Yea, my controller has proportional mixing, and it's impossible to disable the mixing...

Rodney, the plane came ARF, I installed the electronics. The Rx-Tx in question are the ones taken from an E-Flite Blade CP Pro Helicopter. Flaps are optional on my plane and I don't intend to use them.

Thanks a lot for the fast informative and speedy answers Rodney and Matthews.

So here's another question: assuming that it IS my 6ch mixing Tx that's causing the problem, will another 4ch or 5ch Tx work or must it be a 6ch? Feel free to post a link to a specific Tx you you would recommend (price over performance).
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