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/287214
36 Vol.3 No.4 / April 2014 FLIGHT TESTING TECHNIQUES LAST MONTH WE TALKED about your airplane's trim-speed band—that range of airspeeds the airplane can maintain with- out any force applied to the stick. Prior to that, we introduced non-maneuvering static longitudinal stability (static long- stab, for short), which is indicated by how much back- or forward-stick you must hold to fl y slower or faster than the speed for which the airplane is trimmed. Now we'll explain a basic test technique for checking your airplane's static long- stab and show how it relates to the trim-speed band and control system friction. First, you need to equip your stick with force and displace- ment sensors, then—just kidding. We're going to take a more simplistic and practical approach that identifi es whether your airplane is stable, whether the friction in the control system is signifi cant, and how these characteristics can af ect the ease or dif culty of accomplishing fl ying tasks. No equations and no math. Woo-hoo! Apparent longitudinal static stability is what it appears to be to the pilot while fl ying. That may sound like stating the obvi- ous, but we're interested in the pilot's perception because that's what shapes the pilot's opinion. For instance, an airplane might be extremely sensitive to tiny changes in elevator defl ection, but if that plane's longitudinal control system has a massive spring in it, the stick force needed to make that tiny defl ection change could be high. To the pilot it might feel like a strongly stable situation because it takes so much stick force to move the airplane in pitch. So, we'll focus on what the plane feels like when fl ying of -trim and how that perception impacts your fl y- ing precision and workload. Recall that last month's example started with the airplane trimmed for level fl ight at 120 knots. We stepped through the pro- cedure and determined this plane had a trim-speed band between 114 and 124 knots, so we'll stick with that example here. After you determine the trim-speed band, you can assess the static stability at speeds slower and faster than the trim-speed band. Using only back-stick, slow down a few knots to, say, 110 knots. Make sure the speed you choose is slower than the slow end of your trim-speed band. Do not retrim or adjust the engine or propeller controls. When you're stabilized at a steady 110 knots, note how much pull force it takes to hold that airspeed. There's no need to measure this force for our basic approach because you're just going to ensure that the stick force gets higher as your airspeed deviations get bigger. We'll explain how to deal with control system friction next month. After you get a feel for how much pull force it takes to fl y 110 knots, slow down to 100 knots. Stabilize there, and the pull force should be higher than it was at 110 knots. You can continue this process until you reach stall warning speed, but that's probably taking the test a little too far for the cruise-fl ight condition. For cruise fl ight, the idea is to learn how much control stick ef ort is needed to fl y slower or faster than your trim speed. Opera- tionally speaking, this would be a temporary situation because you'll eventually retrim for the new airspeed. The motivation for this testing under landing pattern conditions, however, is dif erent. Here you want to assess your plane's stick-force cues for an airspeed change, so you can use this tactile feedback as another means of notifying you of an inadvertent airspeed devia- tion. It's a good idea to perform this test all the way down to stall warning airspeed—at a safe altitude, of course. When you've mapped the range of speeds as slower than the slow-end speed of the trim- speed band, repeat the process for airspeeds Longitudinal Static Stability A basic approach BY ED KOL ANO E A A E X P _ A p r 1 4 . i n d d 3 6 EAAEXP_Apr14.indd 36 3 / 3 1 / 1 4 9 : 4 6 A M 3/31/14 9:46 AM