Four stroke engine operation

LAST EDITED ON Oct-30-03 AT 09:57 AM (CST)
 
I have read with interest the thread on "Back side........ In many respects it is far beyond my experience and technical knowledge. But, from running my own Saito in my BBQB Bearcat, I have some of my own observations which seem to contradict some of the opinions in that thread.

First, as for "valve float" I haven't experienced it at the RPM I run. More about RPM later.

Second, as for running lean, I break in my engines at 9000 after the first 20 minutes at lower revs. I give the engine 3 hours on the bench with the needle set a full turn rich, then I continue to operate a full turn rich for the first 30 flights (3 hours of flight time) to insure a safe break-in. All subsequent operations are at 1/4 to 1/2 turn rich, never less than 1/4 turn. I don't see much difference in flight performance between full lean and 1/4 rich, certainly not enough to justify potential harm to the engine. I read it but don't understand the theory of running over lean and burning engines down.

Third, I'm not sure I understand the logic in running 4S engines at the peak or beyond in an attempt to duplicate characteristics of piped 2S engines. There was some talk of 4S engines losing thrust on up lines and unloading on down lines and this seems to be the driving force behind these experiments. The problem for me is that I don't experience these characteristics with my 4S operations. My engines tend to run with the authority of a John Deer tractor, just growling around up, down and sideways. I have strong up lines on the hourglass after a hard entry with no apparent deceleration in the corner or on the climb. Line tension in the overheads and cloverleaf is all that I could hope for and much better than I have ever experienced before. I don't notice any unloading on the hourglass down line, just the same old "chug, chug" with very little acceleration coming down, less acceleration, in fact, than was typical with my 2S airplanes. My point is (finally) that I don't see any need to attempt "back side" or pipe characteristics with 4S engines.

Fourth, I think that loss of speed on up lines may be due to having too little thrust for the drag encountered in the corner and the demands of the vertical climb. My thought here is that instead or trying to make 4S engines "break" maybe the engine might be happier with just supplying more thrust all the time to avoid these slowing effects.

I think the answer for 4S engines lie in their torque. This ability to swing large props offers the possibility of raising thrust through the use of relatively large propellers. I have never run a prop smaller than 14" or RPM greater than 9500, so my experience begins at 8800 RPM and 14", about 5 to 6 pitch. Props of about this size ran well and gave me all the desired characteristics described above.

Like everyone else, I guess I never stop wondering if there is more performance to be had somehow. I am currently using a two pronged approach to obtain an even stronger pattern from my airplane and engine. One road to obtaining more thrust is to use longer props which explains the inverted gull wings of the Corsair. Increasing disk area with a larger diameter prop is a theoretically correct path to greater thrust. Applying greater torque/horsepower to the prop is another way to increase thrust. To this end I have been running 14.5" and 15" props to increase disk area. I am also reducing the blade area to unload the engine so it can operate higher on the torque/horsepower curve. With an increase in RPM, it was necessary to reduce prop pitch to maintain the desired lap speed. This reduction in pitch may offset, somewhat, the need to reduce blade area. Last year I was running 8800 RPM, this year I am running 9500 RPM in an effort to get a little more power out of the engine without pushing into the truly high RPM operations where the benefits are problematic and the wear on the engines certain. Well, don't I lose thrust when I reduce blade area while running longer props? Well yes, certainly, but the question is whether this loss of thrust from reduced blade area is more than compensated for by increased disk area. So far the results are promising. Many Merlin powered fighters used props with very little blade area. Perhaps we typically use too much blade area and props shorter than optimum for maximum thrust. So far I am pleased with the results. the thrust is apparent to anyone launching my airplanes, particularly when compared to piped 65s. My vertical and overhead maneuvers seem as strong, perhaps stronger.

Maybe we need longer props instead of pipes on our 4S airplanes. I doubt there is an easily found "Magic Bullet" or "Holy Grail" out there to be discovered to improve 4S operations. I am fairly certain we don't need one. The readily available performance is all that we need. Competition patterns should now depend on the consumption of fuel and practice.

A final word, with the wind at this years NATs on everyone's mind, I would also add that the 4s Bearcat flies very well in the wind. Up to 20 MPH, which is the strongest that I've flown in so far, it hasn't been necessary to distort the pattern or run the engine harder. I don't feel any need to get more thrust when climbing or to take drastic measures to avoid unloading on down lines beyond logical experiments with props.

