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/254584
44 Vol.3 No.2 / Februar y 2014 FLIGHT TESTING TECHNIQUES advertent airspeed change are wind noise, engine noise, vibration changes, coordinating rudder requirements, and stick force. During cruise flight, this stick force cue has limited utility because you're not typically holding any stick force here. Once established at your cruise altitude and airspeed, you trim out the stick force. If your airspeed deviates, you wouldn't know it through stick-force feedback because you're flying "hands-off." Under these conditions, the air- speed indicator and altimeter will most likely be your first clues to an inadvertent airspeed deviation. There are other phases of flight where stick-force feedback can be your primary cue to an airspeed deviation. Let's say you're on a long final approach to land at Oshkosh during EAA AirVenture Oshkosh. Three airplanes are ahead of you, and who knows how many are behind you. The radio is jammed with the controllers telling: the plane on the runway to take it to the end; the plane about to touch down that it needs to remain aloft until the orange dot; the guy behind him to land on the runway's left side; and the guy between that guy and you to slow down and take the right side. He's also barking out similar orders to a few planes behind you. Naturally, most of your visual attention is directed outside the cockpit. You're sacrificing your normal instrument scan to enhance seeing, avoiding, and following the controller's directions. If you find yourself holding back-stick during all this head-on-a-swivel activity, it probably means you've slowed down. Your airplane's positive static stability tells you that through stick-force feedback. Another example is approaching minimums during an actual instrument approach. While in the clouds, you establish a steady airspeed/power/glideslope relationship, and all of your visual attention is directed toward your flight instruments. As you approach your go-around deci- sion, you start to look outside for the runway. If you're barely emerging from the cloud base, catching an occasional glimpse of ground beneath you but still not seeing the runway ahead, you might devote more attention looking for the runway and less attention to your instru- ments. Noticing you're pushing on the stick tells you you're flying faster than your established approach speed and you've probably pushed your plane below the glideslope or minimum descent altitude. Piloting chores during these critical phase-of-flight examples are more difficult in an airplane without positive static stability. NEUTRAL AND NEGATIVE STABILIT Y An airplane with neutral static stability provides no force cue at all to a changed airspeed. Such an airplane maintains whatever speed you like hands-off. In this case, you have to rely on alternate cues, such as noise and vibration changes, and keep a diligent eye on the air- speed indicator. On the plus side, you never have to trim a neutrally stable airplane. You'll have to move the stick forward or aft to cause an airspeed increase or decrease, but once at the desired airspeed, you can simply release the stick. Some military fighters are designed with this feature. Not hav- ing to retrim as the pilot accelerates 300 knots to engage a target reduces the pilot's workload. Not really the same motivation in the homebuilt world. Let's say you've trimmed for straight and level flight at 100 knots. Then you slow to and maintain 80 knots without retrimming. Your airplane has positive static stability, so you must hold, say, 5 pounds of back-stick force. If your air- Illustration courtesy of Ed Kolano E A A E X P _ F e b 1 4 . i n d d 4 4 EAAEXP_Feb14.indd 44 2 / 3 / 1 4 3 : 2 0 P M 2/3/14 3:20 PM