And that's the end of the Prologue.
CHAPTER I
FLIGHT
Air has weight (about 13 cubic feet = 1 lb.), inertia, and
momentum. It therefore obeys Newton's laws[[14]] and resists
movement. It is that resistance or reaction which makes
flight possible.
[[14]] See Newton's laws in the Glossary at the end of the book.
Flight is secured by driving through the air a surface[[15]]
inclined upwards and towards the direction of motion.
[[15]] See ``Aerofoil'' in the Glossary.
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.
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