286 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



We must also understand that the air exerts a force against the wing 

 only when there is movement between them — either by the wing mov- 

 ing through the air or by the air moving past the wing. 



In its simplest form a wing can be thought of as a flat plate sticking 

 out sideways from the body of a bird or from the fuselage of an air- 

 plane. If this flat plate be held edgewise in a current of air, the force 

 of the current will tend to drag the plate with it downstream ; and 

 only an external force tending to pull it upstream can hold it in posi- 

 tion. If, however, we slightly raise the edge that faces upstream (or 





due io 





'-'{t..!'^ ^^rodun, three 



Tttlaiwe VJvnd 



Figure 1. — When a wing is held at a slight angle to an air current the air flows faster over 

 the upper surface than the lower, thereby creating suction above the wing and pressure 

 below the wing; the suction and pressure together cause the "lift." At the same time 

 the moving air tends to "drag" the wing backward. The total effect is to lift the wing 

 and drag it backward. 



lower the downstream edge) the plate will tend to rise bodily in the 

 air, although, at the same time, the force tending to drag it along 

 downstream increases. The moving air is thus exerting two forces 

 on the inclined plate (fig. 1) : (1) a lift force tending to raise the 

 plate; and (2) a drag force pushing it back in the direction of the air 

 stream. True gliding flight becomes possible when these two forces 

 are so adjusted as to be exactly equal to the weight of the bird or air- 

 plane. 



