Polar diagrams and L/D

[Jim[_33_]]

I sure need help here.

As far as I understand polar diagrams the airspeed axis is for indicated airspeed, not true airspeed. This certainly makes sense.

BUT, I have also come to understand that an estimate of L/D for a given airspeed can be obtained by dividing a given airspeed on the polar curve by the associated sink rate for that point on the curve.

BUT AGAIN, if the airspeed on the polar diagram is IAS wouldn't an L/D derived from a polar diagram be reasonably accurage only at sea level in standard conditions in still air?

If a glider pilot is interested in the distance traveled for a given loss of altitude wouldn't TAS be more useful than IAS - especially at higher altitudes? Of course headwind/tailwind would still be a factor in how much distance the glider can cover for a given altitude loss.

I've likely just bunged up this entire thing.

Jim

[Scott Williams]

On Monday, September 18, 2017 at 11:16:37 AM UTC-5, Jim wrote:
> I sure need help here.
>
> As far as I understand polar diagrams the airspeed axis is for indicated airspeed, not true airspeed. This certainly makes sense.
>
> BUT, I have also come to understand that an estimate of L/D for a given airspeed can be obtained by dividing a given airspeed on the polar curve by the associated sink rate for that point on the curve.
>
> BUT AGAIN, if the airspeed on the polar diagram is IAS wouldn't an L/D derived from a polar diagram be reasonably accurage only at sea level in standard conditions in still air?
>
> If a glider pilot is interested in the distance traveled for a given loss of altitude wouldn't TAS be more useful than IAS - especially at higher altitudes? Of course headwind/tailwind would still be a factor in how much distance the glider can cover for a given altitude loss.
>
> I've likely just bunged up this entire thing.
>
> Jim

Hey Jim,
My less than technical use of IAS/sink rate is a moment by moment comparison of airspeed indicator (in Knots) vs vario indication, also in Knots. the result is compared to conditions and how far away from my turnpoint or landing site.
Generally, if I'm worse than polar, I'm in less buoyant air, or outright sink and might entertain a course divergence, if better, I'm happy.
For Me, during soaring flight I find changes in lift/cruise too dynamic to really consider the difference of TAS vs IAS. Of course other than the limits imposed by lots of altitude. (I wish that was my problem more often)

I hope I have not revealed some fundamental ignorance on my part,
Good lift,
Scott, standard cirrus

[Dan Marotta]

Your vario sink rate is an indicated rate, as is the airspeed indicator,
so your calculated glide ratio, /_in the airmass_/ should be a simple
calculation.Â* Having said that, the air is too dynamic to care what your
instantaneous glide ratio is and, like thermals, and investments, past
performance is no guarantee of future performance.Â* It's all "golly,
gee-whiz" stuff at the end of the day.

Have fun!

On 9/18/2017 10:36 AM, Scott Williams wrote:
> On Monday, September 18, 2017 at 11:16:37 AM UTC-5, Jim wrote:
>> I sure need help here.
>>
>> As far as I understand polar diagrams the airspeed axis is for indicated airspeed, not true airspeed. This certainly makes sense.
>>
>> BUT, I have also come to understand that an estimate of L/D for a given airspeed can be obtained by dividing a given airspeed on the polar curve by the associated sink rate for that point on the curve.
>>
>> BUT AGAIN, if the airspeed on the polar diagram is IAS wouldn't an L/D derived from a polar diagram be reasonably accurage only at sea level in standard conditions in still air?
>>
>> If a glider pilot is interested in the distance traveled for a given loss of altitude wouldn't TAS be more useful than IAS - especially at higher altitudes? Of course headwind/tailwind would still be a factor in how much distance the glider can cover for a given altitude loss.
>>
>> I've likely just bunged up this entire thing.
>>
>> Jim
> Hey Jim,
> My less than technical use of IAS/sink rate is a moment by moment comparison of airspeed indicator (in Knots) vs vario indication, also in Knots. the result is compared to conditions and how far away from my turnpoint or landing site.
> Generally, if I'm worse than polar, I'm in less buoyant air, or outright sink and might entertain a course divergence, if better, I'm happy.
> For Me, during soaring flight I find changes in lift/cruise too dynamic to really consider the difference of TAS vs IAS. Of course other than the limits imposed by lots of altitude. (I wish that was my problem more often)
>
> I hope I have not revealed some fundamental ignorance on my part,
> Good lift,
> Scott, standard cirrus

--
Dan, 5J

[Charlie M. (UH & 002 owner/pilot)]

To me, if whatever is shown and you're cutting it close, you're screwed.
Period.

Start finding better air, be looking for landing spots on a long final glide. Better to land in a controlled environment than waiting for a last minute, "hope my great piloting precludes a broken sailplane"!

This from someone that has a lot of off airport landings, one was in a spot (in a borrowed 1-26) that even local hang glider pilots said was impossible.

Just my thoughts.......

