Dynamic Stability. Dynamic stability is the movement of an aircraft with respect to time.
If an aircraft has been disturbed from its equilibrium position and the maximum displacement
decreases with time, it is said to have positive dynamic stability. If the maximum displacement
increases with time, it is said to have negative dynamic stability. If the displacement remains
constant with time, it is said to have neutral dynamic stability. Both static and dynamic stability
are usually desired in an airplane.
Static stability does not guarantee that an airplane will also be dynamically stable; however, the
airplane must first be statically stable before it will oscillate at all and, thus, exhibit any kind of
dynamic stability. Possible cases of stability can be summarized in Figure 2-4.
Figure 2-4 Cases of Stability
In addition to positive static stability, the T-34C has positive dynamic stability. Over a period of
time, it will tend to return to most conditions for which it was trimmed.
Let us again consider the ball in the saucer shown in Figure 2-3. If you move the ball to the
displaced (dotted) position and then let go, it will not go directly to the equilibrium point and
stop. Rather, its inertia will carry it well past the bottom and up the other side. As gravity
overcomes its inertial acceleration, it rolls back the other way towards the original displacement
point until it succumbs again to gravity. This motion could go on indefinitely if there were no
friction present between the ball and the dish. Eventually, friction will reduce the momentum of
the ball sufficiently so that it finally settles down once again to the bottom of the dish. A
pendulum that is pushed to one side and released displays the same action. The resulting motion
damps out the motion.
The T-34C aircraft has both positive static and dynamic stability and thus is an "inherently
stable" aircraft. The design specifications required the trainer to be capable of aerobatic flight
with responsive controls. However, since the T-34C is a primary trainer, it also had to possess
the qualities which would be "forgiving" to the early Contact student.
Generally speaking, the longitudinal axis in the T-34C is less stable than the lateral and vertical
axes. When trimmed for equilibrium, less force is required to displace the control surfaces,
resulting in a rolling moment. It can be stated that the ailerons are sensitive in the T-34C,
particularly in high-speed flight.
INTRODUCTION TO T-34C AERODYNAMICS 2-7