Page 58 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 45

How Airplanes Fly 45

energy consumed must be minimized. That is, we want to For maximum lift efficiency one
produce the desired lift for the least induced power. Thus must accelerate a large amount
one must make m as large as possible and reduce v to as of air at as low a velocity as
small as possible. For maximum lift efficiency one must possible.
accelerate a large amount of air at as low a velocity as pos-
sible. This gives the desired lift with the least energy given
to the air.
Because the rotors on a helicopter are quite small for the weight of
the aircraft, they must accelerate a relatively small amount of air to a
high velocity (high kinetic energy) to produce the needed lift. This is
inefficient. A similar argument can be made to understand why an
airplane cannot “hang” on its propeller. Although the engine is
producing sufficient power to lift the airplane with the wings, the
propeller accelerates too little air at too high a velocity to produce the
necessary lift on its own. One airplane that can hang on its propellers
is the Bell-Boeing V-22 Osprey tilt-rotor aircraft shown in Figure 2.18.
The extreme propellers, or proprotors, divert a great deal of air,
allowing the engines to produce enough thrust to lift the craft
vertically off the ground before they are rotated forward for
horizontal flight.

Lift Requires Power
Sometimes one encounters the misconception that a wing
requires very little power to produce lift and that there is no
net downwash behind the wing.This misconception is easy to
understand. In many computations in aerodynamics the
calculations are done with two-dimensional airfoils.These are
in fact infinite wings.This is done because an infinite wing is
much easier to calculate than one of finite span.The efficiency
of a wing increases with the span of the wing, since the
amount of air diverted increases with area.Thus, an infinite
wing diverts an infinite amount of air at zero velocity to
produce lift, and thus is infinitely efficient.The net vertical
velocity of the downwash is zero.Therefore, the infinite wing
requires no power to produce lift. Of course, this is not the
situation for a real three-dimensional wing.