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What advantage does a delta wing have over a swept wing with high AR for supersonic flight? And what disadvantage does a delta wing have over a swept wing with high AR for transonic flight?

The Concorde is made to cruise at supersonic speed--delta wings.

Most commercial airliner--swept wing with high AR.

Would you design a supersonic airplane with the conventional airliner swept wings? On the other hand, would you design a transonic flight with delta wings?

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Benefits of a delta wing:

  • Due to the large root chord, a delta wing combines low relative wing thickness with a sufficiently thick wing spar for a lightweight structure. Since a low relative thickness keeps wave drag down (a drag component which occurs only in supersonic flow), this makes delta wings especially attractive for supersonic aircraft.
  • The large root chord gives the delta wing a high internal fuel volume even at a low relative thickness.
  • The large root chord also provides it with a large surface area which helps to bring the minimum speed of the aircraft down.
  • With sufficient leading edge sweep, a delta wing produces vortex lift, so flow separation can be turned into a means of increasing lift.
  • A delta wing is naturally stable in pitch, therefore it does not require a separate tail surface.

Drawbacks of a delta wing:

  • The large wing area causes more viscous drag for the same amount of lift compared to a high aspect ratio wing. Swept wings have a better lift-to-drag ratio (L/D) than delta wings.
  • High-lift devices like fowler flaps are hard to integrate into delta wings. The higher relative thickness of regular airliner wings allows to integrate large flaps more easily, and the rearward location of the trailing edge of a delta wing will produce intolerable pitching moments if such flaps would be deflected.

Generally, the moderately swept high aspect ratio wings of subsonic airliners are ideal for flight at high subsonic Mach numbers. Delta wings are superior only in supersonic flight, and due to their good low speed characteristics they offer the best overall compromise for supersonic aircraft.

XB-70 in flight

North American XB-70 in flight, a classical delta wing design (picture source). Note the canard wing, which has little sweep and a low aspect ratio: This kind of wing is ideal for supersonic flight, provided it has a low relative thickness. The main wing of the XB-70 is a delta wing to keep the take-off and landing speeds relatively low and offers the best compromise of low and high speed characteristics. Also, the wing tips could be deflected downwards to improve directional stability at supersonic speed and to improve L/D by using the waverider effect.

Boeing 787 wing

Airbus 350 wing

The wings of both the Boeing 787 (top, picture source) and the Airbus A350 (bottom, picture source) show their powerful high-lift devices in these pictures, which allow them to fly with less than half the wing area of a comparable delta wing aircraft. Also, the lower surface area translates into lower drag at high flight speed, and the higher possible wing span helps to reduce induced drag, making those wings far superior for efficient flight at high subsonic speed.

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  • $\begingroup$ With regards to flaps, delta wings typically make use of their natural stability at high angles of attack to perform a flaring maneuver as an alternative to flaps. Like flaps, a flaring delta wing is both high lift and high drag. This is part of the reason for the drooping nose design of the concorde. The one downside to flaring vs flaps is that you can use your elevator to correct pitching changes when deploying flaps but since you're using elevators for flaring you can't do that. Therefore flaring needs to be done gently if you don't want to end up gaining altitude by mistake. $\endgroup$ – slebetman Feb 10 '16 at 5:23
  • $\begingroup$ @slebetman: Also, a delta wing experiences much more ground effect during landing. But still, the maximum lift coefficient is maybe 40% of that of a fowler-flapped wing, so area-wise it is much less efficient. Also, look at the outer wing of the SR-71: It is pulling around a lot of extra drag from the drooped leading edge just to make sure the plane doesn't roll off during flare. But leading edge devices just won't fit into such a thin wing. $\endgroup$ – Peter Kämpf Feb 10 '16 at 7:15
  • $\begingroup$ The fuel argument isn't meaningful in the supersonic and high transonic case, though, because the cross sectional area is essentially fixed with the Sears-Haack body if I'm not mistaken. I suppose the distribution of volume could be advantageous if you want to keep fuel away from the cabin and maybe there are situations where you'd want to distribute the fuel weight to a wing. $\endgroup$ – Gus Dec 25 '16 at 19:20
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    $\begingroup$ @Gus: Since the delta wing helps to stretch out the thickness distribution, it allows a bigger cross section. The Sears-Haak body of a delta winged aircraft is larger - just compare a F-106 or Dassault Mirage to the EE Lightning! The delta wing is larger but the local gradients are the same. On the Lightning they had to add (as a belly tank) what they cut out from the wing initially. Wouldn't it be better to have that volume contribute lift as well? $\endgroup$ – Peter Kämpf Dec 26 '16 at 0:13
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Delta wing advantages

  • Can fly at high angles of attack, which that's why fighter jets use a delta wing to fly at very high angles of attack

  • Provides good amount of drag for landing. On landing you need drag and that's the purpose of flaps is to increase drag and lift and you can make the flaps a little bit smaller

Delta wing disadvantages

  • Lots of drag, which is caused by vortex lift that a delta wing uses instead of conventional lift. Instead of just suffering drag from a wing tip vortex you suffer even more with 2 extra vortices that form on a delta wing.

With a swept wing you reduce drag by decreasing the lift. The problem with that type of wing at supersonic speeds is that you surfer from wave shock drag. Wave shock drag forms when air on top of the wing exceeds the speed of sound. Even at subsonic speeds that a 787 travels at you suffer from wave shock drag. Air is moving so fast on top of the wing at subsonic speeds that it causes the air to break the sound barrier. If you try to fly a swept wing at supersonic speeds your aircraft is really going to suffer from wave shock drag and any faster speeds could result in the whole plane splitting apart and is very serious. A delta wing doesn't have a slope on top of the wing like a swept wing does so the air doesn't travel any faster than the aircraft does. The reason airliners use swept wings is to reduce the lift to reduce the amount of high pressure air that can rotate around the wingtip. The swept wings reduce wake vortex and wake vortex can actually decrease an aircrafts lift and you want as much lift as possible. The major companies that build airliners main focus is to decrease drag. With a supersonic aircraft your main focus is to reach supersonic speeds and aren't designed to reduce drag. So you wouldn't design a supersonic aircraft that has swept wings, but you would design it with delta wings.

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    $\begingroup$ Hi Ethan. Sorry to say but your answer is rambling, contains many inaccuracies and misunderstandings and is not salvageable. We are all aware that you are only 14 and we admire your thirst for knowledge and your dreams. This site though expects high quality accurate answers and does not distinguish who the poster is. If Peter Kampf posted a wrong answer, it would be downvoted. This is how Stack Exchange works. If you clean my car for me, and you spend 1 hour doing it but do a lousy job or you spend 5 minutes doing it and do a great job, which one am I going to like the most? $\endgroup$ – Simon Oct 22 '15 at 7:07
  • $\begingroup$ @Simon "If Peter Kampf posted a wrong answer, it would be downvoted." can you pass by chat? $\endgroup$ – Federico Apr 6 '17 at 15:07

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