Newton's laws[14] and resists movement. It is that
resistance or reaction which makes flight possible.

Flight is secured by driving through the air a surface[15] inclined
upwards and towards the direction of motion.

[Illustration]

S = Side view of surface.

M = Direction of motion.

CHORD.--The Chord is, for practical purposes, taken to be a straight
line from the leading edge of the surface to its trailing edge.

N = A line through the surface starting from its trailing edge. The
position of this line, which I call the _Neutral Lift Line_, is found
by means of wind-tunnel research, and it varies with differences in
the camber (curvature) of surfaces. In order to secure flight, the
inclination of the surface must be such that the neutral lift line makes
an angle with and _above_ the line of motion. If it is coincident with
M, there is no lift. If it makes an angle with M and _below_ it, then
there is a pressure tending to force the surface down.

I = Angle of Incidence. This angle is generally defined as the angle the
chord makes with the direction of motion, but that is a bad definition,
as it leads to misconception. The angle of incidence is best described
as the angle the neutral lift line makes with the direction of motion
relative to the air. 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, if, as seems reasonable, we regard the surface from the point
of view of the neutral lift line. 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 criticized adversely
is then appli