Worked-Out Example: Configuring and Positioning the Engine and Nacelle in Civil Aircraft
This section provides an example for configuring the nacelle based on an engine bought from an engine manufacturer. (Figure 4.9 gives the relationship between MTOM and engine thrust. Chapter 10 gives more details of engine dimensions).
Figure 6.13. Statistics in the aircraft class: the uninstalled thrust of a turbofan
coarse; however, Figure 6.13 provides similar information in finer detail confined to the aircraft class. The author recommends that readers produce graphs in higher resolution for the aircraft class under consideration. Unlike aircraft in general, the external dimensions of variant engines in a family do not change – the thrust variation is accomplished through internal changes of the engine (see Chapter 10). The same nacelle geometry can be used in all variants. For major variations, the engine size changes slightly, with minimal changes affecting the nacelle mould lines.
From the statistics in Figure 6.13, for a MTOM of 9,500 kg, a typical uninstalled engine thrust for this aircraft class indicates that TSLs/engine = 3,800 lb ± 25% for the derivative variants for the aircraft family to be offered. This may be considered a smaller engine. For better fuel economy, a larger BPR is desirable. Not many engines are available in this class. It is important that a proven, reliable engine from a reputable manufacturer be chosen; of interest are the following:
Honeywell (originally Garrett) TFE731 turbofan-series class.
Pratt and Whitney (Canada) PW 530 series class (not many variants available)
(In the small engine class, Williams is coming up but is still below the required size.)
The Rolls Royce Viper and the Turbomeca Larzac have a low BPR and are suited to a military application. This leaves the Honeywell TFE731-20 turbofan class as practically the only choice. It has a fan diameter of 0.716 m (28.2 inches), a bare engine length of 1.547 m (60.9 inches), and a dry weight of 379 kg (836 lb). At this stage, a generic long-duct nacelle pod to house is used (see Figure 6.13).
Using the relationship given in Equation 6.6, the maximum nacelle diameter =
1.5 x 0.716 = 1.074 m (5.52 ft).
Using the relation given in Equation 6.7, the nacelle length = 1.5 x 0.716 + 1.547 = 2.62 m (8.6 ft).
The nacelle fineness ratio = 2.62/1.074 = 2.44.
Being a small aircraft, the engines are aft-fuselage-mounted, one at each side. At this stage, a horizontal plate may represent the pylons that support the nacelles. The pylon length = 2.44 m (8 ft) with a thickness of 25 cm (9.8 in) and having a symmetrical cross-section aerofoil-like structure for ease of manufacture. Inlet and exhaust areas are established in Chapter 10.
Figure 6.14. Three-view diagram and a CAD drawing of the preliminary aircraft configuration