You will, however, find that in nearly all rigging
specifications the angle of incidence is taken to mean the
angle the chord makes with a line parallel to the propeller
thrust. This is necessary from the point of view of the
practical mechanic who has to rig the aeroplane, for he could
not find the neutral lift line, whereas he can easily find the
chord. Again, he would certainly be in doubt as to ``the
direction of motion relative to the air,'' whereas he can
easily find a line parallel to the propeller thrust. It is a
pity, however, that these practical considerations have
resulted in a bad definition of the angle of incidence becoming
prevalent, a consequence of which has been the widespread
fallacy that flight may be secured with a negative
inclination of the surface. Flight may conceivably be
secured with a negative angle of chord, but never with a
negative inclination of the surface. All this is only applicable
to cambered surfaces. In the case of flat surfaces the neutral
lift line coincides with the chord and the definition I have
criticised adversely is then applicable. Flat lifting surfaces
are, however, never used.
The surface acts upon the air in the following manner:
As the bottom of the surface meets the air, it compresses
it and accelerates it DOWNWARDS. As a result of this definite
action there is, of course, an equal and opposite reaction
UPWARDS.
The top surface, in moving forward, tends to leave the
air behind it, thus creating a semi-vacuum or rarefied area
over the top of the surface.
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