Page 158 - Understanding Flight

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Airplane Propulsion 145

Fig. 5.23. The thrust reverser partially turns the
exhaust forward to produce negative thrust.

and forces it forward. The net result is that the engine pro-
In 1933, William Boeing wanted
duces some negative thrust to slow the airplane down.
to purchase a luxury airplane.
A thrust reverser can only redirect a portion of the air
He purchased an airplane
forward. The net forward thrust is just a very small
manufactured by Douglas. In
component of what it can produce in the forward direction.
1940, he upgraded to another
But even if the net forward thrust were zero with thrust
Douglas airplane.
reversal, the large, non-thrust-producing engines produce a
great deal of drag. This alone would help slow down the airplane on
landing.

Thrust Vectoring

Sometimes jet nozzles can be pointed in a particular direction other
than straight back. This is known as thrust vectoring. Figure 5.24 illus-
trates how a hinged nozzle can redirect the jet exhaust from horizon-
tal to an angle. The idea, similar to a thrust reverser, is to redirect the
jet to any desired direction. The Harrier is an extreme example of
thrust vectoring. The Harrier is able to hover by directing all of the
thrust down. The exhaust jet is routed to four nozzles (shown in Fig-
ure 5.25) that can swivel from horizontal, for forward thrust, to verti-
cal, for hovering. The Harrier also uses the high-pressure gas from the
engine for attitude control as shown. Modern fighter designs, such as
the Lockheed-Martin/Boeing F-22 shown in Figure 3.36, use thrust
vectoring to increase maneuverability.