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What are the rationale behind this anhedral high mounted wing configuration? Is that usual for very large carge aircraft?

enter image description here

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Being definitely an ultralight guy, I ask for this time a question related to a huge one :-) –  menjaraz Jun 12 at 6:51
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i think that the effect is exaggerated on the ground, wings will flex upwards in flight. i.imgur.com/lqN2xaW.jpg –  MikeFoxtrot Jun 12 at 7:27
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6 Answers 6

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The An-225 was initially developed to be the transport aircraft for the russian space shuttle, the Buran

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Image from Wikipedia

This design criteria most notably led to the particular tail design, as the wake of the Buran would have made a classical single rudder/empennage design totally uncontrollable.

The requirement for the cargo compartment has probably led to the high-mounted wings (shorter gear legs, easiness of un/loading).

The predicted Buran wings interference has thus probably led to the anhedral part, but I cannot find any official source on this. Another possibility is that this is a by-product of structural limitations of such large high-mounted wings.

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I find your anhedral explanation hard to believe, as the An-124 (which the -225 was based on) has the same degree of anhedral. –  egid Jun 12 at 18:08
    
@egid I don't require anyone to believe them, I think that I clearly stated probably (but I cannot find any official source on this) and Another possibility. Plus, the An124 is another high-mounted wing cargo plane, the second hypothesis would hold. –  Federico Jun 12 at 18:40
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The anhedral has absolutely nothing to do with the dorsal cargo mount. Only the twin tail does (Shuttle Carrier Aircraft has additional vertical stabilizers for the same purpose). –  Jan Hudec Jun 12 at 19:56
    
In fact the dorsal mount would require less anhedral, because high centre of gravity makes the aircraft less stable and so does the anhedral. And your second theory makes no sense as well. There are no structural limitations for amount of anhedral/dihedral; the wing spars carry the same load either way. –  Jan Hudec Jun 12 at 20:04
    
@JanHudec thank you for the clarifications, but please note as I am not considering simply a dorsal cargo mount, but the dorsal cargo's wings. While this might still be incorrect, is not exactly what you wrote. –  Federico Jun 13 at 7:51
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Anhedral wings will induce roll instability and improve roll maneuverability.

In a large/heavy airplane with a high-wing configuration there is usually excess roll stability, so this type of wings can be pretty common.

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+1. The C5 Galaxy is another aircraft with a noticeable anhedral. –  voretaq7 Jun 12 at 14:34
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So it's a matter of trade off between stability and maneuverability? –  menjaraz Jun 12 at 15:02
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Exactelly. It's the matter of anhedral vs. dihedral, of roll maneuverability vs roll stability, of dutch roll vs tendency to spiral dive. Most of aerodynamic design is trading one benefit(or penalty) for another –  Radu094 Jun 12 at 19:02
    
Even not so large high-wing aircraft have significant anhedral. For example BAe146/AvroRJ. –  Jan Hudec Jun 12 at 19:51
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The anhedral is rather exaggerated in the top photo.

If you look for pictures of the Mriya in flight the wings are more-or-less level. On the ground, fully fueled, the wings with 3 engines each are heavy and will bend down a very noticeable amount. The B-52 has a similar issue, to the point where it has outrigger wheels near the end of the wings to keep them from scraping the pavement.

photo showing the B-52 outrigger wheels

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If you could find a picture illustrating this point I think that would be very helfpul. –  Jay Carr Jun 12 at 15:03
    
@JayCarr b52 landing with 1 wing wheel on the ground: cdn-www.airliners.net/aviation-photos/photos/1/0/8/1400801.jpg –  Dan Neely Jun 12 at 15:48
    
I would rather say the wings are flexible to some extent but not rigid ones. –  menjaraz Jun 12 at 16:14
    
@JayCarr my complements on the photo –  paul Jun 13 at 2:27
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@menjaraz ALL aircraft wings are flexible. Rigid wings tend to break. Heavy plane -> bigger bend. –  paul Jun 13 at 2:29
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Yes, this is common in heavy cargo haulers.

