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APART Study Guide > Aerodynamics > Flashcards

Aerodynamics Flashcards

1
Q

What is Relative Wind?

A

Airflow relative to an airfoil. It is parallel and opposite the direction of movement of the airfoil.

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2
Q

What is Rotational Relative Wind?

A

It is just how we refer to relative wind in a helicopter. Rotational refers to the method of producing relative wind.

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3
Q

What is Resultant Relative Wind?

A

At a hover, resultant relative wind is rotational relative wind modified by induced flow. When the helicopter has horizontal motion, airspeed further modified resultant relative wind.

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4
Q

Where are vortices created during a hover?

A

Vortices are created at the rotor tip.

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5
Q

What distance is considered to be OGE?

A

1 Rotor Diameter.

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6
Q

Describe airflow during a hover IGE.

A

Air flows down through the rotor system and vortices are created at the rotor tips. Because of the close distance to the ground, the downward and outward flow of air tends to restrict vortex generation. In addition, due to the proximity to the ground, the induced flow velocity is reduced because the ground interrupts the airflow which increases AOA, reduces induced drag, allows for a more vertical lift vector, and increases rotor efficiency.

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7
Q

Describe airflow during a hover OGE.

A

Air flows down through the rotor system and vortices are created at the rotor tips. These vortices build much greater than during IGE hover because they are not being restricted by ground interference. In addition, induced flow velocity is higher than during IGE hover causing a decrease in AOA. This means that a higher pitch blade angle is required on the blades, so that creates more drag.

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8
Q

Why does it require more power to hover OGE than IGE? (2 main reasons)

A

1) Increased induced flow velocity OGE meaning a lower AOA.
2) Bigger rotor tip vortices decrease the surface area of the blade that is producing lift.

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9
Q

Does the angle of attack change from an IGE hover to an OGE hover?

A

No.

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10
Q

What is TAF?

A

It is the resultant force of lift and drag. It acts as the center of pressure on the airfoil.

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11
Q

What is TAF the sum of?

A

It is the sum of lift and drag.

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12
Q

What direction does TAF act in normal flight?

A

TAF is normally inclined up and to the rear.

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13
Q

What are the 3 types of drag, and what causes them?

A

1) Parasite Drag - Incurred from the non-lifting components of the aircraft. Increases as airspeed increases and is the main cause of drag at high airspeeds.

2) Profile Drag - Incurred from the frictional resistance of the blades passing through the air. It increases moderately at high airspeeds.

3) Induced Drag - Incurred as a result of the production of lift. It decreases as airspeed increases.

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14
Q

What 3 conditions must be present for dynamic rollover to occur?

A

1) Pivot Point
2) Rolling Motion
3) Exceeding the critical angle

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15
Q

What are the 3 types of dynamic rollover?

A

1) Rollover on level ground
2) Rolling downslope
3) Rolling upslope

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16
Q

What are the physical factors associated with dynamic rollover?

A

1) Main Rotor Thrust
2) CG
3) Tail Rotor Thrust
4) Crosswind Component
5) Ground Surface
6) Sloped Landing Area
7) In some aircraft the presence of a low fuel condition that causes the CG to move upward.

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17
Q

What are the human factors associated with dynamic rollover?

A

1) Inattention
2) Inexperience
3) Inappropriate Control Input
4) Failure to take Timely Corrective Action
5) Loss of Visual Reference

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18
Q

What is the best action if recovery is possible from dynamic rollover?

A

Make smooth moderate control inputs.

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19
Q

What is Translating Tendency?

A

The tendency of the helicopter to drift in the direction of tail rotor thrust (to the right).

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20
Q

How is Translating Tendency compensated for?

A

It is compensated for in the MMU, AFCS, and pilot input.

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21
Q

What is Dissymmetry of Lift?

A

Dissymmetry of lift is the unequal lift between the advancing half and retreating half of the rotor disk caused by different wind flow velocities across each half.

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22
Q

How is it compensated for?

A

Blade flapping and cyclic feathering.

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23
Q

What happens when the blades flap? Is AOA increased or decreased on advancing and retreating blades?