[Eric Greenwell[_4_]]

Dan Marotta wrote on 9/18/2017 10:01 AM:
> Your vario sink rate is an indicated rate, as is the airspeed indicator, so your
> calculated glide ratio, /_in the airmass_/ should be a simple calculation. Having
> said that, the air is too dynamic to care what your instantaneous glide ratio is
> and, like thermals, and investments, past performance is no guarantee of future
> performance. It's all "golly, gee-whiz" stuff at the end of the day.

Unless the vario manual declares the calibration is in "indicated" units, I've
always assumed it's calibrated in true rate-of-climb. I recall the old mechanical
(and maybe the thermistor flow sensor varios), flow driven vane-type varios did
read "indicated", but for decades, varios have used pressure sensors and I think
they are calibrated in "actual rate of climb".

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/Guide-to-transponders-in-sailplanes-2014A.pdf

[Bob Gibbons[_2_]]

On Mon, 18 Sep 2017 09:16:34 -0700 (PDT), Jim >
wrote:
>I sure need help here.
>
>As far as I understand polar diagrams the airspeed axis is
>for indicated airspeed, not true airspeed. This certainly makes sense.
> ... text deleted

If generated from flight test data, the polar plots from all the
flight test measurement programs I have been involved with include the
reduction of all data (including airspeed) to sea level standard
atmospheric conditions, (e.g., 29.92 mb, 59F).

In addition, if measured data, airspeed is corrected for instrument
erros as well as pitot/static errors. Altimeter data is similarly
corrected for instrument errors.

If the polar is from a theoretical calcuation rather than a flight
measurment, I'd assume they do similar correction to standard
atmospheric conditions.

Anyone interested in the full details of flight preformance
measurements can consult Dick Johnson's comprehensive article in April
1968 SOARING magazine. Also, Dick published a more general update in
May 1989 SOARING.

Bob

On Mon, 18 Sep 2017 09:16:34 -0700 (PDT), Jim >
wrote:

>I sure need help here.
>
>As far as I understand polar diagrams the airspeed axis is for indicated airspeed, not true airspeed. This certainly makes sense.
>
>BUT, I have also come to understand that an estimate of L/D for a given airspeed can be obtained by dividing a given airspeed on the polar curve by the associated sink rate for that point on the curve.
>
>BUT AGAIN, if the airspeed on the polar diagram is IAS wouldn't an L/D derived from a polar diagram be reasonably accurage only at sea level in standard conditions in still air?
>
>If a glider pilot is interested in the distance traveled for a given loss of altitude wouldn't TAS be more useful than IAS - especially at higher altitudes? Of course headwind/tailwind would still be a factor in how much distance the glider can cover for a given altitude loss.
>
>I've likely just bunged up this entire thing.
>
>Jim
>

---
This email has been checked for viruses by AVG.
http://www.avg.com

[jfitch]

On Monday, September 18, 2017 at 9:16:37 AM UTC-7, Jim wrote:
> I sure need help here.
>
> As far as I understand polar diagrams the airspeed axis is for indicated airspeed, not true airspeed. This certainly makes sense.
>
> BUT, I have also come to understand that an estimate of L/D for a given airspeed can be obtained by dividing a given airspeed on the polar curve by the associated sink rate for that point on the curve.
>
> BUT AGAIN, if the airspeed on the polar diagram is IAS wouldn't an L/D derived from a polar diagram be reasonably accurage only at sea level in standard conditions in still air?
>
> If a glider pilot is interested in the distance traveled for a given loss of altitude wouldn't TAS be more useful than IAS - especially at higher altitudes? Of course headwind/tailwind would still be a factor in how much distance the glider can cover for a given altitude loss.
>
> I've likely just bunged up this entire thing.
>
> Jim

To the first order anyway, the glider only knows IAS. If you are high, the air is thinner, you are flying faster to generate the same lift (and the same pitot pressure), and also sinking faster (since you are gliding at the same angle). L/D is Lift/Drag ratio, and is numerically equal to the glide ratio. Lift and drag both vary the same with increasing altitude (proportional to air density x velocity ^2). The only thing that changes is your speed over the ground - TAS, if no wind.

[Dan Marotta]

A man who has one vario knows exactly the airmass movement rate.Â* A man
who has two is never quite sure.

This discussion sounds like the military way:Â* measure with a
micrometer, mark with a grease pencil, cut with an ax.Â* Do you really
believe the current altitude record is accurate to 5 significant
digits?Â* Is that in meters or in feet?Â* I know they claim centimeter
accuracy with GPS, but is it repeatable?Â* How about the thickness of the
line on a barograph, a kink in a pneumatic tube or a spot of sunshine on
the tube?

We went into space mostly with three significant digit accuracy (think
slide rules), why is a glide ratio so important when it's such a
transient thing in an active airmass?Â* I have witnessed a 25:1 glider
(or less) beat a whole gang of 40+:1 gliders.Â* It's the pilot, in the
glider, at that moment, and at that location that makes the performance,
not a number on a graph.

