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

On Aug 11, 10:07*am, akiley wrote:
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.

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

I would think that a good portion of the error is that you have not
converted your Indicated Airspeed to True Air Speed. Of course has you
have mentioned you would need the air temp to do that accurately.

On Thursday, 11 August 2011 02:59:20 UTC+1, 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

Why not making it simple (and more accurate and realistic) and use SeeYou to analyze your IGC file which can give you the glide ratio in every segment? Granted it will give you glide ratio over the ground which is fine if there is no wind or if you average the up wind and down wind legs (avoid cross wind as suggested).

Ramy

Go and read all the Bickle and Johnson articles on how they went about going these tests. They had to go to a LOT of trouble to test performance accurately. Measuring the performance of a glider is not simple or easy. Johnson did very high tows usually on very calm winter mornings and he still found there were runs which were rendered unusable due to vertical air mass motion.

That said, one thing to take into account, especially with a Standard Cirrus is that Johnson discovered that the static system on this ship had fairly significant errors by design (many gliders do) and that this has to be taken into account when using the A.S.I. readings to calculate performance. Even if the A.S.I. itself is perfect it likely will still be giving readings that are significantly at variance with calibrated airspeed.

I think it was George Moffatt who suggested that a glider designer who claimed a 45:1 L/D should be towed to one mile altitude and released....45 miles OFFSHORE!

That's a good one. They would probably make it to shore, fly to California City, then north to Minden and land back in California city all before sunset.