The propulsive efficiency (ηp) of a turbojet engine is a measure of how effectively the engine converts the energy from the combustion of fuel into useful thrust for propulsion. In SI units, the formula for calculating the propulsive efficiency is as follows:
ηp = (2 * V * (Vj – V)) / (g * (Vj^2))
Where:
- ηp is the propulsive efficiency (unitless, a dimensionless ratio).
- V is the airspeed of the aircraft in meters per second (m/s).
- Vj is the jet exhaust velocity of the engine in meters per second (m/s).
- g is the acceleration due to gravity, approximately 9.81 m/s².
The propulsive efficiency equation quantifies how effectively the engine converts the energy in the fuel into useful thrust for propulsion. It considers the difference between the velocity of the jet exhaust (Vj) and the airspeed of the aircraft (V) relative to the ground. The greater the difference between Vj and V, the higher the propulsive efficiency.
It’s important to note that for turbojet engines, the propulsive efficiency is typically lower compared to more modern engine types, such as turbofan engines, because a significant portion of the air bypasses the engine core, reducing the effectiveness of the exhaust jet in producing thrust. Modern aviation has transitioned to more efficient engine designs like turbofans with higher propulsive efficiencies.