wrote in message
ups.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