Fig. 4.—Various Forms of Dihedral
[Fig. 5] is a side elevation of two surfaces fixed to a spar, and shows how stability is obtained longitudinally. The surfaces of the elevator or tail, according to whether the machine is “canard” or tractor (canard being the term for propeller-behind or “pusher” machines), is placed at a positive angle with the horizon. The correct angle can, of course, only be found by experiment.
Now, from the foregoing certain laws can be deduced. Firstly, in order to be stable longitudinally, the centre of pressure must be kept as near to the centre of gravity as possible, and secondly, the main surface of the aeroplane must be inclined to preserve lateral stability. With full-size aeroplanes there are, however, several exceptions to this rule, as the faster a machine travels the more stable does it become, and hence the dihedral angle is really unnecessary.
It may be well at this point to describe the action of a plane. Strictly speaking, the terms “plane” and “aeroplane” are misnomers, since no full-size machine has surfaces which even approximate to planes.
Fig. 5.—Disposition of Angles
Fig. 6.—Air-flow Round Plane