The only four stroke aircooled airplane engines I've had experience were R2800s and R3350s.
very little, mind you, but to gain in specific range, the engines were leaned out VERY carefully using a set formula to get a 12bmep drop in cruise. Any more and valves would burn and other bad things. We had cylinder head temp gages, ignition analyzers, mixture controls, etc., and sophisticated carbs that did a lot of things automatically. It just occurred to me to be very careful of running on the lean side. Hmmm.. just remembered- you have to be careful of running on the rich side, too. As in landing at Denver 5000 ft. msl if you weren't quick with the mixtures, the engines would flood out and quit- leaving everyone in the dark. Gotta admit, I'm trolling for a Ted reply. All the best ,

That is all right Al, but what Godzilla wrote is not out of sense. Look, you say go down to max. torque to get large prop having large disk diameter. But efficiency of prop is not in area only. (now I mean efficiency as ability to keep speed, or how much thrust it brings if you slow it down to some degree – not energy efficiency). You are right that large prop brings lot of static thrust. It is because tip of prop has small AoA if diameter is large. For example 6” pitch prop of diameter 12” has tip AoA 9deg. It is probably over its stall point. It means that prop clearly does not make too much static thrust. If you go to 14” prop of the same pitch, you have AoA on tip only 7 deg. It can well be under or at top of its lift. So you have BIG difference in static thrust – exactly what you explored. But it is different in flight at working speed.

Godzilla says we can use it past the max power point – it is far over max torque so no doubt it needs smaller prop. But not only smaller prop, it needs also smaller pitch (because we want the same speed at high rpm). Probably 4”. The question now is, if that smaller prop has really worse response to speed change (compared to your large). If you judge only by prop disk area, then yes, but it is not enough to judge really ONLY by prop area. I can tell you that smaller pitch has BIG effect to that efficiency and number of blades also. I cannot say now where is the best point, but there is certainly some optimum. I would say that it is close max power peak point – but it is only my guess – it can be also far over, if torque is flat enough. What I can say for sure (or by math I did a year ago), that if you have two blade prop 10x4 and another prop of the same construction 12x6 both twisting at constant speed giving the same unloaded speed of the model!!!!, then if you slow down to some degree, both props make the same thrust. It also means that if you have engine keeping optimal speed with 12x6, and that engine will allow you to use 11x4 you will get better pull with the second – even smaller that 12x6.

That is what I wrote to Godzilla – it will be probably true, that combination of low pitch prop past the power peak will be probably very stable, but prop could be so small that even low pitch does not bring enough aerodynamic efficiency and result can be worse than solution at little lower rpm and thus less stable rpm. There IS certainly some optimum and it depends on torque curve.

I add a table I did – it shows theoretical thrust of prop in Newtons after slowing down from 22m/s to 20m/s – means by 10%. 22m/s is close to our regular speed. 1N is I think little under 4oz

10x4....8.2
10x6....7.3
11x6....7.7
12x6....8.2

>Third, I'm not sure I understand the logic in running 4S
>engines at the peak or beyond in an attempt to duplicate
>characteristics of piped 2S engines. There was some talk of
>4S engines losing thrust on up lines and unloading on down
>lines and this seems to be the driving force behind these
>experiments. The problem for me is that I don't experience
>these characteristics with my 4S operations. My engines
>tend to run with the authority of a John Deer tractor, just
>growling around up, down and sideways. I have strong up
>lines on the hourglass after a hard entry with no apparent
>deceleration in the corner or on the climb. Line tension in
>the overheads and cloverleaf is all that I could hope for
>and much better than I have ever experienced before. I don't
>notice any unloading on the hourglass down line, just the
>same old "chug, chug" with very little acceleration coming
>down, less acceleration, in fact, than was typical with my
>2S airplanes. My point is (finally) that I don't see any
>need to attempt "back side" or pipe characteristics with 4S
>engines.

Except that the description you are giving proves beyond any reasonable debate that you in fact ARE running it that way. It sounds stable - i.e. small perturbations don't make it jump to some new, higher rpm - so you are already there. The fact that you can do it more pitch than someone else doesn't change the nature of the situation. That's what I beleive got 4-strokes off to such a good start with some of the non-tech/anti-tech guys. They could slap just about any prop on there, and at least get something that worked, because the peak was at such low revs that it was stable pretty much no matter what you did. That sure doesn't work on most current 2-strokes.

