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Is there any benefit to sweep back on an ultralight of:

  1. 15 deg
  2. 30 deg
  3. 45 deg.
  4. other?
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There are two primary benefits of wing sweep at ultralight speeds. The first is that it acts as variable dihedral -- the higher your angle of attack, the more dihedral effect you get from sweep.

Secondly, when you see ultralights (or hang gliders) with a lot of sweep, it's generally for a tailless design that needs to get the tip fins/rudders and twisted tips and/or elevons back behind the CG location so they have a useful moment arm.

The benefits of sweep in transonic flight (delay of Mach effects onset) obviously don't apply at ultralight aircraft speeds.

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    $\begingroup$ You sometimes end up with too much dihedral effect and are forced to build in geometric anhedral, like the Swift ultralight glider, and most high wing swept wing transports. $\endgroup$ – John K Jan 13 at 19:16
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    $\begingroup$ Possible restructuring: put second paragraph first, deleting first 3 words. Put first paragraph second, dropping first 8 words and saying "Also, sweep acts as a variable dihedral-- the higher your angle of attack..." Add second sentence to this paragraph saying "This may or may not be beneficial. Many ultralights actually have some anhedral to counteract this effect." Motivation for restructuring: in practice, wing sweep in ultralights is usually there for the benefit of the pitch axis dynamics, not for the the purpose of increasing the dihedral effect. Something like that, you get the idea. $\endgroup$ – quiet flyer Jan 13 at 22:47
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For an ultralight aircraft with a wing and tail design and a top speed of less than 60 mph, sweep of greater than 15 degrees does not serve a useful purpose.

However, some sweep can help solve a discovered center of gravity issue by moving the center of lift rearwards. This was commonly done on bi-planes such as the Tiger Moth well before the supersonic age.

Another benefit of sweep is increased yaw stability. This is not to be confused with the Dutch Rolling nightmare of large aircraft. A light aircraft will benefit from the stability if needed. Interestingly, this means a smaller vertical stabilizer but a larger rudder! Forward sweep has also been tried in gliders to make them more responsive to rudder inputs.

Sweep also increases the cross-wind "dihedral effect", which rolls the plane away from the crosswind component. For highwings, this can be used to balance the "anhedral effect" of side area under the CG, creating more desirable cross wind handling characteristics. The Cessna 172 is a fine example of near perfect cross wind effect balance.

But sweep can be detrimental to the stalling characteristics of an aircraft, as swept wings tend to start stalling near their tips. Washout or leading edge slats help prevent this. So, it is to be approached with caution, and full understanding of its effects. 7 degrees would be more like it, for starters.

The Electric Wren motorglider from the 1920s is a great ultralight reference from the days when slow flight was thoroughly understood and the pilots were glad of it.

Although highly swept and delta type designs were considered for this answer, in slow flight straight wings are more efficient, requiring less power, there for a lighter engine. The Electric Wren flew on less than 10 horsepower!

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