best way to measure actual polar of a glider?

I would like to measure and compare the actual polar of my glider to
the factory polar. I would also like to compare the polar with 16.6
meter tips vs winglets. Are there practical methods for doing this
with a flight logger and normal instruments? What airspeeds should be
measured, and how many points on the curve are needed to get
meaningful results?

Any experience or suggestions for a procedure would be greatly
appreciated. The glider is a Ventus C

THanks,

Matt Herron (Jr.)

I had run some L/D tests on my DG808B several years ago using GPS
waypoints on a moving map for accurate distance control and measured
altitude loss during long early morning two way runs. While no method
is going to be perfect I consider it to be adequate for choosing polar
data for the flight computer. I believe the factory estimates are too
optimistic.

The results can be found in our ASA Newsletter history here.
http://tinyurl.com/262m9pt
See the 2003 Jan/Feb issue.

On Jan 7, 11:43*am, "Matt Herron Jr." wrote:
.*Are there practical methods for doing this what airspeeds should be
measured,


It's fun to do, but to do it right is not cheap, is frustrating, and
never perfect.
Dick Johnson's articles on the subject are your best source.
Things start with an accurate knowledge of your system errors.
Particularly static system and instrument errors. These can be a big
surprise.

On Jan 7, 7:29*pm, n7ly wrote:
On Jan 7, 11:43*am, "Matt Herron Jr." wrote:

.*Are there practical methods for doing this what airspeeds should be
measured,

It's fun to do, but to do it right is not cheap, is frustrating, and
never perfect.
Dick Johnson's articles on the subject are your best source.
Things start with an accurate knowledge of your system errors.
Particularly static system and instrument errors. These can be a big
surprise.

Dick was a master craftsman at L/D measurement. He was not shy about
using new technology when it offered benefit. I expect he's be right
in the middle of GPS and video camera data acquisition systems.

Sometime in the late '60's I was a party to just such a conversation
which included Dick, Paul McCready, and Bruce Carmichael among
others. Paul pointed how critical air motion was to valid results and
agreed with Dick that East Texas in Fall often offered possibly the
best US conditions aloft for polar measurement. All noted that as
glider performance increased, accurate measurements would become ever
more difficult.

Then someone, I can't recall who, suggested an alternative drag based
method. It was to set up a measured course over which a test glider
would fly perfectly level as the airspeed bled off. Drag would be how
fast the airspeed decreased. The advantage was that the difficult
slow speed range would be more accurate as the speed decreased at an
ever slower rate as the glider lost speed. The higher the glider
performance, the longer it would take to lose speed so accuracy
actually increases with performance.

Of course, the practical problems were huge. The measured course
would have to be very long and end at a runway were the glider could
safely land. Determining 'perfect level' flight was another
difficulty. Here, I chimed in as I was involved in scientific balloon
flights at the time. A string of tethered balloons, I suggested,
could mark the course each at a precisely equal height MSL and spaced
at carefully measured intervals. The pilot would just be required to
follow the balloon trail The high speed portion of the course could
be over rough terrain and the slow end could be over one of the
Mojave's dry lake beds. A movie camera would record the airspeed and
balloon passages.

The tethered balloons would show any air motion which would affect the
data. If they all stood perfectly in line without waving around, the
data could be considered valid.

The group agreed it wold theoretically work but who, they asked,
wanted to spend all that time tethering balloons in the desert -
looking at me as they spoke. AFAIK, no one ever did it.

On Fri, 7 Jan 2011 18:29:27 -0800 (PST), n7ly
wrote:


Particularly static system and instrument errors. These can be a big
surprise.

Not as surprising as airmass movements.
THIS is the thing to know - anything else can be measured by GPS with
sufficient precision.


Dick Johnson did a phantastic job - but some of his results are far
off because of his limited ressources.

So far the German Idaflieg get the most accurate performance numbers -
they measure performance by flying the glider-to-be-tested next to a
glider (the "holy" DG-300/17) whose performance is precisely known
(calibrating this DG-300 takes about half a year!).

By flying through the same airmass thermal convection can be ruled
out.


I'm afraid but I'm pretty sure this is the only chance to get halfways
accurate performance measurements with today's technology, since at
the moment it is not possible to get information about thermal
convection of the airmass with sufficient precision (one inch/sec of
rising/sinking air is about two points of L/D).


