Aerodynamics Flashcards
What is aerodynamics?
The study of the properties of moving air and the interaction between the air and the solid bodies moving through it
What is the continuity equation?
As speed increases, the cross sectional area decreases so long as density remains constant
What is the relationship between the velocity and the cross-sectional area in the continuity equation?
They are inversely proportional to one another
Give the continuity equation formula
A1V1 = A2V2
What is the property of air in the subsonic region?
Incompressible
Why is air incompressible in the subsonic region?
The flying body does not attain speeds necessary to compress the air
If there is no disturbance, air streamlines are what to one another?
Parallel to each other
When the diameter increases and speed decreases, what type of outlet is this classed as?
A diffuser outlet
What is a jet outlet?
An outlet where the diameter decreases and the speed increases
What does Bernoulli’s Principle state?
Total Pressure = Static Pressure + Dynamic Pressure
Total Pressure is always constant
If the valve is closed, describe static, dynamic and total pressure
Static pressure is at its maximum
Dynamic pressure is 0
Total pressure is at its maximum
As a valve starts to open, describe what happens in relation to pressure
Static pressure decreases as dynamic pressure increases
Total pressure remains at its maximum
What is the Point of Stagnation?
The point on the leading edge where dynamic pressure is 0 or the total pressure is equal to the static pressure
How do you calculate the total pressure?
Total pressure = Static Pressure + Dynamic Pressure
How is lift produced?
Higher pressure below the wing
Lower pressure above the wing
The difference in static pressure acts on the surfaces of the wing to create a lifting force
What is up-wash?
The upward flow of air directly ahead of the Leading Edge
What is down-wash?
The downward flow of air directly ahead of the Trailing Edge
What is the Magnus Effect?
Mechanically induced circulation
Describe the pressure waves using the Magnus effect when the cylinder is stationary
Static pressure on the upper surface of the cylinder is equal to the static pressure on the lower surface
No differential pressure means no lift
Describe the pressure waves using the Magnus Effect on a rotating cylinder
The circulatory flow causes an increase in local speed on the upper surface and a decrease in local speed on the lower surface
Difference in pressure generates lift
If the cylinder rotates at a higher speed, what happens to the pressure?
Decrease
What relation does speed have to pressure?
They are inversely proportional
What is the general rule for vortices?
They come in a pair, clockwise and anti-clockwise
What is a wing profile?
A cross-section of a wing
What line connects the leading edge to the trailing edge?
Chord line
Describe the mean camber line
The line drawn halfway between the upper and lower surfaces of the profile
What is the displacement of the mean camber line from the chord line called?
Camber
What is a maximum camber and where is it located in a typical low speed profile?
5% of the chord line
45% aft of the Leading Edge
What is the fineness ratio?
The ratio of the length to diameter on a wing profile
Or
Maximum thickness as a fraction
What is the maximum thickness of a typical low speed profile and where is it located?
18% of the chord
30% aft of the Leading Edge
What is the Flight Path Velocity?
The speed of the aircraft in a certain direction through the air
What is relative wind?
The speed and direction of the air acting against the aircraft which is passing through it
What does the relative wind depend on?
The flight velocity therefore it is not always horizontal
E.g banking
What symbol is used for the angle of attack?
Alpha
What is the angle of attack?
The angle between the chord line and the relative wind
What is the angle of incidence?
The angle between the chord line and the longitudinal axis of the aircraft
What is the symbol used for the angle of incidence?
Gamma
What is the wing area?
The projection of the outline on the plane of the chord
What does the wing area include?
The fuselage between the wings
How do you calculate the wing area?
Wing span x average chord
b x c
What is the taper ratio?
The ratio between the tip chord and the root chord
How do you calculate he taper ratio?
Tip chord / root chord
What symbol is given to the taper ratio?
Lambda
What is the average chord?
The geometric average of all the chords
What is the aspect ratio?
The ratio between the wing span and the average chord
How do you calculate the aspect ratio?
Wing span / average chord
b / c
What is another way to calculate the aspect ratio?
AR = b^2/A
What is the meaning of a high aspect ratio?
It has longer and narrower wings
What is the angle between the line 25% chord and a line perpendicular to the root chord known as?
Sweep angle
What is dihedral?
The increase of the angle attack for lateral stability
What is a positive dihedral wing?
When the wing tips are higher than the wing base
What is a negative dihedral wing known as and what are they?
Anhedral
When the wing tips are below the wing base
What is the span wise flow?
The air flow that moves from the leading edge to the trailing edge
Airflow flowing what to the chord line accelerates?
Parallel
What are some disadvantages of a swept wing?
Reduce amount of lift produced for a given flight speed, wing area and angle of attack
Contribute to drag
What are some advantages of a swept wing?
Designed to fly close to or above the speed of soung
Provides stability
What does a small dihedral allow?
More manoeuvrability
Give the 2 aerodynamic forces
Lift and drag
What is the center of pressure?
The point at which all pressures and forces act on
What do lift and drag depend on?
Shape of the aerofoil
Angle of attack
Surface area
Dynamic pressure
Give the equation for theoretical lift
Dynamic pressure x surface area
What is the coefficient of lift?
Measured lift/ theoretical lift
What is the coefficient of drag?
Measured drag / theoretical drag
Why can one not rely on only the theoretical lift?
Too many variables changing
If the measured lift is less than the theoretical lift, how do we account for the difference?
Introduce the Coefficient of lift to the lift equation
What is the difference between the theoretical lift and the. Lift equation?
