Why, generally speaking, do large military planes have wings on the top of the fuselage, but on large commercial planes they are on the bottom?
Eric Grandfield
Almondsbury, Gloucestershire, UK
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Large military aircraft are usually designed to carry bulky cargo, so the fuselage needs to be as low to the ground as possible for quick and efficient loading, usually via the rear fuselage but sometimes via the front. From time to time, their operations need to be on rougher terrain, so undercarriages need to be as short as possible and attached to the fuselage. Top-mounted wings allow these.
In general, the most efficient way to mount engines is on the wings. This way, the weight of the aircraft is spread over the wing, enabling a lighter structure at the wing roots, where the wing and fuselage meet. For ease of maintenance, engines need to be under the wing. Military transport aircraft require a slower minimum airspeed to enable shorter take-off and landing. Hence, they are often powered by propeller engines, which offer this advantage, rather than jets. However, this requires engines higher from the ground 鈥� easier with top-mounted wings.
There are, however, some disadvantages to a high wing. A key one is that this design results in two areas of high load: the wing root and the undercarriage mountings. So where bulky cargo and the previously mentioned airfield performance needs aren鈥檛 present, we go for a low wing. As well as easier engine maintenance, the low wing gives more general maintenance advantages.
Chris Daniel
Glan Conwy, Conwy, UK
The most efficient wing position on an aircraft is at the base of the fuselage to minimise structural weight and maximise occupant safety and comfort. There must therefore be compelling reasons for military transport planes to depart from this design and have high wings instead.
One good reason is that these aircraft typically need to carry large loads such as vehicles and pallets, so it is desirable to have unobstructed floor space the length of the fuselage. This is achieved by mounting the strong and bulky wing attachment to the top of the fuselage.
A secondary benefit of this position is to reduce the risk of damage to the wings and engines from debris on the ground when the plane operates from rough or unprepared airstrips. The tactical advantage of good downward visibility is also possible in aircraft with this configuration.
The undercarriage on such planes has to be accommodated in pods on the lower fuselage, which add extra weight and aerodynamic drag to the aircraft, but these considerations aren鈥檛 as important for large military planes as they are for civilian ones. The landing gear on some of these aircraft are designed to 鈥渒neel鈥� to lower the fuselage, making loading easier and less dependent on ground facilities.
Civilian jet airliners, with a few exceptions, have low wings with the engines mounted below them and the undercarriage accommodated inside the wing structure.
In such cases, the obstruction created by putting the wing root in the lower part of the fuselage doesn鈥檛 cause a serious problem, as access for loading baggage and other small cargo items can be gained from appropriately placed hatches in the side of the fuselage.
What鈥檚 more, the floor of the passenger cabin is above the hold near the middle of the generally circular cross-section of the fuselage, which is where the width is greatest.
In addition, low-mounted wings and engines give a quieter ride for the passengers and make servicing and refuelling easier for the ground staff.
Civilian aircraft mostly operate from well-maintained airports where the risk of foreign object damage is low. And in the event of a wheels-up landing, passengers are protected to some extent by the engines and wings, which will absorb some of the impact.
When ditching in water, low wings can also provide buoyancy, as shown by the famous forced landing of an Airbus A320 in New York鈥檚 Hudson river in 2009.
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