Aerodynamic questions

[KevWal]

Hi

Couple of questions:

On the following page:

http://members.aol.com/greenhawk/midstate/combat.htm

it says:

"set up elevator so as to eliminate snap rolling out of loops with full deflection. This depends on the weight and speed of the plane and can be carefully tested once the initial trim flight is underway. "

But I dont understand! Why would the plane want to snap roll when coming out of a loop and how would you stop it through setup of the elevator?


Secondly, I have had two different aileron planes now that end up climbing almost vertically when coming out of a tight aileron turn at speed. I dont believe it is over control on the elevator on my part, it is far too drastic and I am letting go of the elevator at that point as I come out of the turn. The two planes are a .15 powered mini limbo dancer and a .49 powered high wing, no dihedral SPAD.


All comments welcome. Any other info I need to add to help just ask.

Thanks in advance
Kev

[Ollie]

Hi Kev,

Elevator deflection controls the angle of attack of the wing. If the wing is highly tapered and not washed out sufficiently it will stall at the tip first resulting in a snaproll. If elevator deflection is excessive, it will cause the angle of attack to exceed the stalling angle of attack of the wing.

In a level banked turn the wing lift can be resolved into a vertical component that is equal and opposite to weight and a horizontal component that is equal and opposite to the centrifugal force of the turn. To produce the greater lift necessary to provide for both the vertical and horizontal components of lift in the turn the air speed must be greater than if the plane were flying in a straight level line where the lift is equal to the weight. When exiting the turn, the excess kinetic energy of the high airspeed is converted into potential energy of increased altitude unless some down elevator is applied and gradually reduced or the engine is throttled back to bleed off the excess kenetic energy of the excess airspeed through drag.

[vintage1]

If you pull to much elevator at high speed, and try to pull too many G's the plane will stall. yes, even tho its doing 70mph! The instructions are telling you to reduce elevator throw to a level that will not allow you to pll that many G's.

On the other point, you need to first of all cheack what happens in full throttle level flight. If teh model tends to climb a lot under full power, buit is fairly level ayt say hal;f throttle, then chences are the CG is a bit too far forward, and you have used up trim to compenstae. As the model goes fatser, the up trim has more effect.

On the other hand, it may be that you are simply overcontrolling teh model. I always overcontrol until I get used to a model. So I usually test fly with reduced rates and lots of expo.

[KevWal]

Hi Kev,

Elevator deflection controls the angle of attack of the wing. If the wing is highly tapered and not washed out sufficiently it will stall at the tip first resulting in a snaproll. If elevator deflection is excessive, it will cause the angle of attack to exceed the stalling angle of attack of the wing.

Good stuff, understand that, so it's a tip stall due to the angle of attack of the wings, and the angle of attack is effectivly set by elevator deflection. So if the elevators move to much you can induce a tip stall and hence a snap roll.


In a level banked turn the wing lift can be resolved into a vertical component that is equal and opposite to weight and a horizontal component that is equal and opposite to the centrifugal force of the turn. To produce the greater lift necessary to provide for both the vertical and horizontal components of lift in the turn the air speed must be greater than if the plane were flying in a straight level line where the lift is equal to the weight. When exiting the turn, the excess kinetic energy of the high airspeed is converted into potential energy of increased altitude unless some down elevator is applied and gradually reduced or the engine is throttled back to bleed off the excess kenetic energy of the excess airspeed through drag.

This is an interesting one, and one id like to 'solve'. On the mini limbo dancer there was a large gap at tthe hinge point which I thought must not be doing much good, so I taped over the hinge line - that made it worse.

On the SPAD (No taper, no washout) I removed slop on the elevator linkage, and stifened both tail surfaces with carbon spars, that seems to have made it quite a bit better.

The climb on both aircraft was/is quite a lot, ending up with the nose very sudenly pointing verticaly at the end of the turn. I guess the excess kinetic energy takes most effect around the areas of mass - ie the engine at the front, and thats way it ends up vertical?

