Page 197 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 184

184 CHAPTER SEVEN

Thrust available—prop.
Thrust available —jet

Thrust Drag

(Thrust
required)

Speed
Fig. 7.7. Excess thrust as a function of speed.

Airplanes need a lot of fuel. A This produces a climb angle of 6 degrees and a rate of climb
car is about 5 percent fuel by of 770 ft/min.
weight; a city bus, 2 percent; a The thing to remember is that the best rate of climb is
passenger train, 1.1 percent; a associated with excess power and the best angle of climb is
freight train, 0.4 percent. A Boeing associated with excess thrust. Next we take a look at how high
747 is 42 percent fuel by weight. an airplane can climb.

Ceiling

As an airplane climbs, the air becomes less dense. The power available
from the engine decreases at the same time that more power is needed
to produce lift. What happens is illustrated by Figure 7.8. As the air-
plane climbs, the minimum speed that the airplane can fly increases.
This is because the air becomes less dense so the minimum speed at
which the wing will divert enough air without stalling increases. The
speeds for the best rate of climb and the best angle of climb are increas-
ing because the minimums in the power and drag curves are shifting to
higher speeds. As the figure shows, at some altitude, the minimum
flight speed and the two climb speeds meet. At this point, the airplane
cannot fly higher and full power and thrust are needed to sustain
straight-and-level flight. This altitude is the absolute ceiling.