TABLE OF CONTENTS To sustain flight for any appreciable amount of time, aircraft need some type of propulsion system. The purpose of the propulsion system is to produce a controllable force called thrust, which is made to act in concert with the other forces on the aircraft (lift, drag, and weight) to produce the desired translational motion. In most cases thrust is used to accelerate the aircraft along the flight path and to counteract drag; however, it is certainly possible for thrust to be used in other ways such as to augment lift (for example, V-STOL aircraft like the AV-8A Harrier).
A number of different propulsion choices exist for the aircraft designer. This section provides a brief overview of different propulsion systems and their advantages and disadvantages.
The piston (or reciprocating) engine-propeller combination is probably the most efficient propulsion system for low-speed aircraft (Fig. 2.1). This is true because the propeller produces thrust by increasing the momentum of a relatively large amount of air. Also, the reciprocating engine is efficient in terms of fuel consumption. Fuel consumption is usually expressed as pounds of fuel per hour per brake horsepower and given the name of brake-specific fuel consumption (BSFC). Typical values of BSFC for aircraft reciprocating engines in use today are 0.4 to 0.5 lb/bhp-h for cruise power. At flight speeds above approximately Mach 0.3, the efficiency of the propeller starts to drop off because of compressibility effects on the blades. The engine size necessary to produce...
