TimePilot From Switzerland, joined Sep 2005, 295 posts, RR: 0 Posted (3 years 6 months 1 week 3 days 2 hours ago) and read 8248 times:
Yesterday I was out having a cigarette when a 737 flew overhead. I notice that on it, (and indeed other jets) the engines stick out forward from the wings.
Is this for effeciency? Asthetics? If the hung off the back would it interrupt air flow?
Speedracer1407 From United States of America, joined Dec 2004, 333 posts, RR: 0 Reply 1, posted (3 years 6 months 1 week 3 days 1 hour ago) and read 8230 times:
Two reasons come to mine.
1. Ground clearance: Mounting the engines forward of the wings means they can also ride pretty high. Ever since big-fan engines arrived, they've been cantilevered way out in front of the wing. The 737 is a perfect example. Originally designed with low bypass, small diameter JT8-D mounted in the traditional (for the time) place directly under the wing, the MLG was nice and short. When the higher bypass, larger diameter CFM-56 was fitted to the -300s, they were mounted way the hell forward of the wing and high enough than the top of the nacelle was nearly level with the upper surface of the wing to allow adequate clearance.
2. Aerodynamics: I seem to recall reading on this forum that moving the engines forward of the wing had the initially unexpected effect of improving airflow under (and possibly over) the wing. I'm sure someone more knowledgeable than me can elaborate.
I guess it's worth noting that aesthetics would never influence something as important to aircraft design as engine placement.
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
Pilotpip From United States of America, joined Sep 2003, 3084 posts, RR: 12 Reply 2, posted (3 years 6 months 1 week 3 days 1 hour ago) and read 8229 times:
Mainly for aerodynamics. They also aren't truly "straight out", they're canted to allow the best airflow at their location. A good example of this is the DC-9/MD-80.
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 4, posted (3 years 6 months 1 week 3 days ago) and read 8168 times:
Quoting TimePilot (Thread starter): Is this for effeciency? Asthetics? If the hung off the back would it interrupt air flow?
In addition to all of the above, it keeps most of the rotating bits from being in line with the fuel tank. You still find dry bays sometimes to protect particular areas of the wing, but there's no real way to meet the rotor burst requirements without taking a big fuel capacity hit if you park the engine right under the wing.
TSS From United States of America, joined Dec 2006, 2870 posts, RR: 5 Reply 5, posted (3 years 6 months 1 week 2 days 23 hours ago) and read 8136 times:
Some early jet aircraft like the De Havilland Comet
had their (low-bypass) engines mounted inside the wing (I'm sure our own 2H4 can think of other examples of aircraft with engines mounted inside the wing, but the Comet is the only one I could remember).
[Edited 2009-11-17 22:13:30]
Able to kill active threads stone dead with a single post!
DocLightning From United States of America, joined Nov 2005, 16834 posts, RR: 57 Reply 6, posted (3 years 6 months 1 week 2 days 22 hours ago) and read 8106 times:
Quoting Speedracer1407 (Reply 1):
2. Aerodynamics: I seem to recall reading on this forum that moving the engines forward of the wing had the initially unexpected effect of improving airflow under (and possibly over) the wing. I'm sure someone more knowledgeable than me can elaborate.
Now, these are models of aircraft that use the Whitcomb Area Rule of maximum efficiency for transsonic and high-subsonic aircraft. The basic idea is that the area of the cross-section of the aircraft should change gradually, even if locally it chances abruptly. So you can stick big pods out in front or in back of the wing to soften up the sharpness of the increase in cross-section starting at the wing root. This is also why the A380 has that odd shape to its wing box.
The Comet posted above was not very good about the Area Rule. The wing was completely smooth and the engines were embedded, which looks really pretty (and that's all engineers had to work with then; the true value of the Area Rule wasn't appreciated), but is actually really unaerodynamic.
Today, those odd pods that you see have been replaced by massive engines slung well forward of the wing. They soften the sudden increase in the cross-section of the aircraft. And those huge flap fairings stick out behind the wing, way too large to house the machinery they contain, but they serve as antishock bodies, soften up the decrease in the cross-section of the wing.
