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GHU201 > Aerodynamics > Flashcards

Aerodynamics Flashcards

1
Q

1. How does an airfoil generate lift?

A

air flows across a curved upper surface and is accelerated→ a low pressure region is formed; ⊥ to the relative wind → lift is the reaction

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2
Q
  1. The angle of attack of a helicopter rotor blade is defined as the angle between the:
A

blade’s chord line and the relative airflow.

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3
Q
  1. State the lilft formula!
A

FL = cL • ½ • ρ • v2 • S

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4
Q
  1. The lift coefficient of an airfoil section:
A

increases with an increase in angle of attack up to the stall.

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5
Q
  1. What is the magnus effect?
A

superposition of translational and rotational velocities of a rotating body (e.g. drum) with the result of pressure differences which cause a lift force

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6
Q
  1. The blade pitch angle of a rotor blade element is:
A

the angle between the chord line and the tip path plane.

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7
Q
  1. Which factors determine the magnitude and direction of the relative airflow in a still air hover?
A

Induced airflow velocity and rotational velocity of the blade element.

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8
Q
  1. The chord line of an airfoil section is the line:
A

drawn between the leading and the trailing edges.

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9
Q
  1. The center of pressure of an airfoil element:
A

is the point where the total aerodynamic force is acting.

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10
Q
  1. The centre of pressure of a symmetrical airfoil section is behind the leading edge approximately at the following % of the section chord:
A

25% (0,25)

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11
Q
  1. The force which acts at right angles to the relative airflow is.
A

lift

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12
Q
  1. The Centre of Pressure of an aerofoil section is:
A

the point on the chord line through which the resultant of al aerodynamic forces acts.

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13
Q
  1. The chord line of a blade section is:
A

a straight line from leading to trailing edge.

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14
Q
  1. The camber line of a symmetrical airfoil section is:
A

common with the chord line.

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15
Q
  1. In the case of a symmetrical aerofoil:
A

pitching moment varioations due to centre of pressure movement are small.

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16
Q
  1. Thickness/chord ratio of an aerofoil section is expressed in percentage of:
A

chord

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17
Q
  1. The resultant force from pressure envelopes around an aerofoil can be described as:
A

the total reaction

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18
Q
  1. That point where airflow leaves the surface of an aerofoil is known as:
A

the separation point

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19
Q
  1. A current requirement for the main rotor blade section is that:
A

changes in angle of attack produce minimum centre of pressure movement.

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20
Q
  1. The total rotor thrust is:
A

a component of total reaction acting at right angles of the aerodynamic forces on the blades, and perpendicular to the plane of rotation.

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21
Q
  1. State the drag formula!
A

FD = cD • ½ • ρ • v2 • S

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22
Q
  1. cL varies with:
A

angle of attack.

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23
Q
  1. What is the advantage a symmetrical aerofoil section as related to helicopter blade design?
A

The centre of pressure moves little in the normal angle of attack range

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24
Q
  1. An increase in angle of attack of a rotor blade would cause an increase in:
A

drag and lift forces.

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25
Q
  1. On a symmetrical blade element with a positive angle of attack lift is produced by:
A

airflow velocity increasing over upper surface giving decreased pressure and verlocity decreasing over lower surface giving increased pressure.

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26
Q
  1. Rotor blades profile drag is:
A

a component of total reaction the aerodynamic forces, acting parallel to the plane of rotation and backwards at 90 degrees to total rotor thrust.

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27
Q
  1. The amount of lift produced by a given helicopter rotor blade element is dependent upon:
A

angle of attack of the blade, the square of the air velocity relative to the blade element and the air density.

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28
Q
  1. The technical term “geometric twist” can be described as:
A

a reduction in blade angle towards the tip to give a more equal distribution of lift along the span.

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29
Q
  1. Rotor blade sections are designed so that the center of pressure:
A

is normally positioned close to the feathering axis to reduce control system loads.

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30
Q
  1. The term “washout” means:
A

that the used airfoil varies in design (f.e. thickness,camber) from blade root towards blade tip

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31
Q
  1. The in-ground-effect is caused by:
A

the airflow through the disc creating a divergent (spread out) duct with higher pressure beneath the rotor.

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32
Q
  1. The in-ground-effect on a hovering helicopter is greatest on:
A

level ground with no wind.

33
Q
  1. What is the aerodynamic result when a vertical climb is initiated by raising the collective pitch? Explain by means of the blade element theory!
A

ϑ ↑ AoA ↑ cL↑ FL↑ FV ↑ Σ FV ↑ FT ↑ > FW → accelerated motion

34
Q
  1. In a hovering helicopter, recirculated air at the main rotor blade tips will cause:
A

a reduction of lift.

35
Q
  1. The effects of recirculation are at their worst:
A

close to building-type obstructions.

36
Q
  1. In a constant speed vertical climb outside ground effect, if the effects of parasite drag on the helicopter fuselage are ignored:
A

total rotor thrust will equal aircraft weight.

37
Q
  1. The “vortex ring state“ which may develop under conditions of a power-on descent at low forward airspeed is:
A

an unstable condition which may result in an uncontrolled rate of descent.

38
Q
  1. In a free air hover how does Vi vary along the blade?
A

it is greater at the tip because of tip vortices.

39
Q

x9. What are the preconditions for vortex ring state?

A

High rate of descend, no or low airspeed and driven rotor.

