Ultra-high by-pass (UHB) engines, prop-fans and unducted fans

In the quest for improved efficiency, engines with much larger by-pass ratios than the early turbo-fans have been designed. Both ducted and unducted designs have been devised, and examples are illustrated schematically in Fig. 6.27. In some of the designs a gearbox is incorporated in order to reduce rotational, and hence, blade tip speed. Contra-rotating fans are normally used.

There is no standard classification of such engines, and the term ‘prop-fan’ is used loosely to describe various different types. There is also something of a grey area in terms of whether a design should be classified as an unducted fan, or simply an advanced propeller.

The major problem with very high by-pass ratio engines, is the noise that their supersonic blade tips produce. Rear mounting of the engines can reduce the cabin noise and noise-induced structural fatigue. It also eliminates the pos­sibility of the pressurised fuselage being punctured in the event of a blade shear­ing off. Unfortunately, from practical considerations, rear mounting limits the number of engines to two, or possibly three.

For really large transatlantic aircraft, four engines are normally preferred, and in this case, wing mounting must be used. The ducted design shown in Figs 6.27(c) and 6.28 is intended for use on such aircraft, cruising at high sub­sonic Mach numbers. The reduced intake Mach number that can be obtained

Ultra-high by-pass (UHB) engines, prop-fans and unducted fans

Fig. 6.28 The Rolls-Royce contra-rotating very high by-pass fan concept. No gearbox is used, and the fan blades are connected directly to contra-rotating turbines

Ultra-high by-pass (UHB) engines, prop-fans and unducted fans

Fig. 6.29 Unducted fan propulsion

The Macdonnell-Douglas MD-80 unducted fan demonstrator aircraft fitted with the gearless General Electric UDF® engine; front-runner in the race to develop this type of powerplant. Dispensing with the gearbox considerably reduces the weight and mechanical complexity of the engine (Photo courtesy of General Electric Co.)

with a duct is an advantage for flight at such Mach numbers, as explained earlier. The duct also affords some noise shielding and containment in the event of shedding a blade.

For twin-engined transports and for cruise Mach numbers up to 0.86, the even higher-efficiency unducted designs are preferable. Figure 6.29 shows the General Electric gearless unducted fan (UDF®) installed in the MD-80/UDF demonstrator, where it has shown exceptionally low fuel consumption and low community noise, as well as a cabin environment equal to or better than today’s turbo-fans. Its commercial success still needs to be demonstrated.

Ultra-high by-pass (UHB) engines, prop-fans and unducted fans Ultra-high by-pass (UHB) engines, prop-fans and unducted fans
Ultra-high by-pass (UHB) engines, prop-fans and unducted fans

Ultra-high by-pass (UHB) engines, prop-fans and unducted fansFig. 6.30 A reheat chamber or afterburner

The exhaust from a gas turbine still contains a large proportion of oxygen which can be used for burning additional fuel in the reheat chamber. This can produce a considerable amount of extra thrust

Reheat is often used for take-off on combat aircraft with high wing loadings. It is also used at high supersonic speeds, and for rapid acceleration and climb