21
$\begingroup$

I've observed that almost all military transports have high wings (with a pronounced anhedral) and almost all civilian airliners have low wings (with a pronounced dihedral) — why is this?

The high-wing configuration gives the military the advantage of easier loading and unloading (especially when operating from irregular airfields), and lower overall height for the aircraft, but then why do civilian aeroplanes hate this design?

Except in the erstwhile USSR, I haven't really seen high-winged military aircraft being used as civilian airliners (Boeing 737 size and beyond), though I can't think of any reasons why not.

Improve this question
$\endgroup$
1

3 Answers 3

Reset to default
14
$\begingroup$

For military transport aircraft:

  • Wings further off the ground will mean that the engines will also be further off the ground, reducing the risk of foreign object damage.
  • You can move vehicles under the wing, and taxi across smaller taxiways.
  • The fuselage can be closer to ground, facilitating transport.
  • No wing spar crossing the cargo floor.

Reasons why most commercial transport jets don't have it:

  • Many are converted passenger jets.
  • They are mainly pallet loaded, less vehicles and bulky objects.
  • They normally use prepared airports, which military jets cannot assume they will.
  • They often loaded from a side door, and it makes little difference I think if you go up one meter or three meters.

but then why do civilian aeroplanes hate this design?

Airlines don't 'hate' this design, look at the BAe146 Quiet Trader for instance. It's just not that many commercial aircraft from which cargo jets are derived are built this way.

Except in the erstwhile USSR, I haven't really seen high-winged military aircraft being used as civilian airliners (Boeing 737 size and beyond), though I can't think of any reasons why not.

Military jets are very expensive to operate and the east block Antonov 124 probably satisfy the most demand. There are a few commercial C130 (L100) around though. It's also not like the US airforce will give us C-5 Galaxy to a commercial company to do whatever it wants with it.

Improve this answer
$\endgroup$
3
  • 5
    $\begingroup$ US airforce won't give C-5 Galaxy to a commercial company, but Lockheed would happily build and sell them L-500 if they wanted. None did. $\endgroup$
    – Jan Hudec
    Commented Jun 15, 2014 at 19:56
  • 2
    $\begingroup$ AFAIK, the primary reason military transports are so expensive is because their volumes are so low. Volumes are low because only militaries buy them. Catch-22. $\endgroup$
    – markvgti
    Commented Jun 18, 2014 at 9:10
  • 1
    $\begingroup$ Was just reading Sutter's book on the genesis of the 747. He reports that the C5 was considered as the first jumbo airliner. However, the design, being optimized for military use, was just too fuel hungry for efficient commercial use. $\endgroup$
    – tj1000
    Commented Jun 22, 2017 at 12:47
10
$\begingroup$

There are probably two main factors that come into play with the placement of the wing:

Height of fuselage from ground
Especially for military airlifts, the ability to load cargo is extremely important. They need to be able to load large objects and vehicles, and with little or no ground support available. This means that the fuselage should be as close to the ground as possible to allow roll-on/roll-off of cargo with built-in ramps. Placing the wings on top of the fuselage allows the bottom of the fuselage to be closer to the ground.

This is less of a problem for airlines. Commercial cargo can usually count on ground equipment for loading and unloading cargo containers on aircraft.

Ease of maintenance
Since airliners don't need easy access to the fuselage to load cargo, maintenance should be as simple as possible. Especialy with the larger aircraft like the C-5, a high-wing puts the engines and wings far above the ground. This makes it much more difficult to work on these components. Keeping the wings and engines low to the ground makes it much easier to replace an engine or access the wings, such as for refueling.

The military wants planes to be easy to maintain as well, but they are willing to make some tradeoffs for the better cargo loading configuration.

There are of course many other tradeoffs in areas like structure and stability. However, it's clear that both configurations are perfectly feasible. The most important factors come down to the usability of the design.

Improve this answer
$\endgroup$
7
$\begingroup$

Low wings are favored for civil aviation because this puts the passenger cabin on top of the wing. Damage from non-normal landings (gear up, no runway, missed the runway, etc.) will happen to the wing, first. Thus protecting passengers. Fuel tanks are usually in the wing and having them also underneath is considered good for the same reason. A low wing allows engines to be near the ground for easy servicing. And main gear legs fit conveniently into low-wing structures. The legs are tidy and short with low wings.

High wings are favored for military cargo and passenger planes because the high wing keeps the engine, propeller, fan, etc, away from the ground. Military cargo and passenger planes don't always have paved runways- a flatish spot may be as good as they get. High wings make landing gear design more difficult, they're either taller, or stick out from the fuselage and are narrow, maybe mounted in pods on the sides of the fuselage, with extra weight and drag. High wings require strong structure to keep them from crushing the passenger space, in a crash, which is extra weight and complexity. High wings make fuel tank access and engine access difficult. But high wings protect the engines, and that allows high wing airplanes to operate in rough conditions. Meaning it can take off again, after each landing.

Dihedral provides stability to low wing airplanes by giving negative feedback to stabilize straight and level flight. Anhedral reduces stability, allowing a high winged airplane to be maneuvered more easily, but doesn't enhance straight and level stability.

Improve this answer
$\endgroup$
3
  • $\begingroup$ Why not give military (transport) planes low wings with engines mounted on top of the wing (like in the Honda business jet)? This would seem to combine the benefits of both... $\endgroup$
    – markvgti
    Commented Oct 29, 2016 at 5:49
  • $\begingroup$ @markvgti it's been tried to build such. See the VFW614 for example which was a civilian regional jetliner intended for export to places with unpaved runways. It failed for lack of orders. Problem is the engines become hard to reach for maintenance purposes (same reason trijets are disliked by maintenance crews). And there's still the risk of stones and stuff being kicked up by the landing gear damaging the undersides of the wings. $\endgroup$
    – jwenting
    Commented Oct 31, 2016 at 7:17
  • $\begingroup$ How is having the fuel tanks closer to the ground, where they're much more likely to be damaged in a crash, considered good? $\endgroup$
    – Vikki
    Commented May 27, 2018 at 1:33

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .