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

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

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.

On Friday, August 12, 2011 10:58:58 AM UTC-4, Mike the Strike wrote:

Errors in your ASI could account for a good chunk of your problems.

The ASI in the Standard Cirrus uses static air from underneath the wings. That will produce some error. (Dick Johnson mentions this in one of the test reports, I think.)

Your data points are all below expectation, though, so the more likely explanation would be error from other factors. I don't know about yours, but the gelcoat on my Std Cirrus looks pretty rough in some places. Also, if you look at other people's measurements, you'll notice quite an error, so multiple takes are necessary to get more reliable results.

PS.: Have you tried to put tape on the underside of your wing? I hear that'll give you 5 L/D points! ;-)

It's been a while since we last heard about the turbulators project. Would be interesting to hear any updates.

Ramy

I think we may do more tests this fall. Was just at Williams Soaring Center renting there excellent ships, an ASW27 and 24. The air was very stable and I had high tows to 8000 over the mountains. I also now have a Colibrii II logger so I may have more accurate altitudes compared to GPS only. I may just look at segments in SeeYou and work out the performance using altitude lost over time. This takes wind out of the equasion. I will do the same for flights I have done in the Standard Cirrus if I have some resonable segments. But we had an amazing season in Michigan so most of that data will be totally polluted by volitile air.

Since I did a range of speeds with the older Cirrus tests, it seems I should have found reasonable performance at ONE of the speeds. BTW, Our Cirrus finish looks excellent to me.

.... Aaron

On Tuesday, October 23, 2012 9:15:33 PM UTC-7, akiley wrote:
I think we may do more tests this fall. Was just at Williams Soaring Center renting there excellent ships, an ASW27 and 24. The air was very stable and I had high tows to 8000 over the mountains. I also now have a Colibrii II logger so I may have more accurate altitudes compared to GPS only. I may just look at segments in SeeYou and work out the performance using altitude lost over time. This takes wind out of the equasion. I will do the same for flights I have done in the Standard Cirrus if I have some resonable segments. But we had an amazing season in Michigan so most of that data will be totally polluted by volitile air.



Since I did a range of speeds with the older Cirrus tests, it seems I should have found reasonable performance at ONE of the speeds. BTW, Our Cirrus finish looks excellent to me.



... Aaron

How do you plan to take the wind out of the equation? If you take ground distance out of the equation, and use altitude loss, you need to know your true airspeed. SeeYou will claculate it for you, but depend on the igc file you have, it is usually deriving TAS and IAS from the ground speed and calculated wind, but unfortunately the calculated wind can be very inaccurate, so your TAS will be far from accurate. The only accurate information that SeeYou has is distance flown and altitude loss. Altitude loss may be pressure or GPS. Pressure Altitude is more accurate, although GPS altitude is the closest to true altitude. SeeYou calculates L/D based on distance flown and altitude loss.

Ramy