromethane. That is reason why the temperature, and pressure is higher. That is all.

The nitromethane we use is mononitromethane. We know also di-, tri- and tertra- nitromethane. Tetranitromethane is explosive. It has so much FREE oxygen that 5% in fuel brings 1.5 times higher torque (similar to 50% of nitromethane). Di tri and tetra nitromethan is not included more than 0.1% in technical nitromethane, so you can think we use pure CH3NO2. It’s burning to oxygen needs only 1.5 atoms of extra oxygen from air, while methanol CH3OH needs 3 atoms. So you can burn twice more nitro than methanol, it gives more thermal energy, higher temperature, higher pressure, and thus also higher burning speed. No tricks, no magic

LAST EDITED ON Oct-29-03 AT 04:50 PM (CST)
 
There seem to be a few misconceptions floating around concerning torque peaks, horsepower peaks, whether or not California is going to burn to the ground, and if this is a bad thing should it take place.

With any given engine, peak torque hits where the engine breathes best. It's as simple as that. Yes, you can feed the engine more potent fuel and increase the torque. But the peak will still be at the point in the rpm range where it breathes best. Or easiest. However you wish to phrase it.

Depending upon the application, where peak torque hits can be manipulated. In the case of a two-stroke for PA it seems that a good number of fliers have (finally!) realized what the pipes are actually doing when it comes to peak torque and/or where this peak hits. Looked at very simplistically, the pipes have the capability of altering the breathing ability of the engine, and it's kinda neat because one needn't go rooting around inside engines, getting all dirty and oily. Although that is an option, of course...

With the four-stroke it's a different deal. Yes, yes, exhaust tuning is possible with four-strokes, even the very mildly timed Saito engines, but in the real world, or at least our little world, the potential gains are so small as to not be worth the effort. At this time.

So with a four-stroke we need to either work with what we've got, coming to understand as well as possible what's going on and then to work with it--the PW and Godzilla approach--or we're going to have to start making chips. Well, that's not true, as we can also adopt the Zambelli Approach, which seems to center upon shrugging and professing to not caring about understanding why some four-strokes work so well in a PA application, but that also makes the learning curve really steep in that cut 'n try becomes our only development tool.

(Didn't everybody see Frank's side comment about Stunt cams? The man is not a jokester; what do you think he meant?)

I say this because if the goal is to manipulate the torque peak, or horsepower peak, of these four-stroke engines we must get in there and alter the breathing characteristics. Cams. Valve train geometry. Valve sizes. Fewer 90-degree turns for the flow. (It's still hard to believe Saito thought it a good idea to make the flow bend through TWO sharp turns on the 40 when just one on the 56s and 72s ought to make all of us retch.)

While I'm at it, torque peak--even in a competition-tuned four-stroke engine (reference: My second Stunt News article on four-strokes) where we went so far afield as to discuss Harley flat-track motors and their tuning--can hit way, way down in the rev range with peak horsepower coming in a lot later. Even I was stunned by what I found here.

Or skilled engine tuners can go the other way. Back to two-strokes but still with motorcycles, GP 500 bikes have always fascinated and reliable sources indicate that Doohan's "Big-bang" setups had the V-4 engines developing peak torque and peak horsepower within 500 to 1,000 rpm of each other. Talk about a rush when it all hit at once...

Dan Rutherford

>I say this because if the goal is to manipulate the torque
>peak, or horsepower peak, of these four-stroke engines we
>must get in there and alter the breathing characteristics.

I think the first thing would be to understand exactly to what end would altering the peak realize. My guess is that we don't even know where we are at now, or at least very few people realize this. Doug and I have been having some discussions, and I think we are simply right on the verge of even grasping what it is we are doing, what we like, why we like it, and how to reproduce it.

Most of what we have gleaned has been from what most stunt flyers have, which is practical experience. It takes a real hard commitment to truly think "out of the box" and explore areas that "should not work" (Richard Oliver said that) and abandon your practical experience crutches.

Once you have firmly established in your mind what you want, it is definitely easier to think more linearly to solving the system to work to end that you choose. The first thing for me is to understand the engine.

BTW I think the Saito may be timed just about right to take advantage of the available props and the 70' radius. We will see...

It takes a real
>hard commitment to truly think "out of the box" and explore
>areas that "should not work" (Richard Oliver said that) and
>abandon your practical experience crutches.

Yeah like running an Eather 12X3.6 4blade on a Saito 72 at 11200 rpms...Who would have thunk it????

Deffinently not what is called for in the manuel and or by anyone else who has run them.

Brad is really reaching outside and I commend him on it for sure. He gives me good info to "Steal" and use against him next year...

Thanks in advance...

>>>Yeah like running an Eather 12X3.6 4blade on a Saito 72 at 11200 rpms<<<

Why not? Look to picture it is my first 4C model from 1997. It is with Os 52 Surpass. Prop is my regular pipe prop BE 12x3.5 3 blade UCT. It worked for first try with wide open stock carb without any modification.

