Stability Flashcards
(81 cards)
1
Q
What is the aerodynamic centre?
A
The point along a chord of an aerofoil, about which the pitching moment nearly does not change with AoA, and the moment remains as the relatively negative values at the “0” lift angle, if the airspeed is constant.
2
Q
Where is the aerodynamic centre located on the aerofoil?
A
Quarter of the chord length from the leading edge.
3
Q
How does the moment coefficient about the LE and TE change? What about the moment coefficient about the aerodynamic centre?
A
- About the LE and TE, it changes with AoA.
- About the aerodynamic centre, it does not change with AoA.
4
Q
What is the zero-lift angle?
What happens to the lift at this angle?
A
- It is the AoA where the total lift produced is “0”.
- At this AoA, the magnitudes of the positive lift and the negative lift are the same, but are situated at different locations on the chord, so they act as a force couple.
5
Q
Is the zero-lift angle for a cambered aerofoil at 0º AoA?
A
No
6
Q
Is the pitch moment “0” at the zero-lift angle?
A
No, however the total lift is “0”.
7
Q
Does the zero-lift angle change with the location of the pivot point?
A
No, it is independent of the location of the pivot point, meaning that it does not change with the location of the pivot point.
8
Q
What can the pitch moment at the zero-lift angle be approximately treated as?
A
The same as the pitch moment about the aerodynamic centre of the aerofoil, meaning that the pitch moment at the zero lift angle nearly does not change with AoA.
9
Q
What is the pitch moment about the CoP?
A
0
10
Q
What happens to the pitch moment with regards to AoA at both the T.E. and L.E.?
A
- Pitch moment about the leading edge decreases with AoA
- Pitch moment about the trailing edge increases with AoA
11
Q
When is an aircraft in equilibrium?
A
- Total force on the aircraft is 0.
- Total moment about the CoG of the aircraft is 0.
12
Q
What do stick fixed and stick free mean?
A
- Stick-fixed means that the control surfaces are held in their neutral position.
- Stick free means the pilot releases the control column and allows the control surfaces take up their own position.
13
Q
How does an aircraft produce a unexpected pitching motion with respect to longitudinal stability?
A
When an aircraft is in an equilibrium position, and an unexpected force, which exerts on the aircraft, produces a LONGITUDINAL moment, the aircraft will be in an unexpected pitching motion.
14
Q
When is an aircraft said to be longitudinally stable?
A
If the aircraft is able to produce a restoring pitching moment to return to the equilibrium position.
15
Q
List the 4 factors to the pitch moment of an aircraft?
A
- Location of CoG.
- Forces generated on the mainplane/pitch moment of the wings.
- Pitching moments of the fuselage.
- The tailplane.
16
Q
How does the location of CoG affect the pitch moment of an aircraft?
A
CoG should not be too far back, otherwise it could produce difficulty for the tailplane to produce sufficient restoring pitching moments to stabilise aircraft longitudinally.
17
Q
How does the mainplane/wing affect the pitch moment of an aircraft?
What do the magnitude and location of the mainplane forces changes with?
A
- They are the main contributors to the pitching moment about CoG.
- Magnitude and location change with AoA.
18
Q
How does the tailplane affect the pitch moment of an aircraft?
A
Designed to give aerodynamic forces by deflecting the elevator, producing a moment which can neutralise any excessive pitching moments.
19
Q
How does the fuselage affect the pitch moment of an aircraft? (2)
A
- Can produce a pitch moment, which will affect the longitudinal stability.
- The pitch moment produced by fuselage changes with AoA, and usually this moment is nose-up “+” moment.
20
Q
What does the criterion of longitudinal stability state?
A
The tailplane restoring moment must remain greater than any unstable moment from the mainplane/wings.
21
Q
Explain the effect wings have on longitudinal stability? (3)
State if wings are longitudinally stable?
A
- When A/C is in level flight, the CoP on its wings is designed to be behind the CoG of the aircraft.
- If there is a nose-up disturbance, AoA increases, the wings will therefore produce more lift.
- This additional lift will produce a restoring nose-down moment, which overcomes the disturbance to return the aircraft back to equilibrium.
- This feature has a stable effect.
22
Q
Explain the effect of a forward Centre of Gravity has on longitudinal stability? (3) What are the adverse effects of a forward CoG?
A
- A forward CoG position provides a long leverage to the tailplane to produce a restoring moment.
- It is likely to set CoP before CoG on the wings longitudinally
- This is a stable configuration since wings can always produce a restoring moment to a pitching disturbance.
