The Principle Of Flight
An airplane flies because air, moving over and under its
surfaces particularly its wings, travels at different
velocities producing a difference in air pressure low above
the wing and high below it. The low pressure exerts a
pulling force and the high pressure a pushing force. The
lifting force, usually called lift, depends on the shape,
area, and tilt of the wind, and on the speed of the
aircraft. The shape of the wing causes the air streaming
above and below the wing to travel at different velocities.
The greater distance over which the air must travel above
the curved upper surface forces that air to move faster to
keep pace with the air moving along the flat lower surface.
According to Bernoulli's principle, it is this difference
in air velocity that produces the difference in air
Wing area influences lift; the more of the wing that is
exposed to the air, the greater the lift. The up or down
tilt of the wing, usually called its angle of attack,
contributes to or detracts from lift. As a wing is tilted
upward, that is, as its angle of attack is increased, its
lift increases. The air passing over the top of an uptilted
wing must travel a greater distance and therefore produces
a greater pressure differential between the upper and lower
surfaces. Aircraft speed has a great influence on lift. The
faster the air moves over and under the surfaces of an
airplane, the greater the pressure differential and, as a
result, the greater the lift.
As an airplane flies on a level course, the lift
contributed by the wind and other parts of the structure
counterbalance the weight of the plane. Within certain
limits, if the angle of attack is increased while the speed
remains constant, the plane will rise. If the angle of
attack is decreased, that is, the wing is tilted downward,
the plane will lose lift and start to descend. An airplane
will also climb from level flight if its speed is
increased, and it will dive if its speed is decreased. Lift
varies directly with speed.
Factors that contribute to lift in airplane flight also
contribute to undesirable forces called drag. Drag is the
force that tends to retard the motion of the airplane
through the air. Most drag is a result of the resistance of
the air to objects moving through it. This type of drag can
be reduced by streamlining the aircraft. It is also reduced
by placing slots in the Wing so that the boundary layer or
"Wall of air" building up in front and around the wing can
flow through it.
One form of drag, however, known as induced drag, is a
direct result of the lift produced by the wing. Induced
drag is the penalty extracted from lift. Great differences
in the pressure of the air flowing over and under a wing
can cause whirlpools or Eddies of air to billow up along
the trailing edges of the wings. These whirlpools produce a
breaking forced toward the rear that must be overcome by
the forward thrust of the engines. As the angle of attack
of an airplane is increased, the plane gains lift, but the
lift is limited because air turbulence spreads over the
wing. Then at a certain critical point (an angel of about
14 in many airplanes) , the wing loses lift and the plane
stalls, nosing over into a dive.
Airplane designers try to design aircraft with high
lift-to-drag ratios, that is, much more lift than drag.
They are limited, however, by factors such as speed and the
weight that the plane must carry. Faster planes usually
have a lower lift-to-drags ratio. A subsonic modern
Transport has a lift to drag ratio of about 15 and a late
private plane may have a lift to drag ratio of about 25.
Supersonic transports have a lift to drags ratio of about
The Supersonic age that aviation entered after World War II
presented a number of problems, so revolutionary, that the
arodynamisists found themselves resorting to
experimentation as dangerous and adventuresom as any faced
by early pilots.