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 39 of 44

F li g h t Te s t in g Te c hn i q u e s stall AOA does not change. Let's look at the lift equation to see why. L is lift. ρ (the Greek letter rho) is air density. V is true airspeed. S is wing area. CL is the wing's coefficient of lift. If we limit our discussion to one altitude, the air density doesn't change. Wing area certainly doesn't change, and neither does the number 1/2. That means true airspeed and lift coefficient determine lift. Lift coefficient is a convenience term aerodynamicists use. The value of C L depends on the AOA as shown in Figure 2. You can see how higher AOAs produce larger C L values up to a point where the C L drops off—sometimes dramatically—if the AOA increases any further. On the C L versus AOA curve, the AOA corresponding to the highest point (CL max) is the stall or critical AOA. This plot is valid for all flight conditions—climbing, descending, turning, or level. No matter what airspeed you fly, your airplane will always stall at the same AOA. Because there's only one CL that corresponds to the stall AOA, your airplane will always stall at the same CL. During 1g flight, lift equals weight. When you slow down, V (true airspeed) decreases, so C L must increase to maintain enough lift to support the airplane's weight. You've done this many times during slow flight. To compensate for the decreasing air- Tis plot is valid for all fight conditions—climbing, descending, turning, or level. No matter what airspeed you fy, your airplane will always stall at the same AOA. speed, you apply even more back stick to increase the AOA. When you reach CL max, increasing the AOA any further results in a lower C L and a loss of lift, and the wing stalls. Final Approach Let's put some real-world numbers into the lift equation. Let's say our airplane weighs 1,000 pounds and has a wing area of 100 square feet. We're flying the traffic landing pattern at 1,000 feet pressure altitude, where the air density is 0.0023 slugs per cubic foot. Our airplane's CLmax is 1.8. Plugging these values into the lift equation and solving for V, we get a 1g stall speed of 69.5 feet per second or approximately 41 knots. A typical landing approach speed is 1.3 times the stall speed, or 53 knots in this case. If we add a passenger, some luggage, and top off the fuel tanks, our airplane would weigh 1,400 pounds. At this weight the stall speed would be about 49 knots. If we used our landing approach speed based on the lighter-weight airplane, we'd be flying just 4 knots faster than stall speed. In this case a 5-knot wind gust could be trouble. With the heavier loading, the recommended approach speed would be 64 knots (1.3 x 49 = 64). If we flew this speed in the lighter airplane, assuming we'd touch down just as the plane reached its stall speed, we'd float a long way down the runway while dissipating that extra 23 knots (64 – 41 = 23). If our airplane had an AOA indicator, we could have flown the same landing approach AOA at both weights. The airspeeds still would have been 53 knots 40 Vol.2 No.2 / February 2 013

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