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/126719
F li g h t Te s t in g Te c hn i q u e s leave the power controls (throttle, prop, and mixture) at one setting. Setting the controls for sufficient power for level flight at your typical final approach airspeed is a good place to start. If you normally fly power-off approaches, you can test that way, but it will take longer, as you'll soon read. Here's the basic idea. You're going to record airspeed and descent (or climb) rate at several airspeeds. How many test points you record is up to you, but more data will give you better results. You'll want to map the range of airspeeds faster and slower than your final approach speed. For example, if 75 knots is the proper approach speed and your plane stalls at 60 knots and its maximum gear-down speed is 100 knots, you might target eight airspeeds at 5-knot increments between 65 and 100. Nuts and Bolts At a safe altitude, establish level flight and trim for hands-off flight. Note the observed airspeed (what you read on your airspeed indicator) and outside air temperature (OAT) because you'll use these to calculate your true airspeed after the flight. You'll also need your pressure altitude for this calculation, so set your altimeter to 29.92. Here's the basic idea. You're going to record airspeed and descent (or climb) rate at several airspeeds. How many test points you record is up to you, but more data will give you better results. Using only back stick, slow down a few knots. Your first target speed is 70 knots, but you can accept a couple of knots faster or slower. You don't have to be exactly on speed here, because you're going to record data over a range of airspeeds and draw a curve to fill in the airspeeds you don't test. Just make sure you maintain whatever speed you pick within +/-1 knot and the spacing between test points—i.e., the difference between the airspeeds where you record the data—is reasonably consistent. At this new, slower airspeed you'll probably be climbing. That's okay. Stabilize your airplane at the new airspeed. Your plane will be stabilized at the test condition when its airspeed needle is rock steady, your pitch attitude 42 Vol.2 N o.5 / M ay 2013 is unwavering, and your pull on the stick is constant. Naturally, none of these things will happen as you slow down, but once you arrive at the test point airspeed, they must be stabilized. Technically, you should not retrim if your horizontal tail uses a movable trim device like a tab or movable stabilizer. If the trim mechanism is an internal spring/friction system, retrimming is okay. When you're sure you're stabilized, record your observed airspeed and OAT and time your altitude change. Don't rely on the vertical speed indicator value to determine your descent or climb rate. The VSI is too coarse for this test. Timing the altitude change will give you more accurate, refined data. Your airplane's performance will dictate how long your timing should be. It should be long enough for you to have confidence in your data and short enough for you to maintain the rock-steady test point flight condition. Generally, time for 30 seconds or an altitude change of 500 feet, whichever occurs first. (You can record the VSI reading to corroborate your timing.) After you've recorded the data, relax your pull on the stick and take a break. Then apply a push to the stick and establish your next target speed, which in our example would be 80 knots. You'll most likely be descending now, which is good because it will bring you back toward your initial level flight altitude. Perform the same test at this new, faster airspeed. Take another break, and then perform the 65-knot test point, and so on until you have data spread at approximately 5-knot increments from 65 to 100 knots. That's all there is to it. By the Numbers 1. Establish a level flight condition with the airplane trimmed for hands-free flight; set the altimeter to 29.92. 2. Record the altitude, observed airspeed, and OAT. 3. Using only the control stick/yoke, establish a new airspeed a few knots slower than the speed recorded in Step 2. 4. When absolutely steady, begin timing, noting the altitude when timing begins. 5. Record the new airspeed and OAT. 6. Time for 30 seconds or 500 feet of altitude change. 7. Record the altitude passing when timing is complete. Record the elapsed time. 8. Using only the control stick/yoke, establish a new airspeed a few knots faster than the speed recorded in Step 2.