Geeking Out on Performance

[Dan OBrien]

Hi all. I have a spreadsheet I constructed that takes information on the cruise performance of your plane at given density altitude and predicts performance using standard thrust required/thrust available equations at other power settings and altitudes. (The file is 1.2 Mbytes, so is too large to attach). I suspect this could have been done both more simply and more accurately, but in the small amount of testing I've done, it appears to predict pretty well. Imagine that, basic T=D, L=W, and simple quadratic drag polars work pretty well. A fun thing you can do with this is compare the numbers with the performance charts from the POHs of other planes, e.g., the Columbia 350/400, the Cirrus, the Mooney, etc.

I'm not good enough to know whether IVPs and Evolutions are getting too close to mach 1 for compressibility effects to kick in and have a significant impact. The model I have is a "simple" text book model (e.g., like that in Gundersson's text) that makes simplifications. The calculations assume incompressible flows and a quadratic drag polar in which the coefficient of lift at minimum drag is zero, which typically is not quite right. That said, I plan to use this for the cruise performance charts in my POH (which currently have the Columbia 350 numbers) because the predictions seem reasonably accurate.

This is mainly fun. I fly the same way most of the time, so even I don't use it much. One thing I hope to do with it after I do climb, glide, and descent versions is use the information to set up a foreflight profile.

If anyone is interested in the spreadsheet, just let me know and I'll send it along. I'd be curious how well it works for other planes in the fleet. Also, I'm not an aeronautical engineer. Some of you who are or are closer than I am might have criticisms/suggestions that can make it better, possibly lots better.

[Dan OBrien]

I copied the summary sheet into a jpeg so you can see what comes out of the program. On this one, you obviously can't change the numbers in green. But in the complete workbook, with different sheets for each altitude, you can play with the green numbers to your heart's content.

Cruise Performance Summary Sheet.jpg (78.22 KiB) Viewed 2083 times