best way to measure actual polar of a glider?

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.