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On June 7, 2008 I flew my deturbulated Standard Cirrus parallel against a Dianna 1 (PDF) to compare performance under actual soaring conditions. The Diana 1 belongs to Bob Moore of Memphis Soaring Society, who piloted his aircraft. On one out and return, we flew two legs that add up to 20 minutes of parallel flying. That's more than enough time under normal soaring conditions for a performance comparison between gliders.
The first graph shows altitude vs. time as the two aircraft cruised at about 51 kts indicated for the Std. Cirrus. The duration of the first leg of the comparison was 12 minutes. We then turned around and flew for 8 minutes. Finally, we climbed together in a strong thermal for a 3000 foot gain. This is shown in the second graph. In these graphs, the RED trace represents the deturbulated Std. Cirrus and the AQUA trace represents the Diana 1.
In each case, the deturbulated Std. Cirrus and the Diana 1 performed equally. However, in 2006 Dick Johnson measured this Std. Cirrus to have a glide ratio of 32.5 in standard configuration at 49 kts calibrated (51 kts indicated). And in 1999, he measured a Diana 1 to have a glide of about 40 at 49 kts calibrated. The difference from 32.5 to 40 corresponds to 23% improved performance for the deturbulated Std. Cirrus.
This was a realistic comparison in typical turbulent atmospheric conditions, so the performance figures above are not in fact the performance levels of the aircraft. Concerning the Diana 1, its extremely laminar wings probably suffer losses in turbulent conditions. However, the deturbulated Std. Cirrus has a very narrow performance peak at about 49 kts calibrated and actual performance losses 5 kts on each side. (See Johnson Effect Confirmed) For these parallel flights the airspeed indicated in the Std. Cirrus bounced around in the neighborhood of 46 kts to 52 kts calibrated. So, neither ship performed up to the levels measured in smooth air. Nevertheless, the Std. Cirrus should have lost altitude to the Diana 1 over the 20 minutes that they flew in formation. The Std. Cirrus should have lost altitude at 149 fpm while the Diana 1 lost altitude at 125 fpm, for a difference of 24 fpm. Over the 12 minutes of the first leg of the parallel flight, the Std. Cirrus should have lost 288 feet, and it should have lost another 192 feet over the 8 minute second leg. All together the Std. Cirrus should have lost 480 feet to the Diana 1.
The performance measurements published by the manufacturer, PZL-Bielsko-Diana Sailplanes, of the Diana 1 show a better glide ratio than Dick Johnson measured. I take the Johnson measurements to be more realistic. Nevertheless, if the manufacturer's data are accepted, then the improved performance of the deturbulated Std. Cirrus is about 38%. I point this out only to say that the afore mentioned 23% is a conservative figure.
For anyone who would like to replay these flights in their IGC compatible post-flight analysis software, I offer the following log files for download. For each glider there is the fill log file and also one that has been cropped down to just the pertinent section.
|867C3R21.IGC||6/7/2008||Cherry Valley, Arkansas||Std. Cirrus|
|867GNII1.IGC||6/7/2008||Cherry Valley, Arkansas||Diana 1|
This is hard stuff for many people to swallow, and one path to denial might be to claim that the second aircraft in the test was not in fact a Diana 1. For this reason, I am posting below the soaring report for 6/7/08 at Memphis Soaring's giderport. Besides Bob Moore, pilot of the Diana 1, many of those present witnessed the launch of both aircraft and overheard the radio communications between Bob and me while conducting our test. I will be happy to supply contact information for those persons mentioned below who will bear witness to what they know.
MSS had a fairly light turnout despite very good soaring conditions yesterday. The winds were about 15 kts out of the south-southwest, I am pleased to report that everyone's crosswind techniques were up to the challenge and the soaring was well worth the effort.
Mark Warren broke the ice and reported strong lift and nice cloud streets. It wasn't long before a line formed for a tow, and we launched Dick Cadieux, Bob Moore, Phil Morgan, Mark Eshleman and Jim Hendrix. They basically all landed when they got tired or hot, or just had to leave. Joy and Charlene were there to help launch and keep us in good-behaviour.
Back in the hangar Tom Maher (? I hope I got that right) with Steve Aulman's and Steve Garner's help worked all day doing annuals on the club ships. I don't know the exact status of the ships but they seemed to be accomplishing what they set out to do. We all owe these guys a pat on the back!
Later on Steve A. got airborne for a late afternoon cool-down.
In between the 7 tows, Robert Williams and I weighed his glider on his new scales and overstressed some brain cells working through the weight and balance procedures outlined in his Polish aircraft manual. Diplomatic considerations prevent any further mention of our opinions of that manual.
No food or cooks were at hand so we settled for some cool drinks on the porch before calling it a day. It was a good one, sorry if you couldn't make it out.
Oxford Aero Equipment
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