- A forward CoG will make the flight feel “nose-heavy”.
23
Q
Explain the effect of an aft Centre of Gravity has on longitudinal stability? (2)
A
- An aft CoG position results in a short leverage to the talplane.
- It can also result in a position too far back which can result in the CoP being in front of the CoG, which is an unstable configuration, since the wings can produce pitch moment during a disturbance to promote the disturbance.
24
Q
What is the general practice for where the CoG position should be located?
A
CoG should be set in a forward position within the limits of the aircraft.
25
Explain the effect of the tailplane on longitudinal stability? (3)
- The tailplane should be able to produce a pitch moment.
- The tailplane deflects in different directions to generate either positive lift or negative lift to balance the nose-up or nose-down moment from the wings and other parts of the aircraft.
- When the aircraft is in a nose-up disturbance, the tailplane also produces a nose-up disturbance. The tailplane produces more lift when the tailplane produces a greater AoA, greater lift, therefore nose down pitch moment.
26
Is the fuselage stable in an airflow? Why/why not?
- No, even if it does not produce lift.
- Because the forces on the fuselage act like a force couple meaning that the total force is "0", but the sum of the moments produced by the pressure distribution is not "0".
27
Does the moment produce by the fuselage vary with AoA? Why/why not?
Yes, because the pressure distribution changes with AoA.
28
What is the pitch moment like on the fuselage? (negative or positive)
Pitching moment is positive (nose-up) moment when AoA is positive (+) and vice versa when the pitch moment is negative.
29
Describe what longitudinal dihedral is? Give an example? (4)
- The incident angle of the chord of the tailplane can be different from the incident angle of the chord of main planes.
- An example is where the incident angle of the mainplane could be 3º, while the incident angle of the tailplane is 1º.
- There is a 2º difference and this difference is known as longitudinal dihedral.
- ESSENTIALLY the incident angle between the chord of the tailplane is different to that of the incident angle of the chord of the wing, known as longitudinal dihedral.
30
How does longitudinal dihedral affect the longitudinal stability of an aircraft? (3)
- Longitudinal dihedral makes the airflow around the aircraft form the effective AoA to the wing, which is smaller than the geometric AoA (actual setting of AoA), thus the magnitude of the disturbance can be decreased. (Essentially, effective AoA lower therefore magnitude disturbance is decreased).
- In normal cruise, the tailpane needs to produce a small down-wash force for a "nose-up" moment to keep the plane in equilibrium.
- The longitudinal dihedral will help the tailplane operate more effectively.
31
Explain how sweepback wings affect longitudinal stability? (3)
- The CoP position is further rearward for sweepback wings than that of straight wings.
- This ensures that the CoP is behind the CoG, ensuring the wing produces a restoring moment when there is a disturbance from the equilibrium.
- Heavily sweptback wings act like a tailplane.
32
Which is better, stick fixed or stick free configuration and why?
- Stick-fixed is more stable than stick-free.
| - Stick-free might cause low-level oscillation, even though it can avoid an excessive force on the control column.
33
Use the longitudinal stability diagram to demonstrate if sweepback wings are stable?
Use diagram from book.
34
How does an aircraft produce a unexpected rolling motion with respect to lateral stability?
When an aircraft is in an equilibrium, and an unexpected force exerted on the aircraft which produces a rolling moment, the aircraft will be in an unexpected rolling motion.
35
When is an aircraft said to be laterally stable?
If the aircraft is able to produce a restoring rolling moment to return to the equilibrium position, this is known as "keel" effect.
36
What is sideslip?
- Because lift and the weight of an aircraft do not act in the same direction, the sum of these forces (Fy, shown on Figure 6-14 page 89) is called "sideslip" force, causing the aircraft to move sideways.
37
What is sideslip angle with regard to lateral stability?
When the aircraft is in a sideslip, its heading and the relative airflow form a sideslip angle.
38
When is the sideslip angle positive with regards to lateral stability?
When the relative airflow comes from the right of heading, otherwise it would be negative.
39
When is the rolling moment positive? When is it negative? LATERAL STABILITY
- If it rolls to the right - right wing down.
| - If it rolls to the left - left wing down.
40
What is dihedral and anhedral?
- The wings are in the inclination from its horizontal position.
- Upward inclination is called dihedral, and has a positive dihedral angle.
- Downward inclination is called anhedral and has a negative dihedral angle.
