7.1. NEWTON AND PROPULSION

Aircraft propulsion systems are applications of Newton's second law. The most familiar form of the second law is F = ma (Force equals mass times acceleration). In the case of propulsion systems, the derivative form of Newton's second law is more appropriate:

Here the force (or thrust), F, is equal to the rate that mass (in the form of propwash or jet exhaust) is expelled from the propulsion system, dm/ dt = , times the velocity of that mass, v. To increase the thrust of an engine one can increase , v, or both.

The objective of a propulsion system is to produce the needed thrust or propulsive power as efficiently as possible. As will be discussed in the following section, thrust is the appropriate parameter for jets and propulsive power (thrust times speed) is the appropriate parameter for propellers and rotors. In the production of propulsion, kinetic energy (1/2 mv ²) is given to the exhaust or propwash. This is wasted energy, and it is desirable to minimize it. The rate that energy is wasted to kinetic energy is the wasted power:

Therefore, in order to maximize the efficiency of a propulsion system, one must maximize and minimize v. Ideally, one would accelerate an almost infinite amount of gas at almost zero velocity to produce the desired propulsion.

An understanding of the physics of propulsion efficiency makes it easy to understand why fixed-wing aircraft...