Is there any benefit to using swept wings for speeds up to 250km/h ?

(By the way..Why do some fast birds like falcons, swifts and most migratory birds use swept wings or raked wingtips since they fly at low subsonic speeds?)

Peregrine falcon - swept wings

Alpine swift - swept wings enter image description here

Albatros - straight wings with raked wingtips

Common swift - straight wings with raked wingtips enter image description here

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    $\begingroup$ Don't all birds fly at low subsonic speeds, even the really fast ones? $\endgroup$ – FreeMan Aug 28 '20 at 18:10
  • $\begingroup$ Subsonic is every speed below Mach 0.8 (987km/h), birds belong in this category.Fastest bird in horizontal flapping flight is swift (170km/h) and peregrine falcon in dive (390km/h) $\endgroup$ – member2017 Aug 28 '20 at 18:40
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    $\begingroup$ Note that birds have variable geometry wings, and all—including swallows, swifts and falcons—usually fly with straight wings when their intent is best glide range. They sweep the wings for higher speed, but it is more because of which adjustments they can make than a goal of its own. $\endgroup$ – Jan Hudec Aug 28 '20 at 21:04
  • $\begingroup$ @JanHudec Swept wing has lower drag coefficient compare to straight wings?Is this reason why birds sweep wing for high speed? $\endgroup$ – member2017 Aug 29 '20 at 4:35
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    $\begingroup$ @member2017, by partly folding the wing the bird changes some parasite drag, which increases with speed, for some induced drag, which decreases, moving the optimal point to higher speed. The drag is higher at this point than with fully stretched wing at its lower optimal speed, but lower than keeping the wing stretched at the higher speed. $\endgroup$ – Jan Hudec Aug 29 '20 at 9:27

Birds adjust wing area by pulling in their humerus and ulna so the wing sweeps first forward, and to compensate for that, have to sweep their outer wing backwards which is accomplished by folding the digits backwards. Instead of adjusting lift coefficient, they adjust wing area to not expose more surface to air friction than necessary.

The picture below shows how the wing is opened for maximum lift at low speed. As Zeiss Ikon correctly observes, folding the digits back and fore is the bird's way of pitch control, so the sweep is a consequence of pitch control, not aerodynamics.

Red-tailed Hawk Buteo jamaicensis, captive bird, Bacara, Santa Barbara, California.

Red-tailed Hawk, captive bird, Bacara, Santa Barbara, California. By Steve Jurvetson / CC BY (https://creativecommons.org/licenses/by/2.0)

  • $\begingroup$ Does swept wing has lower Cd than straight wing?Every bird at high speed put wing back,isnt this reducing drag force? $\endgroup$ – member2017 Aug 29 '20 at 5:55
  • $\begingroup$ @member2017 It certainly has a lower wing area. Drag coefficient might be the same or a bit lower due to the higher Reynolds number, but it is the product of area, dynamic pressure and drag coefficient that counts. And this product is certainly lower with a smaller wing area, so yes, pulling the outer wing back because of a pulled-in inner wing is reducing the drag force. $\endgroup$ – Peter Kämpf Aug 29 '20 at 7:45
  • $\begingroup$ Birds can also pull outer wing back when keep inner wing straight,(albatros at my post).All pointed wings birds has zero lift at wingtips-bell lift distribution. $\endgroup$ – member2017 Aug 31 '20 at 5:14
  • $\begingroup$ in my topic you say that sweep primary feathers dont have nothing to do with aerodynmics,but in this threads you explain raked wingtips with birds wings.can you explain why?.aviation.stackexchange.com/questions/8544/… aviation.stackexchange.com/questions/19073/… $\endgroup$ – member2017 Sep 1 '20 at 13:29
  • $\begingroup$ @member2017 Raked wingtips work best when restricted to wingtips and not when they constitute most of the wing. Birds use them, too, but when they pull in the inner wing the result does not make a "raked wing". $\endgroup$ – Peter Kämpf Sep 1 '20 at 20:24

Swept wings may be useful to adjust the center of lift when, for instance, a change in power plant or other equipment has resulted in an aft center of mass location, or when it's desirable to allow additional after cargo storage. They can also provide roll stability similarly to dihedral when flying at significant angle of attack.

Birds that have wing sweep generally seem to have the ability to vary this condition in flight -- which they appear (to my eye -- I'm not an ornithologist or even a bird-watcher) to use similarly to managing center of lift as above. In fact, almost all birds do this to some extent, along with tail movements, for pitch control.

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    $\begingroup$ When you say "additional after cargo storage", do you mean like, an unlucky chipmunk now gripped in talons? $\endgroup$ – Harper - Reinstate Monica Aug 29 '20 at 7:11
  • $\begingroup$ @Harper-ReinstateMonica Please tell me you're capable of at least distinguishing a shift from airplanes to birds in the discussion... $\endgroup$ – Zeiss Ikon Aug 30 '20 at 11:28

I was shown a hang glider with mild sweep to the wings, and a few degrees of tip washout, that is, the angle of attack was slightly lower at the wingtips (maintained by tensioning the rigging, I think).

The idea of the washout was that the central part of the wing stalled while the wingtips (with lower AOA) were still flying.

And the idea of the sweep was that the wingtips being aft of the centre section, the centre of lift moved aft when that happened, allowing the nose to drop, automatically recovering from the stall.

So, sweep can be used to help stability, and I'm pretty sure that hang glider stayed well below the transonic region.

EDIT : Tip washout and mild sweep visible here around 0:40 and 1:20, and clearly low Mach number.

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    $\begingroup$ I'm pretty sure that hang glider stayed well below the transonic region Citation needed? :P $\endgroup$ – DarcyThomas Aug 30 '20 at 5:34

Swept wings increase something known as the critical mach number. When air travels over the top half of an airfoil, it gains speed. If the incoming air far upstream from the wing is at a high subsonic speed, the speed gained as it travels over the wing can induce transonic, sonic, or even supersonic flow which can lead to unpredictable wing effects such as control reversal and loss of control as the P-38 pilots of WWII faced.

Swept back wings increase the amount of span-wise flow, decreasing the amount of velocity that the air picks up as it travels over the airfoil and allowing the wing to fly closer to the speed of sound.

Also check out supercritical airfoils which delay mach effects as well.

Hope this helps!

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    $\begingroup$ Given the description of the question, you may precise your answer to focus more on low-subsonic speeds. $\endgroup$ – Manu H Aug 29 '20 at 13:53

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