A question about tim setup.

Here are some specifics...

Billy Wing 710 sqr in.
8.75" nose moment
17.75" tail moment
26% stab and elevator
3" Wide fuse
Saito 72 swinging 14" 2 blader
Rabe Rudder, adjusted for a medium range of movement
Overall weight 64oz.

.014 solids
Full 70'

A few things I plan to try but if anyone has anymore suggestions to get the pull out of it please jump in.

Go from a 5.1 to a 5.4 laptime.
Move from solids to cables.
Close venturi down to kill some speed.
Possibly a shorter prop with more blades to keep the load.

All the things I know to do will just slow it down. I have had one slow flight on it but wow it still pulled like a TOW TRUCK! I dont want to have to carry an anchor with me to the field every time I go flying.

Any ideas??

DMoon

PS I know I do this every time I get a new plane out but this place is just to easy to get tips and tricks from. Thanks for the tips ahead of time.

"Never criticize a man until you walk a mile in his shoes...then you are a mile away from him and you have his shoes"

That one's for you Larry. I dont know where you get all those sayings but they great. Keep em coming.

I am a total newbie, but would like to try and apply my aviation and physics background and write what I would do.

I think I am going to be a big fan of the slower laptimes, so I'd slow the motor down some or re-prop first to get the laptime you like since your lines are already at the maximum.

Then the following, assuming it also pulls too hard overhead. If it is weak up-top I would think there is a fundamental a weight issue (though I am new and 64 oz sounds pretty good to me)?

Anyway:

Any offset on the motor? If so go toward zero degrees.

Move lines forward (but don't do this the same time as going to stranded cable from solids as I would think the increased drag would also provide a general feeling of moving the leadouts forward.

So much fun to think about this stuff! Okay experts jump in here!

Let us know what you try!

Work out.
Put on weight.
Take lots of Aleve.
Sell all your 4 strokes------to me...

First remove some tip weight. Tip weight has more effect on line tension than anything.

Moving the CG back also lightens the pull pretty quickly.

Skinny! Oh My God!

[photo not recovered: nats201.jpg]

Eat a burger every now again boy!

At a weight of 141 lbs

Dead lift 385
Bench press 245(in a Smith rack)
Squat 300(in a Smith rack)

Horrible back spasms removed me from the gym in 96 only to return this past fall. The strength is returning rather quickly but the weight issue remains.

Yes as skinny as I appear there is a little something there. I now tip the scales at a hefty 153. I worry from time to time when my 32 waist on my jeans is getting tight. I use the 12 flight stairs at work from time to time to keep the growing waist at bay. But sooner or later a full fledged workout will ensue. And all the work will not add many pounds to this frame. Beleive me I have tried!!

Brad, I eat a burger now and than, I just dont eat two such as yourself....(Grin, all in fun)

DMoon

Does it pull just as hard up top?

One of the all time great head scratchers about stunt (c/l in general, but stunt is where must of us are at now) is the conundrum between *this perceived pragmatic reality* and the scientific physical *surety* that line tension is simply the mathmatical result of weight, speed and line length: i.e. *G*= weight times velocity squared (appropriately tuned for MPH, FPS etc) divided by the radius of the action. This, of course, based on the fact that a rock on a string always wants to continue in a straight line and thus the tether (contol lines) force an acceleration into the circular flight path with the resultant centrifugal/centripital (let's not get into that debate!) forces dependably and predictably resulting. As Wild Bill liked to say..."wrong, nitro breath".

The science tells us that a stunt ship should only pull so hard and that the amount is predictable based on the *G* formula. What we've learned pragmatically is that nothing could be further from concrete! There is something else going on with powered/tethered flight which has great impact (no pun intended)on just how hard the blamed thing want's to get away from the pilot.

Certainly an adjunct to line tension is the degree to which the thrust is *vectored* away from tangent. Thus, both engine offset and line rake can have some, albeit...I think...modest effect. Just remember that if the total vector of the thrust was directly away from the pilot airspeed would be zero and the static thrust of Dougie's engine would be in the four to six pound range...hardly enough to drag him around the field.

'zilla mentions wing tip weight and that, too, can have a modest effect. This comes (IMHO) solely as a result of banking the aircraft either more or less away from the pilot and thus vectoring a small component of the wing's lift *away* from circle center. We must remember, however, that in level flight the total lift of the wing is only enough to keep the ship from rising...in Doug's case this is only four pounds of total lift. If you banked the airplane away with wing tip weight the total required lift would increase only by the geometric amount of the outward vector plus the still modest four pounds to keep the thing airborne. Once again, unlikely to make Doug look like the bank end of a monster truck pull.

After all this high falutin' pontificating the bottom line is that this is a subject about which I refuse to make a definitive statement. I don't understand why the phenmenom exists and I wait with barely bated breath for the learned cabal of SSW to enlighten all of us.

This'll be a good one!

Once you factor in centripetal and centrifugal force, you can see how the pull on the pilot is escalated.

It doesn't matter what the object is, it is just a different set of vectors when we put thrust on it that can be used to both accelerate the object, as well as direct a force vector away from the pilot.

Would it be a proper exercise to setup the plane to pull what is minimially comfortable overhead (since we have a 1g reduction) and then what you have at level flight you deal with?

If this is the case, then weight becomes imperative.

Again, a total newbie exploring a new space with other applied knowledge.

Thanks!

I know Doug is looking for opinions other than mine, as he gets mine all the time, but I thought I might share some of my experiences with the line tension issue and how it relates to the 4 cycle.

First of all, I have found that the main issue at hand with overall line tension is associated with the weight of the airplane. The overall weight appears to have the biggest effect, even more than trim, if all things are equal from the output of the engine.

Second, the more thrust your engine and prop puts out, the more line tension. No engine I have ever used puts out more raw static thrust than the big 4 cycles, especially with the right prop. I am talking about useable thrust too, not 3 second lap time thrust. I was truly concerned that my wife wanted to pit some for me at the Nats. The Saito 72 pulls SOO MUCH harder than my old ST 60’s. Static thrust means increased force tangent to the circle.

Tip weight appears to have no useable function in level laps. I actually lost my tipweight box (twice) after the Nats (broken aluminum pad under the engine causing vibration) and I really could not tell the difference until about 40 degrees. If the airplane has TOO MUCH tip weight, the pull at 5 feet is noticeably increased. The proper amount of tip weight feels very even, and 45 degrees and 90 degrees feel much closer to level lap line tension.

Nose heaviness equates to added line tension. Move the CG back and the line tension begins to fade. Then we typically add tip weight to try to get the line tension back. Then the tip weight gets excessive and feels like nose weight, then we move the CG back….blah, blah, blah… I would rather have a more forward CG and use much less tip weight, as the airplane flies much cleaner and overall line tension is much more easily obtained. Producing the corner is simply a function of the tail. I had one airplane set up properly in this fashion, at it was the simplest airplane to fly and trim. The line tension was a result of the further forward CG and tip weight was very nearly not required. The Berringer airplane is designed to fly this way.

Doug has never used an adjustable rudder. For the first time he will experience the ability to trim the yaw angle without having to move the leadouts. I expect good things. I feel that the adjustable rudder is MANDATORY with the Saito 72 and a 14” wood prop, and more specifically, you are really missing the boat if you do not use a substantial Rabe rudder. A lot of the negatives associated with large diameter props can be simply trimmed away as easily as adding tip weight or moving leadouts. My only complaint about the Rabe rudder in the past has been that I made the first ones too small, and that I have tendency to rely on the darn thing so much because it is so easy to change from flight to flight. I only wish there were more properly trimmed Rabe rudders out there. It seems everyone associates the Rabe rudder with inducing yaw. The fact is, with the Rabe rudder you can eliminate yaw.

Oh, boy, 'zilla.

I can't wait for Brett to get on this one! What exactly does *static* thrust have to do with anything at cruise speed whereat thrust is equal to drag regardless of the propulsion unit?

