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Part 4 Aerodynamics > Module 1 - Stability & Control > Flashcards

Module 1 - Stability & Control Flashcards

1
Q

About which axis does the pitching moment act? What control surface influences this?

A
  • Rotating motion about the lateral axis

- Controlled by Elevators

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2
Q

About which axis does the rolling moment act? What control surface influences this?

A
  • Rotating moment about the longitudinal axis

- Controlled mainly by Ailerons

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3
Q

About which axis does the yawing moment act? What control surface influences this?

A
  • Rotating moment about he vertical axis

- Controlled by the Rudder

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4
Q

What is the definition of the Aerodynamic centre? Where is it normally located?

A

A point along the chord of an aerofoil, about which the pitching moment remains nearly constant with changes in AoA, and the moment remains at a small negative value at the zero-lift AoA. It is normally about 1/4 of the chord from the leading edge

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5
Q

What is a force couple? What are the requirements for it to be in equilibrium?

A

“System of two forces of equal magnitude, in opposite directions”

-The net force and moment must be 0 for it to be in equilibrium

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6
Q

What is a moment? What does it depend on?

A

The rotation force that results because of a force acting at a distance from a pivot point.
-Depends on the distance of the force from the pivot, and the magnitude of the force

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7
Q

If you wish to superimpose a force of a system to another point, what must you do?

A

Must ad an equal an opposite force at the point you have superimposed the force to

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8
Q

Explain what causes a pitching moment of a wing?

A

Wing sits at an angle of attack to an airflow
Aerodynamic force (total reaction) is produced from the CoP
Depending on the pivot chosen, it will create a pitching moment (positive or negative) about the LE, TE, Aerodynamic centre or CoG.

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9
Q

How does the coefficient of pitching momnent and the pitching moment change about the TE, LE and AC as AoA increases?

A

LE: Cm and pitch moment decreases
TE: Cm and pitch moment increases
AC: both remain constant

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10
Q

What is the zero lift AoA? What forces are present at this AoA?

A

The angle of attack where the total lift produced is 0. AoA is normally about -4˚.
There is still forcr acting up and down, from different locations on the aerofoil creating a force couple (normally nose down pitch moment)

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11
Q

When drag is mimimal, what can be assumed about the zero lift pitch moment and the pitch moment about the AC?

A

Assume that they are approximately equal

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12
Q

Where is the AC normally located along the chord?

A

1/4 chord

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13
Q

Where are the AC and CoP located in relation to each other along the chord on a cambered vs symmetrical aerofoil?

A

Cambered: AC is forwards of CoP
Symmetrical: AC and CoP are at the same location

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14
Q

What features affect the longitudinal stability of an aircraft?

A
  • Force generated by the tailplane is of sufficient distance from the CoG, CoG is not too far rearwards.
  • Total reaction on mainplane varies in magnitude and location with changes in AoA.
  • Elevator produces sufficient forces to overcome any residual or excessive pitching moments.
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15
Q

What is the creterion of longitudionaly stability?

A

The tailplane restoring moment must remain greater than any unstable moment from the mainplane/ wings

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16
Q

What is the equation for total pitch momnet in the longitudional axis? What does each component represent?

A

M = Mm + Me + Ma

Mm: moment due to mainplane pitching moment
Me: moment due to the elevator
Ma: Moment due to fuesalge pitching forces

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17
Q

How do wings affect longitudional stability? What will happen in flight if there is a pitch distrubance?

A

CoP is designed to be behind the CoG.

If there is a nose up disturbance, the AoA increases, increasing the lift, creating a restoring nose down moment

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18
Q

How does the centre of gravity affect the longitudinal stability? Describe what will happen when it is at different extreme locations?

A

-CoG is the point about which all the pitching moments act.

  • If it is too far forward the aircraft will feel nose heavy.
  • If it is too far aft it may be behind the CoP and create a further nose up pitch moment, and will not provide a long arm for the tailplane pitching moment to act at.
  • CoG shpould always be in front of the CoP but not too far forwad it is outside limits
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19
Q

How does the tailplane affect the longitudional stability ?

