Engine Pylon Positioning
 

Forgive my ignorance, I'm not the technical type! But I have a question regarding the POSITIONING of jet engines on modern airliners. For which I am talking ONLY of aircraft that have WING MOUNTED engines. WHY are the engines held on pylons that are positioned FORWARD of the wing? One would have thought there'd be more strain on the pylon than if the engine was mounted DIRECTLY under the wing surface itself. (Hope I'm making sense here!) Might sound like a daft question but I'm looking for a not-daft answer, if anyone can educate me? Thanks awfully.

By mounting the engine forward of the wing you to some extent minimize the aerodynamic interference between wing and nacelle (engine) - since the thickest fart of the wing ends up behind the thickest part of the engine.

More importantly, by hanging mass off the front of the wing the aeroelastic characterists of a thin wing are improved, which means that less additional structure is required to stiffen the wing, so there is an element of weight saving, even accounting for the additional structure to suspend the engine forward of the leading edge.

(Some older aircraft have 'flutter bodies' on wings or tail surfaces - ballast weight forward of the leading edge in faired bodies - in lieu of engines hung forward.)

ROFL :D :D :D :D

I'd try to blame a typo but since the "p" and "f" keys are miles apart it must be Freudian instead
:O

Hi Satco!!!
Except for the reasons that mad scientist gave in a very detailed manner may I suggest that by mounting the engines forward of the leading edge you reduce wing flutter,especially to higher speeds.This is accomplished because the engine acts like a mass balance for the wing as the AC moves closer to the flexural axis.

It may have something to with a little thing called
CofG

eg If you take both engines and T/R cowls off a 767 it sits on its ass

The answer is to prevent aileron reversal at high speeds. When ailerons are used at high speed the wing tends to flex, leading edge down, due to the forces acting on the main wing section and aileron. It also aids in keeping the airflow over the wing smooth and, in the event of an accident where the engine breaks away from the pylon, the front mounting point is weaker than the rear. This means the engine will tend to fall away from the wing rather than take the wing out.

Having the engines mounted forward of the wing flexural axis won't affect aileron reversal.

Aileron reversal can be either purely aerodynamic, or it can be aeroelastic.

If the former, it occurs because the ailerons are disrupting the flow in some fashion that causes the local lift to behave in the 'wrong' fashion. An example of this generally occurs at low speed/near the stall, where deflecting the ailerons downwards causes the local flow to separate, and thus the lift reduces instead of increasing, and the aileron operates opposit to the desired sense.

If the latter, the wing stiffness is too low and the torsion induced by the aileron causes the wing to bend, as mentioned by pigs, and if the resulting change in local AoA is more powerful than the trailing edge deflection then the aileron reverses in effect. But the mass distribution on the wing doesn't matter - it's purely driven by stiffness in this case.

re other suggestions: cg isn't a player, if the wings aren't carrying the engines then the basic design allows for that - aft-mounted engine aircraft don't sit on their tails.

giorgino's answer is basically the same as mine except I called it aeroelasticity and he called it flutter, which are two names for similar things. (flutter is basically divergent dynamic aeroelasticity)

you'd best get in touch with the CAA then if aileron reversal isn't right. That was the correct answer when I did my B1

If the correct answer is aileron reversal, then how do you explain the B-47?

Engines mounted well forward of the wing leading edge but it is the poster child for aileron reversal.

Mad (Flt.) Scientist is correct, basic wing elasticity combined with wing sweep is the aileron reversal culprit.

I believe the reason is so that in the event of an uncontained engine failure (turbine disc bursting) the bits that come out like a bomb going off wont go in to the fuel tanks or sever hydraulic lines or destroy the integrity of the wing.

I think I understand the aerodynamics but I am curious about the mass of the engine being so far removed from the neutral axis of the wing, thereby generating a large polar moment of inertia about that axis.

Does not some of the constructional weight saved in the pylon system have to be put back to resist torsional factors?:confused:

It's more like the other way round.

There is a mass penalty in having a pylon extending well forward of the leading edge, compared to one more directly under the wing, since it's a larger piece of structure and has to take cantilever loads. In other words, such a PYLON is heavier.

But there is a mass SAVING in the basic wing structure, because the fact that a lump of mass (the engine) is so far forward of the wing flexural axis means that the wing can actually be less stiff and still have acceptable flutter margins. So the WING gets lighter.

On balance, it's usually a net structural weight saving to have the engines suspended a decent distance forward, because the weight saving in the WING tends to be greater than the PYLON penalty.

While this scenario does occasionally happen, it is very rare, and turbine discs undergo extreme scrutiny in certification, manufacturing quality, and periodic maintenance inspection to preclude this sort of failure, which is literally impossible to contain. In addition, such highly-loaded rotating parts have life limits placed on them at the time of certification.

And all aircraft types with tail-hung engines have had very serious failures that have either penetrated the cabin or caused major aircraft systems damage. So, you may not be any better off with engines in back.

On a swept wing, if the disc element heads inboard it can easily get to the various systems in the inboard wing.

barit1 is right, an uncontained engine failure is bad news wherever your engine is.