Strangely enough, lower static thrust is probably more easily related to high ultimate lap times...much like starting out in high gear.

Your arguments in support of forward CG would seem to have more merit in terms of increasing line tension although, it must be noted, that since thrust is equal to drag at a constant speed even a lot of offset could never increase line tension more than the margin resulting from that amount of thrust vectored outward. Since the vast majority of the thrust (that producing the forward motion) is exactly the same regardless of the powertrain any additional line tension from offset is cursed to be only a fraction (large or small) of that producing the forward motion. Remember, if you tie the strings to the tail of the ship you will have zero forward motion and the line tension will be equal to merely the static thrust. I guarantee that the static thrust in that situation will be way less than the force with which Dougie is dealing.

If you want to know what the maximum line tension thus available by virtue of using the static thrust of the engine/prop combo, mail me your set-up and I'll give you a measured result. You're going to be disappointed with the result.

I still see no smoking gun to explain the unarguably greater line tension which accompanies these big bore four strokers even in moderate weight stunters at moderate lap times.

Hmmmm?

I think I have the answer to this question. It came to me late last night as I couldnt sleep again as usual.

Nose weight has always been equated to line tension. At least that is what I hear. Bolt 17.5 oz right on the nose of your plane then get it up to 60MPH and it is going to create some outward pull. No matter what the size of the prop that weight at speed will generate pull on its own. The further away from the LE of the wing more outward pull it will create.

Ted you said you didnt see this with the smaller motors. Well I havent either, 26 52 56 all seem to have line tension relative to the 2 stroke counter parts. They all weigh in the same area also. Just a bit heavier not a bunch heavier.

It has not been a normal thing ever in stunt to see planes with 9.5-10.75 nose moments with 17.5oz or higher engines moutned up front. And I am talking about right up front. The Saito has a relatively short shaft also placing the motor even further forward. There is no pipe or header that displaces some weight back on the frame. It is ALL in the nose.

You can set the CG further aft but this lets it turn easier right? Well there is still 17.5oz in the nose trucking along at 60 MPH. That is going to create some pull wether I want it or not.

I truly beleive this has something to do with it directly. It only makes sense to me. The PA motor(no pipe or header) I am used to having 9" ahead of the LE is somwhere around 4oz or more lighter than the Saito 72. This has to have some effect on it. The only way I can prove it is to build shorter noses, I guess. Al R uses 7.5" I think on his new BBQB. I wonder how that pulls?

I too like to fly on finger tip feel so this is going to be one long road to get this monster to calm down. I guess I should have flown Brad's a few more times before I built for this motor.

I know tip weight adds line tension. I cant see how removing .5oz of TW, which is where I will probably end up, is going to take it from Truck pull to finger tip pull. I just cant see that happening from .5 oz out of the tip.

That's my explaination for a the smoking gun.

DMoon

While not disagreeing entirely, I think that the CG is where the mass of the aircraft is centered and where the individual components which make up that mass are located should be irrelevant in terms of line tension. In the pitch axis, of course, weights at the extreme are a different animal in that their inertia make it harder to start a pitch motion as well as harder to stop it. If we were actively accelerating the entire aircraft in the yaw axis as we do in the pitch axis I think your suggestion would have more intuitive merit.

I can make some visual perception argument (perhaps Igor could quantify this thought) for during hard cornering the mass at extremes from the CG might have an outward component during the pitch acceleration. Not sure exactly how that would play out in numbers since the acceleration is taking place during level flight as well. I simply haven't the background in physics to make a determination that would satisfy me...and especially those who know so much more than me about such things.

Brett???

Doug, I would bet a dollar your lost sleep just answered your question....CG back would have been my first instinct from my own experiences. Even with a shortened nose section, 17 oz. on the front of your bird vs. a total of 17 oz with pipe on your usual setup where at least 3 oz. is behind the CG will surely equal some serious line tension. Sure tip weight will add a little tension but I am quite sure you just answered your own question.

AS A RESULT THE ABILITY TO MAINTAIN CONSISTENT LINE TENSION IN ALL PHASES OF FLIGHT DETERIORATES WITH AN INCREASE IN AIRCRAFT WEIGHT. I THINK THIS IS WHY OLDER, LIGHTER (CLASSIC ERA) SHIPS OFTEN HAVE SUPERIOR OVERHEAD AND VERTICAL PERFORMANCE THAN TODAY'S HEAVY IRON. BY THE TIME YOU TRIM THE LEVEL FLIGHT TENSION TO A COMFORTABLE LEVEL WITH TODAY'S PRO-STUNTERS THE LOSS OF FOUR TO FIVE POUNDS OF PULL OVERHEAD IS A MUCH GREATER PERCENTAGE DECREMENT THAN WHEN FLYING A TWO TO THREE POUND CLASSIC SHIP.

>Second, the more thrust your engine
>and prop puts out, the
>more line tension. No
>engine I have ever used
>puts out more raw static
>thrust than the big 4
>cycles, especially with the right
>prop. I am talking
>about useable thrust too, not
>3 second lap time thrust.
> I was truly concerned
>that my wife wanted to
>pit some for me at
>the Nats. The Saito
>72 pulls SOO MUCH harder
>than my old ST 60’s.
> Static thrust means increased
>force tangent to the circle.
>

COVERED THIS IN ANOTHER RESPONSE. I WOULD LIKE TO ADD, HOWEVER, THE FOLLOWING SUGGESTION FOR A TEST OF RELATIVE STATIC THRUST. TIME THE FIRST LAP FROM LAUNCH OF YOUR PANIC WITH A FIVE OR SIX PITCH PROP AND DIVIDE IT BY THE LEVEL LAP TIME AFTER COMING UP TO SPEED. DO THE SAME TEST WITH DOUG'S TUNED PIPE SHIP WITH THE FOUR +/- PITCH PROP. THE SMALLER THE RESULTING NUMBER THE GREATER THE RAW STATIC THRUST AT LAUNCH. ONCE AGAIN, AT A STEADY STATE CONDITION THRUST WILL BE EQUAL TO DRAG WHETHER IT COMES IN THE FORM OF A SIX PITCH AT 8K OR A FOUR PITCH AT ELEVEN. HOW RAPIDLY YOU ACHIEVE THAT EQUILIBRIUM FROM A STANDING START IS A PRETTY GOOD MEASURE OF HOW MUCH THE STATIC (AT REST) THRUST EXCEEDS THAT NEEDED FOR STEADY STATE FLIGHT.
>
>Tip weight appears to have no
>useable function in level laps.
> I actually lost my
>tipweight box (twice) after the
>Nats (broken aluminum pad under
>the engine causing vibration) and
>I really could not tell
>the difference until about 40
>degrees. If the airplane
>has TOO MUCH tip weight,
>the pull at 5 feet
>is noticeably increased. The
>proper amount of tip weight
>feels very even, and 45
>degrees and 90 degrees feel
>much closer to level lap
>line tension.
>
>Nose heaviness equates to added line
>tension. Move the CG
>back and the line tension
>begins to fade. Then
>we typically add tip weight
>to try to get the
>line tension back. Then
>the tip weight gets excessive
>and feels like nose weight,
>then we move the CG
>back….blah, blah, blah… I
>would rather have a more
>forward CG and use much
>less tip weight, as the
>airplane flies much cleaner and
>overall line tension is much
>more easily obtained. Producing
>the corner is simply a
>function of the tail.
>I had one airplane set
>up properly in this fashion,
>at it was the simplest
>airplane to fly and trim.
> The line tension was
>a result of the further
>forward CG and tip weight
>was very nearly not required.
> The Berringer airplane is
>designed to fly this way.
>

NOT SURE I ENTIRELY BUY THIS TRAIN OF THOUGHT. A FORWARD CG DEMANDS A FORWARD LEADOUT PLACEMENT. IF THE TWO REMAIN IN THE SAME POSITION RELATIVE TO ONE ANOTHER I WOULD EXPECT THE ONLY INCREASE IN LINE TENSION TO BE THE RESULT OF THE GROSS WEIGHT OF THE PLANE BEING HIGHER.

