Lehpron From United States of America, joined Jul 2001, 7028 posts, RR: 20
Reply 3, posted (12 years 6 months 6 days 2 hours ago) and read 2147 times:
I don't think the tilted wings upward would do anything, although it is a good point I did not take into consideration.
Optical illision, yes; and my butt. Slats or no-slats, the engines look off aligned; ( I am concentrating on a line from the center of intake to turbine cone) where the inboard engines appear to blow into the flaps and the outboard's blow into the ground.
Can someone find another pix of this exact plane in another view to help this guy?
The meaning of life is curiosity; we were put on this planet to explore opportunities.
AJ From Australia, joined Nov 1999, 2399 posts, RR: 25
Reply 4, posted (12 years 6 months 6 days 2 hours ago) and read 2134 times:
They might well be rigged to reduce wing bending moment, allowing lighter materials to be used. By toeing the outboard engines down they will produce a downwards force on the wing, balancing the upwards lift bending moment. Most jets with wing mounted engines toe the engines in to reduce the outward pulling effect, so I can understand why they'd apply the same to the upward movement.
PW100 From Netherlands, joined Jan 2002, 2705 posts, RR: 17
Reply 7, posted (12 years 6 months 5 days 9 hours ago) and read 2012 times:
Lehpron To me it looks as if 1) the fuselage is tipped forward, 2) the inboard engines are tipped downward and 3) the outboard engines are tipped upward. Is this right?
Your observations look pretty correct to me.
Before going into detail, please be reminded that this is a [ultra] long range aircraft. Aircraft configuration is fully optimized for long range cruise flight.
Furthermore be reminded that "tipped forward", "tipped downward", "tipped upward" are all relative terms. When the aircraft is in cruise, most likely the fuselage will have a couple of degrees incidence [typically 2-5 for commercial airliners] where the fuselage will generate a small amount of lift as well.
Usually the landing gear of an aircraft will be set up in such a way to position the fuselage a little nose down when the aircraft is on the ground. This reduces the angle of attack of the wing during take-off roll, thus reducing drag as speed increases upto the point of rotation, thereby reducing the length of the take-off roll. Drag is a byproduct of lift. So during the take-off roll you really want to reduce lift and thus drag.
Generally the engine nacelles [at least the inlet portion of them] will be angled to allow the airflow to enter the inlet in a straight line along the axis of the fan, to optimize inlet airflow. Now it's important to bear in mind that the inboard section of the wing generates more lift than the outboard part of the wing. As a result of the generation of lift, the air approching the wing will experiance an up-wash. That upwash is greater at the inner engine pair than at the outer engine pair. To cope with this upwash [in cruise flight] and have the air enter the inlet in a nice straight and level manner, the inner engine pair is "tilted downward". The upwash in front of the outer engine pair is much smaller, therefore they are angled differently in relation to the inner pair ["tipped upward"].
This also applies to lateral offset of the inner engines. If you would look at a top view, you'll notice that engines [especially those close to the fuselage, like 767/777] are a little towed in, to cope with the air that is beign forced sideways outboard by the fuselage. On rear fuselage engines you'll see that the engines ar towed out, since the fuselage shape curves the airstream back to the fuselage centreline due to the narrowing of the fuselage at the rear of the aircraft.
This "tipping" has nothing to do with wing bending moment. Only the weight of the engine [and fuel load off course among others] will significantly affect wing bending moment. Imagine that you tilt the engine downward in order to reduce wing bending moment. Now when the engine is thrusting downward, it will eat up part of the lift that is being generated by the wing. Therefore the wing now needs to generate even more lift to balance out the 'negative lift' being produced by the downward thrusting engines!! This doesn't make sense at all in terms off alleviating wing bending moment.
Wing tilting when loaded is an interesting concept. Although I doubt that a wing will tilt sufficiently to align the inner and outer engine pair.
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