Part 4 Flashcards
(99 cards)
1
Q
What is the aim of the lesson?
A
To introduce the basic concepts of an aerofoil
2
Q
List the objectives of the aerofoil lesson.
A
- Define geometric characteristics of an aerofoil
- Understand how differences in aerofoil shape influence performance
- Define lift, drag, moment, and pressure coefficients
3
Q
What is the chord line of an aerofoil?
A
A straight line connecting the leading and trailing edges of the aerofoil (length, c)
4
Q
Define the mean camber line of an aerofoil.
A
A line drawn halfway between the upper and lower surfaces of the aerofoil
5
Q
What does the thickness of an aerofoil refer to?
A
A straight line connecting the upper and lower surfaces of the aerofoil, which is perpendicular to the chord line (length, t)
6
Q
What is relative wind in the context of an aerofoil?
A
The direction of the free-stream velocity of air far upstream of the aerofoil
7
Q
What is the angle of attack (α)?
A
The angle between the relative wind and chord line
8
Q
Define lift (L) in aerodynamics.
A
The component of the aerodynamic force perpendicular to the relative wind
9
Q
What is drag (D)?
A
The component of the aerodynamic force parallel to the relative wind
10
Q
What factors influence lift (L), drag (D), and moment (M)?
A
- Free-stream velocity, V∞
- Free-stream density, ρ∞
- Size of the aerodynamic surface – typically wing area, S
- Angle of attack, α
- Shape of the aerofoil
- Viscosity, µ∞
- Compressibility of the flow, a∞ (M∞ = V∞/a∞) – for high-speed flow only
11
Q
What is the lift coefficient (cl)?
A
A dimensionless coefficient representing the lift characteristics of an aerofoil
12
Q
What is the drag coefficient (cd)?
A
A dimensionless coefficient representing the drag characteristics of an aerofoil
13
Q
What is the significance of the zero-lift angle of attack (αL=0)?
A
The value of α when lift is zero
14
Q
True or False: For cambered aerofoils, there is still a positive value of cl when α = 0.
A
True
15
Q
What happens to cl at large values of α?
A
The linearity of the lift curve breaks down; cl peaks at some maximum value, cl,max and then drops as α is further increased.
16
Q
Define the pressure coefficient (Cp) around an aerofoil.
A
Cp = (p - p∞) / (0.5 * ρ * V∞²)
17
Q
What is the NACA 4-series in aerofoil design?
A
A series of ‘turbulent’ aerofoils developed in the 1930s
18
Q
What distinguishes the NACA 6-series aerofoils?
A
They are ‘laminar’ aerofoils developed around WWII, designed to maintain a laminar boundary layer over most of the wing section.
19
Q
What is the maximum camber in a NACA four-digit section?
A
Indicated by the first digit, which represents the maximum camber in % chord
20
Q
How is the maximum thickness of a NACA aerofoil represented?
A
By the last two digits in the four-digit code, indicating maximum thickness in % chord
21
Q
What is the Buckingham Pi Theorem?
A
A technique for reducing the number of dimensional variables of a problem into a smaller number of dimensionless groups
22
Q
What does dimensional analysis accomplish?
A
Regroups all variables affecting a problem into dimensionless parameters, reducing the number of experiments needed
23
Q
What are the three fundamental dimensions important in fluid mechanics?
A
- Mass (M)
- Length (L)
- Time (T)
24
Q
What is a dimensionless parameter?
A
A parameter that has no dimensions and is used to capture the properties of a physical system meaningfully
25
True or False: The Reynolds number (Re) is a dimensionless parameter.
True
26
What is the principle of dimensional analysis?
Identifying dimensionless groups that govern the characteristics of a physical phenomenon
27
What is the significance of dimensional homogeneity in equations?
Each additive term in the equation must have the same dimension
28
What is the relationship between drag force (D) and parameters like free stream velocity (U) and density (ρ)?
D depends on U, ρ, viscosity (µ), and diameter (d) of the cylinder
29
How is Buckingham Pi Theorem applied in dimensional analysis?
By expressing the relationship between variables in terms of (n-k) dimensionless parameters
30
What is the role of repeating variables in finding dimensionless parameters?
They are selected to form dimensionless groups that yield the most useful results
31
What are dimensionless parameters in aerodynamics?
Dimensionless parameters are groups of variables that do not have units, allowing for the comparison of different physical phenomena.
32
How many groups of three dependent variables can be selected for dimensionless parameters?
More than one group of three dependent variables can be selected.
33
Which repeating variables produce the most useful results in dimensional analysis?
Density (ρ), velocity (U), and a length scale (d).
34
What is the general rule for selecting repeating variables in aerodynamics?
Often select U, ρ, and a length scale; avoid μ.
35
What is the significance of gravitational acceleration (g) in dimensional analysis?