>I think the first thing would be to understand exactly to
>what end would altering the peak realize. My guess is that
>we don't even know where we are at now, or at least very few
>people realize this. Doug and I have been having some
>discussions, and I think we are simply right on the verge of
>even grasping what it is we are doing, what we like, why we
>like it, and how to reproduce it.
>
>Most of what we have gleaned has been from what most stunt
>flyers have, which is practical experience. It takes a real
>hard commitment to truly think "out of the box" and explore
>areas that "should not work" (Richard Oliver said that) and
>abandon your practical experience crutches.
>

The most important thing in successfully "thinking out of the box" (and I think this is true of almost any technical venture) is to understand fully why "the box" existed in the first place. Only then can you begin to exploit the weakness of the basic assumptions that led to it.

The two paragraphs above tell me you are on the verge of *big step* forward in grasping the larger picture.

The backside of my ears were burning this morning, and I had some time to kill, and anyway the IT security guys need something to do.

I think after 20-30 pages it all came around and ended up all right.

The "power system" has to be a picture with a little dotted line encircling the prop, venturi, engine, and exhaust (pipe).

The Saito 56 was a pretty flat power curve. Al's Saito 72 definitely had a humpy back to it. Although I don't know for sure, I don't necessarily think the 56 would have such a shape if I had run it on 20% nitro instead of 10%. My gut feel is that the "timing" of the two are different enough to matter. Although, as I've said, "you can take all of my 4cycle experience and put it in a thimble". Brad seems to feel from experience that the nitro is a driver with 4cycles. At any rate, when I saw the shape of the 72 curve, it was obvious to me that it (the 72) would be "stunt happy" at a higher rpm. The only difficulty is to match the equation of absorbing the power at that rpm without producing too much thrust.

Richard elluded to the different "stages" of a piped two cycle. Well, he didn't ellude, he flat out described the fact that in a piped 2cycle there are multiple "humps". The racing quys are looking for the tops of the humps, we stunt guys are wanting to operate on the "decending" back side of the power humps. The fai cl speed flyer really goes through two humps. If you've ever seen them take off, get out of the dolly, stabilize at maybe 15k and then start whipping the daylights out of the plane. What they are doing is unloading the engine (system) so that it can get through the first "valley" of the power curve. As soon as it gets past the bottom of the first, the system now is on the ascending side of the second hump. The rpm quickly climbs to 30k+.... the fun part here is watching the pilot at this point ...little cartoon feet are just a blur .... Anyway, the "full wave" pipe for stunt has always to me been a more friendly way to run a pipe. The "backside " of the curve is very steep. This means that it will react faster to external changes, as opposed to the baffled system that does descent, but just not as steeply. All of the control mechanizms that we employ for stunt runs take measurable amounts of time to take effect. Small venturi, muffler pressure, etc.
but the backside of the power curve works instantly!

The stunt pipe effect is not to add power, but to simply reshape the power curve. The motorcycles, pylon racers, speed flyers are using piped engines to get increased hp out. But we have more power than we know what to do with. We just want to control it. The piped 2 cycle or the backside of the 4cycle power curve are a very effective means of providing a feedback and control to the power system.

Leonard is right that the 2 cycle piped power system is regulating rpm, and maybe we'd like to really regulate airspeed. That leads me to some stuff I've been doing recently ...... props. The other side of the "box".

I had a phone conversation a while back with a FF guy named Hewitt Phillips (no not the Phillips entry guy). We both marveled at the fact that our model props work as well as they do. We've got 14% airfoil sections operating in the 100k Re region. The data here for these sections is terrible. Mr. Phillips suggested that the centirfugal forces were clearing (slinging) the boundary layer off. Could be.

I've been experimenting with the premise that we missed the boat with our neat carbon fiber props. We simply made copies of our wood props. The best part of the carbor fiber is that we should be able to make much thinner sections and still keep the prop together. We just blindly made the sections look just like the wood ones. So why not thin the sections out? .... But, at the same time, or maybe instead of getting them paper thin, ... keep about the same thickness and increase the chord. .... Yes thats the ticket ..... Increasing the chord does two things that might be good. .. We automagically increase the Re number, which means that we almost all of the time get to a lower drag situation .... and at the same time we get a lower percentage section, which will be a lower drag situation also ... So I've got some stunt props now with 5% sections operating at two to three time the Re number. .....I'm talking like 2inch+ chords ... yes its time to reach out and touch that side of the "box". The downhill line is forever, needs some more work though .... more later ....

Brad just called and wanted the description of my HP props. I'll dig that stuff up and post it here as soon as I find it.

Frank

Frank,

Brad and I were talking and he commented how you found the 56 to have a flatter curve. I then started putting 2 and 2 together and I am beginning to think that is why my 56 liked the 14X5 better than my 72 did. My 72 would spin it up just fine but it would heat up in the maneuvers and lose some power by the end of the sqr eight. Now the 56 seemed to like the same prop pretty good with less burn down effect. Maybe the more broad torque curve was allowing it to keep rpm for a longer period of time. Whereas the 72 will heat up quickly due to a spikey curve and when it is down on the curve way down and still tryning to spin the prop it is heating and the cycle just goes on and on.

