February 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/108002

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Page 40 of 44

with the lighter loading and 64 knots with the heavier loading, but we'd have had the same stall protection in both cases. Pretty handy, huh? Turning Stalls In a turn, the wing produces more lift than the airplane weighs, and the effect on the stall speed is identical to loading the airplane to a heavier weight. The effect on stall AOA is also identical—the stall AOA stays the same. Pilots know that a level-flight, 60-degree bank turn generates 2g. This means the wing is generating an amount of lift that's twice the airplane's weight or two times what it is during straight-and-level flight. This is true for any 2g maneuver, whether the airplane is in a turn or inverted at the top of a loop. If you let the airplane slow down while maintaining your 2g pull during this turn, the wing still stalls when its AOA exceeds its critical value. Because CLmax doesn't change, the only other variable that can change is the airspeed at which CLmax occurs, i.e., the stall speed. In this example, the stall speed is 1.4 times faster than during 1g flight. It doesn't matter what the airplane's weight is, what the 1g stall speed is, or what the altitude is (altitude determines ρ) to know how maneuvering the airplane affects stall speed. Because the stall AOA stays the same, the only thing that affects stall speed in our example is how much lift the wing is producing or, said another way, how hard you're maneuvering. In the steep turn, we increased the lift by a factor of 2, which means the stall speed squared (V2) increased by a factor of 2, and the square root of 2 is approximately 1.4. In a 3g maneuver, the stall speed would be the square root of 3 (approximately 1.7) times the 1g stall speed and so on. As Figure 3 shows, a level, 60-degree bank angle turn would generate 2g (blue arrows), and the airplane If we used our landing approach speed based on the lighter-weight airplane, we'd be fying just 4 knots faster than stall speed. In this case a 5-knot wind gust could be trouble. will stall at 1.4 times its wings-level, 1g stall speed (red arrows). This chart is valid for every airplane. This relationship holds true for any airplane at any weight at any altitude. We used a level turn in our example, but the argument is just as valid for any 2g maneuver. The wing doesn't care about its orientation to the ground. The stall speed is the same during a 2g pull-up, a 2g level turn, or a 2g pull-down during inverted flight at the top of a loop. Changing your airplane's configuration can change its stall AOA, and flaps are the perfect example. You know that lowering the flaps lets you fly slower. What the flaps really do is enable the wing to generate more CL, usually at a lower AOA (Figure 4). With the higher CL capability, the wing can fly slower and still produce enough lift. Changing configurations changes the stall AOA and CL max from the previous configuration, but the stall AOA and CL max for the new configuration don't change. In other words, a 2g stall with the flaps down will occur at 1.4 times the 1g stall speed with the flaps down. The bottom line is, for EAA Experimenter 41

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