41
Explain how lateral dihedral affects lateral stability? (5)
- If the aircraft has a roll disturbance - left wing is down and it sideslips to the left, the left wing has a greater AoA than that of the right (upward) wing.
- Thus, the left wing will produce more lift than the other wing.
- In turn, the rolling-to-the-right moment by the left wing is greater than the rolling-to-the-left moment produced by the right wing.
- The rolling-to-the-right moment assists the aircraft to roll right to its original position.
- Thus a stable feature.
42
Describe how anhedral affects lateral stability?
- Has the opposite effect - providing the rolling moment in the same direction as the disturbance.
43
Explain how shielding affects lateral stability? (2)
- Fuselage shields the trailing (upgoing) wing from the RAF, the effect like that of dihedral.
- Because AoA is the angle between the chord and the RAF, there is less effective RAF on the upgoing wing due to the shielding effect therefore less effective AoA therefore less lift.
44
Describe how a high wing position affects lateral stability?
Above the CoG of the aircraft, has the shielding effect to produce a higher restoring moment (increased effective dihedral, keel effect).
45
Describe how a low wing position affects lateral stability?
Under the fuselage, thus there is no shielding effect and has a lower restoring moment (lower dihedral or anhedral).
46
Describe how fin area affects lateral stability?
- Fin area produces a drag to side-slip.
| - This drag is located above the longitudinal axis of the fuselage - producing a restoring moment.
47
Describe how sweepback wings affect lateral stability? (2)
- Sweepback wings have an increased effective AR of the lower wing while the aircraft is in rolling sideslip.
- It increases the lift of the lower wing, producing the restoring moment.
48
Explain with the aid of a diagram if fin area and sweepback wings are laterally stable?
PAGE 91 AND TOP OF PAGE 92.
49
How does an aircraft produce an unexpected yawing motion with respect to directional stability?
When an aircraft in an equilibrium position, and an unexpected force exerted on the aircraft produces a directional moment, the aircraft will be in an unexpected yawing motion.
50
When is an aircraft said to be directionally stable?
If the aircraft is able to produce a restoring yawing moment to return the aircraft to the equilibrium, it is known as "weathercock" effect, which is when the nose turns into the wind.
51
What is sideslip angle with regards to directional stability?
The angle between the air flow v and the aircraft's heading is called sideslip angle.
52
When is the sideslip angle positive with regards to directional stability? When is it negative?
If the airflow is from the right side of the heading, the sideslip angle is positive, otherwise it is negative.
53
When is the yawing moment positive?
If the aeroplane yaws to the right.
54
Explain how fins affect directional stability on an aircraft? (3)
- When a fin is in a yawing disturbance, an angle of attack is formed between the coming airflow and fin, and then the "lift" - the side-force produced by the fin (F).
- This side-force on the fin produces restoring yawing moment about the CoG of the aircraft, which in turn, puts the nose back into wind.
- This is the weathercock effect by fin.
55
Does dorsal fin increase directional stability? If so, how? (3)
- Yes, it CAN increase directional stability.
- It has a relatively large area along the fuselage, and has relatively low equivalent parasite area, so it can delay the boundary layer separation and increase the stall angle of tailplane (rudder).
- The sideslip force (drag force) will tend to roll the aircraft away from the direction of the slip, thus providing a stabilising influence.
56
How can dorsal fin increase lateral stability?
The increased area above the CoG of the aircraft, then greater restoring moment can be produced.
57
Describe how a sweepback fin increases directional stability?
- Sweepback fin increases the stall angle of the fin, and increases the critical mach number (Mcrit) of the fin, so the fin can work more effectively to overcome a relatively large disturbance in yaw.
58
KEEL/SURFACE AREA: What force is produced when the aircraft is in a sideslip in yaw? What happens to drag when the area after the CoG is greater than before CoG?
- Drag will be produced in the side area facing to the RAF, when the aircraft is in a sideslip in yaw.
- If the area after the CoG is greater than that before the CoG, the drag produced in the area would be greater.
59
What happens if the area after the CoG is greater than before the CoG? Explain the effects this side/"keel" area has on directional stability? What moments are produced before and after the CoG? (3)
- If the area after the CoG is greater than before the CoG, the drag produced in the area after the CoG would be greater than that before CoG.
- The drag on the side area after CoG produces a restoring moment about CoG, while the drag on the side area before the CoG produces the yawing moment which encourages the disturbance, thus making it an unstable feature.
- Thus, the greater side area after CoG is a directionally stable feature.
60
How does the CoG position affect directional stability?
