Ailerons not even - no roll problem

[John RV-6]

Would appreciate any input on trying to get the ailerons to track even with the flaps in straight and level flight.

My RV-6A is flying with no roll problem but the left aileron is about 1/4 inch low at the trailing edge and the right aileron is up 1/4 inch at the trailing edge. Aircraft is also flying straight in yaw, no correction needed.

On the ground both ailerons are even with the tip and flaps at the trailing edge.

Thank you in advance,
John

[Kevin Horton]

If I understand you correctly, to maintain straight and level flight, the ailerons must be deflected slightly. That suggests that the ailerons must be deflected to counter a rolling moment that is coming from something else.

The problem may be due to small differences in wing incidence, or wing twist. But both those problems are difficult to correct now.

Another possible cause is a difference in flap angle. But even if the flaps are perfectly rigged now, you could misrig them slightly so that the ailerons would be lined up with the flaps in straight flight. I.e., lower the left flap slightly, and raise the right flap slightly (this will probably cause grief at the inboard end of the flaps where they meet the fuselage). Now, in order to have straight and level flight, the left aileron will need to be slightly higher than it was before, and if you get the just the magic amount of flap misrig, the ailerons will now line up with the flaps. But they won't be lined with the wing tips - you would need to split the wing tips at the trailing edge and do some fibreglas work to move the wing tip trailing edges up and down as required. Moving the wing tip trailing edges will likely cause a small rolling moment, so now the ailerons will be slightly deflected again for straight and level flight. So, you go around the block again, rerigging flaps, redoing wing tips, etc.

I'm not convinced this is worth the effort, but that is your call.

[rvbuilder2002]

Originally Posted by Kevin Horton

If I understand you correctly, to maintain straight and level flight, the ailerons must be deflected slightly. That suggests that the ailerons must be deflected to counter a rolling moment that is coming from something else.

The problem may be due to small differences in wing incidence, or wing twist. But both those problems are difficult to correct now.

Another possible cause is a difference in flap angle. But even if the flaps are perfectly rigged now, you could misrig them slightly so that the ailerons would be lined up with the flaps in straight flight. I.e., lower the left flap slightly, and raise the right flap slightly (this will probably cause grief at the inboard end of the flaps where they meet the fuselage). Now, in order to have straight and level flight, the left aileron will need to be slightly higher than it was before, and if you get the just the magic amount of flap misrig, the ailerons will now line up with the flaps. But they won't be lined with the wing tips - you would need to split the wing tips at the trailing edge and do some fibreglas work to move the wing tip trailing edges up and down as required. Moving the wing tip trailing edges will likely cause a small rolling moment, so now the ailerons will be slightly deflected again for straight and level flight. So, you go around the block again, rerigging flaps, redoing wing tips, etc.

I'm not convinced this is worth the effort, but that is your call.

I agree with everything Kevin said... and will add...Since it is an RV-6 (non match hole construction) it is possible that any one (or more) of the aileron hinge brackets are misaligned vertically. This can cause unusual roll trim problems because the aileron is not properly in trail with the main portion of the wing. This can be difficult to detect (ailerons look like they align just fine to teh flap and wing tip). I takes some careful measuring. If you do decide to check, you should know that the aileron is actually taller in thickness than what the true shape wold be to match the air foil profile. This helps promote flow attachment. This needs to be taken into account when checking the aileron alignment. There should be an equal amount (approx. 1/32" to 1/16") protruding below and above the profile of the wing.


<a bit off topic> How have you been doing Kevin? How is the recovery going?

[John RV-6]

Right now flying hands off the stick the plane flies straight including yaw and roll with no input. So I do not need to hold any correction in the stick to fly level. I would like to move the ailerons (left up and right down) without making the airplane roll because the airplane flies straight and level right now.

So making any changes to flaps or wings would make the airplane roll and I would need to hold in correction to keep the airplane level. That is why I do not know what change to make. I am guessing I need to make two changes, one to the left airleron and the opposite to the right.

What does moving a aileron up on the hinges do? Does it make more or less lift?


John

[rvbuilder2002]

Originally Posted by John RV-6

Right now flying hands off the stick the plane flies straight including yaw and roll with no input. So I do not need to hold any correction in the stick to fly level. I would like to move the ailerons (left up and right down) without making the airplane roll because the airplane flies straight and level right now.

So making any changes to flaps or wings would make the airplane roll and I would need to hold in correction to keep the airplane level. That is why I do not know what change to make. I am guessing I need to make two changes, one to the left airleron and the opposite to the right.

What does moving a aileron up on the hinges do? Does it make more or less lift?
John

It would not be a good idea to move an aileron to make a trim adjustment. It should only be moved if you determined it was not positioned correctly. If one is positioned incorrectly it can cause the aileron to free float to some position other than neutral (in trail).
I had assumed you meant that it flew straight if you held the ailerons deflected.
Do you have any aileron trim system installed? If so, you should disconnect that before doing anything else so that you are only working with the aileron system (not something else causing an influence).

If the airplane fly's in proper trim now with the ailerons deflected while your r hand is off the stick, then I would be looking at the possibility that your flaps and wing tips also may not be properly align, or that there may be a hinge bracket (or two) misaligned vertically. The problem is, it sounds like what ever is happening, it may be countering another trim issue if it fly's straight while the ailerons are deflected by themselves.

