geezer From United States of America, joined Aug 2010, 713 posts, RR: 1 Posted (3 months 2 weeks 1 day 11 hours ago) and read 2451 times:
Reading about the various types of flaps on airliners has reminded me of something I have long wondered about; I'm thinking, when designing the wings for a 4 engine plane I'm sure the engineers know how big, and how heavy the whole plane will be, and how much length and area the wings must have to handle that much load..........so, I have always wondered why the outboard engines are so far out the wing from the inboard engines. I have worked around cranes a lot, and as any crane operator knows, for every foot of boom extension, ( at a given up-angle ), there is more leverage, so there is a reduction in lifting capacity. ( Just as every degree in up-angle gives an increase in lifting capacity. )
The wings of a plane being at a "fixed" angle, relative to the fuselage, it would seem that the farther away from the fuselage you place the load from the engines, the more strength the wings would need, and the heavier they need to be to support that amount of load. Obviously, there's a huge difference between airplane wings and crane booms, but any load you place on any structure must have a certain amount of strength to support it. With all this in mind, why do the outboard engines need to be so far from the inboard ones, and for that matter, why do the inboard engines need to be so far from the fuselage ? I realize that every single thing on modern day airplanes is exactly where it needs to be, and that certainly holds true for engine placement as well as every other component of the airplane, but not being an airplane designer or an engineer, I can't keep from wondering about this.
I also realize that the wings create the lift necessary for the plane to be able to fly, so I'm sure much of the answer to my question is that it isn't the fuselage supporting all that load while the plane is flying, but rather the lift; but when that fully loaded 747 is sitting at the gate being loaded with tons and tons of fuel, cargo, and people, obviously there is no lift before the plane takes off. Thinking about all of this, my conclusion is........airplane wings must surely be the most technologically marvelous "structures" ever created by man. ( My "hat" is off to anyone who designs them ! )
Starlionblue From Greenland, joined Feb 2004, 15102 posts, RR: 69 Reply 1, posted (3 months 2 weeks 1 day 10 hours ago) and read 2423 times:
Quoting geezer (Thread starter): ..so, I have always wondered why the outboard engines are so far out the wing from the inboard engines. I have worked around cranes a lot, and as any crane operator knows, for every foot of boom extension, ( at a given up-angle ), there is more leverage, so there is a reduction in lifting capacity. ( Just as every degree in up-angle gives an increase in lifting capacity. )
The wings of a plane being at a "fixed" angle, relative to the fuselage, it would seem that the farther away from the fuselage you place the load from the engines, the more strength the wings would need, and the heavier they need to be to support that amount of load.
Your analogy of a crane doesn't really work because the crane does not have lift acting on it, or to be more accurate your example only works while the aircraft is on the ground. The strain on the wings while at the gate full of fuel is not very big compared to the strain in flight. In flight, the further out the engines, the more they provide bending relief, so actually this contributes to a lighter wing.
Think of it this way. The wing is being lifted along its entire span while in flight. However it is only significantly weighed down at the fuselage junction. Putting the engines further out means a more even weight distribution along the wing, and less requirement on the structure.
The opposing force in design is engine out rudder authority. The further out the engines, the more rudder authority you need. So there's a sweet spot.
Tact Is For People Who Aren't Witty Enough To Be Sarcastic
PapaChuck From United States of America, joined Aug 2010, 112 posts, RR: 0 Reply 2, posted (3 months 2 weeks 1 day 6 hours ago) and read 2322 times:
Engines are also isolated away from the fuselage and from each other for protection. If an engine decides to let go of a fan blade and tear itself apart, it's best to keep it as far from other critical structures/systems as possible. In a paired configuration like the B-52, if one engine goes, there's a good chance the one right next to it could be damaged. Additionally, being positioned under the wing can help protect the fuselage from damage as the wing itself can absorb some of the flying debris.
