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Module 13 EASA Part 1 > 13.1(c) Theory of Flight > Flashcards

13.1(c) Theory of Flight Flashcards

1
Q

What is the definition of a helicopter?

A

A heavier than air machine that derives both lift and thrust from one or more horizontally rotating rotors

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2
Q

What are the disadvantages associated with helicopters?

A
  • Limited forward speed
  • Limited operating ceiling (altitude)
  • Limited load carrying capabilities
  • Vibration
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3
Q

What might the pitch axis also be called?

A

Feathering axis

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4
Q

What are the 5 forces acting upon helicopter blades?

A
  • Lift
  • Weight
  • Thrust
  • Drag
  • Centrifugal force
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5
Q

How is the lift imbalance of a rotor blade overcome by blade design?

A
  • Tapering the blade
  • Washout
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6
Q

What are helicopter blades normally made of?

A

Composite materials

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7
Q

What problems could lift imbalance cause on helicopter blades?

A

Build up of stress, leading to fatigue and failure

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8
Q

How is the upward bending of rotor blades reduced?

A

Fitting of flapping hinges

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9
Q

What is the coning angle?

A

Angle between the blade Tip Path Plane and the actual blade angle

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10
Q

What is the purpose of automatic droop stops?

A

Prevent the rotors from flying too low when lift and centrifugal force is lost from the blades

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11
Q

What is the purpose of the flapping restrainers?

A

Prevent excessive upward rotor blade movement in windy conditions when the rotor blade is at a standstill

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12
Q

Why is it important that all rotor blade tips follow the same path, ie tip path plane?

A

Excessive vibration will occur

Blades will be flying through the preceding blades tip vortices

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13
Q

What is the rotor blade which is moving forwards, towards the nose of the aircraft, known as?

A

Advancing blade

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14
Q

What is the rotor blade which is moving backwards, towards the tail of the aircraft, known as?

A

Retreating blade

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15
Q

How does the flapping to equality deal with the dissymmetry of lift?

A

Advancing blade rises on flapping hinge and therefore the relative airflow gives a shallower angle of attack, thus decreasing the lift produced

Retreating blade lowers on its flapping hinge, the relative airflow creates a higher angle of attack, thus increasing the lift on this blade

Lift across the whole rotor disc will be in equilibrium

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16
Q

What is the coriolis effect with regards to rotor movement?

A

Blade which is higher in its flapping hinge has a centre of gravity closer to the shaft axis of the rotor hub, which leads to it increasing in speed

The blade which is falling in its flapping hinge will have a centre of gravity further from the shaft axis, and will therefore attempt to slow down

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17
Q

How is the stress created by the coriolis effect relieved?

A

Vertical hinge called a lead-lag or dragging hinge

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18
Q

What problem is caused by lead-lag hinge?

A

Both blades will be in the same half of the rotor disc, causing a weight imbalance and severe vibration

In one half rotation, the blade positions will be reversed from leading to lagging, in their drag hinges, therefore causing a sustained and severe vibration to develop

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19
Q

How is the problem of vibration caused by dragging hinges overcome?

A

Dragging damper

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20
Q

What are the two types of rotor blade drag dampers?

A

Hydraulic or elastomeric type

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21
Q

What are the advantages of the semi-rigid and rigid rotor heads?

A
  • Simplicity of construction
  • Rapid response to control inputs
  • Fewer moving parts to cause vibration
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22
Q

What are the main disadvantages of the semi-rigid and rigid rotor heads?

A
  • Complex control system is required to provide equalisation of lift across the rotor disc, due to an inability to flap to equality
  • Rotor blades are subject to more severe stresses and must be made much stronger and more damage tolerant
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23
Q

What would a main rotor head have which would classify it as fully articulated?

A
  • Feathering bearings
  • Flapping hinges
  • Dragging hinges
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24
Q

What is the tail rotor induced drift?

