8.4 Flight Stability and Dynamics Flashcards
What is meant when referring to stability?
We refer to how the aircraft is able to follow a
planned straight and level course without pilot action
What are the two types of stability?
Static stability
Dynamic stability
What is meant by negative static stability?
An object when disturbed tends to continue in the displacement direction
What is meant by neutral static stability?
When displaced the object will not show any tendency to move back or move away from its original position
What is meant by positive static stability?
The object when disturbed will tend to return to its
original point of equilibrium
When does dynamic stability apply?
It only applies if the object has positive static stability
If it assumed that there are no friction forces, an object that oscillates forever after displacement, what is this effect called?
An undamped oscillation
If we assume friction forces, an object that is displaced that eventually damps out is called?
A damped oscillation, it is dynamically stable
Explain negative dynamic stability
If a ball is disturbed, and continues to depart away from its original equilibrium position, a divergent oscillation is formed , this is dynamically unstable
What occurs if an objects c of g and its pivot point are in the same place?
The object will have neutral stability and will not return to its original position
What does an object have if its pivot point is below the c of g?
Negative static stability, a disturbance will cause the object to move further away from its original position
What does an object have if its pivot point above the c of g?
The object will return to its original position should it be disturbed
What influences stability?
The distance between the pivot point and the c of g
What is the effect of a large distance between the pivot point and the c of g?
The longer the distance the greater the stability, and thus the tendency to return to its original position is directly linked to this length
With dynamic stability, describe positive stability
(damped) the aircraft will eventually return to the original attitude
With dynamic stability, describe negative stability
(divergent) the aircraft will continue to experience larger and large oscillations
With dynamic stability, describe neutral stability
(un-damped) the aircraft will continue on its new trajectory, not getting ant worse or better with regards to oscillations
What are the three aircraft axis?
Lateral
Longitudinal
vertical
Which axis deals with roll?
longitudinal (roll)
Which axis deals with pitch?
lateral ( pitch )
Which axis deals with yaw?
Vertical ( directional )
What axis is directional/weathercock?
The vertical
What affects the directional stability?
The fin of the aircraft and on the sweepback of the wing
What happens to the fin when it has been deflected in flight?
The fin on one side has a higher pressure than the other, the subsequent turning motion behind the c of g is greater than the turning moment in front of the c of g, causing the aircraft to return to its original flight path
What additional surfaces can be fitted to an aircraft to increase directional stability?
Keel surface and dorsal fin
Describe the effect of swept swings on directional stability
When an aircraft is displaced and a yawing motion occurs, the forward going wing presents a larger surface to the relative airflow, this increases its drag, this additional drag then creates the opposing yaw required to bring the aircraft back on track
lateral stability acts around the … what axis?
Longitudinal
Effect of dihedral when the aircraft is disturbed?
The angle of the each wing heading into the relative airflow is different, the down going wing has a higher angle of attack, the up going wing has a lower angle of attack, this creates less lift on the upwards wing and more lift on the down going wing, this creates a tendency for the aircraft to return itself to the original attitude
Directional stability acts around the … what axis?
vertical, normal, yaw
What is side slip angle?
The angle between the aircraft centre line and the slip direction
When in a slip, where is the wind?
Opposite to the slip direction
When in a slip, which wing creates more lift?
The wing in the slip direction
When is the effect of a spiral dive strongest?
When there is a greater effect on directional stability than lateral
When is Dutch roll likely to occur?
When there is a greater effect on lateral than directional stability
How can you partially overcome Dutch roll?
By having less of a sweep angle, and by improving directional stability
How can you improve the directional stability?
By having a bigger fin, at the cost of added weight and increased drag
