Page 155 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 142

142 CHAPTER FIVE

Fig. 5.20. A turbofan engine. (Photo courtesy of Pratt and Whitney.)

The engines on the Boeing 777 the output velocity. The result of this is that the 100,000-lb
have a diameter that is within thrust engines on the Boeing 777 have a fan so large that the
inches of the fuselage diameter engine’s diameter is within inches of the fuselage diameter of
of a Boeing 737. a Boeing 737. One can fit six seats and an aisle in one of these
engines, though these would be very uncomfortable seats.
Figure 5.21 shows a photograph of the engine of a Boeing 777 next to
a service truck. This gives a feeling for their size.
Unfortunately, there are practical limits to the size of the engines.
But there are always clever engineers out there who find ways
to expand these limits. The engines used on the Boeing 777
To be certified a jet engine must
were unimaginable just two decades ago.
survive the “bird-strike” test. The
Turbofans are the engine of choice for commercial
engine must survive the impact
transports and business jets. Military jets also use turbofans
of a chicken shot at it with a
but must compromise efficiency if the airplane is to be capable
special cannon. The same cannon
of supersonic flight. The huge inlets of the fanjets are a
is used for testing airplane
detriment when it comes to supersonic drag. Military fighters
windshields.
typically use engines with bypass ratios on the order of 3.

The Turboprop

A turboprop follows the same concept as a fanjet. The excess power
in the turbine is used to turn a propeller, rather than a fan. Figure