IF, ON THE OTHER HAND, YOU MOVE THE CG FORWARD AND LEAVE THE LEADOUTS AFT YOU WILL IN FACT INCREASE LINE TENSION DUE TO THE INCREASED OUTWARD THRUST VECTOR ASSUMING THAT YOU KEEP THE SAME FORWARD SPEED SO THAT THE LINE TENSION FROM CENTRIFUGAL FORCE DOESN'T DETERIORATE WHILE DOING SO. IN ADDITION, YAWING THE AIRCRAFT OUT INCREASES DRAG WHICH COULD WELL SLOW THE AIRPLANE MORE WHILE MANEUVERING ABOVE LEVEL FLIGHT ELEVATION THUS REDUCING AIRSPEED AND LINE TENSION COMMENSURATELY.

CONVERSE TO THE EARLIER PREFERENCE FOR SMALLER LIGHTER SHIPS, TODAY'S POWERPLANTS MITIGATE NICELY FOR THE LOADS OF FORCING NOSE HEAVY SHIPS AROUND SEMI-TIGHT CORNERS.

MY GUT REACTION, BY THE WAY, IS THAT YOU ARE ALL TOO WILLING TO THROW TIP WEIGHT INTO A SHIP TO PERFORM A FUNCTION IT IS NOT WELL SUITED FOR AND THE DOING OF WHICH MIGHT WELL RESULT IN INFERIOR ROLL RESPONSE UNDER HIGH G MANEUVERING.
>

TED

P.S. KEEP IN MIND THAT WHILE ALL OF THE FOREGOING MAKES A LOT OF SENSE TO ME THEORETICALLY, I MUST RESTATE MY ABSOLUTE BAFFLEMENT REGARDING THE REALITIES OF THE ALMOST EXCESSIVELY HIGH LINE TENSION WHICH THE FOUR STROKES GENERATE. I'VE FLOWN SEVERAL THAT I'M ALMOST UNWILLING TO MANEUVER WITH SIMPLY BECAUSE MY PREFERRED METHOD OF CONTROL (LIGHT FINGERTIP INPUTS) SIMPLY ISN'T AN OPTION. PRETTY MUCH HAD MY HANDS FULL JUST HOLDING ON.

STRANGELY ENOUGH, NONE OF THE THE SEVERAL PROFILES OF MY OWN I'VE FLOWN WITH FOUR STROKES THE LAST FEW YEARS HAD REMOTELY HIGH LINE TENSION. THIS INCLUDES THE DOCTOR ON THE .40 SURPASS AND THE COYOTE AND RINGMASTER ON THE .26 SURPASS...NEITHER NOTED FOR THEIR SIMILARITY TO A MACK TRUCK.

Yes it does pull hard up top too.

However when entering the Hourglass it has line tension across the top but when making the dreaded and feared third trun it feels like it will all just go away. It doesnt, well a little bit, but it isnt to good to have that feeling.

Does this make sense? Is it possible to have good line tension but not much power. I mention this from time to time at the field and most look at me like I am nuts. But Bob G. knows exactly what I am talking about. I guess I could equate it to a scenario like this. You are flying along and all tension is good and in only a few spots, really tough ones, it faulters miserably then recovers and all tension comes back. The overall shape of the maneuver really sufferes. In situations like these, I have seen it from time to time at the field and flown on some planes, the speed of the thing is keeping the tension but in a hard corner where the speed is gone and you are totaly relying on the power of the motor to pull the plane through it just isn't there. I see this often at the top of the Hourgalss in the third turn. So many pilots think they cant fly an Hourglass when really it is the equipment letting them down at the wrong time. I have seen it all to many times. I somehow feel this new plane might fall prey to the very phenomenon if I dont find just the right setup. Sorry to ramble so. It's just that once I got enough power to drive through every spot of the pattern especially the hard ones WITHOUT any speed up flying the pattern became much much easier to really learn and get a good grasp on.

Now back to my plane at hand.

I feel there are two things going on here that are contributing to this. Speed and adjustment of the rudder. I will start with the speed thing first then try the rudder. But I am most curious about what others around have to say about this.

Bra...er.Godzilla is in full belief that it is the tipweight causing this(toooo much tension). I am at 1 7/8 oz TW at this point and the plane is pretty clean everywhere. A wiggle here and there but I cant tell if it the rudder or the TW doing it so here we go.....

The plane seemed a hair turnerd out while in level flight so I moved forward on the LO and got even more tension but better flights.

Who knows???

DMoon

>
>Bra...er.Godzilla is in full belief that
>it is the tipweight causing
>this(toooo much tension). I
>am at 1 7/8 oz
>TW at this point and
>the plane is pretty clean
>everywhere. A wiggle here
>and there but I cant
>tell if it the rudder
>or the TW doing it
>so here we go.....

>
>Who knows???
>
>DMoon

WOW....WOW.....WOW.....Doug....Doug......1 7/8 ounces of tip weight!!!!!! That is almost Windyish in amount. That is a HUGE amount of tip weight. What happens when that weight is reduced by say 7/8 of an ounce?

After having flown 4 strokes for two years now, and having a little success with them, I have found two things that create line tension. They are engine size and tip weight. Strangely, I have found the prop to be a minor player in this equation. Obviously, speed determines line tension, but that is so obvious I didn't add it to the list.

The big 72 is, bar none, the most line tension producing engine I have ever used. (Note to other 4 stroke users: Nothing comes close to a B-17 for line tension though.) Maybe I am going to try the 90 some day, but that might take another fuselage to keep the CG in some "normal" range, what ever that is!

Interesting thing. I switched from a 56 to a 72 in the Mustang, and the 72 is 2 ounces heavier, and I did nothing to the trim, and in a matter of a few flights, I was fully adjusted to it and the corners were not effected by that change. Explanation: ( blank space here ). Try that with a 2 stroke. That result can't be duplicated with a 2 stroke. Don't know exactly why.

Yes the CG if more forward with the 72, but adding 2 ounces to the 56 doesn't make the same line tension. Go figure. Obviously, the added power is the difference.

Bottom line for me: Bring on the line tension. That's one of the reasons why I went to the 4 strokes. But, it does require a program of some weight training to keep up for a full season!

Tim

Tim

Assuming all your pull is simply coming from the centripetal force you need to exert for your plane going round and round.

5.1 second laps=13.2 lbs of pull
5.4 second laps= 11.8 lbs of pull.

The pull scales as the inverse square of your lap time.
To this you must add any other forces coming from offset thrust lines ect.
I have used your numbers above.

What is interesting is that if the plane is not flying tangent to the circle, you will pick up an extra force vector which is proportional to thrust *the angle. For a two degree offset for example, the added force is just

f(thrust)=thrust*.034. If your engine is outputing 4 lbs of thrust (for example), this will add and extra 0.13 lb to your pull--pretty insignificant.

However if you also need to compute what the aerodynamic contribution is if the plane is cocked outward. This will add to the pull. Any pull coming from an offset rudder (right rudder) will subtract. However I don't have much of a feeling how much of the pull is here.

I guess that I'd play with trying to decrease the offset, since it appears like that is perhaps the largest effect. Other than lightening the plane by throwing out that heavy 4 stroke ( throw it my way! )

Alan

1) Reduce lap speed. Simple. If going from 5.1 to 5.4 isn't enough, go to 5.6 or 6.0 or... Somewhere along this route you are going to start complaining about not having enough line tension.

2) Reduce rudder offset. You can even go to inset. I KNOW this will reduce line speed (and perhaps give you a bit of jogging experience while back pedaling around the circle).

3. Move the leadouts forward. If the leadouts are too far forward, moving them back will increase line tension. If they are too far back, moving them forward will help in overheads. But move them too far forward and you have a bit of the same effect as item 2 above except that you might introduce some jerkiness along with it. However,

Starting with number 1 above and a little of 2 you should lose all of the tension that you care to lose. Then adjusting number 3 will get you into the ball park.

