On Monday, February 24, 2014 2:20:47 PM UTC-5, Dan Marotta wrote:
If I understand your question correctly you need to have your polar curve

plotted on a grid which has an origin, e.g., (0,0). You get min sink speed

by drawing a horizontal line from the Y (vertical) axis which is tangent to

the curve. Record the speed and sink at that point. You get best L/D by

drawing a line from the origin (0,0) which is tangent to the curve. Record

that speed and sink rate. Pick your third point at a reasonable cruise

speed, say 80 kts and pick the sink rate off the Y axis for that speed.



I don't mean to insult you with the above, I just don't know if you know.



"ES" wrote in message

...

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

The conventional wisdom goes back to Reichmann's Cross Country Soaring textbook.
Remember that his "day" job was teaching math! In the book, he describes using
a quadratic equation to estimate the sink rate at any speed. This is a pretty
good method since the drag on a sailplane is dominated by the parasite drag in
the range of speeds you'll use for final glide, and parasite drag is
proportional to v**2. There are some noted weaknesses with the approach:
drag at lower speeds has a larger amount of induced drag, and flapless
gliders have more drag at higher speeds than predicted by the quadratic
(a result of the optimization of the airfoil for climb).

I've gone through the exercise a number of times. (At one point I had a cool
PostScript program that would print out one of those circular sliderule
glide computers, but alas the diskette containing it was bad!) Picking the
3 speeds is more crucial than you think. It helps to make a spreadsheet
that shows the difference between measured and computed sink rates, and
the resulting l/d, and then try picking different sets of points to
generate the quadratic.

In general the range you should use is best l/d speed up through the fastest
you'd expect to fly final glide. Lower speeds include too much induced drag,
and higher speeds stray into the drag bucket. For my ASW-19 I used 55-85kts
as the range. Best L/D is about 39 at 50kts, but only drops off to 38.5 at
55kts.