Engine location under aircraft wing

[Jason2000]

Hi,

I was wondering if someone could explain to me why aircraft engines are attached to pylons which stick out forward from the wing, as opposed to hanging the engines directly under the wing? (ie what determines the longitudinal location of the engine)

Is the reason to do with aerodynamics or maintenance of CG location or what??

Thanks in advance,

Jason

[Onan the Clumsy]

I always thought it was to do with CG, but I could be wrong. I guess there's a difference between piston and turbine as turbines seem to get mounted on pylons (apart from the Comet of course) whereas recips get mounted in line with the wing (apart from the Ford Tri Motor of course). I think a recip is forward of the leading edge so the the engine can be changed/maintained easily and it doesn't interfere with the internal structure of the wing.

So basically, I haven't got a clue.

[jar66_b2]

Possibly to do with Airflow into the Compressor?

Probably wrong on that but after doing my Turbine Theory for my maintenance exams and reading about compressor stall, it would make sense to have it away from anything that could prevent a good airflow entering the intake (Such as disruption flowing over the aerofoil)

Just my 2p's worth. Feel free to correct me

[Departures Beckham]

I remember reading in an AAIB report that the engines are designed to cleanly detach in the event of sudden ground impact, so as not to cause further damage to the wings and fuel tanks. That may have something to do with it.

[Onan the Clumsy]

an exception to the rule perhaps? (http://www.airliners.net/open.file?id=213589&WxsIERv=QmVyaWV2IEJlLTIwMA%3D%3D&WdsYXMg=QmVyaWV2IERlc2lnbiBCdXJlYXUvTUNoUw%3D%3D&QtODMg=TW9zY293IC0gWmh1a292c2t5IChSYW1lbnNrb3llKSAoVVVCVyk%3 D&ERDLTkt=UnVzc2lh&ktODMp=QXVndXN0IDE3LCAyMDAx&WNEb25u=RmFlcmJlcmcgTGVvbmlk&xsIERvdWdsY=UkEtMjE1MTE%3D&MgTUQtODMgKE=TUFLUyAyMDAxIGFpcnNob3cuIFRoZSBmb2xsb3ctdXAgdG8 gdGhlIEEtNDAgd2l0aCBsb2FkcyBvZiBhZHZhbmNlZCB0ZWNobm9sb2dpZXM gYWRkZWQuIFRoZXkgcmVjaWV2ZWQgdGhlIGNlcnRpZmljYXRlIG9mIHRoZSB 0eXBlIGZvciB0aGlzIHBsYW5lIHRoZSBvdGhlciBkYXkgSSB0b29rIHRoaXM gcGljdHVyZS4%3D&YXMgTUQtODMgKERD=MTQ5Ng%3D%3D&NEb25uZWxs=MjAwMi0wMS0xNA%3D%3D&static=yes&size=M) though it does keep the inlets clear of aerodynamic disturbances (and waves).

[A-V-8R]

As explained to me in 747 and DC-8 Schools, the engings are put on pylons on the wings so the wings can be made thinner. (Stopping the upward flexing during flight).

There are fuse bolts that will allow the engine to be slung so as not to rip the wing off in flight during a high g loading.

Many years ago, 737-200's were dropping engines all over the place. While they worked on a fix, a red line was painted on the aft end of the engine mount and engine; if it didn't line up a fuse bolt broke.

Many carriers have slung engines in flight.....I can recall two, one over Denver and one over Anchorage, both in mountain wave turbulence.

I'm not sure why they are mounted forward.....the early jets didn't do that though.....seems to me the closer the thrust line is to the center of gravity the less pitching moment with the application and reduction of power.

Generally, airlines prefer a loading with an aft cg as it burns less fuel.

[Notso Fantastic]

See if you can track down 'Handling the Big Jets' by D. P. Davies. An amazing book brought out at the dawn of the jumbo age describing the design philosophy. Amazon might be a good place to start, but if libraries don't have it, used bookshops should eventually track it down.

[lomapaseo]

Forget about the ole wifs tale of engines designed to fall off and save the wing.

The engines are designed and demonstrated to stay attached up to and including the worst fan blade failure. The wing critical structure (fuel box structures, spar beams etc.) are designed to sustain a specfic gust load. and turbulence. If you take the aircraft outside these specified envelops and margins than things begin to fail according to design margins.

