Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/21



§ 1. Introductory.— A body in motion through a fluid of any kind, whether liquid or gaseous, experiences resistance, and work has to be done in its propulsion.

Such resistance is due to two clearly distinct causes, the independent nature of which may be illustrated by considering a few commonplace instances.

If a piece of cardboard be moved briskly through the air, the resistance, though quite sensible, is very much less than that experienced when a similar movement is attempted under water. In this case the difference is evidently due to the very much greater density of water, which at 10° C, is 800 times that of air.

If now we similarly compare water with any ordinary grade of heavy lubricating oil, or with common treacle, we again find a great difference, but this time the density is approximately the same, and we recognise that the resistance is due to an entirely different cause; a certain stickiness of the medium, otherwise viscosity. All fluids are viscous to a greater or lesser degree; the viscosity of water is small, that of air still less, whilst lubricating oil and treacle are highly viscous substances. Now just as in the study of ordinary mechanics it is found expedient initially to neglect the effects of friction, so, in connection with the present subject, we can afford to ignore the fluid friction to which the viscosity of the fluid gives rise, and in the first instance deal with resistance as a function of density alone.