Since heat is the 4s main demon I would think the ultimate goal would be to go away from getting it go WAY over the curve but keep it at bay, just over the top, with a light small load. I must admitt I didnt start thinking this way until Brad tried the different props. But we did spend many hot days at the field this summer racking our brains. The main culprit was on the way up to the top of the hourglass you can just feel the power go down. It still has good tension that is not what I mean. You feel the speed drop a tad and you know it is overworked. You can feel it heat up and over it goes. First we kept after the fuel draw and issue. Pressure niples of different sizes seemed to do something but it was a bandaid for sure. Fuel draw is not the issue. The load is. Getting the load away from the equation. Letting the motor spin the same all the time and not overheating it is the key.

Anyway, the
>"full wave" pipe for stunt has always to me been a more
>friendly way to run a pipe. The "backside " of the curve is
>very steep. This means that it will react faster to external
>changes, as opposed to the baffled system that does descent,
>but just not as steeply.

Exactly. The steeper the slope, the faster the reaction. Frank keeps pushing to go to the steepest slope of the backside of the curve. Now I get it...

All of the control mechanizms that
>we employ for stunt runs take measurable amounts of time to
>take effect. Small venturi, muffler pressure, etc.
>but the backside of the power curve works instantly!

Too many variables. A more predictable curve would eliminate the need for so many...

One I like about the 4 cycle, it only has one curve, but it is a really good one. Not all this extraneous crap to temp you to mess with (pipe length, compression, venturi, fuel, etc). For me the 4 stroke offers one curve, and this can be manipulated with the fuel. Pretty easy.

Frank finally understands one of the variables I want to test. He feels that the power curve will never move regardless of the nitro, I believe it moves up and to the right as the percentage nitro increases (same with the venturi). I want to dyno the engine and get the results. Finding the right RPM to match to a prop would simply be a matter of matching the fuel and venturi to achieve the curve of the greatest slope and putting the prop in that RPM range.

>Frank finally understands one of the variables I want to
>test. He feels that the power curve will never move
>regardless of the nitro, I believe it moves up and to the
>right as the percentage nitro increases (same with the
>venturi). I want to dyno the engine and get the results.

I guess that there will be a *small* shift in the point of the maximum torque. On engines with fixed timing (like, a spark plug), the torque peak is the peak of the breathing, and only changing the breathing changes it (like Danny Dirt said earlier). I bet that there will be a slight change in the torque peak RPM based on the fact that the ignition timing will change as the result of the different thermal environment.

>
> I guess that there will be a *small* shift in the point
>of the maximum torque. On engines with fixed timing (like, a
>spark plug), the torque peak is the peak of the breathing,
>and only changing the breathing changes it (like Danny Dirt
>said earlier). I bet that there will be a slight change in
>the torque peak RPM based on the fact that the ignition
>timing will change as the result of the different thermal
>environment.
>
> Brett

Wouldn't there also be a change in "breathing" by virtue of adding nitro? In effect you are "turbocharging" the engine with more oxygen and ability to burn more fuel. I, too, would think that would move the torque curve.

>
>>
>> I guess that there will be a *small* shift in the point
>>of the maximum torque. On engines with fixed timing (like, a
>>spark plug), the torque peak is the peak of the breathing,
>>and only changing the breathing changes it (like Danny Dirt
>>said earlier). I bet that there will be a slight change in
>>the torque peak RPM based on the fact that the ignition
>>timing will change as the result of the different thermal
>>environment.
>>
>> Brett
>
>Wouldn't there also be a change in "breathing" by virtue of
>adding nitro? In effect you are "turbocharging" the engine
>with more oxygen and ability to burn more fuel. I, too,
>would think that would move the torque curve.

Len,

In a word, no. When it comes to torque peaks and what effects them, you are simply thinking too much. Step back a bit and look at our engines are air pumps. Because that is all they are. And it makes it easier to understand certain aspects to the operation of internal combustion engines.

>In a word, no. When it comes to torque peaks and what
>effects them, you are simply thinking too much. Step back a
>bit and look at our engines are air pumps. Because that is
>all they are. And it makes it easier to understand certain
>aspects to the operation of internal combustion engines.

I think the fact that the ignition point is variable, based on thermal and catalytic effects, it's not as simple as it might be on a regular engine. But there's one sure way to find out. I think you might be oversimplifying it a bit in this case. It won't be grossly different, but I would be surprised if it just moves up and down, and not side-to-side.

I think you might be oversimplifying it a bit
>in this case. It won't be grossly different, but I would be
>surprised if it just moves up and down, and not
>side-to-side.

One of Igor's chrats shows the effect of nitro on the YS-120.
The peak of the curve goes up (.1 KW) and to the right (more than 1000 RPM).

[photo not recovered: 3f9fad4f2313b6fa.gif]

Brett and Godzilla,

I am more than a little surprised. In considering Brett's input, reconsidering my view of fixed-timing engines versus our fire-when-ready two- and four-stroke engines, I was willing to concede the torque peak might well move around a bit when feeding them fuels with varying amounts of volatile ingredients. I mean, our engine are compression-ignition pieces, after all.

But a shift of 1,000 rpm! And this with a fairly low rpm piece to begin with.

However, I still think it wise, at least at one level, to look at our engines as mere air pumps which, if the fuel variable is controlled, develop peak torque exactly in the rpm band where they breathe the best and most deeply.

Thanks to all...

Godzilla,

Do you know if the cam from a Saito 72 will fit into a 56 or even if there are any differences between the two when it comes to when the bumps tickle the pushrods or how big the bumps are?

