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421 Advanced Aero > Exam 1 > Flashcards

Exam 1 Flashcards

1
Q

Describe the shape/thickness of LOW SPEED airfoils

A
  • Leading edge is thick, rounded and blunt
  • Trailing edge is thin and sharp
  • Greater camber on top of wing
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2
Q

The Area of Influence refers to:

A

The area AHEAD of, around and behind the airfoil where pressure waves affect airflow

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3
Q

As air pressure decreases, the temperature and density ____

A

decrease

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4
Q

How is static pressure applied to the airfoil?

A

Static pressure vectors will always act PERPENDICULARLY to the solid (airfoil)

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5
Q

When airflow is accelerated, STATIC pressure ______

A

decreases

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6
Q

When airflow is accelerated, DYNAMIC pressure ______

A

increases

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7
Q

The lift vector is always perpendicular to:

A

The CAMBER of the wing

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8
Q

What happens to drag if the speed is doubled?

A

Drag increases by the square of speed

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9
Q

If the surface area is cut in half, what happens to the drag?

A

Drag decreases in half, as well.

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10
Q

What causes wing vortex drag?

A

Lift/Drag

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11
Q

What is the actual measurement of a super sonic shock wave?

A

1/10,000th of an inch

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12
Q

Drag is always _____ to RW, and Lift is always _____

A

Drag is Parallel

Lift is Perpendicular

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13
Q

Where is lift anchored along the chord of a SUBsonic airfoil?

A

25% chord

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14
Q

Total Lift is always ______ to the RW/Flight Path

A

Perpendicular

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15
Q

The stagnation point is a region of high ____ pressure

A

STATIC

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16
Q

What is the Adverse Pressure Gradient

A
  • Where air molecules begin to flow from Low to High pressure (vs the natural way H to L)
  • 1/2mV^2 gives them enough velocity to bust through the higher pressure at the Adverse Pressure Gradient
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17
Q

The difference in chambers on the top and bottom of an airfoil causes what phenomenon?

A
  • Air molecules going over the top of the wing will travel faster, creating lower static pressure
  • Air molecules traveling under the wing will go slower, creating higher static pressure
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18
Q

The speed at which 1 air molecule will go supersonic (meaning a/c is entering transonic flight) is known as:

A

Mcrit (the critical mach number)

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19
Q

Does temperature affect the speed of sound?

A

Yes, as temperature increases the speed of sound increases.

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20
Q

True or False: Sound waves are Pressure Waves.

A

TRUE

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21
Q

What is the number one thing that affects the speed that sound waves travel?

A

TEMPERATURE

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22
Q

The ___ Mach is read on the dial in the cockpit and used for Mcrit

A

Local Mach (vs Flight Mach)

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23
Q

What is a shock wave?

A
  • A sharp change in pressure caused by an object moving faster than the speed of sound
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24
Q

What is a bow wave?

A

The bunching up of sub-sonic air out in front of the wing!

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25
Q

In regards to a shock wave, when supersonic air pushes through to subsonic, the ____ and ____ _____ INCREASE

A

Temperature and STATIC pressure increase

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26
Q

What is Transonic flight? Give an example of a Transonic speed.

A
  • When there is a mix of subsonic and supersonic airflow acting on the airfoil
  • Mcrit (~M1.3)
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27
Q

True or False: You will encounter Compressability issues below Mcrit.

A

FALSE

- Compressability issues will only occur in Transonic flight (speeds at or above Mcrit)

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28
Q

True or False: Airflow behind/past the Bow Wave is ALWAYS subsonic

A

TRUE (think of picture)

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29
Q

What are the Mach ranges for Sub, Trans, Super and Hypersonic flight?

A

Subsonic: 0 - Mcrit (~0.75)
Transonic: Mcrit - 1.2
Supersonic: 1.2 - 5.0 (low super 1.2-3.0)
Hypersonic: 5.0+

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30
Q

In subsonic flight, what are the negative effects of compressability?

A

None, compressabilty does not exist at SUBsonic speeds.

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31
Q

What is the reason Supersonic flight is divided into Low-Super and High-Supersonic?

A

Because at Mach 3.0 THERMODYNAMICS come into play

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32
Q

True or False: The speed of sound is constant.

A

FALSE

- Temperature causes the speed of sound to vary

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33
Q

____ is Parallel to the Relative Wind

A

Drag

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34
Q

____ distribution changes with AOA, and is always Perpendicular to the Relative Wind

A

Lift

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35
Q

For subsonic airfoils, the Center of Pressure is located at ____ chord and the origin of the vector (aerodynamic force) remains constant there

A

25 percent chord

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36
Q

What is Bernoulli’s part in airflow? (Over a wing)

A

The velocity distribution over the wing (faster->slower) and beneath the wing (slowest) creates a downward deflection of airflow

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37
Q

What is Newton’s part in airflow? (Over a wing)

A
  • Equal and opposite reactions law
  • When upwash (positive) is out ahead of wing, then transitions to downwash, slicing down through the airfoil around 25% chord and ending beneath/behind (negative)
  • Downward deflection of airflow = Upward deflection of the airFOIL
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38
Q

What kind of airflow is incompressible and the density does not change?

