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Helicopter Special Items > Helicopter Aerodynamics > Flashcards

Helicopter Aerodynamics Flashcards

1
Q

Define Newton’s first law

A

The law of inertia. An object in motion remains in motion until acted upon by an outside force. The same occurs with an object at rest

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2
Q

Define Newton’s second law

A

Law of acceleration, an object acted upon by a force will continue to accelerate in proportion to the magnitude of the force and the mass of the object

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3
Q

Define Newton’s third law Newton’s third law

A

Equal and opposite reaction. When an object receives a force it pushes back with an equal opposite force.

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4
Q

How does Newton’s first law act upon a helicopter?

A

Helicopter will continue to move unless acted upon by an outside force such as weight or drag.

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5
Q

How does Newton’s second law impact a helicopter?

A

The amount of lift or thrust required is proportional to the weight of the aircraft

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6
Q

How does Newton’s third law act upon an aircraft?

A

Tail rotor counteracts the main rotor torque effect

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7
Q

What is the definition of a scalar?

A

A scalar is a value with a magnitude only (Speed)

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8
Q

What is the definition of a vector?

A

Vectors have both magnitude and direction (Velocity)

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9
Q

What is the vector known as if it is the result of two separate vectors?

A

The resultant vector

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10
Q

The anti torque pedals control what in a helicopter?

A

The tail rotor which impacts yaw

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11
Q

The cyclic controls what two axes of motion in a helicopter?

A

Pitch and roll

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12
Q

What is known as the venturi effect?

A

Fluid velocity increases in direct proportion to the reduction of area of flow

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13
Q

What is Bernoulli’s principle?

A

As fluid velocity increases, the dynamic pressure also increases, so to keep total pressure constant static pressure must decrease

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14
Q

How does bernoulli’s principle act upon an airfoil?

A

As the air moves around the airfoil the dynamic pressure increases due to the increase in velocity above the airfoil resulting in a decrease in static pressure generating lift

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15
Q

The symbol CL corresponds to what in the lift equation?

A

A combination of angle of attack and wing shape

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16
Q

The symbol P corresponds to what in the lift equation?

A

Air density

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17
Q

The symbol V corresponds to what in the lift equation?

A

Airfoil speed

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18
Q

Symbol S corresponds to what in the left equation?

A

Wing surface area

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19
Q

Temperature lapse rate is defined as what?

A

2 degrees per every thousand feet

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20
Q

As humidity increases, what happens to overall air density?

A

Overall air density decreases resulting in a increase in density altitude

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21
Q

An airfoil is defined as what?

A

A surface body or structure designed to produce a lift or thrust force when subjected to an airflow

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22
Q

The front of the airfoil is known as what?

A

A leading edge

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23
Q

The rear of the airfoil is known as a what?

A

Trailing edge

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24
Q

A line from the leading edge to the trailing edge is known as what?

A

The chord

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25
Q

What line is the average between the upper surface and lower surface of the airfoil?

A

The camber line

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26
Q

What are the characteristics of a symmetrical airfoil?

A

Equal camber on each side of the cord, each half is a mirror image of the other.

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27
Q

What are some advantages of a symmetric airfoil?

A

Constant center of pressure, ease of construction, lower cost.

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28
Q

What are some disadvantages of a symmetric airfoil?

A

Less efficient at a given angle of attack, undesirable style characteristics

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29
Q

What is the description of a non symmetric airfoil?

A

A cambered or curved area on the upper surface of the airfoil

30
Q

What are the advantages of a non symmetrical airfoil?

A

Increase lift drag ratios and better stall characteristics

31
Q

The center of pressure is defined as what?

A

The point at which the lifting force from the airfoil is created

32
Q

Centrifugal force is defined as what?

A

An outward force produced whenever a body moves in a curved path

33
Q

Centrifugal force provides what for a helicopter rotor system?

A

Rigidity

34
Q

Centripetal force is defined as what?

A

The normal force of the centrifugal force pointing towards the center of the rotation

35
Q

What is rotor blade coning?

A

Upward flexing of the rotor blades which is a compromise between lift and centrifugal force, when lift is stronger than the centrifugal force of blade will cone upward

36
Q

Excessive coning results in what?

A

A loss of disk area with a resultant loss of total lift as well as increased stress on the blades

37
Q

Why are rotor blades twisted?

A

To even the amount of lifting force along the length of the blade

38
Q

A lower pitch angle at the tip of the blade results in what?

