Those of you who have followed my postings about Jim Smith's RV-6 and the big boost he has gotten in performance with his new wingtips know that I was somewhat taken by surprise at how much better they performed than what I had predicted. It finally came to me this morning what factor might be making the difference.
For those of you familiar with the Oswald efficiency factor of a wing and how it enters into the calculation of a wing's CL and induced drag, you know that a rectangular wing planform, with a really good tip shape, will have an OEF of about 0.8 to 0.82 for the best. Since that is in the denominator of the equations, that means that a wing's required CL and its CDI will be about 22% to 25% worse.
But were you also aware that an elliptical wing has an OEF of 1.0. That got me thinking about whether the excess improvement in Jims' performance may be do to an OEF that is greater than the 0.81 I had used. I had done a determination of Jim's before and after lift distribution using the Schrenk method, and the after version was much closer to the elliptical ideal than the before.
So I did some calculations based upon his multi flight averages at 10,000' dalt with the original tips and the new tips. When I ended up, after making power adjustment for his different speed and rpm, I came out with an Oswald efficiency factor of 0.91; that's 12.4% better than what I had assumed in my original performance estimates, and can be shown to be the difference factor for my estimates.
Coupled with this are the pix Jim took of tufts on his wing when approaching a stall that show the tufts straight back up to and just before the stall. This indicates that there is almost no spanwise flow and a very minimal tip vortex.
So it appears that by proper shaping and extension of the tip region, the wing can be made much more efficient both in terms of lift and drag than can be obtained by just one of the range of curved under, flat, or curved up shapes that are available.
For those of you familiar with the Oswald efficiency factor of a wing and how it enters into the calculation of a wing's CL and induced drag, you know that a rectangular wing planform, with a really good tip shape, will have an OEF of about 0.8 to 0.82 for the best. Since that is in the denominator of the equations, that means that a wing's required CL and its CDI will be about 22% to 25% worse.
But were you also aware that an elliptical wing has an OEF of 1.0. That got me thinking about whether the excess improvement in Jims' performance may be do to an OEF that is greater than the 0.81 I had used. I had done a determination of Jim's before and after lift distribution using the Schrenk method, and the after version was much closer to the elliptical ideal than the before.
So I did some calculations based upon his multi flight averages at 10,000' dalt with the original tips and the new tips. When I ended up, after making power adjustment for his different speed and rpm, I came out with an Oswald efficiency factor of 0.91; that's 12.4% better than what I had assumed in my original performance estimates, and can be shown to be the difference factor for my estimates.
Coupled with this are the pix Jim took of tufts on his wing when approaching a stall that show the tufts straight back up to and just before the stall. This indicates that there is almost no spanwise flow and a very minimal tip vortex.
So it appears that by proper shaping and extension of the tip region, the wing can be made much more efficient both in terms of lift and drag than can be obtained by just one of the range of curved under, flat, or curved up shapes that are available.