I am trying to estimate the required servo torque for my design using hinge moment at the elevator.

XFOIL results were as follows-

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I am getting a Hinge moment of about 0.06-0.08 Nm. for the given deflection. How can I relate this to the servo torque I require? Will they be equal?

  • 2
    $\begingroup$ Its a perfectly valid question with a good and to-the-point answer. Hinge moments are part of flight control design. $\endgroup$
    – Koyovis
    Commented Aug 7, 2019 at 13:38

1 Answer 1


The servo torque depends on the mechanism. If it has a gear ratio of 1, they will be equal. If the servo is driving larger angular deflections than its own, the servo torque will be higher.

Another important point is to consider the dynamic forces. XFOIL only computes results for steady conditions. Dynamically, the servo may have to bear even larger torques; due to acceleration or gravity.

Finally, unless you really trust your computations, some uncertainty factor can be used to reduce the risks of redesigning the control system solution.


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