You could also go to a smaller prop and move the CG back, but the above three items should do the trick admirably.

Leonard Neumann

Never, never, never, have I seen this to be true, Len. Maybe in level flight, but the tension actaully diminishes at 45 degrees.

Ted, said something to the effect that the leadout position is a function of CG (I hope that is right). If that is the case, I don't know if I swallow that whole or not. I think line rake is a function of sweep of the lines. This explains why cables require further aft positions than solids, even if the CG is constant. I think Windy and Big Jim would side with me on this one. Windy bench trims his leadout locations based on cables and solids, and the length of the lines, not on the CG location.

The extra line tension of a further forward CG is caused by the airplane naturally wanting to yaw outard due to the moment imposed by the offset CG. The line rake is not affected by the CG and does not need to change if the lines have not been changed. The heavier the airplane, the more this effect is magnified. For a given drag on the lines, the leadout location stays basically the same, and the line tension increases.

Like Ted said, if you put the leadouts on the tail the airplane, it is flying away from you and the line tension is equal to the thrust of the airplane. Which, I think pretty much explains why thrust leads to line tension.

The airplane is yawed out in the real world. Even if just a little. The thrust of the engine is actually yawed outward in some small quantity. So some small quantity of thrust equates to line tension. It must! Why does everyone spend so much time and money on props and engines?

Long story short, I just don't think trimming is that easy.

Oh and Ted, nothing I have ever seen goes from a standing start to steady lap speed faster than our Saito 72's with the 14" prop.

>Oh and Ted, nothing I have
>ever seen goes from a
>standing start to steady lap
>speed faster than our Saito
>72's with the 14" prop.
>

UH....What....There is a certain old man that gets his to lap speed as about as fast as there is. His pull on the ground may not feel like yours but getting to speed is instant. Some people launch that thing and they say "The alchol dragster isnt that fast out of the hole" His plane is the fastest to lapspeed out of the ones at our field. At least I see it that way. Maybe I am wrong.

I wanted to edit this by saying Brad's plane is the hardest pulling plane at the Field by far. When you let it go it just jumps. But from looking at the first laps comparitively The old mans seems to get there fastest. Even my Nats plane once it's in there air there is no build up of speed it is just on the 5.8 laptime and ready to go the second it is in the air. Even on my new on that we are discussing it takes it some time to get to speed. But once it gets there it never goes away!

Brad next time we are out I want to try that experiment.

DMoon

I have the means to accurately measure static thrust and could make some comparisons. In fact, if Brett is up he took some static thrust measures the other day comparing large diameter props at different pitches and revs appropriate for launch. The four pitch props produced appx 50% more static thrust than did the six pitch...again at revs appropriate for desired lap times.

>Lots of stuff goes into the
>test; not the least of
>which is the aircraft weight.
> Prop disc diameter has
>a lot to do with
>it as well. The
>bottom line, however, is that
>at a given diameter of
>prop, spinning a low pitch
>at adequate revs to reach
>the desired lap times produces
>dramatically greater static thrust than
>spinning the same diameter prop
>at a higher pitch which
>results in the same lap
>times.
>
>I have the means to accurately
>measure static thrust and could
>make some comparisons. In
>fact, if Brett is up
>he took some static thrust
>measures the other day comparing
>large diameter props at different
>pitches and revs appropriate for
>launch. The four pitch
>props produced appx 50% more
>static thrust than did the
>six pitch...again at revs appropriate
>for desired lap times.

It was actually more like a factor of two. The question was - does a 12-6 @8000 have a different static thrust than a 12-4 @12000. I sort of already knew the answer, but Ted has a thrust stand, so I hooked up a 40VF set for about 13000 rpm, and tried it.

12-6 Rev-Up at 8000 = 32 oz
12-4 Rev-Up at 12000 = 80 oz.
or a factor of 2.5

The RPM choice was an attempt to equal out the theoretical advance in either case - rpmxpitch. In actual fact, a more realistic launch rpm to get the same inflight speed would be about 8300 and 11400. Interpolating from the data taken in the test at several RPM points, this would likely result in about 36 oz and 62 oz respectively (+- a pretty big error bound). This certainly corresponds to the subjective experience. This is an inevitable consequence of prop blade Cl going up linearly with pitch, but the dynamic pressure going up with the square of the rpm. And corresponds pretty well to the expected HP difference (a measurement of this difference will probably be the next test).

I also know that 12-4 Rev-Up at 11400 doesn't fly the airplane nearly as well as other props, some of which have more static thrust, and others of which have less static thrust.

I have a small data set, but I don't particularly care to post it. I think that a lengthy discussion of static thrust and release acceleration, would be counterproductive in the sense that it would tend lead people to jump to false conclusions. The relationship of static thrust to in-flight performance is not strong, and is grossly overstated. PW's Impact with a 40VF/11.3-4 Bolly, while having respectable static thrust, would certainly not be confused with a competition pulling tractor. But it did manage to win 7 National Championships. Same with the Infinity - it flies reasonably close to the same with a 40VF/11.5-3.75 as it does with a PA61/12.5-4.1, and the static thrust is, well, different.

Not so with a 4 stroke.

Incorrect. Magic pixies that violate the laws of physics are not involved. We did this test the other day, too. Some data points were taken with both a PA61 and a OS52 Surpass. Exactly the same to the ounce.

I want to make sure I follow this.

If an os 91 two stroke spins a 14x5 prop at 8500 it makes the same thrust as the Saito 72 spinning the exact same prop at the same RPM? Thus meaning the range of power for the particular motor means nothing?

Brad and I were talking and he said the Saito makes all it torque down low around 8500-9000. 2 Stroke motors dont make as much power down low like the 4 stroke.

You and Ted are saying that the 2s spinning a 12x6 at 8000 has less thrust then the same motor spnning 12x4 at 11000. This makes sense all the way.

Brad is arguing for the thrust of the Saito 72 spinning 14X5 at 8700. Does that make more thrust than PA 65 spinning Eather 4blade 12X3.6 at 10800?

What would be your geuss?

DMoon

PS where did you get your thrust finder?

Means nothing *with regards to static thrust*, which was the topic. A particular propellor spinning at a particular RPM will *always* generate the same static thrust. The air has no idea what's spinning the prop, be it electric motor, two-stroke, two-stroke set to be 4-stroking, or a 4-stroke, Briggs and Stratton or Saito.

Of course the engine torque characteristics matter in the ultimate in-flight performance. That's the reason that just static thrust is pretty useless as a measure of performance.

>
>Brad and I were talking and
>he said the Saito makes
>all it torque down low
>around 8500-9000. 2 Stroke
>motors dont make as much
>power down low like the
>4 stroke.

I lay no claim to any eternal truths, but this is also probably incorrect. We did about half of that test, too. A PA61 set up for flight with a 12.5-3.75 3-blade (optimized for about 11000) can spin a 14-6 Rev Up at 8800 rpm, which is probably *way too fast* for a decent level flight speed. It would be loafing at 8000. We were very impressed that it was putting out quite a bit more ponies than a ST60 could manage even at low revs, and with grossly sub-optimal settings for the test.

The output of the PA61 as configured and set for flight needle settings has an extremely steep torque dropoff with speed. With the pipe at 17.5, it plummets very quickly as the speed goes from about 9000 to 12000. It's putting out about .9 hp at 9000, .45 at 10800 (launch revs) and it's down to about .25 hp at 12200. This is the essence of the regulation effect.

Of course, this is with it set for an in-flight rpm of around 10800-11000. Pull the pipe out, and the characteristic it had at 11000 now appears at 10000, or 9500, and the low-rev torque shoots to the sky.

