3/19/2011 Sink Rate Data
j...@oxaero.com
Jari,
Here is today’s sink rate data (red). Conditions were poor, but I wanted to get a rough idea whether moving the forward edge of the lower surface-only tape down from the nose would fix the severe loss for airspeeds below 60 KIAS (AOA above zero), the orange curve. This time I used 3/8” wide Tesa 4101 tape that is .003” (76 um) thick. The forward edge of the tape is located 45 degrees below the nose of the wing. As I mentioned in my prior message, this is extremely easy to install. Anyone can do it in less than an hour.
The dotted curve is a third order polynomial fit. The waviness of the data is likely due to the poor conditions today. But I am confident that the polynomial fit is roughly true. I plan to fly this way this season and try to get some parallel flights.
This should be good for practical flying. I look forward to your measurements and competition results this season.
This data is very interesting because it confirms that the notched polars (below) are top surface phenomenon after all.
I questioned this when the first lower-surface only data replicated the performance notch (red curve above). I was so sure that the notch would disappear that I didn’t take the critical notch speed and I forgot to take the 55 KIAS speed too, so the notch is not well defined above, but it can be seen anyway. What is curious about this is that the top surface effects were occurring from the forward tape edge on the nose of the wing. I would never have thought that flow could detach from such a forward position. The cause of the general depression of performance below 60 KIAS is not clear to me, except that it comes from the forward tape edge being so high that for AOAs above zero the flow sees a step down instead of a step up.
Unlike the top surface mods, this one is not temperamental in-flight, is extremely easy to install using nothing but some Tesa tape of the type we use for smoothing the forward edge of pre-curved Mylar gap seals. And, the payoff is very much worthwhile. It will be interesting to see how it works with other airfoils. I would not be surprised if it produces good results on most glider airfoils.
Jim J