8.4 Flight Stability and Dynamics

Question 1

What is meant when referring to stability?

Answer 1

We refer to how the aircraft is able to follow a

planned straight and level course without pilot action

Question 2

What are the two types of stability?

Answer 2

Static stability

Dynamic stability

Question 3

What is meant by negative static stability?

Answer 3

An object when disturbed tends to continue in the displacement direction

Question 4

What is meant by neutral static stability?

Answer 4

When displaced the object will not show any tendency to move back or move away from its original position

Question 5

What is meant by positive static stability?

Answer 5

The object when disturbed will tend to return to its

original point of equilibrium

Question 6

When does dynamic stability apply?

Answer 6

It only applies if the object has positive static stability

Question 7

If it assumed that there are no friction forces, an object that oscillates forever after displacement, what is this effect called?

Answer 7

An undamped oscillation

Question 8

If we assume friction forces, an object that is displaced that eventually damps out is called?

Answer 8

A damped oscillation, it is dynamically stable

Question 9

Explain negative dynamic stability

Answer 9

If a ball is disturbed, and continues to depart away from its original equilibrium position, a divergent oscillation is formed , this is dynamically unstable

Question 10

What occurs if an objects c of g and its pivot point are in the same place?

Answer 10

The object will have neutral stability and will not return to its original position

Question 11

What does an object have if its pivot point is below the c of g?

Answer 11

Negative static stability, a disturbance will cause the object to move further away from its original position

Question 12

What does an object have if its pivot point above the c of g?

Answer 12

The object will return to its original position should it be disturbed

Question 13

What influences stability?

Answer 13

The distance between the pivot point and the c of g

Question 14

What is the effect of a large distance between the pivot point and the c of g?

Answer 14

The longer the distance the greater the stability, and thus the tendency to return to its original position is directly linked to this length

Question 15

With dynamic stability, describe positive stability

Answer 15

(damped) the aircraft will eventually return to the original attitude

Question 16

With dynamic stability, describe negative stability

Answer 16

(divergent) the aircraft will continue to experience larger and large oscillations

Question 17

With dynamic stability, describe neutral stability

Answer 17

(un-damped) the aircraft will continue on its new trajectory, not getting ant worse or better with regards to oscillations

Question 18

What are the three aircraft axis?

Answer 18

Lateral
Longitudinal
vertical

Question 19

Which axis deals with roll?

Answer 19

longitudinal (roll)

Question 20

Which axis deals with pitch?

Answer 20

lateral ( pitch )

Question 21

Which axis deals with yaw?

Answer 21

Vertical ( directional )

Question 22

What axis is directional/weathercock?

Answer 22

The vertical

Question 23

What affects the directional stability?

Answer 23

The fin of the aircraft and on the sweepback of the wing

Question 24

What happens to the fin when it has been deflected in flight?

Answer 24

The fin on one side has a higher pressure than the other, the subsequent turning motion behind the c of g is greater than the turning moment in front of the c of g, causing the aircraft to return to its original flight path

Question 25

What additional surfaces can be fitted to an aircraft to increase directional stability?

Answer 25

Keel surface and dorsal fin

Question 26

Describe the effect of swept swings on directional stability

Answer 26

When an aircraft is displaced and a yawing motion occurs, the forward going wing presents a larger surface to the relative airflow, this increases its drag, this additional drag then creates the opposing yaw required to bring the aircraft back on track

Question 27

lateral stability acts around the ... what axis?

Answer 27

Longitudinal

Question 28

Effect of dihedral when the aircraft is disturbed?

Answer 28

The angle of the each wing heading into the relative airflow is different, the down going wing has a higher angle of attack, the up going wing has a lower angle of attack, this creates less lift on the upwards wing and more lift on the down going wing, this creates a tendency for the aircraft to return itself to the original attitude

Question 29

Directional stability acts around the ... what axis?

Answer 29

vertical, normal, yaw

Question 30

What is side slip angle?

Answer 30

The angle between the aircraft centre line and the slip direction

Question 31

When in a slip, where is the wind?

Answer 31

Opposite to the slip direction

Question 32

When in a slip, which wing creates more lift?

Answer 32

The wing in the slip direction

Question 33

When is the effect of a spiral dive strongest?

Answer 33

When there is a greater effect on directional stability than lateral

Question 34

When is Dutch roll likely to occur?

Answer 34

When there is a greater effect on lateral than directional stability

Question 35

How can you partially overcome Dutch roll?

Answer 35

By having less of a sweep angle, and by improving directional stability

Question 36

How can you improve the directional stability?

Answer 36

By having a bigger fin, at the cost of added weight and increased drag