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

On 8/10/2011 6:59 PM, 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? ...

A 500' drop is a much more common number. With 1 minute runs at best
L/D, you will be dropping about 120 feet, too small to measure accurately.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

At 03:29 11 August 2011, Eric Greenwell wrote:
On 8/10/2011 6:59 PM, 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? ...

A 500' drop is a much more common number. With 1 minute runs at best
L/D, you will be dropping about 120 feet, too small to measure
accurately.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

AS all the numbers were unreasonably low, I would suspect
general and fairly strong airmass subsidence; Blipmaps will
give you a good estimate of this for NA.

JMF