A polar calculation question for you number crunching enthusiasts out there

I'm trying to use Paul Remde's very handy "Reichmann to Cambridge and SeeYou" spreadsheet to derive some usable SeeYou polar numbers for my Diana-1. But I'm having a heck of a time picking three speed/sink-rate pairs from the data set that result in Max L/D and Best Glide Speed numbers that pass the sniff test.

For example, I can enter 3 pairs of values from early, middle, and late in the data set, and it tells me I have a max L/D of 49 (wouldn't that be nice!) at ... 38 km/h. Uh, that's well below stall speed!

Is there any conventional wisdom on this?

Are there any other tools available that can take a set of sink rates (hopefully more than 3) and produce some decent numbers?

Wishing the flight computers would just start with lookup tables,

tuno/ES

Ted,

Does the rest of the curve look like a reasonable match to the polar? If so that is likely good enough. You are not going to use a speed to fly to fly too slow. You will only fly down to best l/d speed anyway.

If you really want to force the lower portion of the curve you need to use more pairs of points and likely will need to go to a third or fourth order polynomial. The problem is most flight software is designed around the second order fit.

Tim

ES wrote, On 2/23/2014 3:49 PM:
I'm trying to use Paul Remde's very handy "Reichmann to Cambridge and
SeeYou" spreadsheet to derive some usable SeeYou polar numbers for my
Diana-1. But I'm having a heck of a time picking three
speed/sink-rate pairs from the data set that result in Max L/D and
Best Glide Speed numbers that pass the sniff test.

For example, I can enter 3 pairs of values from early, middle, and
late in the data set, and it tells me I have a max L/D of 49
(wouldn't that be nice!) at ... 38 km/h. Uh, that's well below stall
speed!

Is there any conventional wisdom on this?

Are there any other tools available that can take a set of sink rates
(hopefully more than 3) and produce some decent numbers?

Wishing the flight computers would just start with lookup tables,

Put in the values for an ASW 27. If after flying a few flights that
seems to pessimistic, put in the values for the next step up. Accuracy
in the polar isn't important to success.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

Accuracy in the polar isn't important to success.

Your position is stipulated -- my OP is ONLY about the accuracy of the polar.

On Sun, 23 Feb 2014 15:49:16 -0800, ES wrote:

Are there any other tools available that can take a set of sink rates
(hopefully more than 3) and produce some decent numbers?

Does your flight computer accept polars in the Winpilot format?

If so, are the numbers for an SZD-56-2 Diana-2 of any use, either
directly or for use as a starting point? If so the SZD-56-2 polar is
included on the LK8000 polars database.

Wishing the flight computers would just start with lookup tables,

Well, both XCsoar and LK8000 do just that and both load polars from
Winpilot format polar files, - but neither currently has a polar for the
SZD-56-1 Diana-1.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

I'm using SeeYou Mobile with an LXNAV V7. Both take polars in the same form: stall speed, A, B, and C coefficients, min (wing) loading, min weight, max weight, empty weight, and pilot weight.

I'm also confused about the weights. If "pilot weight" is provided, then I can only assume "empty weight" means just the glider without the stick actuator, but why would the computer care about that? Why not just "dry take-off weight"?

On Monday, February 24, 2014 5:15:14 AM UTC-8, ES wrote:
I'm using SeeYou Mobile with an LXNAV V7. Both take polars in the same form: stall speed, A, B, and C coefficients, min (wing) loading, min weight, max weight, empty weight, and pilot weight.



I'm also confused about the weights. If "pilot weight" is provided, then I can only assume "empty weight" means just the glider without the stick actuator, but why would the computer care about that? Why not just "dry take-off weight"?

Hmmm...

Do they have provision for different pilot profiles to fly the glider?

You could also throw Dick Johnson's polar test data into an Excel spreadsheet and do a curve fit - that might be a good starting point.

Mike

At 02:58 24 February 2014, Martin Gregorie wrote:
On Sun, 23 Feb 2014 15:49:16 -0800, ES wrote:

Are there any other tools available that can take a set of sink rates
(hopefully more than 3) and produce some decent numbers?

Does your flight computer accept polars in the Winpilot format?

If so, are the numbers for an SZD-56-2 Diana-2 of any use, either
directly or for use as a starting point? If so the SZD-56-2 polar is
included on the LK8000 polars database.

Wishing the flight computers would just start with lookup tables,

Well, both XCsoar and LK8000 do just that and both load polars from
Winpilot format polar files, - but neither currently has a polar for the
SZD-56-1 Diana-1.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

To add to what Martin says, LK8000 (don't know about XCS), has the facility
to add the wing area field to the polar file. You can then set your wing
loading, for actual flight, and the polar will be adjusted automatically.
Dave

At 10:20 25 February 2014, David Salmon wrote:
At 02:58 24 February 2014, Martin Gregorie wrote:
On Sun, 23 Feb 2014 15:49:16 -0800, ES wrote:

Are there any other tools available that can take a set of sink rates
(hopefully more than 3) and produce some decent numbers?

Does your flight computer accept polars in the Winpilot format?

If so, are the numbers for an SZD-56-2 Diana-2 of any use, either
directly or for use as a starting point? If so the SZD-56-2 polar is
included on the LK8000 polars database.

Wishing the flight computers would just start with lookup tables,

Well, both XCsoar and LK8000 do just that and both load polars from
Winpilot format polar files, - but neither currently has a polar for the

SZD-56-1 Diana-1.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

To add to what Martin says, LK8000 (don't know about XCS), has the
facility
to add the wing area field to the polar file. You can then set your wing
loading, for actual flight, and the polar will be adjusted automatically.
Dave



Use the excellent polar spreadsheets buy Paul Remde he
http://www.cumulus-soaring.com/soarmn/soaring_files.htm
Tables are not used by LK8000, and the quadratic approximations are not a
great way to do it, look-up would be much better!