Stability Flashcards
Static stability
Initial response of the a/c
Dynamic stability
Change over time
Statically neutral
Push the ball and remains where it is pushed
Positive static
Pushed the ball it wants to return to original position
Negative static
Push the ball, it will continue to roll
2 different types of dynamic stability
A-periodic without oscillation
Periodic with oscillation
Neutral dynamic and static
Remains in the position it was pushed to
Positive static and neutral dynamic
Tries to get back to equilibrium but it doesn’t get better and it doesn’t get worst, constant oscillations at the same heights
Positive static and negative dynamic
Tries to get to equilibrium but the oscillations get worst over time
Negative static, negative dynamic
Continues to get worst
A-periodic
Longitudinal stability
Stability in the pitch
Lateral stability
Stability in the roll
Directional stability
Yaw
Point on the wing where the force acts
AC
M Cm
Pitching moment
Lateral axis
Positive is up
L Ci
Rolling moment
Longitudinal axis
Positive is right
N Cn
Yawing moment
Normal axis
Positive is right
How can you change the tail plane to make it more stable
Increase the surface area
Move the tailplane further away
Further aft CG
Decreases stability
Further fwd CG
Increases stability
Directional stability
Stability in the yaw
Airflow to the side of the nose, a/c will want to yaw into the airflow as it hits the tail
To improve directional
Increase the size of the tail
Increase the distance of the tail from the CG
Lateral stability
All about the wings
Lateral - AoA of the wings
Upgoing, Reduced, wants to pitch down
Downgoing, increases, wants to pitch down
Improving lateral stability
Low mounted wing has to be dihedral to make it more stable
High mounted wing has to be anhedral to reduce the dihedral effect
Increase the sweepback
A keel surface above the CG
Decreases lateral stability
Anhedral wings
A keel below the CG will destabilise - vortral fins
Low wings
Longitudinal stability
Disturbance in the pitch
Absolute AoA
From the RAF to the zero lift line
Using absolute AoA makes it easier to determine the reaction force
Cm formula and was is it and the most important factor
Cm = m/qsL
Natural ability for the a/c to pitch in response to a disturbance
Most dominant is IAS square (q) which means the faster we go the less Cm
In S and L with a reduction in speed, what must be done to the elevator and trim tab to make the stick force 0
Elevator is deflected upwards, trim tab further downwards
Which direction does a bob weight move the control stick
Forwards
The level of effect on stick force by a change in speed from a trimmed position
High speed small change in speed = less effect on stick force
Low speed small change in speed = bigger effect on stick force
For an a/c to exhibit static longitudinal stability where do you want the lift to act
Aft of the CG, extra lift will push the nose down again
Fwd of the CG, the extra lift will push the nose further up
Why is excessive lateral stability undesirable for transport a/c
During sideslip, e.g during a cross wind, it may impose excessive demands on roll control
Reducing sweepback
Decreases static lateral stability
On fully powered controls what happens to the neutral position of the control column when trimming
Remains unchanged
