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 38 of 44

F li g h t Te s t in g Te c hn i q u e s Angle of Attack That single number that works for you By Ed Kolano Last month we finished explaining how to calibrate your airplane's airspeed indicator. We laid the foundation with a little theory, explained the test procedures, and finished with the data reduction that created plots (or charts) of observed airspeed (what you read on your airspeed indicator) versus calibrated airspeed. No doubt about it, airspeed is important. The Federal Aviation Regulations (FARs) define more than two dozen "V speeds," and aviation texts define dozens more. These include stall speed, maneuvering speed, maximum range speed, best glide speed, and on and on. All are handy numbers for pilots, but they all depend on your airplane's weight or altitude or flight condition. Stall AOA All pilots know that a wing stalls when it exceeds its critical angle of attack. And as the Airplane Flying Handbook (FAA-H-8083-3A) says, this can happen at any airspeed, at any attitude, and at any power setting. Regardless of the airplane's flight condition, the wing always stalls at the same AOA. If a wing stalls at the same AOA, why does your airplane stall at a faster speed when it's heavier than when it's lighter, or at a faster speed when turning than when flying straight? Your airplane stalls at different speeds precisely because the What if you had a single number you could fly that would guarantee maximum range regardless of your airplane's weight? Or a single number to replace stall speed that would be correct whether you're straight and level or in a hard turn? This number exists, and it's called angle of attack. Angle of attack (AOA) is the angle formed by the wind and the wing. Specifically, it's the angle between the relative wind and the wing's chord line, the imaginary line between the wing's leading and trailing edges, as shown in Figure 1. EAA Experimenter 39

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