A

Subsonic Air

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39
Q

Dynamic pressure + Static Pressure = ________

A

Total Pressure

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40
Q

Total Energy is constant but will change forms. The most common change will be from _____ Energy to _____ Energy during shockwave formations

A

Kinetic Energy -> THERMAL Energy

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41
Q

Explain the incorrect “Fred and Barney” lift theory.

A

The distance traveled across the upper chamber is greater, therefore air on top is moving faster in order to “meet back up” with air traveling across the bottom of the wing

42
Q

Explain the incorrect “Venturi” lift theory.

A

The airfoil and airstream act as a Venturi that CONSTRICTS the flow ABOVE the wing…but not the airflow below

43
Q

List the ranges for the different sonic speeds

A 
Subsonic: 0 - Mcrit (.75)
Transonic: Mcrit (.75) - 1.2
Supersonic Low: 1.2 - 3.0
Supersonic High: 3.0 - 5.0
Hypersonic: Faster than Mach 5.0
44
Q

As temperature increases, the speed of sound:

A

Increases!

45
Q

As air flows across a shockwave, what happens to the temperature, static pressure and velocity of the air?

A
  • Temperature increases
  • Static Pressure increases
  • Velocity decreases
46
Q

True or False: Airflow in-front of a Normal Shockwave is ALWAYS supersonic

A

TRUE

47
Q

True or False: Airflow behind a Normal Shockwave is ALWAYS subsonic

A

TRUE

48
Q

What are the compressibility affects in Transonic flight? (5)

A
  • They are all Negative
    1. Loss of trim/rudder effectiveness
    2. Mach Tuck
    3. Control Reversal
    4. Buffeting/buzz
    5. Aeroelastic Twisting
49
Q

The Adverse Pressure Gradient is an area of high _____ Pressure

A

Static pressure

50
Q

How do we get air molecules to flow from Low to High Static Pressure?

A

MOMENTUM

51
Q

Expansion waves ______ airflow

A

Increase

52
Q

Why is airflow affected way out in front of the airfoil? Is it affected in front at all speeds?

A
  • Pressure Waves (in subsonic flight)

- In Transonic flight shockwaves will cut of Pressure Waves when formed on the airfoil

53
Q

Mcrit varies inversely with _____ and ______

A
  • Gross Weight

- Angle of Bank

54
Q

Is it better to have a Low or High Mcrit?

A

Increased Mcrit = good

Decreased Mcrit = bad

55
Q

What are the 3 kinds of shock waves?

A
  1. Normal
  2. Expansion
  3. Oblique “attached” and “detached” to leading edge due to sharp or blunt shape
56
Q

Name the 3 main characteristics of Normal Shock-waves?

A
  1. Perpendicular to airflow
  2. Flow direction does not change
  3. Airflow behind is ALWAYS subsonic
57
Q

Draw .95 mach airfoil and the airflow over it.

A

Draw it!

- Top shock-wave cannot attach to wing because at .95 the disturbance is so strong

58
Q

For academic purposes, at .95 Mach (high transonic) is the shock-wave attached or detached from the wing? Why?

A

Detached due to the disturbed airflow

59
Q

At .82 Mach is the shock-wave attached or detached from the wing?

A

Attached

60
Q

At Mach 1.0 where is the disturbed airflow located? Why?

A
  • Back behind the wing

- Oblique shock-waves have made it all the way past the airfoil

61
Q

Bow wave is approx. 90 degrees with subsonic airflow behind it at ____ Mach

A

1.02

62
Q

Pure supersonic flow is achieved at ____ Mach

A

1.3

63
Q

How can you avoid the negative affects of a forward stagnation region/point?

A

Have a sharp/pointed leading edge (vs a blunt/rounded leading edge)

64
Q

Is going from Subsonic to Supersonic a big deal?

A

No, it is a smooth transition

65
Q

Is going from Supersonic to Subsonic a big deal? Why?

A

Yes, there are LARGE changes is pressure, temperature and density across the shock-wave

66
Q

What are 2 “big” difference with Oblique Shock-waves that differ from Normal Shock-waves?

A

With Oblique:

  1. Slows airflow down
  2. Changes direction of the airflow
67
Q

Who cares about Mcrit?

A

Engineers

68
Q

Who cares about Mmo? (max operating speed)

A

Pilots

69
Q

Why should pilots care about Mmo?

A
  • It is negative to a/c controllability

- Compressability issues arise

70
Q

What 4 main affects do Shock-waves have on airflow?

A
  1. Increased Density and Static Pressure
  2. Increased Temperature
  3. Decreased Velocity
  4. Decreased Kinetic Energy
71
Q

How is Wave Drag impacted with airspeed?

