The propulsive efficiency (ηp) of a piston engine in an aircraft measures how effectively the engine converts the energy from the combustion of fuel into useful thrust for propulsion. Piston engines are typically found in small aircraft and operate on the principle of converting reciprocating motion into rotational motion. In SI units, the formula for calculating the propulsive efficiency of a piston engine in an aircraft is as follows:
ηp = (2 * V * (Vp – V)) / (g * (Vp^2))
Where:
- ηp is the propulsive efficiency (unitless, a dimensionless ratio).
- V is the airspeed of the aircraft in meters per second (m/s).
- Vp is the piston exhaust velocity in meters per second (m/s).
- g is the acceleration due to gravity, approximately 9.81 m/s².
The propulsive efficiency formula accounts for the airspeed of the aircraft (V), the piston exhaust velocity (Vp), and gravity (g). The piston exhaust velocity represents the velocity of the exhaust gases expelled by the piston engine.
Piston engines are used in small general aviation aircraft and have different characteristics compared to jet engines. They are less efficient at high speeds and altitudes, and the efficiency is influenced by factors such as the engine design, compression ratio, and specific operating conditions.
The specific values of V, Vp, and other parameters would depend on the piston engine design and aircraft characteristics. These values are determined through engineering analysis and testing for the specific aircraft and engine combination.