Lapse Rates, Glide Calc, IAS

[Chris G]

Andy reminded me that I had written up a quick-n-dirty spreadsheet that calculated temp spread, thermal index, lapse rates, glide ratios with a bunch of parameters, altitude lost, time to goal, and indicated airspeed at altitude. I did it in Numbers v.8 but exported a copy to Excel too. It's not nearly as pretty in Excel but it should work the same. Everyone should know how to calculate the T.I., Lapse Rate etc by hand but this is a quick way to check it in the morning if you happen to be at a computer. It's also interesting to play with the glide ratio figures to see just how little you gain by stepping up to that DHV 2 with a .5 better glide..;P

[Chris G]

I've been prodding this thing and getting it to output more and more interesting information. In the process of double checking my work I remembered the chart that Zach Hoisington made up for his flight deck that tells you your Speed-To-Fly for either best altitude, or fastest to the next climb with a given climb rate. The spreadsheet will output all the numbers (you still have to do some manual entering until I can figure out how to make it automatic) but I was able to duplicate his chart, updated for the current crop of top end DHV 2 gliders, and adjusted it so you didn't have to be at 1/2 bar before deciding how much you needed. It's also imperial instead of metric. I've attached the result with the directions below. Maybe someone will find it useful. At least it doesn't require batteries! This one is based on the Sigma 7 28, middle of the weight range at @ 4921 ft Density Altitude.

If folks dont have Numbers 2.0 and want a custom version, shoot me a polar curve for your glider, the weight range, and your weight. It should be modifiable for hangies too.

Reading The Graph
To arrive highest at goal, find your indicated Glide Ratio in orange on the left (glide ratio is indicated on GPS). Follow it right to where it intersects the orange line. Read down from that point to the Bar % you should use.

To arrive the soonest, with a given climb rate at the destination, find your expected climb rate in blue on the left. Follow it to the right to where it intersects the blue line that most closely represents your indicated sink rate at trim. Read down from that point to the Bar % you should use.

Example 1: Your GPS indicates a glide ratio of 4:1. You should be using a touch over 50% bar.
Example 2: Your expected climb rate at the next thermal source is 280 fpm and your current sink rate is 380 fpm. You should be using 75% bar.

If people want the full spreadsheet that generated the graph, the latest one is always up at http://destinyaltitude.blogspot.com/2009/01/performance-ias-instability-calculator.html