change the total lift. If the aircraft weighs 10,000
airfoil (fig. 8-4). It follows that large, subsonic
pounds, 10,001 pounds of lift causes the aircraft
aircraft have thick wings to produce a great
to climb; 9,999 pounds of lift causes the aircraft
amount of lift at slow speeds. Supersonic aircraft
to descend.
must have very thin wings to decrease drag at high
speeds.
To fly at a constant airspeed, the forces of
Many airfoils have devices attached to them
thrust and drag must be equal. When one force
to increase or decrease lift in various flight
is greater than the other, the aircraft accelerates
conditions or attitudes. These devices may mount
or decelerates.
on the leading edge, trailing edge, rounded
To turn, place the aircraft in a bank angle
surface, or flat surface. If the trailing edge
(fig. 8-7). The lift developed by the airfoils
attaches by a hinge and has controls to move the
can then be broken down into components of
trailing edge, you control lift by changing the
horizontal and vertical lift. The horizontal
angle of attack (fig. 8-5). When the trailing edge
component of lift pulls the aircraft around in the
moves into the higher pressure air on the airfoil's
turn. The vertical component of lift must be equal
flat side, the angle of attack is effectively
and opposite to gravity for the aircraft to remain
increased. This causes more lift and drag.
at a constant altitude. (Note that total lift must
Conversely, if the trailing edge moves into the
be increased to prevent a loss in altitude.)
airfoil's low-pressure side, the angle of attack
decreases. Lift and drag decrease accordingly.
When centrifugal force equals horizontal lift,
the aircraft is in a constant-rate turn. For a faster
In flight, each aircraft has certain forces acting
rate of turn, increase horizontal lift by increasing
upon it (fig. 8-6). To sustain flight at a constant
the bank angle. When all lift is vertical to gravity,
altitude, the total lift of all airfoils must equal the
any turning motion is called a skid.
aircraft weight. To change altitude you must
Figure 8-4.-Induced drag.
8-4
