Configuring a Civil Aircraft Nacelle: Positioning and Layout of an Engine
The nacelle pod size depends on the choice of engine. At this design stage, a statistical value of uninstalled TSLS per engine is considered to determine the size of an engine. A formal engine sizing and matching is accomplished in Chapter 11. For better fuel economy, a large bypass ratio is desired. Dialogue with engine manufacturers (that can offer the class of engines) continues with “rubberized” engines (i. e., engines scalable and finely tuned to match the aircraft performance requirements for all variants). There are not many engine manufacturers from which to choose.
Numerous engine accessories (see Chapter 10) are part of the engine power plant. They are located externally around the casing of the engine (i. e., turbofan or turboprop). In general, these accessories are located below the engine; some are distributed at the sides (if the engine is underwing-mounted with less ground clearance). Therefore, the nacelle pods are not purely axi-symmetric and show faired bulges where the accessories are located.
Long-duct nacelles, chosen for the example, appear to be producing a higher thrust to offset the weight increase of the nacelle, while also addressing environmental issues of substantial noise reduction. Also, long-duct designs could prove more suitable to certain types of thrust reverser designs. This book only considers long-duct design but it does not restrict the choice of short-duct nacelles.
For this example, the maximum nacelle diameter « <1.5 x engine-face diameter
In general, the intake length in front of the engine face « <1.0 x engine-face diameter, and the exhaust jet-pipe length aft of the last stage turbine disc « <1.5 x engine-face diameter.
The total nacelle length « (engine length) + (k x engine-face diameter) (6.7)
where 1.5 < k < 2.5. For smaller engines, the value of k is lower.
For long-duct nacelles, the fineness ratio (i. e., length/maximum diameter) is between 2 and 3.
Pylons are the supporting structures (i. e., cross-section streamlined to the aerofoil shape) of the nacelle attaching to the aircraft and carrying all the linkages for engine operation. Aft-fuselage-mounted pylons are generally horizontal but can be inclined if the nacelle inlet must be raised. For wing-mounted nacelles, the pylon is invariably vertical. The depth of the pylon is about half of the engine-face diameter; the pylon length depends on the engine position. For an aft-fuselage-mounted installation, the pylon is nearly as long as the nacelle. For a wing-mounted installation, the nacelle is positioned ahead of the wing LE to minimize wing interference. In general, the t/c ratio of the pylon is between 8 and 10%.
The nacelle size is determined from the matched-engine dimensions. Using the considerations listed in Section 6.3.4, the following stepwise approach is suggested. The engine-thrust level indicates engine size (Figure 6.13). It is best to obtain the engine size from the manufacturer as a bought-out item.
Step 1: Configure the podded nacelle size.
The maximum engine diameter determines the maximum nacelle diameter. The ratio of the maximum nacelle diameter to the maximum engine diameter is given statistically in Chapter 10. Similarly, the length of the nacelle is established from the engine length. The keel cut is typically thicker than the crown cut to house accessories. In this book, the nacelle is symmetrical to the vertical plane but it is not a requirement.
Step 2: Position the nacelle relative to the fuselage.
The nacelle position depends on the aircraft size, wing position, and stability considerations (see Section 4.10). Subsequently, CFD analysis and wind-tunnel testing will fine-tune the nacelle size, shape, and position.
Step 3: Use pylons to attach the nacelle to the aircraft.
A worked-out example follows in the next section.