Generally, it's easier to make things strong in compression than in tension.

In a low-wing plane, the weight of the aircraft is on top of the wing; in a high-wing aircraft, it hangs from it.

It seems to me (I'm not an engineer) that the area of attachment in the latter case has to do a lot more difficult work (suspending the rest of the plane by its bolts) than in the former (bearing the weight from below).

And since in a high-wing aircraft all the structure is in tension (everything is hanging from something above it), presumably it's not just the wing and its attachment points that are affected, but most of the fuselage that has to withstand this tension.

Are these intuitions true, and if so, what are their engineering implications?

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    $\begingroup$ I dispute your premise - compression is only easier than tension when you're building in something like brick, stone or concrete, which is much stronger in compression than tension. Compression causes buckling of long members and needs careful design, while the strength of a wire in tension is almost unaffected by its length and is easy to design. Compare, for example, the tower of a tower crane with its lifting cable. $\endgroup$ May 15 '19 at 8:28
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    $\begingroup$ If you remove the first sentence it makes a lot better question. "Generally, it's easier to make things strong in compression than in tension" detracts from your question as it is not true in every instance. $\endgroup$
    – ghellquist
    May 15 '19 at 15:13
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    $\begingroup$ The cynical answer is a well known aircraft designer's joke (though like all good jokes there is some truth to it). Civil aircraft have low wings because the passengers would get scared if they saw the cracks on the underside of the wing opening up when the plane took off :) $\endgroup$
    – alephzero
    May 15 '19 at 17:32
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    $\begingroup$ @DanieleProcida The not "always" case include sheet material that is stronger in tension than compression like paper (try compressing a sheet of paper and you will end up folding it). And sheet material includes things like sheet aluminium - you know, what airplanes are made of? $\endgroup$
    – slebetman
    May 16 '19 at 0:45
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    $\begingroup$ @DanieleProcida I wasn't being sarcastic. I was just reminding you that your non-always cases include airplanes $\endgroup$
    – slebetman
    May 16 '19 at 11:59

Actually, in aircraft construction tension is preferable to compression: aeroplanes are thin walled structures, and compression forces introduce buckling.

In a low wing aircraft, the fuselage is pressing downwards on the top half of the wing, the bit that is under compression. In fact, quite complicated frame structure members are required for the fuselage/wing intersection for low wing aircraft: they need wing dihedral, so the wing looks pre-buckled at the spot of largest bending moment.

enter image description here

So although high vs low wing does have some differences in structural implementation, those are not the deciding factors in the layout. The design considerations for operational use are what drives the choice high-low-mid wing. Picture below from Torenbeek, depicting the Galaxy C-5.

enter image description here

  1. High wing designs are usually applied for aircraft that need quick loading/unloading, and/or operate from airports with limited ground equipment.

    • Wing out of the way: good for loading/unloading, and for long extended flaps on STOL aircraft.
    • Floor close to the ground: easy cargo handling, good access for passengers, no need for airstrips.
    • More room for propellers.
    • Lowest induced drag at high lift.
    • Self stabilising roll behaviour.
  2. Mid wing has the lowest drag of the three layouts at high speed, but poses a particular problem in running the wing through the fuselage. The wing must be a complete structure, because it has the highest bending loads in the wing centre and we don't want any holes there, best to make the hole in the fuselage to lead the wing through. But this bit of the fuselage cannot be used for passengers or cargo.

  3. Low wing is usually applied in passenger aircraft because:

    • The undercarriage can be easily retracted.
    • The wing forms an impact structure that absorbs energy in case of a crash. Although the fuel tanks are in the wing...
    • The wing is fully underneath the floor and does not impede on the thoroughfare in the fuselage.
    • Optimal use of ground effect during take-off and landing.

In the 1950s, for the F27 the decision was taken to implement a high-wing design for its intended successor to the DC3: market analysis showed a significant market share outside Europe and the USA, at airports without the latest facilities. From wikipedia:

Wikipedia is our friend

while a high-mounted wing had been selected as it produced a higher lift coefficient than a lower counterpart, it also enabled easier ground loading due to a lower floor level and provided unfettered external views to passengers without any weight increase.

And who wouldn't want unfettered external views?

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    $\begingroup$ Low wing for passenger jet aircraft is more specifically because the high speed of jet fans means they basically explode when they fail. Fast-moving shrapnel from shattered engine blades is often not contained within the engine housing, and would easily penetrate the fuselage. In contrast the wing is already strong, and can take significant damage without affecting airworthiness. Putting the wing between the engines and the passengers is a major improvement for safety. $\endgroup$
    – Graham
    May 15 '19 at 8:09
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    $\begingroup$ @Graham except a good number of airliners do not have the wing between the engines and the fuselage. Since the advent of high BPR turbofans, the engines tend to be ahead of the wing, with line of sight to the fuselage. That is the reason for the regulations concerning blade-off behavior of fans. I am pretty certain the reason for high vs. low wing has more to do with military transports requiring better ground clearance and FOD avoidance on unprepared fields. $\endgroup$ May 15 '19 at 11:10
  • $\begingroup$ @Graham yes safety is better with a low wing. There are many other considerations though. $\endgroup$
    – Koyovis
    May 15 '19 at 12:29
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    $\begingroup$ @Graham: I challenge you to prove the statement that low wing is choosen to protect against shrapnel from shattered Engine blades. $\endgroup$
    – ghellquist
    May 15 '19 at 15:10
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    $\begingroup$ @Graham your premise about containment is not backed up by the facts. The number of blade failures is low in any case, and most of them are contained and never become media stories. Another counter-example is propellers, which have no containment system at all when they fail. $\endgroup$
    – alephzero
    May 15 '19 at 17:30

The intuitions depend on the application. Wood is very strong in compression, steel in tension. And we must also consider G loading forces, which only add to the situation.

