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Senior Design > CH2: Basic Aerodynamics > Flashcards

CH2: Basic Aerodynamics Flashcards

1
Q

Define “skin friction” drag.

A

The drag on a body resulting from viscous shearing stresses over its wetted surface.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 37

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2
Q

Define “pressure (or form)” drag.

A

The drag on a body resulting from the integrated effect of the static pressure acting normal to its surface resolved in the drag direction.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 37

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3
Q

Define “profile drag.”

A

This type of drag (is usually) defined as the sum of the skin friction drag and the pressure drag for a two-dimensional airfoil.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 37

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4
Q

Define “viscous drag due to lift.”

A

The drag that results due to the integrated effect of the static pressure acting normal to its surface (resolved in the drag direction) when the airfoil angle-of-attack is increased to generate lift. (Note: It is present without vortices.)

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 37

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5
Q

Define inviscid drag due to lift.

A

The drag resulting from the influence of a trailing vortex (downstream of a lifting surface of finite aspect ratio) on the wing aerodynamic center.

Note: It is present with or without viscosity

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 37

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6
Q

Define “interference drag.”

A

The increment in drag resulting from bringing two bodies into proximity to each other; for example, the total drag of a wing–fuselage combination will usually be greater than the sum of the wing drag and fuselage drag independent of one another.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 37

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7
Q

Define “trim drag.”

A

The increment in drag resulting from the aerodynamic forces required to trim the aircraft about its center of gravity; usually this takes the form of added drag-due-to-lift on the horizontal tail.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 37

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8
Q

Define “base drag.”

A

The specific contribution to the pressure drag attributed to a blunt afterbody.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 38

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9
Q

Define “wave drag.”

A

Only exists with supersonic flow; this is a pressure drag resulting from non-canceling static pressure components on either side of a shock wave acting on the surface of the body from which the wave is emanating.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 38

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10
Q

Define excrescence drag (a.k.a, protuberance drag)

A

The drag associated with antennas, total pressure probes (part of the air data system), and other protrusions above the aircraft’s exterior. External fuel tanks, missiles, and bombs are also considered excrescence drag items.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 38

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11
Q

Define “cooling drag.”

A

The drag resulting from the momentum lost by the air that passes through the powerplant installation (i.e., the heat exchanger) for cooling the engine, oil, and so on.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 38

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12
Q

Define “ram drag.”

A

The drag resulting from the momentum lost by the air as it slows down to enter an inlet.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 38

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13
Q

Air molecules flowing over a body in layers are called _______.

A

Streamlines.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 38

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14
Q

What is the characteristic behavior of a separated boundary layer?

A

The flow near the surface can reverse direction and flow upstream.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 38

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15
Q

True or False
The velocity gradient at the surface is smaller for the laminar boundary layer than for the turbulent boundary layer.

A

True

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 38

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16
Q

Whether a boundary layer is laminar or turbulent is dependent upon what parameter?

A

The Reynolds number.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 38

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17
Q

The Reynolds number is a ratio of what two phenomena?

A

The inertial forces in the boundary layer to the viscous forces.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 38

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18
Q

For what value of the Reynolds number does the flow transition frame laminar to turbulent?

A

~5E10

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 39

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19
Q

From incompressible theory, what is the value of the lift curve slope per radian for a subsonic airfoil?

A

2pi / radian

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 40

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20
Q

From incompressible flow theory, where is the aerodynamic center located with respect to the chord of an airfoil?

A

At the quarter chord point.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 40

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21
Q

True or False
From incompressible flow theory, for a thin symmetric airfoil, C_m_ac is constant with respect to the angle of attack, and Cl

A

True

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 40

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22
Q

What is the Prandtl-Glauert transformation?

A

A compressibility correction. It allows incompressible properties to be approximately converted to compressible values.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 43
Modern Compressible Flow, Anderson, Pg 329

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23
Q

The effect of compressibility is to ______ the section lift curve slope.

A

Increase

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 43

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24
Q

True or False
A swept wing aircraft will be required to take off and land at higher angles of attack than a straight wing aircraft of the same aspect ratio.

A

True. Swept wings have a lower CL_alpha

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 45

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25
Q

What is the characteristic feature of flow past a slender body of low aspect ratio?