Andreas

On Jan 7, 12:43*pm, "Matt Herron Jr." wrote:
I would like to measure and compare the actual polar of my glider to
the factory polar. *I would also like to compare the polar with 16.6
meter tips vs winglets. *Are there practical methods for doing this
with a flight logger and normal instruments? *What airspeeds should be
measured, and how many points on the curve are needed to get
meaningful results?

Any experience or suggestions for a procedure would be greatly
appreciated. *The glider is a Ventus C

THanks,

Matt Herron (Jr.)

Hi Matt, if you look at http://www.oxaero.com/Oxaero-Performance.asp,
and download the pdf file on that page, you will get everything you
could want to know about it; for the price of the CD (about 2 tows),
you get the whole thing ready to go. Good work on Jim Hendrix's part!

I've been thinking of doing it, but our season is so short, it seems
never to be the time. Also, Dick Johnson did a polar for my glider
that is very usable...

On Jan 7, 10:55*am, Gary Evans wrote:
I had run some L/D tests on my DG808B several years ago using GPS
waypoints on a moving map for accurate distance control and measured
altitude loss during long early morning two way runs. While no method
is going to be perfect I consider it to be adequate for choosing polar
data for the flight computer. I believe the factory estimates are too
optimistic.

The results can be found in our ASA Newsletter history here.http://tinyurl.com/262m9pt
See the 2003 Jan/Feb issue.

Thank you Gary for the link to your write up.

On Jan 8, 8:32*am, Dan wrote:
On Jan 7, 12:43*pm, "Matt Herron Jr." wrote:

I would like to measure and compare the actual polar of my glider to
the factory polar. *I would also like to compare the polar with 16.6
meter tips vs winglets. *Are there practical methods for doing this
with a flight logger and normal instruments? *What airspeeds should be
measured, and how many points on the curve are needed to get
meaningful results?

Any experience or suggestions for a procedure would be greatly
appreciated. *The glider is a Ventus C

THanks,

Matt Herron (Jr.)

Hi Matt, if you look athttp://www.oxaero.com/Oxaero-Performance.asp,
and download the pdf file on that page, you will get everything you
could want to know about it; for the price of the CD (about 2 tows),
you get the whole thing ready to go. *Good work on Jim Hendrix's part!

I've been thinking of doing it, but our season is so short, it seems
never to be the time. *Also, Dick Johnson did a polar for my glider
that is very usable...

Thanks Dan. This is really good stuff. My enthusiasm is restored.

There is one untried principle so far. A little vane on a trailing
bomb could measure angle of descent. All it needs is accurate
measurement of heading relative to straight down. Angle of descent is
unaffected by smoothly rising or sinking air. Has anyone tried this?

John Cochrane

On Jan 9, 8:03*pm, John Cochrane
wrote:
There is one untried principle so far. A little vane on a trailing
bomb could measure angle of descent. All it needs is accurate
measurement of heading relative to straight down. Angle of descent is
unaffected by smoothly rising or sinking air. Has anyone tried this?

John Cochrane

John, it's been seriously discussed several times - once in the
McCready/Carmichael group I mentioned before. L/D is indeed the angle
between a horizontal plane and the free air flow. However, for 40:1
it's around one degree which is a very small angle for experimental
measurement. A long carbon fiber nose boom with a pitch vane could do
pretty well with the free air flow part. The other part requires a
very good "stable table" inertial guidance system to provide a perfect
horizontal plane. There's been a lag between the technology to
measure L/D this way and the ever increasing L/D to be measured.
Today it could probably measure 40:1 pretty well but anything more
than that would be lost in the 'noise'. There are cheaper ways to
measure less than 40:1 L/D's.

Going back to the speed course idea. There are huge basins in the
western US where pools of cold air collect in winter. These pools of
heavy, cold air form under an intense inversion several thousand feet
thick which isolate them from air motions above it. These cold air
pools appear to be utterly free of all air motion. There are usually
perfectly flat dry lakes in the center of these basins where a glider
could land.

Laying out a carefully surveyed speed course where a glider could
enter the course at near redline speed and coast along well above
ground effect slowly losing speed in perfectly level flight wouldn't
be that hard. A recording laser range finder altimeter could assure
the flight was indeed level. An auto tow from the dry lake could
easily get a glider high enough to run the course. It would cost so
little per flight that scores of runs could be done to reduce
'scatter' of data points.