The forward position of CoG along the longitudinal axis increases the leverage (moment arm), in turn, increasing the restoring yawing moment.
61
Explain how sweepback wings affect directional stability? (3)
- Increased effect Ar on the outward wings, while the aircraft is in yawing sideslip.
- The outer wing has a greater effective span, meaning it produces more drag than that of the inner wing (greater effective area, thus greater skin drag) (also, greater AR = greater AoA = greater lift = greater drag).
- Thus, the net moment by the drag of the wings turns the aircraft into the wind, e.g. alignment with the airflow.
62
Explain with the aid of a diagram if fin area and sweepback wings are directionally stable?
USE DIAGRAM ON PAGE 94
63
What causes spiral instability?
Very strong directional stability and relatively weak lateral stability could cause spiral instability, which happens if the aircraft is in a disturbance in roll.
64
Explain how spiral instability occurs? (3)
- If it is in a left roll disturbance, the RAF comes from the left because of the sideslip, lateral stability will try to roll the plane back to right, but directional stability will try to turn the plane into wind - yaw left, the same direction as the disturbance.
- When the directional stability is too strong, and the yawing into the "wind" dominates, the disturbance is enhanced, and the plane rolls even deeper.
- This is unstable, and can turn into a dangerous "graveyard dive", if the plane rolls too deeply.
65
What causes dutch roll?
Very strong lateral stability and relatively weak directional stability could cause an unstable oscillation known as Dutch Roll.
66
Explain how dutch roll occurs? (6)
- Dutch roll happens if the plane is in a disturbance of left yaw, and the RAF comes from the right.
- This yaw causes a left roll because of the faster outer wing.
- When the lateral stability is very strong, a right roll moment will be produced, and the plane would roll to the right immediately.
- The roll then causes yawing back to the right, which leads to a sideslip to the right, and then a strong roll to the opposite direction, and so goes on.
- The aircraft would yaw and roll at the same time in alternate directions.
- Dutch roll is a complex unstable oscillation.
67
What is a more pronounced yawing oscillation of Dutch roll called?
Snaking.
68
State the design requirements for lateral and directional stability? (4)
- Lateral and directional stability should be correctly matched. None should dominate each other too much.
- Small amount of spiral instability and preferred than Dutch Roll since it can be controlled.
- Dutch Roll should be dampened by design to prevent any structural damage on the aircraft.
- If lateral and directional stability cannot be correctly matched, it is preferred to have more lateral stability, since a relatively strong moment of yaw can be generated by rudder.
69
What is phugoid mode?
The long period mode of longitudinal dynamic oscillation, which is usually poorly dampened is called phugoid mode.
70
How long does phugoid mode last for?
Period of oscillation is relatively long, about 20 to 60 seconds.
71
What is considered constant and what is varied in phugoid mode?
- AoA along the flight path can be treated as constant.
| - Pitch moment, altitude and airspeed can vary widely.
72
What happens when the aircraft oscillates and reaches a higher and lower altitude?
- Higher altitude = speed decreases.
| - Lower altitude = speed increases.
73
Regarding phugoid mode, in terms of motions dynamics of the aircraft, it can be created as the energy exchanges between ____ and ____?
Potential energy and kinetic energy.
74
What is the oscillation of phugoid mode like in terms of speed and motion?
Slow and not violent
75
Why is phugoid mode dangerous? (2)
- Can go unnoticed by the pilot due to its slow and non-violent nature.
- Can cause crashes like Japan Airlines Flight 123.
76
How can phugoid mode be reduced? (4)
- Phugoid mode can be reduced by reducing the throttle position.
- Because phugoid involves the interchange between kinetic and potential energy, anything that will dissipate energy in the process will add damping.
- Reducing throttle will reduce the energy associated between the interchange of kinetic and potential energy, this will add damping, which reduces oscillation.
- Because oscillation is reduced as a result of added damping, phugoid mode will decrease.
77
How long does short period mode last for?
Period of this mode of oscillation is relatively short, about 0.3 to 1.5 seconds.
78
What is considered constant and what varies in short period mode?
- Airspeed can be treated as constant.
| - AoA along its flight path varies in this oscillation.
79
Why is the short period mode better than phugoid mode?
Because the oscillation of this mode is usually well dampened by design.
80
What causes the oscillation in short period mode?
Caused by (stick-free) elevator flapping pitch moment.
81
How can a pilot recover from short period mode?
- Holding the control at its neutral position.
| - Releasing the control can make the aircraft recover from this oscillation.
