Videns From Argentina, joined Mar 2004, 133 posts, RR: 0 Posted (8 years 9 months 1 week 1 day 16 hours ago) and read 3569 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?
A319114 From Netherlands, joined Aug 2004, 541 posts, RR: 3 Reply 1, posted (8 years 9 months 1 week 1 day 16 hours ago) and read 3530 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
Lorm From United States of America, joined Jun 2004, 409 posts, RR: 1 Reply 2, posted (8 years 9 months 1 week 1 day 16 hours ago) and read 3524 times:
I'm not gonna give an airflow explanation, as that's beyond my education . 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 .
-Mike
KDTWFlyer From United States of America, joined Jun 2004, 807 posts, RR: 1 Reply 3, posted (8 years 9 months 1 week 1 day 15 hours ago) and read 3482 times:
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.
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
Auae From United States of America, joined Apr 2004, 296 posts, RR: 4 Reply 5, posted (8 years 9 months 1 week 1 day 10 hours ago) and read 3234 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
Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66 Reply 6, posted (8 years 9 months 1 week 1 day 8 hours ago) and read 3143 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." - from Citadel by John Ringo
N328KF From United States of America, joined May 2004, 6222 posts, RR: 3 Reply 7, posted (8 years 9 months 1 week 1 day 6 hours ago) and read 3036 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
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." - from Citadel by John Ringo
Auae From United States of America, joined Apr 2004, 296 posts, RR: 4 Reply 9, posted (8 years 9 months 1 week 1 day 6 hours ago) and read 3001 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
BuyantUkhaa From Mongolia, joined May 2004, 2779 posts, RR: 3 Reply 10, posted (8 years 9 months 1 week 1 day 5 hours ago) and read 2988 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.
Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66 Reply 11, posted (8 years 9 months 1 week 1 day 5 hours ago) and read 2981 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." - from Citadel by John Ringo
Auae From United States of America, joined Apr 2004, 296 posts, RR: 4 Reply 12, posted (8 years 9 months 1 week 1 day 3 hours ago) and read 2952 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
Thrust From United States of America, joined Sep 2003, 2673 posts, RR: 11 Reply 14, posted (8 years 9 months 1 week 1 day ago) and read 2913 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.
Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66 Reply 15, posted (8 years 9 months 1 week 2 hours ago) and read 2821 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 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 But I'm sure someone here has a degree in physics!
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo
MERSPACE From United States of America, joined Mar 2004, 56 posts, RR: 0 Reply 16, posted (8 years 9 months 6 days 21 hours ago) and read 2781 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.
L-188 From United States of America, joined Jul 1999, 29350 posts, RR: 62 Reply 18, posted (8 years 9 months 6 days 14 hours ago) and read 2737 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.
SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 71 Reply 20, posted (8 years 9 months 5 days 22 hours ago) and read 2665 times:
Thrust From United States of America, joined Sep 2003, 2673 posts, RR: 11 Reply 21, posted (8 years 9 months 5 days 4 hours ago) and read 2563 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.
Sponsor Message: 
From Argentina, joined Mar 2004, 133 posts, 
From Netherlands, joined Aug 2004, 541 posts, 
From United States of America, joined Jun 2004, 409 posts, 
. 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 
From United Kingdom, joined Feb 2002, 2184 posts, 
From Hong Kong, joined Feb 2004, 15870 posts, 
From Mongolia, joined May 2004, 2779 posts, 
I stand corrected.
But I'm sure someone here has a degree in physics!
From Canada, joined Aug 2002, 3371 posts, 
Sponsor Message: 
Printer friendly format
