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Aerodynamics > P4 Stability > Flashcards

P4 Stability Flashcards

1
Q

What is the pitch moment?

What is the equation for it?

A

PITCHING MOMENT results in a ROTATING MOTION about the LATERAL AXIS, CONTROLLED by ELEVATORS;
PM = Cm x 1/2ρv^2 x S x c

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2
Q

What is the roll moment?

What is the equation for it?

A

ROLLING MOMENT results in a ROTATING MOTION about the LONGITUDINAL AXIS, CONTROLLED by AILERONS;
RM = Cr x 1/2ρv^2 x S x b

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3
Q

What is the yaw moment?

What is the equation for it?

A

YAWING MOMENT results in the ROTATING MOTION about the DIRECTIONAL AXIS, CONTROLLED by RUDDER;
YM = Cy x 1/2ρv^2 x S x b

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4
Q

What is the aerodynamic centre of an aerofoil?
Where is it?
What happens when airspeed is constant?

A

A POINT along the CHORD of aerofoil ABOUT WHICH the PITCHING MOMENT NEARLY does NOT CHANGE with AoA;
About a QUARTER of the CHORD from LEADING EDGE;
MOMENT is relatively SMALL NEGATIVE VALUE at the 0 LIFT ANGLE

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5
Q

What is the magnitude and direction of moment dependant on?

A

MAGNITUDE of FORCE and relative POSITION of FORCE to PIVOT POINT

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6
Q

When is a moment couple constant?

What does it mean for the force and moment when it is constant?

A

A FORCE COUPLE is formed when F and -F are EQUAL MAGNITUDE at a DISTANCE APART;
Total FORCE is 0 but MOMENT is NOT

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7
Q

How can the force of a system be moved without changing total force/moment?

A

SUPERPOSITION: By adding a FORCE PAIR with same magnitude a distance away from the ORIGINAL FORCE at the PIVOT a force COUPLE is formed with the ORIGINAL FORCE and DIRECTIONALLY OPPOSING added FORCE, leaving the added force in the SAME DIRECTION as ORIGINAL force to be equal to the ORIGINAL but in a DIFFERENT LOCATION. The moment will remain constant since the PIVOT where the forces were added is SAME and the ORIGINAL FORCE is constant

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8
Q

How does pitch moment change about leading edge, trailing edge, CoP?

A

About LEADING EDGE DECREASES with AoA;
About the trailing edge increases with AoA;
About CP is 0

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9
Q

How does the pitch moment about AC change?
Explain its relation to coefficient of zero lift pitch moment?
What is the zero lift pitch moment?

A

Moment COEFFICIENT does NOT CHANGE about AC with change in AoA;
Pitching moment at ZERO LIFT angle is the AoA where TOTAL LIFT is 0N, it does NOT CHANGE with LOCATION of PIVOT when DRAG is relatively SMALL as the moment is DOMINATED by moment of LIFT FORCE COUPLE which is INDEPENDENT of PIVOT;
Therefore at the ZERO LIFT pitch moment when DRAG is SMALL can be TREATED as pitch moment ABOUT AC, if we know Cmac, we know Cmzl will be approximately the SAME provided ρ and v remain CONSTANT

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10
Q

Where is the location of AC and CoP relative to one another in a cambered and symmetrical aerofoil?

A

CAMBERED: AC is FORWARD of CoP;
SYMMETRICAL: AC and CoP are in SAME LOCATION

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11
Q

How is the location of AC determined?

How is the location of CoP determined?

A

M(TE) or M(LE) +/- ML +/- MD = Mac;
M(TE) or M(LE) +/- ML +/- MD = 0

M(TE/LE) = Cm + 1/2.ρ.v^2.S.c
M(L) = CL + 1/2.ρ.v^2.S.xcosθ
M(D) = CD + 1/2.ρ.v^2.S.ysinθ
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12
Q

When is an aircraft said to be in equilibrium?

A

The TOTAL FORCE on the aircraft is 0N and TOTAL MOMENT ABOUT CoG is 0

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13
Q

What is longitudinal stability?

What is the criteria of longitudinal stability?

A

An inherent tendency to RETURN to the SAME PITCH ATTITUDE after a DISTURBANCE by producing a RESTORING pitching moment;
The TAILPLANE RESTORING MOMENT must remain GREATER than any UNSTABLE MOMENT from the WINGS

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14
Q

What are the 4 factors which affect the pitching moment of an aircraft?
Why are they important?

