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Hobbit
2nd Mar 2004, 00:04
Big interview coming up and I would like to know why the engines on an under-wing, podded installation are so far forward. Any suggestions please?

Crossunder
2nd Mar 2004, 00:26
1. Undisturbed airflow into the engines
2. Mass balance for the wings (prevent flutter)

JAFCon
15th Mar 2004, 16:44
a few reasons spring to mind,

1) should the engine have an uncontained failure, parts should not enter the wing fuel tank area of the wing making a bad situation worse.

2) should the engine decide that it no longer wishs to go round and seizes and due to the torque of the engine it should rip its self off the wing the fuse pins should fail in such a manner that as it departs from the wing it should do so in a manner that due to its forward position will not excessivlly damage the wing and wing fuel tanks.

747FOCAL
15th Mar 2004, 16:56
JAFCon,

Then what about Airbus airplanes? Boeing designs the engines such that they can leave the airplane. Airbus designs the strut such that they are strong enough to hold the engine no matter what. Don't look at flight 587 though as both engines left the airframe in a hurry way before the crash.

Crossunder got it right. But it also augments lift. :)

Crossunder
16th Mar 2004, 12:22
Yup. Forgot the augmented lift part. I believe the B737-200 had to split the flaps in order to acommodate the fuel-to-noise-converting pipelines and didn't get the (partially) blown flaps like the later 737 versions have...

stagger
16th Mar 2004, 13:26
747FOCAL wrote...Then what about Airbus airplanes? Boeing designs the engines such that they can leave the airplane. Airbus designs the strut such that they are strong enough to hold the engine no matter what.
Err...are you sure about this? Don't they both have fuse pins?

747FOCAL
16th Mar 2004, 14:32
stagger,

Thats what I was always told. Not really any way for me to check it out without putting one of the Airbus planes through some 9g moves. :E Could be wrong though. Somebody around here should know for sure.

Crossunder,

Actually, they had to extend the tailpipe exit plane beyond the flaps. The first time Boeing flight tested the 737 it went airborne again after the pilots applied reverse thrust on landing. :} Bet that got their hearts thumping as if that was done at Renton Field their is no buffer between the end of the runway and city streets and buildings, at Boeing Field about the same. :ooh:

Jhieminga
16th Mar 2004, 14:43
I recall that after the 747 design Boeing also went back to wanting to keep the engines on the aircraft no matter what. I think it was one of the questions that also popped up after the El Al 747 crash on Amsterdam.

Flight Detent
17th Mar 2004, 02:37
Errr......

Centre of lift / centre of gravity considerations.

those engines are heavy 'mothers',

Cheers

Jet II
17th Mar 2004, 08:16
If the engines are put directly under the wing -- apart from the problems of cluttering up the airflow surface you also start to get problems with ground clearance. To alleviate this you would need longer landing gears which lead to larger gear bays and therefore more weight.

Have a look at the 737 - if you put the engine directly under the wing the landing gear would need to be extended by about 2 foot.

Another point is that with the engine so far forward it allows easy access all around the engine for maintenance.

arcniz
18th Mar 2004, 10:20
Isn't there some easy gain in overall stability from having the thrust moment forward of the wing chord?

Jhieminga
18th Mar 2004, 10:34
Errr......

Centre of lift / centre of gravity considerations.

those engines are heavy 'mothers',

Cheers

True, but remember that this design philosophy emerged in the 707 days. A JT3 weighs less than a CF6 does, and obviously worst case scenario from the designers point of view would be one leaving, not the whole lot (experience has taught us different opinions since then). In such a case the CofG would move aft but not by so much to make an emergency landing impossible.

And with the then available knowledge of jet engine reliability and behaviour when misbehaving the standpoint "I don't want that on my airplane anymore" would perhaps have its merits.