Just my two cents worth.

Dan

On 9/18/2017 9:06 PM, Eric Greenwell wrote:
> Dan Marotta wrote on 9/18/2017 10:01 AM:
>> Your vario sink rate is an indicated rate, as is the airspeed
>> indicator, so your
>> calculated glide ratio, /_in the airmass_/ should be a simple
>> calculation.Â* Having
>> said that, the air is too dynamic to care what your instantaneous
>> glide ratio is
>> and, like thermals, and investments, past performance is no guarantee
>> of future
>> performance.Â* It's all "golly, gee-whiz" stuff at the end of the day.
>
> Unless the vario manual declares the calibration is in "indicated"
> units, I've always assumed it's calibrated in true rate-of-climb. I
> recall the old mechanical (and maybe the thermistor flow sensor
> varios), flow driven vane-type varios did read "indicated", but for
> decades, varios have used pressure sensors and I think they are
> calibrated in "actual rate of climb".
>

--
Dan, 5J

[Frank Whiteley]

On Tuesday, September 19, 2017 at 9:13:54 AM UTC-6, Dan Marotta wrote:
> A man who has one vario knows exactly the airmass movement rate.Â* A man
> who has two is never quite sure.
>
> This discussion sounds like the military way:Â* measure with a
> micrometer, mark with a grease pencil, cut with an ax.Â* Do you really
> believe the current altitude record is accurate to 5 significant
> digits?Â* Is that in meters or in feet?Â* I know they claim centimeter
> accuracy with GPS, but is it repeatable?Â* How about the thickness of the
> line on a barograph, a kink in a pneumatic tube or a spot of sunshine on
> the tube?
>
> We went into space mostly with three significant digit accuracy (think
> slide rules), why is a glide ratio so important when it's such a
> transient thing in an active airmass?Â* I have witnessed a 25:1 glider
> (or less) beat a whole gang of 40+:1 gliders.Â* It's the pilot, in the
> glider, at that moment, and at that location that makes the performance,
> not a number on a graph.
>
> Just my two cents worth.
>
> Dan
>
The following recognition was approved at the recent SSA BOD meeting. I do not know if differential GPS was used or available with the GPS flight recorders measurements.

<quote>
Todd Walter and Duncan Eddy - I hereby nominate Todd Walter and Duncan Eddy for the SSA Exceptional Service Award for their willingness to take their personal time and doing the work which resulted in the Perlan flight recorders (FRs) achieving the first successful certification of high altitude flight recorders.
When a last-minute-type of message came in early summer 2015 from the Airbus-sponsored Perlan project for help to get their FRs certified, a network of people were contacted for assistance. Although several people offered aid, a decision was made to utilize the positive response from Todd Walter, senior research engineer in the Department of Aeronautics and Astronautics at Stanford University, to help by taking on the work immediately. Todd oversaw the operation, setting up an arrangement with Stanford Professor Simone D'Amico to involve his grad student Duncan Eddy who was experienced in operating the GPS simulator.
Considerable effort was needed by Todd and Duncan to find proper connecting cables which were then successfully utilized and testing began. Readings were taken every 5,000' in ascent/descent. However. at 49,000', the run ceased getting readings from the FRs. Both FRs were tried to no avail. Todd searched through the FR documentation and discovered there was an electronic switch in the FR that prevented operation above 15,000 meters. Since operation of the switch on the FR to permit operation above 15,000 meters could not be achieved in the GPS simulator testing lab being utilized at Stanford, the FRs were sent back to the manufacturer in Europe. They reset the switch to permit operation above 15,000 meters and the FRs were sent back to Stanford where runs were successfully completed to above 90,000 feet on both FRs.
The complete evaluation of the FRs was completed in a very short, but intensive work effort on their part, resulting in the issuance of approval for use of that model of FR for high altitude flight verification. The FRs were then shipped to Argentina for use in the Perlan flights of last year. It is believed that this
marked the first time FRs were calibrated for flight above 15,000 meters and that it was done utilizing GPS simulators.

Submitted by Bernald Smith
SSA Honorary Vice Chair
</quote>

Frank Whiteley

[[email protected]]

"Considerable effort was needed by Todd and Duncan to find proper connecting cables..."

- yeah, that's usually the hardest part! :-)

[[email protected]]

Jim -- space does not permit a full reply, if you email me at I can send you a fuller set of notes ... but there is a fundamental thing about "polar plots" that most people don't get: the two axes on a polar plot must be HORIZONTAL and VERTICAL speeds, so that the axes are orthogonal, that is the very definition of what the plot means, and it is key to the use of the plot to find best L/D etc by graphical methods.

Because the L/D of sailplanes are high the error that results from the common lazy use of the glider's airspeed for the horizontal axis aren't too big, but it's mathematically incorrect.