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CH05_Anderson 7/25/01 8:58 AM Page 139

Airplane Propulsion 139

ditioning by the diffuser helps bring the airspeed to the compressor to
its optimum speed, regardless of the speed of the airplane. A typical
speed entering the compressor might be half the speed of sound
(Mach 0.5). So, in a transport that cruises at roughly Mach 0.8, the dif-
fuser slows the air down considerably. When standing still at
the end of the runway, the diffuser speeds up the air. This
The Sophia J-850 is a very small
might lead you to believe that the diffuser is an active device.
jet engine. It weighs 3.08 lb
It is the compressor that “demands” how much air must be
(1.4 kg) and produces 18.7 lb
sucked into the engine. For most jets, the diffuser is thus pas-
(85 N) of thrust. It is a fully
sive, making sure the air is uniform when it hits the compres-
functional turbojet engine
sor at the right speed. At supersonic speeds it is important that
designed for instruction and
the diffuser slow the incoming air to subsonic speeds as effi-
model airplanes.
ciently as possible.
At the other end of the turbojet is the nozzle, which “conditions”
the exhaust gas as it exits the engine. The ideal situation is to expand
the gas back to atmospheric pressure such that it exits at the greatest
possible velocity. This gives the greatest thrust. The design of the
nozzle depends on the pressures and velocities of the gas after they
leave the turbine.
There are two fundamental problems with the turbojet. First, the
turbojet produces thrust with a very high exhaust-gas velocity. We
have shown that this requires more power and is thus inefficient.
Another problem is that the higher the exhaust-gas velocity the
more noise the engine produces. This noise is unacceptable today
and FAA noise standards do not permit turbojets to operate at many
airports.
Older airplanes, such at the Boeing 707, that used turbojets, are
now a rare sight at most airports. The Boeing 727 and 737
originally had turbojets, and many of the earlier versions of
The Concorde burns about
these airplanes cannot be flown into many airports because of
500 lb (225 kg) of fuel per
noise restrictions. In the case of the Boeing 737 the airplane
passenger seat per hour. The
has gone through two major redesigns to improve efficiency
Boeing 777 burns only about
and noise. Because of its unique requirements, the Concorde
35 lb (16 kg) per passenger
uses turbojets. It is noisy and is considered a “gas-guzzler” by
seat per hour.
any standard.