A

POSITION of CoG: Not too far back so that it is DIFFICULT for the TAILPLANE to PRODUCE RESTORING MOMENT;
Pitch MOMENT of MAINPLANE (Mm): Most significant FORCES on aircraft which CHANGES MAGNITUDE and LOCATION with AoA and SPEED;
Tailplane (Me): PRODUCES aerodynamic FORCES by DEFLECTING ELEVATOR which can NEUTRALISE excessive pitching MOMENTS;
Pitching MOMENT of FUSELAGE (Mα): Pitch MOMENT CHANGES with AoA and usually NOSE-UP;
TOTAL pitching MOMENT: M = Mm + Me + Mα, when = 0 aircraft is STABLE

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15
Q

What effect does the wings have on longitudinal stability?

A

In LEVEL FLIGHT CoP usually sits AFT of the CoG;
If there is a NOSE UP disturbance, AoA INCREASES and the wings produce MORE LIFT;
The lift will produce NOSE DOWN MOMENT restoring EQUILIBRIUM/STABLE EFFECT

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16
Q

What effect does CoG have on longitudinal stability?

A

FORWARD CoG provides LONG LEVER to TAILPLANE to produce RESTORING MOMENTS/STABLE effect;
Positioned FORWARD of CoP allowing WINGS to produce RESTORING MOMENT/STABLE effect;
CoG TOO far FORWARD will make aeroplane NOSE HEAVY and TOO far AFT will SHORTEN LEVER to TAILPLANE and sit BEHIND CoP making WINGS and TAILPLANE LESS EFFECTIVE/UNSTABLE effect

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17
Q

Using an equation show how the tailplane assists in longitudinal stability?

A

Tailplane DEFLECTS in different directions to generate +LIFT or -LIFT to BALANCE aerodynamic MOMENTS from WINGS and other parts of plane;
L(tailplane) x y(force on tailplane to CoG) > L x z(aerodynamic force to CoP)

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18
Q

What effect does the fuselage have on longitudinal stability?

A

Due to the SHAPE of the FUSELAGE, the PRESSURE DISTRIBUTION means that the TOTAL FORCE is 0N but the SUM of the MOMENTS is NOT 0;
The FORCES act as a force COUPLE meaning the fuselage is NOT STABLE in an airflow;
The MOMENT CHANGES with AoA since the PRESSURE CHANGES with AoA, and is USUALLY POSITIVE when AoA is POSITIVE

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19
Q

What is longitudinal dihedral?

What is the effect of it on longitudinal stability?

A

The DIFFERENCE in INCIDENCE ANGLE between CHORD of TAILPLANE and CHORD of MAINPLANES;
Makes the airflow form EFFECTIVE AoA to the WING which is SMALLER than the ACTUAL AoA which DECREASES MAGNITUDE of DISTURBANCE;
During CRUISE TAILPLANE produces small DOWNWASH to maintain equilibrium and LONGITUDINAL DIHEDRAL helps this OPERATE more EFFECTIVELY

20
Q

What effect does sweep back wings have on longitudinal stability?

A

CoP is more REARWARD so it ensures it remains AFT of CoG;

If wing is HEAVILY SWEPTBACK it may ACT like a TAILPLANE

21
Q

What is the difference between stick fixed and stick free conditions?

A

Stick FIXED: CONTROL SURFACES are held in their NEUTRAL POSITION;
Stick FREE: CONTROL SURFACES take up their OWN POSITION if RELEASED

22
Q

What effect does stick control have on longitudinal stability?

A

Stick FIXED is MORE STABLE as stick FREE may cause LOW LEVEL OSCILLATION

23
Q

For the features of lateral and directional stability to have an effect what conditions must be met?
What are the features? (5) and (7)

A

There must be a SIDESLIP TOWARDS the DOWN GOING WING where the RAF CHANGES DIRECTION;
LATERAL: DIHEDRAL/ANHEDRAL, SHIELDING, WING VERTICAL POSITION, FIN AREA, SWEEPBACK WING;
DIRECTIONAL: FIN, DORSAL FIN, SWEEPBACK FIN, SIDE/KEEL AREA, CoG position, SWEEPBACK WING, RUDDER FIXED

24
Q

When is an aircraft said to be laterally stable?

A

It produces a RESTORING ROLLING MOMENT to return to the EQUILIBRIUM position known as KEEL EFFECT

25
Q

What definition of sideslip?
What is the sideslip angle? How do we determine the sign (+/-)?
How do we determine the sign of the rolling moment (+/-)?
How do we determine the sign of the yawing moment (+/-)?

A

The SUM of the LIFT and WEIGHT vectors of an aircraft in a DISTURBANCE in ROLL;
The ANGULAR DIFFERENCE between HEADING and RELATIVE AIRFLOW;
When AIRFLOW comes from RIGHT of HEADING it is POSITIVE (+);
When aircraft ROLLS to the RIGHT, the ROLLING MOMENT is POSITIVE (+);
When aircraft YAWS to the RIGHT, the YAWING MOMENT is POSITIVE (+);

26
Q

Describe the lateral stability diagram?