Brad seems to be experimenting with something tangential to your experience. Diameter is not the only regulating factor on airspeed - pitch can be just as if not more effective. Smaller diameter, with associated lower pitch, can be just as if not more effective than larger diameter and higher pitch. Whether 4" of pitch ends up working better or not, or if the engines can be changed to improve it, can probably only be found by trying it with an open mind, and objectively assessing the performance. That's not saying that 30-year-old concepts were not workable - just that they aren't the only concepts that may work, and they may not be optimum. I think it's well worth experimenting, even if the end result is not improvement.

There's more than one way to skin a cat.

>A final word, with the wind at this years NATs on everyone's
>mind, I would also add that the 4s Bearcat flies very well
>in the wind. Up to 20 MPH, which is the strongest that I've
>flown in so far, it hasn't been necessary to distort the
>pattern or run the engine harder.

And, for the most part, at 20, no one else needed extraordinary measures either. That's one thing that's different from the good old days - one actually has to pay attention to shape, size, intersections, etc, even in reasonably decenbt amounts of wind. Most people, myself included, were getting through the maneuvers just fine at 30. But I can tell you right now that going 48 mph into a 35 mph headwind is going to cause a problem. The one, and only solution, was to make sure that you kept enough speed nosed into the wind to keep the airplane flying. Guys flying extraordinarily slow, like less than about 5.4, were DEAD. All the disk area and "torque" in the world ain't gonna fix that.

>

>
Diameter is not the only regulating
>factor on airspeed - pitch can be just as if not more
>effective. Smaller diameter, with associated lower pitch,
>can be just as if not more effective than larger diameter
>and higher pitch. Whether 4" of pitch ends up working better
>or not, or if the engines can be changed to improve it, can
>probably only be found by trying it with an open mind,
>and objectively assessing the performance. >
>>> >
> Brett
Brett,

I tried to make it clear that I was only interested in obtaining as much thrust as possible from my engine prop combination. I was/am satisfied with the character of the run. In no way was I using diameter to regulate airspeed. 4 strokes seem to respond primarily to pitch changes for airspeed/lap times. Increasing RPM to move up the torque/horsepower curve required less pitch to fly the same speed/lap time. Curiously enough adding diameter without changing pitch or blade area slowed RPM dramatically but made the airplane fly faster. Convince me that the greater disk area wasn't also dramatically increasing thrust. Flew OK but loading the engine down to less than 7500 caused concern about harming the engine from excessive load and insufficient cooling.

As for your final conclusion about disk area and torque, I beg to disagree. Disk area is thrust. Thrust is required not only to penetrate upwind in level laps but to penetrate into the wind in maneuvers, particularly, the hourglass, vertical eight, overhead eight and cloverleaf. Torque is a major factor in providing maneuvering line tension, particularly in maneuvers like the overhead eight where centrifugal force is minimal. A relatively powerful airplane should always have a decided advantage in a strong wind. As for windy competition, I don't recall you being at the '79 NATs in Olatha which also had extreme winds. People who were there will recall that Snagggletooth handled the winds well. It is impossible to compete successfully for more than a decade without demonstrating an ability to fly in the wind. To suggest that judging standards were lower then than now is insulting.

>>
>
>>
> Diameter is not the only regulating
>>factor on airspeed - pitch can be just as if not more
>>effective. Smaller diameter, with associated lower pitch,
>>can be just as if not more effective than larger diameter
>>and higher pitch. Whether 4" of pitch ends up working better
>>or not, or if the engines can be changed to improve it, can
>>probably only be found by trying it with an open mind,
>>and objectively assessing the performance. >
>>>> >
>> Brett
>Brett,
>
>I tried to make it clear that I was only interested in
>obtaining as much thrust as possible from my engine prop
>combination. I was/am satisfied with the character of the
>run.

Right, and I was merely pointing out that the acceptable nature of your run makes it absolutely certain that you, too, were already running on the back side of the curve (i.e. stable). Front side IS UNSTABLE.

I also submit the most static thrust is not necessarily associated with diameter. It is only if you hold the RPM and pitch constant - which in general you can't, with a given engine of finite horsepower and defined pwer delivery characteristics.