A

When the blades flap, the upward and downward motion changes the induced flow velocity, in turn changing the AOA. AOA is decreased on the advancing half and increased on the retreating half.

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24
Q

Why does blowback occur? How is it compensated for?

A

When the blades flap, the maximum upflap position is at the nose and the maximum downflap position is at the tail. This causes blowback. It is compensated for by cyclic feathering from the aviator (forward cyclic).

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25
Q

Describe Retreating Blade Stall.

A

When the helicopter is in forward flight, forward speed of the helicopter decreases the relative velocity of the retreating half of the rotor disk. Because of the decrease in a velocity, a higher AOA is required to generate the same lift as the advancing side. In addition, there is a smaller area of the disk creating lift (due to the no lift areas) on the retreating side which places an even greater demand on that area to create the lift required. Eventually, the AOA becomes so high that it stalls.

26
Q

What are the 2 main factors that cause retreating blade stall?

A

1) High forward airspeeds (decreased relative velocity on the retreating side)
2) Increased no-lift areas on the retreating side (reduced surface area creating lift)

27
Q

Describe the 3 no-lift areas. What happens as speed increases?

A

Inboard to Outboard:
o Reverse Flow - Air flows backward from the trailing edge to the leading edge of the blade.
o Negative Stall - The blade stalls with a negative AOA.
o Negative Lift - The blade has a negative AOA creating negative lift.
As airspeed increases, the areas grow larger.

28
Q

What indications would be experienced during retreating blade stall?

A

Vibrations and buffeting and a roll left and nose pitch up.

29
Q

What are conditions conducive to retreating blade stall?

A

1) High Blade Loading (High Gross Weight)
2) High DA
3) High G-maneuvers
4) Low Rotor RPM
5) Turbulent Air

30
Q

How do you reduce or stop retreating blade stall?

A

1) Reduce Collective
2) Reduce Airspeed
3) Reduce the Severity of the Maneuver
4) Reduce Altitude (Descend if possible)
5) Increase Rotor RPM to normal limits

31
Q

What on the PPC relates to retreating blade stall?

A

Vne and Max Angle

32
Q

Describe the compressibility of air at low and high airspeeds.

A

At low airspeeds, air is incompressible. Air density remains relatively constant. At higher airspeeds, as air compresses its air density changes.

33
Q

What is transonic airflow?

A

Transonic airflow is when some parts of the airflow on the airfoil is subsonic and some is supersonic.

34
Q

Why does transonic airflow occur on an airfield?

A

As air flows over the airfoil, it speeds up and slows down. When the airfoil (the tip of the rotor blade) is traveling near the speed of sound, the airflow will exceed the speed of sound (become supersonic) when it naturally speeds up going across the airfoil.

35
Q

What is the critical Mach number? What happens if it exceeded?

A

Critical Mach is the highest airspeed that can be attained without reaching supersonic flow. If it is exceeded, the flow will become supersonic.

36
Q

What forms when airflow slows from supersonic to subsonic?

A

As airflow slows from supersonic to subsonic airspeed, a shockwave is formed.

37
Q

What happens to the aerodynamic center of the airfoil when the shockwave moves aft? What does this cause the blade to do?

A

The aerodynamic center of pressure moves away from its normal location and moves aft. This causes the leading edge to be deflected downward, which may result in structural failure of the blade.

38
Q

What indications would be experienced during compressibility?

A

Rotor roughness, vibration, cyclic shake, nose pitch down, and right roll.

39
Q

What are conditions conducive to compressibility?

A

1) High Airspeed
2) High Rotor RPM
3) High Gross Weight
4) High DA
5) High G-Maneuvers
6) Low Temperature (the speed of sound decreases as temp decreases)
7) Turbulent Air

40
Q

How is compressibility different from retreating blade stall?

A

Retreating blade stall occurs with Low RPM.

41
Q

How do you recover from compressibility effects?

A

1) Reduce blade pitch (lower collective)
2) Reduce Rotor RPM
3) Reduce the severity of the maneuver
4) Reduce airspeed

42
Q

What is another name for Settling with Power?

A

Vortex Ring State

43
Q

Describe what happens during Settling with Power.

