On Saturday, June 13, 2015 at 9:25:06 AM UTC-7, Andy Blackburn wrote:
On Friday, June 12, 2015 at 6:49:47 AM UTC-7, wrote:

Okay, I got out my dog-eared copy of "Cross Country Soaring" and pulled the right formula out of the math section. Here is a spreadsheet that takes three points off of a polar of your choosing, adjusts it for actual takeoff weight versus the weight the polar was measured at and creates a quadratic formula for the polar. It then calculates the locations on a speed ring for any given speed to fly. If you assume Mc=0 it will give you speed for best L/D, but it also works if you want to make a ring that rotates for different Mc values.

It does the calculation for zero wind and for a wind velocity you can input. Everything is in knots and pounds. Headwinds are negative numbers. Inputs are in red and a graph of the polar is produced for the weight used to create the polar, for the actual takeoff weight as well as for the designated wind condition at actual takeoff weight. It's in 1-knot STF increments, so you can either place even speed increments around the ring at irregular intervals or put irregular increments of STF at even increments around the ring, whatever is easier for you to use in the cockpit.

Here's a link to download the Excel file:

https://drive.google.com/folderview?...ms&usp=sharing

Building this was an interesting review of some basic cross-country soaring principles. It reminded me that I should never, ever fly best L/D if I can avoid it when racing. The speed penalty huge and the benefit is marginal..

9B

Also - the polar pre-loaded in the spreadsheet is for an ASW-27. You need to provide the polar for the glider you are flying - preferably a polar where you know the takeoff weight and the takeoff weight for the glider you are flying as it makes a bit of a difference.

9B