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/113663
F li g h t Te s t in g Te c hn i q u e s Distance 1 Distance 2 Figure 1 Angle of Attack and Maximum Range By Ed Kolano Last month we explained how your airplane's wing always stalls at the same angle of attack (AOA) regardless of its attitude. We used the lift equation and the lift coefficient (CL) versus AOA curve to illustrate how airplane weight and accelerated maneuvering affect stall speed but not the stall AOA. Using a landing pattern example, we compared the stall protection of an AOAbased final approach with a one-speed-fits-all shortcut or miscalculation of a weight-based approach speed. In short, we attempted to demonstrate the simplicity of using AOA for stall protection. This month we'll take the AOA discussion to another safety arena—maximum range glide. Yes, a single AOA value will always yield the maximum, no-wind glide distance. Believe it or not, that same AOA will also provide you the maximum cruise range for your airplane. Glide angle is the difference between your airplane's flight path and the horizontal. If your glide angle is zero, you're flying level. Your airplane would never contact the ground and give you an infinite glide range. If your glide angle is 90 degrees, your airplane is going straight down, and it has a zero glide range. Reality is between these extremes, and the shallower the airplane's glide angle, the farther it will glide. During an engine-out glide, you can fly an airplane at a variety of glide angles. In Figure 1, both airplanes begin their glide at the same altitude. The airplane on the right has a shallower glide angle, or flight path angle (γ, Greek letter gamma), and therefore travels farther than the other plane. We did not mention glide speed because how far an airplane glides depends only on its flight path angle. Yes, your airplane has an optimum glide speed that produces the shallowest glide angle, but that speed depends on your plane's weight. Gliding at a speed faster or slower than the optimum for that weight results in a steeper flight path angle and less range. In Figure 1, both airplanes could be exactly the same but gliding at different speeds, and the plane on the left could be at a speed faster or slower than the other plane. What's important is that there is only one optimum glide speed for a particular airplane weight, but there is one AOA that ensures the optimum glide speed and maximum range for every airplane weight. Last month we explained how stall speed depends on airplane weight because the stall AOA doesn't EAA Experimenter 43