how to compute parameters for newer gliders?

If manufacturers stopped providing polars for their new gliders, how are the computer manufacturers supposed to calculate data for those gliders, like STF ?

On Tuesday, January 21, 2014 2:04:40 PM UTC-5, Lucas Pirolla wrote:
If manufacturers stopped providing polars for their new gliders, how are the computer manufacturers supposed to calculate data for those gliders, like STF ?

Get a high tow into still air. Go on a sled ride. Determine the stall speed, then fly steady just above stall speed. Increase speed by 5 knots and fly steady. Repeat until you reach VNE.

Use the computer to log airspeed, altitude and elapsed time during this flight. Plot the logged results and connect the dots. Voila! A polar for your glider flown by you.

Thanks to Richard (Dick) Johnson for doing this the hard way on so many gliders.

On Tuesday, January 21, 2014 6:14:33 PM UTC-6, son_of_flubber wrote:
On Tuesday, January 21, 2014 2:04:40 PM UTC-5, Lucas Pirolla wrote:

If manufacturers stopped providing polars for their new gliders, how are the computer manufacturers supposed to calculate data for those gliders, like STF ?



Get a high tow into still air. Go on a sled ride. Determine the stall speed, then fly steady just above stall speed. Increase speed by 5 knots and fly steady. Repeat until you reach VNE.



Use the computer to log airspeed, altitude and elapsed time during this flight. Plot the logged results and connect the dots. Voila! A polar for your glider flown by you.



Thanks to Richard (Dick) Johnson for doing this the hard way on so many gliders.

All soaring software packages I know calculate the polar from only 3 speed/sinkspeed data pairs. The commonly used software has extensive databases of common gliders, often back to pre-historic times. If you don't find your glider, pick a similar one and go flying. Over time you can tell if the polar chosen is about right - if not, make corrections. For a glider manufacturer to offer a new glider without providing a polar would be foolish to say the least.

On Tuesday, January 21, 2014 7:14:33 PM UTC-5, son_of_flubber wrote:
Get a high tow into still air...

The likelihood of measuring meaningful data yourself is ZERO.
Johnson published measurements are in some cases badly off,
as the CG was not controlled during testing, and the datapoints
and reduction inadequate. It is REALLY HARD to do this properly
and only Idaflieg (?sp) measurements are generally useful.

Choose a similar polar and dial in a few bugs until final
glides generally match...

Hope that helps,
Best Regards, Dave

On Tuesday, January 21, 2014 2:04:40 PM UTC-5, Lucas Pirolla wrote:
If manufacturers stopped providing polars for their new gliders,
how are the computer manufacturers supposed to calculate data
for those gliders, like STF ?

Some of us work with the glider manufacturers and are not allowed
to release the raw data we use to generate the polars. In some
cases we take the published polars and adjust out the marketing
factor (which can be quite large). Other cases we adjust based
on results we observe during extended competition flights...

Probably not the answer you wanted !
Hope this helps,
Best Regards, Dave

On Wednesday, January 22, 2014 8:36:38 AM UTC-8, Dave Nadler wrote:
On Tuesday, January 21, 2014 7:14:33 PM UTC-5, son_of_flubber wrote:

Get a high tow into still air...



The likelihood of measuring meaningful data yourself is ZERO.

Johnson published measurements are in some cases badly off,

as the CG was not controlled during testing, and the datapoints

and reduction inadequate. It is REALLY HARD to do this properly

and only Idaflieg (?sp) measurements are generally useful.



Choose a similar polar and dial in a few bugs until final

glides generally match...



Hope that helps,

Best Regards, Dave

This is imprecise at best for a whole bunch of reasons relating to measurement errors and differences between the configuration of your glider and whatever measured or analytical approach was used to make the polar you get from whatever source.

For a modern glider you really care about a speed just around (preferably above) best L/D and one that is in the middle of your most typical cruise speed.

Fly some long final glides (still air is best of course) one of these two speeds and see if you gain or lose on the arrival altitude. Move the point down of you are consistently losing, move the point up if you are consistently gaining. It is possible that you will adjust the points in opposite directions. Remember to note your wing loading.

Simpler is to pick your most common glide speed and use the bugs adjustment.. There are quadratic formulas widely available to make polars from three data points.

9B

Then you will have a polar that works for you.

On Wednesday, January 22, 2014 11:36:38 AM UTC-5, Dave Nadler wrote:
On Tuesday, January 21, 2014 7:14:33 PM UTC-5, son_of_flubber wrote:

Get a high tow into still air...

The likelihood of measuring meaningful data yourself is ZERO.

If I cannot empirically measure an accurate polar by for my body in my glider (wearing my shoes), then how is it possible to use a published polar to accurately predict the performance of my glider (with me in it)?

