Aerodynamic Stability

Question 1

What does aerodynamic stability refer to?

Answer 1

The tendency and subsequent behavior of an aircraft when it is disturbed from its trimmed condition.

Question 2

What does static stability refer to?

Answer 2

The initial tendency of an object to return to its original position after being disturbed by an external force.

Question 3

What are the three different types of static stability?

Answer 3

1) Positive static stability
2) Negative static stability
3) Neutral static stability

Question 4

What is dynamic stability?

Answer 4

The response over time to a disturbance

Question 5

What are the three modes of dynamic stability?

Answer 5

Positive, neutral or negative dynamic stability,

Question 6

What is directional stability?

Answer 6

Concerned with the yawing movement about the vertical axis.

Question 7

What is provided for directional stability?

Answer 7

1) Vertical stabiliser

2) Other keel areas

Question 8

What type of aerofoil is the vertical stabiliser?

Answer 8

Symmetrical aerofoil

Question 9

What can occur if the aircraft yaws too far?

Answer 9

It is possible for the AoA on the vertical stabiliser to go beyond the point where it can produce a horizontal lift vector.

Question 10

What is a spiral dive/divergence? When does it occur?

Answer 10

• 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.

Question 11

What are the signs of entering a spiral dive?

Answer 11

1) Increasing airspeed
2) Rapid increase in RoD
3) Rapid decrease of altitude

Question 12

What should the pilot do generally to recover from a spiral dive?

Answer 12

• 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.

Question 13

What is the aerodynamic centre?

Answer 13

• 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

Question 14

What does lateral static stability refer to?

Answer 14

The tendency of an aircraft to return to level flight, after a disturbance in roll.

Question 15

What design feature is used to influence lateral stability in roll?

Answer 15

Wing anhedral or dihedral, depending on whether it is a high wing or low wing aircraft.

Question 16

Why do high wing aircraft have anedral?

Answer 16

To counteract the positive stability the high wing aircraft already has through the mass pendulum and pockets of high pressure air.

Question 17

What does the mass ahead of the CoG do regarding directional stability?

Answer 17

Increased mass/keel ahead of the CoG will reduce the directional stability of the A/C.

Question 18

What can be fitted to further increase the area rearwards of the CoG?

Answer 18

Strakes

Question 19

What can occur if the aircraft yaws too far?

Answer 19

Possible for the AoA on the vertical stabiliser to go beyond the point where it can produce a horizontal lift vector.

Question 20

What do designers often need to balance regarding spiral divergence and Dutch Roll?

Answer 20

Balance between strong yaw or strong longtinduinal stability.

Question 21

What is stability about the lateral axis called?

Answer 21

Longitudinal static stability.

Question 22

What would occur in an aircraft with positive static longitudinal stability and neutral dynamic longitudinal stability?

Answer 22

• 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.

Question 23

What does stick free refer to?

Answer 23

Stick free means that after a disturbance, an aircraft’s control surfaces are able too freely trail, aligning with the RAF.

Question 24

What does stick free mean WRT stability?

Answer 24

• 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.

Question 25

What is the opposite of stick free?

Answer 25

Stick fixed.

Question 26

What type of stability does Dutch Roll have?

Answer 26

• Positive directional and lateral stability
• Neutral dynamics directional and lateral stability
• Lateral stability is dominant.

Question 27

Do swept wings add to lateral stability? Does that make them more prone to dutch roll?

Answer 27

Yes

Question 28

What are yaw dampers used for?

Answer 28

To contribute to increasing directional static stability to reduce the tendency for dutch roll.

Question 29

Will highly swept wings have a lot of dihedral/anhedral?

Answer 29

No, because of their tendency to have strong lateral stability anyway.

Question 30

What is the difference between passive and active stability?

Answer 30

• Passive stability refers to design features on the A/C (dihedral, shape of vertical fin) that require no input from the pilot.
• Active stability refers to artificially provided stability from either the pilot or the autopilot.