Experimenter

JAN 2013

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.

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F li g h t Te s t in g Te c hn i q u e s emergency action preparation, etc. During the data reduction you should exercise good engineering judgment. For example, if you noted on your test data card that your airspeed varied 5 knots during a particular timing run, throw away that data. If some data points seem to be well off the faired line, go back to your data card to see why. Perhaps you were not as confident in your timing on this run or not as steady at the start of the timing as you were for the other runs. "Quality" notes on your test cards can be very useful after the flight to help explain data that don't seem to fit. If your line fits the data points well, you can extend it with a dashed line to show the predicted VO/VC correlation at speeds slower than your slowest tested airspeed. Remember, however, this is only an extrapolation, and the difference between VO and VC generally gets bigger at slower airspeeds. Don't rely on this extrapolation as a safe indication of how much faster than stall speed you are flying. Your stall speed testing will provide the observed stall speeds for different configurations, weights, and center of gravity locations. The more data you collect during the flight, the more confidence you'll have in your results. Fly at least five or six airspeeds during your test. More is better. The speeds you select should cover the entire airspeed range for that configuration. The test speeds don't have to be exact, but they should be close to your target speed. For example, if your plan calls for a 130-knot test run, but you find yourself stabilized at 126 knots as you approach the start checkpoint, that's okay. Remember, you'll fair a line through these points anyway, and there's no good reason to abort an otherwise good setup just because you're off a few knots. Just make sure you fly the reciprocal heading run at the same speed as the first. What you should not do is start the run at 126 knots and try to "make it up" by ending the run at 134 knots. The goal is a rock-steady airspeed for the entire run. Make a note of any potentially interfering events that occur during the run because that information may come in handy after the flight to explain apparent data anomalies. Don't forget to perform the same data reduction for those airspeeds you spot-checked with your airplane at a different weight. You can plot these data on your VO/VC graph to see how close they are to your original weight line. If your spotchecked data points plot significantly above or below the line, you can fly another full airspeed calibration test at the second weight. Plot this line on the same graph, and don't forget to clearly label which is which. You can also plot lines for different configurations on the same graph, if it's not too cluttered. That's it. The data reduction may seem a bit cumbersome at first, but you'll master it in no time. This month we started with elapsed time, observed airspeed pressure altitude, and outside air temperature. We removed the wind effects by averaging reciprocal heading runs, then turned that true airspeed into calibrated airspeed with the aid of an altitude table and a calculator. Finally, we created a useful plot of calibrated versus observed airspeed. Not a bad day's work. You now have a handy plot for your operator's manual for cross-country planning and in-flight reference. Remember, if you modify your airplane externally, you may have to fly another airspeed calibration if the modification affects the airflow near the static ports of your pitot-static system. It's a good idea to perform a spot check of a few airspeeds after the modification. If the spot-checked data points don't fall on the line, fly another complete airspeed calibration. » Questions about flight testing for Ed? Send an e-mail to Experimenter@eaa.org with the words Flight Testing in the subject line, and we'll forward your questions to him. corresponding pairs of VO and VC. Fair a line through the data points you just plotted as shown in Figure 3. A Few Words About Judgment During your test flights you exercised good piloting judgment concerning test site selection, minimum test altitude, 48 Vol.2 No.1 / January 2013 Ed Kolano, EAA 336809, is a former Marine who's been flying since 1975 and testing airplanes since 1985. He considers himself extremely fortunate to have performed flight tests in a variety of airplanes ranging from ultralights to 787s.

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