Aircraft stability

Question 1

What happens when a trimmed aircraft is disturbed?

Answer 1

the behaviour of the aircraft will depend on the change in the pitching moment acting on the aircraft

as shown in the schematic, the up-gust will increase AOA

lift from the wing and tailplane will increase resulting in the pitching moment around CG changing

Question 2

Why are aircraft not made very stable?

Answer 2

large control forces are required to manoeuvre the aircraft

Question 3

Stable aircraft

Answer 3

L behind W nose up pitching moment

Question 4

Unstable aircraft

Answer 4

L infront of W nose up pitching moment

Question 5

Neutrally stable

Answer 5

L=W

Question 6

Longitudinal stability

Answer 6

Question 7

Lateral stability

Answer 7

for lateral stability, positive sideslip provides a negative roll

dihedral improves lateral stability

Question 8

Why does dihedral improve lateral stability?

Answer 8

an aircraft banks whilst sideslipping

slideslip induces a normal velocity to the leading wing

AOA can be expressed as normal velocity divided by the forward speed of the aircraft

lift on the leading wing is increased and reduced on the trailing wing and therefore the aircraft rolls away from sideslip

Question 9

Directional static stability

Answer 9

keeping the aircraft into the wind