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National Advisory Committee for Aeronautics, Technical Notes - Bench Test Investigation of the Transient Response Characteristics of Several Simulated Airplanes Incorporating an Autopilot Sensitive to Yawing Accelerations

An autopilot sensitive to yawing acceleration was used to control
four simulated airplane (mass-spring) configurations and a series of
bench tests were conducted to determine the response characteristics of
the combinations. The occurrence of an unstable mode of motion was
predicted in previous theoretical analyses, NADA TN 2006 and TN 2005, in
which the autopilot was assumed to have a constant time lag and a
constant amplitude ratio. The transient characteristics of the closed—
loop system oscillating in yaw were measured and the results show that,
with the actual autopilot, no unstable mode of motion was present
throughout the frequency range investigated. Examination of the
frequency—response characteristics of other existing autopilots indicates
that this unstable mode of motion would probably not exist if these
autopilots were used, because constant time lag and amplitude ratio do
not satisfactorily approximate the frequency-response characteristics
of such autopilots in the frequency range where the unstable mode is
predicted.

The frequencyhresponse characteristics of the test autopilot were
measured separately and were combined with the calculated frequency;
response characteristics of the four airplane configurations through use
of the method reported in NACA Rep. 882, which was extended to facili—
tate the determination of the actual transient characteristics of the
combined system. The damping and resonant frequency of the system ‘
calculated by this method were compared with the damping and resonant
frequencies measured from the transient oscillations of the closed-loop
airplane-autopilot systems.

At the wing loadings, airspeeds, and altitudes at which present-
day airplanes are operating, lateral oscillations with insufficient
damping are being encountered. Means have been sought to control these
oscillations without necessitating a modification in design which would
detract from some desirable feature of the airplane. Rate—sensitive
autopilots have been used in some airplanes and have given satisfactory _
results; however, a rate-sensitive autopilot is limited in satisfactory
operation within a certain range of phase angles of lag (less than 90°).

In order to extend the range of phase angles of lag over which
satisfactory damping of these oscillations can be obtained, an autopilot
sensitive to yawing accelerations was investigated in reference 1. Such
a control has the added feature over a rate—sensitive autopilot of not
opposing the pilot in steady maneuvers. The results of reference 1,
in which the autopilot was considered to have a constant time lag, show
that, for a large range of values of time lag, satisfactory damping to
the Dutch roll mode can theoretically be supplied by an acceleration-
sensitive control; however, in references 1 and 2 a high—frequency
unstable mode of motion is predicted to be introduced by the presence
of the autopilot.