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/90184
Fl ight Testing Techniques Smooth air is essential for obtaining good data. Early morning is usually the best time for calm conditions, and the sun's low position also ensures your shadow won't be under you. The FAA recommends less than 10 knots of wind for this test, but I'd stick with less than 5 knots with no gusts. Calm is best. The effects of your airplane's center of gravity should not affect your data. Weight, however, can affect data. The heavier your airplane, the higher its angle of attack must be to produce lift equal to its weight for a given airspeed. Because higher angles of attack create stronger upwash and downwash around the wing, which can affect the pressure sensed at the static port, weight can have an infl uence in your calibration. To check this, perform the en- tire test profi le at a heavy weight, then spot-check several airspeeds at a near-minimum weight for comparison. If there is a signifi cant difference, you may want to perform the entire test profi le at maximum and minimum weights. The FAA recommends testing several speeds between 1.3VS1 and maximum level flight speed. VS1 is your air- plane's stall speed in the tested configuration. The 1.3 factor is there for safety. Remember, you'll be low and slow, and that means you won't have a lot of options should something go wrong. Select a ground course whose length is compatible with your airplane's speed range. FAA Advisory 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. Circular 23-8C recommends a 5-mile course for air- speeds faster than 250 knots and a one-mile course for airspeeds less than 100 knots. For an airplane with a test speed range of 65 to 150 knots, a 1-½-mile course is probably a good choice. Course length is up to you, but longer courses require very demand- ing flying for longer periods, and shorter courses can mean larger airspeed errors if your timing is off. For example, a 1-second timing error on a 1-mile course flown at 150 knots produces a 6-knot airspeed error. That same 1-second timing error on a 5-mile course causes an airspeed error of less than 2 knots. That should about do it for the flight-test procedural considerations. Next month we'll get into how to fly the ground course, the data you'll need to record, and set- ting the stage for crunching that raw data into useful charts for your operator's handbook. Glasair II 52 NO. 3/NOVEMBER 2012