Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.
Issue link: http://experimenter.epubxp.com/i/178050
F li g h t Te s t in g Te c hn i q u e s density altitude should supply enough confidence to extrapolate this line to zero. Speaking of errors, our data show a single VX observed airspeed for all three tested altitudes. In theory, VX should increase about one-half percent for each 1,000 feet of altitude. There's 5,900 feet between our lowest and highest test altitudes, so a VX of 79 knots at 3,800 feet would imply a theoretical VX of (79 + 0.005 x 79 x 5800/1000 =) 81 knots at 9,600 feet. To see whether this two-knot discrepancy matters, we took another look at the data from the 6,700and 9,600-feet tests. Here's where that flight path angle column on the worksheet comes in. For all three tested altitudes, flying up to 5 knots slower or faster than the VX for that altitude results in less than one-quarter of a degree climb angle penalty. At 3,800 feet density altitude, that one-quarter degree translates to 22 feet additional distance needed to clear a 50foot obstacle. At 9,600 feet density altitude, it's 20 additional feet. So what should you do with this information? Well, you could repeat the entire test series and data reduction. You could review your test cards to see if you included data you shouldn't have or omitted data you should have included. You could scrutinize your original data reduction, searching for errors. Or you could accept the fact that your VX results may be off by a couple of knots between zero and 10,000 feet. Considering we're talking about a distance penalty of a little more than the length of the typical airplane and that it should only take 428 feet (638 at 9,600 feet density altitude) of no-wind horizontal distance (after takeoff and accelerating to VX) to clear that 50-foot obstacle, you might just accept the results and enjoy the fact you won't have to memorize different VX for different altitudes. We think that last option sounds pretty good, but we'll spot-check by repeating a few climbs, anyway. That about does it for the climb performance. Next time we're going to take a break from the "how" stuff and talk about a few aviation "whys." Stay tuned. 42 Vol.2 N o.9 / September 2013