I'm thinking--without any way to know, admittedly--that other than displacement the biggest difference between the two motors is the cam timing. The valves are no doubt larger, but they have a larger cylinder to fill and so are assumed to be in some reasonable proportion to those used in the 56.

>Godzilla,
>
>Do you know if the cam from a Saito 72 will fit into a 56 or
>even if there are any differences between the two when it
>comes to when the bumps tickle the pushrods or how big the
>bumps are?
>
>I'm thinking--without any way to know, admittedly--that
>other than displacement the biggest difference between the
>two motors is the cam timing. The valves are no doubt
>larger, but they have a larger cylinder to fill and so are
>assumed to be in some reasonable proportion to those used in
>the 56.
>
>
>Dan Rutherford

OK, when is the first engine tuner going to show up with a retimed camshaft for the Saitos?

>
>OK, when is the first engine tuner going to show up with a
>retimed camshaft for the Saitos?

Len,

I believe I asked that question, or one very much like it, almost two years ago in the January/February 2002 Bad Boy Stunt column, the first in a (short) series of BBS columns dealing with four-strokes as used in a CL PA application.

While it seemed to whiff right over the heads of a number of people, some of whom have been active on this very thread, part of the thinking behind that and subsequent articles was to get a jump-start on how these things work so we can make them better.

Remember the discussion in March/April 2002 BBS column about "the right kind of power versus just more power"? I knew at the time the Saito 56 most certainly did not have more horsepower than, for example, your run-of-the-mill ST 60. Now it seems to be common knowledge, Pat Johnston being the latest to do the test, that the Saito 56 must squeak and grunt just to equal an ST 60 in gonna-fly-some-Stunt tune.

(Pat uses the APC 13 x 4W on his Saito 56. Launches peaked--as in there ain't no more revs to be had--at 9500 rpm. He recently bolted one of these props on one of his proven ST 60s. With the needle fat, as it must be for an effective Stunt run on this motor, it turned the APC 13 x 4W at 9500 rpm. With a whole bunch more horsepower only a twist of the needle away.)

Anyway, and truly I am not tooting my own horn, it seems to me that many would benefit from reading the above named columns again. No, they were not 100% accurate as to predictions, my guesses as to the best revs to run were short and I may have genuflected too frequently at the altar to pure, raw torque.

But it's pretty obvious to me that I saw a few things back then which we are still struggling with today. And in some ways I'm tired of seeing us spin our wheels, wish we could all get on the same page and go forward with the development of better four-stroke motors for PA applications.

I mean, "Wire the carb open," advice and the literal shrugging of the shoulders as in, "I don't care how they work, they just work!" is pure nonsense.

Plus there are not very many of us seriously interested in further development of four-strokes. Look at the 2003 Nationals. Wasn't Gordan the only Top-20 guy with a four-stroke? While PW did super at this year's FAI Team Trials, wasn't he the only one who had poppet valves in his motor?

Two years ago I thought we would be a lot further along than we are today.

Dan, I remember your column and was even thinking of it at the time I asked my question.

(Hey, isn't it nice to be remembered?)

>>I'm thinking--without any way to know, admittedly--that
>other than displacement the biggest difference between the
>two motors is the cam timing. The valves are no doubt
>larger, but they have a larger cylinder to fill and so are
>assumed to be in some reasonable proportion to those used in
>the 56.

I would bet that the Saito 56 and Saito 72 are timed exactly the same (just guessing that the engineers at Saito are not necessarily that clever to have unique curves for each engine).

I told Frank that I believed the big difference betweeen the two engines that he tested was that the Saito 56 was tested at 10% nitro and The Saito 72 at 20%. Hence the big peaked curve of the Saito 72. I would say the Saito 56 would be of very similar shape on the same nitro, even though I do believe the Saito 72 to be of slightly higher compression (lower deck).

As far as custom cams, I would really like to see more intake duration or a larger intake valve. Anybody want to make one? I will test it for sure...

Actually, the biggy for me would be a way to retard or advance the cam to move the curve.

Godzilla,

You may be right in thinking the primary difference between the 56 and the 72 is compression ratio. I've now got both sizes in flyable models, the 72 is a bit nastier to handle and increased squeeze would explain some of this.

Yes, the Saito engineers do seem to take the easy way out at times, do they not? I'm still (mildly) peeved about the goofy intake plumbing on the new 40, especially as I am considering using one in a future model.

Guess I'm just going to have to take one of these things apart, as I had been assuming one could indeed easily advance or retard the cam one tooth. The primary value in doing this--and I vote for retarding the cam as a first step--would be in determining the specs for a new cam.

Still, I would have to steal some ideas before going much further. While I cannot think what it might be, surely there is an application out there where a mildly-timed four-stroke--one with an abrupt drop-off in torque and horsepower at fairly low revs--works better than anything else.

If we could identify such a powerplant it seems as if our quest for a more appropriate spread of power would be a lot easier.

>If we could identify such a powerplant it seems as if our
>quest for a more appropriate spread of power would be a lot
>easier.
Dirt,

I understand some of your frustration, but I think there is a little room for reflection.

