Why not more high-wing aircraft?

A few large planes have been designed and built with wings that attach at the top of the fuselage instead of the bottom. Examples include the C-5, C-130, and C-17. Having the wing high off of the ground reportedly allows the fuselage to be closer to the ground, which aids cargo loading/unloading:

All of the large commercial passenger jets I’m familiar with are low-wing aircraft. If the high-wing configuration has an advantage for cargo aircraft, presumably it must have some disadvantage that discourages its use in commercial passenger aircraft. So what would that be? Why aren’t more passenger jets high-wing?

Having the wing high off the ground also allows the planes to take off and land with worse runway conditions. The high wings will pass over brush and other obstacles. This is the same advantage of small light ‘bush’ planes with high wings. The lower wing also aids in landing and takeoff by creating ground effect lift when the planes are near the ground. For small planes lower wings provide more visibility. There are structural advantages to locating the fuselage on top of a large wing spar instead of hanging from it, though at the moment I can’t recall what those advantages are.

It depends on what you’re trying to see. A low wing allows you to see what’s beside you and above you, but a high wing gives you a better view.

This may be it. In general, airliners will always be taking off in the best of conditions; a long, clean runway. There would be no need for a high wing then, and the high wings make the engines less accessible.

I would also venture a guess that the engines are louder (to the passengers) when mounted on a high wing than low wing.

I don’t know that there is any one single thing that is the dominant factor, but it seems like a cleaner functional arrangement for a jetliner. The wings attach to the fuselage at a structural center called the wingbox. In a low wing arrangement, the wingbox is below the passenger cabin, so you have a clean uninterrupted passenger cabin front-to-back. It’s fine for the baggage holds to be separately located to the front and rear of the wingbox.

The low wingbox & inboard low wing naturally provide a substantial structure to accommodate massive main landing gear. Whereas something high wing like the C130 or ATR 72 requires additional structure sticking out of the fuselage to accommodate the landing gear:

The engines, which require frequent inspection, are low and accessible. Lower wings are more accessible for de-icing, refueling.

In a semi-controlled crash landing, low wings rather than fuselage are in a position to absorb more of the impact.

Finally, the decisive factor may be that a low wing arrangement allows for much easier gremlin-spotting.

A major consideration for airliners is that the main passenger cabin be easily accessible without large obstructions or interruptions. On a large “tube & wing” commercial aircraft, the wings attach to the fuselage through the wing box which is located in the lower fuselage area below the main cabin, allowing an essentially continuous load-bearing structure members to span the width of the fuselage and attach ton the longitudinal structure of the fuselage as shown in the CAD image below:

On military cargo aircraft like the C-5A, C-17, C-130, et cetera, where it is necessary to have a low aft (and for the C-5A, forward) loading ramp, in order to have an unobstructed cargo bay the wing box needs to be as high as possible, in addition to keeping the wings and attached engines up high to avoid obstructions on the ground. In this case, the longitudinal structure is primarily built into the top of the fuselage (although these cargo aircraft have a more robust fuselage structure overall due to the need to support landing on unmaintained runways).

It is true that cargo aircraft are louder because of the placement of the engines under the wing and with a direct acoustic view to the fuselage, but because cargo aircraft are primarily intended for hauling cargo it isn’t a major concern. Cargo aircraft are sometimes equipped with installed row seating for transporting troops, and are often used for dropping paratroops, in which case the people need to wear hearing protection because the intensity of the acoustic environment exceeds occupational safety noise limits.

I’ll say that at least smaller cargo aircraft like the C-130 are substantially less comfortable for long distance transport because the effects of wing flexure are more apparent (for subtle structural dynamic reasons that I won’t go into here); having wings mounted essentially through the lower middle of the fuselage is much better for transporting people, hence why all commercial aircraft use the low wing configuration.

Stranger

I can’t say with 100% certainty, but here’s some semi-informed speculation …

For airplanes larger than single engine Cessnas, the high wing design requires either very tall landing gear extending from the engine nacelles (see TU-95, DHC-7, DHC-8, Fairchild/Fokker F-27, etc.) or podded gear on the lower lobe of the fuselage (see C-130, C-141, C-5, C-17, A400M, ATR-42/72, etc.)

The former solution is heavy, the latter is draggy. Neither are desirable unless having high engines and a low deck height are themselves offsetting benefits. Having engines high off the ground is helpful when operating on dirt or gravel. But is a PITA for routine ground servicing and for the minor occasional repairs & inspections.

Wings on Boeing-scale airplanes are already high enough off the ground that fuel servicing requires ladders or workstands. If the wings were mounted atop the fuselage they’d be that much higher requiring more complicated equipment. Yes, the fueling ports could be moved lower onto the fuselage from their current underwing location, but that too has issues matching up with existing infrastructure. And with the general issue of turn servicability, that lots of different dedicated vehicles need to all suckle against the pig more or less simultaneously, and there’s only so much surface area to go around for them all.

