Denker connects decalage with pitch stability in relating a change in air flow across both. A change causing the plane to pitch into the flow is stability and visa-versa. If the wing is flying at +3 deg straight and level and the stab is at +2 deg (1 deg of decalage), a 1 deg change in the airflow into the plane (upward) will have a greater effect on the stab (2+1 = 50% increase) than the wing (3+1=33% increase). The lift coefficient will thus change more dramatically in the stab that the wing. Causing the stab to lift into the changed airflow. Think of what would happen in the reverse - an updraft would cause the plane to involuntarily pitch upward. The difference here is the difference between pitch stability and instability. I agree, that application to reduced size RC changes some things, but the aerodynamic principals don't change. The point about trim is quite valid too. I am not a free-flighter, but their configurations call for significant decalage to provide high pitch stability. When doing so, the airframe becomes very sensitive to trim with change in power level and speed. More power/speed, more tendency to pull up. All about compromises.

What I was looking for here was something other than all the first-line issues we think of with regard to pitch stability that the OP had already checked.

Bedford

Regarding decalage and stability..

There is a mix up between cause and effect and between stability and trimgoing on here. The cause of stability is having the CG located ahead of the plane's neutral point, simple as that. The effect of having the CG ahead of the neutral point other than producing stability is that youneed toadd some decalage to achieve balanced (a.k.a. 'timmed') straight and level flight (i.e. to trim the plane). Decalage creates a nose up pitch that balances the nose down pitch caused by the forward CG.

Decalage is not the cause of the stability, it's an effect of stability and isrequired if the plane is to be trimmed for level flight.

I'll give you a couple of examples that hopefully illustrate this point.
<ul>[*]Take a plane and add weight to the nose to make a forward 'stable' CG. Now set the decalage to zero. Drop the plane from a great height.. It wont fly as such, it will plummet vertically down, but it IS stable. i.e. whatever way you dropped it, and however it's disturbed on it's way down, it will always return to a vertical dive. A vertical dive with zero AoA is it's trimmed state and it's stability ensures that it always returns to it's trimmed state. This plane is stable in the same way a bomb dropped from a B-17 is 'stable'.[*]Now take another identical plane (the first having been destroyed when it dived vertically into the ground) and add some decalage. Now the plane is trimmed to fly at some positive AoA. It's stability (due to forward CG)ensures that whatever way it's disturbed it will return to it's trimmed state. By adding decalage we did not change the stability, we changed the trim of the plane.[/list]The point here is <u>stability is the tendency for a plane to return to it's trimmed state after being disturbed</u>. Decalage sets what that trimmed state will be. Decalage does not make the plane stable, it's CG location that does that.

A third example;<ul>[*]Take the same plane and move the CG way back. You can set the decalage to whatever you like. If you are very careful is should be possible to come up with a balanced (trimmed) condition even with the CG way back,howeverany disturbance will see the plane deviate away from this balanced state. This plane will be incapable of maintaing steady state flight. This plane is unstable and nothing you can do with decalage will make it stable, only CG location can do that.[/list]<u>Anyone who doubts any of this I'll repeat my challenge</u>: If you truly believe that decalage makes a plane stable -Take a plane that you know flies well. Add a lump of weight in the tail so that the CG moves maybe half the wing chord back from it's normal position. Do whatever you like with the decalage to makethe plane stable. Now go fly it and tell me it's reallystable

Steve

PS.. i accept the point made by rmh earlier that these days it's possible to add 'artificial stability' with gyros and the like but that's another topic, lets assume we are talking conventional aerodynamic stability here...

sorry, double post

Your comments about stability are fine - -all basic rules of stability.
What these guys are doing -having seen it and discussed the issues at length, is trying to improve stability with aft cg locations.
A contradiction in basics.
The std "trick" is to add some effective downthrust -which must be contradicted by adding elevator trim.
Some do this by adding incidence to the wing -which results in the fuselage now flying angled up a bit higher at the tail
This then needs a bit of elevator/stabilizer trim to compensate
The angles between thrust,wing and tailplane have now changed .
Thrust is now "pulling down" as wing "pulls up" and hopefully the whole result is a plane which "locks in" better to straight flight-even tho cg is aft.
Does this really work?
some guys swear by it .
IF -we have forces working against each other, we can create a more stabilized arrangement . That much is fact.
The energy required to keep this new setup moving must have increased -tho it is slight - as is the difference in stability.
If you have flown competitive aerobatics and or speed models - this stuff is all familiar to you . If not - it likely sounds just plain ol weird.

