Page 161 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 148

148 CHAPTER FIVE

propeller accelerates air back, which pulls the airplane forward. The
same is true of a jet engine. The jet accelerates air back to produce
thrust. The efficiency of the engine is determined from a compro-
mise between the amount of air flowing through the engine per sec-
ond and the jet velocity. For subsonic speeds the fanjet maximizes
the airflow through the engine and is the most efficient for trans-
ports traveling just below the speed of sound. Fanjets are fuel-effi-
cient and relatively quiet. At supersonic speeds the turbojet is more
efficient.
Next we look at high-speed flight. There are profound differences in
flight at or above the speed of sound where information cannot be
communicated forward and the compressibility of air becomes very
significant.