Airplane designers, over the years, have learned to use sound fundamental structural concepts to advance from opposing tension cables (very strong, not aerodynamic) to cantilever design (loaded triangles in both tension and compression), distribution of load (stressed skin), and tubular design (arch strength), as well as improved building materials such as aluminum, steel alloys, and titanium.

Although attachment to a high wing as opposed to resting on a low wing does make sense, the greatest loads are on the wings themselves, and the parts of the fuselage bearing the bending force of elevator and rudder.

So you have a very strong fuselage either resting on or suspended from the wing spars. Military transports seem to favor high wings, airliners low wings. No strong evidence for either case. But a lot of bolts will make it strong.

  • 3
    $\begingroup$ Actually, wood is stronger in tension than in compression. $\endgroup$ May 15 '19 at 17:59
  • $\begingroup$ Beg to differ (I usually don't with your work). Trees are made by nature to support great weight (really tons) and also be flexible to the wind. The concentric growth rings of a tree, combined with its composite matrix of cellulose polymers and natural resins gives it great strength to weight ratio. The fibre has to be pulled apart sideways against the rings in order to fail. No doubt also strong in tension as well, but designed for compressive strength. Early aircraft designers complimented this trait with the tensile strength of steel cable. I would not mind building one from scratch. $\endgroup$ May 15 '19 at 18:41
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    $\begingroup$ @RobertDiGiovanni Have you considered that the "be flexible by wind" require wood to be very strong under tension - sometimes/often even stronger than under compression - to prevent wood from snapping in half? In my experience with model aircraft structure in a crash or structural failure (flutter, high G forces, etc) it is ALWAYS the inner part buckling that cause the failure rather than the outer part tearing - this proves to me that the wood I use (balsa) is much stronger in tension than compression. Foam on the other hand behave the opposite where it is ALWAYS the outer part tearing $\endgroup$
    – slebetman
    May 16 '19 at 0:55
  • $\begingroup$ OK, what KIND of wood. The average stick broken over your knee fails tension first. Even more so with "green stick" fracture. Balsa is a very soft wood and would certainly not be something I would use for structural strength with a full scale aircraft. The ancient Greeks knew how to complement steel cable and wood when they built their ships with a steel cable from stern to bow. A wave at the ends would bend and pitch the ship, but one in the middle would break its keel without the cable. So the BEST of both are used together. $\endgroup$ May 16 '19 at 1:49
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    $\begingroup$ This document suggests that wood is is stronger in tension than in compression. (Page 4-24 has a table of tensile strength) Often by a factor of 3 or 4. $\endgroup$
    – Arsenal
    May 16 '19 at 6:33

the tensile-versus-compressive stress issues have been worked out to a satisfactory degree many years ago, meaning that the loadpaths for high-versus-low wing aircraft really aren't design differentiators- but there are other issues, as follows.

Low wings furnish a natural location for a wide-stance main landing gear, making for stable landings and easy ground handling. But high wings are less prone to damage from striking rocks or bushes on the ground.

In a low wing layout you can position the pilot and copilot seats over the main wing spar so they do not reduce cabin room, whereas a main spar carry-through in a high wing layout might reduce headroom in the cabin. However, a low wing interferes with the pilot's view of the ground whereas a high wing does not.

These differences- which do not have anything directly to do with stresses in the airframe- affect the pilot's decision-making process with respect to buying and flying a low wing instead of a high wing plane.

I invite the experts here to add their comments.

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    $\begingroup$ Flying a turn will make the high wing obstruct the pilot's view; only ground view favours the high wing. $\endgroup$ May 15 '19 at 18:02

For structural weight efficiency, tension wins because stiffness isn't a factor. This means, if structural efficiency is your top priority, a high wing, braced with struts, or for even less weight cables, wins.

With strut bracing, the major structural attachments are simple pin joints, and the highest stress component, the wing strut, is in tension except during reverse or negative loading where it's in compression, but where the requirement is less. There is moderate compression loading along the spar axis directed to the wing root, and along the upper spar cap at the strut attachment, but nothing like the compression stress in a fully cantilevered structure at the wing root.

And for best visualization, really take it to the extreme. Look at a paraglider. You can't compress a string. The wing is "high" and everthing is under tension load. And the whole thing weighs maybe 10lbs but can lift 200+, or 20+ times its weight.

Note that on cantilever high wing airplanes, like a military transport or a Dash 8, the placement of the wing has little structural advantage and there are other issues to favour one or the other, like loading etc.

  • $\begingroup$ Cessna Cardinal is high wing with cantilever beam, no struts. Fixed gear and retractable. Pilot sits slightly ahead of the wing for a great view, and there are no struts to block the side view. Very nice plane to fly. Cessna 210 and P210 are also high wing strutless. I know the pins that hold the ends of the wing spar to the fuselage are pretty big. crossroadsfencing.com/airplane/painting%20pics/IMG_0563.JPG $\endgroup$
    – CrossRoads
    May 15 '19 at 1:49

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