1

So far, we have only seen a few civilian jet transports with high wings. All of them BAe 146, Do 328JET, seem to be regional jets. Yet military jet transports are almost completely high-wing. Why aren't we seeing high-wing large jetliners?

Some advantages of high wings:

  • shorter landing gear
  • easier access to the fuselage
  • a better location for the center-of-thrust.
  • better stability
  • better ground clearance for upcoming high-bypass turbofans. Lack thereof caused the design changes that ended in 737 MAX crashes
  • Increased flap efficiency
  • better able to take advantage of strut-bracing, if any.

Some disadvantages:

  • less efficient location of landing gear. (probably not mounted on the wings)
  • more likely to tailstrike
  • passengers may mock
  • belly landings could be tricky
  • water landings may be even more tricky

I don't know how these pros and cons add up though. Can someone give some more info?

Abdullah is not an Amalekite
  • 3,571
  • 1
  • 15
  • 58
  • The 737MAX would had become unstable regardless of wing position. The forward shift of larger engines is the problem, and the solution would be a larger tail. If only Boeing "management" would accept that simple truth. – Peter Kämpf Jun 17 '20 at 10:12
  • 2
    Also, the MAX fiasco has nothing to do with center of thrust. It's all about aerodynamics, where the new nacelles generate lift at high enough AoA, causing an unwanted pitch-up moment. – TooTea Jun 17 '20 at 10:26
  • 1
    The larger engines couldn't be hung below the wing, so they had to be moved up and forward. Then, what you say happened, – Abdullah is not an Amalekite Jun 17 '20 at 10:36
  • 2
    @PeterKämpf if MAX was a high wing design, there might not have been the need to place the bigger engine more forward than the older and smaller ones. This would all depend on ldg gear design, of course... – Jpe61 Jun 17 '20 at 16:21
  • "passengers may mock" -- hi, Larious. – quiet flyer Jun 17 '20 at 16:47
  • @Jpe61: You're right. – Peter Kämpf Jun 17 '20 at 16:48
  • 1
    Why would a high wing mean shorter landing gear? I'd think just the opposite. – jamesqf Jun 17 '20 at 18:51
  • 1
    If gear is placed on the bottom sides of the fuselage (like C-17 for instance) then yes, they would be shorter. There is no reason to attach ldg gear to wings on a high wing jet. – Jpe61 Jun 17 '20 at 21:33
  • "passengers may mock"?? I always prefer high-wing planes, as a passenger or a pilot. Better view, for one. (Should be in the list of advantages, for sure). – Zeus Jun 18 '20 at 01:22
  • @Jpe61: Sure there is, to have the gear a reasonable distance apart for stability. Compare for instance the wing-mounted gear of a low-wing Piper Cherokee to the fuselage-mounted gear of a Cessna 172, which sticks out at an angle to either side. – jamesqf Jun 18 '20 at 04:29
  • @jamesqf I'd argue the C-17 is in substantial need of stability, especially with it's outboard engines. – Abdullah is not an Amalekite Jun 18 '20 at 10:04
  • @jamesqf "...a high wing jet..." – Jpe61 Jun 18 '20 at 13:18
  • @Jpe61: Same principle at work. I just don't happen to own or fly any jets, so can't speak from experience about them, or FRM really call any particular ones to mind. – jamesqf Jun 18 '20 at 16:54
  • I got duped and one of the questions I'm linked to is also duped haha. I suppose a difference between those questions and mine may be that mine focuses on large jetliners, while they're more general (turboprops, pistons). – Abdullah is not an Amalekite Jun 19 '20 at 09:43

2 Answers2

7

Large commercial aircraft have low wings

  • to easily stow away their long landing gears in the wing root,
  • to hide the wing spar carry-through below the passenger deck,
  • and to improve the accessibility of wing-mounted engines.

Long gears make it possible to stretch the fuselage and still be able to rotate during take-off. Stretching makes it possible to tailor one basic design to a wide range of sizes, lowering the development cost of a single aircraft.

Pressurized fuselages need to have their passenger floor a bit below the center of the fuselage tube, leaving ample space below for cargo, systems and structure. Placing the wing high would produce a hump on the fuselage, which adds wave drag at the transsonic cruise speed of typical airliners.