You can get more static thrust other ways, that you appear to have not explored. I would also submit that you could "Get more static thrust" with props with which you would get very poor inflight performance - from which it logically follows that getting the most static thrust may not lead to the best inflight performance.

>In no way was I using diameter to regulate airspeed.
>4 strokes seem to respond primarily to pitch changes for
>airspeed/lap times. Increasing RPM to move up the
>torque/horsepower curve required less pitch to fly the same
>speed/lap time. Curiously enough adding diameter without
>changing pitch or blade area slowed RPM dramatically but
>made the airplane fly faster.

Of course it did - for two reasons. One, to hold the same ground RPM you had to get more poop out of it somehow, which later manifested itself as MORE INFLIGHT RPM because it unloaded more. Two, the larger diameter increased the advance ratio.

> Convince me that the greater
>disk area wasn't also dramatically increasing thrust.

In-flight thrust was only different because of the speed difference. Even out the speed, and the inflight thrust IS THE SAME unless somehow increasing the diameter also alters the laws of physics.

If you think about it a while, I believe you will agree that high static thrust, and the same inflight speed, helps because the rate of change of thrust with changes in forward velocity is steeper. Once several people realized that fact, it first - explained what the fundamental principle of all succssful stunt power systems was, and second - suggested other ways to acheive the same goal that didn't involve ever larger props and Harley-Davidson power curves.

>Flew
>OK but loading the engine down to less than 7500 caused
>concern about harming the engine from excessive load and
>insufficient cooling.
>
>As for your final conclusion about disk area and torque, I
>beg to disagree. Disk area is thrust. Thrust is required
>not only to penetrate upwind in level laps but to penetrate
>into the wind in maneuvers, particularly, the hourglass,
>vertical eight, overhead eight and cloverleaf. Torque is a
>major factor in providing maneuvering line tension,
>particularly in maneuvers like the overhead eight where
>centrifugal force is minimal. A relatively powerful
>airplane should always have a decided advantage in a strong
>wind.

Yes, Al, as far as the maneuvers go, I am generally with you. You're of course only conditionally right about "disk area" since you haven't acknowledged the concept that low pitch and high revs (even if it takes a smaller disk area to achieve the necessary RPM) can work well and/or "get more static thrust" (two entirely different, only marginally related things). That's why my ultra-strong PA61 airplane didn't have much problem in the maneuvers.

The problem at this year's NATs WAS NOT (at least not for me and the others having signifcant problems), GETTING THROUGH THE MANEUVERS. It was FLYING AT 45 MPH AIRSPEED INTO A 35 MPH HEADWIND. Anything that flies at a reasnably constant speed (the sole purpose of getting a lot of thrust/performance, whatever you call it) that was TOO SLOW, was not going to do very well. If you showed up with a 250CC Motorcycle engine swinging a 3 foot prop, and it ran 6 seconds a lap , you, too, would have had all sorts of problems because the GROUNDSPEED WOULD BE INADEQUATE FOR ACCEPTABLE LINE TENSION.

>As for windy competition, I don't recall you being at
>the '79 NATs in Olatha which also had extreme winds. People
>who were there will recall that Snagggletooth handled the
>winds well. It is impossible to compete successfully for
>more than a decade without demonstrating an ability to fly
>in the wind. To suggest that judging standards were lower
>then than now is insulting.

Horsecrap, Al, I never suggested any such thing. What I suggest is that what used to happen was that in 20 mph of wind, a few guys did well enough, and thus won, and a lot of people were just hanging on and hoping. I submit there are a lot fewer guys just hanging on and crossing their fingers, now, than there used to be.

I just find it unfortunate that you seem to be taking a tack that discourages people from experimenting, and trying to learn the underyling principles. You skinned the cat one way very successfully - but I get the impression that you consider it to be the only way.

Brett,

Disk loading is a major element in static thrust, ask any helicopter pilot (and I are one, a helicopter flight instructor, no less). If small props were so effective in making thrust then why do helicopters have such long blades if not to reduce disk loading to get maximum thrust from minimum horsepower. First comes disk area, then blade area. The ratio of blade area to disk area is the solidarity ratio and depends on the power of the engine. The whole reason for engine gearing is to obtain more thrust from engines of a given size than than would be possible direct drive. There are very few instances where fixed wing airplanes use smaller diameter props than structure or configuration allows. I have been associated with most phases of aviation for 52 years. If I seem somewhat fixed in my opinions it is because some of these popular ideas for stunt ships violate my experience as a pilot, flight and ground instructor. I am not easily persuaded by technobabble no matter how respected the source.

