Propulsive Efficiency

Propulsive efficiency, not to be confused with propeller efficiency, is the energy imparted to the aircraft, as a percentage of the energy produced by the propeller, or jet engine.

An inspection of Diagram 11, Propulsive Efficiency V. KTAS, compares the efficiency of the different types of aircraft propulsive systems, referred to here as the propulsor, piston-

prop engine, turboprop, Propfan and turbofan engines. At the average jet cruise speeds of around 500 KTAS, the efficiency of the turbofan is reaching its peak efficiency while the turbojet’s efficiency is still increasing and is good for speeds up to about 2000 knots and altitudes of 90,000 feet, making it more suitable for military aircraft such as supersonic fighters. The turbofan’s limit is reached around Mach 1.0 (575-661 knots where it fits the slot nicely between the turboprop’s and turbojet’s speed range making it ideal for Bizjets and air transport aircraft. Although the jet engine is very efficient for high cruise speeds at high altitudes, its fuel consumption is uneconomical at low speeds and low altitudes.

At the low end of the speed range the piston-engine/ propeller reigns supreme. However, like any airfoil, the prop obeys the laws of aerodynamics and its performance is limited by the constraints of decreasing air density with altitude and the effects of high speed The prop achieves its greatest propulsive efficiency at around 330 KTAS or so, depending on the type of engine driving the propeller The different engine

types are listed below to match the numbers above the curves on Diagram 11, Propulsive Efficiency V. KTAS, thus:

1 . Propulsor

2 .Piston-engine

3 . Turboprop

4 . Propfan and turbofan.

The curves are representative of the approximate average cruise speeds for each engine/propeller combination.

Propulsive efficiency is the product of propeller efficiency and the engine’s brake thermal efficiency expressed by the following formula:

Propulsive efficiency =

Where Va = aircraft speed in FPS (feet per second)

Vj = propwash speed in FPS

This also will be covered in the section on ‘Propwash – thrust’. For propeller powered aircraft, the propwash velocity during cruise is nearly the same as the aircraft’s cruise speed. Therefore, the propeller and propulsive efficiency are both identical