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National Advisory Committee for Aeronautics, Report - Flight Investigation of the Effectiveness of an Automatic Aileron Trim Control Device for Personal Airplanes

A flight investigation to determine the efiectioeness of an auto-
matic. aileron trim control device installed in a personal airplane
to augment the apparent spiral stability has been conducted.
The device utilizes a rate—gyro sensing element in order to switch
an on-ofi type of control that operates the ailerons at a fixed rate
through control centering springs. An analytical study using
phase-plane and analog-computer methods has been carried out
to deter-mine a desirable method of operation for the automatic
trim control.

Results indicate that the device is capable of maintaining the
airplane in equilibrium over its operational speed range under
directional outpof—trim conditions that would cause rapid diver-
gence of the basic airplane. The device also prevents excessive
heading wander and airplane gyrations in turbulent air without
pilot control. A means for holding the airplane in a stabilized
turn to facilitate mild maneuvering through the automatic control
is pro-aided.

As a result of the present interest in the spiral—stability
problem associated with most personal-owner airplanes, the
National Advisory Committee for Aeronautics has under—
taken a program to investigate the effectiveness of a spiral-
stability augmenting device. The specific problem facing
the pilot of a- personal—owner airplane is to maintain his air—
plane in wings-level flight during times when he has no natu-
ral-horizon reference and to keep the airplane from diverging
spirally while he may be preoccupied with navigational prob-
lems.

It is demonstrated in reference 1 that the pilot’s sense
of orientation is unreliable in the absence of a visual refer-
ence, as may be the case when inadvertently or unavoidably
encountering instrument weather. Also, many personal air-
planes are equipped with only the basic instruments for in-
strument flight (turn indicator, ball—bank indicator, alt-i-
meter, and airspeed meter). Considerable proficiency in
instrument flying is required to interpret the indications of
these instruments properly and, in many cases, personal-
airplane pilots are not sufficiently skilled in instrument flying
to undertake it with safety.