[Mel]

This is actually a pretty common occurrence with RVs. Most people just don't talk about it because only you, your passenger and close formation guys will ever know.
It's really not a problem.

[Bill Wightman]

Hi John,

This kind of rigging/trim problem can be confounding, as you know. I'll try and give you my point of view, to add to the comments from Kevin and Scott.

So making any changes to flaps or wings would make the airplane roll and I would need to hold in correction to keep the airplane level. That is why I do not know what change to make. I am guessing I need to make two changes, one to the left airleron and the opposite to the right.

Lets look at this as two separate problems that are offsetting each other. Kevin pointed out there's something "external" to the ailerons that's influencing the roll trim of the airplane. This problem, separate from the aileron rigging issues, will be called "Problem #1"

Now, Problem #2 is that the ailerons are misrigged. Lets look at this a different way: The ailerons are mechanically "bussed" together, and work as one unit. Therefore, its not a matter of trying to adjust the left/right aileron independently of each other. In flight they aerodynamically balance against each other through the mechanical bus. So, resetting one aileron will only cause the balance point between them to shift, and won't solve the problem. Right now, your ailerons are generating different loads between them, which is causing them to trail off-center. This is precisely what an aileron trim tab would do.

It just so happens that your plane flies hands-off with the ailerons trailing as they do. So, Problem #2 is offsetting Problem #1, and the result is your plane is trimmed but out of rig. They're "offsetting penalties" so to speak. If we only fix one of the Problems, the airplane won't fly in trim. We need to fix both Problems. I hope this makes sense...

*********************************

You said your plane flies "straight including yaw and roll". I'm not 100% sure that means its in trim in yaw. So be sure to note, in wings level flight, that the ball is centered. If it is, then the only thing I can think of that would introduce that much roll moment would be wing or flap incidence problems. Take a real close look again at your wing incidence. Lets solve this problem one part at a time...

edit: Agree w/you Mel. This is a pretty common issue, but John wants the plane to fly in closer rig, in trim. And I think we can find the solution to this, with a little work.

[Rivethead]

Aileron, Elevator and for that matter the Rudder all work the same way to create different desired results = control. A lifting body with a given shape creates X amount of lift. Change the camber of that lifting body, a wing for instance, and you change the amount of lift created. When a control surface such as an aileron is raised as in the right aileron in a right turn it acts as a spoiler and spoils the lift in that section of the wing the Aileron being lowered increases the camber of the wing and generates more lift. So an Elevator works the same way lowering the elevator control surface increases the camber of the horizontal tail surface and increases the lift generated by the tail surface. Raising the Elevator spoils the lift and the tail drops. Rudders do the same thing by adding a left or right sideways lift to the vertical surface. As an aside: Notice that there are no Newtonian forces of any significant amount at work here.

[roee]

Whoa, I don't know how this turned into a general aero discussion, but I think a few important points need correcting on the previous post. Now, I'm no aerodynamics expert myself, so the real experts please chime in (and please correct me if I'm wrong).

Originally Posted by Rivethead

A lifting body with a given shape creates X amount of lift. Change the camber of that lifting body, a wing for instance, and you change the amount of lift created. When a control surface such as an aileron is raised as in the right aileron in a right turn it acts as a spoiler and spoils the lift in that section of the wing the Aileron being lowered increases the camber of the wing and generates more lift.

Camber is only one of many parameters that influence the generation of lift. Angle of attack is another. By deflecting a control surface, you're also changing the angle of incidence, and thereby the angle of attack, which I think is the more important effect than camber. Consider the case of a stabilator, on a Piper Cherokee or an F-18 for example, that deflects without altering camber at all, it's all AoA.

Originally Posted by Rivethead

So an Elevator works the same way lowering the elevator control surface increases the camber of the horizontal tail surface and increases the lift generated by the tail surface. Raising the Elevator spoils the lift and the tail drops.

On an airplane with a conventional aerodynamic configuration (such as an RV) the horizontal tail surface is actually rigged at an angle of incidence that will normally cause it to generate lift down, not up, to push the tail down, i.e. generate a pitch up moment. That counterbalances the pitch down moment caused by the fact the the CG is forward of the wings' center of lift. That's a key component in achieving positive stability.

Originally Posted by Rivethead

As an aside: Notice that there are no Newtonian forces of any significant amount at work here.

I think Sir Isaac Newton might take offense to that statement . A lifting body acts on a mass of air accelerating it downwards (and to a lesser degree in other undesirable directions), and thereby experiences an equal and opposite reaction creating lift (and the non-vertical component contributing to drag). I think that's about as Newtonian as it gets.

[John RV-6]

Thank you for the comments!! The airplane flies great so just leaving it is an option. Just trying to make it as good as I can. The plane is painted so I don''t realy want to start cutting the tip trailing edges apart.

The wings are as even as I could get them with a digital protractor and match up nice and even to the planes bottom.

I will measure the wings again to see if that is the issue.

I was hoping that maybe moving one aileron up and one down would maybe hide the problem that I am having.

Maybe be to picky but trying to make it as good as I can (and maybe pick up a knot or two in the process).

John