_Fig. 3. Lift and Drift_

In Fig. 3 the plane A is assumed to be moving
forwardly in the direction of the arrow B. This
indicates the resistance. The vertical arrow C
shows the direction of lift, which is the weight
held up by the plane.

NORMAL PRESSURE.--Now there is another term
much used which needs explanation, and that is
normal pressure. A pressure of this kind
against a plane is where the wind strikes it at
right angles. This is illustrated in Fig. 4, in
which the plane is shown with the wind striking
it squarely.

It is obvious that the wind will exert a greater
force against a plane when at its normal. On the
other hand, the least pressure against a plane is
when it is in a horizontal position, because then
the wind has no force against the surfaces, and
the only effect on the drift is that which takes
place when the wind strikes its forward edge.

_Fig. 4. Normal Air Pressure_

_Fig. 5. Edge Resistance_