PETER GARRISON
TABULATING
TAKEOFF If the runway’s too short, at least it’s virtual. GALEN HANSELMAN, who has published several guidebooks and charts for pilots interested in landing on something other than 5,000-foot paved runways, sent me his twovolume flyer's guide to Utah and the associated "Supplemental World Aeronautical Chart," which resembles a WAC chart but includes a slew of backcountry airstrips not on the WAC. The quality of his productions and the amount of effort that he has put into them are impressive. One volume of each set covers the airstrips themselves in great detail, with frank and often rather disconcerting comment about their condition and risks; the other concerns why you would want to go there – history, lore, hikes, scenery, fishing and so on.
Mountain Flying Bible, which is, if you’ll pardon the redundancy, the bible of mountain flyers. I responded that I had a computer program for calculating takeoff distance that incorporated those variables and more, and I ought to be able to come up with something. Ah, the vanity of human wishes! It turns out that takeoff distance is very difficult to calculate with anything like precision. And, for that matter, how would you know that you had the answer right? The best you can hope for is to simulate a few existing aeroplanes and see how well your results match the POHs.
Ru n n i n g u p to full power agains t t he brak es .
I exchanged a few emails with Hanselman, and at one point he commented that he wished he had something more to tell his readers about effects on takeoff performance of wind, altitude, runway gradient and surface and so on than just the rules of thumb found in the late Sparky Imeson's
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December / January 2021/22
Here’s how the program works. At intervals of a tenth of a second it calculates thrust from a few known – or guessed – propeller characteristics. From this it subtracts resistance due to friction, drag and runway slope, and applies what’s left to the mass of the aeroplane. This results in an acceleration and a certain distance travelled. The process repeats itself, reporting speed, distance, acceleration,