You will not convince me that vertical and overhead maneuvers in the wind are less demanding than plodding upwind in level flight. If todd Lee's Mustang had a bit more thrust for upwind penetration or torque for line tension, maybe Todd would still have his airplane and maybe a NATs win. Since when doesn't thrust and torque improve capability in the wind?

Stunt aerodynamics were fairly well determined by the late 70s. Many Classic airplanes fly well enough to win Open Stunt today. The primary differenced was/is in the engines. If you had to fly now, with the engines we used then, I think your pattern would also lack a bit of crispness. Who gave you the idea that patterns then had poorer shapes and intersections than now? Stunt competition was and is largely a culmination of effort. The top flyers of the 70s counted practice in the thousands. Fuel was counted not by the gallon or case but by the drums of methanol. I have receipts. Substandard flying wouldn't win then or now. Judging was relative. The best pattern won. We did the best we could with the equipment we had to work with. I am anxious to fly my 70s Snaggletooth design with a modern 65. It could be killer!

I read it
>but don't understand the theory of running over lean and
>burning engines down.

That is very interesting you would say that. Doug has some pretty interesting results on the subject of "burning down". Maybe he will post them.

>Third, I'm not sure I understand the logic in running 4S
>engines at the peak or beyond in an attempt to duplicate
>characteristics of piped 2S engines. There was some talk of
>4S engines losing thrust on up lines and unloading on down
>lines and this seems to be the driving force behind these
>experiments. The problem for me is that I don't experience
>these characteristics with my 4S operations. My engines
>tend to run with the authority of a John Deer tractor, just
>growling around up, down and sideways.

My point is (finally) that I don't see any
>need to attempt "back side" or pipe characteristics with 4S
>engines.

I guess that just speakes to the versitility of these engines. Like the PA and RO-JEtt they can be ran successfully in several different configurations.

> My thought
>here is that instead or trying to make 4S engines "break"
>maybe the engine might be happier with just supplying more
>thrust all the time to avoid these slowing effects.

I have no interest in "break, I have interest in "brake". I don't need any more thrust, that is for sure.

>Maybe we need longer props instead of pipes on our 4S
>airplanes. I doubt there is an easily found "Magic Bullet"
>or "Holy Grail" out there to be discovered to improve 4S
>operations.

I guess I will continue to dream, or I guess maybe "go for broke", huh?

I don't see the connection between torque and line tension.

Al,

Why would you think that Brett is taking shots at older times. He simply stated what is true about then and now. Then you had a handful of guys who kicked ass and the rest. The handful found power where others didnt or want to take the time to suck every drop out of their motor to get through 20 MPH winds. Now any old Joe and can get his hands on a perfectly crafted stunt motor and pipe and get a setup with relative ease and a design that is well proven and whol-la you have a good perfomer in almost any condition. That simply means there are alot more pilots ready to tackle the winds. I think that is all he meant. I have watched a film conversion to video of the 62 NATS. It was very windy. Only ONE plane could fly level inverted. All the rest were hunting like mad. Today in winds like that inverted flight is not a question.

Comparing HELO's to our planes come on. I know where you were going but it doesnt even begin to compare. They dont have wings remember? I know you were making an example about diameter and thrust. But the HELO has no wing so it will need that very large diameter.

You and Brett's bantering about is very enlightening for sure but why does it always turn into someone gettng offened? I dont understand..

I can rotate my engine test stand from horiz. to vert. while the motor is running. Here is what I found.

I took a 72 and put on a 14X5 wood prop that I carved from a zinger.

I sarted the motor and ran it at peak rpm. The tach read 8800 steady.

Then I rotated the running motor to 90 degrees. The thought would be gained RPM right. WRONG...after about 10-15 secs of this the motor lost rpm very steadly. It got to 8400 and I rotated it back to hor.. It gained to 9000 in a burst then leveled at 8800 again. I rotated it again and it did the very same thing.

Then I bolted on a 12X3.5 Eather 4 blade.

I ran this to max RPM 10500.

Then I rotated the running motor to 90 degrees. The motor spun the prop at the same rpm for as long as I would leave it in that position. Almost a minute. Then I would take it back to hor. and it would spin the same.

