If pitching moment creates drag, how can high and low moment airfoils have similar drag polars?

Let's call a symmetrical airfoil like NACA0015 a low moment airfoil having no moment at an AOA of 0.

Let's call NACA 2415 a medium moment airfoil. Let's say it has a medium moment at an AOA of 0.

Let's call Eppler399 a high moment airfoil, and it has a high moment at an AOA of 0.


I understand pitching moment creates drag. While the above airfoils have vastly different moment polars at an AOA from -10 to + 10, they almost identical drag polars,at an AOA from -10 to +10.

How can this be?

See pics below

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1 Answer 1


A high Cm airfoil does not mean it will be draggy by itself. However, it does mean that when combined with trim devices (e.g. tailplane, elevator, canard, elevon), the overall drag may be higher due to the trim drag component.

Since 25% MAC, about which pitching moments are generally cited, generally gives around 20% static margin for low speed aircraft, it's a rough first indication of how much trim would be required.

  • 3
    $\begingroup$ The absolute value of Cm does not matter much as you can add a constant moment by moving the centre of gravity (that is, choosing where to mount the wing). The difference between highest and lowest is what will have to be compensated by the elevator and thus create trim drag. $\endgroup$
    – Jan Hudec
    Nov 5, 2019 at 21:56

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