Page 188 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 175

Airplane Performance 175

Glide

A pilot of a powered airplane must be prepared for the loss A commercial jet can glide for
of power. Contrary to what many think, an airplane does not about 100 miles (160 km) if it
fall out of the sky if the engine stops. In fact, an airplane can loses its engines at cruising
fly quite a distance without power. Without an engine, the altitude.
airplane is just a poor-performing glider. So, upon loss of
power, what should the pilot do?
The pilot may want to maximize the amount of time in the air. This
will give more time to search for an emergency landing field, more
time to attempt to restart the engine, and more time to communicate
with air traffic controllers. To achieve maximum time in the air, or
endurance, the objective is to minimize the rate of altitude loss. Since
the altitude loss is the source of power keeping the airplane flying, the
best endurance will occur at speed for minimum power required
(Figure 2.13). At this speed the rate of descent is lowest. On the other
hand, the speed is slow and the airplane does not cover much ground
as it descends.
What if the pilot is less interested in the time remaining in the air
but wants to glide for the largest distance, say, to make it to a better
place to land? In this case the pilot wants to increase the glide ratio,
which is the distance traveled per loss in altitude. As was introduced
with the toy glider example earlier, the distance covered divided by
the change in height is the L/D of the airplane. Thus, the maximum
glide ratio is equal to the maximum L/D. Since the weight of the
airplane is constant, the maximum L/D translates to minimum drag
(Figure 2.16). The speed for a glide of maximum range (minimum
drag) is typically about 20 percent faster than the speed for greatest
endurance (minimum power).

In the case of an engine failure, reducing the weight can
increase the airplane’s range and endurance.This reduces the
load on the wing and thus the induced drag. In WWII flying
movies there is sometimes the dramatic scene of the crew
throwing excess weight overboard on the return flight due to
a shot-up engine.