Low test numbers on Standard Cirrus, what could it be?

On Wed, 10 Aug 2011 18:59:20 -0700 (PDT), akiley
wrote:

Hi All,

I'm fairly new to soaring, but thought it might be fun to test our clubs Standard Cirrus to see if it was getting advertised L/D.
I jumped in with not enough understanding of all the factors that effect performance, other than reading how Dick Johnson does his tests.
I took a tow to 6000 agl on a calm early morning when ground temps were about 70f. I raised the gear, closed both vents and
flew one minute legs after I stabilized the IAS at 40, 50, 60 and 70 KIAS. Then I turned in the opposite direction and flew the series again.
I video taped the gauges and got an IGC file from SeeYou/iPaq.

I did NOT factor temps and didn't compute calibrated airspeed. My average of 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 Cirrus.
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

I don't see any mention of calibrations of the airspeed pitot, static,
and instrument errors.

Did you use a water manometer to calibrate your airspeed instrument?
Did you use a static bomb and kiel tube to calibrate your pitot/static
system?

All detailed in Johnson's articles.

Our efforts to use GPS for alititude interval measurements have been
troubled by noise in the GPS data. We continue to use a calibrated
altimeter.

At some point, you'll also have to undertake the conversion to
standard atmosphere.

Bob