All of this begs the question of why a few short haul jetliners are high wing. I think a high wing arrangement allows more flap for short field landings. I’m not sure if there are other reasons?

An advantage to high winged small airliners is the cargo and passenger loading take place at ramp level, not needing extra conveyor belts, lift trucks, etc. For short flights and hence a workday with lots of stops, the higher drag configuration is paid for by the lack of need for support infrastructure. And in a workday dominated by ground time, the relatively slower cruise speeds of that configuration (all else equal) are less impactful.

Which is why we mostly see such aircraft (BAE-146, DHC-7/8, and ATR-42/72) now used in austere service, not in e.g. Des Moines to Chicago or Birmingham to London.

Quite right. I was supposed to write something like ‘improved visibility below’. Obviously the degree of visibility wouldn’t be much different with the wing above or below.

But of course this is only relevant to light aircraft. In a jetliner cockpit, the position of the wing only affects visibility when you stick your head out of the window to check behind you in reverse gear.

Plus the additional structural strength (weight?) to extend from the wing spar to the lobes for something like the C-130 - essentially a big box capable of sustaining a heavy landing “thump” as well as the weight of the aircraft - over and above the structure required to hold the cargo. Similarly, the much longer gear for a wing landing gear means overall heavier construction too.

When the cargo is a tank or some such concentrated weight, presumably “stronger structure” is already part of the design. At least for passenger aircraft, the weight is distributed along the full length.

Plus, the wings are usually the main fuel tanks (and the wing box at the fuselage) so the extended box to the pods must also accommodate that weight. (I read of some aircraft dumping fuel for emergency landings, so full fuel is significant weight). With a low wing, the structure between landing gear and the main weight-bearing spar is much shorter.

As others have mentioned - with low wing, the fuselage weight-bearing is below the cabin attached to the spar - the ceiling needs minimal structural strength. With high-wing, the top and bottom need strength, or the same box between floor and wing spar that would support the wheel pods needs to be stronger, even if using engine nacelle landing gear. That box has to be braced for sway in every direction, not just the weight of the wing. But… you don’t want triangular braces across the middle of the cabin, so it has to be even stronger (aka heavier) to compensate.

A high wing aircraft, all else being equal, will have more roll stability than a low wing aircraft due to the pendulum effect. If thr fuselage is slung below the wing, when the aircraft rolls in either direction the weight of the fuselage wants to pull it back to level. If the fuselage sits on top of the wing, when the plane rolls the weight of the fuselage wants to keep it rolling.

This is not crucial in transport planes, but in fighters a low wing makes the olane more manoeverable.

In carrier operations, a high wing can make for long landing gear, which would need to be extra strong to handle the force of carrier landings. The F4U Corsair was a low wing plane, but had such a huge propeller that required a lot of ground clearance. If the Corsair had a straight wing it would have had gear too long and spindly. That’s why it has an inverted gull wing - purely to get the gear structure closer to the deck so it would be stronger.

Or not. :wink:

All of @md-2000’s cogent points about structure are well founded as applied to transports / bombers.

Interestingly, the F-15 is high wing. The F-16 & F-18 are best described as mid-wing, but higher up than the 50% level. So mid/high-ish. For fighters, the whole damn fuselage is a beefy structural core except for some hollow spaces for fuel & electronics and, nowadays, stealthy weapon bays.

Modern fighter aircraft are special case because the wingbox and engine pods are so tightly integrated into the fuselage that they are not readily distinguishable structures, instead essentially being a monocoque frame in which the skin panels are actual structural members. Commercial aircraft are not built this way because it is more difficult to maintain and they do not need the kind of structural rigidity and body lift, not to mention how much of then engine vibration is transferred to the cockpit.

Stranger

Of course. I said ‘can’, not ‘always do’. Fuselage mounted gear can solve the problem. But in the taildraggers of the Corsair’s era, narrow fuselage mounted gear made for a squirrely airplane to land. The Grumman Wildcat solved the big prop problem with fuselage mounted gear however, and it was a carrier-based fighter.

The B-17 Flying Fortress and B-24 Liberator were two four engined bomber aircraft made in the tens of thousands in World War 2 but the B-17 had a low wing and the B-24 had a high wing.

Iirc the B-17 was significantly studier compared to the B-24 due to the higher wing putting more stress on the air frame. The higher air frame made the B-24 more aerodynamic and this faster but the B-24 could take far less damage especially to the wings. A B-17 could very easily make a no gear landing and survive, while B-24s doing no gear landings tended to completely break apart.

Also note that in smaller high-wing aircraft - Cessnas, Piper Cub, etc - the lateral flex of the box between the landing gear and the wing spar is limited by those diagonal struts for extra bracing. This also means the whole wing spar does not need to be quite as strong.

For bigger higher speed aircraft, I assume the extra drag is less acceptable.

Stick your head out the window? Didn’t your dad teach you to back up using the mirrors? :wink: