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When I extend my flaps to 10 degrees, what exactly is the 10 degrees measuring? Is this referring to the angle of the flap blades themselves, the new angle of the wing chord, the change in the new critical angle of attack or something else?

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    $\begingroup$ Selecting the first and/or second detent of flap extension may also extend leading edge devices. Also, in addition to 'lowering' the flaps, it may extend them aft of their retracted position. $\endgroup$ – Terry Jan 21 '14 at 3:20
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The "Degrees" of flap deployment generally refers to the deflection from the neutral (0° - flush with the rest of the wing) position of the flaps.

For most GA aircraft this is an actual angular measurement - 10° of flaps corresponds to the flaps lowering 10° from their neutral position, but the degree of travel can also be given as a percentage (0-100%). The aircraft's maintenance manual will specify the actual travel (and specify how it is to be measured), and if specific position detents are provided there will usually be "rigging limits" specified for where each position stop must be & how accurate the control position must be at each stop.

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    $\begingroup$ Is it just me, or does this not explain from what part of the flap is this angle measured? I understand that 0° is the neutral flap position, but is it a measurement to the tip of the flaps or some other method that dictates this angle? $\endgroup$ – Qantas 94 Heavy Jan 21 '14 at 10:38
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    $\begingroup$ @Qantas94Heavy The aircraft's maintenance manual will specify the actual travel (and specify how it is to be measured) though if it's a plain flap and an angular measurement the "where" doesn't really matter - the angle doesn't change, you're just using a different-sized protractor to measure it... $\endgroup$ – voretaq7 Jan 21 '14 at 17:00
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    $\begingroup$ As I understand it, in small aircraft the degrees of flap is unambiguous: it refers to the angle of the flap to the rest of the wing. For larger aircraft with special flaps (slats, leading edge), the degrees measurement is simply a holdover terminology. $\endgroup$ – Hugh Feb 11 '15 at 21:38

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