I personally, have been very apprehensive to advocate the modification of the 4 cycle engines due to the simple fact that I am not sure what I want them to do. I think this may be more of a universal problem than simply with the 4 cycle engines. Very few (if anybody) seems to understand the cause and effect of what it is we like about ANY KIND of power system used in our sport. There are reams of practical information, but very little in the way of understanding what makes a stunt engine good or bad. Why go modifying engines when the outcome is unpredictable in the first place. I think if someone could better identify the positive and negative aspects of power delivery for stunt, then certainly a cam, or head, valve, or whatever could be effectively designed.

What exactly would we be designing for?

Output? My Saito 72 already pulls harder than anyone around here likes to fly, I am already testing with airplanes that are bigger and heavier than I would ultimately like. Why would I need more?

RPM? I am already going above the practical limit of Brian Eather’s popular selection of props.

Power Curve? This one is a little confusing. I can see Frank W.’s point about creating the steep curve, but I can also see Brett’s point about subtle power delivery being even more attractive. I simply do not know.

Ease of use? This one I know something about. The easiest to use 4 stroke setups that perform the best over a large range of external conditions do not perform the best patterns, period, (at least with conventional stunt planes). At least from my practical experience. I can give you a setup that will perform flawlessly day in day out in terms of consistency (in any kind of crappy hot air), but it will be beaten soundly by a modern piped engine in pattern performance. I can give you an engine run that will rival a pipe run, but it work on again and off again. We simply do not understand what we are doing.

To this end I am seriously considering fitting the Dr. Jekyll with a piped engine, as I am simply growing tired of living in the “go for broke� bubble. There certainly are no bonus points for originality in our sport any more (if there ever were), and frankly I feel I am actually at a higher scrutiny working outside the norm. I simply get the feeling that everyone is watching at times, waiting to see if I fail. There is very little support for rebel causes in stunt. It can be very frustrating to spend all this time trying all these new things, with almost no one to copy. Then show up to a contest and get beat by a guy who just built a new plane, ordered a prop from Brian, an engine from Richard, and is getting good runs on the first day. There simply are much more resources being poured into the current 2-stroke status quo and an infinitely larger database of information from which to COPY.

Then, at the same time, all this energy is wasted debating about what does and does not “work� in the 4-stroke world. Heck, I went all week at the Nats and never touched my needle. I flew all my maneuvers straight downwind, and ripped through the wind with no problems, and I do not think there were two words said about the performance of my engine because I failed to reached the Top 20.

I guess “work� is a relative term.

I think that infinitely more good could be gleaned from the understanding of the external parts of the total power system while leaving the engine alone. Frankly, how many venturis, manifolds, needle valves, fuels, oils, nitro, tanks, bladders, pumps, pressure systems, tank plumbing, props, needle settings, etc. have you tried and shared your results with the group? I certainly think there are plenty of things that have yet to be tried and optimized that do not involve cutting on the engine, which are very delicate pieces and will involve certainly more than can be accomplished with a Dremel tool. Please feel free to break this new ground, I am getting a little tired myself.

I think I will just practice my pattern, as the judges don’t seem to be counting the strokes…

Please, do not take this rambling as an affront, it is merely my view of myself, and has nothing to do with you…

Brad

>But a shift of 1,000 rpm! And this with a fairly low rpm
>piece to begin with.

More than I expected.

LAST EDITED ON Nov-05-03 AT 09:16 PM (CST)
 
> I think you might be oversimplifying it a bit
>>in this case. It won't be grossly different, but I would be
>>surprised if it just moves up and down, and not
>>side-to-side.
>
>One of Igor's chrats shows the effect of nitro on the
>YS-120.
>The peak of the curve goes up (.1 KW) and to the right (more
>than 1000 RPM).

[photo not recovered: 3f9fad4f2313b6fa.gif]
Did you notice the fuel formula, other than the increase in Nitro? The 4% Propylene Oxide is interesting. General rule for 2 strokes is to NOT run ANY Propylene with ANY % of Alcohol...it causes pre-ignition. When I did my speed fuel testing in the '60's, I found I could boost power by using Propylene Oxide by balancing it with Nitro Benzine, which kills the pre-ignition. I used up to 8%(!!!!), and it went like, ummmm, STINK! Later, I found that Amyl Acetate does the same in much smaller doses, without the bad run characteristics and the physiological twitches. It might be interesting to discuss the use of P.O. in 4S engines with Frank W., and then test it. FWIW, Steve

EDIT: Thinking about this after posting, I suspect that the P.Oxide was to get/keep the nitro lit up, which is a problem with 4 strokes, requiring very hot glowplugs. Maybe a single small glowplug is not up to the job? Maybe the YS 120 has compression on the low side, or loses cyl. pressure by hot cam syndrome? It looks like the base fuel was zero nitro. Yes?

By the way, if you were under the impression that my old speed mix had only 8% P.O., no, it was 8% Nitro Benzine, and 5% PO. It made 65% nitro run 800 rpm better than K&B Speed Fuel, which was 60%, admittedly.

Interesting. I've seen fuel formulae which had copious amounts of propylene oxide (something like 15%, plus up to 60% nitro), and it was meant for 1/2-A competition free flight. I had Red Max make me up a gallon of this, and it ran pretty well in my 1/2-As.

Steve,

Why let Frank W. have all the fun? Come on by sometime. I've got some dark brown glass bottles (from a medical supply house, they don't even look like beer bottles!) in the downstairs refrigerator. They've all got P.O. in 'em.