Vc10 From United Kingdom, joined Nov 2001, 1382 posts, RR: 17 Reply 7, posted (3 years 6 months 1 week 2 days 19 hours ago) and read 8041 times:
Quoting TSS (Reply 5): had their (low-bypass) engines mounted inside
There was no by-pass low or otherwise on these engines, as they were just good old straight through turbo jets. Even the Comet 4 with RR Avons was a straight through jet
The British V bombers all had their engines buried in the wing
If they hung off the back they'd have to be above the wing, otherwise the aircraft wouldn't be able to rotate to take off, unless the undercarriage was a lot longer. Engines above the wing reduce wing efficiency and the mounting structure needs to be stronger. They're also more likely to be prone to the effects of "bad air flow" generated by the wing, in a stall for example.
The VFW614 is the only example of a civil plane with engines above the wings.
Starlionblue From Hong Kong, joined Feb 2004, 15873 posts, RR: 66 Reply 9, posted (3 years 6 months 1 week 2 days 16 hours ago) and read 7974 times:
Quoting Oly720man (Reply 8): Apparently there is some structural benefit in having the wings forward in that they act as a mass balance against flutter.
Quite. Hanging the engines up front like that makes them counter the wing's tendency to twist.
Oly720man, thanks for posting the dreaded VFW-614. Only of my favorite ugly planes.
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo
F14D4ever From United States of America, joined May 2005, 319 posts, RR: 4 Reply 10, posted (3 years 6 months 1 week 2 days 16 hours ago) and read 7941 times:
Quoting Oly720man (Reply 8): Apparently there is some structural benefit in having the wings forward in that they act as a mass balance against flutter.
Quoting Starlionblue (Reply 9): Quite. Hanging the engines up front like that makes them counter the wing's tendency to twist.
Flutter and twist are two distinct phenomena. Flutter is periodic wing flex due to bending moment along the span. Twist is torsional deformation of the wing along its span under static (more or less) load.
Hanging the engines forward of the wing relieves twist moment, as Starlionblue pointed out. Oly, I'm not sure how an axial (forward) shift of engines relieves flutter. Can you elaborate or provide reference? Thanks.
F14D4ever From United States of America, joined May 2005, 319 posts, RR: 4 Reply 13, posted (3 years 6 months 1 week 2 days 14 hours ago) and read 7878 times:
No go on the google books link, but the Hondajet paper was interesting.
Thanks.
Ex52tech From United States of America, joined Dec 2006, 559 posts, RR: 1 Reply 14, posted (3 years 6 months 1 week 2 days 4 hours ago) and read 7582 times:
The Avro C102 jetliner was developed in canada in the late 40's but didn't go into production. It's engines were podded and mounted through the wing similar to the B-45, Tu16/Tu104.
The C102 was smaller than the Comet.
"Saddest thing I ever witnessed....an airplane being scrapped"
1. Ground clearance: Mounting the engines forward of the wings means they can also ride pretty high. Ever since big-fan engines arrived, they've been cantilevered way out in front of the wing. The 737 is a perfect example. Originally designed with low bypass, small diameter JT8-D mounted in the traditional (for the time) place directly under the wing, the MLG was nice and short. When the higher bypass, larger diameter CFM-56 was fitted to the -300s, they were mounted way the hell forward of the wing and high enough than the top of the nacelle was nearly level with the upper surface of the wing to allow adequate clearance.
2. Aerodynamics: I seem to recall reading on this forum that moving the engines forward of the wing had the initially unexpected effect of improving airflow under (and possibly over) the wing. I'm sure someone more knowledgeable than me can elaborate
Not to mention handling and balance benefits: when your wing is installed substantially on the center of the fuselage (as it should be to facilitate CG limits management and improve control harmony) forward engine installation provides more weight towards the nose of the airplane thereby contributing to better longitudinal stability. This facilitates handling/recovery in the stall.