We haven't done this test explicitly yet, but what your 4-stroke appears to do is simply take a similar torque curve and *cut it off* as the RPM goes up. It doesn't matter what prop you put on it, it won't run faster than about 10500 as timed. We'll soon see, but I wager there's less rpm regulation in the 4-stroke than the 2-stroke. This would explain the difference in the lap airspeed and maneuver airspeed, and the piped engine low pitch just keeps putting out more and more thrust as the speed drops/load goes up, and the 4-stroke with a 5" pitch just slows down.

The primary advantage of the 4-stroke seems to be that the torque curve is really smooth around the operating point. It can't suddenly burst into a 2-stroke, which has been the bane of my existence with the PAs. And this was pretty easily solved.

I would hasten to add that I think you can easily get better speed regulation with the stock PA setup than with my constant 4-stroke setup. But almost by definition, better regulation means more variable power, and there's a compromise to be made between these two.

But based on testing, I would guess it would be pretty easy to recreate the 4-stroke torque curve with a PA61, and in fact I've heard rumors of people taking their best 4-stroke props, and adjusting the engine/pipe to try to get the same feel.

>You and Ted are saying that
>the 2s spinning a 12x6
>at 8000 has less thrust
>then the same motor spnning
>12x4 at 11000. This
>makes sense all the way.

And that's *all* I was saying. I would make np claim at all to anything else.

>Brad is arguing for the thrust
>of the Saito 72 spinning
>14X5 at 8700. Does
>that make more thrust than
>PA 65 spinning Eather 4blade
>12X3.6 at 10800?
>
>What would be your geuss?

This is precisely why I didn't want to get into a static thrust discussion, because I feel the momentum building for a "static thrust bragging contest" in the making. Which would be a meaningless distraction to some more useful pursuit. But...

I don't know for sure, but a 13-5 at 8700 is about 45 oz. At the same rpm and pitch, hand calcs suggest that the 14-5 would be in 60-65 oz area (diameter^4 effect, (14/13)^4). This is probably conservative, so figure 70-ish. Of course the HP required is monumentally higher, too. I don't have any 4-blades, but as I fly it, the 12.5-4.1 3-blade Green @10600 puts out about 80 oz.

These are all in close to STP atmosphere, so figure less everywhere else for all readings.

Ted can tell you where he got the thrust stand, but I while I beleive the number I am giving, I would once again urge everyone not to make any conclusions about the "goodness" of various systems based on the static thrust.

A vastly, monumentally, better use of the thrust stand would be to put in the back of a pickup, with a flow straightener in front of the prop. Then run the engine at a constant setting, and drive at different speeds down the road to get the *dynamic thrust*, which *would* be very useful.

BTW, if anyone wants to compare launch accelerations, it would be very easy to simply mark out with cones 15 degree markers around the circle, tape it with a video camera from the center, then note and record the time history. I would be very interested in the results if anyone does it. Note also the weight of the airplane fully fueled for each test.

Timing the first lap or two from launch is a really crude way of measuring it. Ted did this the other day, and one low-powered 4-2 break model took ~9.5 seconds for the first lap, and ultimately reached 5.0 flat. This compared to 95% of ultimate velocity in 30 feet or so for David's PA51 models (based on exhaust-plop measurement)

Brett

p.s. oh, what the heck, it'll get out of hand anyway, so here is our existing small thrust data set, with assumed pretty big error bars:

rpm static thrust
12-6 Rev-Up Pro Series 3
8000 32
9000 43
10000 66

12-4 Rev-Up Special Pro 2
10000 33
11000 66
12000 80

12.5-4.1 3-blade, Brian Eather "Green", helical
8000 25
9000 51
10000 77
10600 89
11000 97

13-5 Rev Up Special Pro 2
7500 21
9000 50

> I lay no
>claim to any eternal truths,
>but this is also probably
>incorrect.

How is this incorrect?

For a 4 cycle, not a 2 cycle?

I would not argue that a PA 65 has more output at 10,500 than at 8000 RPM. But so what?

We did about half
>of that test, too. A
>PA61 set up for flight
>with a 12.5-3.75 3-blade (optimized
>for about 11000) can spin
>a 14-6 Rev Up at
>8800 rpm, which is probably
>*way too fast* for a
>decent level flight speed. It
>would be loafing at 8000.
>We were very impressed that
>it was putting out quite
>a bit more ponies than
>a ST60 could manage even
>at low revs, and with
>grossly sub-optimal settings for the
>test.
>
> The
>output of the PA61 as
>configured and set for flight
>needle settings has an
>extremely steep torque dropoff with
>speed. With the pipe at
>17.5, it plummets very quickly
>as the speed goes from
>about 9000 to 12000. It's
>putting out about .9 hp
>at 9000, .45 at 10800
>(launch revs) and it's
>down to about .25 hp
>at 12200. This is the
>essence of the regulation effect.
>
>
> Of course, this
>is with it set for
>an in-flight rpm of around
>10800-11000. Pull the pipe out,
>and the characteristic it had
>at 11000 now appears at
>10000, or 9500, and the
>low-rev torque shoots to the
>sky.

You seem to understand the pipe setup very well.

>This would explain the difference
>in the lap airspeed and
>maneuver airspeed, and the piped
>engine low pitch just keeps
>putting out more and more
>thrust as the speed drops/load
>goes up, and the 4-stroke
>with a 5" pitch just
>slows down.

You really are coming to a lot of conclusions. How many 4 stroke flights have you observed this phenomenon? How many different setups? One, maybe two, three perhaps?

How many guys have you launched for that run a 4 cycle? I do not know what you are calling thrust (obviously) but for shear pull nothing beats the 4 cycles.

Maybe you should get one, put an Eather 3.7 three blade on it and launch it at 10,500. It sounds real interesting when the valves float.

A torque curve is a torque curve. You seem to be bordering on attributing magical properties to 4-strokes.

My point was that the PA 61 we tested seems to be more than capable of swinging a "4-stroke prop" at more than adequate speed to fly the model. For instance, a 14-6 at 8800 instead of a 14-5 at 8700, like Doug suggested. Unless you are relying on elves and/or wizards, the PA61 is putting out quite a bit more torque than your 72 at the same low revs. A PA61 set up *way off optimal* for the conditions (implying that you could get far more with a proper setup).

Next time I get a chance, I'll run a Rev-Up 14-5 at 8700, and then we'll all see if it takes less or more torque than a 14-6 at 8800. That seems to be what you are arguing. But of course you already know the answer. I won't tell you what engine I use, *since it doesn't matter*.

>>This would explain the difference
>>in the lap airspeed and
>>maneuver airspeed, and the piped
>>engine low pitch just keeps
>>putting out more and more
>>thrust as the speed drops/load
>>goes up, and the 4-stroke
>>with a 5" pitch just
>>slows down.
>
>You really are coming to a
>lot of conclusions. How
>many 4 stroke flights have
>you observed this phenomenon?
>How many different setups?
>One, maybe two, three perhaps?

I also only suggest an answer to this puzzling issue, not declare it at the eternal truth.

Well, between yours, and Pauls, and Uncle Jimby's, and Teds, and Larry's, quite a few. Actually, it seemed to be true in *all* cases.

One of the noted appeals of the 4-stroke is that it goes through the maneuvers slowly. But it goes fast in level flight. My assesment explains this. But I have an open mind, if you have a better idea.

>
>How many guys have you launched
>for that run a 4
>cycle? I do not
>know what you are calling
>thrust (obviously) but for shear
>pull nothing beats the 4
>cycles.
>

Well, that's not supported by any evidence (subjective or objective) that I have seen. I had numerous reliable witnesses to the data, and to the identical readings with a 4-stroke and a 2-stroke at the same RPM, which seems to underlying your argument.

What I mean by thrust is forward pull on the propellor shaft, which is exactly the same as what you mean.

I would guess I've launched around 50-60 flights of serious 4-stroke planes,and innumerable little ones. None, including Uncle Jimby's, has come close to David's 51 in terms of static thrust. It's quite respectable, and in fact about the same at most PA61 models, but it's not significantly different and certainly not significantly more.

