Control Surfaces & Servo Torque

The errors this method actually invites, and what it does about each. Every one of them is already a warning somewhere — in a note that appears when your own figures trip it, in the scope statement, or in the equations behind the page. Collected here so you can read them before you need them.

Growing the surface chord and expecting the torque to follow it. Chord enters twice — once in the area and once as the moment arm — so the hinge moment grows as the square of the chord fraction. It also grows as the square of the speed, so a surface sized at cruise is undersized at the dive speed.

Sizing on a catalogue torque figure. Every maker quotes stall torque, at about six volts, which is not what the servo will hold all day against an air load. The classes here are compiled from catalogues across several makers and share that optimism.

Leaving the hinge gap open. An unsealed gap multiplies the hinge moment by 1.25 and costs effectiveness on top. Sealing it is tape.

Entering an all-moving tail as a 100% chord flap. It will be sized, and the answer will be wrong. Such a surface hinges about its own quarter chord and its hinge moment depends on where you put the pivot, which is not something a flap model can express.

Expecting the roll rate to a few per cent. Strip theory ignores the induced flow a rolling wing makes and treats each strip of aileron as if it worked alone. The two errors push opposite ways, which is why it survives as a preliminary tool — expect the real aeroplane within about 30%.

Enlarging the fin here and believing the crosswind. A bigger fin in this box raises what the rudder can make and leaves the weathercock stiffness it works against exactly where it was, because that figure comes from Sheet 12 and not from the fin alone — on a deep-nosed model the fuselage takes back nearly half of what the fin makes. Change the fin, run Sheet 12 again, bring the new figure back. Otherwise the gain is counted once and the cost of it not at all.

Reading the forward limit as the whole balance range. It is one point and one condition: the elevator on the stop at the stall. The other end is Sheet 03's neutral point less whatever static margin you want, and only the two together are a range. On a model the forward limit is almost never what binds, which is the opposite of full-size practice — twenty-five degrees of throw on a tail a fifth of the wing is an authority no certificated aeroplane has.