Keta From Germany, joined Mar 2005, 440 posts, RR: 0 Reply 18, posted (3 years 6 months 1 week 1 day 13 hours ago) and read 7335 times:
Quoting Faro (Reply 17): forward engine installation provides more weight towards the nose of the airplane thereby contributing to better longitudinal stability. This facilitates handling/recovery in the stall.
Forward or aft the wing, it doesn't matter in terms of location of center of gravity. I mean, it's not that you have your fuselage and wing balanced, and then you put the engines thus moving the CoG; rather, you would think about where the engines are going and then move the wing so the center of lift and the center of gravity are where you want. I don't know if I'm being sufficiently clear... What you're saying would mean that aircraft with tail-mounted engines would have worse stall behavior? Or am I missing something?
Western727 From United States of America, joined Jan 2007, 579 posts, RR: 4 Reply 19, posted (3 years 6 months 1 week 1 day 11 hours ago) and read 7282 times:
Quoting DocLightning (Reply 6): The Convair 990, I believe, was the first to have this
Fascinating...however, didn't the 880 come out before the 990 did?
DocLightning From United States of America, joined Nov 2005, 16834 posts, RR: 57 Reply 20, posted (3 years 6 months 1 week 1 day 6 hours ago) and read 7205 times:
Quoting Western727 (Reply 19):
Fascinating...however, didn't the 880 come out before the 990 did?
Yes, but the 880 didn't have the antishock bodies on the wings.
Western727 From United States of America, joined Jan 2007, 579 posts, RR: 4 Reply 21, posted (3 years 6 months 1 week 1 day 4 hours ago) and read 7179 times:
Quoting DocLightning (Reply 20): Yes, but the 880 didn't have the antishock bodies on the wings.
9VSIO From United Kingdom, joined Dec 2006, 643 posts, RR: 2 Reply 24, posted (3 years 6 months 6 days 5 hours ago) and read 6821 times:
Quoting Avt007 (Reply 23): Yes, but they used the Spey, which only has a 1:1 bypass ratio. RR, however, describes it a "medium bypass engine".
Not the latest Nimrod, the MRA4. Rolls-Royce BR710. Means the intakes are now a lot wider than the old Nimrod.
Me: (Lining up on final) I shall now select an aiming point. || Instructor: Well, I hope it's the runway...
25 Avt007: They must be a lot quieter than the old Nimrods. That aircraft is the loudest I've ever heard at an airshow, including the Harrier and Concorde!
26 Cpd: But this had its own problems. How do you predict airflow into the engines mounted in the wing-root? Further more, the structure here would be quite
27 Tdscanuck: Wind tunnel. Today you'd do it with CFD, but that probably wasn't an option then. True, although that might be an advantage because the structure at
28 Zkpilot: Mounting them forward also helps with an aircraft's CoG, not to mention the coupling between thrust/drag and lift/gravity in flight.
29 Cpd: I think they figured back then it was a way of improving aerodynamics, but the B707/Dash-80 with heavily swept back wings was seriously speedy - prov
30 Scooter01: Here, they freed up space inside the wing for more fuel by mounting 2 of the 6 Allison J-35s on pylons, leaving 4 engines inside the wing: The one and
31 Keta: Again? (See reply 17.) I'll repeat myself, unless I'm missing something it has no effect. Could you elaborate this? I don't get what you mean.
32 TSS: Perhaps he's referring to how with engines mounted in pods beneath low-mounted wings an increase in thrust tends to make the aircraft want to assume
33 Zkpilot: Sorry missed your post. Part of what I was talking about is that if you removed the engines from a lot of aircraft they will actually tip back onto t
34 Keta: That's right, moments should be balanced. But even if the engines weren't mounted in front of the wings, you could equally balance the plane, I don't
35 413X3: but wing design on tail mounted engines is different than engines mounted on the wings. So yes engines mounted at different places has a different eff
36 Keta: Wing design indeed is different and I never said the contrary. But the difference relies on structure and aerodynamics, not on the CoG being located
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What you're saying would mean that aircraft with tail-mounted engines would have worse stall behavior? Or am I missing something?
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