vega2614

I've custom-built a foamy and every time I do an aileron turn (roll right or left, then up elevator) the plane pitches up way too much. Sometimes this has minimal effect on flight and actually allows the plane to turn tighter, but sometimes the plane stalls. The plane is a typical high wing with vertical and horizontal stabilizers on the tail. The ailerons are at the rear of the wings. It's sort of like a cross between a Cessna and a P-51.

I noticed that my aileron deflection wasn't equal between pushing (aileron down) and pulling (aileron up). During pushing (aileron down), the pushrod bends a bit, causing less deflection. Thus, instead of aileron movement canceling each other out, I get a net up deflection. I was thinking maybe that's why the plane keeps pitching up, the net up aileron deflection acting like an elevator. However, I'm unsure whether a net up deflection would cause pitch up or pitch down.

Also, would an improper CG (too tail heavy) cause pitching up during aileron turns? Has anyone else run into this problem? Thanks.

P.S. I reinforced the pushrods so they get about equal deflection, and adjusted the CG, but I won't be able to try the plane out for a while and I'm curious.

ozrcboy

Hi Vega,

One thing you can get is adverse yaw - the yaw axis runs vertically thought the model and so the model rotates around the yaw axis to turn left or right, which when banked will tend to make the nose pitch up or pitch down (because the aircraft is on it's side relative to the ground).

One way to fix adverse yaw is to use the rudder to correct it when flying. However you can normally fix it by playing with the amount of deflection of each aileron (assuming ailerons are served by separate servos) - the normal scenario is that you get adverse yaw tending to pitch the noise of the model in towards the ground which is corrected by reducing the amount the "down" aileron goes relative to the "up" aileron.

Two ways to do this - if you have a computer radio and at least 5 channel RX set your plane up for Flaperon (so right and left ailerons are on separate channels) then find your differential mixing control, which allows you to vary the amount one the ailerons go down/up relative to one another.

The other way is mechanical - you may have noticed the servo arms don't, at neutral tend to be completely perpendicular to the servo. The servos move in opposite directions and when you look at the arc the arm traces you will see that one generates more movement in the backwards/forwards plane than the other due to the geometry of it. So, if you can't mix it on your radio then change the neutral angle of the servos to minimise the adverse yaw.

My experience with adverse yaw was on a low wing Tucano (kinda like a PC9) and she pitched in badly when rolled. I reduced the aileron deflection of the down wing and all came good. With your high wing foamie I wouldn't expect much adverse yaw, but the designers probably intended for the model to deflect ailerons at approximately equal displacement, so potentially you have induced positive (as in away from the ground) adverse yaw.

Now - all that being said it is normal for planes under good power to climb through a corner, and you wouldn't really be harshly affected by adverse yaw until you roll past at least 45 degrees, so if that doesn't describe your situation do check things like CoG.

Cheers,
Oz.

Matt Kirsch

My Feedback: (21)

If it's pitching up when you apply up elevator, then you're simply applying too much up elevator.

There could be a couple reasons for that:
1. Too tail heavy. You'd notice that it was very touchy on the elevator.
2. Too much elevator throw. Try dual-rates set at 50% and see if that helps.

The uneven aileron deflection is being caused by the flexing pushrods. It's definitely not causing your pitch-up problem. Ailerons deflected upwards would actually make the plane sink like spoilers on a sailplane. Uneven aileron throw is actually used quite often to make a plane roll without yawing (see: aileron differential).

vega2614

Ah, ok. Thanks for the help Matt. I'll give those suggestions a try.