Like Iskandar, I used to mix 1/2A fuel with some heavy hits of P.O., although never anything like 15%. This was back when we were developing, and then I was importing, the first version of the VA .049, and the little guys kinda liked the stuff.

Unfortunately, I was also dabbling in RC Pattern at the time and we once got fuel bottles mixed up out at Marymoor. The O.S. Hanno (Not So) Special 60 really honked. For a very short period of time. Hey, some of those boys thought 45%-nitro fuel was really killer; we showed 'em what was hot. And what was shot.

LAST EDITED ON Nov-05-03 AT 10:39 PM (CST)
 
I read where some would like to change the cam. Yes Dan I remember your article also. What a big stink over nothing, The engine already runs great with the cam that is supplied. Very smooth makes good power all the across the rpm range. You cant really seperate HP and Torque. I guess you could grind a cam that really runs well in a narrow rpm range but I bet it wouldnt work out to well. We dont want wild specifics we need a mild setup that has the "Potential" to really knock out the ponies then just use a little bit of it. I guess I understand what you are driving at. Getting the big ponies right where we want them. But as brad says it already makes plenty.

Brad and I were talking the other night after a pretty uneventful evening of flying, while the Cowboys somehow overcame 4 turnovers to whip the Redskins.

IMHO the real issue with the 4s is gravity. When the nose is pointed up on a 2s it just jumps up into a 2s from a 4s hence the 4-2-4 or it just speeds up in the 4s or 2s it is running in. On a 4s when the nose is pointed there is really nowhere else for it to go. We already run the engines at or a little past the peak rpm on take off so when the engine has to draw fuel uphill it leans and this builds heat very fast. The leaning is the main culprit. A new cam isnt really going to fix it. Tried fuel pumps and they had virtually no effect. I dont know how to get past this one problem. I need the motor not to lean as the nose is pointed up. Is that possible? The power curve issues will take care of themselves with the changing rpm but we really need the engine not to lean as the nose is pointed up. Let the load casue the rpm to change not gravity. As Brad and I have really tried lessening the load with low pitch props it has really lessened the effect of the leaning out has on the engine. The leaning out is there. You cant get around it. I know someone out there can think of it. Would a bladder do it? I personally dont know. I tried the YS approach, supercharged system with its own pump, regulator, and supercharger, leaning still there. You can try anything you want but that leaning will be there and when it gets darn hot it has much more of an effect on the setup.

So tell me how am I going to keep it from leaning out?

I guess I could fly 4s all the time and just get used to it, truth is I fly both piped motors and 4s all throughout the year. This leaning is always the first thing I notice right away. I always hear it on the others I watch also. I just wish I could figure out a away around it.

Help me out.

I agree that the way people set up 2 strokes in Stunt, they pick up power when the nose points up (though I wonder if they really do run faster - it might be an aural illusion), but when you compare that to 4 strokes, I think it's apples and oranges. If you took the same 2 stroke and ran it the way you're running the 4 stroke - i.e., with a largish prop, a largish venturi, and on the verge of lean, then it's going to behave the same way the 4 stroke does. Point the nose up, and it'll go lean and slow down.

So you COULD run the 4 stroke with a smaller prop, possibly a smaller venturi, and rich, and avoid the problems with leaning out, but you'd make less power. Just as the 2 stroke makes less power when you run a smaller prop, a smaller venturi, and rich.

>So you COULD run the 4 stroke with a smaller prop, possibly
>a smaller venturi, and rich, and avoid the problems with
>leaning out, but you'd make less power. Just as the 2 stroke
>makes less power when you run a smaller prop, a smaller
>venturi, and rich.

Once again Isky hits closest to the mark, and defines the circle of frustration.

The 4 cycle is like a 2 cycle running in a flat 2 stroke. It is really not that different at all when comparing the two.

Once we figure out how to make the sucker run at exactly the same mixture for the entire flight, we will have it. Almost ALL OF THE PROBLEMS ASSOCIATED WITH THE 4 STROKE ARE RELATED TO PROVIDING FUEL TO THE ENGINE AND HEAT. Same as with a combat motor I suppose. Not cams, torque curves, or any of that other stuff. I think the RC guys have known this for quite some time.

Power curves are cool, but apparently we do not understand them at all.

>I agree that the way people set up 2 strokes in Stunt, they
>pick up power when the nose points up (though I wonder if
>they really do run faster - it might be an aural illusion),
>but when you compare that to 4 strokes, I think it's apples
>and oranges. If you took the same 2 stroke and ran it the
>way you're running the 4 stroke - i.e., with a largish prop,
>a largish venturi, and on the verge of lean, then it's going
>to behave the same way the 4 stroke does. Point the nose up,
>and it'll go lean and slow down.

Yep it sure does. Just fly an FP 40 and get a runaway yet keep after the pattern. Long about verticle eights it is just sagging horribly...been there done that many a time..

>
>So you COULD run the 4 stroke with a smaller prop, possibly
>a smaller venturi, and rich, and avoid the problems with
>leaning out, but you'd make less power. Just as the 2 stroke
>makes less power when you run a smaller prop, a smaller
>venturi, and rich.

