You said that it was a biplane. Most if not all bipes exhibit this. If you bank left and are turning left, the nose wants to pull to the right. If this is what you are experiencing, it is called adverse yaw. It is caused by the aileron that is down creating more drag than the other aileron that is up. In other words, you bank left, pull some up elevator to make the turn, and the airplane wants to go to the right. You can take care of the problem in a couple of different ways. If you have a computer radio, dial in some differential, where you use only about 30 or 40 % of down aileron when compared to the amount of up aileron. You can also do this by resetting the control arm on the servos. The other way to correct this is to fly, using the appropriate amount of rudder when making a turn.

You often see this same thing in other airplanes that use a flat bottom wing. Notably, Trainers.

Bill, Waco Brother #1