Compare Polars

With today's computers, 6th order polynomials are easy to deal with, so have at it. But - getting a fit to within 1% of data that has errors of 5 or 10% is a fools errand. Idealized drag equations, even complex ones, still do not account for many large unknowns: how accurate is the wing profile and how does it change with temp/humidity, how good are the seals, what are the disturbances around surface intersections and how does that change with speed, how dusty or buggy, how much of a slip is the pilot flying in. Any or all of these can easily amount to a few percent, and may vary by individual glider, not just design. Go for it - but be prepared for reality to strike..

If you could just solve the problem of correctly handicapping for wing loading in known conditions, that would be a much greater advance than fitting the polar curve better.

On Saturday, December 23, 2017 at 10:31:46 AM UTC-8, wrote:
I'd appreciate it if somebody would answer "what is G/F" ... is that the wing loading and if so in what units?

As to all of this dealing with ms-format xls spreadsheets ... if it's your tool of choice, fine ... but it's not mine, and I just wanted the data out, and other than the issue of the gross weight or wing-loading ... I've got that done.

I would make the following points however, as a working scientist/engineer:

* you'll find (if you look) that journal standards for "supplementary material" and funding-agency data retention requirements (NSF, NIH, DOE, NASA) ban, at least strongly discourage, proprietary or spreadsheet data formats, in favor of ascii-delimited, or NetCDF

* "can't help you there" ... is not accepted

On Saturday, December 23, 2017 at 11:44:41 AM UTC-5, jfitch wrote:
I'm pretty sure there is no value in fitting a 6th order polynomial. The data isn't that good.


Ah, now this is a more interesting issue, that has several aspects. Some things to think about:

* for almost every test, the minimum sink speed and the best L/D points are given, and in some cases this is all the data for which you get actual numbers without digging, or trying to digitize data from squinchy graphs etc..

* It is important for general application that the fit get the minimum sink and best-L/D right. If you don't believe me on this point, we could discuss further. But you'll find that you cannot get a polynomial that does these right AND gets the higher-speed data right ... without 5 or 6 terms. Go away and give it a try..

The fundamental reason for all of this is that the drag is reasonably described as the sum of drag terms, with the dominant terms are the induced drag and the "profile drag."

The induced drag is proportional to the lift-coefficient squared, and when you grind that out it falls as 1/v ... and polynomials don't fit that well with few terms ... that's the real reason you need more.

This leads to the following point/idea ... nothing magical a about a polynomial, and if you want something with few terms that fits a polar pretty well and has a "reasonable" basis" consider (Ax + 1) ( B/x + C * x^2) ... easy to derive the standard best-glide and speed-to-fly equations with this. (and A is generally small)

Related to the above issues is "yes, the data aren't that good" in many cases, but OK, we know physics (aerodynamics) and our job isn't just to get "some line" through the data, our job is to get the best physically plausible estimate of the polar, from the data we have. Another reason for this is that often the data don't extend to higher speed ranges, etc.

Now this leads to one of the things that I am playing with to deal with these data, that consists of doing a drag fit to a model that includes terms for induced drag (including the epsilon term), a wing washout term vs AOA, using the airfoil drag-bucket curve if known (or a generic one if not), and a simple fuselage profile drag vs AOA function.

This yields a physically-realistic polar, given the physics we know (gets to be a much bigger nuisance for flapped sailplanes though, without getting handbook data and applying those) , and in principle allows us to "fix" not so good data to a a degree. It's also the most physically-plausible way to extrapolate the data to higher airspeeds if you need to, though doing is is always a reach.

The resulting system is something of a mess that one doesn't want to use as the function for speed-to-fly etc (it's messy and derivatives are messier), so it is easier to fit the result with a 5 or 6 term polynomial for subsequent use ... and this is just another way of saying what I have said above -- takes a polynomial with a fair number of terms to approximate a real drag model, because the physics has that pesky induced drag term, and some sort of drag-bucket approximation.

I'd appreciate it if somebody would answer "what is G/F" ... is that the wing loading and if so in what units?

The help sheet says G/F is wing loading in kg/sqMeter

On Saturday, December 23, 2017 at 1:31:46 PM UTC-5, wrote:
I'd appreciate it if somebody would answer "what is G/F" ... is that the wing loading and if so in what units?

I haven't really looked deeply into this spread-sheet but most likely G/F stands for 'Gewicht / Flaeche' or weight(mass)/surface area. The typical UoM would be kg/m^2

Uli
'AS'