Then my tach took a dump and doesnt work anymore. Stinking Glo-Bee...ERRRRR...

I was going to run it again to make sure but now I will have to wait.

I now fully understand what Brad is talking about removing the load from the equation.

I have run the 4s like Al and many others are running them for a long time. Loaded up and on the torque curve. It does work. I have been messing with these things since 97. No super effort but I have owned OS 26, OS 26C, Enya 53, Saito 50, Saito 56, Saito 72, YS 63 on a pipe sub 4 second laps.., and helped out on some of the motors Brad is running and Steve's Saito 40. The one thing they do well is fly at one nice seemingly controlled speed. Like Brett said you can really just about bolt any old prop and get in some good patterns. But there is a power loss in using this setup. There is still plenty of tension but you can "feel" the motor heat up in the maneuver. This equals a slowing effect due to in large part, We think, to heat buildup in the motor.

I never tried to run a any lower pitched props, 4 and under, for fear of tearing up the Valve train. But That is not really an issue anymore after what I have witnessed with Brad's 72. Also we have noticed another thing. The motors are much cooler after the run. MUCH cooler. The OS 52 on a 13X5 at 8400 rpms was on fire after the run. The spinner would be literally to hot to touch. After going to 11 7/8X3.75 Eather 3 blade at getting 10700 on the tach it was nice and cool upon landing. The spinner is just warm not even really that hot. Brad is finding the very same thing. Motors HATE heat it just kills them. Running it faster with less load seems to be much easier on the motor. As long as you have enough oil in it it should be fine for a long long time. Fly a square 8 with the big loaded prop and you will be able to notice the slowing effect at the end of the maneuver. That is a massive heat buildup in the motor that is causing this. Unload it loose all that heat and it will hang in there for as long as you want. This is also why lap times and Maneuver speeds are much more in line with each other. Running a fast lap like some do and a slow maneuver is just "Burning Down" motor.

>I can rotate my engine test stand from horiz. to vert. while
>the motor is running. Here is what I found.
>
>I took a 72 and put on a 14X5 wood prop that I carved from a
>zinger.
>
>I sarted the motor and ran it at peak rpm. The tach read
>8800 steady.
>
>Then I rotated the running motor to 90 degrees. The thought
>would be gained RPM right. WRONG...after about 10-15 secs
>of this the motor lost rpm very steadly. It got to 8400 and
>I rotated it back to hor.. It gained to 9000 in a burst
>then leveled at 8800 again. I rotated it again and it did
>the very same thing.
>
>Then I bolted on a 12X3.5 Eather 4 blade.
>
>I ran this to max RPM 10500.
>
>Then I rotated the running motor to 90 degrees. The motor
>spun the prop at the same rpm for as long as I would leave
>it in that position. Almost a minute. Then I would take it
>back to hor. and it would spin the same.

THIS I don't understand. On a moving airplane, yes, when you put the airplane in a vertical climb, there will be an added load on the prop because you're hauling the airplane upwards against gravity, using the prop instead of the wings. On a test stand? Where's the additional load coming from? You can't even blame gravity, if anything, moving air downwards is moving it _with_ gravity (not really true, since we have to make Archimedes happy).

What I think is actually happening is that the venturi size is too big for that RPM with the bigger prop. With the needle set on the lean side of correct. I'll also assume the tank is mounted somewhere behind the engine's backplate. So when you turn the entire engine/tank assembly skywards, the engine slowly but surely starts leaning out, because the engine is now above the tank.

Substitute the smaller prop. More RPM = more air moving through engine = more air moving through venturi = more suction = more ability to keep up the fuel flow.

Try the larger prop again, but run the engine decidedly rich, and let us know what happens.

>>I don't see the connection between torque and line tension.
>To try to make this simple, if you take two engines,
>identical except that one is weaker than the other (worn,
>perhaps), and fly them both at the same RPM the stronger
>engine will have noticeably more line tension. Wouldn't the
>stronger necessarily have more torque at any comparable RPM?

On the same prop? If they are swinging the same prop at the same RPM, they are producing *the same* torque. And (while I am familiar with the apparent line tension difference from "stronger" or "weaker" engines) I still don't see the mechanism that underlys it, and whether it's linked to potential (but not realized in level flight) torque.