A

Strong cross flow leads to the separation of the flow at the sides of the body or the wing edges. This forms free vortices on the upper surface (see Fig. 2.12 on page 47).

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 46

26
Q

For slender bodies of low aspect ratio, why is there a nonlinear relationship between lift, pitching moment, and angle of attack?

A

The formation of free vortices on the upper surface of the wing/body.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 46

27
Q

What is the critical Mach number?

A

The freestream Mach number at which the sonic condition first appears on the airfoil.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 54
Modern Compressible Flow, Anderson, Pg. 345

28
Q

True or False
One of the principal effects of the normal shock wave is to produce a large decrease in the static pressure of the airstream behind the wave.

A

False. It creates a large increase in the static pressure.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 54

29
Q

What is the “force divergence Mach number?”

A

The Mach number that produces a large increase in the drag coefficient (drag divergence) when an aircraft is transitioning from subsonic to supersonic.

Note 1: This is not the same thing as the “Critical Mach Number.”
Note 2: The book does not specify, but it is assumed that it is a freestream Mach number.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 55

30
Q

How can the critical Mach number be increased?

A
  1. Decreasing the wing thickness ratio.
  2. Increasing the leading edge sweep
  3. Decreasing the aspect ratio
  4. Using a supercritical airfoil

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 56

31
Q

Explain how swept wings increase the critical Mach number.

A

For swept wings, there are two components of velocity. One parallel to the leading edge, which contributes nothing to the magnitude of the pressure distribution, and one perpendicular to the leading edge, which does contribute. Since the perpendicular flow component is less than the free stream velocity, the pressure distribution is effectively reduced, which increases the critical Mach number.

Note: This is why a large sweep angle is more effective at increasing the critical Mach number than a small angle.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 57

32
Q

True or False
Sweepback wings help to delay the onset of compressibility effects however they have no effect on the aerodynamic coefficients (e.g., Cl, Cd, etc.)

A

False. Sweepback will help to reduce the magnitude of the changes in the aerodynamic coefficients when transitioning from incompressible to compressible flow regimes.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 57

33
Q

True or False
Small sweep angles are more effective than larger ones.

A

False. If wing sweep is to be used, large angles (35-45 degrees) are ideal.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 58

34
Q

What is the primary disadvantage of using swept wings?

A

The lift curve slope is decreased.

Note: Other cons are a reduction in C_L_Max and tip stall occurring at lower speeds
(Pg. 59).

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 58

35
Q

Describe the shape and function of a supercritical airfoil.

A

The supercritical section has a much flatter shape on the upper surface that reduces both the extent and strength of the normal shock, as well as the adverse pressure rise behind the shock, with corresponding reductions in drag.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 59

36
Q

Why does a supercritical airfoil have a highly cambered trailing edge?

A

To compensate for the reduced curvature on the upper chamber and the corresponding loss of lift.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 59

37
Q

True or False
The supercritical airfoil will have a similar critical Mach number as a conventional airfoil for a given thickness ratio but will delay the divergence Mach number.

A

True

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 59

38
Q

What is the “thrust pinch” with respect to the transonic regime?

A

For many propulsion units, the thrust generated around M = 1.0 grows slower than the drag. This creates a minimum value of (T-D).

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 62

39
Q

Define “Mach wave.”

A

The locus of all wavefronts from an infinitesimal pressure disturbance.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 62

40
Q

An infinitesimal pressure disturbance propagates at what speed?

A

The speed of sound.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 62

41
Q

In general, the pressure distribution over a surface is primarily a function of the flow ______ to the leading edge.

A

Normal.

Fundamentals of Aircraft and Airship Design, Nicolai, Pg 63

42
Q

What is the difference between the center of pressure and the aerodynamic center with respect to the chord line of an airfoil?

A

The center of pressure is the point along the chord about which the resultant moment is zero. It is essentially the centroid of the net pressure distribution over the airfoil.

The aerodynamic center is the point along the chord, about which the change in the resultant moment with respect to the angle of attack is zero.

Mechanics of Flight, W. Phillips, Pg. 5

43
Q

Why can viscous forces be ignored (usually) for high Reynolds number flows?

A

Because the Reynolds number is defined as the ratio of inertial forces to viscous forces. A high Reynolds number implies very small viscous forces relative to inertial forces.

