Getting an ornithopter to fly is only half the battle. These things are notoriously difficult to steer. Admittedly, much of the trouble arises from trying to use strange steering methods other than the tried-and-true rudder and elevator system found in many RC airplanes. But the ornithopter does have some unique characteristics you'll need to understand.

Typically, the ornithopter will have a horizontal stabilizer in the back, like most airplanes. Usually, the tail provides a downforce to keep the nose up, and therefore the tail incidence relative to the wing is much more than you would find in an airplane. This angle is typically about 15 degress, but it may be less or more depending on where the center of gravity is located and other aspects of the ornithopter design.

downforce stabilizer

lifting stabilizer

In certain rubber-powered ornithopters, the motor stick has been elongated in an effort to increase duration. Since the center of gravity is farther to the rear, the stabilizer provides lift instead of downforce, and the incidence is decreased. Obviously, the lifting stabilizer is more efficient, and this is what birds use. However, the more rearward center of gravity location can decrease the directional stability of the ornithopter. Unlike birds, which are actively stabilized by the nervous system, your ornithopter might need a vertical fin.

Because of their aeroelasticity, and because of inertial effects, slight differences between the two wings can cause the ornithopter to turn left or right. For example, the carbon rods used for wing spars may bend more easily along one axis than the other. It may be necessary to rotate the spars in their sockets so both sides bend equally under load.

In general, if the ornithopter wants to turn to one side, this can be corrected by adding weight to the wingtip on the outside of the turn. Through inertia, the added weight acts to supress the flapping motion of the weighted wing, while increasing the flapping angle of the unweighted wing. The difference in travel range results in a difference in thrust that will compensate for any unexplained pull to one side. If the wing spars are made of wood, their weight may naturally differ, and the technique of adding weight to one wingtip is a perfectly logical correction.

swinging tail

tilting tail

Steering is usually done by the tail. The wings can be used for steering, but this is less consistently successful and more difficult to implement. A simple elevator and rudder system is very effective for ornithopter steering. For a more birdlike appearance, a flat, fan-shaped tail is more often used. For steering, the tail may swing out to the left and right sides, so that the downforce of the tail causes a rolling moment on the ornithopter. Alternatively, the tail may rotate about its long axis. In this case, the downforce is redirected in a way that provides yaw control. In some RC ornithopters, the tail rotates about an oblique axis, combining the two motions described here.

If you're at all serious about building an ornithopter, be sure to read the Ornithopter Design Manual. The additional information on ornithopter stability will make your task much easier.