Probably stronger is richer and can jump on better ... ??? ... (if we speak abot 2 cycles )

The stronger engine will react more
>favorably to the increased demands and will EVIDENCE GREATER
>LINE TENSION.

Sounds like you are wanting a very steep slope. Most definately descending.

>I also doubt that the larger side area of semi-scale
>airplanes is much more affected by wind than slimmer
>airplanes. On casual inspection one might think so but, I
>haven't experienced any problems directly attributable to
>configuration. Downwind, an airplane with greater side
>area, typically concentrated aft of the CG will experience a
>slight inward yawing, relieving, somewhat, the force of the
>wind. Upwind, this airplane would tend to yaw outward,
>relieving, somewhat, the inward drifting effect of the wind.

There was a lot of talk about this after the NATs - where yours truly crashed, and several others (Bear, Todd, and a few more) had a lot of problems. I am with you - I really don't think that the larger side area was a significant contributing factor - going upwind at 5 seemed to the the crux of the problem. It flew just fine on the downwind side. I really don't see designing for once per decade 35 mph winds if it involves any other compromises.

Your first flight was excellent. You did get a little pokey westbound on the second.

>Your first flight was excellent. You did get a little pokey
>westbound on the second.

I was in the ballpark of making it. I think that had I done any one of half a dozen things a little differently, I would have had no problem. I had *no difficulty* making the maneuvers on the first flight - or really on the second flight (when the conditions were MUCH worse) either, other than teh mild concern over the wings, bellcrank, and/or my wrist breaking from the loads.

Doudg,

I've got five tapes for you but you gotta fix my LAN.

Al. Try a single blade, Add 1in disc and 1in pitch or 2in disc and same pitch. Better everywhere. It will only look funny when parked.
MM

I will take you up on that but I cant commit to when I can do it. I start school next week. My latest employer is putting me through home inspection courses. Maybe sunday evening...

I can see Brett's point.

All the power in the world isn't going to help you if your airplane is set up to fly too slowly given the speed of the wind.

Imagine having a .90 in the nose, and running a low enough pitch that your level flight airspeed is the same speed as the wind. You'll never make it past the middle of the upwind leg, let alone up high where you do the overhead 8s, no matter how much power you have. You'll have to go up in pitch to use some of that power to give you enough airspeed to get around the circle.

Even worse, supposing you set the pitch so that your airspeed is, say, 5mph faster than the wind. Now you WILL make it past the halfway point, but you'll immediately lose line tension and crash immediately afterwards.

>You have got it. The rotated stand leans the setup out.
>Over time the larger load prop drags the motor down in a
>lean situation. BAD. The smaller prop, less load, spins
>the prop at the same rpm for an extended amount of time even
>in the very lean situation.
>
>You do this to a 2s and it starts breaking or goes full 2s.

Unless you're also running the 2s with a large venturi, a large prop, and 2C to begin with!

>I am sure it will grab rpms as it leans out. That
>prediction is simple. What I am driving at is running the
>motor on a smallish venturi at peak rpm with it fully loaded
>can result in a run that can be less than desirable. The
>lean out in the maneuvers just heats the motor and causes it
>to struggle.

It's the same deal with the 2C, except you're already running it with the small prop rich.

But yeah, I see your point.

The other thing one MIGHT do is pressurize the tank... or use a pump. YS engines, anyone?

(I do have a YS .45 sitting in a box..)

>>I don't see the connection between torque and line tension.

>To try to make this simple, if you take two engines,
>identical except that one is weaker than the other (worn,
>perhaps), and fly them both at the same RPM the stronger
>engine will have noticeably more line tension. Wouldn't the
>stronger necessarily have more torque at any comparable RPM?

Well, that's what you observe (and I'm not disputing that), but what I don't understand is WHY it happens. I don't see a connection. Torque acts to roll the plane. It doesn't yaw the airplane out, or pull the airplane away from you. Nor does it necessarily make the plane fly faster (which would increase the centripetal force the lines exert on the plane).

All I know is when I first started in the airplane biz, I asked some of the more experienced engineers and designers about prop diameter. Everyone answered with the same exact quote:

"Keep it as long as you can, for as long as you can." I guess that's the mantra for guys who props for a living.

Thankfully, I moved into supersonic combustion research and never had to deal with the prop guys. After working together for tens of years, they were still always arguing with each other at the lunch table!