The low position of the wing-mounted engines makes them much better accessible for maintenance; a major reason this configuration was selected in the early jet transports. Civilian freighters use mostly converted passenger planes, and their small number precludes custom designs. Initial concerns about foreign object damage (which resulted in a number of early jetliners with tail-mounted engines) proved to be unfounded.

Large high-wing airplanes with their low fuselage position are easier to load and unload, at the price that the fuselage taper has to start shortly aft of the landing gear, so no stretching is possible. The military doesn't mind and prefers the high-wing variety. Also, the high wing reduces the risk of engine damage by foreign objects on improvised runways. That Lockheed likes to stretch their transports (C-130, C-141) anyway is the exception that proves the rule.

Turboprop-powered civilian airplanes may have a high wing in order to provide more space for the propellers. Here, the fuselage-mounted landing gear can be made short and light (which is how the Do-328 JET evolved). Still, both versions exist. And the ones where the engineers did not know how to attach a jet close to the wing*.

Aerodynamically a mid-wing position would be best. This is used when the payload is compact and needs little space, or is hung externally. In bombers, in other words. Civilian transports will always either have a low or a high wing since nobody wants their passengers to climb across a wingspar carry-through on their way to their seat. At least since 1933.

* To be fair to the Avro engineers: The high wing position gives that airplane a very high maximum lift coefficient without requiring slats.

Peter Kämpf
  • 231,832
  • 17
  • 588
  • 929
  • The 328 was an odd kind of dead end design, mostly because of its straight wing, and it could only cruise at m .66, up to FL350. That cruise speed made it too slow for the FLs above 300 in busy airspace, so ATC would put them in the high 2s all the time where the fuel burn was sub optimal. – John K Jun 17 '20 at 14:11
  • Dash 8 have been stretched repeatedly; but its gears extend from the engine nacelles rather than the belly. – JZYL Jun 17 '20 at 14:15
  • @JohnK: As a turboprop (as intended) it was excellent. Only a desperate attempt to make it into a jet turned it into that odd design. Dornier by that time had a 728 on the drawing board with swept wings and all, but they wisely decided not to enter that crowded market. – Peter Kämpf Jun 17 '20 at 15:05
  • 1
    Peter, is it not also true that a high-wing approach moves both the wing and the engine nacelles up away from the ground in military transports, thereby minimizing foreign object damage when operating out of unimproved airstrips? – niels nielsen Jun 17 '20 at 17:38
  • 1
    @nielsnielsen yes, that's another advantage of the high wing. – FreeMan Jun 17 '20 at 17:49
  • 1
    @nielsnielsen: You are right. I focused on jetliners here as the question suggested and only gave the most important reason for high wings in military transporters. And in some turboprops. And in one British oddball. – Peter Kämpf Jun 17 '20 at 21:26
2

The really big show stopper in the high wing/low wing issue for airliners is bullet point #3 in Peter's answer; the need to place the wing box for the high wing mostly outside the circumference of the fuselage, so overall frontal area is higher and you have this big projection above the fuselage. This is driven by the need for headroom in the cabin as shown below: enter image description here

The DeHavilland Dash 7 had insufficient headroom in the center aisle under the wing because they were trying to get the wing box as low as possible, and it had to incorporate a dip in the aisle below the surrounding floor, where it ran under the floor, to gain an extra inch or so of headroom.

Another minor factor is the wing to fuse attachment structure has to accommodate tension loads since the load is hanging, whereas with the low wing, the fuselage just sits on the wing box with much simpler structural connections, so the high wing's arrangement is a bit heavier.

John K
  • 130,987
  • 11
  • 286
  • 467
  • High wing attachment is indeed heavier, but mainly because it needs reinforcement between the wing [spar] and the landing gear (if it is attached to the fuselage). Otherwise, tension loads for thin/slender structures like airplane's are actually better than compression. – Zeus Jun 18 '20 at 01:51
  • Yes but you need complex fittings and hockey stick frames that fork at the top to distribute loads with the more narrow interface at the top. Much more elaborate connection. In the 70s I did wing marry-up on Dh -7s. A low wing just has maybe 6 links or lugs joining the wing box to the fuse. – John K Jun 18 '20 at 02:19