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National Advisory Committee for Aeronautics, Report - The Dynamic Response Characteristics of a 35° Swept Wing Airplane as Determined from Flight Measurements

The longitudinal and lateral-directional dynamic-response
characteristics of a 35 ° swept-wing fighter-type airplane
determined from flight measurements are presented and compared
with predictions based on theoretical studies and wind—tunnel
data. Flights were made at an altitude of 85,000 feet covering
the Mach number range of 0.50 to 1.04. A limited amount
of lateral-directwnal data were also obtained at 10,000 feet.

The flights consisted essentially of recording transient responses
to pilot-applied pulsed motions of each of the three primary
control surfaces. These transient data were converted into
frequency-response form by means of the Fourier transformation
and compared with predicted responses calculated from the
basic equations of motion. The equations, or transfer func-
tions, that best describe the various measured responses were
evaluated by a curve-fitting process involving the use of tem-
plates and an analog computer. By this method it was generally
possible to find equations, of simple form, that closely matched
the experimental frequency responses between 1 and 10 radians
per second and at the same time adequately described the re-
corded time histories.

Easperimentally determined transfer functions were used for
the evaluation of the stability derivatives that have the greatest
eject on the dynamic response of the airplane The values of
these derivatives, in most cases, agreed favorably with predictions
over the Mach number range of the test.

In the design of automatic-control equipment for high
performance aircraft, the dynamic response characteristics
of the aircraft must be considered. It is desirable to express
these characteristics as transfer functions which are expres-
sions that describe the motion of the airplane for the various
flight conditions of interest. The airplane can then be rep-
resented as a single element in a more complex closed-loop
system.