A

The opposing force from the tail rotor to the torque reaction creates a sideways vector

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25
How is tail rotor induced drift overcome?
Mounting the main rotor gearbox and head assembly at a small angle opposite to the induced drift or Biasing the main rotor controls so that when the control column is neutral, the main rotor disc is offset
26
Why must the designer ensure that the tail rotor is mounted as close as possible to the same height as the main rotor?
Rolling couple will be produced
27
What are the advantages of the tandem main rotors concept of cancelling torque reactions?
* Greater load carrying capability * Long and spacious fuselage
28
How is the coanda effect in a notar helicopter achieved?
Similar principle to slats/slots used on some fixed wing aircraft leading edges Slots running horizontally along the tail boom allow air to escape from one side Air flowing down from the main rotor is stimulated by this air, causing it to adhere to one side of the tail boom for much longer
29
Why does the ducted fan concept of cancelling torque reaction have an aerodynamically shaped aerofoil?
Provide a side force during forward flight above certain speeds
30
How does a helicopter increase total rotor thrust?
Increasing the pitch of all the main rotor blades, equally and simultaneously
31
What is the 'ground effect' also known as?
Ground cushion
32
When is the 'ground effect' greatest?
Within half a rotor span of the ground
33
Explain the 'ground effect'?
Downwash of air is displaced from the main rotor much more quickly than it can escape from beneath the aircraft Creates an air cushion
34
When will the 'ground effect' be lost?
When the helicopter moves from the hover at a speed greater than 3 to 5 mph
35
Why is the 'ground effect' useful?
Provides additional lift and reduce the power required to maintain the hover
36
How is forward flight achieved in a helicopter?
Decreasing the pitch on the rotor blades as they pass the nose of the aircraft Increasing the pitch as they pass over the tail boom Effectively tilting the rotor disc
37
What is necessary when the rotor disc is tilted to achieve forward (or otherwise) flight?
Increase the rotor thrust
38
What is the result of the induced flow into the main rotor caused by increased forward speed?
It is more efficient and the helicopter will be begin to climb
39
What will be applied a few metres in the air whilst in autorotation and why?
Flaring which is maximum pitch to all main rotor blades, equally and simultaneously Reduce descent speed to prevent excessive force being felt through the structure on landing
40
What must the pilot do when entering autorotation?
Reduce the pitch on all main rotor blades to their absolute minimum Minimise control inputs to minimise energy loss
41
What hazardous flight conditions are there which are unique to rotorcraft?
* Vortex ring state (power setting) * Rotor blade stalling * Ground resonance * Recirculation * Blade sailing
42
What is vortex ring state?
Rotorcraft enter a near vertical powered descent 2 opposing airflows, air passes downward through the rotor disc near the tips, upward through the rotor disc at the blade centre Lift is reduced leading to effective free-fall
43
At what rate of descent is vortex ring state likely to occur?
At rates above 500 feet per minute
44
What must the pilot do with the helicopter to avoid vortex ring state?
Limit the vertical or near vertical powered descent to approximately 250 feet per minute Input a forward speed of approximately 15 to 30 knots
45
What might be the result of reversed rotor blade airflow?
Severe buffeting and vibration, and the aircraft will pitch violently nose up, as airflow is lost over the retreating blade
46
What conditions may cause retreating blade tip stall?
* High airspeed * Low rotor RPM * Aircraft at, or close to, maximum permissible weight * High density altitude * Accelerated flight leading to high rotor disc load factor * Flight through turbulent air or gusts * Excessive or abrupt control deflections during manoeuvres
47
What is ground resonance?
Self excited vibration while the aircraft is on the ground caused by unbalancing of the rotor disc due to the leading and lagging of the rotor blades about the dragging hinge becoming out of phase
48
What can cause ground resonance?
Inefficient drag dampers
49
How can ground resonance be further aggravated?
The reaction of the aircraft's landing gear Incorrect tyre pressures and shock absorber extension will cause the vibration to increase in frequency
50
What should the pilot do if ground resonance occurs?
