What are the advantages/disadvantages of in-fuselage engines (eg most fighter jets) vs wing/fuselage mounted engines?

Question

Most modern jet fighter designs incorporate their engine(s) within the fuselage, while most other medium/large aircraft mount them either under the wings or on the rear fuselage.

What are the advantages and disadvantages of in-fuselage jet engine placement, compared to those mounted on the fuselage or wings of an aircraft?

Advantages and disadvantages compared to what? Earlier piston-engined warplanes? Please [edit] your question so we can give you the kind of answer you're looking for. — Dan Hulme, Apr 02 '19 at 08:20
An advantage of wing mounting the engines is that it frees up fuselage space for cargo, be it boxed or self-loading. — FreeMan, Apr 03 '19 at 18:46
Related: How does the mounting location of a jet engine affect aircraft performance? — fooot, Apr 03 '19 at 19:05

Peter Kämpf · Answer 1 · 2019-04-04T00:22:16.693

The answer is roll inertia.

Fighter aircraft need to maneuver quickly, and one of the most important parameters is roll speed. In order to achieve a high roll rate quickly, low moment of inertia in roll is helpful. Placing the engines on the wing would increase roll inertia without much benefit in air combat.

Wing-mounted engines can only be found in designs which were optimized for raw speed, such as the P-38 or the YF-12. Even the Me-262 was initially designed with fuselage-mounted engines, hence its triangular fuselage cross section. Only when the engines grew in diameter during development had they to be relocated to the wing.

Disadvantages of fuselage-mounted engines:

Worse accessibility for maintenance.
Variants using a different engine cost much more to develop.
Engine fires or loss of a turbomachinery blade pose a much bigger threat to the structure.
No bending moment relief or flutter damping for the wing.
Complex intake geometry with boundary layer splitter plate, unless the intake is in the fuselage nose. The air requirements of high bypass ratio engines cause high losses with side intakes.

Note that at supersonic speed the engine can actually benefit from the proximity of a fuselage: The precompression of the forward fuselage of the F-16 or the Dassault Rafale help to increase thrust, especially when it is most needed at high angles of attack (well, those relatively high angles in supersonic turns, that is).

Advantages of fuselage-mounted engines:

Low moment of inertia in roll.
Lower overall surface area, thus lower friction drag, when compared to podded engines.
Precompression of the forward fuselage can be used to increase supersonic thrust.

I'll add a disadvantage to the list, difficulty in swapping the engine for a different model. As an airplane ages it is common to be updated with new engines and electronics. Newer electronics are often smaller and lower power so those updates are rarely a problem. New engines models though must fit in a very constrained space, as opposed to hanging from the wing and not dragging on the ground. An advantage is stealth. With a smoother outline it's easier to minimize radar return. — MacGuffin, Jun 20 '21 at 11:22
@MacGuffin Right, and this was especially hard for the British and Russian designs with wingroot-mounted engines (Comet, Tu-104, Victor, Valiant). The first jet engine with bypass (RR Conway) was restricted to a bypass ratio of 0.25 so it would fit into the existing airplanes. — Peter Kämpf, Jun 20 '21 at 14:22

score 1 · Answer 2 · answered Apr 03 '19 at 19:25

Advantages of placement in the fuselage:

low drag, which enables high speeds

disadvantages:

difficult installation (so more time spent on maintenance)
lots of noise
no flexibility in engine installation (you can't easily update the aircraft with a new, larger engine)
lots of space taken up by the engines and ducting
constraints on the center of gravity (heavy engine in the back, lots of empty ducting in front of the engine)

disadvantages of engines under the wing:

on aircraft with a low wing (e.g. to make the wing box pass underneath the cabin), you need longer landing gear
more drag due to extra frontal area

Therac · Answer 3 · 2019-04-09T17:10:06.707

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Most fighter jets operate at much lower bypass ratios than commercial jets. At low bypass ratios and high speeds, drag is important, and it's simply more efficient to embed the engine within the airframe. At high bypass ratios, the engine is a large assembly that's difficult to embed.

The A-10 has separated nacelle engines, while being a combat (attack) jet. The very large Concorde had nacelles flush to the fuselage. Both were tradeoffs, and in this case supersonic flight does call for reducing the cross-section.

edited Apr 09 '19 at 17:10

answered Apr 03 '19 at 19:28

Therac

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Are you sure about the location of the Concorde engines? They still are in nacelles, but those are directly attached to the lower wing. – Peter Kämpf Apr 03 '19 at 23:48
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Had you mentioned the XB-70 rather than Concorde you'd have been almost right. – jwenting Apr 04 '19 at 05:23
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@PeterKämpf Right, nacelle but not pylon mounted. – Therac Apr 04 '19 at 06:08

score 0 · Answer 4 · answered Apr 03 '19 at 19:19

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You may notice that the primary difference between the two classes of aircraft you mention is maximum designed airspeed. To allow supersonic performance, low drag considerations such as the area rule are more easily met with in-fuselage engines. Minimizing radar reflections, withstanding high G loads and other effects also apply. The fighters are also smaller and lighter than many medium or larger aircraft as well.

answered Apr 03 '19 at 19:19

Jim Horn

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So what about the B-58 Hustler? – Peter Kämpf Apr 03 '19 at 23:03
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Like the XB-70A or the SR-71, it's not a "jet fighter" as described in the original question. My initial answer included all three as exceptions but I deleted that. Good point, though, Peter! – Jim Horn Apr 04 '19 at 20:15

What are the advantages/disadvantages of in-fuselage engines (eg most fighter jets) vs wing/fuselage mounted engines?

4 Answers4