Glider performance testing

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

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

I'm looking forward to seeing what else comes from this post.

Can't the pressure altitude recorded in the log file be used? The
altitude used doesn't need to be accurate in the absolute sense (does
it?); rather just show the accurate altitude delta over the measurement
window.

On the topic of flight tests, I usually hear crickets when I ask this
question, but does anyone know where the Ventus 2 polar came from in
the SeeYou database (Tools | Polar)?

~ted/2NO

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.

"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.

wrote:
Anyone have suggestions or caveats about doing it?

Myles Bradley

I would contact Dale Kramer from Canada. He has done the same thing
(using computer spreadsheet number crunching) for "Pilot Performance".
I bet if you contact him, you wouldn't have to waste a lot of time
re-inventing the wheel (oops, I meant the measurement criteria, and
number crunching)
Micki

wrote:
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

Reply:
The other columns on the spread sheet will be for airmass movement.
Figure out how to deal with this and it will be as easy as it looks.
This is why Idafleig uses a "master" glider in formation and compares
performance.
UH