Chimborazo From United Kingdom, joined Sep 2011, 48 posts, RR: 0 Reply 3, posted (3 months 2 weeks 1 day 6 hours ago) and read 2322 times:
And the further out you can put the engines also helps reduce the landing gear length- and therefore weight- due to the dihedral of (most) wings lifting them further from the ground the further out you go from the A/C centreline.
tdscanuck From Canada, joined Jan 2006, 11033 posts, RR: 72 Reply 4, posted (3 months 2 weeks 1 day 4 hours ago) and read 2254 times:
Quoting geezer (Thread starter): I have worked around cranes a lot, and as any crane operator knows, for every foot of boom extension, ( at a given up-angle ), there is more leverage, so there is a reduction in lifting capacity.
Exactly the same thing happens with wings...except the force diagram is upside down and "backwards" compared to an airplane. You've got a distributed gravity load pulling down on your crane boom plus a point load for the end for the thing you're picking up. A wing has a distributed gravity load, a distributed (non-uniform) lift load, and a point load at the *fuselage* end for the thing the wing is picking up. In flight the lift load is significantly larger than the gravity load so it dominates. The best "crane-boom" model of an aircraft wing is the idea that the fuselage is the thing you're picking up, the engines are sliding weights on your boom, and the "pivot" for the boom is the wingtip. In that frame, it should be clear why having the engines outboard is structurally good.
Quoting geezer (Thread starter): when that fully loaded 747 is sitting at the gate being loaded with tons and tons of fuel, cargo, and people, obviously there is no lift before the plane takes off.
Absolutely. In this case it's better to think of the wing/boom as pivoting at the landing gear attachment point with the distributed gravity load and the engines as point loads...having the engines outboard in this scenario is worse but, since the lift load is bigger than either the wing gravity or engine loads, it's not the dominant load case.
jetblueguy22 From United States of America, joined Nov 2007, 1518 posts, RR: 0 Reply 5, posted (3 months 2 weeks 22 hours ago) and read 2086 times:
Quoting Starlionblue (Reply 1): In flight, the further out the engines, the more they provide bending relief, so actually this contributes to a lighter wing.
Think of it this way. The wing is being lifted along its entire span while in flight. However it is only significantly weighed down at the fuselage junction. Putting the engines further out means a more even weight distribution along the wing, and less requirement on the structure.
If it does provide bending relief why don't twins have engines further out. Would it just put too much stress on the wing and snap it? Or are placing engines closer to the fuselage on a twin just more efficient.
Blue
DC8FriendShip From United States of America, joined Apr 2005, 237 posts, RR: 3 Reply 6, posted (3 months 2 weeks 21 hours ago) and read 2046 times:
Quoting PapaChuck (Reply 2): In a paired configuration like the B-52, if one engine goes, there's a good chance the one right next to it could be damaged.
The B-52 engines were also paired for thrust and aerodynamics. When the air Force was looking at re- engining them with turbofans, they were going to have four high bypass engines in place of the paired tf-33's.
Starlionblue From Greenland, joined Feb 2004, 15102 posts, RR: 69 Reply 8, posted (3 months 2 weeks 16 hours ago) and read 1859 times:
Quoting jetblueguy22 (Reply 5): If it does provide bending relief why don't twins have engines further out. Would it just put too much stress on the wing and snap it? Or are placing engines closer to the fuselage on a twin just more efficient.
As CitationJet says, there's a trade-off. The further out the engines the bigger the asymmetric thrust in an engine out scenario.
It's all compromises. One of my faves is comparing the L-1011 vs the DC-10. Due to its engine placement DC-10 has a smaller rudder. Therefore it had to have engines closer to the centerline, therefore it had to have a beefier wing than it woudl have had with an engine in the fuselage like the L-1011
There's a special case with the 330 and 340, which share a wing. The engines on the 330 are where they are because that's where the pylon is for both 330 and 340. If there had been no 340 perhaps they would have been further out.
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