Aerodynamic Stability Flashcards
What does aerodynamic stability refer to?
The tendency and subsequent behavior of an aircraft when it is disturbed from its trimmed condition.
What does static stability refer to?
The initial tendency of an object to return to its original position after being disturbed by an external force.
What are the three different types of static stability?
1) Positive static stability
2) Negative static stability
3) Neutral static stability
What is dynamic stability?
The response over time to a disturbance
What are the three modes of dynamic stability?
Positive, neutral or negative dynamic stability,
What is directional stability?
Concerned with the yawing movement about the vertical axis.
What is provided for directional stability?
1) Vertical stabiliser
2) Other keel areas
What type of aerofoil is the vertical stabiliser?
Symmetrical aerofoil
What can occur if the aircraft yaws too far?
It is possible for the AoA on the vertical stabiliser to go beyond the point where it can produce a horizontal lift vector.
What is a spiral dive/divergence? When does it occur?
- Occurs with strong directional stability but weak lateral stability.
- If the pilot banks the A/C into the turn without applying any rudder, a sideslip towards the lower wing will occur.
- A/C being directionally stable will yaw into the turn, resulting in even more bank and more yaw with pitch down
- End result is a spiral dive.
What are the signs of entering a spiral dive?
1) Increasing airspeed
2) Rapid increase in RoD
3) Rapid decrease of altitude
What should the pilot do generally to recover from a spiral dive?
- Reduce power to control airspeed and deploy airbrake if required.
- Roll wings level with the horizon
- Pitch the nose up to the level or climbing attitude, maintain wings level
- Avoid overstress or overspeed of the aircraft during recovery.
What is the aerodynamic centre?
- The point on the aerofoil where the aerodynamic force can be assumed to act through for all angles of attack
- This limits the problem of moving Cp for designers
What does lateral static stability refer to?
The tendency of an aircraft to return to level flight, after a disturbance in roll.
What design feature is used to influence lateral stability in roll?
Wing anhedral or dihedral, depending on whether it is a high wing or low wing aircraft.
Why do high wing aircraft have anedral?
To counteract the positive stability the high wing aircraft already has through the mass pendulum and pockets of high pressure air.
What does the mass ahead of the CoG do regarding directional stability?
Increased mass/keel ahead of the CoG will reduce the directional stability of the A/C.
What can be fitted to further increase the area rearwards of the CoG?
Strakes
What can occur if the aircraft yaws too far?
Possible for the AoA on the vertical stabiliser to go beyond the point where it can produce a horizontal lift vector.
What do designers often need to balance regarding spiral divergence and Dutch Roll?
Balance between strong yaw or strong longtinduinal stability.
What is stability about the lateral axis called?
Longitudinal static stability.
What would occur in an aircraft with positive static longitudinal stability and neutral dynamic longitudinal stability?
- After a disturbance in pitch, the aircraft’s initial reaction will be to return to level flight, but after that will be to overshoot the level flight attitude and continue to oscillate nose up and nose down, at a constant amplitude.
What does stick free refer to?
Stick free means that after a disturbance, an aircraft’s control surfaces are able too freely trail, aligning with the RAF.
What does stick free mean WRT stability?
- There will be a reduction in the tailplane area due to control surface aligning with the RAF.
- As a result the correction and damping of a disturbance will be less effective.