Moving forward, are we pretty sure it is 'aerodynamics' thats causing this effect, and not anything else? If so, what can we do the 'improve' the aerodynamic design?

Thanks
Kev

[KevWal]

Hi again Vintage, Thanks for the reply.

If you pull to much elevator at high speed, and try to pull too many G's the plane will stall. yes, even tho its doing 70mph! The instructions are telling you to reduce elevator throw to a level that will not allow you to pll that many G's.

On the other point, you need to first of all cheack what happens in full throttle level flight. If teh model tends to climb a lot under full power, buit is fairly level ayt say hal;f throttle, then chences are the CG is a bit too far forward, and you have used up trim to compenstae. As the model goes fatser, the up trim has more effect.

On the other hand, it may be that you are simply overcontrolling teh model. I always overcontrol until I get used to a model. So I usually test fly with reduced rates and lots of expo.

The elevator trim changes required between idle and full throtle are quite a bit, especially on the mini limbo dancer. However when the plane is doing a fast turn I'm not changing the throttle setting, full throtle in, full throtle round and full throtle out, airspeed will slow during the turn and speed up again on exit, but it will never be faster than when it's straight and level?

The suden vertical at the end of a turn feels a lot more than just out of trim, it really is a suden vertical climb.....

All ideas welcome, feel free to prove that I'm wrong :)

Cheers
Kev

[Ollie]

The climb on both aircraft was/is quite a lot, ending up with the nose very sudenly pointing verticaly at the end of the turn. I guess the excess kinetic energy takes most effect around the areas of mass - ie the engine at the front, and thats way it ends up vertical?

Moving forward, are we pretty sure it is 'aerodynamics' thats causing this effect, and not anything else? If so, what can we do the 'improve' the aerodynamic design?

Thanks
Kev

It sounds to me like you have way too much pitch stability with a very forward CG. This results in a pitch trim condition with a lot of angular difference between the wing and stab and a strong tendency to nose up with excess air speed. If you move the CG back until the decalage can be reduced a lot, then the plane's pitch trim won't change much with airspeed and it won't pitch up so violently when coming out of a turn. When the stqbility is neutral and the controls are neutralized, the plane will go where it is pointed without any tendency to return to the original flight path or diverge from the new flight direction. The other thing you can do is add lots of drag which will dissipate the excess kinetic energy quicker when exiting a turn.

Good aerobatic designs have light weight, neutral stability, lots of drag and huge amounts of thrust to overcome the drag. This results in a plane that doesn't accelerate much on the down line and whose airspeed is controlled almost entirely by the throttle and almost not at all by the elevator.

[BMatthews]

...However when the plane is doing a fast turn I'm not changing the throttle setting, full throtle in, full throtle round and full throtle out, airspeed will slow during the turn and speed up again on exit, but it will never be faster than when it's straight and level?

The suden vertical at the end of a turn feels a lot more than just out of trim, it really is a suden vertical climb......
Kev

If this is accurate then you may be holding the up elevator for the turn longer than you think you are or you are holding it in too long as you start to bank the model back to level. The amount of elevator required to maintain a LEVEL turn varies as the bank angle varies. So it's important to ease off the elevator as the model starts to reverse it's bank back to level. You may think you are doing it but in practice you may be holding the sticks until the model is almost level and then letting it come back to neutral elevator all at once. But by then the damage is already done and up the nose comes.

Properley coordinated and with a CONSTANT airspeed the model will end up flying the whole turn perfectly level. If it does not then it's you and not the airplane. Sorry but it's just physics.

But having said that if you DO pick up some speed in the turns and your model is "overly stable" with the forward CG as many here, including me, suspect then it will want to nost up sharply as it comes back to level flight with the excess airspeed. The more forward the CG and up trim to compensate you have the worse this will be.

I find that many folks dive or climb in turns without realizing that they do this. Perspective is a funny thing and it's often confused. I found that flying gliders for a few years and learning to judge the airspeed by relating the fuselage lengths per time travelled helped a lot. Using this sight trick you can better guage your airspeed at any distance.