§ 9. A vertical fin in the rear, or something in the nature of a "keel," i.e. a vertical fin running down the backbone of the machine, greatly assists this movement.
If the model be of the tractor screw and tail (Blériot) type, then the above remarks re elevator apply mutatis mutandis to the tail.
§ 10. It is of the most vital importance that the propeller torque should be, as far as possible, correctly balanced. This can be tested by balancing the model transversely on a knife edge, winding up the propeller, and allowing it to run down, and adjusting matters until the torque and compensatory apparatus balance. As the torque varies the mean should be used.
In the case of twin propellers, suspend the model by its centre of gravity, wind up the propellers, and when running down if the model is drawn forward without rotation the thrust is equal; if not adjustment must be made till it does. The easiest way to do this may be by placing one propeller, the one giving the greater thrust, slightly nearer the centre.
In the case of two propellers rotating in opposite directions (by suitable gearing) on the common centre of two axes, one of the axes being, of course, hollow, and turning on the other—the rear propeller working in air already driven back by the other will require a coarser pitch or larger diameter to be equally efficient.
CHAPTER X.
THE LAUNCHING OF THE MODEL.
§ 1. Generally speaking, the model should be launched into the air against the wind.