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Aviation > Rotary Wing aerodynamics > Flashcards

Rotary Wing aerodynamics Flashcards

1
Q

Pitch angle

A

is the angle between the chord line of the rotor blade and the plane of rotation

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2
Q

Tip-path plane

A

imaginary circular surface formed by a plane passing through the path of the tips of the rotor blades

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3
Q

Rotor disc

A

is the circle of air swept by the rotor

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4
Q

What axis does feathering occur on ?

A

The Longitudinal axis

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5
Q

What axis does flapping occur on ?

A

The normal axis ( vertical )

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6
Q

What axis does lead lag occur on ?

A

The lateral axis

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7
Q

Induced airflow

A

air that is sucked in through the top of the rotor blades and blown out the bottom.

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8
Q

Increased induced airflow

A

reduces angle of attack

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9
Q

Coning

A

is the upward flexing of the rotor blades

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10
Q

Resulting from

A

the forces of lift distributed along the blades

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11
Q

Tips of the blades produce

A

more lifting force than the roots

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12
Q

Rotor disc starts to

A

take up weight of fuselage , the disc begins to cone

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13
Q

Factors that increase coning and cause a high coning angle

A

The RPM of motors ( Low rotor RPM ) , The weight of the vehicle ( High weight increases coning ) Any G forces experienced in flight ( High G manoeuvres )

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14
Q

Excessive coning results in

A

Undesirable stresses on the blades , A decrease in effective disk area and hence lift. Tip path plane decreases therefore an overall decrease in lift

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15
Q

Helicopter has three controls

A

Collective , cyclic , tail rotor pedals

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16
Q

Collective pitch lever causes

A

it to rise and descend

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17
Q

Produces simultaneous

A

increase or decrease in angle of pitch of rotor blades by same amount

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18
Q

Cyclic pitch control

A

causes it to tilt in the desired direction

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19
Q

Resembles

A

control stick , alters pitch of rotor blades by different amounts on each side , differential in lift causes disc to tilt

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20
Q

Tail rotor pedals

A

cause it to yaw

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21
Q

Anti torque pedals are

A

foot controls which change the pitch of the tail rotor and the thrust which it is producing.

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22
Q

Concentric shafts a pair of

A

helicopter rotors mounted one above the other on concentric shafts

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23
Q

with the same

A

axis of rotation but turning in opposite directions

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24
Q

Gyroscopic procession

A

means a force applied to a gyro (rotor disc ) will produce an effect up to 90 degrees forward of the applied force.

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25
Q

Rotor spinning Anti Clockwise , want to tilt rotor forwards

A

force on disc needs to be applied up to 90 degs before hand

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26
Q

Tail Rotor Drift also called

A

Translating Tendency

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27
Q

If a rotor spins ACW, fuselage of helicopter will spin

A

CW (Newton’s 3rd law. Think of drill bit getting stuck)

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28
Q

This CW motion will also cause a sideways force

A

to the right, shown at the rotor mast as drift.

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29
Q

Tail rotor provides

A

anti-torque for fuselage

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30
Q

which also causes a net sideways force

A

to the right

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31
Q

These two forces provide an overall

A

net movement of the helicopter to the right.

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32
Q

Pilot actions to counteract: Pilot uses

A

cyclic movement in opposite direction to TRD

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33
Q

Dissymmetry of Lift , Relative airflow is different

A

on advancing side and retreating side of rotor disc

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34
Q

Compensated automatically

A

in rotor system through flapping.

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35
Q

Retreating side flaps

A

down

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36
Q

advancing side flaps

A

up

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37
Q

This flapping causes relative airflow

A

to be different on either side and consequently angle of attacks are different on either side.

38
Q

Retreating side flaps down. Causes an

A

increase in AoA, creating more lift, causing blade to flap back up.

39
Q

Advancing side flaps up. Causes a

A

decrease in AoA, diminishing lift, causing blade to flap back down.

40
Q

This is called

A

flapping to equality.

41
Q

If flapping wasn’t allowed

A

DOL would cause FLAPBACK.

42
Q

Advancing side would produce

A

more lift

43
Q

Gyroscopic precession would transfer this increase in lift

A

to the front of the helicopter (approx. 90 degs forward) causing uncontrollable rise to the nose…. Could be fatal.

44
Q

Pilot actions : As a helicopter moves forward,

A

constant force must be applied to keep the disc tilted forward.

45
Q

As helicopter moves faster,

A

disc tilts more, causing retreating side to continually experience higher AoA than advancing side

46
Q

Eventually AoA will

A

exceed critical angle, causing retreating blade stall

47
Q

The stall will be experienced at the

A

rear of helicopter due to GP, causing severe vibration and pitching up!!

