The first is offered by the planes of the machine itself and results directly from the pressure of the air supporting the model during flight.
The second is set up by the framework, the edges of the planes, the wires, etc., while the last is caused solely by the air in traveling over the surfaces of the various members composing the machines.
FIG. 6. The disturbance caused by a triangular body moving through the atmosphere.
The head and surface or skin resistance, as it is sometimes called, can be reduced, but the aerodynamical resistance cannot.
Air is no less a fluid than water, and the same considerations apply to it, subject, of course, to certain conditions and with due regard for such factors as density, viscoscity, etc.
Plate II.
When an object, such as a square stick of wood, is moved through the air, the latter flows around it leaving behind a region of "dead air." The dead air represents so much waste energy or unnecessary resistance to overcome because it requires an expenditure of power to drag it along.
