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The L/D benefits of laminar airfoils seem hard to pass if flying within their Reynolds number. But what would happen if you used them for canard aircraft? Could they be used on an aircraft like the Rutan Quickie or would it not be wise to use them for a main lifting surface because of the risk of stall...

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Well designed laminar airfoils have very benign stall characteristics. They are designed to not stall even when transition is happening very early. This can be seen in their lift curve slope which has two peaks, the first and lower one for mostly laminar flow, and the second at a higher angle of attack and lift coefficient, with a fully turbulent upper side. This second peak is shallow and wide, so lift will not drop off a cliff as it does in 5-digit NACA airfoils when going beyond the angle of maximum lift coefficient.

Two things need to be considerd when designing a canard with laminar airfoils:

  1. Wing sweep should be kept low. Sweep will cause a rotation of the local speed vector within the boundary layer which encourages early transition. With sweep below 20° or better 15°, laminar airfoils should work well. Less sweep will also allow to use a higher aspect ratio in order to stay below the sweep - aspect ratio limit for poor wing stall characteristics.

  2. Make sure that the wake from the canard will stay above or below the wing. The turbulence in the wake will destroy laminar flow. Of course you can design your canard such that its wake will start to hit the wing as the configuration approaches stall. But this will affect overall lift much more as when the wing wake hits the tail in a conventional design (a rather poular natural stall warning) because the main wing is so much larger than the canard.

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  • $\begingroup$ I helped a friend build a Q2 many years ago. One thing we learned was that rain had a serious negative affect on the laminar flow. Even a light rain would cause significant loss of lift. While he had no options, I don't know how you could account for it in the design. $\endgroup$
    – Gerry
    Commented Apr 6 at 21:19
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    $\begingroup$ @Gerry That was a common problem with laminar airfoils from the Sixties and Seventies. Everyone who has ever flown a Pik-20 or a Janus A in rain will agree. Those airfoils reached their maximum lift coefficient only when clean. Modern designs achieve their maximum lift coefficient with a fully turbulent upper side and fly almost as well in rain - turbulence will lower L/D, but not affect minimum speed. $\endgroup$ Commented Apr 7 at 18:26
  • $\begingroup$ Oh I see thank you. You can see the two peaks in the NLF airfoil lift curve., that's interesting. The GU airfoil drop dramatically, more so than the LS1. $\endgroup$
    – Josh Carr
    Commented Apr 8 at 2:26
  • $\begingroup$ Hi @PeterKämpf I have a design for a VTOL aircraft I am designing based on the Quickie. Do you still do any consulting? If possible please let me know!! Kind regards $\endgroup$
    – Josh Carr
    Commented Apr 28 at 10:24
  • $\begingroup$ @JoshCarr Never seriously did consulting, but keep asking! $\endgroup$ Commented Apr 29 at 6:30

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