Airplanes won't fly straight! What am I doing wrong!?

I used to play a lot more and haven't played as much as I would have liked the last few months and I've apparently forgotten something important. When I built the engine in these planes my original creations flew straight and level. Now anything with this engine just won't sly straight! Through some minor testing it seams that it isn't in the engine at all but with the wings. It's weird because with any sort of thrust the planes either roll or yaw and I can't figure out why. However, if I drop it from really high and just let it glide it goes straight. If someone with experience could take a look at these 2 planes and show me what I'm doing wrong and how to fix it that would be awesome!


Although you have 2 rotors the friction will make them spin at different speeds which will make your plane roll. The best solution is to have control surfaces. They will help with all kinds of stability issues.
Those propellors turn the same direction, right? That will produce a nonstop twist on the aircraft. If they turn opposite directions, that twist is balanced out.