Hans Meij

I designed a model for the first time myself. It has a span op 190 cm and a length of 170 cm, 4 kg weight. Symmetrical airfoil, shoulder-wing configuration, wing and elevator angle 0 degrees to the center-line.
Loops, rolling, downlines etc. that is all perfect like a F3A plane. But when I apply rudder it acts horrible.
In the Hammerhead it kicks the tail back, wen I roll to knife -edge and apply rudder it makes a turn and with a lot of rudder in the knife edge it will snap. It is like a lot of up is applied all the time.

I searched this forum and got the idea that this is called rudder pitch coupling.

OK but what did I do wrong in the tail configuration?
Is the stab placed to low in relation to the wing.
Is it the shape of the rudder with the lage part in front of the hinge-line.
Or has it to do with the angle of the rudder with the fuselage that is 90 degrees.

Ben Lanterman

I am not sure about the terms you are using - "kicks the tail back" and "makes a turn". Could you define the maneuver and resulting action in terms that give the initial attitude of the airplane, all the inputs to control surfaces and pitching toward the canopy or belly with rudder, etc..

I don't believe there is anything inheretly bad about your setup. It could be a result of too much rudder deflection but you need to say how much you are givng also.

Mark Winstanley

I have the same problem in a couple of planes I have designed. One is a small scale Spacewalker which acts totally different from my other 1/4 scale version. When rudder is applied there is a definite pitch down effect. Stall turns have to be completed with a lot of up elevator otherwise the turn gets out of plane due to the pitch down. Knife edge is just as troublesome with pich away from the canopy with rudder application.

The spacewalker has a semi symmetrical wing aerofoil and flat plate (sheet) tail surfaces.

Any thoughts gratefully received.

Montague

It looks to me like you have the opposite of the pitching problem that is common to models of the CAP series of planes. Which makes sense, because they have stabs mounted higher than the wing, you've basically reversed the "problem". If you raise your horzontal stab, say to the hight of the wing, you should see a dramatic lessening of this effect. (see threads in here about CAP performace questions for more details)