The propulsive efficiency (ηp) is typically defined for air-breathing engines like turbofan or turboprop engines, which are used in aircraft. Turboshaft engines, on the other hand, are typically used in helicopters and rotorcraft and have different characteristics. These engines are used to turn a rotor to generate lift, and their efficiency is generally measured differently from the propulsive efficiency of aircraft engines.
In the context of turboshaft engines used in rotorcraft, the efficiency is often referred to as the “rotor efficiency” or “rotor mechanical efficiency.” This efficiency accounts for how effectively the engine’s mechanical power output is converted into the lift generated by the rotor.
The formula for rotor efficiency is as follows:
Rotor Efficiency (ηr) = (Useful Power Output / Power Input)
Where:
- Useful Power Output is the power delivered to the rotor to generate lift.
- Power Input is the power produced by the turboshaft engine.
Rotor efficiency is specific to the design and performance of the rotorcraft and the turboshaft engine, and it measures how effectively the engine’s power is utilized to generate lift. It doesn’t directly measure how the engine converts fuel energy into thrust, as in the case of propulsive efficiency for aircraft engines.
To calculate rotor efficiency, you would need to know the power output to the rotor (which is influenced by the engine’s power output and transmission losses) and the power input from the engine. These values are determined through engineering analysis and testing of the specific rotorcraft and engine in question.