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To my knowledge the only aircraft featuring a crescent wing that was built in numbers was the Handley Page Victor.

It seems that a crescent wing offers the possibility of a high subsonic/transonic aircraft with a thick wing root but not too much drag at the root. It seems that a crescent wing would work well at airliner speeds.

  • Why there were not more crescent wing aircraft been built?
  • Why are they not used on airliners?
  • Is there any application where a crescent wing would be an ideal or even a good solution?
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You are right, it helps to de-sweep the outer wing. The main advantages are (read this Flight article from 1965 here for a much more detailed explanation):

  • An identical critical Mach number over the whole span as sweep is reduced together with relative thickness.
  • A smooth cross-section distribution in the direction of flow which helps to reduce transsonic drag.
  • A tendency to pitch up in ground effect such that the wing will essentially land itself, especially when combined with a T-tail.

However, as long as wings are made of aluminum, the manufacturing effort is significantly higher.

The basic problem is the needed two-dimensional bending of the skin panels. Using a straight spar line and a straight leading edge requires bending only in one dimension, but when sweep changes, the skin panels would need to stretch in some corners while being bent. This requires much more expensive tooling, or a break in the structure which increases structural mass.

Even one composite wing I know of has a straight leading edge, even though the aircraft would have benefited from de-sweeping. Both the TKF-90 and the X-31, which were designed by the same people before, had reduced sweep on the outer wing, and for good reason. When the multinational project started and they talked to the BAe engineers, the British side insisted on a straight leading edge for easier manufacture, ignorant of the fact that using composites made their point moot. Yes, that was practically the same company that had built the Victor before.

Another disadvantage is the fact that any sweep change will convert bending moment into torsion. Normally, this reduces the incidence of the outer wing when a swept-back wing is bending under lift loads. De-sweeping the outer wing will introduce an uptwisting torsion which can cancel the torsion from sweep-back, and Handley Page claimed that the Victor wing was close to aero-isoclinic. However, this reduces flutter damping, resulting in a lower wing flutter speed (PDF!). If the wing's aspect ratio is low, this is manageable, but with higher aspect ratios the wing needs to be stiffened.

To answer your question directly: In the majority of cases it is aerodynamically advantageous to vary sweep with relative thickness, but it is not structurally and economically efficient. In the delta wings of combat aircraft which need to operate over a wide range of angles of attack, de-sweeping the outer wing produces indeed the best wing planform, even when economics is included.

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  • $\begingroup$ Is it correct to say the compound delta is a crescent design at heart? I was under the impression the higher sweep angle at the root was more to do with vortex control and the extensions at the end for better aileron performance. $\endgroup$ – Maury Markowitz Mar 14 '18 at 15:29
  • $\begingroup$ @MauryMarkowitz: Both your points are correct, each at a specific angle of attack region. It is the combination of all the benefits which makes the compound delta so attractive for fighter aircraft. And yes, the compound delta is one variant of a crescent wing. $\endgroup$ – Peter Kämpf Mar 15 '18 at 10:41
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The Avro Vulcan B2 also featured higher sweep angles at the wing root. In modern aircraft, while not as extreme as the Victor, the Boeing 737 also incorporates a slightly higher sweep angle at the wing root than at the tip.

enter image description here

One of the reasons the Victor engineers chose this was that the more extreme sweep angle enabled the design team to house engines and landing gear near the wing root since they could make that section fatter without incurring additional drag. This video ("The Crescent Wing" - Handley Page Builds the Victor V Bomber) explains some of the science behind it, plus a good explanation of stall conditions as they develop based on different wing geometries.

The most probable cause for non-development in this arena is because it's more expensive to fly faster. As noted in this question, Mach 0.8-0.85 seems to be the sweet spot. And on that subject, Peter's answer has a good idea why.

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    $\begingroup$ When you look at leading edge sweep, you are correct. But if you look at the sweep angle of the quarter-point line (where the angle-of-attack-dependent lift forces act), the 737 wing has constant sweep. And thanks for the video link. This movie puts 99% of what is shown today on Discovery channel to utter shame! $\endgroup$ – Peter Kämpf Feb 17 '15 at 13:46
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    $\begingroup$ Cool video but...it was downright eerie at the end seeing the model of the HP97 with those square windows...looking a lot like the Comet...and hearing him say it "will bring the advantages of the crescent wing to civil aviation." "It will cross the Atlantic three times a day." Really, really eerie. $\endgroup$ – DrZ214 Jan 16 '16 at 21:23

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