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• December 6, 2016 Create Date
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National Advisory Committee for Aeronautics, Technical Notes - Flight Investigation of a Combined Geared Unbalancing Tab and Servotab Control System as Used with an All Movable Horizontal Tail

Results are presented of a flight investigation of a Curtiss ZIP-112
airplane equipped with an all-movable horizontal tail having a control
system incorporating a combination of geared unbalancing tabs and servotabs.
Previous investigations of the configuration had been made with a servotab
control system for which the variations of elevator hinge moment with
elevator deflection and with angle of attack were near zero. The stick-
force variations with airspeed and acceleration were supplied by means of
a bobweight. This system offered the possibility of having a constant
control-force gradient regardless of altitude or center-of-gravity position.
The servotab control system tested was found to be unsatisfactory, however,
because of the light stick forces accompanying rapid elevator deflections.

In order to increase the stick forces for rapid deflections without
affecting the forces in gradual maneuvers, the servotab linkage was modi-
fied by incorporating a viscous damper in one of the links so that the
tabs deflected as geared unbalancing tabs for rapid stick motion. ‘For
slow motions, the danger allowed the tabs to operate as servotabs.

A flight investigation of this arrangement showed that the control
characteristics were generally satisfactory in rapid. maneuvers as well as
in steady flight.

An investigation was initiated to study a type of longitudinal-
control system which would provide satisfactory control characteristics
over a‘wide center-of-gt‘avity range. In the development of such a control
system, it is necessary to keep the value of Che (the variation of

hinge-moment coefficient with control deflection) near zero and it is
also desirable to keep Chou (the variation of hinge-moment coefficient

with angle of attack) near- zero and to supply the stick-force variation
with speed and acceleration by means of a bobweight. This arrangement
would be expected to eliminate the variation in stick-force gradient with
center-of- gravity position and altitude and yet provide adequate static
longitudinal stability and force per g.