Turbo-props

Simple turbo-jet propulsion is inefficient at low speeds, and when high flight speeds are not required, it is better to use the gas turbine to drive a propeller, producing a turbo-prop system, as shown in Fig. 6.20.

In the turbo-prop, most of the energy available in the exhaust gases is extracted by the turbine, and fed to the propeller. Nearly all of the thrust comes from the propeller, rather than directly from the engine as jet propulsion. The turbo-prop, therefore, has a much higher Froude propulsion efficiency than a pure turbo-jet.

The turbo-prop retains many of the advantages and characteristics of turbo­jet propulsion including a high power-to-weight ratio, and a power output that rises with flight speed. Its main disadvantage is that, when used with a conven­tional propeller, it is limited to use at Mach numbers of less than about 0.7.

Because of the high rotational speed of the turbine, turbo-props normally use a reduction gearbox to connect the propeller shaft to that of the turbine. For large engines, the gearbox becomes a very large, heavy and complex item, reducing some of the theoretical advantages of the system.

Despite early scepticism about the economic viability of pure turbo-jet air­craft, the first jet airliner; the De Havilland Comet (Fig. 9.3) started a revolution in air transport when it entered service in May 1952, some two years after the first commercial turbo-prop flights by the Vickers Viscount. It was found that there was no shortage of passengers willing to pay the price premium for a significantly faster service provided by the turbo-jet. The introduction of the bigger, faster and more efficient turbo-jet Boeing 707 in 1958 caused the demise of the large turbo-prop for long-range civil transport, and started the continuing battle to produce ever more efficient jet airliners. The turbo-prop, however, retains a place on shorter routes, where increased speed does not pro­duce such a significant shortening of the overall journey time. The higher cabin noise level of the turbo-prop is also more acceptable on short flights.