A thought inspired by sglynn's aileron trim thread

[Aviator168]

In sglynn's aileron trim thread. Some drivers prefer spring loaded trims. Others prefer the more natural trim tabs. However, since the aileron reacts differently aerodynamically (passive for trim tabs and active for spring loaded trims), would they a different Vne? I have not done any analysis yet; but my gut tells me they would.

[Bill Wightman]

I'm one of those guys who prefer to trim out the ailerons with an aerodynamic tab. I think the Rv's have some of the nicest ailerons anywhere, and wanted to preserve the feel that (maybe) has made the RV's such a success on the market.

Placing a servo + pushrod + trim tab inside the aileron does add mass to the control surface. That wouldn't have an adverse effect on flutter, at least not control surface flutter. The RV ailerons aren't fully mass balanced anyway. But adding lots mass at the trailing edge of the wing (in an attempt to balance ailerons) can cause a divergent twisting motion of the wing under load. In this case, we're talking about fluttering the entire wing, which normally is really bad.

The other factor is the nose down pitching moment the aileron has on the wing structure. Ailerons impart twisting loads on the wing that try and force the wing in the opposite direction to the input the pilot is making. If the wing is long enough and the ailerons powerful enough, the aileron will actually act like a trim tab on the back of the wing. Loads like this try and twist the wing opposite the direction the pilot is wanting the airplane to roll. This is called "roll divergence". Its not a problem for us, since our wings are so torsionally stiff. Adding a tab to the aileron would not change this either.

Therefore, the thing to be careful of (from a design point of view) is control surface flutter. This typically occurs due to slop in the hinges, the actuation system, or movement in the control surface structure itself (oil canning in the aileron skins for example). The frequency of this type of flutter is normally higher than wing flutter. One RV pilot reported a "buzz" in the stick, which was thought to be elevator flutter.

So, if you're putting an aero tab on the aileron itself, I'd do the following:

- Keep it light
- Put as much as the mass of the system over the aileron hinge axis to reduce trim inputs under load
- Keep everything tight

I used a 4" long by 1" chord aileron on my RV4, with +/- 20 degree deflection stops. It was on the left aileron, inboard. It worked nicely!