A

-The tailplane is the main thing affecting the longitudional stabilty.
The tail plane should be able to produce a moment (Fd) greater than the moment produced by the mainplane (Fd)

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20
Q

How does the fuselage affect the longitudinal stability? Explain why is unstable/stable?

A
  • In an airflow the fuselage is unstable
  • It does not produce lift, as the force vectors act in different locations to create a nose pitch up moment.
  • It can be treated as a long, soild cylinder “cigar” shape
  • Normally produces a positive pitch moment when the AoA is positive
21
Q

Describe what longitudinal dihedral is? How does it influence the longitudional stability?

A

Longitudional dihedral is where the tailplane is set at a lower angle of incidence than the mainplane. The difference between the two angles is longitudional dihedral
-Creates a smaller AoA on the tailplane and allows the tailplane to operate more effectively.

22
Q

How does sweepback affect the lopngitudional stability?

A

Sweptback wings will have a CoP which is further rearwards, so it is always behind the CoG. This will ensure a restorning moment when a disturbance is felt.
Heavily swept wings will act like a tailplane

23
Q

What is the difference between stick fixed and stick free?

A
  • Stick fixed means the controls are held in their neutral position
  • Stick free means the pilot can release the control column and allow the control surfaces to take up their own position.
24
Q

How does the stick control affect the longitudinal stability? Why?

A

A stick fixed configuration is more stabile than a stick free configuration as it will stop low level oscillations but will require more force to hold it in place

25
Q

Describe positive, negative and neutral static and dynamic stability?

A
  • Positive static: resists displacement, returns to origional position
  • Neutral static: will remain at position it is displaced to
  • Negative static: will trey and move away from origional position and keep moving away
  • Positive dynamic: osciallation will reduce over time
  • Neutral dynamic: osciallation will remain constant over time
  • negative dynamic: oscillation will worsen over time
26
Q

What is positive lateral stability also known as?

A

The keel effect

27
Q

What is the force required for all lateral stability devices? What is it the result of?

A

-Sideslip force

Sideslip is the sum of the lift and weight vector after the initial displacement.

28
Q

What is sideslip angle? What is positive and negative sideslip angle and rolling moment?

A

Sideslip angle is the angle formed betweent he heading of the aicraft and the relative airflow from a sideslip angle.

β = Positive if RAF is from right of HDG
β = Negative if RAF is from left of HDG
M(roll) = Positive if roll is to the right
M(roll) = Negative if roll is to the left
29
Q

What is lateral dihedral and how does it affect lateral stability?

A

It is when the wings have an upwards inclination from the horizontal.
If there is roll disturbance the downgoing wing will have a higher AoA due to the sideslip angle. This will increase the lift onthe downgoing wing and so create a rolling moment to restore the aircraft to its level position

30
Q

What is the differnece between dihedral and anhedral? What is the purpose of anhedral?

A

Anhedral has the wings set at a negative inclintion to the horizontal
It will create a rolling moment in the same direction as the dirturbance and can be used to prevent unstable dynamic oscillatins like dutch roll

31
Q

How does shielding affect the lateral stability?

A

When the aircraft is in sideslip the upgoing wing is shielded slightly from the RAF and so will produce less lift creating a restoring rolling moment.

32
Q

How does the vertical wing position affect lateral stability?

A 
High wings (above CoG) will have more shielding effect and increased effective dihedral
Low wings (under the fuselage) will have less shielding effect and lower restorning moment
33
Q

How does the fin area affect lateral stabilty?

A

The vertical stabiliser will create a drag due to the sideslip. Due to the area being above the longitudional axis this drag froce will act to produce a restoring moment

34
Q

How do sweepback wings affect lateral stability?

A

When the aircraft is in sideslip it will have an increased aspect ratio on the downgoing wing, increasing th elift on that wing relative to the upgoing wing and so will produce a restorning moment

35
Q

What is the term used to describe the effect of a directionally stable aircraft?

A

The “weathercock” effect - the nose will turn into the wind

36
Q

What is a positive and negative sideslip angle with respect to directional stability? What about positive and negative yawing moment?

A 
β = Positive if RAF is from right of HDG
β = Negative if RAF is from left of HDG
M(yaw) = Positive if moment makes aircraft yaw right
M(yaw) = Negative if moment makes aircraft yaw left
37
Q

How does the fin affect directional stability?

