|We have defined static stability as the initial tendency
that the airplane displays after being disturbed from its trimmed condition.
Occasionally, the initial tendency is different or opposite from the overall
tendency, so we must distinguish between the two. Dynamic stability is
the overall tendency that the airplane displays after its equilibrium is
disturbed. The curves of Fig. 17-23 represent the variation of controlled
functions versus time. It is seen that the unit of time is very significant.
If the time unit for one cycle or oscillation is above 10 seconds' duration,
it is called a "long period" oscillation (phugoid) and is easily controlled.
In a longitudinal phugoid oscillation, the angle of attack remains constant
when the airspeed increases and decreases. To a certain degree a convergent
phugoid is desirable but is not required. The phugoid can be determined
only on a statically stable airplane, and this has a great effect on the
trimming qualities of the airplane. If the time unit for one cycle or oscillation
is less than one or two seconds, it is called a "short period" oscillation
and is normally very difficult, if not impossible, for the pilot to control.
This is the type of oscillation that the pilot can easily "get in phase
with" and reinforce.
A neutral or divergent, short period oscillation is dangerous because structural failure usually results if the oscillation is not damped immediately. Short period oscillations affect airplane and control surfaces alike and reveal themselves as "porpoising" in the airplane, or as in "buzz" or "flutter" in the control surfaces. Basically, the short period oscillation is a change in angle of attack with no change in airspeed. A short period oscillation of a control surface is usually of such high frequency that the airplane does not have time to react. Logically, Federal Aviation Regulations require that short period oscillations be heavily damped (i.e., die out immediately). Flight tests during the airworthiness certification of airplanes are conducted for this condition by inducing the oscillation in the controls for pitch roll, or yaw at the most critical speed (i.e., at Vne, the never exceed speed). The test pilot strikes the control wheel or rudder pedal a sharp blow and observes the results.