Page 135 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 122

122 CHAPTER FIVE

of fuel to engine power. This is the engine efficiency. The losses here
are primarily due to inefficiencies in the burning of the fuel and fric-
tion within the engine. Energy is also used for engine support such as
powering fuel and lubricant pumps and the generation of electricity.
These losses reduce the engine’s output efficiency.
Once the engine has converted the chemical energy of the fuel into
mechanical energy, it must convert the mechanical energy into
propulsion. The difference between the engine power and propulsive
power gives us the propulsive efficiency. The total efficiency of
the propulsion system, which is engine efficiency times
Because of the oil embargo, the
propulsive efficiency, is a measure of how much power the
price of 1000 gallons of Jet A1
system develops for a certain quantity of fuel burned. So,
fuel went from $100 in 1970 to
what contributes to the propulsive efficiency? The same
$1100 in 1980.
arguments used for lift efficiency can be used here.
Remember from Chapter 2 that the lift of a wing is proportional to
the momentum mv that is transferred to the air per time. The kinetic
2
1
energy given to the air by the wing ( ⁄2mv ) is an energy loss to the
airplane. For the most efficient flight one wants to produce the
necessary lift while transferring as little energy to the air as possible.
Therefore, one wants to divert as much air as possible at as low a
velocity as possible. That is why the efficiency of a wing increases
with size. Increased lift efficiency requires the increase in the amount
of air diverted, not an increase in the speed of the diverted air.
Propulsion systems produce thrust and deliver their power to the
surrounding environment in the same manner as a wing. Thus, for the
most efficient thrust one wants the engine to accelerate as much air as
possible at as low a velocity as possible. This minimizes the
wasted power. If a propeller or a jet could discharge a very large
Air transportation consumes 8.9
amount of air or exhaust at a relatively low velocity, it would
percent of the world’s
take much less power than another system that developed the
transportation energy budget.
same thrust but by discharging a small amount of air at a high
Highway transportation consumes
velocity. Keep in mind that the energy given to the air producing
72.4 percent.
the thrust is lost energy that must be paid for by the engine.
The propulsive power divided by the engine power is the propulsive
efficiency. For the best propulsive efficiency you would want to have
almost zero kinetic energy in the air left behind. In this case the wasted