I guess this is an option. Thing is whenever I have tried to run a 4s say 1/2 a turn rich it just seems so inconsistant in level flight. It really sputters and farts around. Does anyone else have these issues when it is at really rich setting? I know Al runs his pretty rich but I havent seen anyone else getting away with it...

Why wont a 4s run steady in a rich setting and I mean real steady like it does on the bench? In the air you can hear it all over the place when it is rich. I am not talking about a click or two but say 1/2 turn. A 2s will run steady rich but a 4s wont...why?

PS I saw some pics of the Japanese running the Ultimate 70 on a venturi. Looked really cool made from Delron...rad stuff. Looks awesome with the Blue valve cover.

I suppose the "4 cycling" type run you get when you run a 2 stroke rich isn't going to work on a 4 stroke, simply because the cylinder scavenging works in a totally different manner.

If I recall from an earlier discussion about "4 cycling" a 2 stroke, the engine fires only every other stroke, BUT the power you get from each power stroke is a good deal more than what you get from a power stroke when the engine is two cycling. So you end up with 70% of the power rather than 50% when the engine is 4 cycling.

Brett speculated (and I tend to agree with him) that this was due to more thorough scavenging - you have more of the cylinder filled with intake mixture (i.e. unburned fuel and air) rather than exhaust gas when it fires because you're firing once every two revolutions, which gives an extra pump stroke to evacuate the exhaust gases and introduce new intake gases. The engine probably fires on the second revolution, even if the mixture is rich, BECAUSE you have a bigger volume of mixture to burn, rather than intake mixture mixed with exhaust. So you have a built-in mechanism to make the engine fire every other cycle.

With a four stroke engine, if you run the engine rich, you don't get the same effect, because the scavenging is more thorough to begin with, and it doesn't matter whether the mixture ignited the previous cycle or not. If you run it rich enough to cause the engine to miss, there's no mechanism to make sure it fires every alternate power stroke (that means, once every 4 revolutions). So it'll miss, but it might miss 4 times in a row, and then follow that up with 6 consecutive power stroke, so what you end up with is a rough-running engine, and nothing more. Even if you could get the thing to fire once every 4 revs, then you'll have 50% power, not the 70% you want (a leap from 50 to 100 percent when going up is probably too much to use in Stunt).

If you were using spark ignition, I suppose you might be able to program the ignition computer to miss every nth stroke... this is supposedly part of the traction control systems in some racing cars.

Everyone read what Isky wrote. Dead on...

>If you were using spark ignition, I suppose you might be
>able to program the ignition computer to miss every nth
>stroke... this is supposedly part of the traction control
>systems in some racing cars.

I don't know that you would need to program it to miss (8-4-8 anyone?) but merely to fire on a rich mixture. The mixture would lean out overhead and the engine could then pick up power without going over lean. With today's light ignition systems, it should be feasable. It would add more weight, however.

>
>>If you were using spark ignition, I suppose you might be
>>able to program the ignition computer to miss every nth
>>stroke... this is supposedly part of the traction control
>>systems in some racing cars.
>
>I don't know that you would need to program it to miss
>(8-4-8 anyone?) but merely to fire on a rich mixture. The
>mixture would lean out overhead and the engine could then
>pick up power without going over lean. With today's light
>ignition systems, it should be feasable.

If you're going to program it to fire on a rich mixture, why not program it to hold an RPM instead?

Besides, the "leaning out" is largely an illusion. When a 4-2 break motor goes from a 4-2 and back, it' s not really like you suddenly closed the needle, and then openeed up again. It goes to a 2-stroke, and sounds "lean", but in fact it's mostly unrelated to the mixture. Its a load effect. It effects 4-strokes, too, by altering the timing even at a fixed mixture. But we've been down this road before...

I suppose it's possible that you could find a range at which it might 8-stroke, based on the tiny bit of exhaust that sticks around even after the exhaust stroke. But I bet this is so tiny a range between "fires clean but doesn't fire slightly dirty" and "doesn't fire at all" that it's useless for our purposes.

Brett, you wrote:

>>>If you're going to program it to fire on a rich mixture, why not program it to hold an RPM instead?<<<

Do you think it is good idea to keep constant Rpm? Do you think it is enough? Do not you think it will need some amount of active "acceleration" uphill?

I am longer time thinking about such unit, it is not difficult to make, but I do not know what is that proper amoung of feedback and if it should be positive or negative.

If 4C guys are happy with almost constant rpm, than active regulator keeping really constant rpm should be good way ... ???

May be not legal for new FAI rules, but can bring some answers.

>
> If you're going to program it to fire on a rich mixture,
>why not program it to hold an RPM instead?

Because if you have ever dealt with rev limiters that hold the engine at a certain rpm and allow no more you get a serious misfire when you rev past the allowed rpm until it comes back then it fires up again. To control rpm would have to be done with a programed carb the would shut as the motor trys to rev or open when the motors needs it. Doing it with ignition would result in misfires right where you wouldnt want it.

>
> Besides, the "leaning out" is largely an illusion. When
>a 4-2 break motor goes from a 4-2 and back, it' s not really
>like you suddenly closed the needle, and then openeed up
>again. It goes to a 2-stroke, and sounds "lean", but in fact
>it's mostly unrelated to the mixture. Its a load effect. It
>effects 4-strokes, too, by altering the timing even at a
>fixed mixture. But we've been down this road before...

