4.1 Introduction

The introductory fundamentals of aircraft propulsion systems are covered in this chapter. Emphasis is placed on propulsion systems that operate on the so-called Brayton cycle. Such systems include turbojets, turboprops, turbofans, ramjets, and combinations thereof. Before taking up the thermodynamic processes involved in these systems, we consider the forces acting on a propulsive duct and the effect of installation on the net propulsive force.

4.2 Thrust Equation

We define a propulsion system as a unit submerged in a fluid medium about and through which the fluid flows. The propulsion system contains an energy-transfer mechanism that increases the kinetic energy of the fluid passing through the system. This mechanism is called the engine. In Fig. 4.1, the engine is shown schematically in a nacelle housing that forms the second portion of the propulsion system. Thus the propulsion system contains:

1. An engine (the nozzle is considered to be part of the engine in our terminology),

2. Housing about the engine (nacelle or duct).

Figure 4.1: Propulsion system.

Several different aircraft systems may use the same engine submerged in different-shaped nacelles. Thus one propulsion system may use engine X in a pod installation hanging from a wing while, in another system, engine X may be surrounded by a nacelle that is, in fact, the airplane's fuselage examples are the F-15 vs F-16 propulsion systems that use the F-100 turbofan engine. The thrust of a propulsion system will depend on 1) its engine and 2) its nacelle.

As a result,...