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Wing Mounted Engine Question  
User currently offlineVidens From Argentina, joined Mar 2004, 133 posts, RR: 0
Posted (10 years 1 month 2 weeks 19 hours ago) and read 4302 times:

I searched the forum, and even though I think it's there, I couldn't find it.
Why are wing-mounted engines mounted forward of the wing? Is it because of structural issues, airflow, etc...
I'd love to know the answer (or the link to it).

thanks;

videns


Travel? Why would i travel if I can watch it on TV?
23 replies: All unread, jump to last
 
User currently offlineA319114 From Netherlands, joined Aug 2004, 541 posts, RR: 3
Reply 1, posted (10 years 1 month 2 weeks 19 hours ago) and read 4263 times:

Although I'm no engineer or something like that, I would guess it has something to do with balance. If you would place the engines more to the back, you also would have to place the rear landing gear more to the back. And since the wingroot is the most economical place to stow the rear gear (no significant amount of cargo space will go lost) the engine has to be placed forward of the wing to prevent tipping over.

Again, I don't know whether that's the reason but it would seem logical to me...



Destruction leads to a very rough road but it also breeds creation
User currently offlineLorm From United States of America, joined Jun 2004, 409 posts, RR: 1
Reply 2, posted (10 years 1 month 2 weeks 19 hours ago) and read 4257 times:

I'm not gonna give an airflow explanation, as that's beyond my education  Wink/being sarcastic. But I would suspect that as most modern wings are built to flex and give quite a bit, that the majority of the weight of the engine is placed ahead of the leading edge, to counteract the flexing during flight, and to minimize wing flutter. I suspect it also has structural implications and also deals with how a plane deals with it's center of gravity. You'll notice a fully fueled plane's wings will droop quite a bit and I'm sure this also is one of the reasons they are positioned more forward of the wing to distribute weight. Earlier aircraft wings weren't built to flex as much as more modern airfoils and that's probably why some earlier designs didn't have podded engines. Podded engines seem to be all the norm these days with commercial airliners, and wing root mounted engines ala Comet have gone the way of the dodo .Let's see what the techno-wizards of the tech forum have to say  Wink/being sarcastic.
-Mike



Brick Windows
User currently offlineKDTWFlyer From United States of America, joined Jun 2004, 830 posts, RR: 1
Reply 3, posted (10 years 1 month 2 weeks 18 hours ago) and read 4215 times:

I found this http://www.cheburashka.5u.com/history.html page that may help. In one part it says ....

To increase the surface of wing under jet stream engines were placed forward of the wing, and the rear of the nacelles were given a form to spread the exhausting gases over the wing more effectively.

This following link deals mostly with props but may also be of use.


http://www.experts-exchange.com/Miscellaneous/Math_Science/Q_21047543.html

Note that the engines are assumed to be mounted forward of the wing’s leading edge in typical fashion to help delay the onset of flutter.
.... I found that in the link below

http://www-psao.grc.nasa.gov/Library/Abstracts/overwing.html

in the paragraph following the first equation



REgaRds,
Tim



NW B744 B742 B753 B752 A333 A332 A320 A319 DC10 DC9 ARJ CRJ S340
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 4, posted (10 years 1 month 2 weeks 14 hours ago) and read 4038 times:

Flutter it is. The wings aren't in the jet wash in the first place.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineAuae From United States of America, joined Apr 2004, 296 posts, RR: 3
Reply 5, posted (10 years 1 month 2 weeks 13 hours ago) and read 3967 times:

Pretty good question,

Seems like there are a variety of positive reasons to place the engines there, but I am not sure that one overrides the other. Airflow would seem to be important, if the engine were further back the inlet would surely get caught in the upwash of the wing. Balance is probably important too, as the engine pylon helps to put the engine ahead of center of gravity. It must help flutter as mentioned above, and I would think it helps subdue wing twist.

From a safety standpoint, it is good to have the first stage fan blad out in front of the wing too. The first stage is the most likely to become uncontained in the likelyhood of a failure, and you definately dont want it ripping into the wing.

What about the flip side? Seems like some negative reasons might be :

In the event of an engine fire, more wing is exposed to danger the further the engine is in front of it. And pylon oscilation, leads to structural fatigue at the wing to pylon joint.

Shawn



Air transport is just a glorified bus operation. -Michael O'Leary, Ryanair's chief executive
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17054 posts, RR: 67
Reply 6, posted (10 years 1 month 2 weeks 11 hours ago) and read 3876 times:

And pylon oscilation, leads to structural fatigue at the wing to pylon joint.

True, but this is more than compensated for by the positive effects of decreased wing twist. The leading edge wants to twist upward due to aerodynamic effects and putting the engine way forward allows for a weaker, lighter wing.


The pylon is usually supported on self-aligning bearings which do not transfer thrust forces to the wing (and thus the airframe). These bearings allow the engine to "hang" loosely off the wing and flop about without twisting metal, thus the issue of metal fatigue is relatively minor. Actual thrust forces are transferred through a thrust link, which also has two hinges to allow the pylon to move independently of the wing.