48
Q

Transverse flow effect : In forward flight, the rotor disc is tilted

A

forwards and the air enters it from above.

49
Q

The air entering the rear of the disc is acted upon

A

by the blades for a longer period of time and is accelerated to a higher speed than that coming through the front of the disc.

50
Q

This results in a

A

greater downwash from the rear of the disc and a decreased angle of attack, hence decreased lift experienced by the rear of the disc.

51
Q

Combined with

A

gyroscopic precession, this difference in lift causes the helicopter to roll in the direction of the advancing blade.

52
Q

Countered by use of the

A

cyclic

53
Q

This effect is most evident at

A

low airspeeds and exaggerated forward pitch angles of attack.

54
Q

Effective translational lift : 3 stage of ETL , 1st stage

A

Stationary hover (0kts forward speed) , Helicopter is operating in its own wingtip vortices, with tips of the blade ineffective at producing lift

55
Q

2nd stage ,

A

Translational flight (moving from hover to forward flight)

56
Q

Rotor tilted

A

forwards

57
Q

There is still some

A

wingtip vortices but they are starting to get pushed away from rotor system by forward airflow

58
Q

causing rotor disc

A

to receive greater efficiency.

59
Q

Bonus in lift from this.

A

Same lift can be achieved with less engine power

60
Q

3rd stage :

A

ETL (12-40 kts airspeed)

61
Q

Helicopter has moved

A

out of its wingtip vortices and into ETL

62
Q

Greater

A

Lift can be produced with less power until 40 kts

63
Q

After 40 kts

A

parasite drag from the fueslage negates ETL

64
Q

Coriolis effect states that due to

A

conservation of angular momentum, coning will cause the centre of mass of the blades to come closer to the mast,

65
Q

increases

A

rotor rpm

66
Q

This increase in RPM

A

if not managed could overspeed the engine and cause damage to rotor and engine.

67
Q

In semi rigid rotor systems

A

automatic flapping down of one side and up on the other diminishes the effect of coriolis and avoids over-RPM of engine

68
Q

In a fully articulated head with more than

A

two blades

69
Q

a drag hinge is used

A

to allow backward and forward movement of blade

70
Q

Retreating blade

A

flaps down, moves slower) lags

71
Q

advancing blade

A

(flaps up, moves faster) leads

72
Q

Ground effect becomes non existent once

A

the helicopter achieves a translational speed of more than 5 knots

73
Q

When operating away from the ground, helicopter experiences

A

recirculation of air at the blade tips

74
Q

Similar to

A

wingtip vortices

75
Q

This recirculation makes an area

A

around the blade tip ineffective at producing lift.

76
Q

When in ground effect

A

within a rotor span from ground

77
Q

recirculation is

A

disrupted and not allowed to flow back into the rotor system, so less of the blade tip is affected.

78
Q

This results in bonus

A

lift production IGE.

79
Q

Helicopters can seem to carry

A

heavier loads when IGE and use less power to produce same amount of lift as when OGE.

80
Q

Know the dangers of ground effect: If helicopter is operating IGE on top of a peak

A

and accidentally drifts off the peak or ledge and ends up OGE, the helicopter will immediately lose lift.

81
Q

If lift is not restored by raising

A

collective, the helicopter will sink and possible collide with terrain.

82
Q

Autorotation : Is the state of flight where rotor is driven

A

by relative airflow rather than power of the engine.

83
Q

All helicopters must have the ability to

A

disconnecting the main rotor from the engine so that the rotor can turn even if the engine cannot.

84
Q

tail rotor also needs to be

A

turning during autorotation to provide some counter to the frictional forces in the gearbox which would otherwise induce the fuselage to turn in the same direction.

85
Q

1st step

A

Disengage the rotor system from the engine.

Lower the collective pitch to reduce pitch of blades to a minimum, reducing torque

Nose is pushed down to give an angle of glide and to provide some translational lift

86
Q

2nd step As the helicopter descends,

A

the air coming up through the rotor disc causes the blades to rotate, and as they do, progressively store more energy.

87
Q

3rd step When the ground approaches, the pilot:

A

flares the helicopter in a similar manner to a normal landing and during this phase,

the rotor will usually increase speed because of the flare.

The final speed increase provides enough extra lift to arrest the helicopter’s descent

88
Q

4th step : helicopter has lost

A

its forward kinetic energy and has no air flowing upwards to keep the rotor turning.

89
Q

the helicopter settles on the ground, with the pilots last action:

A

As rotor speed decays , Extract the remaining lift from the rotor by increasing the collective pitch to its maximum.

This increases the lift, but also increases the drag and the rotor quickly loses speed as the helicopter settles gently on the ground.

90
Q

Cyclic acts 90 degrees

A

before the total effect is achieved by the rotor

Aviation (4 decks)

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