You seem to be bent out of shape about this. I think this is rhe result of jumping to precisely the sort of conclusion that I feared when I did not post the thrust data originally.

Measure the static thrust at various RPM, then you might have something to argue about. But you need a better argument than the laws of physics being repealed because you have a valve train.

Brett
AKA Gamera

This is exactly what I got to thinking later after I posted this. It really does make not a bit of difference what turns the prop. It will only make X thrust at Y RPM period.

>
>
> Of course the
>engine torque characteristics matter in
>the ultimate in-flight performance. That's
>the reason that just static
>thrust is pretty useless as
>a measure of performance.

Yep.

>
>>
>>Brad and I were talking and
>>he said the Saito makes
>>all it torque down low
>>around 8500-9000. 2 Stroke
>>motors dont make as much
>>power down low like the
>>4 stroke.

Yeah this has no bearing on the Static thrust conversation which really means nothing I guess. I just find it interesting.

>
> We haven't done this
>test explicitly yet, but what
>your 4-stroke appears to do
>is simply take a similar
>torque curve and *cut it
>off* as the RPM goes
>up. It doesn't matter what
>prop you put on it,
>it won't run faster than
>about 10500 as timed. We'll
>soon see, but I wager
>there's less rpm regulation in
>the 4-stroke than the 2-stroke.

I thought there would be more RPM regualtion in the 4s. It just spins to a certain point and stops. Thing is when it is in level flight it is fast at the RPM. That same RPM in a Maneuver is slow. Just a thought.

>This would explain the difference
>in the lap airspeed and
>maneuver airspeed, and the piped
>engine low pitch just keeps
>putting out more and more
>thrust as the speed drops/load
>goes up, and the 4-stroke
>with a 5" pitch just
>slows down.
>
> The primary advantage
>of the 4-stroke seems to
>be that the torque curve
>is really smooth around the
>operating point. It can't suddenly
>burst into a 2-stroke, which
>has been the bane of
>my existence with the PAs.
>And this was pretty easily
>solved.

Yep they cant just jump on you and that is nice.

>
> I would hasten
>to add that I think
>you can easily get better
>speed regulation with the stock
>PA setup than with my
>constant 4-stroke setup. But almost
>by definition, better regulation means
>more variable power, and there's
>a compromise to be made
>between these two.
>
I found the opposite to be true as we headed down the constant 4 stroke setup on our PAs. At least it feels that way when you are flying a full 4s run. It just pretty much stays the same everywhere. I can run much lower pitch props and get really no hard speed up. It just revs to point and seems to stop.

I see 2-4 runs on PAs that look like a nightmare. But the pilot likes it, somehow.

> But based on
>testing, I would guess it
>would be pretty easy to
>recreate the 4-stroke torque curve
>with a PA61, and in
>fact I've heard rumors of
>people taking their best 4-stroke
>props, and adjusting the engine/pipe
>to try to get the
>same feel.
>
>>You and Ted are saying that
>>the 2s spinning a 12x6
>>at 8000 has less thrust
>>then the same motor spnning
>>12x4 at 11000. This
>>makes sense all the way.
>
> And that's *all*
>I was saying. I would
>make np claim at all
>to anything else.

Yep.

Thanks for the reply I found it interesting.

DMoon

What formulas are you using? The only formulas I can find for thrust only use the diameter and the RPM. Pitch is not even a part of the equation, neither is the HP or torque of the engine.

'zilla, (and Doug)

I think you can answer your own question if you think about the following. What would the thrust be if the pitch of the prop pitch was *zero*? Would the thrust be any different if the zero pitch prop was five inches in diameter or 15 inches...or fifteen feet?

Note that Brett and I are always comparing props of the same diameter at different pitches and at RPMs appropriate for the level lap times we desire. If, for instance, you spun the six pitch 12 inch prop at the same revs as the four pitch you will, in fact, generate more thrust. Lap times with each would be dramatically different, however. More on that in a bit.

You are correct in that torque and HP are not part of the static thrust equation...with the exception that the power source must have enough of both to spin the prop at the desired RPM.

The fact is--as you suggested, Doug--that a given prop will produce X amount of static thrust whether it is driven by a four stroke, a two stroke or a turbine.

Other aspects that will enter into the in-flight performance of the prop are, of course, much more diverse and, as we've discovered from playing with props, close to black magic. It isn't, of course (black magic, that is), but because the complexities are so diverse and the measurement of them is probably the task for a lifetime we live in alternate delight and dismay as we stumble through the rosebush of propeller selection for a given airplane.

What's more germane, I think, is that we've been talking about a very simple measurement of a not particularly valuable piece of information...i.e. what the thrust is at a zero airspeed. Since we don't fly our airplanes at zero airspeed the value is interesting but not necessarily definitive of what is good for us and what isn't.

The one thing it will do is give us a pretty good indication of what the initial take-off roll will look like. A prop developing five plus pounds of static thrust *will* lift a four pound airplane vertically. The same engine at lower revs with a higher pitch prop will produce a lot less *static* thrust (see Brett's example) and the vto may no longer be an option.

If you put the same ships on their wheels and launch them there will be a commensurate difference in the manner and rate at which they accelerate to the desired lap time.

It might be helpful to think of the propeller as the "transmission" between the engine and the rear wheels of a car. In the case of the car the rear wheels are the driving force and in an airplane the ambient air is. Neither machine moves until the transmission imparts energy to the driving element. The engine can be putting out tons of power and torque but until the transmission is engaged no useful work is being done. Once the transmission is engaged the amount and rate of work being done depends on the gearing built into the transmission.

Thus cars have gear shifts and real airplanes have variable pitch propellers (technically, the props are constant RPM devices which allow the engine to operate at an optimum power/torque range and the pitch varies to produce different airspeeds appropriate for the phase of flight at that optimum engine power setting...just like the car's transmission. This is not unlike our stunt ships that operate at a constant throttle setting...except that we don't have a constant speed prop.

As a result our engine/prop selections will always be a compromise. That is *exactly* why in the old Fox .35 days a six pitch was almost essential and nowadays (schnerle two stroke days) significantly less pitch is preferable. The Fox was limited in the RPM range at which it did its best work and the modern schnerle's optimum RPM range is way in excess of what works best with "traditional" prop pitches for stunt.

One final observation, it makes no difference what engine you drive a given prop with (in terms of static thrust). Spinning a 15 X 5 at 8K with a Saito .72 will produce exactly the same static thrust as doing so with a 1.2 ci two stroke. You could put that prop on the front of an Pratt and Whitney R-2800 (DC-6 powerplant) and spin it at 8K and still get no more thrust. Of course, the 2800 would blow up long before it got to 8K, ergo the actual thrust avaiable from the 1000 or so horsepower wouldn't be enough to pull the test stand on ball bearing wheels.

Once the airplane starts moving and the load changes on the prop/engine combo the problem gets much more complex and now torque, horsepower and the RPM range at which they are optimized start to have a great effect.

For another time.

Brad, I'm interested in what prop and launch RPM you fly the .72?

Ted

Gosh, sorry about this but this subject is fascinating. Although my statement about thrust increasing with pitch at a given RPM and diameter is generally true. It is also true that pitch can get so steep that a powerful enough engine can actually drive the prop at rest at angles of attack which exceed critical and the blades will actually be stalled thus dramatically reducing thrust. I believe this is a pretty good description of the phenomenom known as cavitation.

Where did you get that thrust stand?

DMoon

Jumping <supreme being of your choice> on a pogo stick! Please, please try to understand fundamentals before arguing. You have some really bad and wrong equations, or are grossly misinterpreting them.

The pitch is intimate part of the thrust equation, and of course the torque required turn a prop is related to the drag on the airfoil and the rpm. The drag on the airfoil is the sum of the induced drag and the parasitic drag. The induced drag is the drag imposed by creating lift, or in this case, thrust! This drag leads to torque required, and the torque x rpm = HP.

