Читать книгу Aeroplane Construction and Operation онлайн

48 страница из 66

NOTE.-Densities marked * are interpolated from a graph, but are close enough for all ordinary purposes.

In aerodynamic laboratory reports, the standard density of air is 0.07608 pounds per cubic foot at sea level, the temperature being 15 degrees Centigrade (59 degrees Fahrenheit). This standard density will be assumed throughout the book, and hence for any other altitude or density the corresponding corrections must be made. Owing to the fact that the temperature decreases as we gain altitude, further corrections must be made in the tabular values, but as the changes are rather difficult to make and are relatively small we will not take the matter up at this point.

Fig. 1. Air Flow About a Flat Normal Plate. Pressure Zone at Front and .#. Turbulent Zone at Rear (H). Arrows Show Direction O OW.

Air Pressure on Normal Flat Plates*

Contrary to the common opinion, the vacuous part of the drag is by far the greater, say in the neighborhood of from 60 to 75 per cent of the total. When a body experiences pressure due to the breaking up of an air stream, as in the present case, the pressure is said to be due to "turbulence," and the body is said to produce "turbulent flow." This is to distinguish the forces due to impact and suction, from the forces due to the frictional drag produced by the air stream rubbing over the surface.