Counter yaw problem

#1
hi guys, I am currently trying to build a single wing propeller plane. but the plane appears to yaw always to the right side, without input from me. because I could not find the solution for the problem, I deleted the entire plane and started again. after awhile, same problem. with previous planes, this issue is simply solved with a vertical tail wing. this time it wont do the job. so this time I decided to find the solution. I used the engine (on which I would rather keep using) and added 2 wing panels on both sides, like this:
20170409105507_1.jpg


started the simulation, zero-gravity on, toggled the engine. at first it flew straight, but sooner or later is started to yaw. Any suggestions?
 
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