A polar calculation question for you number crunching enthusiasts out there

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.