The lift equation includes the coefficient of lift which makes it more reliable
What happens if the maximum angle of attack is exceeded?
Stall happens due to flow separation
Two profiles have the same camber but different thickness. How are they affected by the Coefficient of lift?
Same coefficient of lift but at a lower angle of attack
Higher coefficient of like when the angle of attack exceeds 10 degrees
Two profiles have the same thickness but different camber. How are they affected by the coefficient of lift?
Higher coefficient of lift at very low angle of attacks
Higher maximum coefficient of lift at lower maximum angle of attack
What is the advantage/disadvantage of an aircraft flying at a high maximum Coefficient of lift?
Adv - aircraft able to fly slower
Disadv - may produce high drag and low critical mach
How can ice on the surface be formed?
Change in temperature, pressure, density or humidity
What effect can ice formation on the surface on the wing have?
Reduce maximum coefficient of lift and the maximum angle of attack if it forms on the LE
Give the 2 dimensionless coefficients that can be combined to give information about the performance of profiles
Coefficient of drag
Coefficient of lift
What is the Polar Diagram?
A lift vs drag graph which gives the best glide ratio
What is needed in the Polar Diagram to find the angle for the best glide ratio?
Coefficient of lift
Coefficient of drag
What is the meaning of the best glide ratio?
The best angle for the aircraft to fly efficiently
What is induced drag?
Drag caused by lift
What is the drag caused by pressure distribution known as?
Form Drag
On a finite wing, what are the ends called?
Wing tips
What happens on the wing tips of a finite wing?
Turbulence at the wingtips causes wingtip vortices
What is the circulation around the profile causing upwash and downwash known as?
The bound vortex
What does a finite wing have in comparison to an infinite wing?
Wing tip vortices and the bound vortex
What is induced drag affected by?
Aspect Ratio
Speed of the aircraft
Wing tip design
If the aspect ratio is high, what happens to the bound vortex and wing tip vortices?
They decrease
What is the drag caused by skin friction known as?
Friction Drag
What are the 3 types of Parasite drag?
Form
Friction
Interference
What factors affect form drag?
Area of the wing
Aircraft speed
What is the retarded air hitting the surface of the wing known as?
Boundary layer
What is compressible drag?
Drag caused by shockwaves approaching the speed of sound
What is compressible drag also known as?
Wave drag
How do heavy birds reduce drag?
Use their feathers - act as wingtips
What happens to an aircrafts angle of attack and lift coefficient during low speed and high speed flights?
Low speeds - higher angle of attack and lift coefficient
High speeds - lower angle of attack and lift coefficient
What is used to reduce form drag on aircraft
Make the profile more streamlined
What is the relationship between form and friction drag?
Inversely proportional
How much is the form drag on the profile with the highest length to diameter ratio?
Has the lowest form drag but the highest friction drag
What is the length to diameter ratio known as?
Fineness ratio
What is friction drag?
Air particles that get trapped on the skin of the wing
What is the boundary layer?
Where the air thickens and becomes retarded
Where the air moves from high pressure to low pressure areas and back again
How is friction drag minimised?
Higher form
Making the skin surface smoother
What are the two types of boundary layers?
Laminar
Turbulent
Where is the laminar boundary layer located on the wing?
From the leading edge to the maximum thickness
What does an increase in angle of attack do to the laminar boundary layer?
The laminar boundary layer comes earlier due to a faster airflow
Which type of boundary layer encounters an energy exchange?
Turbulent
How much friction is produced in the turbulent boundary layer compared to the laminar?
3x more than the laminar
What are the roles of a slot in a profile?
To assist air particles reach high pressure areas at the trailing edge
To transfer air with the high energy to the lower side
How can flow separation be prevented on the profile?
Slots
What is interference drag?
Pressure caused by shockwaves
How can we reduce interference drag?
Fearings
What is compressible drag?
Shockwaves formed by the aircraft approaching the speed of sound
What is the cause of flow separation in the compressible drag?
Boundary layers thickening
Adverse pressure gradient across shockwaves
What are the 4 different types of wing shapes?
Tapered
Rectangular
Square
Swept
How is the wing designed so that the root can stall before the tips?
Geometric and aerodynamic twists
What is the difference between the geometric and the aerodynamic twist?
Geometric twist - where the angle of incidence is greater at the root than the tip
Aerodynamic twist - where the camber is greater at the root than in the tip
Where does the total wing lift act?
The center of lift
What does MAC stand for?
Mean Aerodynamic Chord
Where is the MAC located?
30% from the leading edge
Where does the total weight of the aircraft act?
The center of gravity
What happens if the position of the center of lift is the same as the center of gravity?
Straight and level flight
When there is flow separation, at the root of the wing, where will the center of lift be?
In front of the center of cravity
When does the aircraft experience a nose up reaction?
When the center of lift is in front of the center of gravity
What is the device used on small aircraft to prevent the wing tip from stalling?
Stall strips
Vortex generator
What is the disadvantage of the stall strip being mounted at the leading edge of the wing root?
Stall strips disturb lift
What are slats used for?
Prevent wingtip stall on large aircraft
What is span wise flow?
The air flow from the leading edge to the trailing edge
What is the effect of span wise flow?
Boundary layer thickens towards the wing tip
What are wing fences used for?
To keep the air particles in a straight line direction
What are boundary layer fences?
Wing fences
What is another device used to improve boundary layer control?
Saw toothed leading edges
