# Stick Force for a Stabilator

In the case of an all-movable tail, the moment, H, can be expressed in terms of a moment coefficient, CM, about the pivot. For a control surface that is hinged to a surface ahead of it, the “hinge” moment is usually expressed in terms of a hinge moment coefficient, CV

H=pV2ScCh

S refers to the planform area of the control surface (such as the elevator), and c is the mean chord of the control surface.

Let us now consider the stick force (“stick” is used by most pilots and engineers as synonymous with control wheel) for a movable tail with a linked tab. For simplicity a symmetrical airfoil will be assumed, so that the moment coefficient can be written as

Cm, — ba, + b2 Se

The notation, bt and b2, is borrowed from Reference 8.2 and avoids the use of triple subscripts in denoting the partial derivatives. Obviously,

The constant b can be positive or negative, depending on the location of the pivot relative to the aerodynamic center. b2, on the other hand, is usually negative.

To allow the stick force to be trimmed to zero, a constant term, 50, is added to the linked elevator angle so that

Se кelhs So

Substituting Equations 8.31, 8.32, and 8.33 into Equation 8.27 gives P

GqS, c,

Replacing ihs by Equation 8.26 and using

WIS

results in

(8.35)

The gradient of the stick force with velocity at a particular trim speed is found by differentiating Equation 8.35 with respect to V and letting V = VTr.

dP 2 GS, c,(WIS)A

dVr r Vtr

For a given trim speed a positive P, that is, a pull on the control, should result in a decrease in the speed. Thus, for the proper feel, the constant, A, in Equation 8.35 should be positive. This is referred to by FAR Part 23 as a stable stick force curve and is a requirement for all conditions of flight.

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