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/307497
EAA Experimenter 35 SO FAR IN OUR discussion of longitudinal static stability, we explained why this characteristic is important and how it can af ect your perception of your airplane's handling qualities. We described how to test your airplane to determine its trim-speed band—that range of airspeeds the plane will maintain without any force on the control stick. Last month we described a very basic fl ight test for get- ting a general feel for your plane's static stability. This time we'll fi nish the topic with the detailed fl ight test technique for assessing the friction in your plane's longitudinal control system. Still no equations, and the only math is simple arith- metic—and it's optional. From the basic approach explained last month, we pro- duced the plot in Figure 1 and suggested it provides a general idea of the plane's static stability but is overly simplistic. It does not show the trim-speed band or the control system fric- tion, but we're going to fi x that now. Performing the basic assessment will be tougher if there's a lot of friction in your longitudinal control system. The friction raises the force you must apply to the stick to make the elevator move. And it low- ers the stick force required to hold the deflected elevator in its new position because the friction is helping to pre- vent the elevator from returning to its pre-deflected position. It's a good idea to spot-check for friction while performing your first test. Using our example from last time, your first data point target airspeed is 110 knots. To arrive at a stabilized 110-knot condition, you initially apply back-stick to begin the deceleration from your 120-knot trim point. As the airspeed approaches 110 knots, you'll probably vary your stick pull, searching for the pitch attitude that results in a stabilized 110 knots. At this point, you're holding some back-stick. Let's say it's 5 pounds of pull. To check for friction, after you've stabilized at 110 knots, you slowly decrease your pull force ( just like you Longitudinal Static Stability, Part II Friction BY ED KOL ANO FLIGHT TESTING TECHNIQUES E A A E X P _ M a y 1 4 . i n d d 3 5 EAAEXP_May14.indd 35 5 / 5 / 1 4 3 : 2 0 P M 5/5/14 3:20 PM