Aircraft need to reduce weight in order to reduce induced drag. The modern shallow cantilever beam wings carry a massive weight penalty, as most of the material is wasted inefficiency absorbing the bending moment. However, with advancements in materials, their mass has reduced to the point that parasitic drag benefits outweigh induced drag losses.

But I would like to know how much structural efficiency still influences wing design. Compared to the total empty weight of the aircraft, how heavy are the wings if modern cantilever monoplanes? This includes large airliners and small GA planes. An example of a design from each class is sufficient.

  • $\begingroup$ Your use of words "massive weight penalty" suggests you have a preconceived bias. Cantilever monoplanes have been the preferred configuration since the 1930s. $\endgroup$ – Eric S Apr 26 at 13:53
  • $\begingroup$ @EricS Yes I do, for the reason mentioned in the second half of that sentence $\endgroup$ – Abdullah Apr 26 at 14:12
  • $\begingroup$ Which I disagree with, but I’ll let the experts answer your question. $\endgroup$ – Eric S Apr 26 at 14:19
  • $\begingroup$ Study the 1968 Cessan 172 and the 1968 Cessna Cardinal. Strut braced and cantilever, nearly same empty weight, nearly same payload, speed etc. Cessna though they would get a much larger speed benefit using a scaled down Centurion cantilever wing than they thought, but in any case the weight penalty was not so much once you allow for the weight of lift struts and the additional related structure of a strut braced design. $\endgroup$ – John K Apr 26 at 15:18
  • $\begingroup$ Massive weight penalty compared to what? Cantilever monoplanes have been the design default for seventy years. $\endgroup$ – Pilothead Apr 27 at 17:45

Wing structure is roughly 20% of empty weight, which is roughly half of gross weight. I clipped the two sources some time ago and no longer have references.

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Ultimately, the history of the debate was all about engine power and speed.

In the early days engines were pretty pathetic, forcing designers down the braced-wing route, so aerofoils could be made thin with all the stiffness conferred by the bracing.

The thick vs. thin debate over monoplane wings first took reasonably informed shape in the latter part of WWI, when engine powers above 100 hp and speeds above 100 mph became the norm. Thick wings could be made stiffer, reducing or even eliminating the bracing and thus allowing the maximum speed to take full advantage of the powerful engine.

But it was not an open and shut case. For a well-streamlined design with an efficient l/d ratio and a need for speed, such as a scout fighter, the saving in form drag is worth a little induced drag. But for a clumsy bumbling thing with poor l/d, such as a naval torpedo bomber hung about with all kinds of gadgetry, lighter structural weight was the more important.

As engine powers rose inexorably through the next twenty years, they offered the opportunity for ever-increasing speed through thinner wings and reduced form drag. For example the Spitfire owed its success principally to the combination of the 1,000-plus horsepower Merlin engine with an unusually thin wing at 13% t/c ratio. It also had an unusually long chord for a fighter, not only helping to maintain adequate structural depth with that slender profile, but also reducing the stresses in the main forward spar and reducing the wing loading so that it could take off and land slower and out-turn the Messerschmitt Bf 109.

The thick wing advocates fought back, especially for the heavy lifters, and the type lasted right through WWII and into the jet age; check out the Avro Vulcan nuclear bomber.

Indeed, check out the wing root sections of any large modern airliner. Note how these more sophisticated wings are thick at the root where strength with lightness matter most and thin at the tips where drag dominates.

So it's very much horses for courses. Do you want high speed and high manoeuvrability, do you want to heft as much as you can as far as you can, or do you want an optimal blend of speed with payload-range?

  • $\begingroup$ Thick wings also provide a place to put fuel. $\endgroup$ – Eric S Apr 27 at 19:10
  • $\begingroup$ @EricS Fuel was seldom a significant factor. If you needed a thin wing, you just found room elsewhere. $\endgroup$ – Guy Inchbald Apr 27 at 19:13
  • $\begingroup$ it might not have been an issue in WWI, but I think it is a pretty big issue by WWII. Hard to imagine a P-51 being about to escort bombers without wet wings. $\endgroup$ – Eric S Apr 27 at 19:15
  • $\begingroup$ @EricS The P-51, like the Spitfire, had relatively thin wings: compare them to say the B-29 heavy lifter. Just, the laminar-flow section happened not to be quite as thin. Even so, like the Spit it carried only some of its operational fuel load in the wing; it could not perform its escort duties without drop tanks. The fuel location was thus not critical to the design, just a minor consequence of it. $\endgroup$ – Guy Inchbald Apr 28 at 9:11

One way of looking at this (thanks to fine data chart by Pilothead) is wing loading, and it is best based on takeoff weight, speed, and anticipated G loading in manuevers.

Going to the formula Total Drag = CDo + CDi, we can see why many designers pursued flying wings: theoretically, almost all drag is CDi (induced drag).

Looking at the high lift to drag ratios of airfoils, we can see the "massive weight penalty" pales in comparison to CDo, especially as airspeed increases. Wings can lift a lot of weight with minimal drag penalty at optimum angle of attack.

The faster you go, the higher the wing loading can be, which means wings are relatively smaller across the board because lift squares as speed increases.

Additionally, instead of heavy solid "leaf spring" design spars, structures such as D boxes, stressed skin, and "cantilever within a cantilever" beams make for weight and all important form drag savings.

But does this mean "monoplane"? Not necessarily. Early cantilever bi and triplane builders had to add struts so non-believing government ministers would approve and fund their designs.

In all cases, a shorter wing is stronger and rolls faster. So, with the trend towards very high aspect wings (some with rows of electric motors), we may yet see an X biplane as a "new" design.


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