Aircraft Engine Pylon Design

[scudpilot]

Hi All,
Forgive me for the incursion of the tech log forum, and please feel free to post if inappaproriately posted....
I have been working at Farnborough this week and thus spent a lot of time looking at aircraft...
Left me with a question... why are aircraft engines always slung forward of the wing rather than below it.
Apologies if it's a really basic question for you techies....

:ugh:

[Genghis the Engineer]

Offhand, to get the cleanest possible flow into the intake, and to keep as much structure as possible in compression (since metal fatigue is pretty much only a problem with structure in tension).

G

[Flash2001]

Genghis

If one wanted to maximize the amount of metal in compression, shouldn't the engines be on pylons above the wing? It'd cut down on FOD too. It could also make off-airport landings a bit safer as torque around the yaw axis might not be a great during touchdown.

After an excellent landing you can use the airplane again!

[Rainboe]

Keep the intakes out in the clean air ahead of the wing. Keep the exhaust sonics shielded from the cabin by the wing. I think there is an element of balancing the CG of the wing as it is sharply swept backwards. Ease of maintenance, wing bending relief. Cons- pitch power coupling, ground clearance issues, spoiling wing for high lift devices, asymmetry.

Engines above the wing have not been a success. VFW614 had engines perched on top of pylons on top of the wing. Must have been noisy and restricting view. Honda Jet yet to prove configuration works. Risk of severe damage if engine breaks free. Exhaust interference with tailplane.

[Flash2001]

"Tailplane" would be better off as a canard anyway, more efficient.

After an excellent landing you can use the airplane again!

[ChristiaanJ]

scudpilot,
I'm oversimplifying, probably.
The aerodynamic forces on the wing structure tend to twist it upwards and backwards. So you have to design for a stiff enough wing with a 'clean' wing (think VC10).
Hang a few engines under the wing, well forward, and you compensate for that 'twist', so you end up with a lighter wing structure design.

It's only for small jet liners (RJs) and business jets where that doesn't really work because the engines would be practically dragging the runway, unless you increase the size of the landing gear significantly. So there it makes more sense moving the engines to the back.

CJ

[barit1]

737-100/200 had engines directly under the wing - but small diameter. no pylons to speak of.

-300 et subs had a big fan, so needed to move it FWD for ground clearance. Even so, it needed longer gear.

[DC2 slf]

Genghis

If one wanted to maximize the amount of metal in compression, shouldn't the engines be on pylons above the wing? It'd cut down on FOD too. It could also make off-airport landings a bit safer as torque around the yaw axis might not be a great during touchdown.


It would also discourage the use of the engines as jack points, a la the AA DC10 at Chicago.

On the other hand, it's a lot easier to get at them for maintenance and replacement where they are in most planes.

Is it the small Honda twin jet that puts them on top?

[scudpilot]

Cheers Guys!:ok::ok:

[ft]

You will have interference drag due to interactions between the nacelle and the wing. This is minimized by having the engines forward or aft of the wing (Synthesis of subsonic airplane design, Torenbeek, 1976). Clearly the latter is not favourable for a number of other reasons.

It is probably a nice bonus to have as much of wing, flaps and fuel tanks out of the plane of the turbine discs as well.

I also recall reading somewhere that having the nacelles slung forward of the wing counters torsional flutter, but I have no reference on that and it is not my area of expertise. On first look it does make sense.

[Genghis the Engineer]

There are lots of reasons for putting the engines on top of the wings. But the major reason not to, maintenance, trumps the lot.

Re: fatigue - the working loads of the engine must exceed by a large margin the ground/static loads; if they didn't, the engines wouldn't be doing anything very useful.

G

[411A]

You will have interference drag due to interactions between the nacelle and the wing. This is minimized by having the engines forward or aft of the wing

Spot on.
The further forward the better, actually, within reason.

[Terry McCassey]

Believe it is correct to say that only the Rolls Royce RB 211 powered B747-200's could tank extra fuel by utilising the 2 and 3 reserve tanks. The shorter length of the 3 spool engine kept the turbine disc area clear of these tanks in the event of uncontained turbine failure. Enabled CX to help fly non-stop HKG/YVR in the Kai Tak days. I also believe the B732's had reinforcing plates over the tanks for protection. Comments of course, most welcome

Terry

[james ozzie]

Does it not also contribute to controlling the overall cross sectional area of the aircraft especially in the area of the mainplane?

(you know, like that pointy thingy that has optimal transonic/supersonic flow: thin at one end, fat in the middle and thin at the other, like Ann Elks dinosaur?)