Page 195 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 182

182 CHAPTER SEVEN

On February 21, 1979, at Kitty further reduced until the backside of the power curve is
Hawk, former astronaut Neil reached. Eventually the wing would stall if the angle of attack
Armstrong climbed to 50,000 ft were further increased.
in a business jet and set five There are two climb scenarios of interest to pilots. The first
world records. is the fastest climb or the best rate of climb. Airplanes fly more
efficiently at higher altitudes, so pilots generally want to climb
to their desired cruise altitude as quickly as possible. This is the rate
of climb most useful to a pilot. The second scenario is the steepest
climb, or the best angle of climb. Suppose you are in a mountain valley
and wish to clear the mountaintops. You would want to gain as much
altitude in the shortest distance possible, and thus fly at the steepest
angle. These two scenarios lead to different climb paths and airspeeds,
which are shown schematically in Figure 7.5.
The best rate of climb for an airplane occurs when the excess power
is the greatest. The excess power is the difference between the
propulsive power (power providing thrust) and the power required for
flight. This is shown in Figure 7.6 for an airplane fitted with either a
jet engine or a propeller. The power required is just the power curve
discussed in Chapter 2. The arrow connecting the power curve with
the propulsive power for the propeller is the excess power. When it is
the longest possible, it is at the speed for the best rate of climb. The
best rate of climb for a propeller-driven aircraft is at a speed that is
very near the minimum drag. One would guess that it would be near
the minimum power required, but the variation of the propulsive
power with speed causes the best rate of climb to be at a higher speed.
As shown in the figure, the propulsive power of the jet increases
with speed. Thus the arrow marking the greatest excess power for the

Best rate

Best angle

Fig. 7.5. Best angle of climb and best rate of climb.