Subsonic Civil Aircraft Nacelle and Engine Installation

Nacelle design and engine integration typically are the responsibility of aircraft designers. A nacelle is a multifunctional system consisting of (1) an inlet; (2) an exhaust nozzle; (3) a thrust reverser, if required; and (4) a noise-suppression system. The goal of nacelle design is to minimize associated drag and noise and to provide a smooth airflow to the engine in all flight conditions. Therefore, the aerodynamic shaping – slimline as much as possible – is very important for aerodynamicists. Typ­ical nacelle positions in current practice are shown in Figures 4.31 through 4.33.

Except for the Concorde, all civil aircraft currently are subsonic with a maxi­mum speed of less than Mach 0.98. All subsonic aircraft use some form of a pod – mounted nacelle such that the design has become generic. Readers should note that designs with the engine buried in the wing (e. g., the Comet) are no longer practiced. Recently, with the advent of very small turbofans competing with propeller-driven engines, in some smaller jet aircraft the engine can be integrated with the fuselage instead of using pod mounts. The approach of this book continues with the domi­nant pod-mounted nacelles. Figure 10.15 shows a turbofan installed in a civil aircraft nacelle pod. An over-wing nacelle like that of the VFW614 is a possibility that has yet to be explored properly (Honda has reintroduced a jet aircraft). An under-wing nacelle is the current best practice; however, for smaller aircraft, ground-clearance issues force the nacelle to be fuselage-mounted.

There are two types of podded nacelles. Figure 10.16a shows a long-duct nacelle in which both the primary and secondary flows mix within the nacelle. The mixing increases the thrust and reduces the noise level compared to a short – duct nacelle, possibly compensating the weight gain through fuel – and cost-savings. Figure 10.16b shows a short-duct nacelle in which the bypassed cold flow does not
mix with the hot-core flow. Advantages include weight, interference drag, and cost reduction by decreasing the length of the outside nacelle casing. The length of a short-duct nacelle can vary. The length depends on a designer’s assessment; the shortest is about half of the nacelle length. Although larger nacelles can benefit from having short ducts, designers may decide on a smaller nacelle with a short duct if the engine-noise level is low.

Three typical positions of the nacelle relative to the wing are shown in Figure 10.17 (see [3] for details). The top wing represents a B747, the middle wing represents an A300, and the bottom wing represents a DC10. All nacelles are hung over well ahead of the wing to keep interference drag low, almost at zero. There is no quick answer for the degree of incidence, which is design-specific and varies for the type of installation. It depends on aerodynamic consideration, the engine position relative to the wing (e. g., how much inboard on the wing and the flexure of the wing during flight). Post-conceptual design studies using CFD and wind-tunnel and flight tests fine-tune the nacelle geometry and its positional geometry to the production standard. Readers should note the typical gap between the nacelle and the wing.

Aircraft designers must make their best compromises in positioning the engine on the wing. In the coursework, Table 10.4 may be used to position wing-mounted nacelles. The most-inboard engine should be kept at least 30 deg from the nose- wheel spray angle, as shown in Figure 10.18 (the B747 is somewhat widely spaced).

Fuselage-mounted nacelle contours are similar in design but the positioning rel­ative to the fuselage requires special consideration. A gap of at least one half of the nacelle diameter can be left between the fuselage and the nacelle. The vertical position can be close to the fuselage centerline or high up on the fuselage (see Fig­ures 4.31 and 4.33). For the coursework exercise, consider the following points for positioning the nacelle on the fuselage:

• Stay clear of the wing wake.

• Keep the exhaust flow from interfering with the empennage.

Figure 10.17. Typical position of the nacelle relative to the wing

Table 10.4. Wing-mounted nacelle position

2- engine 0.3 to 0.32 of semiwing span from the aircraft centerline

3- engine Same as 2-engine; the third engine is at the aircraft centerline

4- engine Inboard at 0.29 to 0.32 and outboard at 0.62 to 0.66 of the semi-wing span

• Keep the thrust line close to the aircraft CG to comply with the first two points.

• Keep the engine sufficiently forward to satisfy the CG position relative to the aircraft.

In the past, both the internal and external contours of a nacelle were designed by the aircraft manufacturer. Although there was no strict requirement, it gradually became convenient to develop the internal contour in consultation with or even entirely by the engine manufacturer. The external contour of a nacelle is developed by aircraft designers who match it with the lines of the internal contour. The contour of the nacelle cross-section is like that of an aerofoil except that it is not uniform all around – it may be perceived as a wrapped wing around the engine. The crown-cut section is thinner than the keel-cut section, as shown in Figure 10.16. The keel-cut section is thicker in order to house accessories and its fuller lip contour helps avoid separation at a high angle of attack. In principle, it is preferable to have circular cross-sectional areas for the intake throat area, but it may not always be possible – for example, for ground clearance. The Boeing 737 has a flat keel line in order to gain some ground clearance. In this book, the intake areas are considered to be circular.

In principle, the external contour lines of a good nacelle design are not neces­sarily symmetrical to the vertical plane. However, to keep costs down by maintain­ing commonality, many nacelles are designed to be symmetrical with the vertical plane. This allows manufacturing jigs to produce interchangeable nacelles between the port and starboard sides and to be able to minimize the essential difference at the finishing end. Efforts for the nacelle aerodynamic design (i. e., external mould-line shaping and internal contouring) have progressed to a point of diminishing returns and are approaching a generic shape.

Engine designers provide aircraft designers with the engine performance – currently, using a computer program amenable to input of the various off-takes. Air­craft designers must substantiate for the certifying agencies that the thrust available

Figure 10.18. Inboard nacelle position

Retracted lit iron

(a) Scoop Intake Figure 10.19. Typical wing-mounted turboprop installation

from the engine after deducting the losses is sufficient for the full flight envelope as specified. In hot and high-altitude conditions, it becomes critical at takeoff if the runway is not sufficiently long and/or there is an obstruction to clear. In that case, an aircraft may take off with lighter weight. Airworthiness requirements require that an aircraft maintain a minimum gradient (see Chapter 11) at takeoff with its critical engine inoperative (customer requirements may demand more than the minimum).

Following are the obligations of designers when installing an engine and inte­grating it with an aircraft:

• Generate the external and internal contours of the nacelle. Multiengines are either wing-mounted (i. e., larger aircraft) or fuselage-mounted (i. e., smaller air­craft).

• Compute the compressor air-bleed for the ECS (e. g., cabin air-conditioning and pressurization, de-icing and anti-icing, and other purposes).

• Compute power off-takes from the engine shaft to drive the electric generator, accessories, and so forth.

• Substantiate for the certifying agencies that the thrust available from the installed engine is sufficient for the full flight envelope.

Current developments involve laminar flow control over the external surface of the intake duct and technologies for noise and emission reduction.