In a civil aircraft design with more than one engine (i. e., turboprop or turbofan), the engines are invariably pod-mounted on the wing or the aft fuselage. The predominant options in civil aircraft design are shown in Figures 4.25 and 4.27. Here, some military designs are shown because they can be applied to civil aircraft design as well.
Larger aircraft have nacelle pods mounted under the wing (Figure 4.24a), but low-wing small aircraft have fuselage-mounted (Figure 4.24b) nacelle pods because there is insufficient ground clearance. An overwing nacelle pod (Figure 4.24c) on a smaller low-wing aircraft is gaining credence. Four-engine underwing nacelles are shown in Figures 4.27a and 4.27b (i. e., high and low wings, respectively). Introductory coursework may use any combination of these configurations.
Other options for engine positions are shown in Figures 4.28 and 4.29. The first commercial jet transport aircraft, the de Havilland Comet, had engines buried in the wing root (Figure 4.28c). These were not efficiently designed and are not pursued any longer in civil aircraft designs. For an odd number of engines, the odd one is placed in the centerline (e. g., Douglas DC10); if it is buried in the fuselage, then its intake may require an S-duct-type intake (e. g., Boeing 727) (see Figure 4.27d). In the 1970s, the proposed Heinkel 211 (not shown) had two S-ducted engines with the two surfaces of its V-tail. The overwing slipper-nacelle design
has been flown by both Boeing (see Figure 4.27f) and Douglas for STOL performance. The engines on single-engine aircraft are at the centerline (except on special – purpose aircraft), mostly buried into the fuselage. The Boeing B52 bomber has eight engines in four pods slung under the wing. If propeller-driven, an engine can either be a tractor (i. e., most designs) or a pusher-propeller mounted at the rear.
Some unconventional single – and twin-engine positions are shown in Figure 4.28; futuristic nacelle design options are shown in Figure 4.29 and have yet to be built. Figure 4.29a shows a Boeing Super Cruiser and Figure 4.29b is the Silent aircraft BWB proposed by MIT and Cambridge University.
Some helicopter designs have rotor-tip-mounted thruster engines and some VTOL aircraft have wing-tip-mounted tilt engines; all are special-purpose designs. Virginia Polytechnic Institute (VPI) conducted studies on interesting aircraft configurations with potential. Through their MDO studies of high-subsonic aircraft with engines at the tip of a strutted wing (Figure 4.29c), they found better weight and drag characteristics than in conventional cantilevered designs . Although the studies have merit and they have considered the critical issues, more detailed analysis is required using better resolution. The structural weight gain due to a truss – supported wing and the aerodynamic gain due to induced-drag reduction of the wing-tip engines are not coupled even when the former offers structural support for the latter. A major concern will be to satisfy the mandatory requirement of a one-engine inoperative case. This will result in a considerably larger tail, possibly divided in half, depleting some weight benefits. Cost is another factor that the studies did not consider. The proposed aircraft will be more expensive, which may erode the DOC gains. The new aircraft certification will further add to the cost. Until more details are available, the author does not recommend the wing-tip – mounted engine installation, especially during an introductory course. Engines should be kept close to the aircraft centerline but away from any wake effects. The nose-wheel spray may require the nacelle to be at least 30 deg, away from the nose wheel (see Chapter 10). Detailed sensitivity studies are required for comparative
analyses of this novel configuration when a simple winglet provides induced-drag reduction. However, VPI’s study of twin side-by-side engines between the V-tail (Figure 4.29d) concluded that it would be better with a winglet.