To take an absurd example, what if you used a right-had prop instead of a left-hand prop. That's like having negative pitch, and you would get negative thrust. Or just bolt up a paint stirring stick (with 0 pitch) to your Saito and see how much thrust it puts out.

For instance:

For the same propellor blade shape/area/airfoil, the thrust is very roughly proportional to the HP required to turn the prop.

For a given blade shape/area/airfoil and RPM, the thrust is *kind of* proportional to the pitch, with some big caveats about prop airfoil stall angles.

For a given pitch/blade shape/airfoil/rpm, the thrust goes up roughly as the 4th power or the diameter.

Torque/HP/RPM/Static thrust are all *intimately and inextricably linked*.

To begin understanding this from first principles, treat the propellor as a wing with the AoA at each point = alpha angle of the blade, and the velocity at each point as omega*r. Divide the prop in to sections of small increments of radius, and then treat each separately with the lift and drag equations. Then add them up, and you will begin to see the relationships. It won't be right because of lift distribution effects, but it will certainly get you started.

And I emphasize once again *static thrust in and of itself, tells you almost nothing, and makes an exceedingly poor figure of merit*.

Give it a look.

I'll probably get the program, but I don't think I've ever disagreed that for the most thrust *per unit power* will lead you to larger diameters. Larger props are more efficient (in the technical sense) than smaller props, everything else being equal. Note this is not efficiency in the non-technical sense, where efficiency is equated with "best".

Jumping <supreme being of your choice> on a pogo stick! Please, please try to understand fundamentals before arguing. You have some really bad and wrong equations, or are grossly misinterpreting them.

The pitch is intimate part of the thrust equation, and of course the torque required turn a prop is related to the drag on the airfoil and the rpm. The drag on the airfoil is the sum of the induced drag and the parasitic drag. The induced drag is the drag imposed by creating lift, or in this case, thrust! This drag leads to torque required, and the torque x rpm = HP.

To take an absurd example, what if you used a right-had prop instead of a left-hand prop. That's like having negative pitch, and you would get negative thrust. Or just bolt up a paint stirring stick (with 0 pitch) to your Saito and see how much thrust it puts out.

For instance:

For the same propellor blade shape/area/airfoil, the thrust is very roughly proportional to the HP required to turn the prop.

For a given blade shape/area/airfoil and RPM, the thrust is *kind of* proportional to the pitch, with some big caveats about prop airfoil stall angles.

For a given pitch/blade shape/airfoil/rpm, the thrust goes up roughly as the 4th power or the diameter.

Torque/HP/RPM/Static thrust are all *intimately and inextricably linked*.

To begin understanding this from first principles, treat the propellor as a wing with the AoA at each point = alpha angle of the blade, and the velocity at each point as omega*r. Divide the prop in to sections of small increments of radius, and then treat each separately with the lift and drag equations. Then add them up, and you will begin to see the relationships. It won't be right because of lift distribution effects, but it will certainly get you started.

And I emphasize once again *static thrust in and of itself, tells you almost nothing, and makes an exceedingly poor figure of merit*.

'ZILLA, YOU'VE LEFT OUT AN IMPORTANT PART OF THE EQUATION. LEADOUT SWEEP AND CG ARE INEXTRICABLY CONNECTED BECAUSE THEY ARE THE TWO PARAMETERS THAT DETERMINE AT WHAT YAW ANGLE RELATIVE TO THE CIRCLE THE SHIP WILL FLY UNDER STEADY STATE CONDITIONS. WHAT YOU SAY IS ABSOLUTELY TRUE IN TERMS OF SWEEP BEING BASED ON LINE DRAG (LENGTH, DIAMETER, COMPOSITION, ETC), HOWEVER, THE APPROPRIATE POSITION FOR THE LEADOUTS IN RELATION TO THE CG LOCATION IS CONSTANT...ONCE YOU DETERMINE WHAT YAW ANGLE YOU DESIRE.

MOVING THE CG FORWARD OR THE LEADOUTS AFT ACCOMPLISH THE SAME THING IN TERMS OF THEIR EFFECT ON LINE TENSION. WHAT IS LESS OBVIOUS IS THAT DOING SO DOESN'T NECESSARILY EFFECT LINE TENSION IN THE GENERALLY EXPECTED MANNER. MOVE THEM TOO FAR AFT RELATIVE TO THE CG AND THE AIRCRAFT WILL SLOW SO MUCH THAT THE LINE TENSION WILL DROP...IN THE ULTIMATE CASE WHEN THE SHIP IS POINTED DIRECTLY AWAY FROM THE PILOT, AIRSPEED IS ZERO AND THE ONLY TENSION YOU GET IS THE STATIC THRUST OF THE ENGINE NOW MOUNTED ON A PRETTY, STATIONARY TEST STAND.

ASSUMING THAT YOU AGREE THAT INFLIGHT TENSION IS GREATER THAN THAT DERIVED FROM THE STATIC THRUST OF THE ENGINE IS BECOMES A GIVEN THAT LINE TENSION WILL GRADUALLY BUT CLEARLY DECREASE AS THE LEADOUTS MOVE AFT (ASSUMING NO COMPENSATING CHANGES IN POWERTRAIN, ETC.)

AS YOU'VE STATED, THE GREATEST SOURCE OF TENSION IS CLEARLY THE CENTRIFUGAL/CENTRIPITAL FORCES GENERATED BY THE MASS OF THE AIRPLANE IN MOTION TRYING TO FLY OFF TANGENT TO THE CIRCLE. TO THE EXTENT THAT POWERTRAIN MODS CAN MAINTAIN THE DESIRED FORWARD SPEED AS YOU INCREASE YAW (CG/LEADOUT RELATIONSHIP) YOU WILL BE SEE SOME INCREASE IN LINE TENSION. AT SOME POINT IN TIME SUFFICIENT FORWARD THRUST TO GENERATE THAT LAP TIME WILL SIMPLY NOT BE AVAILABLE FROM THE POWERTRAIN AND THEN THE TOTAL LINE TENSION WILL DROP PRECIPITOUSLY TO, AGAIN, THE STATIC THRUST OF THE STATIONARY ENGINE/PROP.
>
>Like Ted said, if you put
>the leadouts on the tail
>the airplane, it is flying
>away from you and the
>line tension is equal to
>the thrust of the airplane.
> Which, I think pretty
>much explains why thrust leads
>to line tension.

I THINK THE ABOVE RESPONSE ADDRESSES THIS AS WELL. AT SOME POINT THERE WILL BE A CROSSOVER BETWEEN THE ABILITY OF THRUST TO CREATE TENSION FROM BOTH FORWARD MOTION AND THRUST OF THE ENGINE (WHICH, BY THE WAY, IS MUCH LESS AT SPEED THAN AT REST). IN ANY CASE, INCREASING LINE TENSION BY YAWING THE CRAFT OUTWARD (ASSUMING THE ORIGINAL CG/LEADOUT COUPLE WAS A MINIMUM DRAG CONDITION...PRETTY MUCH YAW TANGENT TO THE CIRCLE)WILL PRETTY MUCH ALWAYS REQUIRE SOME COMPENSATING ADJUSTMENT TO THE POWERTRAIN TO OVERCOME THE DRAG WHICH RESULTS.
>
>The airplane is yawed out in
>the real world. Even
>if just a little. The
>thrust of the engine is
>actually yawed outward in some
>small quantity. So some
>small quantity of thrust equates
>to line tension. It
>must! Why does everyone
>spend so much time and
>money on props and engines?
>

THE KEY WORD HERE IS..."*SMALL* QUANTITY". REMEMBER, ACTUAL THRUST IN STABLE FLIGHT IS PRETTY MODEST. IF SOME "SMALL QUANTITY" OF A MODEST TOTAL IS VECTORED OUTWARD ONLY A VERY TINY AMOUNT OF TENSION CAN RESULT...AND THEN ONLY IF SPEED ISN'T REDUCED. REMEMBER, THE *G* FORCE EQUATION INCLUDES THE SQUARE OF VELOCITY SO TINY SPEED REDUCTIONS WILL HAVE A MORE SIGNIFICANT EFFECT ON TENSION THAN MIGHT APPEAR ON THE SURFACE.