A

Settling with power is a condition in which the helicopter settles in its own downwash. The helicopter normally produces vortices around the blade tips, however when the helicopter starts a near vertical descent, the upward flow of air creates vortices starting at the hub and moving outward as well. These vortices reduce the rotor disc surface area that is creating lift, eventually not producing enough lift to keep the helicopter from descending.

44
Q

What 3 things must be present for Settling with Power to occur?

A

1) Vertical or near vertical descent of at least 300 FPM. Actual critical rate depends on gross weight, rotor RPM, DA, and other pertinent factors.
2) Slow forward airspeed (less than ETL).
3) Rotor system using between 20% to 100% of the available engine power with insufficient power to arrest the descent. Low rotor RPM can aggravate the situation.

45
Q

Describe the induced flow during Settling with Power?

A

During normal hovering flight induced flow velocity is highest at the rotor tips and lowest at the center of the disk. During Settling with Power, the induced flow starts becoming upward rather than downward starting at the center of the disk and moving outward.

46
Q

What are conditions conducive to Settling with Power?

A

1) Steep approach at a high rate of descent
2) Downwind approach
3) Formation flight approach (where SWP could be caused by turbulence of preceding aircraft)
4) Hovering above the max hover ceiling
5) Not maintaining a constant altitude control during an OGE hover
6) During masking/unmasking

47
Q

How could you prevent Settling with Power?

A

Descend on flight paths shallower than about 30 degrees.

48
Q

How can you recover from Settling with Power?

A

During the initial stage (when large amounts of power are available), a large application of collective pitch may arrest the descent. Otherwise, apply cyclic in any direction to gain airspeed and arrest the upward induced flow. If altitude permits, lower the collective.

49
Q

What is Transverse Flow Effect?

A

Basically, when in forward flight, the air passing over the rear portion of the disk has a greater downwash angle than the air passing through the forward portion. This creates more lift on the forward portion of the disk than the rear portion of the disk.

50
Q

Describe the airflow during Transverse Flow.

A

In the forward portion of the disk, the air is more horizontal, and in the rear portion of the disk, the air is more vertical.

51
Q

How is the angle of attack different on the forward and aft portion of the disk during Transverse Flow? Why is it different?

A

On the forward portion of the disk, the AOA is increased (compared to the rear) because of the more horizontal flow of air, which increases lift. In the rear portion of the disk, the AOA is decreased (compared to the front) because of the more vertical flow of air, which decreases lift.

52
Q

What airspeed would you experience Transverse Flow?

A

Between 10 and 20 knots.

53
Q

What does Transverse Flow cause the aircraft to do?

A

It causes the helicopter to have a right rolling motion.

54
Q

What is an autorotation?

A

When the engine is disengaged from the rotor system, and the upward flow of air through the rotor system drives the rotor.

55
Q

What are the 3 blade region associated with autorotation and where are they located?

A

Stall Region - Inboard 25%
Driving Region - In the middle 25-70%
Driver Region - Approximately the outer 30%

56
Q

What can cause the size of the blade regions to vary during autorotation?

A

Blade pitch setting, rate of descent, and rotor RPM.

57
Q

What blade region(s) during autorotation is/are responsible for thrust/acceleration? Drag/deceleration?

A

The driving region is responsible for acceleration and thrust of the blade, while the driven region creates lift which slows the rotation of the blade, and creates drag.

58
Q

What is ETL?

A

ETL is an increase in efficiency of the rotor blades once the helicopter outruns the circulation of old vortexes and begins to work in relatively undisturbed air.

59
Q

What airspeed does ETL typically occur?

A

Between 16 and 24 knots.

60
Q

What happens to induced flow as ETL is experienced? Drag? AOA?

A

The flow of air through the rotor system becomes more horizontal as the helicopter moves forward which decreases induced flow, as well as induced drag. AOA is subsequently increased.

61
Q

What is experienced in the aircraft during ETL?

A

The nose pitches up and the aircraft rolls right.

62
Q

What other 3 aerodynamic phenomenon cause the effects of a right roll and nose pitch up on the aircraft?

A

The combined effects of dissymetry of lift, gyroscopic precession, and transverse flow effect cause this tendency.

APART Study Guide (7 decks)

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