Aircraft require to thrust themselves to accelerate and thus counteract drag forces. In this chapter, we look at the way aircraft engines work. All aircraft propulsion systems are based on the principle of reaction of airflow through a power plant system. The two means for accelerating the airflow surrounding the aircraft that are presented in this chapter are through propellers and jet expansion, which give rise to the so-called propeller engines and jet engines to be studied in Section 6.1 and Section 6.2, respectively. In Section 6.3 the different types of jet engines will be studied. A third type of propulsion systems are the rocket engines, but they are used in spacecrafts and lay beyond the scope of this course. An introductory reference on the topic is Newman [3, Chapter 6]. Thorough references are, for instance, Mattingly et al.  and Jenkinson et al. .
The design of an aircraft engine must satisfy diverse needs. The first one is to provide sufficient thrust to counteract the aerodynamic drag of the aircraft, but also to exceed it in order to accelerate. Moreover, it must provide enough thrust to fulfill with the operational requirements in all circumstances (climbs, turns, etc.). Moreover, commercial aircraft focus also on high engine efficiency and low fuel consumption rates. On the contrary, fighter aircraft might require an important excess of thrust to perform sharp, aggressive maneuvers in combat.