On Aug 11, 1:31*am, (Alan) wrote:
In article writes:
Hi All,

I'm fairly new to soaring, but thought it might be fun to test our clubs St=
andard Cirrus to see if it was getting advertised L/D. *I jumped in with no=
t enough understanding of all the factors that effect performance, other th=
an reading how Dick Johnson does his tests.

I took a tow to 6000 agl on a calm early morning when ground temps were abo=
ut 70f. *I raised the gear, closed both vents and flew one minute legs afte=
r I stabilized the IAS at 40, 50, 60 and 70 KIAS. *Then I turned in the opp=
osite direction and flew the series again. *I video taped the gauges and go=
t an IGC file from SeeYou/iPaq.

I did NOT factor temps and didn't compute calibrated airspeed. My average o=
f both directions at each speed gave me: 26 L/D at 40 KIAS, 26.5 at 50, 28 =
at 60, and 25 at 70. *These numbers seem to be very low for a Standard Cirr=
us. *Johnson's results were in the neighborhood of 35 L/D. *It had no bugs,=
wing root tape, and yes, the gear was up.

Any comments or ideas other than sending me back to school? *... Aaron

* Well, doing it in both directions probably didn't have much effect, since
you were doing it with airspeed, not groundspeed.

* The suggestion that you do it for a greater time, such as 500 feet of
altitude, seems good. *Since you don't have an engine vibrating the altimeter,
you need to tap it to ensure it isn't lagging behind from minor friction.

* Even so, your results will be affected by air motion -- not horizontal wind,
as you are moving with that -- but vertical motion. *If you are in an area of
sink, your results will be worse. *If in an area of lift, you may get some
really great looking results!

* To get an idea of how you did, plot the sink rate polar from your results.
Then compare those points with the published results. *I would expect that you
may find an approximate vertical offset showing higher sink rates (shifting
the graph down), if you were testing in sink.

* * * * Alan

Thanks for the help. The longer sample makes sense. I forgot to
mention, I also had a Garmin 396 with WASS so altitude should have
been closer. After looking at the Garmin numbers, it was father off
from the iPaq/SeeYou and the panel altimeter. After a rough graph,
The slower speeds were consistantly worse than the 60 and 70 KIAS
speeds. Anyway, pretty bad numbers, and not very consistent either.
Maybe there was overall subsidence that day. Should have taken temp
readings and saved all that day's weather data.

The plots from my three sources compared to a published excel plot for
a clean Standard Cirrus showed fairly wild inconsistency between
sources mainly due to the Garmin's data. It also shows as mentioned
bad results at 40 and 50 KIAS, slowly getting better and 60 and 70.

Taking notes for next try which I won't get to for at least three
weeks. ... Aaron