I can see that measuring my own polar and then using it to calculate my ideal glide slope doubles the source of error.

Factoring out weather, I'm thinking that the biggest source of real world deviation from the idealized polar would be the total weight and the CG, and measuring my glider's polar with me in it factors out that error.

Do glider manufacturers have special inventories of still air for empirically derive polars?

I think if you read one of Johnson's articles carefully, you should be able to identify several other key variables/controls that affect results. An obvious one (in addition to weights and CGs mentioned above) is pitot/static system calibration. Just in that area alone, there are probably a half-dozen related areas to consider if you are talking about accuracy to within a percent or two...

P3

On Wednesday, January 22, 2014 12:25:18 PM UTC-5, son_of_flubber wrote:
On Wednesday, January 22, 2014 11:36:38 AM UTC-5, Dave Nadler wrote:

On Tuesday, January 21, 2014 7:14:33 PM UTC-5, son_of_flubber wrote:



Get a high tow into still air...



The likelihood of measuring meaningful data yourself is ZERO.



If I cannot empirically measure an accurate polar by for my body in my glider (wearing my shoes), then how is it possible to use a published polar to accurately predict the performance of my glider (with me in it)?



I can see that measuring my own polar and then using it to calculate my ideal glide slope doubles the source of error.



Factoring out weather, I'm thinking that the biggest source of real world deviation from the idealized polar would be the total weight and the CG, and measuring my glider's polar with me in it factors out that error.



Do glider manufacturers have special inventories of still air for empirically derive polars?

In this instance calculating the polar for one self would actually be
favourable. The polar will be calculated with exactly the same errors
as were present while the sample data was gathered, therefore precise if
not exactly accurate.

Cheers,
Luke

On 01/22/2014 1:25 PM, Papa3 wrote:
I think if you read one of Johnson's articles carefully, you should be able to identify several other key variables/controls that affect results. An obvious one (in addition to weights and CGs mentioned above) is pitot/static system calibration. Just in that area alone, there are probably a half-dozen related areas to consider if you are talking about accuracy to within a percent or two...

P3

On Wednesday, January 22, 2014 12:25:18 PM UTC-5, son_of_flubber wrote:
On Wednesday, January 22, 2014 11:36:38 AM UTC-5, Dave Nadler wrote:

On Tuesday, January 21, 2014 7:14:33 PM UTC-5, son_of_flubber wrote:



Get a high tow into still air...



The likelihood of measuring meaningful data yourself is ZERO.



If I cannot empirically measure an accurate polar by for my body in my glider (wearing my shoes), then how is it possible to use a published polar to accurately predict the performance of my glider (with me in it)?



I can see that measuring my own polar and then using it to calculate my ideal glide slope doubles the source of error.



Factoring out weather, I'm thinking that the biggest source of real world deviation from the idealized polar would be the total weight and the CG, and measuring my glider's polar with me in it factors out that error.



Do glider manufacturers have special inventories of still air for empirically derive polars?

Fair point :-) Assuming, I guess, that the same errors are reflected in whatever device is used for creating the glide calculations.

It really must be Winter...

P3

On Wednesday, January 22, 2014 2:47:08 PM UTC-5, Luke Szczepaniak wrote:
In this instance calculating the polar for one self would actually be

favourable. The polar will be calculated with exactly the same errors

as were present while the sample data was gathered, therefore precise if

not exactly accurate.



Cheers,

Luke



On 01/22/2014 1:25 PM, Papa3 wrote:

I think if you read one of Johnson's articles carefully, you should be able to identify several other key variables/controls that affect results. An obvious one (in addition to weights and CGs mentioned above) is pitot/static system calibration. Just in that area alone, there are probably a half-dozen related areas to consider if you are talking about accuracy to within a percent or two...



P3



On Wednesday, January 22, 2014 12:25:18 PM UTC-5, son_of_flubber wrote:

On Wednesday, January 22, 2014 11:36:38 AM UTC-5, Dave Nadler wrote:



On Tuesday, January 21, 2014 7:14:33 PM UTC-5, son_of_flubber wrote:







Get a high tow into still air...







The likelihood of measuring meaningful data yourself is ZERO.







If I cannot empirically measure an accurate polar by for my body in my glider (wearing my shoes), then how is it possible to use a published polar to accurately predict the performance of my glider (with me in it)?







I can see that measuring my own polar and then using it to calculate my ideal glide slope doubles the source of error.







Factoring out weather, I'm thinking that the biggest source of real world deviation from the idealized polar would be the total weight and the CG, and measuring my glider's polar with me in it factors out that error.







Do glider manufacturers have special inventories of still air for empirically derive polars?