From: 	Kevin Caldwell
Sent: 	Wednesday, May 14, 1997 8:07 AM
To: 	davis@halcyon.com
Subject: 	RE: Polar Corrections

Davis,

I'm not subscribed to the Digest, and I don't have permission to post,
so I won't be posting the airspeed corrections. I'm not sure I want to
get involved in a public discussion of polar measurement with
uncalibrated instruments anyway. Relatively small instrument errors in
sink rate or airspeed can obviously effect the best L/D number quite
significantly. I agree your polars are probably a reasonably good
comparison between those particular examples of the gliders, but I
wonder if you took some more runs of the XC how close they would be?

The correction for the airspeed depression below the wing should actually
be built into the instrument deck. Since it is actual glide through the
air and over the ground that are of interest for speed to fly, and speed
to get there respectively, then that is the speeds the fancy instrument
decks should be working with. 6% error at best L/D is fairly significant,
and it is bigger at lower speeds.

What polar are you using in your GC? The sink rate numbers your GC
produces seem higher than I have measured and seen on other decks, which 
would account for the lower L/D values you are measuring. It is harder
to check the vario calibration than the airspeed. Any idea how Ball sets 
the vario calibration? If they just rely on some +/- 10% resistors, then 
there could be some big errors. If the GC is incorrectly measuring
airmass and glider sink rates, then it will give you incorrect speeds to 
fly, particularly for speeds to get there. And of course much lower L/D
numbers.

I can understand why manufactures are not keen to have all this data
from uncalibrated instrument decks published everywhere. Even from one
deck and one pilot, the values of the L/D numbers will stick in peoples
minds. And if you add in multiple decks with varying calibration and
different pilots and harnesses, you have huge potential for the less
discriminating pilots to be misled with questionable data. Since
manufacturers are also face with the same calibration and scatter
difficulties, any data they might publish could end up making their
glider "leper of the year" against other manufacturer's uncalibrated data. 
Not a good scenario.

I liked your phrase that good science is expensive. I think we need high 
quality measurements like Dick Johnson does for sailplanes. This would have
been such a far better thing to spend thousands of dollars on than fighting
the waiver, but I guess that would never happen.

Kevin