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Consider a big commercial airliner, like a 727, 747, or a 787.

At cruising altitude, under standard conditions, how much of the lift of the aircraft comes from the wings, and how much from the rest of the airframe -- the tube that holds all the people?

My general impression from watching airplanes in flight is that the velocity vector of the centre of mass of the airplane tends not to be pointing in the same direction as the nose of the aircraft. They're usually a little off, moreso during take-off and landing but it seems like they're almost always a little off.

When I look at a side-view of a 747, I see that the angle of attack of the airfoil doesn't quite match the line of the tube part of the hull. Moreover, the tube has a pretty large surface area relative to the wing area.

enter image description here

My suspicion is that there's maybe stability reasons why you'd want the tube to not be flat in steady-state flight. And perhaps that's part of the reason why the bottom of the tube is more flat towards the front Is something like this what's going on?

enter image description here

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The term for heavier than air vehicles which get a substantial part of their lift from the main body is "lifting bodies", and it is generally used to distinguish that category from most air planes. –  dmckee Apr 22 '12 at 23:59
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The lift fraction given by the fuselage is generally between 0.1 and 0.2. See, for example, the second slide of this presentation or figure 7 of this paper‌​. –  mmc May 20 '12 at 21:12

3 Answers 3

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mmc gave the answer in his comment to the original question above. Generally on such big airliners, between 10% and 20% of the lift is generated by the body rather than the wings. See the paper he linked to, figure 7.

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20% is an extremely large lift fraction for a fuselage. The fuselage is a very inefficient lift generator, meaning it generates much more drag per lift than a wing. The lift generated by the fuselage is not designed that way. Rather you try to minimize drag for the fuselage, and any lift it generates is just a result of its orientation / shape. What I mean is, you don't design an airplane saying it's going to generate x% of its lift through its fuselage. Once the plane has been designed and tested, you find out what portion of the lift is fuselage-dependent –  Pranav Hosangadi Nov 16 '13 at 15:30

Most of the lift comes from the main wing, and in fact the tail lifts down, so the main wing also has to support that. (That's for a stability reason.)

The lift of a wing is roughly proportional to two things:

  • angle of attack, and
  • airspeed squared

so, the slower an airplane is flying, the more it raises the nose.

You will notice this the next time you fly.

At cruising speed, the plane is at around 300 knots (a knot is about 1.16 mile per hour), and it is pretty flat, with an angle of attack in the range of 1-2 degrees. (At altitude, 300 knots corresponds to a much higher ground speed, due to the thinner atmosphere, but that doesn't change the lift relationship.)

When the plane is maneuvering in the approach pattern, its airspeed is more like 150 knots, half of cruise speed. So it has to have roughly 4 times as much angle of attack, anywhere up to about 8 degrees, thus the high nose.

The maximum angle of attack is around 19 degrees, at which the wing stops working. The crew has to stay well below that in order to have reserve lift in case they need to pull up suddenly, like if they hit a downdraft or wind shear, or if they have to turn quickly.

That beautiful photograph of the 747 was taken by another plane flying in formation with it, and for a photo shoot it was probably not travelling at cruising speed. (It's also not very high, unless that's the Himalayas in the background.)

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Minor point: air density has exactly nothing to do with the difference between air speed and ground speed. Air flow velocity is the important parameter there. –  Colin K May 17 '12 at 18:42
    
I also just want to add, some planes are designed so that the wings have a positive angle of attack while the fuselage (or at least the cabin floor) is level. –  Colin K May 17 '12 at 18:45
    
@Colin: I was trying (unsuccessfully) not to say too much. Instead of airspeed I should have said indicated airspeed, which is what matters aerodynamically, and at higher altitude, true airspeed does increase for the same indicated airspeed, simply because the air is less dense. For your second point, you're absolutely right. –  Mike Dunlavey May 17 '12 at 20:27
    
@Colin: Let me draw your attention to indicated airspeed and the statement "the aircraft will always stall at the same indicated airspeed, regardless of density altitude" (which is what I was taught as a pilot). –  Mike Dunlavey May 17 '12 at 20:58
    
Thanks for the response Mike. I'm fairly certain that's and old Canadian Pacific plane, so that's likely the Canadian Rockies, likely Mt. Robson in the background. To get back to the point, is there a smart way one could compute the amount of lift generated by the airframe other than the wings? –  Ryan Budney May 17 '12 at 22:17

What Mike Dunlavey said. Airliners are designed for maximal fuel efficiency in cruise, and having the fuselage generate lift is very inefficient, because is has a much lower lift to drag ratio than the wings. My guess is the designers try to have the fuselage generate close to zero lift during cruise.

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mmc's response indicates this isn't true -- a substantial part of the lift is generated by the non-wing parts of the plane. –  Ryan Budney May 20 '12 at 22:50
    
I stand corrected. –  user1631 May 21 '12 at 17:58

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