notices a sudden increase in the noise level, which can exceed the physical threshold. An AB glow is visible at the exit nozzle; in the dark, it appears as a spectacular plume with supersonic expansion “diamonds.” In the absence of any downstream rotating machines, the AB temperature limit can be increased from 2,000 to 2,200 deg K, at the expense of a significant increase in the fuel flow (i.e., a richer fuel-to-air ratio than in the core combustion).

An AB exit nozzle invariably runs choked and requires a convergent–divergent nozzle for the supersonic expansion to increase the gain in momentum for the thrust augmentation. Typically, to gain a 50% thrust increase, fuel consumption increases from 100 to 120%; that is why it is used only for a short period, not necessarily in one burst. It is interesting that AB in bypass engines is an attractive proposition because the AB inlet temperature is lower. In fact, all modern combat-category engines use a low bypass of 1 to 3.

Losses in an AB exit nozzle are high – the flameholders and so forth act as obstructions. It is preferable to diffuse the flow speed at the AB from higher speed to Mach 0.2 to 0.3, which results in a small bulge in the jet-pipe diameter around that area. A combat aircraft fuselage must be able to house this bulge.

10.6.4 Turboprop Engine: Formulation

Turbo props are described in Section 10.4.4. They are very similar to turbojets and turbofans except that the high energy of the exhaust jet is utilized to drive a pro- peller by incorporating additional low-pressure turbine stages, as shown in Figure 10.7. Thrust developed by the propellers is the propulsive force for the aircraft. A small amount of residual thrust could be left at the nozzle exit plane, which should be added to the propeller thrust. The relationship between the thrust power (TP) and the gas turbine SHP is related to propeller efficiency, ηprop, as:

ESHP is a convenient way to define the combination of shaft and jet power, as follows:

Aircraft at a static condition have an ESHP = SHP because the small thrust at the exit nozzle is not utilized. As speed increases, ESHP > SHP, as there is some thrust at the nozzle. SFC and specific power are expressed in terms of ESHP.

Summary

The formulae provide good reasoning for the gas turbine domain of application, as shown in Figure 10.2. Turboprops provide the best economy for a design flight speed at and below Mach 0.5 and are well suited for shorter ranges of operation. At higher speeds, up to Mach 0.98, turbofans with a high BPR provide better efficiencies (see the comments following Equation 10.5). At supersonic speeds, the BPR is reduced and, in most cases, uses an AB. Smaller aircraft have piston engines up to a certain size (i.e., ≈ ≤500 HP). Above 500 HP, turboprops prove better than piston engines.