Figure 10.14. Installation effects

10.7 Engine Integration with an Aircraft: Installation Effects

Engine manufacturers typically supply bare engines to aircraft manufacturers, which install them to integrate with an aircraft. The same type of engines can be used by different aircraft manufacturers; each has its own integration requirements. Installing an engine in an aircraft is a specialized technology with which aircraft designers must be knowledgeable. Engine integration is accomplished by aircraft manufacturers in consultation with engine manufacturers.

A bare engine at the test stand performs differently than an installed engine on an aircraft. The installation effects of an engine result from having a nacelle – that is, the losses of intake and exhaust plus off-takes of power (e.g., driving motors and generators) and air-bleeds (e.g., anti-icing and environmental control). The total loss of thrust at takeoff could be as high as 8 to 10% of what is generated by a bare engine at the test bed; at cruise, the loss can be reduced to less than 5%. Figure 10.14 shows typical off-takes that are required due to various installation effects. Designers conduct analytical and empirical studies to establish key parameters in order to arrive at a design that produces satisfactory thrust to meet aircraftperformance requirements.

A nacelle is the housing for the engine and it interfaces with an aircraft; typically, it is pod/pylon-mounted in civil aircraft designs. A nacelle on an aircraft with more than one engine is pod/pylon-mounted on the wing and/or the fuselage. Propeller-driven engine nacelles are also similar to podded nacelles, modified by the presence of a propeller (see Section 10.7.2). An aircraft with one engine is aligned in the plane of the aircraft symmetry; engines with propellers can have a small lateral inclination of 1 or 2 deg about the aircraft centerline to counter the slipstream and gyroscopic effects from a rotating propeller. As discussed previously, wing-mounted nacelles are best for relieving wing-bending in the flight load. The engines on military aircraft are buried in the fuselage and therefore do not have a nacelle unless the designer chooses to have pods (e.g., some older designs). Militaryaircraft designers must consider intake design as described in Section 10.8.2. The

Figure 10.15. Installed turbofan housed in a nacelle pod under slung below aircraft wing (Courtesy of Bombardier-Aerospace Shorts)

position of the nacelle relating to the aircraft and the shaping to reduce drag are important considerations (see Section 9.8).

10.7.1 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 podmounted 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 shortduct nacelle, possibly compensating the weight gain through fueland cost-savings. Figure 10.16b shows a short-duct nacelle in which the bypassed cold flow does not

Figure 10.16. Podded nacelle types

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 nosewheel 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