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/118927
F li g h t Te s t in g Te c hn i q u e s angle. We do this by mentally combining the picture through the windscreen with our flight instruments, but this information can be deceptive. Figure 4 is a composite plot that shows flight path and descent rate versus airspeed. Notice that slowing from 62 knots to 55 knots results in a slower descent rate but a steeper descent angle. This apparent contradiction occurs because the change in true airspeed has a bigger effect on the flight path than it does on the descent rate. You'd be coming down slower, but you'd also be traveling forward slower. It will take longer to come down, but you won't travel as far as you would have at the faster speed. Let's take this scenario into the cockpit to see why it's an insidious deception. You're established on short final at 65 knots, but you see that you're not going to reach the runway. In an effort to stretch your glide you nudge the stick back just a bit. As the plane's nose comes up, there's a reassuring balloon effect. Even after the balloon, the vertical speed indicator needle settles onto a smaller decent rate as you continue flying at 55 knots. From the information available to you in the cockpit it looks like you've solved the problem. What you can't see is that your actions have steepened 1 It's the flight path angle that determines whether you'll reach the runway or clear the trees into that forcedlanding field. Unless your airplane has a descent angle indicator, you need to understand your airplane's descent angle sensitivity to airspeed changes; in other words, its flight path stability characteristics. This month we introduced the flight path stability concept, compared different flight path stability characteristics, played a couple of what-if games, showed how different flight path stability characteristics determine your safety margins, and how your plane's flight path stability can affect the way you fly. Next month we'll present the flight testing techniques and follow that up with the data reduction that will enable you to create flight path stability curves for your airplane. Airspeed (knots) 0 20 -1 Flight Path Angle (degrees) the airplane's flight path angle by 1/2 degree. Remember your instructor's mantra: You can't stretch a glide? 40 60 80 100 0 -200 -2 -3 -4 -5 -400 -600 -800 Vertical Speed (f/min) 0 In an efort to stretch your glide you nudge the stick back just a bit. As the plane's nose comes up, there's a reassuring balloon efect. -6 -7 -1,000 Figure 4 44 Vol.2 No.4 / April 2013