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

On Sunday, February 23, 2014 3:49:16 PM UTC-8, ES wrote:
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?


I took a look at the spreadsheet. It uses the normal formulas for solving a quadratic fit to three points as near as I can tell. I built my own just because I like knowing what the assumptions are. Depending on what points you pick, values for L/D and sink outside those three points might be inconsistent with the actual polar - especially at the low end where the actual polar can be quite non-quadratic. If you care about speeds below best L/D you'll need to pick a point off the actual polar at that speed - it's probably a good idea so you don't get unrealistic advice if you set Mc=0 on a glide (not a great idea, but I know people do it). Keep in mind that the quadratic will go exactly through the three points you pick - all other points will depend on how close the factory polar is to quadratic in behavior - it's very glider-specific.

9B

I should add that this is all predicated on the assumption that the factory polar is how the actual glider performs. Nothing like a few long final glides to see how good the polar really is.