Page 141 - Understanding Flight

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128 CHAPTER FIVE

In 1933, Boeing made an something like a governor on the engine. There are two
exclusive agreement with United controls, the engine throttle and an rpm control. The throttle
Airlines for its advanced model controls the power output of the engine and the rpm control
247. Competing airlines, which sets the rotation speed of the propeller and thus the speed of
could not buy the Boeing 247, the engine. If the engine wants to run too fast, the pitch of the
asked Douglas to build a propeller is automatically increased until the engine slows
competing aircraft. Douglas’s down to the preset speed. This allows the efficiency of the
answer was the DC-1, which led constant-speed propeller to look something like the envelope
to the DC-3 by 1936. The Boeing of maximum efficiency shown in Figure 5.6.
247 became instantly obsolete. On takeoff and in climb the propeller is adjusted to have
a fairly small pitch. Because of the airplane’s slow speed,
the angle of attack is still quite large. In cruise the airplane’s speed
causes the propeller to see a reduced angle of attack. Here the pitch
of the propeller is increased to allow the engine to operate at its
optimum performance. Typical constant-speed propellers on small
airplanes improve the overall efficiency of the propeller by about 15
percent.

Piston Engines

The Wright brothers’ first engine Either a piston or turbine engine can power a propeller. Here
produced 12 hp and weighed you will be briefly introduced to aircraft piston engines. We
180 lb. will not go into detail about how they work, but rather focus
on the relationship of the piston engine to the power delivered.
In brief, the piston engine works by converting a certain amount of
energy from a fuel and oxygen mixture into kinetic energy of a piston.
The energy of the piston is then used to turn a shaft. Finally, the shaft
turns a propeller. This is illustrated in Figure 5.7.
If we define the total power as the amount of energy per second
available in the fuel and air mixture, then the total power will be
limited by the amount of air that can be pumped into the cylinders of
a piston engine. Larger cylinders mean a more powerful
engine. The higher the altitude, and thus the lower the air
Modern aircraft piston engines
density, the less power is produced. Because the available
weigh approximately 2 pounds
oxygen decreases with altitude, the normally aspirated piston
per horsepower.
engine generally limits aircraft to low operating altitudes.