Load and draw are directly realted. It is not one or the other. The YS engine with a built check valve to pressure the tank and a diaphram pump on the crank and pulse regulator of the back of the crank at the carb will shove enough fuel in the engine to keep it at a certain rpm no matter what attitude or what load you have on it. I could rotate that thing in any direction on my test stand it would spin what ever prop I had on there at the SAME rpm at any given attitude. Go to a Saito no pump or check valve and you get rpm changes as you rotate the motor. It leans as it has to draw fuel uphill and at a peak setting you get a slight reduction in rpm, and it doesnt smell so good either. At a slight rich setting you get a slight gain in rpm. As long as the fuel is there it could care less about the load. But with no pump and its own drawing power it will always lean out as it looks uphill. Yes the load will have a direct effect. It leans more with more load and leans less with less load but they are interelated. The pumps on the YS proved this out pretty clearly. In pattern they cant have a lean situtation in a 200' verticle eight. That is where all that pump stuff comes in. The nose is up for so long but it still doesnt lean out and get hot. They deffinently knew what they were doing when they setup that beast.

That sure was a cool motor. I wish I could have figured it out for stunt.
>
>
>I suppose it's possible that you could find a range at which
>it might 8-stroke, based on the tiny bit of exhaust that
>sticks around even after the exhaust stroke. But I bet this
>is so tiny a range between "fires clean but doesn't fire
>slightly dirty" and "doesn't fire at all" that it's useless
>for our purposes.
>
> Brett

I agree. I bet you cant even find that spot. I have had so many of these on the stands and planes and all the rpms and props you can imagine and I havent gotten it do that. Maybe I am missing something.

Isky,

Explain scavenging. I dont think a 4s scavenges fuel since it has an intake valve. It doesnt get it fuel through the crankcase it gets it from an intake valve. So where is the scavenging? Maybe I am mixed up.

Yes it will take a seperate ignition system to really run it dead rich and keep it nice and steady on a venturi. Being able to change timing would pretty fun to mess with also.

>Isky,
>
>Explain scavenging. I dont think a 4s scavenges fuel since
>it has an intake valve. It doesnt get it fuel through the
>crankcase it gets it from an intake valve. So where is the
>scavenging? Maybe I am mixed up.

Of course it scavenges - in fact, it dedicates an entire stroke to it, which makes it very, very effective. It explicitly uses a stroke to clear the exhuast, and then uses a the next stroke to charge the cylinder. That's one reason that you get a lot more power per unit fuel consumption than a 2-stroke. It always clears the exhaust very well, and thus always fires a nice clean charge. That's also why it runs so hot!

2-strokes always wind up with a lot of old exhaust hanging around the next time, because it counting on the displacement of the exhaust out the exhaust port by the incoming charge. This never works all that well, so you always have exhaust left in the cylinder and some of the intake charge always goes out the exhaust (since they are open at the same time), All the port fiddling that is done in rework is to change how this works at different RPM and needle settings.

Brett

LAST EDITED ON Nov-09-03 AT 08:30 PM (CST)
 
Yeah, scavenging is the act of evacuating the exhaust gases from the cylinder (and may also involve introducing the new charge, though I'm not certain about this). All internal combustion volume-expansion engines do this (I don't know if the equivalent thing in jet turbines can be called "scavenging").

Len:

I see a couple of problems progamming the engine to fire (or was it not to fire) on a rich mixture. First, you'd need to sense a rich mixture. Would this involve an oxygen sensor, perhaps? The second is that if it doesn't fire at all if the engine is rich (or lean), then it won't run at all during certain parts of the run. Or do you mean have it "8 cycle" on a rich mixture?

Brett:

Yeah, holding a constant RPM was what I had in mind as the use for the misfiring system. And it needn't necessarily misfire every second time, either - you can have it misfire once every nth time, as needed to keep the RPM. Supposedly cars equipped with these things make a popping noise due to backfiring in the exhaust systems.

Igor:

Yeah, setting it up to actually give more RPM when needed (going up, AND going upwind!), and cutting RPM when needed (downwind AND going down) would be ideal. None of the systems we have now does this. All of them work the way you want when going up and down, but not the way you want going up and downwind. How to do it? Remember my radio beacon/doppler shift idea?

>Len:
>
>I see a couple of problems progamming the engine to fire (or
>was it not to fire) on a rich mixture. First, you'd need to
>sense a rich mixture. Would this involve an oxygen sensor,
>perhaps? The second is that if it doesn't fire at all if the
>engine is rich (or lean), then it won't run at all during
>certain parts of the run. Or do you mean have it "8 cycle"
>on a rich mixture?

I wasn't tinking of programming it for a rich mixture as such, just thinking that a spark ignition might be more precise in firing on a rich mixture. If a rich mixture is a problem with a 4-stroke, then an ignition system might allow it to fire a slightly rich mixture that would lean when you would point the nose up, but not go overlean. Dunno, just a thought, and may not be worth the extra weight.

LAST EDITED ON Nov-10-03 AT 07:15 AM (CST)
 
That is! I better start new thread - "What is optimal engine regulation?" - this is somehow long for me