40
Q
  1. Cyclic stick movement:
A

alters the tip path plane attitude.

41
Q
  1. The rotor thrust is always:
A

perpendicular to the tip path plane.

42
Q
  1. In level flight, as forward speed is increased, induced airflow velocity:
A

decreases and the component of the horizontal airflow through the disc increases.

43
Q
  1. What can be noticed during transition from hover to forward flight (anti-clockwise rotor)?
A

significant climb without raising the collective pitch lever

44
Q
  1. Translational lift becomes useful:
A

as airspeed reaches a value of approximately 20 kts.

45
Q
  1. The coning angle is the angle:
A

between the longitudinal axis of the blade and the tip path plane.

46
Q
  1. Transition to forward flight
A

causes a roll towards the advancing side.

47
Q
  1. When the cyclic stick is pushed forward, a main rotor blade will reach its maximum blade pitch angle:
A

on the retreating side.

48
Q
  1. If the collective pitch lever is raised during straight and level flight the helicopter will roll to the (1) because (2)
A

(1) advancing blade (2) the coning angle increases

49
Q
  1. A “transition” in a helicopter is:
A

a change in the flight condition from or to hovering flight.

50
Q
  1. The tip path plane is:
A

the path plane described by the blade tips during rotation and perpendicular to the axis of rotation.

51
Q
  1. For a rotor which turn in an anti-clockwise direction seen from above, a sideways hover to the left with zero wind, the pilot will have the advancing blade:
A

in front of him.

52
Q
  1. For a rotor which turns in a clockwise direction seen from above, in a sideways hover to the right, with zero wind, the pilot will see the retreating blade:
A

behind him.

53
Q
  1. For a rotor which turns in a clockwise direction seen from above, in a backward hover, with zero wind, the pilot will see the retreating blade:
A

on his left.

54
Q
  1. Reverse airflow is associated with:
A

flight at high forward speed and originates at the root of the retreating blade.

55
Q
  1. If the collective pitch lever is lowered during straight and level flight the helicopter will pitch (1) because (2)
A

(1) down (2) of the dissymmetry of lift

56
Q
  1. If the collective pitch lever is raised during straight and level flight the helicopter will roll to the (1) because (2)
A

(1) advancing blade (2) the coning angle increases

57
Q
  1. When the cyclic stick is pushed forward, a main rotor blade will reach its maximum blade pitch angle:
A

on the retreating side.

58
Q
  1. Retreating blade stall is most likely to occur at:
A

high forward speed, at high gross weight, high altitude and temperature.

59
Q
  1. During an autorotation descent the maximum gliding distance will be obtained at:
A

speed greater than that associated with the minimum rate of descent.

60
Q
  1. That point where airflow leaves the surface of an aerofoil is known as:
A

the separation point

61
Q
  1. The purpose of the swept back tip region in some modern rotor blade designs is to:
A

improve high speed performance.

62
Q
  1. What happens to the coning angle if rotor RPM decreases and collective pitch is constant?
A

It increases.

63
Q
  1. How does rotor downwash affect a helicopter with a tail boom mounted horizontal stabilizer in a free air hover?
A

it will pitch nose up.

64
Q
  1. Compared to a straight and level flight to perform a coordinated turn (same altitude and speed) the collective blade pitch angle (1) and power (2) must be:
A

(1) increased (2) increased.

65
Q
  1. Which factors have an influence on the bank angle in turning flights?
A

velocity, curve radius, gravity

66
Q
  1. What is the load factor?
A

Factor which refers to the extend of thrust that must be increased to hold altitude while turning.

67
Q
  1. What sources of energy are available in case of a double engine failure?
A

Potential energy because of the height, kinetic energy because of airspeed and rotor RPM.

68
Q
  1. In a normal forward autorotation descent, if the collective lever is raised by a small amount, the rotor RPM will (1) and the rate of descent will (2):
A

(1) decrease (2) decrease.

69
Q
  1. What is the effect of an increasing airspeed on the region of driving blade elements during autorotation?
A

shifting of the driving blade elements to the retreating blade

70
Q
  1. During an autorotation descent the maximum gliding distance will be obtained at:
A

speed greater than that associated with the minimum rate of descent.

71
Q
  1. The “avoid“ areas in a height/velocity diagram (deadman’s curve) define the height/velocity combinations:
A

from which it is not possible to make a safe autorotation landing.

72
Q
  1. A tail rotor is fitted to most helicopters to compensate for:
A

main rotor torque reaction and give directional control (yaw control).

73
Q
  1. Yawing in a helicopter is the term used to define a rotation:
A

about the vertical axis.

74
Q
  1. A helicopter having an anti-clockwise rotating main rotor (when seen from above), with power on, will have a natural tendency to drift:
A

to the right.

75
Q
  1. Tail rotor drift is corrected by:
A

tilting the main rotor disc in the opposite direction to the drift.

76
Q
  1. In hovering, for a single rotor helicopter whose main rotor turns clockwise from above, the thrust of the main rotor will be mainly vertical but with a slight orientation towards the:
A

right.

77
Q
  1. With a tail rotor positioned lower than the main rotor a helicopter at the hover will:
A

fly left side low if main rotor rotates anti-clockwise viewed from above.

78
Q
  1. During flight, an increase in main rotor torque will require:
A

an increase in tail rotor pitch.

79
Q
  1. An anti-torque rotor is necessary on:
A

a single rotor helicopter.

GHU201 (10 decks)

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