THE MORE IMPORTANT CONSIDERATION IS HOW MUCH THAT THRUST IS ABLE TO RESIST DRAG INCREASES, BY VIRTUE OF THE RELATIONSHIP OF THE RPM TO THE TORQUE PEAK AND THE INCREASE IN THRUST COMPONENT WITH SPEED REDUCTION...IE, THE POWERTRAIN'S RESISTANCE TO SLOWING DOWN; THE GOOD OLD *LOW GEAR* SYNDROME, IDEALLY ON THE BACK SIDE OF THE TORQUE CURVE.

I THINK THE TIME SPENT ON ENGINES AND PROPS IS PRIMARILY THE RESULT OF TRYING TO FIND A PROP/ENGINE SETUP COMBINATION WHICH GETS THE WHOLE MAGILLA WORKING IN THE PROPER TORQUE RELATIONSHIP.
>
>Long story short, I just don't
>think trimming is that easy.
>

BOY, HAVE YOU GOT THAT RIGHT!

>
>Oh and Ted, nothing I have
>ever seen goes from a
>standing start to steady lap
>speed faster than our Saito
>72's with the 14" prop.
>

WE SHALL SEE.

They are two of the factors, but not the only two, I am not sure I buy that they are necessarily linked. I always keep it in mind, because I keep hearing it, but in practice, once the leadouts are set to the lines, varying the CG has its own unique properties. I try not to mess with the leadouts. Moving the leadouts back has never improved tension above about 10' for me or anyone else I have ever helped trim.

Doug's Nats airplane has a super aft CG and the leadouts are way forward. I think that the lines are just a lot straighter with the big modern powerplants. We also run solids. Bob G. said the leadouts were further back with the Fox 35. There was a lot more "bow" in the lines.

Give me a few more years and I will revisit my opinion.

>MOVING THE CG FORWARD OR THE
>LEADOUTS AFT ACCOMPLISH THE SAME
>THING IN TERMS OF THEIR
>EFFECT ON LINE TENSION.

Sorry, disagree. Moving the Cg forward increases line tension everywhere in the hemisphere. Moving the leadouts back does not.

>ASSUMING THAT YOU AGREE THAT INFLIGHT
>TENSION IS GREATER THAN THAT
>DERIVED FROM THE STATIC THRUST
>OF THE ENGINE IS BECOMES
>A GIVEN THAT LINE TENSION
>WILL GRADUALLY BUT CLEARLY DECREASE
>AS THE LEADOUTS MOVE AFT
>(ASSUMING NO COMPENSATING CHANGES IN
>POWERTRAIN, ETC.)

I do not understand this paragraph at all.

>>Oh and Ted, nothing I have
>>ever seen goes from a
>>standing start to steady lap
>>speed faster than our Saito
>>72's with the 14" prop.
>>
>
>WE SHALL SEE.
>
>TED

I have already seen. Bob's airplanes are a pound lighter than mine. The acceleration of the airplane is a relative thing.

The PA 65 and Saito 72 are comparable in static thrust. I just think the Saito has the edge. Windy said the same thing.

DMoon

Oh yes and when you return it throw the prop you ended up using in the box along with the engine

This could explain his findings.

Bri,

I suspect you might be right. I chose not to extend the exchange because we were getting a little nit-picky and there is only so much technical info that can be exchanged via the written word (sans pix and hands on examples)

For instance, when Brad said he had never gained line tension by moving the leadouts aft he clearly wasn't considering the case where the leadouts started out too far forward and actually yawed the aircraft in during flight.

I think it is likely true that for any individual airplane there is a CG/leadout position that will be optimum for the envelope of speeds at which it performs. Any change in that relationship will result in a deterioration in overall performance. The ultimate in the case of aft movement of the leadouts being when airspeed is zero and line tension is equal to the static thrust of the stationary powertrain.

It would be interesting to have one of our carrier fliers chime in with some info on line tension when utilizing the slider leadouts for slow flight. This semi-helicopter form of flight approaches the extremes to which Brad and I have addressed ourselves.

From the outside of the circle it appears as though a good carrier ship in its "hovering" mode requires pretty much full throttle to almost hang on the end of the lines. The leadouts are full aft and the ailerons and rudder are jammed full right...thus approximating the conditions of lots of offset and tons of tip weight in a stunter.

My feeling from a spectator's perspective is that line tension under these conditions is significantly less than that same airplane at full throttle during high speed flight when the aerodynamic cork screws have not been applied...notwithstanding the fact that a very significant portion of the prop's thrust is being vectored away from the pilot.

It should be noted that the carrier example merely illustrates the situation of grossly aft leadouts relative to the CG and is not in and of intself illustrative of anything which contradicts Brad's position. If it illustrates anything relative to this discussion it is that the thrust of the engine in and of itself is a very poor source of line tension.

I try. The ideal is make airplane fly straight all time. The Rabe rudder can help maintain the zero yaw condition.

>I suspect you might be right.
> I chose not to
>extend the exchange because we
>were getting a little nit-picky
>and there is only so
>much technical info that can
>be exchanged via the written
>word (sans pix and hands
>on examples)

I hope it is understood that we are working the "still good" envelope. The airplane is capable of flying good patterns at both settings.

>For instance, when Brad said he
>had never gained line tension
>by moving the leadouts aft
>he clearly wasn't considering the
>case where the leadouts started
>out too far forward and
>actually yawed the aircraft in
>during flight.

No, I mean that no line tension is gained by moving aft of the optimum setting. I think I understand that you did not mean this case, and I think I understand your explanation. I am just saying that moving the leadouts back never GAINED line tension, assuming that the leadouts were not too far forward to begin with... hey, I am trying my best here.

>It should be noted that the
>carrier example merely illustrates the
>situation of grossly aft leadouts
>relative to the CG and
>is not in and of
>intself illustrative of anything which
>contradicts Brad's position. If
>it illustrates anything relative to
>this discussion it is that
>the thrust of the engine
>in and of itself is
>a very poor source of
>line tension.
>
>Ted

I don't think hovering is done anywhere near full throttle. More like half throttle or a little less. I am absolutely no carrier expert though. We have several that fly at our field though, and we see them practicing all the time.

The carrier planes I have seen will not hover at full throttle. They haul ass at full throttle, regardless of where the leadouts exit the wing.

I'll take your word for that until we hear from some carrier flyers. I do know that the current "hover carrier" event is spectacularly different from my carrier career back in the late '50s. Without slider leadouts and hard right yaw and roll controls slow flight was a very low power operation depending on just barely keeping the airplane above a stall. Occassional burst of power were necessary when you exceeded critical AoA but for the most part the throttle was near idle.

Nowadays a carrier ship at full low speed mode would stay airborne only fractions of a second at any close to idle power.

Must admit that a lot of my most recent exposure to carrier was when WAM was still doing a lot of it. Their special rules actually allowed a condition not unlike that we've discussed. The could legally run the throttle line out the tail of the aircraft and pull the tail in to a point where the ship would literally just pull away from them. Couldn't do that in comps, of course, because they have to maintain forward progress but it was not unusual for the slow flight portion of the program to take closer to minutes a lap than seconds.

Still hope some carrier guys will belly up to the bar here. I'm obviusly in way over my head. Dale Gleason, do you lurk out there somewhere????

A heavier airplane will be heavier than one that weighs less. And you do not have to figure out ways to trim for the weight.

Oh you can slow it down, change the tangental angle and a number of other things. But then by doing this are you negatively impacting the flight performance perhaps walking a fine line of the performance envelope?

So I guess if you want to fly the big airumlplanes you's gotta pay the price.