In the DC-10 case, there are two bearings in the forward pylon bulkhead and one in the rear.



[Edited 2004-08-18 16:58:42]


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineN328KF From United States of America, joined May 2004, 6489 posts, RR: 3
Reply 7, posted (10 years 1 month 2 weeks 9 hours ago) and read 3769 times:

Why not do it like the YC-14? Put the engines so that the exhaust runs over the wing, so that you enable the Coanda effect? Greater lift!


When they call the roll in the Senate, the Senators do not know whether to answer 'Present' or 'Not guilty.' T.Roosevelt
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17054 posts, RR: 67
Reply 8, posted (10 years 1 month 2 weeks 9 hours ago) and read 3745 times:

Like the An-72/74 too.


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Well maintenance gets tricky and you complicate the whole wing structure, as well as the aerodynamic environment on the top of the wing. It can be done, but apart from certain STOL applications, the benefits are outweighed by the disadvantages.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineAuae From United States of America, joined Apr 2004, 296 posts, RR: 3
Reply 9, posted (10 years 1 month 2 weeks 9 hours ago) and read 3734 times:

The pylon is usually supported on self-aligning bearings which do not transfer thrust forces to the wing (and thus the airframe). These bearings allow the engine to "hang" loosely off the wing and flop about without twisting metal, thus the issue of metal fatigue is relatively minor. Actual thrust forces are transferred through a thrust link, which also has two hinges to allow the pylon to move independently of the wing. - Starlionblue

*****

You are missing the fundamentals of physics, mainly that every force has an equal reaction force, unless that body is in motion. In the case of an engine, that bouncing is reacted by the wing. If it wasn't, the engine wouldn't stay attached. Albiet that pylon to wing attachments are pretty complex, every force, dynamic oscillation, thrust, reverse thrust and torque must be reacted by something. Most pylons use point loading attachments to transfer those forces. The forces those points transmit are definitely cyclical, and those attachments must be fatigue resistant.

Shawn

edit: Meant to say, that since just about the whole darn plane needs to be fatigue resistant it really isn't much of a negative!!!

[Edited 2004-08-18 19:21:40]


Air transport is just a glorified bus operation. -Michael O'Leary, Ryanair's chief executive
User currently offlineBuyantUkhaa From Mongolia, joined May 2004, 2899 posts, RR: 3
Reply 10, posted (10 years 1 month 2 weeks 8 hours ago) and read 3721 times:

As a civil engineer, I can see one other obvious advantage of the current design: the sign of the thrust load (pressure vs. tension). In the current situation, thrust forces create tension in the pylon (or, as Starlionblue said, a thrust link), which can be strong but thin, as it will not be bent because of the thrust as it is under tension.

In case the engine would be aft of the wing, it would "push" the wing, and the thrust link (under pressure) would have to have a much larger cross-section to compensate for secondary bending moments and flexing (the "plastic teaspoon effect"), and thus be heavier.




I scratch my head, therefore I am.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17054 posts, RR: 67
Reply 11, posted (10 years 1 month 2 weeks 8 hours ago) and read 3714 times:

Auae, yes I understand that forces are transferred to the wing. Oscillations are partially isolated by the pylon to wing bearings. This is like the suspension in a car. If your car did not have suspension it would vibrate apart.

But we were talking about metal fatigue. The pylon to wing bearing is meant to move, which means the actual bending of metal (causing metal fatigue) is minimized.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineAuae From United States of America, joined Apr 2004, 296 posts, RR: 3
Reply 12, posted (10 years 1 month 2 weeks 6 hours ago) and read 3685 times:

The only moving part on a pylon is the vibration isolators, which on some aircraft are rubber. Other aircraft don't even have those. The oscilations I am talking about are when you hit turbulence, and see the engine bounce.

As for bearings, I don't believe that most bearings on a pylon are spherical. The few that are on only spherical to help align a bolt or prevent eccentric loading on a bolt. They don't prevent cyclic loading which is the root cause of fatigue. Fatigue is not only caused by bending metal, such as with a clothes hanger. Take the actual thrust link, in flight it is under some amount of tension. When you hit turbulence, the engine accelerates up or down, creating more or less tension in the thrust link. That is cyclic loading. That thrust link connects to a fitting on the bottom of the wing, through bearigns and bolts. That fitting is most likely sized for fatigue, not ultimate strength.

Shawn



Air transport is just a glorified bus operation. -Michael O'Leary, Ryanair's chief executive
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17054 posts, RR: 67
Reply 13, posted (10 years 1 month 2 weeks 5 hours ago) and read 3671 times:

Ok good explanation  Big grin I stand corrected.


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineThrust From United States of America, joined Sep 2003, 2690 posts, RR: 10
Reply 14, posted (10 years 1 month 2 weeks 4 hours ago) and read 3646 times:

Engines are mounted on the wing for several reasons...number one, they help hold the wing "down" against upward lift forces...number two, they produce less drag than if the were put aft of the cabin. Also, if they are mounted on the wings, the wings can be made structurally lighter, saving money and weight.