A
  • At Mcrit drag levels increase

- At Mach 1.0 drag levels decrease

72
Q

What are “reversible” flight controls?

A

Controls with cables and pulleys

73
Q

What are “irreversible” flight controls?

A

Hydraulically operated controls

74
Q

Control reversal is due to _____

A

Wing-twisting

75
Q

Name 6 Compressability issues

A
  1. Uncontrollable aileron roll
  2. Aileron buzz
  3. Wings twisting
  4. Control reversal
  5. Air-frame buffeting
  6. Mach Tuck
76
Q

What does Mach Tuck result in? Why?

A
  • Causes nose of a/c to pitch down uncontrollably because the aerodynamic center of lift slides aft
  • This causes a pith down because the horizontal stabilizer is in the wash of aerodynamically dead air
77
Q

What is a Mach Tuck solution?

A

A trimable horizontal stabilizer!

78
Q

1 degree of Stabilator movement is equal to how many degrees of Elevator movement?

A

4 degrees of Elevator movement

79
Q

How will the Buffet Boundary (altitude you can fly) be affected as a/c gross weight INCREASES?

A

Buffet boundary will decrease with increased weight

80
Q

What is Area Rule? What did it result in?

A
  • Method invented by Whitcomb to deal with TOTAL DRAG
  • Looks at a/c GLOBALLY (as one whole airfoil)
  • Coke bottle shape fuselage smoothing a/c front to back
  • F-102 was the 1st prototype
81
Q

How do we determine the shape of the Mach Cone?

A
  • The speed and shape of the object

ex: sharper edge = smaller cone

82
Q

What are 2 advantages of Swept Wings?

A
  1. Delays the onset of Mcrit (transonic flight) meaning you can get by with a low-speed airfoil if wing is kept inside the shock cone
  2. Allows higher cruise speed bc of #1
83
Q

True or False: Thicker airfoils are stronger.

A

TRUE

84
Q

What are the advantages of having a low-speed (subsonic) airfoil? (5)

A
  • stronger and can hold more gas*
    1. Allows a larger aspect ratio a/c
    2. Allows Hi lift devices (to aid low speed flight)
    3. Lower TO/LD speeds
    4. Lower TO/LD distances
    5. Allows use of spoilers (to aid in low speed maneuvers)
85
Q

What are 2 advantages of Super-critical Wings?

A
  1. Delays onset of Mcrit

2. Shock-waves are weaker

86
Q

What is Vmo

A
  • The IAS for max operating speed
  • Low Altitudes
  • Has to do with STRUCTURAL limits of a/c
87
Q

What is Mmo

A
  • Mach Number for max operating speed
  • High Altitudes
  • Concerned with aerodynamic compressability affects
88
Q

What does Coffin Corner refer to?

A

The distance between the low speed buffet and (Mmo) high speed buffet (gets smaller and smaller as you go up in altitude)

89
Q

True for False: Full supersonic flight is way more stable

A

True!!

90
Q

If supersonic flight is more stable, why do commercial airliners not fly at those speeds?

A
  • Its nore expensive because drag is increased, which increases fuel burn!!
91
Q

What are 2 ideal supersonic airfoils?

A
  • Biconvex
  • Double Wedge
    (both have sharp leading edges)
92
Q

What are characteristics of airflow when it goes AROUND a corner?

A

Supersonic Airflow:

  • Accelerates
  • Creates “Expansion Fans”
  • NO disturbed airflow (compressability = good)

Subsonic Airflow:
- Becomes disturbed

93
Q

What are characteristics of airflow when it goes INTO a corner?

A

Supersonic Airflow:

- NO disturbed airflow

94
Q

4 characteristics of a Blunt Leading Edge (know for exam)

A
  1. Large forward stagnation point
  2. Large area of slow moving air
  3. Bow wave cannot attach to wing
  4. High drag (no desirable)
95
Q

3 characteristics of a Sharp Leading Edge (know for exam)

A
  1. Virtually no forward stagnation point
  2. Bow wave CAN attach to wing
  3. Much lower drag (desirable)
96
Q

Where is the center of lift located on SUPERsonic airfoils?

A

At 50% chord

97
Q

On supersonic airfoils with a slight AOA, identify:

  • Weak shockwave
  • Strong shockwave
  • Area of fastest airflow/lowest pressure
  • Area of slowest airflow/highest pressure
A

Top-left: Weak shockwave/fast airflow
Top-right: FASTEST airflow/LOWEST pressure
Bottom-left: Strong shockwave/HIGHEST pressure
Bottom-right: Slowed airflow because of stronger shockwave out infront

98
Q

What is the point of Supersonic Diffusers?

A
  • Slows airflow to SUBsonic speeds before it hits the compressors on a gas turbine engine
  • Design forces airflow to produce an oblique shock wave, or several weak shock waves
99
Q

Does airflow have to be subsonic before it hits the compressor?

A

Yes!

100
Q

Do RAM jets require a compressor/turbine?

A

No!

421 Advanced Aero (1 decks)

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