Third Parallel Flight - vs. Diana 1 (6/7/2008)
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,
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.
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.