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Aerodynamic centre

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Aerodynamics, General Aviation

Hello Aerodynamic Center (AC) is that point on the aerofoil where the pitching moment is constant irrespective of angle of attack. Please can any one explain why then it moves when the aerofoil stalls. I have learned from instructor that AC moves at stall otherwise it remains constant.

3 Answers

  1. Brian on Aug 14, 2013

    This is one of those, it’s more than we need to know as pilots, sort of questions. The common definition for AC, as you explained it is: “A point at which the pitching moment is independent of angle of attack.”

    To quote one of my texts: “However, outside the traditional approximation of linear aerodynamics, there may be be no fixed point on a wing or complete aircraft about which the pitching moment is totally independent of the angle of attack.”

    Back on track now; for a pilot, the traditional definition will suffice. For an engineer it would be a gross misconception. An arguably more accurate definition might be: “A point for a given angle of attack where by small changes in angle of attack have no effect on pitching moments.”

    Notice the difference in the traditional and my own take on a more precise definition. The traditional recognizes only one point when in reality a different AC can be calculated for each AOA. In other words, for each AOA there will be a point (AC) where small changes in angle of attack will be independent of pitching moments.

    What should you take away from this? Everything said past the first paragraph, as it pertains to a pilot, is white noise. The purpose of AC, be it for a pilot or engineer, is to simplify the analysis (and calculation for the engineers) of aircraft stability. We are concerned with analysis, and for analysis the traditional definition is sufficient. Worrying about how it changes with stall or with varying AOA’s is again, white noise.

    I’m not sure why your instructor chose to point out this change in stalled flight, though I wouldn’t mind hearing his/her reasoning. It, like CP, shifts forward at higher angles of attack. Again though, this is not important. We use AC specifically so we do not have to talk about this shift. Hope that straightens it out for you.

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  2. eaglectl on Aug 14, 2013

    @ Brian sir thank you so very much for the value able comments

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  3. rizwan on Oct 03, 2013

    a point where small change in aoa is catered by the change in moment arm… so pitching movememt stays constant

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