A

A lower overall angle of attack due to the higher relative winds that the airfoil experiences

39
Q

Profile drag is defined as what?

A

The frictional resistance of the surface of the airfoil

40
Q

Induced drag is defined as what?

A

Rotational eddies of air which are formed when the airfoil produces lift which resulted in drag on the airfoil

41
Q

Parasitic drag is defined as what?

A

Drag from non lifting surfaces the aircraft moving through the air

42
Q

The airspeed at which induced drag and parasite drag are at a minimum is defined as what?

A

Max endurance air speed or the Max l / d

43
Q

Airfoil stall is defined as what?

A

An exceedance of the critical angle of attack of the airfoil where the laminar flow separates from the airfoil next slide

44
Q

During a hover, weight and lift are in a state of what?

A

A state of balance

45
Q

What causes the high power requirement for hovering?

A

The continuous creation of new vortices and ingestion of existing vortices is a primary cause of high power requirement for hover

46
Q

What is the definition of hovering in ground effect?

A

Hovering above the ground within one disc diameter where the rotational vortices or mitigated by the ground

47
Q

What is the definition of out of ground effect hover?

A

Hovering above one rotor disk where the helicopter is subjected to the full force of the induced drag of the lift

48
Q

hover power requirement is typically how much less than out of ground effect?

A

30% less

49
Q

If hovering over tall grass, rough terrain, water, or other rough surface what may occur with in ground effect hover?

A

In ground effect hover may have increased power requirements

50
Q

What is translating tendency?

A

It is a right lateral tail rotor thrust that results in a right lateral drift on the helicopter

51
Q

Compensation for translating tendency is typically what?

A

An application is cyclic to create a main rotor force to the left

52
Q

How else is translating tendency overcome?

A

Helicopter design usually has a preference for left role built into the airframe to compensate

53
Q

What is the transverse flow effect?

A

Downward air flow at the rear of the rotor disk resulting in a condition of increased drag with reduced angle of attack meaning that the rear of the rotor produces less lift. This overall results in a right rolling motion at low aircraft speed

54
Q

Dissymmetry of lift is defined as what?

A

Differential lift produced by the differences in relative wind between the rotating helicopter blades

55
Q

What 2 design considerations prevent adverse effects of dissymmetry of lift?

A

Blade flapping and cyclic feathering

56
Q

what occurs at the retreating blade exceeds the angle of attack?

A

A retreating blade stall

57
Q

What is cyclic feathering?

A

The increase of the pitch of 1 rotor blade while simultaneously decreasing the pitch of the other

58
Q

What is the definition of effective translational lift?

A

Point of which the aircraft out runs its own down wash resulting in both blades experiencing smooth oh unaltered error.

59
Q

At what speed does effective translational lift occur?

A

16 to 24 knots

60
Q

As the helicopter speed increases in translational lift becomes more effective what characteristic is seen?

A

The nose will pitch up which is a tendency called blowback.

61
Q

Blowback is caused by what?

A

The combined effects a dissymmetry lift and transverse flow

62
Q

If the aircraft is allowed to pitch up during blowback while transiting ETL the aircraft may also experience a role to which direction?

A

A roll to the right

63
Q

Auto rotation is defined as what?

A

Rotor spinning through the action of air moving through the rotor disk rather than the engine turning the rotor via the transmission

64
Q

What are some reasons to auto rotate?

A

Engine failure, engine under speed, main drive shaft failure, clutch fails to reengage, complete loss of tail rotor thrust that is uncontrollable

65
Q

Auto rotations are defined by three phases which are known as what?

A

Entry, steady state descent, deceleration and touchdown

66
Q

When entering an autorotation what are the four steps?

A

Collective- adjust, pedals- adjust, throttle- adjust, ear speed- adjust

67
Q

What is the minimum rate of descent airspeed at 94% Nr?

A

58 knots

68
Q

What is the maximum glide distance to the helicopter 94% Nr?

A

106 knots

69
Q

What are the three auto rotation regions of the blade during an autorotation?

A

Driven, driving, Stall

70
Q

Which region produces lift during auto rotation?

A

The driving region

71
Q

The driven region of the blade results in what aerodynamic force?

A

Drag, which slows down the blade

72
Q

Reducing the collective during autorotation results in what change of the driving region?

A

The total size of the driving region increases and results in an increased speed on the blade

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