A 
SIDESLIP ANGLE (x) and ROLLING MOMENT coefficient (y): (-,-)/(+,+) UNSTABLE;
SIDESLIP ANGLE (x) and ROLLING MOMENT coefficient (y): (-,+)/(+,-) STABLE
27
Q

What effect does dihedral/anhedral have on lateral stability?

A

DIHEDRAL: If there is a ROLL DISTURBANCE and begins to SIDESLIP towards the DOWN GOING WING, this wing will have a GREATER AoA due to the LATERAL COMPONENT of the RAF which PRODUCES MORE LIFT and a ROLLING MOMENT towards the ORIGINAL;
ANHEDRAL: If there is a ROLL DISTURBANCE and begins to SIDESLIP towards the DOWN GOING WING, this wing will have a SMALLER AoA due to the LATERAL COMPONENT of the RAF which PRODUCES LESS LIFT and a ROLLING MOMENT AWAY from the ORIGINAL

28
Q

What effect does the fuselage have on lateral stability?

A

It SHIELDS the UP GOING WING from the RAF so the wing PRODUCES LESS LIFT and promotes a RESTORING ROLLING MOMENT to the ORIGINAL position;
ALTHOUGH the fuselage does produce AERODYNAMIC FORCE of DRAG in SIDESLIP acting in the LONGITUDINAL AXIS it does NOT produce a RESTORING ROLLING MOMENT since there is NO DISTANCE between the FORCE and ROTATING AXIS ie: does NOT ALLIGN with PIVOT AXIS

29
Q

What effect does wing position have on lateral stability?

A

HIGH WING: Wings ABOVE the CoG have the SHIELDING EFFECT to produce a higher RESTORING MOMENT ie: INCREASES effective DIHEDRAL;
LOW WING: Wings BELOW the CoG have NO SHIELDING EFFECT which produces LOWER RESTORING MOMENT ie: DECREASES effective DIHEDRAL

30
Q

What effect does the fin have on lateral stability?

A

The FIN PRODUCES an AERODYNAMIC FORCE of DRAG to the SIDESLIP;
When the FIN is located ABOVE the LONGITUDINAL AXIS this PRODUCES a RESTORING ROLLING MOMENT to ORIGINAL position

31
Q

What effect does sweep back wings have on lateral stability?

A

If there is a ROLL DISTURBANCE and begins to SIDESLIP towards the DOWN GOING WING, this wing will have a GREATER effective ASPECT RATIO due to the LATERAL COMPONENT of the RAF which PRODUCES MORE LIFT and a ROLLING MOMENT towards the ORIGINAL

32
Q

What effect does the dorsal fin have on lateral stability?

A

INCREASES AREA ABOVE the CoG which produces a RESTORING ROLLING MOMENT

33
Q

When is an aircraft said to be directionally stable?

A

It produces a RESTORING YAWING MOMENT to return to EQUILIBRIUM known as WEATHERCOCK EFFECT

36
Q

Describe the directional stability diagram?

A 
SIDESLIP ANGLE (x) and YAWING MOMENT coefficient (y): (-,-)/(+,+) STABLE;
SIDESLIP ANGLE (x) and YAWING MOMENT coefficient (y): (-,+)/(+,-) UNSTABLE
37
Q

What effect does sweep back fin have on directional stability?

A

INCREASES the STALL ANGLE of FIN and the Mcrit which allows the FIN to work more EFFECTIVELY to overcome DISTURBANCES in YAW

38
Q

What effect does the side/keel area have on directional stability?

A

If there is a YAW DISTURBANCE and begins to SIDESLIP, the side area facing the RAF will produce an AERODYNAMIC FORCE of DRAG;
The AREA AFT of the CoG will provide a RESTORING YAWING MOMENT while the AREA FORWARD of the CoG will NOT;
This means there must be a GREATER AREA AFT of the CoG for the aircraft to be DIRECTIONALLY STABLE

39
Q

What effect does the position of the CoG have on directional stability?

A

A FORWARD CoG along the LONGITUDINAL AXIS will INCREASE the LEVER from the TAIL which INCREASES the RESTORING YAWING MOMENT

40
Q

What effect does sweepback wings have on directional stability?

A

If there is a YAW DISTURBANCE and begins to SIDESLIP, the aeroplane will CHANGE DIRECTION to the RAF and INCREASE the EFFECTIVE SPAN of the OUTER wing, INCREASING ASPECT RATIO and DRAG which provides a RESTORING YAW MOMENT

41
Q

Why cannot we discuss directional and lateral stability independently?
Explain why this is in a practical example?