A good designer will obviously not attempt to design the wing as the weakest link, thus engines are noted to fall off more often than wings when overloading occurs.

The experience with the b737-200 was not a gust or engine overload, it was a mechanic foud up followed by metal fatigue.

The two crashed B747 was also a fatigue problem and not an overload problem.

The two mountain wave problems were an overload problem which the wing but not the pylon survived. (sounds like an adequate design to me)

[wobblyprop]

Jason2000,

An a/c is subject to many aerodymanic loads (gusts, control inputs, etc.) and those forces have different characteristics at various speeds i.e. rate of change and moment. These forces may interact with the a/c structure and have a negative damping affect (which means a force gets bigger over time) and allow flutter.

Wing flutter can be delayed to a higher speed, for a given structural stiffness by having a mass balance. This mass balance comes in the form of the engine mounted on a pylon. This moves the flexing axis forward onto the aerodynamic centre (and why that works i don't know)

does that help?

[knobbygb]

Exception to the rule II (http://www.airliners.net/open.file/302054/M/)

Surely the risks of debris entering the cabin in an uncontrolled failure would be much greater with this configuration? Also, wouldn't engines mounted so high give a marked pitch down on sudden application of power for, say, a go-around?

On the plus side - better ground clearance and errr..... looks distinctive :confused:

[Volume]

Exception to the rule III (http://www10.plala.or.jp/strgzr/aircraft/aska/qc1_03.jpg)

Normally wing aerodynamics are main cause for placing the engines as far low and front of the wing as possible.
Placing them to far up disturbes the airflow at high AOA reducing high lift performance and making leading edge device design (i.e. slats) difficult.
Placing them to far aft affects transsonic drag by violating the area rule.
I´ve seen a diagram of some NACA report showing CL and transonic CW effects of engine position, with some typical airplanes drawn in as example. This was in the design papers of a 70 seat twinjet project that has been cancelled later on. I can´t find it on the web.

[Jason2000]

Ok...thanks for the replies people.

From what many of you guys have said, it's simply to do with the affect on the wing aerodyamics.

Volume ...surely the violation of the area rule case (Sears Haack approximation) only affects high speed 'sonic cruisers' and not the more ocnventional things flying about?

The reason I ask, ironically, is that we are designing a sonic cruiser as part of our 3rd year Aero Eng course at uni and am trying to work out where to locate the engines! So form here, it looks like working out all the cs areas to approximate a Sears Haack body?

Cheers

Jason

[rwm]

Most of the items I've read here are all about sub-sonic aircraft. Anything that goes beyond about MACH .9 has a very diferent design, to reduce drag. because the faster you go to the sound barrier, the more power it takes. So if you want to have a decent payload, you must keep your drag down. This will allow you to have a smaller engine, less fuel burn. Also the design of the intake is a very critical item, and must be designed to prevent shock waves from entering the engine. Most super sonic A/C still use the area rule, giving the apearance of a coke bottle.

Back to engines designed to come off, the engines on most of the early Boeings were designed to do this. The 737 has to forward mounts that are made with a rubber disk and a cone bolt designed to rotate if the engine creates a runaway. the aft mount is also a cone bolt, and they are all designed to shear if too much strain is put on them. There is also a secondary mount made of either a small wire, or later ones have a crushable disk in it.

[northwing]

I think Knobblyprop is closest here. My understanding of it is that putting the engine forward on a swept wing balances out the mass of the wing and makes the wing root bending moment closer to pure bending as opposed to a combination of bending and twisting. This deals in a structurally more efficient manner with the gust loads and is and hence lighter.

[Fil]

From the book 'Wide-Body' about the making of the Jumbo it is mentioned that when a young lad one of the main desgners observed an engine fire in a B17or B29 that lead to the fire burning through the wing with the wing falling off killing all onboard, including the likes of the then Boeing Chief Engineer.

Said designer whose name escapes me never wanted to see that happen again so when Boeing produced the B47 which was one of their first large jets they thought about putting the engines in pods. This was to try to prevent engine fires resulting in wing structural failure.

Furthermore, as mentioned above, the effect of flutter can be reduced by using mass balances on the wings. According to the book Boeing engineers realised that these two factors could add to the safety of an aircraft which hence helped the adoption of engines in pylons for a number of reasons but particularly these two.