The fact that this is not done is quite sufficient to give an unequal torsion. The friction in both cases must also be exactly equal. Both propellers must be released at exactly the same instant.
Supposing all these conditions fulfilled (in practice they never are), supposing also the propellers connected by gearing (prohibitive on account of the weight), and the air quite calm (which it never is), then the machine should and undoubtedly would fly straight.
For steering purposes by winding up one propeller many more times than the other, the aeroplane can generally speaking be steered to the right or left; but from what I have both seen and tried twin-screw model aeroplanes are not the success they are often made out to be, and they are much more troublesome to deal with, in spite of what some say to the contrary.
The solution of the problem of steering by the use of two propellers is only partially satisfactory and reliable, in fact, it is no solution at all.[45] The torque of the propeller and consequent tilting of the aeroplane is not the only cause at work diverting the machine from its course.
§ 4. As it progresses through the air it is constantly meeting air currents of varying velocity and direction, all tending to make the model deviate more or less from its course; the best way, in fact, the only way, to successfully overcome such is by means of speed, by giving the aeroplane a high velocity, not of ten or twelve to fifteen miles an hour, as is usual in built up fabric-covered aerofoils, but a velocity of twenty to thirty miles an hour, attainable only in models (petrol or steam driven) or by means of wooden or metal aerofoils.
§ 5. Amongst devices used for horizontal steering are vertical "FINS." These should be placed in the rear above the centre of gravity. They should not be large, and can be made of fabric tightly stretched over a wire frame, or of a piece of sheet magnalium or aluminium, turning on a pivot at the front edge, adjustment being made by simply twisting the fin round to the desired angle. As to the size, think of rudder and the size of a boat, but allow for the difference of medium. The frame carrying the pivot and fin should be made to slide along the rod or backbone of the model in order to find the most efficient position.
§ 6. Steering may also be attempted by means of little balancing tips, or ailerons, fixed to or near the main aerofoil, and pivoted (either centrally or otherwise) in such a manner that they can be rotated one in one direction (tilted) and the other in the other (dipped), so as to raise one side and depress the other.
§ 7. The model can also be steered by giving it a cant to one side by weighting the tip of the aerofoil on that side on which it is desired it should turn, but this method is both clumsy and "weighty."
§ 8. Another way is by means of the elevator; and this method, since it entails no additional surfaces entailing extra resistance and weight, is perhaps the most satisfactory of all.
It is necessary that the elevator be mounted on some kind of universal joint, in order that it may not only be "tipped" or "dipped," but also canted sideways for horizontal steering.