Gravitational acceleration is often selected as a variable in dimensional analysis.
36
What should the exponents (a, b, c) and (d, e, f) be chosen to ensure in dimensionless parameters?
They should be chosen such that π1 and π2 are dimensionless.
37
What is the relationship between pressure and velocity described by the Bernoulli equation?
p + ½ρV² = constant.
38
What does conservation of mass imply in fluid dynamics?
The mass flow rate must remain constant through a flow field.
39
What is the role of surface pressure distributions in aerodynamics?
Surface pressure distributions help in estimating lift and drag forces on aerofoils.
40
What is gauge pressure?
Gauge pressure is defined as pgauge = pabs - patm.
41
What does a positive gauge pressure indicate?
A positive gauge pressure is indicated by an arrow pointing towards the surface.
42
What is the Euler equation's role in fluid dynamics?
It relates the change in pressure to the change in velocity at a given point.
43
What is the definition of lift force in aerodynamics?
Lift force is proportional to the pressure difference between the upper and lower surfaces of an aerofoil.
44
What happens to the pressure distribution around a symmetrical aerofoil at a positive angle of attack?
Lift increases and the front stagnation point moves anti-clockwise around the aerofoil's surface.
45
What is the non-dimensional pressure coefficient (Cp) formula?
Cp = (p - p∞) / (½ρV∞²).
46
How can lift coefficient (cl) be calculated from pressure coefficient (Cp)?
By integrating the pressure coefficient distribution over the aerofoil surfaces.
47
What are the three ways to describe how an aerofoil produces lift?
* Differences in air pressure on a wing (Bernoulli theory) * Bending air downwards (Newton's 3rd law) * Turning flow to produce lift (Newton's 2nd law).
48
What is the 'equal time' argument in aerodynamics?
It suggests that air molecules take the same time to travel over the upper and lower surfaces of an aerofoil, which is incorrect.
49
What are the limitations of Bernoulli's theorem?
* Flow along a streamline * Inviscid flow * Incompressible flow * Steady flow.
50
According to Newton's 3rd law, what happens when an aerofoil imparts a downward force on the fluid?
The fluid imparts an upward force on the aerofoil, resulting in lift.
51
What does Newton's 2nd law state about forces and acceleration?
A force is produced when a mass is accelerated; changes in velocity (magnitude or direction) result in a force.
52
What is the relationship between pressure differences and curved streamlines?
Pressure differences create centripetal forces necessary for maintaining curved flow.
53
What does turning a flow of air produce?
A lift force in reaction proportional to the amount of air turned downwards
54
What is required for a particle moving along a curved streamline under constant speed?
A centripetal force acting normal to the direction of motion
55
What generates pressure differences in a curved streamline?
The pressure on one side of the particle is greater than the other
56
What happens to pressure in a curved streamline?
There is a pressure gradient across it, with pressure decreasing towards the center of curvature
57
What occurs at point A around a simple aerofoil?
Streamlines are straight and parallel; hence, there is no pressure gradient
58
What is the pressure at point B compared to atmospheric pressure?
pB << patm
59
What generates a resultant pressure force on the aerofoil acting upwards?
The difference in pressure where pB < pD
60
What is the effect of increasing the angle of attack on lift?
More flow curvature is introduced, generating more lift until flow separation occurs
61
What happens when the flow separates from the surface of the wing?
The amount of streamline curvature above the wing is reduced, causing a sharp drop-off in lift force
62
In generating lift, how does the wing airflow behave?
As though a circular flow around the wing has been combined with a linear flow over it
63
What does Bernoulli's principle state about the velocity above and below the wing?
Velocity above the wing is higher than velocity below the wing
64
How is circulation defined in relation to lift?
Circulation is the line integral of the tangential velocity component around a closed path
65
What is the Kutta-Zhukovsky lift theorem?
It states that lift is generated around any shaped body in a uniform stream with circulation
66
What happens to lift when circulation increases?
More lift is generated
67
What are the two largest sources of drag on an aircraft?
* Viscous drag (profile drag) * Lift-induced drag
68
What is lift induced drag produced from?
The wing having a finite span
69
What is the relationship between CDi and CL?
CDi is high when CL is high
70
What is the role of winglets?
Reduce induced drag by improving lift distribution
71
How is induced drag coefficient related to wing aspect ratio (AR)?
CDi is inversely proportional to AR
72
What is the effect of a finite wing on lift curve slope?
It reduces the lift curve slope compared to an infinite wing
73
What is the taper ratio in wing design?
The ratio of tip chord to root chord
74
What does the total vortex system of a lifting wing consist of?
* The starting vortex * The bound vortices * The trailing vortices
75
What is the significance of the starting vortex?
It is responsible for producing the correct conditions for lift
76
What are the Helmholtz Theorems related to vortex motion?
