Page 196 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 183

Airplane Performance 183

Propulsive power - jet Power

required
Power

Propulsive
power - prop.

Speed
Fig. 7.6. Excess power as a function of speed.

jet is at a considerably higher speed than for the propeller- Sometimes technology does not
driven airplane. This is why propeller-driven airplanes slow rule. During the Vietnam War, 91
down to climb while jets maintain their speed or even percent of all U.S. fighters shot
accelerate a little to climb. down by antiaircraft fire were
The best angle of climb is achieved with the maximum aimed at by hand.
excess thrust. The reason for this is not particularly difficult
but beyond the scope of this book. Take a look at Figure 7.7, which
shows the relationship of thrust and drag (required thrust) to the
speed of jet and propeller-driven airplanes.
The available thrust of a propeller-driven airplane decreases with
increasing speed. Thus, as shown by the arrow, the maximum excess
thrust does not occur at the minimum drag but at a lower speed. In
fact the best angle of climb occurs just above the stall speed of a
propeller-driven airplane. Such an airplane, taking off from a short
runway, with an obstacle such as power lines, will clear them by
climbing at about the takeoff speed.
As can be seen in Figure 7.7, the thrust of a jet is
The best rate of climb is
approximately constant with speed. Thus the best angle of climb
associated with excess power and
for a jet-powered airplane is achieved at the minimum drag.
the best angle of climb is
For a Cessna 172 at sea level the best rate of climb is
associated with excess thrust.
achieved at an indicated airspeed of 84 mi/h (134 km/h).