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Considerations in Configuring the Nacelle
Following are general considerations important for configuring the nacelle (see also Section 6.7):
|
Geometry |
Aerodynamics |
|
(1) diameter (comfort level, |
(1) drag |
|
appeal) |
(2) interference |
|
(2) length, fineness ratio |
(3) surface roughness |
|
(3) wing and fuselage position |
(4) noise/emission |
|
and pylon geometry |
(5) vibration |
|
(4) ground clearance |
(6) thrust and bypass ratio (BPR) |
|
(5) cross-section to house |
level |
|
accessories |
|
|
(6) intake geometry and lip |
|
|
section |
|
|
Structure (affecting weight |
|
|
and external geometry) |
Systems |
|
(1) engine burst considerations |
(1) control linkage |
|
(2) foreign-object ingestion |
(2) fuel system |
|
problems |
(3) electrical |
|
(3) fuel volume |
(4) thrust reverser |
|
(4) weight |
(5) fire prevention |
|
(5) nose gear collapse |
(6) anti-icing |
|
(6) access |
Civil aircraft designs are invariably externally pod-mounted on either the wing or the aft fuselage (smaller low-wing turbofan engines). The demonstration of high engine reliability enables an ETOPS clearance by the FAA for a two-engine
Figure 6.5. Narrow-body, single-aisle fuselage layout (not to scale)
configuration. Three-engine designs (e. g., B727, DC10, and Lockheed Tristar) are no longer pursued except for a few designs. An underwing-mounted nacelle should remain clear of the ground in the event of a nose-wheel collapse. A minimum of 30 deg of separation (see Chapter 9) is necessary to avoid wheel-spray ingestion.
Nacelles should have their thrust lines positioned close to the aircraft CG to minimize associated pitching moments. In general, the nacelle aft end is slightly inclined (i. e., 1 to 1.5 deg) downward, which also assists in takeoff. Because of the lack of ground clearance for smaller aircraft, engines are mounted on the fuselage aft end, forcing the H-tail to be placed higher. Aft-mounted engines are less desirable than wing-mounted engines. Therefore, when aircraft size and wing position allows, engines take the natural position mounted on the wing, generally slung underneath. It is for this reason that the designers of smaller aircraft are currently considering mounting the engine over the wing, as in the Honda small-jet-aircraft design.