I'd like to add a bit more about my pylon racer that was twitching down at high speed. I spent a fair amount of time trying to figure this problem out and had many many many "expert" suggestions along the way. The suggestions included items such as - the servos are not "Holding" center properly, radio glitch, the CG is too far aft, the fuse is flexing etc etc. Well, I went out and spent BIG bucks on the most expensive std size digital servos I could find with the best centering numbers available and guess what. NO CHANGE at all. I added some carbon fiber to stiffen the fuse - NO CHANGE. Switched from a Futaba radio to Spektrum, NO CHANGE. As I mentioned in the earlier post I moved the CG forward a bit and while that seemed to help it, it also introduced problems that were unacceptable for the intended purpose of the plane. (It also had the side effect of slowing the speed slightly) Changing the tail incidence fixed the problem which, after much thought, I suspect was some form of turbulence hitting the stab and causing the twitch. Whatever the cause, changing the stab incidence added stability to my racer. We're talking about a plane that was traveling between 120 and 130 mph so whether or not it would affect one traveling much slower I have no clue. These extremely fast Pylon Racers normally "Groove" extremely well with everything set at 0-0-0 on the incidence angles. This type of plane, AMA - 424/428 AKA Q500, with a Jett or Nelson engine will run between 180 and 190 mph and they usually just go EXACTLY where you point it hence the term "Groove". In the end some really sharp racers over in the pylon forum came up with the stab angle change and viola problem solved. Why? I have no idea, it doesn't make sense. But it worked for mine as well as theirs.

How did you change the stabilizer incidence?
which direction?
-I designed a mount which I shimmed on my patternplanes -others used a screw adj

when you were done with this adj-did you retrim the elevator - very much?
I designed all my pattern types with the thrust line running slightly to a bunch- above th e effective CLof th wings-
the engine/wing and stabs were all 0-0-0 to start
the error allowed in fitting wings to fuselage was 0,to a measured 1/8" positive at the root - the stabs could then be tweaked to get the stab/elevator alignment at "hands off" , about 0-0
depending on the speed desired (pattern) these arrangements varied a little to give the best controllability AND neutral feel.
There is NO perfect setup.

Perhaps part of the confusion here is out different perceptions on what 'stability' means. Stabilityin aerodynamic termshas a very specific meaning, as i tried to explain in my previous post,butthereare various types ofstability, longitudinal, lateral, directional, and different stability modes; static and dynamic.

What I've been talking about is static longitudinal stability. Dynamic stability is a different animal and while i cant see any obvious reason why decalage should effect it, dynamic stability is a complex field and these things are always possible.

Bear in mind in all of this that the original poster was asking why two 'identical' models (that presumably, being 'identical'have identical decalage) behaved differently to elevator input. It was not about pylons racers or stability with aft CG's or any other of the tangential directions the discussion has taken.

Steve</p>

Yes -the basics
I flew a model yesterday which had lateral stability such that you could roll a little and the model continued on in a straight line-
To reword this - banking did not produce a turn.
getting used to this is tricky at first because ALL turns are rudder turns (in level flight)
This type of stuff may seem all wrong -to anyone who has not tried these models with huge lateral areas but we areworking with cgs-close to neutral point and wing loadings of 2-ounces per sq ft -plus power which will allow acceleration from any attitude.

Check this article:

http://www.geistware.com/rcmodeling/...cs/control.htm

Verify any wash out or in of the main wing and tail.

Best!

Hi,
One other thing that I did not mentioned in post #22 is a good assesment of speed your plane will be flying at. The numbers in post #22 are from:

1 Speed 100 ft/sec

By calculating incidence at 200 ft/sec and 300 ft/sec the requirement for incidence is lowered to approx 0.0 deg.

Regards

Well Steve, like I said I expected this type of response. Your anology is interesting. Maybe I should have been more specific. In that 33 years I have designed and flown models in IMAC Pattern, Sailplnes, helicopters and pylon. In all cases I have risen to the top level from a regional standpoint. Except pattern as this really is my first season but then again I started in the advanced class. I dont not mention this to simply blow my own horn but to point out that in order to accomplish this one has to know how to set up an airplane to fly correctly. Nothing is carved in stone here. When I advise someone here it is because I have at some time experienced the same issue and throw out what has worked for me. As far as the OP is concerned, It would be nice if he were to let us know exactly what airplane he has and take some measurements of both and post what he finds. I'm still convinced that both models do not have the same incidence angles.

speedracerntrixie,

My response was not in any way a 'hand slap'. You clearly are a experienced and acomplished flyer and i never suggested otherwise. Howeverthe fact that you are an accomplished flyer and can set up RC modelsin any wayproves theclaim that aerodynamic principals don't apply to models. Unless you have actually conducted specific experiments that prove thatcertain aerodynamic principals don't work? If so i would be genuinely interested to discuss. Claiming that the accepted principals of aerodynamics don't work on models is a very bold claim, i don't think it unreasonable to ask for a bit of proof to back the claim up?

