Balsawings

I’m in the process of trimming a Cermark Javelin II for SPA Novice (haven’t got the Super Kaos built yet) and I’ve noticed the plane has some Yaw when rolling. I don’t seem to be able to remember which way to set up the Aileron Differential to take out the yaw. I know that a high wing plane and a low wing are set up opposite. I’m thinking more up than down but I’m just not sure. Any help is appreciated.

Bob Cox
SPA 374

WEDJ

Normally it is more up than down. However, on a low wing plane, you have to look and see if the yaw is with or against the aileron command.

WEDJ

Check towards the bottom of this link

http://www.rcuniverse.com/forum/m_55...mpage_7/tm.htm

teamvortex

My Feedback: (6)

Bob,

In non-engineering terms, the roll-yaw coupling is caused by uneven drag from the ailerons when deflected.

The aileron with the larger drag yaws the airplane into it. The two coupling terms are adverse yaw and proverse yaw. If the plane yaws in the opposite direction from the aileron input (adverse roll - yaw coupling) ,ie left roll causes right yaw, then you need to reduce the throw of the right hand aileron in the down direction. You would then have more up throw than down throw. You might have to play with it a bit if the coupling is not as severe in the other direction,ie right roll. Proverse roll-yaw coupling is not as common, but can occur. The solution is the reverse of what I just said.

The full story has more to do with the mean camber line of the airfoil and a few other terms related to the roll center, aileron roll and yaw center moments, tail moments and such. Heavily cambered airfoils have the problem a lot more than symmetrical ones. In cambered high wing aircraft, rudder input is required to compensate for the adverse coupling. Rudder mixing can also be use to overcome the adverse coupling effects, but this is more a "Piper Cub" solution than an aerobatic one.

Rich

ps: Bachelors and Masters in Aerospace Engineering from GaTech and a whole lot of designs for models. A good bit of education and lots of experience.

kingaltair

My Feedback: (4)

I easily get lost when the discussion turns to aerodynamics.

The simplest answer I can give came from Dennis Hunt, (who is also "over my head" in "aerodynamics-speak" in no time flat), is to have about 2 degrees on a deflection meter more up than down in the direction you want to turn. For right roll you need the right aileron to deflect about two degrees up MORE THAN the left aileron deflects down. The reverse is true for left roll.

This is a general rule. If after doing this there are still problems, (not caused by the pilot by deflecting the elevator too much at the same time), then you need to call in someone who really knows the score to individually tailor your plane.

Hope this helps--I believe I understood Dennis correctly.

Duane

Balsawings

Thanks everyone for your help. I put 5% more up than down which equates to 2 ½ degrees. This fixed the problem. It’s really tough to tell which way the yaw is occurring but with so little needed, it’s simply a matter of trying differential one way and if that doesn’t work or makes it worse, to reverse the differential.

Thanks again

Bob Cox
SPA 374

JeffH

My Feedback: (43)

I always read/ was told that "creating lift causes more drag than killing lift." The downward moving aileron is creating lift, and the upward moving one is killing it. The drag created pulls that wing back, in effect yawing the aircraft.