Lift off into the hover
51
Why is recirculation more dangerous where the helicopter is operating close to a building on one side?
The effective angle of attack on the building side is drastically reduced, meaning the lift on that side is reduced, and it will drift into the building
52
Where should the helicopter be approached and when?
Only from the advancing blade side Only with the permission of the pilot or other appropriate flight crew member
53
What forces affect the coning angle?
Lift, resultant and centrifugal force
54
Where is 'ground cushion' most effective? Over what sort of surface?
Hard surfaces where most of the energy of the air isn't lost to the environment Within half a rotor span from the ground
55
Explain co-axial contra rotating main rotors as a method of counteracting 'torque reaction'?
Co-axial contra rotating main rotors - 2 main rotor assemblies are mounted on the same gearbox, rotating in opposite directions, directional control provided by variations in main rotor thrust
56
Explain tandem main rotors as a method of counteracting 'torque reaction'?
Tandem main rotors - 2 main rotors, on separate gearbox and head assemblies, rotating in opposite directions
57
Explain notar as a method of counteracting 'torque reaction'?
Notar - Flow of air provided by an internal fan driven by the engine, ducted through a hollow boom, forced from the helicopter by a swivelling cap
58
Explain ducted fan as a method of counteracting 'torque reaction?
Ducted fan - small, multi-bladed fan very like a gas turbine engine compressor, mounted inside a duct built into the tail boom, aerodynamically shaped fin for directional control
59
With the helicopter hovering in 'ground effect', will the power required be higher or lower than that when hovering out of 'ground effect'?
Lower
60
What are the two problems which must be overcome in the flying controls?
* Transferring the control input from the non-rotating control system into the rotating rotor head flying control assembly * Gyroscopic precession
61
What is gyroscopic procession also known as?
Phase lag
62
What are the component parts of a swash plate?
* Rotating scissor link * Non-rotating scissor link * Control actuator input * Pitch change rod * Ball joint * Rotating star * Non-rotating star * Slider sleeve
63
How was phase lag handled in early Sikorsky rotorcraft?
Actuator placed at 45 degrees and the pitch change horn forming the remaining 45 degrees
64
What is advance angle?
Mechanically fixed angle between the pitch link attachments at the blade and the swashplate
65
In a two bladed rotor system, what is the usual method by which to deal with phase lag?
The entire 90 degrees to be achieved by advance angle alone, and the actuators are positioned on the point of maximum desired effect
66
What changes are made to the tail rotor pitch changes?
Collective
67
When turning against the direction of the natural torque, what must be done with the pitch of the tail rotor blades to produce the turn?
Increase, to produce the additional force
68
What is incorporated into the collective lever to allow the pilot/co-pilot to operate other equipment?
Collective friction device
69
If collective is increased on a helicopter with main rotors that rotate clockwise (viewed from above), which way will the fuselage tend to yaw?
Anti-clockwise, assuming power is increased to deal with the increased lift requirement
70
What direction would the helicopter turn if the tail rotor blade pitch angle was increased on a helicopter with main rotors that rotate clockwise (viewed from above)?
Yaw to the right, against torque reaction
71
What will initially tend to happen to rotor speed if collective is increased?
The rotor will tend to decelerate
72
Describe how the control input from the pilot is transferred to the main rotor blades during flight on a typical helicopter?
Collective changes - transferred to non-rotating swashplate by all actuators evenly, transferred to rotating swashplate which pitches all blades evenly Cyclic changes - transferred to fore/aft or lateral actuators which move non-rotating swashplate in a particular axis, transferred to rotating swashplate and to specific pitch change rod 90° before desired position of maximum effect, 45° pitch change horn forming the remaining advance angle transfers the pitch change to individual blades
73
What is the purpose of the mixer unit?
Ensure that all movements will be transferred to the appropriate actuator(s) and that the rotorcraft responds correctly to pilot/co-pilot inputs

Module 13 EASA Part 1 (20 decks)

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