You have touched on the issue that may explain the wild variations in performance values vs. airspeed that I have been measuring. I’ll tell you what I think is going on, but keep in mind it’s only a guess and I may be proved wrong.
The image below shows a significant performance notch around 50 knots indicated. The notch is very, very narrow. Just one or two knots on either side (48 and 51 KIAS) there is a performance increase. The narrowness of this anomaly tells me that we are not looking at the tape tripping the flow. Nor would that explain the big performance increases on each side. My view is that at that precise airspeed, the LE Tape along the most effective span range of the wings is working perfectly. That is, the thin, laminar flow leaves the surface cleanly at the edge of the tape and accelerates with no resistance at all from the surface behind the tape. The result is a too flat trajectory that results in a taller than normal transition bubble and a more violent reattachment at a steeper than normal angle. This would add to form drag, skin friction and turbulent energy loss in the wake.
I believe that on each side of the precise AOA that produces the notch, the flow does not clear the wing surfaced cleanly and so there is just enough skin friction to tame the excessively linear trajectory, causing it to detach a little behind the tape and follow the wing surface in a marginally detached state so that the bubble is flattened and reattachment is at a grazing angle that produces much less turbulence.
This explanation may be totally wrong. Time will tell.
> -----Original Message-----
> From: John Firth [mailto:juliet...@ncf.ca]
> Sent: Saturday, July 31, 2010 8:08 AM
> To: j...@oxaero.com
> Subject: L.E tape
> Hi Jim
> It is likely that location of the step with respect to the stagnation point
> is important; for the FX S 02-196 mod the S pt will move considerably
> with A o A
> so the useful speed range may be quite limited.
> Do you have any idea how critical the distance is?
> For a flapped airfoil the S pt will not move much.
> My oil flow tests will resume when I have sanded the wing and got some
> 50 SAE used oil. I also need an empty airstrip and little wind.
> John F