I intend on calculating the the servo torque required to actuate a 75mmx38mm NACA0012 airfoil. The example at the end of the page in the link https://github.com/build-week/hover-jet/blob/feature/start-design-scripts/design-scripts/jet_vane_design.ipynb mounts the servo at 25% chordlength and then finds the pitching moment. I would like to know how the servo torque required would be found if the servo was offset from this 25% chord location.

Kind regards

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    $\begingroup$ At what airspeed? At what deflection angle? If this is a vane in a pipe (a jet exhaust), how big and what shape is the pipe? $\endgroup$ Commented May 30, 2021 at 15:16
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    $\begingroup$ Hi, airspeed: 73.27m/s , not sure what exact deflection angle but it is within -15 to 15 degrees , it lies by the exit of a rocket fuselage of 90mm diameter ( it is an electric ducted fan rocket) . We are using vanes to deflect the thrust by the way $\endgroup$
    – Johan M
    Commented May 30, 2021 at 15:26

1 Answer 1


See what the L/D curve for NACA 0012 is at 15 degrees AoA, and calculate the Reynolds number for that chord and airspeed.
That estimates the maximum lift, i.e., force at right angles to the duct. Then you know, by leverage, the max force exerted by the servo on the vane, and from the servo arm's length, the max servo torque.

But that estimate of lift will be corrupted by

  • the swirly airflow hitting the airfoil
  • the turbulence between the wall and the "wingtips" that don't quite reach the wall
  • the short distance "above and below" the vane to the duct walls, like the middle wing of a Fokker triplane
  • the interactions at the duct's exit
  • dynamic effects, if the vane's AoA changes quickly

So for a trustworthy answer, you'd have to do some bench measurements, perhaps with smaller ducts and slower efflux.

  • $\begingroup$ Thank you for the informative response! I have actually been playing with Xflr5 to get coefficient values at the Reynolds number. Just a query of the 1st statement: One could find the lift coefficient and find its value with a chosen vane area. Though, I am not certain how one would find the centre of pressure at 15 degrees. Would it be applicable to find the coefficient of moment and use that to size the torque required for the servo alternatively? $\endgroup$
    – Johan M
    Commented May 30, 2021 at 21:58
  • $\begingroup$ Sure, that would work. But to really avoid servo blowback for even a fraction of a second, use a safety factor of two, because it could get really turbulent in there. Any by-the-book corrections for max torque (from, say, how CP varies with AoA) of +-10% would get lost in that noise. $\endgroup$ Commented May 31, 2021 at 18:12
  • $\begingroup$ thanks for the confirmation $\endgroup$
    – Johan M
    Commented Jun 1, 2021 at 6:51

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