A recent scratch built glider kept turning to the right even after painstaking efforts to match the wings perfectly. Extensive addition of fin area to the tail failed to correct it. Closer examination showed the left leading edge had been carved to a sharper point, angled more sharply than the curvature of the wing.

After carving the right LE to a point the glider flew perfectly straight.

This was not expected at the 5-7 mph the glider flies at, and the effect seemed to be stronger at lower speeds.

Could sharper leading edges help even at very low speeds?enter image description hereenter image description here

  • $\begingroup$ Sand it off to match the other wing and see what the effect is. Personally, I don’t think this is causing what you describe. $\endgroup$ – Greg Taylor Dec 26 '18 at 18:39
  • $\begingroup$ @Greg Taylor actually i matched the other wing to it. Both on the same plane made for a quite unexpected "wind tunnel" experiment. Big relief pulling the extra lumber off the tail after it straightened out. Made me think of the "old" B52 using spoilers instead of rudder and aileron. Control surfaces are next. $\endgroup$ – Robert DiGiovanni Dec 26 '18 at 19:21

The leading edge is round to allow a wider range of angles of attack. If the airfoil is operated at one angle of attack only, the leading edge can be sharp. See the leading edges of compressor blades or fowler flaps for example - the stator rsp. the wing ahead of them directs airflow such that the angle of attack variation on the airfoil is small, in case of flaps especially if the flap has only one extended position.

This means that your sharp leading edge works well because (by chance) it is exactly where the stagnation point in trimmed flight goes. Re-trim the aircraft for a different speed and the right-turning tendency will be reversed. Unfortunately, now you made both sides similar, so you cannot run this test directly.

  • $\begingroup$ Thanks so much for responding. Lucky indeed as FF gliders usually fly at Vbg. The right turn (much to my dismay) did not appear until velocity stabilized to trim speed as you have correctly diagnosed. The "spike" may help generate forward lift by creating a less diffuse airstream near the leading edge, similar to what a slat does at higher AOA. Wonder if LEs could be adjusted like inlet spikes to save fuel? Will build more and test. $\endgroup$ – Robert DiGiovanni Dec 27 '18 at 8:43
  • $\begingroup$ @RobertDiGiovanni: The F-18 does it (it's called "living wing") - the slats and flaps are continuously adjusted to the current angle of attack. $\endgroup$ – Peter Kämpf Dec 27 '18 at 19:25

Other factors equal, a thinner ("sharper") leading edge will generally reduce drag in an airfoil. There are tradeoffs, however; first and foremost in this case is the much more abrupt onset of stall as the sharper leading edge makes boundary layer detachment more abrupt. In addition, the coefficient of lift will generally be lower, though peak L/D will often increase.

The combination of lower coefficient and sharper stall onset can make even a model cranky to fly -- in the case of a glider, a bad launch may cause a stall that devolves into a dive into the ground, even though a good launch might outperform the same model with a more conventional airfoil. For competition hand-launch gliders, this is often tolerated and compensated by long practice at launching, in order to get the best possible glide performance. For an R/C or piloted craft, on the other hand, this is a serious safety issue.

  • $\begingroup$ absolutely 100% with you on the launching technique. This is why John Collins had a quarterback throw his indoor record "Suzanne" paper airplane over 226 feet. Thanks! $\endgroup$ – Robert DiGiovanni Dec 26 '18 at 19:26

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