High by-pass ratio turbo-fans or fan-jets

Further improvements in efficiency are obtained by increasing the by-pass ratio; the ratio of the amount of air by-passed around the core engine to that which passes through it. Increasing the by-pass ratio requires making the lowest pressure stage larger in diameter.

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Three independent sets of turbine discs

Fig. 6.24 A triple-spool turbo-fan engine based on the Rolls-Royce RB-211

This high by-pass ratio engine has three concentric shafts or spools running at different speeds. A large proportion of the thrust is provided by the front low speed fan

All of the more recent designs of civil jet transport aircraft use high by-pass turbo-fans

Figure 6.24 shows a high by-pass ratio Rolls-Royce RB-211 turbo-fan which uses three shafts or spools, one being dedicated to the fan drive. A significant proportion of the overall thrust comes from the pressure difference across the fan blades, as with a propeller.

The big primary fan results in an engine of much larger diameter than the earlier simple arrangements. The large diameter of the fan is evident in Fig. 6.25.

In turbo-fan engines, the speed of the air relative to the surrounding shroud walls is subsonic, but relative to the moving fan blades, it is supersonic. As explained earlier, however, losses due to shock wave formation are less severe for fans than for simple propellers. The shroud helps to suppress some of the noise from the fan, and because of the low jet speed, turbo-fan engines can be extremely quiet. Jet noise is related to the eighth power of the jet speed. The British Aerospace 146, shown in Fig. 6.26, is an outstanding example of a quiet turbo-fan-propelled aircraft.

The turbo-fan provides a practical means of propulsion at Mach numbers above the limiting value of about 0.6 to 0.7 for a conventional propeller. It also represents an alternative to the much less efficient turbo-jet.

Low by-pass engines are now normally used for all combat aircraft, even for types designed for flight at supersonic speeds. High by-pass engines are mainly used for subsonic transport aircraft, both civil and military.

Fig. 6.25 High by-pass turbo-fan for efficient transonic flight The large diameter high by-pass engines fitted to the Boeing 777 are evident in this photograph

Fig. 6.26 The quietness of the high by-pass ratio turbo-fan was a major selling point of the BAe 146

(c) Fig. 6.27 Ultra-high by-pass (UHB) ratio engines, sometimes referred to as prop-fans. The term unducted fan is also used for unducted versions (a) Tractor unducted (b) Pusher unducted (c) Pusher ducted prop-fan. The large propulsion fans are directly connected to contra-rotating turbine discs. No gearbox is used