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/101874
F li g h t Te s t in g Te c hn i q u e s and the ET for test run, you can calculate the ground speed by dividing the distance traveled by the time it took to travel that distance. course length recommended in the November "Flight Test Techniques." Okay, you now have a VG of 116.87 and 126.69 for your first run pair. Averaging these speeds removes any wind effect. In this equation, course length is in feet, ET is in seconds, and 0.5925 is a correction factor to have the ground speed (VG) in knots. (Use 0.6818 for VG in statute mph.) Plugging our sample data from the first data row in the grid into this equation, we get Repeat this VG calculation for every test run, and enter the results in the ground speed column. Let's talk about wind for a minute. Comparing the ET and VG for the first set of reciprocal heading runs, you'll note a difference of 3.1 seconds and about 10 knots. That's because there was a steady 5-knot wind during the test—a direct headwind during the first run and a direct tailwind during the reciprocal run. Had this 5-knot wind been a direct crosswind, the actual distance traveled during the run would have been longer than the 7,890 feet, because the airplane would have drifted downwind. Although you could calculate the drift angle and actual distance, it's not necessary. If you test only when the wind is 5 knots or less, the worst-case error in your ground speed calculation will be less than a quarter of a knot for the typical homebuilt airplane flying the 46 Vol.2 No.1 / January 2013 The next trick is to think backwards. When you compute your ground speed during cross-country planning, you apply the forecast wind to your planned true airspeed to determine your expected ground speed. During your airspeed calibration data reduction so far, you removed the wind by averaging VG1 and VG2, so your calculated average VG is also your calculated average true airspeed (VT). We'll assume your airspeed calibration test flight did not occur above 10,000 feet pressure altitude or at an airspeed faster than 200 knots, so we can ignore the compressibility effects on your airspeed indication. Now all you have to do is convert VT to VC, but there's a minor inconvenience. You have to know the ambient air temperature, but your OAT gauge provides total air temperature. Converting total air temperature to ambient air temperature requires you to know your calibrated airspeed, which is what you're trying to find out. Fortunately, most of us don't have to worry about this circular argument, because the airspeed error created by using OAT instead of ambient temperature is typically less than half a knot below 10,000 feet pressure altitude up to 200 knots. You can determine VC by using the table in Figure 2, along with some math. Or you can bypass the table, but the math is slightly more complicated. We'll present both methods.