81 04 Directional And Lateral Stability

Question 1

What factors increase the Directional Restoring moment?

Answer 1

• A more forward CG from the Fin
• Larger Fin area
• Higher IAS
• Higher side slip angle (beta)

Question 2

What effect does sweep back and aspect ratio have on the fin in relation to CL and AOA?

Answer 2

• A straight, high aspect ratio fin will have a higher CL at lower AOA and a higher CLmax, but has a lower critical AOA.
• A swept back fin or a low aspect ratio fin has a lower CL for the same AOA and a lower CLmax, but a much higher critical AOA.

Question 3

What effect does a Dorsal fin have on Directional stability?

Answer 3

A dorsal fin increases the Fin surface area and so increases directional stability

Question 4

What effect does a Dorsal fin have on Latitudinal stability?

Answer 4

A Dorsal fin increases the high keel area and so will increase Latitudinal stability

Question 5

What effect does a Ventral fin have on Directional stability?

Answer 5

A ventral fin increases the Fin surface area and so increases Directional stability

Question 6

What effect does a Ventral fin have on Latitudinal stability?

Answer 6

A ventral fin increases the ‘low’ keel area (fin area under CG) so this decreases Latitudinal stability

Question 7

What effect do wings and swept back wings have on directional stability and why?

Answer 7

• Wings have a stabilising effect on directional stability
• This is because the wing which is turned towards the RAF, now has a bigger ‘effective’ wing span compared to the wing turning away.
• This increases L on the wing towards the RAF, which in turn produces more D creating a moment which returns the AC to its original heading.
• A more swept back wing increases this effect and so is more stabilising.

Question 8

What effect does the Fuselage have on Directional stability and why?

Answer 8

• The Fuselage reduces the Directional stability
• This is because the fuselage acts like an aerofoil. When the AOA increases it generates a ‘lift’ force in the direction of the disturbance

Question 9

How does CN change with changing beta angle for a positively Directionally stable AC?

Answer 9

When Beta angle is positive (RAF to right of nose) then CN is going to be positive (yaw to the right)

Question 10

How does CL’ change with changing beta angle for a positively Latitudinal stable AC?

Answer 10

When Beta is positive (RAF from the right) CL’ is going to be negative (roll to the left)

Question 11

What factors will increase Latitudinal stability and why?

Answer 11

• Dihedral wings: Down going wing will have a greater AOA than the up going wing, so will generate more lift.
• Swept back wings: Down going wing has more ‘effective’ wing span and so will generate more lift.
• High wings: Fuselage increases the AOA on the down going wing and so generates more lift.
• High Keel surface area: A larger surface area of the keel above the CG increases the amount of air impacting creating a dynamic force to level the AC

Question 12

Describe what stability conditions cause Spiral Instability and describe a Spiral Dive

Answer 12

• Spiral Instability is caused by excessive Directional stability and weak Lateral stability.
• A disruption causes a wing to drop and the AC begins to slip
• The excessive Directional stability yaws the AC towards the new RAF
• During the yaw the up going wing increases in speed and so increases lift
• The weak Lateral stability is not able to counter this and the AC will roll further into the dive causing more side slip and yaw

Question 13

Describe what stability conditions that cause and what a Dutch Roll is

Answer 13

• Caused by excessively strong Latitudinal stability and weak Directional stability.
• A disturbance causes a wing to drop
• The excessive Latitudinal stability causes the wing to rise back up
• The rising wing is now generating more lift and so more drag
• This causes the AC to yaw in the opposite direction
• The excessive Latitudinal stability causes the wing to overshoot and so the process continues

Question 14

What is required if an AC is prone to Dutch Roll?

Answer 14

• A Yaw dampener (effectively a control coupling)

- If the Yaw dampener is not serviceable before take-off you cannot fly

Question 15

What is the effect of Pressure Altitude on Dynamic Stability and why?

Answer 15

• Higher Altitude reduces dynamic dampening

- TAS is greater at higher pressure altitudes which results in a smaller side slip angle