Polar Analysis from flight logs?

Andy Blackburn wrote:

Without knowing the technique Dick Johnson uses, or
the specs on a specific pressure transducer, it's hard
to know if measuring pressure altitude through a digital
transducer is more or less accurate than the traditional
method. I'd guess it's a close call, but that has nothing
to do with GPS.

I don't know the specifics of Dick's instrument, but a digitally logged
unit has got to be better, maybe a lot better, than trying to read a
mechanical altimeter with a little vibrator on the panel!


The main source of error, is being able to turn GPS
ground speed (or distance) into IAS reliably by subtracting
wind speed and adjust for altitude.

I think the pilots trying this are not doing that, but instead rely on
their airspeed to give them IAS. This does require calibration of the
ASI for real accuracy.

An even greater
source of error is trying to use fixes from a typical
soaring day with airmass movements and pilot control
inputs, airspeed changes and flightpath deviations.

I agree completely, and expect the best data to come on the same kind of
day Dick uses; that is, not a soaring day!

It's this air mass movement that drives the Akafliegs to using "sacred
cows" to measure the motion of the test glider relative to the "cow".



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Eric Greenwell
Washington State
USA

At 07:30 29 December 2004, Eric Greenwell wrote:

I think the pilots trying this are not doing that,
but instead rely on
their airspeed to give them IAS. This does require
calibration of the
ASI for real accuracy.

Actually the first post in this thread (from Mark)
read:

'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?'

This original idea has now morphed into a suggestion
essentially to replicate the technique used by Dick
Johnson and others, with the main difference being
using the barometric altitude transducer in a flight
computer instead of the mechanical altimeter(?). This
might offer some improvement in accuracy, but is at
least as complex to execute as the flight test techniques
used for the past 40+ years.

There are two main challenges with using flight logs
only:

1) There is no good source for IAS, so you have to
try to estimate it from GPS ground speed.

2) Typical soaring flights don't involve adequately
calm vertical airmass movement and probably not constant
enough airspeed to trust even long glides of many tens
of miles.

9B

Andy Blackburn wrote:

This original idea has now morphed into a suggestion
essentially to replicate the technique used by Dick
Johnson and others, with the main difference being
using the barometric altitude transducer in a flight
computer instead of the mechanical altimeter(?). This
might offer some improvement in accuracy, but is at
least as complex to execute as the flight test techniques
used for the past 40+ years.

There are two main challenges with using flight logs
only:

1) There is no good source for IAS, so you have to
try to estimate it from GPS ground speed.

This data is available from various instruments, like the Cambridge 302
that I use. For the 302 it's actually the TAS, which could be combined
with the air temperature and pressure altitude to compute the IAS. The
TAS and temperature are not recorded in the 302 flight log,
unfortunately, but they could be recorded by the Ipaq connected to it
and inserted in the flight log it keeps, and this log used for the analysis.


2) Typical soaring flights don't involve adequately
calm vertical airmass movement and probably not constant
enough airspeed to trust even long glides of many tens
of miles.

I think this is a show-stopper, even if the IAS is available in a flight
log.
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USA

Andy Blackburn writes:

There are two main challenges with using flight logs only:

1) There is no good source for IAS, so you have to try to estimate
it from GPS ground speed.

2) Typical soaring flights don't involve adequately calm vertical
airmass movement and probably not constant enough airspeed to trust
even long glides of many tens of miles.

You really need a different logger, that records raw GPS carrier phase,
plus static, pitot and temp. And Alpha vane would also be a big help.
Also you would want to run at 10 sample/sec or possible more.

If you go to this much trouble, you may as well ues 3 sets of GPS and
can then extract full position, attitude, and velocities as well.

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The different air masses, the different instruments
ect. ect.
obviously lead to different results. Not being the
physic's expert, I was just wondering if anyone has
tried flight testing Sailplanes in a wind tunnel? Could
you 'hard point' it and measure weights or lack of
weight to get Polar info. Im sure boeing or MD has
done something like that. I suppose that you may not
get a direct glide ratio but could get a common start
point that all gliders then could be compared to. Any
comment from the mathematicians?



At 14:00 29 December 2004, Andy Blackburn wrote:

This original idea has now morphed into a suggestion
essentially to replicate the technique used by Dick
Johnson and others, with the main difference being
using the barometric altitude transducer in a flight
computer instead of the mechanical altimeter(?). This
might offer some improvement in accuracy, but is at
least as complex to execute as the flight test techniques
used for the past 40+ years.

There are two main challenges with using flight logs
only:

1) There is no good source for IAS, so you have to
try to estimate it from GPS ground speed.

2) Typical soaring flights don't involve adequately
calm vertical airmass movement and probably not constant
enough airspeed to trust even long glides of many tens
of miles.

