Instability

Question 1

What is Dutch roll?

Answer 1

Dutch roll is an oscillatory instability associated with swept-wing jet aircraft. It is a continuous yawing and rolling motion.

Question 2

What causes Dutch roll?

Answer 2

Swept wings. Yawing motion caused by the pilot or external factors such as turbulence will allow the outer wing to travel faster and produce more lift causing a rolling action. Increased drag and a possible stall on the outer wing then reverses the action.

Question 3

What is the recovery technique from a Dutch roll?

Answer 3

Applying opposite aileron input to the direction of roll. The use of rudder is not very effective and aggravate the situation due to the quick yawing motion of the aircraft and delays in pilot inputs.

Question 4

What prevents Dutch roll?

Answer 4

Yaw dampers prevent Dutch roll by assisting the rudder in maintaining directional stability. This is done through yaw sensors and motor inputs to the rudder.

Question 5

What is directional stability?

Answer 5

Directional stability is an aircraft tendency to maintain its direction (heading) around the vertical axis. (yaw)

Question 6

What is spiral stability or instability?

Answer 6

Spiral stability is an aircraft’s tendency to resist the motion of rolling into a turn. Spiral instability will allow the aircraft to yaw, roll, pitch, yaw,….into the turn even further until a spiral dive occurs. This is due to the slipping motion of the aircraft and vertical stabilizer surface aiding this process.

Question 7

What is lateral stability?

Answer 7

Lateral stability is and aircraft’s tendency to resist the motion of roll.

Question 8

How can lateral stability be increased?

Answer 8

• Wing dihedral allowing the lower wing to create more lift as the aircraft slides into the roll.
• Vertical stabiliser design creating opposing forces of the roll direction.

Question 9

What is longitudinal stability?

Answer 9

Longitudinal stability is an aircraft’s tendency to resist a pitching moment around the lateral axis.

Question 10

What aids an aircraft’s longitudinal stability?

Answer 10

• The tailplane.

- Movement of CoP during AoA changes (design dependant)

Question 11

How does altitude affect and aircraft’s stability?

Answer 11

In general altitude will affect stability in a negative way since the aerodynamic forces have a lesser dampening effect.

Question 12

What are the four flying qualities that are affected by high altitude operations?

Answer 12

• Restricted operating speed range (coffin corner)
• Reduced manoeuvrability
• Reduced aerodynamic dampening
• Reduced stability