Why, therefore, persist in making a form which, by its very nature, invites danger? Because birds fly that way!
THE TURNING MOVEMENT.—This structural arrangement accentuates the difficulty when the machine turns. The air pressure against the wing surface is dependent on the speed. The broad outstretched surfaces compel the wing at the outer side of the circle to travel faster than the inner one. As a result, the outer end of the aeroplane is elevated.
CENTRIFUGAL ACTION.—At the same time the running gear, and the frame which carries it and supports the machine while at rest, being below the planes, a centrifugal force is exerted, when turning a circle, which tends to swing the wheels and frame outwardly, and thereby still further elevating the outer end of the plane.
THE WARPING PLANES.—The only remedy to meet this condition is expressed in the mechanism which wraps or twists the outer ends of the planes, as constructed in the Wright machine, or the ailerons, or small wings at the rear margins of the planes, as illustrated by the Farman machine. The object of this arrangement is to decrease the angle of incidence at the rising end, and increase the angle at the depressed end, and thus, by manually- operated means keep the machine on an even keel.
CHAPTER IV
FORE AND AFT CONTROL
THERE is no phase of the art of flying more important than the fore and aft control of an airship. Lateral stability is secondary to this feature, for reasons which will appear as we develop the subject.
THE BIRD TYPE OF FORE AND AFT CONTROL.— Every aeroplane follows the type set by nature in the particular that the body is caused to oscillate on a vertical fore and aft plane while in flight. The bird has one important advantage, however, in structure. Its wing has a flexure at the joint, so that its body can so oscillate independently of the angle of the wings.
The aeroplane has the wing firmly fixed to the body, hence the only way in which it is possible to effect a change in the angle of the wing is by changing the angle of the body. To be consistent the aeroplane should be so constructed that the angle of the supporting surfaces should be movable, and not controllable by the body.
The bird, in initiating flight from a perch, darts downwardly, and changes the angle of the body to correspond with the direction of the flying start. When it alights the body is thrown so that its breast banks against the air, but in ordinary flight its wings only are used to change the angle of flight.