* The strength of a vortex filament is constant along its length * A vortex filament cannot end in a fluid
77
What effect do wing-tip vortices have on airflow?
They induce a small downward component velocity, known as downwash
78
What is the effective angle of attack for a local aerofoil section?
αeff = α - αi
79
What is the relationship between induced drag and lift?
Induced drag is the price to pay for the generation of lift
80
What happens to the starting vortex as steady flight proceeds?
It is soon left behind and can be ignored
81
What is the total vortex system of a lifting wing at steady flight according to Helmholtz Theorems?
The total vortex system satisfies the following theorems:
* The strength of a vortex filament is constant along its length.
* A vortex filament cannot end in a fluid; it must either extend to the boundaries of the fluid or form a closed path.
## Footnote
These theorems are fundamental in understanding vortex dynamics in fluid mechanics.
82
What is the horseshoe vortex?
The remaining three-sided vortex after the starting vortex is called the horseshoe vortex.
## Footnote
It is essential for investigating wing effects in close proximity to the wing.
83
What does the Biot-Savart law calculate?
The Biot-Savart law is a mathematical tool for calculating the induced velocity (downwash) produced by a wing-tip vortex.
## Footnote
It involves integrating the effects of an elemental length of a vortex.
84
What is the velocity at point P induced by the elemental length dl in the Biot-Savart law?
The velocity at P induced by the elemental length dl is given by:
* dv = (Γ/(4π)) * (sin(α)sin(β)/r)
## Footnote
Here, Γ represents circulation, and r is the distance from the vortex element to point P.
85
What happens to the induced velocity for a semi-infinite vortex?
For a semi-infinite vortex (like a trailing vortex), the induced velocity is given by:
* hv = (Γ/(2π))
## Footnote
This condition arises when α = π/2 and β = 0.
86
What is the effect of formation flying on induced drag?
In formation flying, each aircraft experiences upward components (upwash) induced by other aircraft, reducing its own drag.
## Footnote
This phenomenon is known as 'vortex surfing'.
87
How much fuel can airliners save by using vortex surfing in formation flying?
Airliners can save up to 15 percent of fuel used by flying in a V-shape to take advantage of 'vortex surfing'.
## Footnote
This technique helps to reduce drag over long distances.
88
What is induced drag and how is it calculated?
Induced drag, Di, is calculated as:
* Di = (e * AR * CL^2) / (π)
## Footnote
It is produced by the wing-tip vortices and is the drag incurred for generating lift.
89
What factors can reduce induced drag?
Methods to reduce induced drag include:
* Using an elliptical wing planform
* Increasing aspect ratio (AR)
* Adding winglets to wing-tips
## Footnote
Each method has its challenges and trade-offs.
90
What are the main types of low-speed aerofoil stall?
The main types of low-speed aerofoil stall include:
* Trailing edge stall
* Leading edge stall
* Thin aerofoil stall
## Footnote
Each type varies in how stall develops with increasing angle of attack.
91
What is boundary layer separation?
Boundary layer separation is the process where fluid elements near a solid surface decelerate and break away from the surface.
## Footnote
This process is highly dependent on the pressure gradient of the flow.
92
What is a favourable pressure gradient?
A favourable pressure gradient occurs when static pressure decreases in the downstream direction (dp/dx is negative).
## Footnote
An example is flow in a wind tunnel contraction.
93
What is an adverse pressure gradient?
An adverse pressure gradient occurs when static pressure increases in the downstream direction (dp/dx is positive).
## Footnote
An example is flow in a diffuser.
94
What is the critical angle of attack?
The critical angle of attack is the angle at which stall occurs, leading to a decrease in lift.
## Footnote
At this angle, flow separation on the wing surface becomes dominant.
95
How can the maximum lift coefficient (cl,max) be increased?
Methods to increase cl,max include:
* Increasing aerofoil camber
* Adding leading edge slats and/or trailing edge flaps
* Using flow control technology
## Footnote
These methods are crucial for enhancing aircraft performance.
96
What is the slat effect in increasing lift coefficient?
The slat effect protects the leading edge of the main element from flow separation, reducing the adverse pressure gradient.
## Footnote
This effect helps to maintain lift at higher angles of attack.
97
What is the flap effect in increasing lift coefficient?
The flap effect increases the velocity on the upper and lower surfaces at the trailing edge, reducing the adverse pressure gradient and increasing circulation.
## Footnote
This results in additional lift.
98
What is the Coanda effect?
The Coanda effect is the phenomenon where a jet flow attaches itself to a nearby surface and remains attached even when the surface curves away from the initial jet direction.
## Footnote
This effect is utilized in blown flaps for flow control.
99
What are vortex generating devices used for?
Vortex generating devices generate streamwise vortices that entrain faster moving fluid into the boundary layer, thereby re-energizing it.
## Footnote
They are essential for controlling flow separation.
