Page 139 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 126

126 CHAPTER FIVE

same for two-bladed vs. three-, four-, five-, or six-bladed propellers,
the efficiency will be close to the same.
Transitioning from two to more blades with the same total area is a
result of subtle tradeoffs. Two-bladed propellers are usually best for
lower-speed airplanes where the power requirements are low. More
blades are used when power requirements are higher, such as faster
climb and higher speeds. Other factors favoring multiblade
propellers are that they produce less objectionable noise and
Wilbur Wright died of typhoid
reduced vibrations. So, why not always use more blades? The
fever in 1912.
simple reason is that they are more expensive.
Propeller Pitch

With the size and speed of the propeller fixed, the airflow through the
propeller is basically fixed. So, in order to get more thrust, the air
velocity behind the propeller must be increased. The pitch of a pro-
peller is analogous to the wing’s angle of attack. With a fixed-pitch
propeller, the angle of the blades is fixed with respect to its rotation
direction. The propeller’s apparent angle of attack is determined by
the pitch of the propeller, its speed of rotation, and the speed of the
airplane through the air. The faster the airplane is traveling the smaller
the apparent angle of attack. This is demonstrated in Figure 5.5. As
the airplane flies faster, the wind due to the forward speed reduces the
angle of attack of the propeller. Thus, the propeller diverts less air,
producing less thrust and requiring less power from the engine.
The efficiency of a fixed-pitch propeller depends on the speed of
rotation and the speed of the airplane. Figure 5.6 shows the efficiency
of a propeller at a single rotation speed for various pitch angles as a
function of the speed of the airplane. From this it is clear that for a
single pitch, the efficiency is the optimum over a fairly narrow range
of airplane speeds. Because of this, a fixed-pitch propeller must have
a fairly high pitch for all-around performance. The rated engine power
available at a given altitude is determined by the engine’s rpm, which
is also the propeller’s rotation speed. Thus high pitch may cause the
engine to run below its optimum speed during takeoff and thus not
produce full power. The same blade at cruise speed may require that
the engine be throttled back to prevent the engine from operating at