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What makes it so attractive to use a stabilator in some small airplanes like Piper Cherokee or Seminole? Its primary purpose is to allow the pilot to control a pitch movement with a less force (which I presume better serve big airplanes), and if so, wouldn't a horizontal stabilizer coupled with an elevator be a better choice for the manufacturers, especially given that a stabilator installed in small airplanes typically requires an antiservo tab to prevent overcontrol by the pilot?

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Simplicity. Yes, a servo tab is helpful, but most control surfaces should include a trim tab, so this can double as a servo tab. Total part count should still be lower than that for a stabilizer-elevator combination. But there are other advantages: Easier adjustment of stick forces and little loss of stability with free-floating controls.

By hinging the stabilator (or full-flying tail) near the quarter point, its control forces can be kept small and tailored with the servo tab. Also, this helps to keep the hinge moment coefficient derivative over angle of attack $c_{r_\alpha}$ low. The ratio between it and the derivative over deflection angle $\eta$ determines the angle to which the control surface will float, and a small $c_{r_\alpha}$ means that the stability with a floating control surface will be close to the stability with a fixed surface.

Also, balancing masses are easier to place when the full surface moves.

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