Page 136 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 123

Airplane Propulsion 123

power would be almost zero and the engine power would equal the
propulsive power, which is the thrust times the speed of the airplane.
Unfortunately this is not possible. To produce thrust, there must always
be some velocity given to the air and thus some waste.
Let us look at the situation of an engineer who wishes to design a
jet engine that is to have twice the thrust of a previous engine. The
engineer can increase the thrust by increasing the amount of mass
discharged by the engine per time, by increasing the velocity of the
exhaust, or by increasing both. However, if one increases the
exhaust velocity, the wasted power increases as the velocity
squared. So doubling the thrust by doubling the exhaust velocity
doubles the propulsive power but increases the wasted power by a
factor of 4! However, if one doubles the thrust by doubling
the mass discharged, the propulsive power has again been
Each engine of the Boeing 777
doubled and the wasted power has only doubled. Basically,
produces 98,000 lb (436,000 N)
the objective of an aircraft propulsion system is to create the
thrust and has an airflow of 1.4
most thrust for the least wasted power, which leads aircraft
tons (1.3 metric tons) of air per
engine design to favor increasing mass flow over exhaust
second.
velocity.
Even for the most efficient propulsion systems, a great deal of
kinetic energy is imparted to the air. There is no ideal propulsion
system for flight. Wings are much more efficient in producing lift
than engines in producing thrust. This is because wings, with their
large size, are able to divert a great deal of air at a slow speed.
Engines do not have that luxury.

Propellers

A propeller is simply a rotating wing. As an illustration of this point,
look at Figure 5.4, which is a photo of slices of a propeller. The airfoils
of the propeller are clearly winglike. We know that the purpose of the
wing is to divert air down to create lift. Likewise a propeller diverts air
back to produce forward thrust. For low-speed flight a propeller is the
most efficient means of propulsion. At peak efficiency 85 percent or
more of the engine power can be converted into propulsive power by
a propeller. Thus only about 15 percent of the power is wasted.