Mechanics of Flight, W. Phillips, Pg. 15

44
Q

For a thin airfoil with no camber in subsonic incompressible flow, the center of pressure lies at the _______.

A

Quarter chord.

Mechanics of Flight, W. Phillips, Pg. 30

45
Q

The deflection of the trailing edge flap changes the effective _______ of the airfoil section.

A

chamber

Mechanics of Flight, W. Phillips, Pg. 39

46
Q

What is the difference between geometric and aerodynamic twist?

A

The geometric twist is defined as a spanwise variation in geometric angle of attack.

The aerodynamic twist is defined as a variation in zero lift angle of attack.

Note: alpha = alpha_induced + alpha_effective

Mechanics of Flight, W. Phillips, Pg. 51

47
Q

True or False
At a given angle of attack, the untwisted elliptic wing produces more lift than any other untwisted wing of the same aspect ratio.

A

True.

Mechanics of Flight, W. Phillips, Pg. 54

48
Q

Washout _________ the zero-lift angle of attack for any wing but the lift slope for a wing of arbitrary platform shape is __________.

A

increases
unaffected

Mechanics of Flight, W. Phillips, Pg. 59

49
Q

True or False
A wing with washout will have zero induced drag when the angle of attack is equal to zero.

A

False. The induced drag at zero lift for a twisted wing cannot be zero.

Mechanics of Flight, W. Phillips, Pg. 59

50
Q

True or False
For unswept wings, the aerodynamic center of the wing is invariant with the span coordinate.

A

True.

Mechanics of Flight, W. Phillips, Pg. 120

51
Q

Where does the center of pressure and aerodynamic center lie with respect to the chord line for a thin supersonic airfoil?

A

The 1/2 chord point.

Mechanics of Flight, W. Phillips, Pg. 145

52
Q

True or False
Most Delta wings have a lift coefficient of zero (or very close to it) at zero degrees angle of attack.

A

True.

Fundamentals of Aerodynamics, Anderson, Pg. 482

53
Q

For a delta-type wing, which of the following statements are true:
1. The lift curve slope is small (~0.05/degree)
2. Lift is still obtained at very high angles of attack (stall is usually around 35 degrees).

A

Both statements are true.

Fundamentals of Aerodynamics, Anderson, Pg. 482

54
Q

What effect do the large tip vortices over the upper surface have on the static pressure for a delta-type wing?

A

The vortex acts to lower the static pressure and increase the total lift.

Wikipedia: https://en.wikipedia.org/wiki/Concorde#Slender_deltas

55
Q

What is the benefit of having an aircraft that approximates a Sears-Haack body?

A

The Sears–Haack body is the shape with the lowest theoretical wave drag in supersonic flow, for a given body length and given volume.

https://en.wikipedia.org/wiki/Sears%E2%80%93Haack_body

56
Q

What is a waverider, and how does it work?

A

A waverider is a hypersonic aircraft design that improves its lift-to-drag ratio by using the shock waves generated by its flight as a lifting surface, a phenomenon known as compression lift.

https://en.wikipedia.org/wiki/Waverider

57
Q

True or False
For most aircraft, the higher the value of CLmax the lower the stalling speed.

A

True

Fundamentals of Aerodynamics, Anderson, Pg. 329

58
Q

True or False
The quarter chord is the center of pressure for a thin chambered airfoil immersed in subsonic ideal flow.

A

False.

Extra Notes: However, the quarter chord point is the theoretical location of the aerodynamic center for a chambered airfoil.

Fundamentals of Aerodynamics, Anderson, Pg. 360

59
Q

Downwash is constant over the wing span for an __________ lift distribution.

A

Elliptical.

Fundamentals of Aerodynamics, Anderson, Pg. 443

60
Q

For an elliptic lift distribution, the plan form must also be _____.

A

Elliptical.

Fundamentals of Aerodynamics, Anderson, Pg. 446

61
Q

The minimum induced drag corresponds to an ________ lift distribution.

A

Elliptic.

Fundamentals of Aerodynamics, Anderson, Pg. 449

62
Q

True or False
The lift-to-drag ratio of a delta wing with sharp leading edges will typically be lower than a conventional wing platform in the subsonic regime.

A

True.

Fundamentals of Aerodynamics, Anderson, Pg. 484

Senior Design (13 decks)

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