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/142883
F li g h t Te s t in g Te c hn i q u e s Sawtooth Climbs What they help us discover By Ed Kolano Way back in the early days of digital cameras—did I say "way back"?—my camera had a feature where you could take several overlapping pictures, and the camera software would stitch them together to make a clean panoramic image. It turns out you can do a similar stitching of several short climb tests to create curves that show your airplane's best climb rate airspeed, VY, and best climb angle airspeed, VX, at any altitude. Sawtooth climbs involve timing how long it takes to climb through an altitude block at a particular airspeed, then immediately descending and repeating the climb through the same altitude block at a different airspeed. This procedure is repeated until the entire range of possible climb airspeeds is covered. The technique is called sawtooth because of the vertical sawtooth pattern you'll fly throughout the test. (which you don't know yet). Like the altitude block discussion, the more airspeeds you fly during your tests, the more accurate your climb plots will be. Your slowest speed should be sufficiently faster than stall speed for safety and to ensure adequate controllability for the tight airspeed tolerance necessary. Your fastest speed need not be any faster than you intend to fly while climbing. When deciding on your test airspeeds, don't forget to include your intended cruise-climb speed and the speed you might use when changing from one cruise altitude to another. The Sawtooth Climb Test Procedure Load the airplane (weight and CG) to represent the way you'll load it for normal flying. This may be maximum gross weight or half fuel with only the pilot aboard or whatever loading you find useful. Record this information. You'll be performing the sawtooth climbs through several altitude blocks. The more blocks you choose, the more accurate your climb performance data will be. If you never plan to fly higher than 10,000 feet, you might choose blocks centered around 2,000, 5,000, 8,000, and 11,000 feet. Which altitude blocks you choose is not important because your final climb performance information will be in the form of plots, which will show your plane's climb performance at any altitude, even between the test blocks. Take off, and when ready to begin the test, set 29.92 in the altimeter. This allows you to record pressure altitude during the test, which will be used along with outside air temperature (OAT) to determine density altitude. Your finished climb performance charts will be in terms of density altitude. This way, you can use them anytime by knowing the density altitude. Otherwise your charts would only be useful on days when the barometric and temperature conditions exactly matched the test day. Because you want to determine your airplane's VY and VX, you'll have to test at airspeeds slower than VX and faster than VY to ensure you bracket these speeds There's no need to perform your first series of climbs through the lowest altitude block. It's probably smarter to start with a higher block for safety reasons. Save 38 Vol.2 No.7 / July 2013