Advantages of underslung engines...

[r2_unit]

Chaps,

Been trying to get as full an answer as poss to this classic tricky-interview-question...

What are the advantages of underslung engines as opposed to tail/fuselage mounted?

My thoughts so far...

1. Shorter longditudinal distance to CofG resulting in smaller pitching moment for given engine mass.

2. Ease of access for maintenance.

3. More flexibility in selection of engine size than fuselage mounted engines (or indeed engines mounted inside the wing!).

4. Reduced damage to structure in the event of an engine fire/ mech fail.

5. Helps reduce wing bending.

Offering it across for a bonus point...

r2 :8

[gas path]

A cleaner wing and a bonus is the pylon acts in part like a fence limiting spanwise flow.
Ref. Your item 5. A lighter wing structure.

[r2_unit]

gas path,

thanks for the swift reply!

how does an underslung engine design result in a cleaner wing?

Nice thoughts re the wing fence vs pylon - hadn't thought of that!

[Rainboe]

Ditching- engines knocked off at pylon reducing weight and damage to fuselage on entry to water allowing plane to float more easily.

Pylon much narrower than engine meaning wing structure less interfered with by engine. Also pylon good place to access fuel/hydraulic/air lines and place firewall.

[HZ123]

Sucks in all the FOD

Potential danger to ground crews / departure / pushback

potential of engine explosion and fire

737 engines probably reponsible for about one death a year caused by careless ground crews being sucked in.

[r2_unit]

Nice one fellas.:ok:

Thoughts on whether the thrust couple from an underslung engine arrangement helps or hinders during a stall recovery???

[smith639]

PRO - Engine intake in clear airflow

[Mad (Flt) Scientist]

4. Reduced damage to structure in the event of an engine fire/ mech fail.


At least the rotor burst zone for an aft mounted engine usually excludes the pressure vessel. If an underwing rotor goes, and one of the segments heads for the fuselage .....

Plus at least the fuel is in the wing, a decent distance from an aft-mounted engine. Fires on underwing engines are a tad close to the fuel tank ...

[411A]

Plus at least the fuel is in the wing, a decent distance from an aft-mounted engine. Fires on underwing engines are a tad close to the fuel tank ...

Not forgetting of course that wing mounted podded engines are jusy slightly more advantageous than those that are in the wing...(hello deHavilland):}

[airseb]

and gravity feeding for a fuel pump problem

[mustafagander]

r2
1 - Think about pitch couples with low thrust lines
2 - Yep
3 - Maybe
4 - Very likely
5 - Big advantage

As has been mentioned, engine struts can act as fences - look at B747 obrd strut inbrd edge. Clean wings (rear engines) often require fences for airflow control anyway.

Engine inlet incidence is easy to design for best ram recovery and short inlet leads to low air handling losses, but short inlet path can mean unstable air doesn't "have enough time" to sort itself out leading to potential problems at the compressor face.

Engine fuel feed lines are short and can be run through tanks for most of the path - an advantage in case of a line leak/fracture. Pod engines can usually produce considerable power without tank boost pumps.

While pods generally require LEDs and flaps to be split into several parts, this is no bad thing for redundancy and, anyway, swept wings often require it, especially flaps.

[r2_unit]

Fellas,

Top gen. Thanks :ok:

[OzExpat]

Ditching- engines knocked off at pylon reducing weight and damage to fuselage on entry to water allowing plane to float more easily.
Rainboe... I was watching a story on the National Geographic Channel about a 767 that ditched a few years ago after a hijack that resulted in the aircraft running out of fuel. The low-slung engines were a distinct disadvantage in that instance because, according to the analysis of the ditching, the left engine pod struck the water before the fuselage.

I readily concede that the aircraft ditched at a higher speed than normal, due to (apparently) loss of power to flaps/spoilers, etc., but the fact seems to remain that the engine pod hit the water first. The result was that the aeroplane broke up quite quickly and dramatically.

This prang might not be typical of the way in which the aeroplane should've been ditched but it's already in the accident records, unfortunately.

[cargonaut]

Have to agree with Mustafagander - there is 0 pitch up with thrust on aircraft with tail mounted engines.

[radicalrabit]

Didn't at least onaircraft suffer total hydraulics loss because a fan blade shredded the hydraulic lines in the rear tail section? and become un flyable?

[Big-Flame-Out]

For structural reasons you want to put the heavy bits (engines) distributed across the bit that generates the lift. This also reduces the moment of inertia in the pitch axis, making the aircraft respond quicker in this axis. (But MI is increased in roll axis, of course)
Also from the flyability angle, the C.G. is nicely below the Center of lift. (stability at constant speed*)

Now look at a DC9
If you start to separate the engines from the wing then you have to make the structure between the two a bit stronger (stronger=heavier->bad)
Also, as you have mentioned, the wing will bend a bit more.
You have to keep the engines low on the fuselage so you end up with a T-tail (inspection and service issues, stall characteristics).
All the control runs from the cockpit are longer, not just the engines but the wings too. The only advantage I can see is that the undercarriage is quite short.

There's another problem - it would be difficult to scale up a DC9 to 777 size. On a 777, the large bypass ratio engines are suspended so there is nearly zero bending moment at the attach point on the wing. If you stuck them at the back you would have to build some sort of triangular structure to partially support them from underneath or suspend them from above because the engines are heavier and stick farther out.

*though the aircraft is more stable at constant speed, thrust changes will cause pitch changes

[055166k]

Sorry....not a pilot.....but I read that in certain circumstances an underslung engine could separate [drop off] but the wing remains undamaged and the aircraft is flyable.

[cargonaut]

Re : Hydraulic lines

True. DC-10. Unlikely on 727
Also killed rear occupants on dc-9 or md-80 I believe.
I guess they could enter the fuel tank on underwing designs. Clearly the underwing design has won for airliners. Handling the Big Jets has a good discussion on the merits and flaws of each design.

Engine separation has been a characteristic of jet engines for a long time. They'll also nicely separate from the tail.