9B

At 20:00 29 December 2004, Cliff Hilty wrote:
I was just wondering if anyone has
tried flight testing Sailplanes in a wind tunnel? Could
you 'hard point' it and measure weights or lack of
weight to get Polar info.

Hey Cliff,

A wind tunnel would allow you to measure lift and drag
forces directly at various speeds, angles of attack
and flap settings. From this you could generate a polar
for any weight. This would be the most accurate way
to generate a polar I believe because it is under lab
conditions with highly calibrated instrumentation.
Of course you'd need a big wind tunnel like the NASA
Ames 80'x120' low-speed tunnel. You can rent it out,
but I bet it's expensive. To cover all the speeds,
flap settings and weights you'd need several dozen
test points, which would require a few hours in the
tunnel.

http://windtunnels.arc.nasa.gov

9B

At 20:00 29 December 2004, Eric Greenwell wrote:
Andy Blackburn wrote:

1) There is no good source for IAS, so you have to
try to estimate it from GPS ground speed.

This data is available from various instruments, like
the Cambridge 302
that I use. For the 302 it's actually the TAS, which
could be combined
with the air temperature and pressure altitude to compute
the IAS. The
TAS and temperature are not recorded in the 302 flight
log,
unfortunately, but they could be recorded by the Ipaq
connected to it
and inserted in the flight log it keeps, and this log
used for the analysis.

Neat idea - is that a configuration item on the 302
and/or WinPilot, or do you need to write software/firmware
to make it happen? Of course you'd still need to calibrate
the 302 IAS transducer for each sailplane type, but
at least then it would automate the data capture for
the kinds of performance testing Dick Johnson does.
Maybe he already has a rig to do this, but maybe not.
It would also allow collecting data from more than
a single test subject since anyone who could take a
high tow on a calm day and hold a constant airspeed
for more than 5 minutes could contribute a test point.

Of course here I'm assuming that all 302s and sailplanes
of a particular type with the same pneumatic setup
have similar airspeed errors, which may or may not
be true.

9B

Andy Blackburn wrote:
The
TAS and temperature are not recorded in the 302 flight
log,
unfortunately, but they could be recorded by the Ipaq
connected to it
and inserted in the flight log it keeps, and this log
used for the analysis.


Neat idea - is that a configuration item on the 302
and/or WinPilot, or do you need to write software/firmware
to make it happen?

I don't know about Winpilot, but it's not part of SeeYou Mobile. I think
it could easily be added by SeeYou and included in the Ipaq flight log
it can already keep, and I've suggested it to SeeYou. It's just another
data "sentence" that the 302 can send out.

Of course you'd still need to calibrate
the 302 IAS transducer

The sensor could be (probably is already) calibrated by Cambridge and
other labs when it is calibrated for flight recorder use.

for each sailplane type,

You wouldn't have to measure your pitot/static system if you were
interested only in your glider, and not in comparing the data with data
from other gliders; for example, if you just wanted to maximize your use
of the flaps and speeds.

but
at least then it would automate the data capture for
the kinds of performance testing Dick Johnson does.

I'm sure he does use an automated logger of some kind. I don't think
he's used a "vibrating panel" for quite a while!

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Eric Greenwell
Washington State
USA

at least then it would automate the data capture for
the kinds of performance testing Dick Johnson does.

"Eric Greenwell"

I'm sure he does use an automated logger of some kind. I don't think he's
used a "vibrating panel" for quite a while!

I the spring of 2003 Dick Johnson when he tested my glider he still used
the hard ware he has been using for a long time.
A vibrator for the Panel, he calibrates the airspeed indicator and the
altimeter. He uses his own instruments and takes notes of temperature
and as he flies trough 500ft timed intervals. He is able to do 3 to six
speeds
for each flap setting on one flight from 12000ft. In my case he made two
flight. The other aids are a stop watch, a note pad and at least two if
not
three pencils. I would be able to provide you with a copy of the numerical
data sheet.
Regards
Udo

Udo Rumpf wrote:

I'm sure he does use an automated logger of some kind. I don't think
he's used a "vibrating panel" for quite a while!


I the spring of 2003 Dick Johnson when he tested my glider he still used
the hard ware he has been using for a long time.
A vibrator for the Panel, he calibrates the airspeed indicator and the
altimeter. He uses his own instruments and takes notes of temperature
and as he flies trough 500ft timed intervals. He is able to do 3 to six
speeds
for each flap setting on one flight from 12000ft. In my case he made two
flight. The other aids are a stop watch, a note pad and at least two
if not
three pencils. I would be able to provide you with a copy of the
numerical data sheet.

I was sure I'd read/heard he was using an electronic logger, at least at
one time. Did he mention if he'd tried one, or if some of the pilots
assisting him used one?

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Eric Greenwell
Washington State
USA