MT2 Flashcards
What is stability of a dynamical system?
Stability is the system motion behavior characteristics about a given equilibrium points. Always takes place about a state of equilibrium where sums of the forces and moments about the cg of the aircraft are zero.
What is static stability?
Static stability deals with the initial response of the system to a disturbance about a desired equilibrium point
What is dynamic stability?
Dynamic stability deals with the Time History of the system’s motion after the initial response
What are the 3 possible cases that can occur when an AC is disturbed from trim?
-Tendency to return to trim position - AC is statically stable (positive stability)
-Stays at disturbed position - AC is statically neutrally stable (neutral stability)
-Tendency to move away from equilibrium - AC is statically unstable (negative stability)
What does dynamically stable mean?
Over time the system stays within a constricting envelope (system may oscillate but they are damped out over time)
What does dynamically marginally stable mean?
Over time the system stays at the same position or oscillates at constant amplitude (no damping)
What does dynamically unstable mean?
Over time the system immediately diverges or has divergent oscillations
Is static stability necessary for dynamic stability?
Yes, however it is not a guarantee of static stability
What happens when the fuselage is added to the wing?
The aerodynamic center is shifted forward, the lift curve slope is increased, and a negative contribution is added to moment about the aerodynamic center.
What are the wing, body, and tail contributions to the longitudinal static stability of an aircraft?
Wing: has negative contribution to LSS
Body: has a negative contribution to LSS
Tail: has positive contribution to LSS
What happens as the COG is moved forward, at, and behind the neutral point
forward: statically stable
at: statically neutrally stable
behind (aft): statically unstable
What is static margin?
Static margin is a direct measure of longitudinal static stability. For static stability, the static margin must be positive, and the larger it is, the more stable the airplane
What is the elevator for?
It can be used to change and control the trim of the airplane, and therefore the equilibrium velocity of the airplane
What is directional stability?
Stability in the yaw
What value must be positive for directional static stability?
Coefficient of yaw
What is directional divergence?
It is the result of a directionally unstable airplane. When the airplane yaws or rolls into a sideslip so that side forces on the airplane are generated, the yawing moments that arise continue to increase the sideslip. This condition may continue until the airplane is broadside to the relative wind
How does the fuselage, wing, and vertical tail contribute to directional stability?
Fuselage is destabilizing, wing is slightly stabilizing, vertical tail is stabilizing
Why is the vertical tail added for directional stability?
It is added to increase the keel surface behind the COG. For directional static stability, the keel surface behind the COG must be larger.
What is lateral stability?
Stability in the roll (x-axis)
What must the coefficient of roll be for lateral static stability?
It must be negative
What is spiral divergence?
It is characterized of an airplane that is very stable directionally (about Z) but not very stable laterally (about X)
