Page 194 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 181

Airplane Performance 181

Lift

Thrust

Drag

Weight
Fig. 7.4. Forces on an airplane in climb.

airplane. The forces on the airplane are rotated, except for the
In a sustained climb the lift of
weight, as shown in the diagram in Figure 7.4. In this rotated
the wing is actually less than
configuration, part of the weight of the airplane is supported
the weight of the airplane.
directly by the engine. As we will see, the airplane is climbing
on the excess thrust and excess power of the engine.
To understand this, let us begin by looking at two extreme
situations. First, take the case of straight-and-level flight. The angle of
climb is zero and the lift on the wing is the weight of the airplane.
Now consider a very powerful jet fighter that can go straight up in a
climb. In this case the angle of climb is 90 degrees and the lift on the
wing is zero. The engine now supports the weight of the airplane. As
the fighter goes slowly from straight-and-level flight to a vertical
climb, the load on the wing smoothly changes from the weight of the
airplane to zero. During this transition the lift produced by the engine
goes smoothly from zero to the weight of the airplane.
Now let us consider what happens when a low-powered airplane
goes into a climb. Consider a small single-engine propeller airplane
flying straight-and-level at full power. In this case the wings produce
the lift and the power is just equal to the induced and parasitic power
requirements. Now the pilot pulls back a little on the controls and the
airplane starts to climb. Part of the engine’s power now goes directly
into lifting the airplane. This leaves less power to overcome drag and
to produce the remaining lift with the wings. Thus the airplane slows
down. As the pilot continues to pull back on the controls, the speed is