Module 1 Directional Stability and Stability questions

Question 1

When is an aircraft directionally stable?

Answer 1

It is directionally stable if the aircraft is able to produce a restoring yaw moment to return it to equilibrium. This is known as a weathercock effect or turning into wind.

Question 2

When an aircraft has a disturbance in yaw what happens?

Answer 2

It turns away from the RAF.

Question 3

What is the angle between the aircraft heading and the RAF?

Answer 3

Sideslip Angle

Question 4

When is the sideslip angle in directional stability + and -?

Answer 4

When the RAF of the aircraft is coming from the right the side slip angle is positive and if it was coming from the left the side slip angle would be negative.

Question 5

What are factors the effect directional stability?

Answer 5

Fin, Side/keel area, Position of CG, Sweepback wing,

Question 6

How does the fin effect directional stability?

Answer 6

When the fin of the aircraft is in a. yawing disturbance, there is a angle of attack formed between the RA and the fin, δ. This creates a side force (lift), which will produce a yawing moment about the CG which will turn the nose of the aircraft back into the RA. (weathercock). Meaning the fin is a stable effect of directional stability.

Question 7

What are the types of fins?

Answer 7

Dorsal fin and Sweepback fin

Question 8

Describe a dorsal fin and its effects?

Answer 8

A dorsal fin is lower in height and wider at the root compared to the slim tall fins while maintaining the same area.
Benefit of the dorsal fin is it can reduce parasite drag because of the fin root can act as a fairing to reduce airflow interference and the lower height can reduce boundary layer separation.
This can lead to a more effective side force.

Question 9

Describe a Sweep back fin?

Answer 9

Sweep back wing can increase the stall angle, so it can produce a restoring moment more efficiently to overcome relatively large yaw disturbances.
Increase Mcrit

Question 10

Describe the side/keel area and how it effects moment in directional Stability?

Answer 10

The side/keel area is the area viewed from the side of the fuselage. Drag will be produced on the side area facing the RAF when the a/c is in a yaw disturbance. Dependent on wether or not the larger area is FWD or AFT of the CG will either restore a restoring or a greater disturbance.
AFT will be restoring, FWD will be enhancing the yaw.

Question 11

Describe Position of CG to directional stability?

Answer 11

Forward CG along the fuselage increases side area restoring moment. So to increase restoring moment of directional stability move CG forward.

Question 12

Sweep back wing effect on directional stability?

Answer 12

Outer wing of a sweep back aircraft moves towards the relative airflow and the inner wing moves away from the RAF when the aircraft is in yaw disturbance.
The outer wing is traveling faster the the inner wing.
The outer wing produces a greater effective wingspan and higher aspect ratio to the RAF.
This will all create more drag on the outer wing and create a restoring moment.
Sweepback wings are a positive feature to directional stability.

Question 13

What could happen if lateral stability and directional stability were not matched.

Answer 13

Lead to unstable oscillation, due to the restoring aspects of the aircraft.

Question 14

What causes Spiral Instability?

Answer 14

Combination of very strong directional stability and weak lateral stability, which happens when the plane is in a roll disturbance.

Question 15

Give an example of spiral instability?

Answer 15

Aircraft is in left roll disturbance and the RAF is coming from the left of the aircraft. Due to the aircrafts strong directional stability it yaws the aircraft left into the RAF, Before the weak lateral stability has time to act to roll it back to equilibrium. The left yaw causes the outer right wing to travel faster and produce more lift to roll the aircraft in the direction of the roll disturbance. Then the directional stability acts again and continues the process which can lead to a grave yard dive.

Question 16

What causes a dutch roll?

Answer 16

Combination of very strong lateral stability and weak directional stability, which happens if the plane is in the disturbance of roll. Dutch roll is a complex unstable oscillation.

Question 17

Give an example how an aircraft could fall into dutch roll.

Answer 17

Aircraft is in a left yaw disturbance which has the RAF coming from the right of heading, this will lead to the outer wing traveling faster creating more lift and inducing a roll disturbance. to the left. The strong lateral stability will correct the roll before the direction stability produces a yawing moment. Then the directional stability and the right roll will cause a right yaw which will lead to a strong left roll due to the strong lateral stability and the oscillation of yawing to one side and rolling to the other continues.

Question 18

What is snaking?

Answer 18

A dutch roll with much more pronounced yawing oscillation.

Question 19

What is the preferred enhanced stability when lateral and directional stability cannot be matched?

Answer 19

Lateral Stability

Question 20

What are the two modes of longitudinal dynamic oscillation?

Answer 20

Long period mode (phugoid mode) and short period mode

Question 21

Explain Phugoid mode?

Answer 21

Is a long period mode of longitudinal oscillations and is usually poorly dampened. Period usually last for 20 - 60 seconds. During the oscillation the angle of attack of the flight path is fairly constant and the flight path is like a sine wave going up and down.
Magnitude of the pitch moment, the altitude and airspeed can vary widely.
Airspeed decrease at high altitude and increases at low altitude.
Oscillation is usually slow and violent and sometimes not noticed by the pilot.

Question 22

Explain Short Period Mode?

Answer 22

Period of this mode is usually short 0.3 to 1.5 seconds long.
Airspeed of the aircraft is usually constant but the angle of attack varies in this oscillation.

Question 23

How is short period mode dampened?

Answer 23

By design, Oscillation can be caused by the pitch moment from elevator flapping.
To hold the control at its neutral position, or release the control can make the plane recover from this oscillation.

Question 24

Does the centre of pressure change with airspeed and AOA?

Answer 24

Centre of pressure changes with AOA not with airspeed.

Question 25

Does fuselage produce restoring moment in rolling?

Is it a stable feature?

Answer 25

Fuselage produces a aerodynamics force which does not the restoring moment.
Fuselage can be a stable feature to lateral stability for a high wing aircraft.

Question 26

Does airspeed affect pitch moment about the aerodynamic centre?

Answer 26

Yes it does

Question 27

Does AOA affect the coefficient of pitch moment about the aerodynamic centre?

Answer 27

The coefficient of pitch moment about the aerodynamic centre is not effected by aoa.