Fly one thing; Fly it well
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17054 posts, RR: 67
Reply 15, posted (10 years 1 month 1 week 6 days 5 hours ago) and read 3554 times:

You are missing the fundamentals of physics, mainly that every force has an equal reaction force, unless that body is in motion. In the case of an engine, that bouncing is reacted by the wing. If it wasn't, the engine wouldn't stay attached. Albiet that pylon to wing attachments are pretty complex, every force, dynamic oscillation, thrust, reverse thrust and torque must be reacted by something. Most pylons use point loading attachments to transfer those forces. The forces those points transmit are definitely cyclical, and those attachments must be fatigue resistant.

I've been thinking about this for about a day now (ok I did break for sleep and work and stuff  Big grin and I figured out why it didn't sound completely right, and yet not completely wrong.

Here are my thoughts: All forces from the engine and pylon must be transferred to the wing but this does not have to happen all at once. The function of the pylon assembly is thus to flop around and slowly transfer energy. If it could be transferred all at once we might as well have a rigid pylon/wing interface, which would have to be much heavier and stronger.

Same thing with the wings. They flap so that their energy is transferred slowly to the fuse (and, incidentally, the engines). Energy is retained in the flapping motion. As the flapping decreases, the energy is slowly transferred to the fuse.

This is like the suspension in a car. It stores energy so that it can be transferred gently to the car instead of just passing along every little bump in the road.


So yes, the force are of course transferred, but the pylon mounting shields the wings from the worst of the forces. Flopping about of the engine is a sort of energy storage. The energy is stored so that it can slowly leech to the wing.


Or I may have it all wrong  Smile/happy/getting dizzy But I'm sure someone here has a degree in physics!



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineMERSPACE From United States of America, joined Mar 2004, 56 posts, RR: 0
Reply 16, posted (10 years 1 month 1 week 6 days ago) and read 3514 times:

Very good thread !! -- some very good technical discussions.
From an aeroelasticity text book some 40 years ago, I seem to remember
the following:
1) Pod mounted engines (B47, B52, 707, DC-8, 747, etc) distribute the engine weight more uniformly along the wing. This reduces the wing's strength requirement, hence lower wing weight.
2) Engines below and in front really reduces the wing twist, as several
people have already noted. Front mounted engines create a stable self correcting dynamic response to gusts, etc. Rear mounted engines could easily lead to a divergent dynamic response, with resultant failure of the wing.
3)The early jets (B47, B52 707, etc.) actually were designed to have the engine pods fall off the aircraft in case of catastrophic engine
probems.


User currently offlineCanadianNorth From Canada, joined Aug 2002, 3390 posts, RR: 9
Reply 17, posted (10 years 1 month 1 week 5 days 18 hours ago) and read 3485 times:

Not all wing mounted engines are placed infront of the plane...


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Just thought I'd point that out...
CanadianNorth



What could possibly go wrong?
User currently offlineL-188 From United States of America, joined Jul 1999, 29802 posts, RR: 58
Reply 18, posted (10 years 1 month 1 week 5 days 17 hours ago) and read 3470 times:

Actually the 737-200 engine installation is a mod CanadianNorth.

The orginal tailpipe did not extend past the trailing edge of the wing on that aircraft, however when it entered service it was discovered that the orginal engine nacelle design was causing the aircraft to get light on the mains when the reversers where used, Extending the tailpipe aft allowed the reversers to dump their air aft of the wing, and not cause the airplane to "Float"

It also had the added bennie of making the engines exhause quieter since it has a longer pipe to run through.


Before the mod


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After the mod


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Sorry I could only find a photo of a sister ship.



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User currently offlineCanadianNorth From Canada, joined Aug 2002, 3390 posts, RR: 9
Reply 19, posted (10 years 1 month 1 week 5 days 6 hours ago) and read 3427 times:

L-188... I know, but still  Smile



CanadianNorth



What could possibly go wrong?
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 20, posted (10 years 1 month 1 week 5 days 1 hour ago) and read 3398 times:

Because it is prettier than this:

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Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineThrust From United States of America, joined Sep 2003, 2690 posts, RR: 10
Reply 21, posted (10 years 1 month 1 week 4 days 7 hours ago) and read 3296 times:

Yea, I remember reading that engines in case of failures had their pylons designed to break so they could fall free of the wing in case of a catastrophic engine failure. But this was a fatal mistake...I thought several Boeing 747s were downed because engines came off the wing in flight??? The new pylon design keeps the engine on the wing even after a catastrophic failure.


Fly one thing; Fly it well
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17054 posts, RR: 67
Reply 22, posted (10 years 1 month 1 week 3 days 23 hours ago) and read 3253 times:

The most prominent 747 to go down following the discarding of engines was the Israeli one in The Netherlands

http://aviation-safety.net/database/1992/921004-2.htm



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineL-188 From United States of America, joined Jul 1999, 29802 posts, RR: 58
Reply 23, posted (10 years 1 month 1 week 3 days 21 hours ago) and read 3242 times:

There was that one, and then there was the one here in Anchorage that dropped one on take off.


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They made it back though.






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