Other types of powerplant

So far we have restricted discussion to two of the most common types of power – plant in order to illustrate the way in which conditions for best economy

change according to the type of powerplant which we decide to employ. We must not forget, however, that other types of powerplant are used, and these alternatives are mentioned in Chapter 6.

Of these alternatives perhaps the most common is the turbo-prop. This tends to be something of a ‘half-way house’ between the piston engine and the turbo-jet. The basic efficiency of the gas turbine will rise with increasing speed, but the propeller efficiency will deteriorate as the speed increases because of the effects of compressibility. The use of the more advanced type of propeller and the unducted fan mentioned in Chapter 6 promises to overcome some of these problems and extend the speed range over which such a powerplant can be used.

At higher speeds other forms of propulsion such as the ramjet or turbo-rocket may start to look attractive, particularly if we take a more comprehensive view of the economics of an aircraft than the simple measure of the fuel required to accomplish a particular journey for a given payload.