A

They are CLOSELY RELATED and AFFECT EACH OTHER;
When aeroplane ROLLS it SIDESLIPS due to SUM of AERODYNAMIC FORCES so RAF comes FROM LOWER WING, because of WEATHER COCK effect the plane will YAW TOWARDS the direction of RAF;
When aeroplane YAWS the OUTER WING must travel FASTER which produces MORE LIFT, this produces a MOMENT about the LONGITUDINAL AXIS where the DIFFERENCES in LIFT cause a ROLL

42
Q

Explain the conditions and process of spiral instability?

A

Very STRONG DIRECTIONAL and relatively WEAK LATERAL STABILITY;
Occurs due to a DISTURBANCE in ROLL, if the roll is to the left, the RAF comes from the left because of the sideslip created by the SUM of AERODYNAMIC FORCES;
The LATERAL stability will try to ROLL the aeroplane to the right but the DIRECTIONAL stability will try to WEATHERCOCK to the left into the RAF, if the DIRECTIONAL stability is too STRONG the YAWING will dominate and the DISTURBANCE is ENHANCED causing a DEEPER ROLL;
If the ROLL DEEPENS too much the aeroplane may experience a GRAVEYARD DIVE

43
Q

Explain the conditions and process of Dutch roll?

How does snaking differ from this?

A

Very STRONG LATERAL and relatively WEAK DIRECTIONAL STABILITY;
Occurs due to a DISTURBANCE in YAW, if the YAW is to the left, RAF will come from the right and the OUTER wing needs to TRAVEL FASTER generating MORE LIFT and ROLLING to the left;
The STRONG LATERAL stability will cause a ROLL back to the right to CORRECT the motion;
The ROLL causes a YAW back to the right due to WEATHER COCK effect which LEADS to right SIDESLIP and a ROLL to the left to CORRECT the SIDESLIP and the CYCLE REPEATS causing YAW and ROLL at the SAME TIME in OPPOSITE DIRECTIONS;
DUTCH ROLL with more PRONOUNCED YAWING than ROLLING is known as SNAKING

44
Q

What are the requirements to achieve a good balance between lateral and directional stability?

A

LATERAL and DIRECTIONAL stability should be CORRECTLY MATCHED, NONE should PREDOMINATE too much;
SPIRAL instability is TOLERATED and PREFERRED to DUTCH ROLL since it can be CONTROLLED;
It is PREFERRED to have slightly STRONGER LATERAL stability since relatively STRONG MOMENTS of YAW can be generated by RUDDER;
DUTCH ROLL should be DAMPENED by DESIGN to PREVENT structural DAMAGE

45
Q

What is phugoid mode?
Describe what will be experienced?
Can the pilot control this?
Does the pilot need to control this?

A

A LONG PERIOD mode of LONGITUDINAL DYNAMIC oscillation usually POORLY DAMPENED;
MOTION is SLOW, DAMPING FORCE is LOW and PERIOD is 20-60 seconds;
AoA can be treated as CONSTANT and PITCH MOMENT, AIRSPEED, ALTITUDE VARY widely;
SLOW interchange between POTENTIAL and KINETIC ENERGY;
Pilot can REDUCE THROTTLE setting which REDUCES KINETIC energy and therefore TOTAL energy which DAMPENS the MAGNITUDE and DECREASES OSCILLATION;
If the pilot did not do anything the ENERGY would eventually DISSIPATE and the plane would return to EQUILIBRIUM

46
Q

What is short period mode?

Describe what will be experienced?

A

A PERIOD mode which is relatively SHORT (0.3 - 1.5 seconds);
AIRSPEED can be treated as CONSTANT and the AoA VARIES along flight path;
Can be CAUSED by STICK FREE elevator flapping PITCH MOMENT;
Usually well DAMPENED by DESIGN and RECOVERED by holding the control at NEUTRAL position or RELEASING the control

47
Q

What effect does the dorsal fin have on directional stability?

A

INCREASES AREA ABOVE the FUSELAGE, with relatively LOW equivalent PARASITE AREA which can DELAY the BOUNDARY LAYER SEPARATION and INCREASE the STALL ANGLE of the TAILPLANE (RUDDER)

48
Q

What effect does the fin have on directional stability?

A

If there is a YAW DISTURBANCE and begins to SIDESLIP, there is an AoA formed BETWEEN AIRFLOW and FIN;
A AERODYNAMIC SIDE-FORCE is produced which provides a RESTORING YAWING MOMENT

Aerodynamics (11 decks)

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