A

When there is a yaw disturbance there is an AoA formed with the fin. This produces an aerodynamic force acting to yaw the aircraft into the RAF, producing a moment to restore the aircraft

38
Q

How does a dorsal fin affect directional stability? What other stability does it help? how?

A

Dorsal fin has a larger area along the fuselage, but not a lot of area to be affected by parasite drag. This can increase the stalling AoA of the vertical fin by delaying the boundary layer separation.

Can also affect the lateral stability by increasing the area above the CoG to create a greater lateral drag force to restore the aricraft in lateral disturbance

39
Q

How does a sweepback fin affect directional stability?

A
  • Sweepback fin will increase the stall angle over the fin and rudder
  • This makes it more effective at overcoming large yaw dirsturbance by allowing it to produce a greater aerodynamic force
  • Moves CoP of the fin further rearwards behind the CoG
40
Q

How does side/ keel area affect directional stability?

A

When the aircraft is in a sideslip the side area of the fuselage forwardsa and rearwards of the CoG will produce a drag force which will act to yaw the aircraft. The area exposed to the sideslip behind the CoG is larger than ahead of it due to the vertical stabiliser and so this will create a yawing moment to weather cock the aircraft

41
Q

How does the position of the Centre of Gravity affect directional stability?

A

A more forward position of CoG is desirable as it increase the arm of the force created by the fin to increasing the magnitude of the restiring moment

42
Q

How do sweepback wings affect directional stability?

A

When disturbed in yaw, one wing will have a greater aspect ration than the other.
The wing on the outward side of the disturbance will ahave the higher AR and so will also produce more drag.
The difference between the drag on the outer wing and the inner wing will create a yawing moment to restore the aircraft to balanced flight

43
Q

What is spiral instability? Describe what would happen if their was a roll disturbance?

A

It is a combination of very strong directional and relatively weak lateral stability

  • If the aircrat is in a roll disturbance to the left, the relative airflow will be coming from the left, due to the sideslip
  • The lateral stability will act to roll the aircraft back tot he right and the directional stabilty will act to yaw the aircraft into the wind.
  • When the directional stability is too strong, the yawing moment will cause the aircraft to weathercock, which will further deepen the roll disturbance.
  • This is laterally unstable and can lead to a graveyard/ spiral dive
44
Q

What is dutch roll? Describe what would happen if there was a disturbance in yaw? What is snaking?

A

It is a very strong lateral stability and relatively weak directional stability. Snaking is when this oscillation is much more pronounced

  • If the aircraft is a left yaw disturbance, the relative airflow will come from the right and due to the sideslip
  • This left yaw will cause left roll due to the increased speed of the right wing
  • Because the lateral stability is so strong, a right roll moment will be produced as soon as a left roll is experienced
  • This right roll will create a yaw to the right, which will cause the latreral stability features to correct this immediately
  • This will leave the aircraft in a state of simultaneous roll in yaw in opposite directions
45
Q

What are the two types of longitudional dynamic oscillations?

A

Long Period - Phugoid mode

Short Period

46
Q

What is phugoid mode? Describe how the airspeed, altitude, angle of attack and energy chnge throughout the oscillation?

A
  • Pitching oscillation with a relatively long period of 20-60 seconds
  • AoA can be assumed to remain constant throughout
  • As ALT increases, speed decreases, and the potential energy increases and kinetic energy decreases
  • As ALT decreases, speed increases and potential energy decreases as kinetic energy increases.

This oscillation may not be noticed by pilots by the energy will eventually dissapate and return to equilibrium

47
Q

What is short period mode? how does AoA, airspeed and altitude vary?

A

A period of short oscillations between 0.3-1.5 seconds

  • Airspeed can be assumed to remain constant as AoA varies along the flight path
  • It is usually dampened well by design
  • Can be recovered by holing the contorl in the neutral position or by releasing the controls
48
Q

Is stick free or stick fixed more stable for each of the primary control surfaces?

A

Elevator: Stick fixed
Rudder: Stick fixed
Ailerons: Unknown

Part 4 Aerodynamics (6 decks)

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