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

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

If you want another set of data, take a look at this video I made of
the Open Cirrus. All the instruments were calibrated, you can see the
30 sec digital averager on the panel top, the timer on the lower left
with the altimeter will give you a timed measurement, and the Winter
on the left is a 2 min averager. This flight is on OLC if you want
the GPS data.

I don't have time to comment on you results, except to say reread the
Johnson and Bikel articles on test measurement.

http://www.youtube.com/user/aerodyne.../2/GJLeuUYbWdM

aerodyne

On Aug 11, 10:53*am, aerodyne wrote:
If you want another set of data, take a look at this video I made of
the Open Cirrus. *All the instruments were calibrated, you can see the
30 sec digital averager on the panel top, the timer on the lower left
with the altimeter will give you a timed measurement, and the Winter
on the left is a 2 min averager. *This flight is on OLC if you want
the GPS data.

I don't have time to comment on you results, except to say reread the
Johnson and Bikel articles on test measurement.

*http://www.youtube.com/user/aerodyne.../2/GJLeuUYbWdM

aerodyne

Thanks, interesting video. I just uploaded my video in two parts.
I'm pretty sure I collected the data correctly. Here is part 1.

http://www.youtube.com/watch?v=MgzhZDN_9RY

Here is part 2.

http://www.youtube.com/watch?v=Ic4DNgox7MA

I find the Excel spreadsheets hard to understand. There is one called
Performance.xls that is a blank. More homework I guess.

.... Aaron

If anyone's interested in attempting to figure out, or give feedback
on my glide video, I'd be interested in comments. As I mentioned, the
numbers were not good.

http://www.youtube.com/watch?v=MgzhZDN_9RY

Here is part 2.

http://www.youtube.com/watch?v=Ic4DNgox7MA


.... Aaron

On Aug 12, 6:14*am, akiley wrote:
If anyone's interested in attempting to figure out, or give feedback
on my glide video, I'd be interested in comments. *As I mentioned, the
numbers were not good.

http://www.youtube.com/watch?v=MgzhZDN_9RY

Here is part 2.

http://www.youtube.com/watch?v=Ic4DNgox7MA

... Aaron

Errors in your ASI could account for a good chunk of your problems.
Have you tried a calibration with another known source? With the
knowledge of air density, you can calculate true airspeed and
indicated airspeed from your GPS data.

As others have also mentioned, you don't need much vertical air
movement to screw things up either.


Mike

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

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

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.