"Bill Daniels" bildan@comcast-dot-net wrote in message
. ..
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.


All this stuff has been used by the Idaflieg in the 80's, and they now work
on a differential GPS measurement system.

The point with the vane obviously works only if its base is in the free
flow, and if it is the base which is levelled horizontally. If the base is
connected to the fuselage, it will give you the fuselage pitch and nothing
else.
The point with all measurements done without a reference glider is that you
can't cancel vertical airmass movements. That's ok if you are measuring a
2-33, but on anything like a 40+ ship an airmass movement of a few cm/s will
result in substantial differences. An error of 1 cm/s in the sink of a 50:1
ship gives ore takes almost 1 point. A couple of cm/s sink is something you
often have associated with high pressure regions over a wide area (that's
actually how they function).
That is the main reason the Idaflieg measurements are usually trusted far
more than Dick's measurement. Dick is putting an enormous effort in getting
good results, but he just can go so far.