Mutually centering in-surface actuators

Martin Newell

August 2004

Actuators provide more positive control if some method is used to center them. Typically this is done either with elastic hinges, or by using the magnets in the rudder and elevator actuators to magnetically center each other. This is usually done only with remote actuators, not with in-surface actuators. This note shows how in-surface actuators can be made mutually centering.

The geometry of mutually centering actuators is quite constrained, so it is necessary to design the tail section of the plane around the actuators. The actuators need to be arranged with the magnetic axes of the two magnets aligned with the fuselage axis, and the rotational axes of the two actuators must be at right angles, just as in remote mutually centering actuators, to avoid unwanted interactions between the actuators. The distance between the actuators depends on the strength of the magnets and the amount of centering desired. For the actuators on my 10 gram planes 1/2" works well. The need for the rotational axes of the actuators to be a right angles fits the fact that the horizontal and vertical stabilizers are at right angles. So we just arrange the hinge lines of the horizontal and vertical control surfaces to go through the centers of the the two actuators.

For balsa planes the actuators can be let into suitable cutouts in the fixed stabilizers. With carbon rod construction this cannot be done. The rudder actuator can be positioned directly below the rudder hinge. However, it is desirable to have the hinge line of the horizontal stabilizer be a single carbon rod, for rigidity and alignment of the hinge line. So, if we cannot let the actuator coil into the fixed horizontal stabilizer, we must let the stabilizer into the coil. This can be done by making the coil in two parts connected in series, and mounting them sandwiching the horizontal stabilizer hinge rod. The rod doesn't go through the center of the actuator coil because you want the effective center of the hinge to go through the center of the coil, and this is usually immediately behind the hinge rod. The magnet in the actuator will be connected to the two elevator surfaces, and will rotate around the hinge rod. There's not a lot of room, but there is enough to make this work.