Gas turbine efficiency

The overall efficiency of a gas turbine propulsion system depends on two major contributions, the Froude efficiency which, you may remember, is related to the rate at which energy is expended in creating a slipstrean or jet, and a thermal efficiency, which is related to the rate at which energy is wasted by creating hot exhaust gases.

As noted earlier, the Froude propulsive efficiency of the pure turbo-jet is low, because thrust is produced by giving a small mass of air a large change in velocity. However, for a fixed amount of thrust, as the speed of a jet or gas – turbine-propelled aircraft increases, the air (mass) flow rate through the engine also increases. A smaller change in velocity is needed for this larger mass of air, and the Froude efficiency thus improves. However, for an aircraft in steady level flight, the thrust required is equal to the drag. Since the drag varies with speed, the thrust required must similarly vary, so the overall efficiency of propulsion depends on the drag characteristics of the aircraft. This inter­dependence between the propulsion device and the aircraft aerodynamics is an important feature of aircraft flight and is described further in Chapter 7.