I calculated the control hinge-moment coefficient due to tab by using the dimensions of a tail and its elevator. However, the trim tab dimension parameters in the calculations were approximated values corresponding to similar aircrafts' tail geometries. Thus, what are the next steps to find the precise trim tab dimensions, and what is the role of the control hinge-moment coefficient due to tab in sizing a trim tab?


  • the dimensions of the tail and elevator are precise and could not be changed
  • hinge-moment coefficient due to elevator is known
  • $\begingroup$ Since it operates in the thickest part of the boundary layer, the effectiveness of a trim tab is notoriously hard to compute. The usual procedure is just as you did: Copy what others have done before. The next step is to fine-tune the design in flight test. $\endgroup$ Dec 31, 2020 at 20:40

1 Answer 1


The trim tab is sized first, and then the tab hinge moment is computed. The trim tab is sized to the conditions required for maximum trimmed position of the aircraft at:

  • low speeds;
  • all relevant CG positions;
  • all relevant flap settings;

The contribution of the trim tab to the elevator hinge moment is tiny, see the right hand graph which is measured data from the F-27.

from an old uni book


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