When selecting an aircraft engine, the following are key considerations ¹:

- *Cruise Requirement*: The first step is to estimate the power requirement of the aircraft at the specified cruise speed. At a constant cruise speed, the aircraft forces will be in balance; thus the thrust produced by the engine-propeller combination must be equal to the total drag of the aircraft.
- *Power*: Power is defined as the rate at which work is done, and work is simply a force multiplied by a displacement. Therefore, the total power required to keep an aircraft flying at a constant speed is simply the product of the drag force and velocity.
- *Propeller Efficiency*: The ratio of the power available for thrust generation to that output by the engine shaft is referred to as propeller efficiency. A well-designed propeller operating at its most efficient point has an efficiency of between 0.80 and 0.85, and so the thrust power required above must be factored by the propeller efficiency to determine the shaft engine power required to maintain the chosen cruise speed.
- *Propeller Forces*: A propeller blade is essentially a wing with a twisted airfoil section designed to produce aerodynamic forces as a result of the motion of the blade through the air.
- *Propeller Diameter*: A propeller works by accelerating a tube of air with a diameter approximately equal to that of the propeller. This results in a reaction thrust force which propels the aircraft forward. The rotation of the propeller adds momentum to the air tube, where momentum is equal to the mass of air (function of diameter) multiplied by the rearward velocity.
- *Number of Blades*: The number of blades, the width (chord), and the diameter of the propeller determine the total blade area. Generally larger engines that produce more power require a greater number of blades to extract that power and convert it into useful thrust.
- *Propeller Pitch*: Broadly propeller pitch refers to the ease with which the propeller can spin through the air. There are a few different concepts lumped under the term “pitch” which all describe the same phenomenon. Pitch refers to the angle of the propeller blade relative to a flat plane. Pitch can refer to the distance that the propeller advances through the air during one revolution. Pitch can be thought of as the size of the “bite” the propeller takes out of the air.