Ido agree that models due to their lower Reynold Numbers can behave differently to full size planes but this difference is entirely as the theory would predict. Basicallyall I'm saying is the laws of physics still apply to models, the laws are the same ones which apply to full size planes. Unless you have evidence otherwise?.


Steve

<span style="font-size: small">Jet Plane (Steve)
I agree with your posts above. Physics is physics, and it would take a very brave self appointed expert to attempt to change the accepted laws thereof.
Attempts to brush aside the accepted teachings of Aerodynamics on the basis of "I'm an amazing experienced flier and I find that....." have to be treated with scepticism.

However I have thought of another possibility that does fit in to Aerodynamics.
If the Trailing Edge of the tailplane/elevator is rounded off, instead of square cut, then a very small change in the airflow, or a tiny elevator movement, can cause a disproportionate change in pitch trim. This will not cause a wickedly tight loop, as a very aft CG might, but it may make the model seem a touch sensitive around neutral.

Ideally the airflow around a symmetrical tail at zero degrees would be symmetrical. There would be a small wake right on the centreline.

But in practice the wake can be drawn up (or down) around a rounded TE, causing a small down (or up) force as in the diagram.

Cutting the TE off square will lead to a thicker wake and more drag but it should be stable, leading to a proportionate elevator response. With elevator neutral the wake position is symmetrical, and small elevator movements progressively move the wake up or down yielding small progressive control forces.</span>

That's a pretty cool pictorial comparison of things for sure. It brings to mind a bunch of similar pictures from many many years ago.

The Precision Aerobatics guys from the dim past spent some years discussing aft airflow and how to control it. They didn't stop with the trailing edge. Well, actually, they started slightly ahead of it.

Some of them noticed the aileron/flap/elevator/rudder gap. As with everything, the line drawn created two different camps. A bunch of guys decided the TE wasn't really as important as the other side thought because the gap turbulated the air before it hit the active surface, much less before it hit the TE. Then LOW AND BEHOLD, a bunch of guys decided that every wing profile developed up 'till then showed the air turbulated before it was even close to the gap.

Then a bunch of guys started screwing around trying different ideas and did their best to explain it all with aeronautical terminology. Some guys "discovered" that if they made the leading edge of the surface significantly wider than the trailing edge of the wing, their models flew awesome. Of course anothe bunch "discovered" that making the LE of the surface significantly narrower than the TE of the wing, THEIR modesl flew even awesomer. Good thing was both bunches had the same theory. The suddenly wider LE, and the suddenly narrower LE both acted to reattach the boundary layer or somesuch.

All the while, a smaller, less vocal bunch found that making the surface's TE both square AND narrow made their models fly more accurately, faster, more efficiently and many other awesome things that have since slipped my mind.

I believe there has been some relatively scientific documentation that somewhat proves the square and narrow TE treatment. One thing for sure about it, it's easier for a modeler to create accurately, and far easier to tell when it's gotten hangar rash.

One problem us modelers have that affects the whole situation is accuracy. I maiden a bunch of models all year long. One thing that is almost guaranteed is that almost every one will have at least one surface hinged inaccurately. I'd put $20 on a bet the models in question are going to be affected more by less than perfect alignment of active surfaces to TEs than by anything else. But discussing theory is fun for sure. The beauty of modeling is we so very seldom have any really accurate ways to measure our results.

Yeah, I remember all that stuff...I had a C/L aerobatics flyer tell me very earnestly that he incorporated around a 1/32 slop at the trailing edge of his carefully contoured, 1/32 thinner elevator to minimise the tiny bobble made by his hand exiting a manoeuver to S&L. Then, on TV last nite a full size aerobatic pilot showing off the full trailing edge strip aileron of his Edge/Extra/Giles, don't remember which, and it had a square, sharp cornered T/E at least 1/2 inch thick...
Evan, WB #12.