I have tried using a $10,000 carrier phase GPS receiver with 0.1m
precision (post-processed) for glide testing. The GPS data was so
precise, you could clearly see the antenna move a few cm when the wing
was raised for takeoff. Even when flying in the calmest conditions,
with no discernable airmass movement, the vertical motions are
significant.

In analysing the data, I could not precisely fit a straight line to the
data points from 1-2 min glides at constant airspeed, even after
correcting for slight airspeed variations using total energy. Johnson
does not have this problem because he only has two data points, one at
the beginning and one at the end of each glide. When you have about a
hundred data points, one every second, you can really see the problem.
You need a lot more data points to average out the noise.

Based on this, I doubt that you could get useful data from a less
precise GPS, with a slower sampling interval, in uncontrolled
conditions. There is just too much noise to get useful results without
an impossibly huge data set.

Eric Greenwell wrote:
Mark Zivley wrote:

We all know what the manufacturer's polars look like, but what
about
our individual planes. Has anyone done any work to develop a
program
that would look at some flight logs and determine what a particular
glider's actual polar is? At one point Ball was making a vario
system that would determine the aircraft's polar over time just by
flying.

For someone who already had some algorithms for computing wind from

ground track drift during thermals could take this info and then be
able to back figure from GPS ground speed what the IAS was during a

particular phase of the flight. By isolating longer sections of
cruise flight at varios airspeeds it should be do-able. Question
is,
has it been done.

I haven't heard of it being done, and I can't imagine how one would
compensate for air motion, both vertical and horizontal, just using
the
GPS info. Both motions change with location, altitude, and time.
Perhaps
if the flight record included the airspeed, like some varios can
supply,
there would be some hope of doing it. I don't think you could count
on
the vertical motion averaging to zero during the cruises, since we
typically adjust our path to include as much up air as possible.

You can get some good info using a flight recorder, but you have to
do
it when the air is calm. If you are really interested, invest in a
few
high tows and make the measurements. Take a look at this test done on
a
DG 800:

http://groups.yahoo.com/group/ASA-NewsGroup/message/59

You don't have to be a group member to read the message.


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Eric Greenwell
Washington State
USA