AOPA Turbine Pilot Magazine - August 2017 - 96

P&E

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TECHNIQUE

That resulting longitudinal motion is a
combination of two fundamental and
independent oscillations: the phugoid
(or long-period) oscillation and the
short-period oscillation. Lateral and
directional stability are important, but
let's restrict this discussion to longitudinal stability.
The phugoid oscillation features
a nearly constant angle of attack and
changing airspeed and altitude, which
can take several minutes to settle down.
The short-period oscillation is characterized by a changing angle of attack
and constant airspeed, and typically
lasts less than a second. Both these
oscillations must be demonstrated during certification of an aircraft under
Part 23. Although there is no requirement that the phugoid oscillation be
stable, it "must not be so unstable as
to increase pilot workload or endanger
the airplane." A reasonably skilled pilot
should be able to intervene to return
the airplane to its trimmed condition.
In contrast, the short-period oscillation
happens so quickly that it is required to
be heavily damped. While the stability
characteristics of each airplane are different, most general aviation airplanes
feature stable phugoid and short-period
oscillations.
The University of Tennessee Space
Institute Navion is no ordinary airplane.
The aircraft is used to simulate flight in
icing conditions, and its variable stability system enables it to be configured
to mimic the stability characteristics of
another airplane. That day, the Navion
behaved as the NASA Twin Otter does
with ice accretions. Ice buildup on the
wings and tail can seriously degrade the
stability characteristics of an aircraft.
In this case, the short-period oscillation was slightly unstable, so keeping
the aircraft under control was like trying to stand on a basketball. Pilots who
load their aircraft outside the weight and
center of gravity envelope may witness a
similar stability degradation.
PILOT-INDUCED OSCILLATIONS

People are not particularly adept at arresting longitudinal oscillations, and a breezy
96 | AOPA PILOT August 2017
http://www.methodseven.com/aopa http://www.aviationseminars.com/pilot
AOPA Turbine Pilot Magazine - August 2017

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