At 00:00 28 February 2006, Bill Daniels wrote:

wrote in message
oups.com...
I recently reread Richard Johnson's article, 'Sailplane
Performance
Flight Test Methods' in the May, 1989 issue of Soaring
magazine. In
the article Johnson mentions the Deceleration Measurement
method of
determining a performance polar. It had occured to
me a couple of
years ago that such a method could be used in conjunction
with GPS data
as a fairly simple way of measuring glider performance.
I envision
importing the GPS time, altitude, and position data
into a spread sheet
application to do the number crunching. Anyone out
there ever tried
anything like that? Anyone have suggestions or caveats
about doing it?

Myles Bradley



I don't know if it has been actually tried but I have
heard it discussed
several times. If, and it's a big if, you know that
the glider can be flown
perfectly level as it slows down, then you measure
the rate at which
airspeed is lost, the whole polar can be computed from
a single pass.

Back in the 1960's it was proposed to build an 'alley'
of helium balloons
staked out in two parallel rows exactly1000 feet above
a dry lake to avoid
ground effect. The glider pilot would use the balloons
as an eyeball guide
in flying a level path. An on-board strip chart recorder
would record
airspeed against time as it decayed from Vne to stall.

I don't think it was ever tried. Obviously, there
are considerable problems
with this approach not the least is that it will require
a dead calm day and
very cool hand on the controls. If there is an advantage
it is that only
one tow is needed and higher performance glider take
longer to slow down so
resolution improves. I don't see how GPS would help
unless, somehow, GPS
altitude could be made to provide centimeter level
accuracy. Vertical motion
in the atmosphere would still skew the results.

There is a third potential method with its own difficulties.
L/D is the
angle between exact level and the free stream flow.
If you had an inertial
reference unit that could establish exact level and
a pitch vane on a long
nose probe, the angle of the vane measured against
level is the L/D. The
problem is that the angles are very small. (40:1 L/D
= 1.43 degrees , 60:1
= .0.95 degrees) and small angular errors generate
large L/D errors.

All considered, the timed descent series method that
Dick Johnson uses is
the least difficult and if done carefully, produces
very good results.


Bill Daniels

I have calculated speed to fly/flap postion for my
DG808B by running a pre-planned gps course of 5 mile
per setting in a straight line and calculating L/D
for each speed from the data log. While the actual
L/D may be off slightly due to wind the relative values
should be accurate. This provides the best speed to
fly each flap position. I have also run pure L/D tests
also using a pre-planned gps course over a 15 mile
course in both directions to cancel out wind effect
which is what is loaded into my flight computer polar.