Experimenter

April 2013

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

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F li g h t Te s t in g Te c hn i q u e s 1 Airspeed (knots) 30 40 50 Flight Path Angle (degrees) 0 60 B 70 90 C D -1 -2 80 A E -3 -4 -5 -6 Figure 1 Flight Path Stability By Ed Kolano Stability is one of those aviation terms you hear all the time. I've seen it applied to everything from control feel to electronic displays. And so it is with this month's topic, flight path stability, because it's not really about stability in the aviation sense. It's about what changing your airspeed on final approach can do to your flight path angle. Let's assume for now that your power is constant during your final approach. Not entirely unreasonable, since that's the situation we all strive for, and it's the situation we're stuck with following an engine failure. Now we can explore how changing only airspeed affects vertical flight path. The Curve You're on glideslope at the recommended final approach speed with the power set. Point E in Figure 1 depicts this condition. In this example, Point E represents a 75-knot approach speed along a 2-degree glide path. Without changing the power, if you pull the stick back a bit and fly slightly slower than 75 knots, your flight path angle becomes shallower. If you fly, say, 70 knots, your flight path angle would be about ½ degree below horizontal flight, as depicted by Point D. At 66 knots (Point C), you'd be flying level. Slow to Point B, and you'd be in a slight climb in our example airplane. So far your plane is behaving as you probably expect because it's been flying on the front side of the flight path stability curve, where all airspeeds are faster than the airspeed for the curve's peak at Point B. When operating on the front side of the flight path stability curve, you pull the stick back to fly slower and shallow your flight path, and push the stick forward to fly faster and steepen your flight path. Airspeeds to the left of Point B are on the back side of the curve, where the slower you fly the steeper your descent angle or flight path will be, unless you add power. Here's why. Suppose you establish your airplane on final approach at Point A, about 47 knots. Notice that your flight path angle is the same as if you were flying EAA Experimenter 41

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