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Top or Bottom?

Top or bottom: where do you like your wing? Placed at the top of the fuselage, it’s a “high-wing”. Fixed at the bottom of the fuselage, it’s a “low-wing”. For these two variations in design, there’s a series of performance factors considered by the aircraft’s designer in order to optimize the results for the mission for which the aircraft was created. Each wing’s vertical location has pluses and minuses. What follows is a review of a few of them.

 A popular remark favouring high-wings is this one: have you ever seen a low-wing bird? If Mother Nature made all birds into high- wings, she must have had a good reason. She did: it’s inherently more stable to have weight suspended below the wing. Another compelling benefit of a high-wing is surely one of the most obvious: in straight and level flight, cockpit occupants have an unobstructed view of the ground. For sightseeing flights, there’s no better way to view the Earth below.

The external struts used on high-wings can lower the weight of the wings. Though struts add significantly to drag, they have less negative impact on the overall lift of a high-wing since more of the lift is produced by the wing’s upper surface than is forthcoming from its lower surface where the strut is attached.

For an aircraft designed with short takeoff and landing (STOL) intentions, high-wings provide more room for larger flaps required to generate greater lift. They also, when landing, tend to reduce the floating that’s induced along the runway by ground effect. Wing- mounted engines on high-wings also suffer less damage from runway debris owing to their higher position. High-wings do, of course, have their down sides, not least of which is the complication association to designing an undercarriage that retracts into the fuselage. When banking a high-wing, the pilot’s visibility becomes obscured toward the direction in which the pilot is turning. In a climb, the pilot of a high-wing cannot see upward, a contributing factor to mid-air collisions between climbing high-wings and descending low-wings.

Low-wings offer advantages in the design of their landing gears. On a low-wing, the landing gear assembly can be attached directly to the wing box which, given its required strength, will support the weight of the undercarriage without requiring extra strengthening to absorb gear loads. The landing gear on a low-wing retractable can also be readily stowed into the wing, avoiding the more complex solutions required of high-wing retractables.

Low-wings provide a wider gear stance, giving the aircraft a more firm footprint for ground manoeuvring. Multi-engine low-wings need greater fuselage clearance to protect their engines and propellers. Although this adds to the weight of the landing gear as a result of it having to be larger, the positive upshot of this clearance requirement is an aft-fuselage design with less upsweep towards the tail. This affords more space in the rear of the cabin, and has the aerodynamic benefit of producing less drag.

A low-wing without dihedral can have a one-piece flap that passes under the fuselage. Such a design eliminates the risk of an asymmetric flap failure. By contrast, dihedral designed into a low-wing can often be a compromise between aerodynamics and the need to reduce the risk of a wingtip striking the ground on a botched landing.

Refuelling a high-wing can sometimes be a pain, especially without a step ladder. Refuelling a low-wing is a breeze and can allow you to relax and work a few extra minutes into your tan. You’ll never hit your head getting into a low-wing; the same, unfortunately cannot be said of an airplane with its wings fixed up high.

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