As you probably are aware, a dihedral wing configuration provides roll stability. Roll the plane, and it will naturally roll back to level. A center of mass well below the center of lift provides the same effect. Put a lot of weight down below the center of lift, and you'll get the roll-back effect again.

While that's nifty, you can get too much of it. Too much stability makes the plane very hard to turn or otherwise control. You can also get a sort of aerodynamic pendulum-like effect called a "Dutch roll" as the "roll back" action over-corrects and causes a swing back the other way, and back and forth with a combination yaw and roll that is surprisingly effective at inducing air-sickness. Putting an anhedrial angle on the wing counteracts the "too-much-stability" problem created by the weight distribution, making the plane easier to control again and reducing unwanted oscillations.

Why not just make the wings lower? That may solve the stability problem, but it would create other problems as well. Just how low would the wings have to be? Would that design affect safety for ground operations or unpaved runways? And what about cargo -- how well does your new design perform in empty versus loaded configurations? How tall would the landing gear have to be to make such a design safe... and how does that affect loading and unloading?

Some aircraft, even the comically large Airbus A300-600 Super Transport "Beluga", opt for the low-wing solution, which invariably leads to a dihedral. A300-600ST

Airbus A300-600ST - photo credit: Airbus

But in certain cases, the high-wing plus pronounced anhedrial design has won out for some heavy cargo hauling designs, and is particularly popular with military where they may have to operate out of dirt runways and need to get the wings up away from the ground, and where short landing gear is desirable.

C5 Galaxy takeoff CG Galaxy at takeoff -- Photo Credit: USAF

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The best explanation so far. Just keeping the gear short is mainly important to make loading easier; An-224 is too heavy for unpaved runway anyway. –  Jan Hudec Jun 12 at 20:10
    
Sorry to disagree, Jan, but Radu has my vote this time. Dutch roll damping is the main reason for anhedral with high-mounted wings. Once you start yawing, the low fuselage helps to roll the aircraft, so roll maneuverability is not the issue. –  Peter Kämpf Jun 12 at 20:28
    
@PeterKämpf dutch roll is yet another side-effect of excessive stability. You can solve it with several options, but anhedral wing mounting is the one that keeps the wings up high. Other planes, like the airbus a380 have low-mounted wings with a dihedral, and still no dutch roll. –  tylerl Jun 13 at 0:04
    
@tylerl: Sure, that is due to the fuselage influence on wing clbeta. The Antonov has high wings so tanks can drive all the way through, and the 380 has low wings so it looks familiar to passengers, although it would have benefitted from a mid wing configuration. The fact stays, the 225 anhedral is mainly due to dutch roll damping. –  Peter Kämpf Jun 13 at 16:41
    
As far as I (or Wikipedia) know, freighter version of A380 is planned, but not in service anywhere yet. –  Jan Hudec Jun 13 at 20:32
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Both the high wing location and sweepback increase roll stability. Too much roll stability not only reduces maneuverability but is likely to lead to Dutch roll, so anhedral is added to compensate.

Airplanes with high but unswept wings generally have neither anhedral nor dihedral.

Edit: Here's a discussion of stability, dihedral and Dutch roll, albeit in the context of RC gliders.

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It is not unusual for a dedicated cargo plane design to have a high mounted wing design. It allows for the fuselage of the plane to sit lower on the tarmac while maintaining the ground clearance for the engines and wingtips.

It also makes it easier to allow the cargo door to be opened in flight without compromising the structural integrity because the main strength of the fuselage comes from the spine in the top where the wings and tail attach to.

Looking at the list of military transport aircraft, most use the same wing design. The only exceptions are those based on a civilian aircraft.

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You've overlooked the "anhedral" side of the question. Is it really so or not as argued @MikeFoxtrot? –  menjaraz Jun 12 at 9:50
    
@menjaraz on ground, especially for large wings, the angle is always different from the flight one, but, afaik, the angle is measured and defined on ground. –  Federico Jun 12 at 10:42
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