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

[akiley]

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

[Bastoune]

Aaron,

A couple of questions regarding the approach used: For the
calculation, did you use the GPS distance and GPS altitude to
calculate the L/D ratio? The altitude reading of the GPS may not give
you the tolerance needed on such a short run. You should loose only
approximately ~150 feet on a minute run at 50kts. This is not much. I
do not know the tolerance on altitude of GPS except that I undertand
that they are often poor. Extending the length of the run (2-3
minutes) may reduce the height measurement errors.

If you used the altimeter, did you have a vibrating gizmo attached to
the case to "free" the needle? My altimeter only moves 100feet at a
time when left to its own...

As for the distance covered, the GPS again my throw things off if
there was some wind. If the run was up and then down the down, the
wind factor could be eliminated from the calculation. If it was a side
wind, it would be very difficult to extract.

No real answer but perhaps areas to explore. Good luck.

Bastoune

[Eric Greenwell[_4_]]

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)

[Alan[_6_]]

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

[aerodyne]

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/aerodyneservices#p/u/2/GJLeuUYbWdM

aerodyne

[akiley]

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

[Tony[_5_]]

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.

[akiley]

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/aerodyneservices#p/u/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

[akiley]

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

[Mike the Strike]

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

[Bob Gibbons[_2_]]

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

[John Firth]

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

[Brian[_1_]]

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.

[[email protected]]

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

[Ramy]

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

[[email protected]]

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.

[David Reitter]

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! ;-)

[Ramy]

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

Ramy

[akiley]

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

[Ramy]

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

[Roel Baardman]

> How do you plan to take the wind out of the equation?

I have performed succesful tests with wind and thermals (in Condor
though, not real life) where by alternating the direction you fly in
and then averaging, you get pretty okay results.

The only huge flaw I can think of in this setup is the assumption that
the wind field is the same for the entire altitude-range.

Roel

[akiley]

Yes, I realized after I wrote the post that calculating LD can't be done without speed, distance and altitude lost. You can figure out feet per minute sink, but then there is the airmass wild card.

The original test with the Cirrus was flown in a straight line at the following IAS. 70, 60, 50, 40. Then I turned the opposite direction and flew the same speed sets. Might be better to fly one speed, do an immediate 180 and fly the same speed. Probably for longer that the 45 seconds per speed that I used.

Also realize that TAS has to be computed, ASI calibrated, winds and airmass.. ... Aaron

[[email protected]]

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!

[akiley]

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.