why different knife edge?
 

I have been flying my Goldberg Super chipmunk with an OS91 surpass (not 3d, I know). It does a great knife edge moving left to right across the sky with the canopy facing you. Then, coming back, moving right to left across the sky ,also canopy facing you, it takes a lot more rudder input (max) and needs a lot more up elevator.
Why is this? is it due to engine torque? could it be I don't have the right degree thrust offset with the engine?

I have a pull pull set up on the rudder and have it set up with a lot of throw.

Thanks for any input.

Rob

RE: why different knife edge?
 

I would say it is a thrust issue. Mine is about equal on both sides. I have loads of right thrust though.

RE: why different knife edge?
 

i have this problem on a small .12 powered extra. it will knife edge one way but not the other, i forget which.

there is no blowback on the linkage and the throw is equal, i think as sprink suggested , it is a thrust issue

RE: why different knife edge?
 

You also have to pay attention to the amount of rudder you are giving the plane. If you are adding too much rudder it will cause all kinds of things to happen. The rudder can stall the tail. Try to use a little less rudder and see if it helps. Then check the right thrust.

RE: why different knife edge?
 

I've tried the less rudder technique and it drops like a stone.
The more I think about it, the more I think I need to add some more right thrust and try it.
My only fear is that it won't knife as well in the direction that's okay now!

Thanks.

RE: why different knife edge?
 

I agree its thrust, what does it do on take off?

Another thing to think about is propwash. The propwash curls around the body to strike the fin on one side more than the other. Altering the thrust will also alter the propwash somewhat.

RE: why different knife edge?
 

Thats why you have right thrust, to counter the unequal propwash forces on the rudder.

RE: why different knife edge?
 

thing is propwash depends on the length of the body among other things. Have a long enough (or short) enough body and the propwash will either be equal or reverse direction so that may play a bigger part in unequal KE than thrust line. I imagine prop pitch also plays a part.

RE: why different knife edge?
 

Could also be your rudder servo, Are you using a pull-pull set up or a single rod? The pull-pull set up will give you equal throw on each side. The single rod set up (if it is a weak rod or a long way from servo to control surface) the servo will pull more than push.
Disregard I went back and re read your post about the pull-pull set up.

RE: why different knife edge?
 

Hi,
the greater rudder deflection you need during knife-edge with starboard wing up sounds much like too little or no sidethrust. (slipstream effect)
Are you sure you need up-elevator during knife-edge? Normally you need slight down-elevator (amount depending on CG) during knife-edge, a little bit more when flying k.e. with the starboard wing up. This is due to he different angle of attack of the propblades as the flight direction differs considerably from propaxis. Known as P-factor it is mostly experienced when flying hi-power and high angle of attack situations like glidertowing where the tractorplane yaws to the left.
Hope the explanation was helpful,
Andre