Aeroplanlære - Kap 2 (Basics) Flashcards
When is the only time an aircraft can become and stay airborne?
When it does work on the air to produce a force to oppose gravity
Why do wings work according to newtons law?
Because they are driven through the air at a speed that allows them to oppose gravity
Why do we use a propeller?
To provide at least a an equal and opposite force to oppose drag
Describe the forces that works on an aircraft
Which 2 methods does a wing have to create force?
Flat plate lift
Differential pressure effect
How does flat plate lift work?
A flat surface is angled so that is pushes air down and creates and opposing force upwards
How does differential pressure effect work?
The shape of the wing creates a low pressure on top of the wing and a high pressure underneath, this “sucks” the wing up
When does differential pressure effect work on an object?
When the object is:
1. Curved
2. Has a curved leading edge
3. Has a sharp trailing edge
4. Is set to a relatively low angle to the airflow
What is the requirement for an aerofoil?
A relatively smoothly curved leading edge
An upper surface that is convex
A sharp trailing edge
Describe the pressure around an aerofoil
What is absolutely critical to produce a low pressure above an aerofoil?
An angle of attack
(Maximum around 16°)
What is the effect that bends the air over the wing called?
Coanda effect
What happens to the air at an angle of attack exceeding 16°?
The air breaks away
What are the two concepts fundamental to an understanding of the properties and behavior of flowing air?
The equation of continuity and bernoulli’s theorem
What is the basic assumptions of an ideal fluid?
Incompressible - This is approximately true for flight at slower speeds
Has no viscosity
What is the equation of continuity?
Mass can neither be created nor destroyed
Is mass flow a constant value?
Yes
When mass flow is put through a venturi tube, what is constant at all points?
The density of the air
What happens to the velocity when put through a venturi tube?
Velocity increases and then decreases
What is the bernoulli’s theorem described with words?
Mass can neither be created nor destroyed, it can only change from one form of energy to another
What is the bernoulli’s principle?
In a flow of ideal fluid, the sum of pressure and kinetic energy (per unit volume) remains constant
What does the bernoulli’s theorem have to do with pressure?
It describes that:
Static pressure + Dynamic pressure = Total pressure (Constant)
Describe the pressure in a venturi tube according to bernoulli’s theorem
Which types of airflow is there?
Laminar
Turbulent
Vortex
Force = ? x ?
Pressure x area
What will exist if there is an asymmetry in the pressure acting on an object?
A net force
What is a two dimensional flow?
A flow that moves in 2 directions:
Left and right
Up and down
Describe the pressure around a cylinder
Where streamlines are close speed is faster
Where flow is faster static pressure is reduced
Where flow is slower static pressure is increased
Where can total pressure be measured on an object moving through the air?
At the stagnation point
Describe static pressure on a cylinder
What is the chord and chord line?
The chord is the 2 furthest points from leading edge to trailing edge and the line between them
What is the Angle of attack and what is the symbol?
Which 4 different aerofoil conventions is presented in the powerpoint?
What is camber?
Camber is the asymmetry between the two acting surfaces of an airfoil
What is chord ratio?
The ratio between the chord length and the aerofoil’s maximum thickness
Describe how airflow moves across an aerofoil
Faster on top and slower underneath
Where is the accelerated airflow located around the wing?
Above
What does the differences in surface pressure at each point around the aerofoil create?
An unbalanced force
What is all the unbalanced forces around the aerofoil added together equal too?
The total reaction
Where does the total reaction originate from?
The centre of pressure (CP)
When a wing is lifted and the angle of attack increases where is the total reaction pointing?
Upwards and backwards
What is the component which is perpendicular to the free stream flow called?
Lift
What is the aerodynamic total reaction?
Aerodynamic total reaction = The vector for lift + The vector for drag
What is the stagnation line and stagnation point?
The line is the airflow towards and away from the stagnation point
The stagnation point is the point where fluid’s velocity is zero in the inviscid approximation
What does the magnitude and direction of the total reaction depend on?
The pressure distribution created around the aerofoil
The overall area of the aerofoil
What does the pressure distribution depend on?
The speed of the flow
The angle of attack
Camber
What happens to the total reaction when airspeed doubles?
The total reaction is 4 times greater
Where is the total reaction pointing on a symmetrical aerofoil with zero degress angle of attack?
It’s pointing backwards
Where is the total reaction pointing on a symmetrical aerofoil with positive degress angle of attack?
It’s pointing upwards
Where is the total reaction pointing on a positively cambered aerofoil with zero degress angle of attack?
It’s pointing upwards
Does the CP change if a cambered aerofoil moves upwards?
Yes, the magnitude of the total pressure increases and moves it forward
What does an increase in camber do?
Increase the pressure differential between the upper and lower surfaces of the aerofoil
Describe the 3 flows of three dimensional flow?
1st dimension - Forward and backwards
2nd dimension - Up and down
3rd dimension - Left and right (Spanwise)
What does spanwise flow involve?
Vortices
What is the direction of airflow influenced by?
The inertia of the mass of air in the flow
Pressure differentials
Why is a vortex created on the wing and where?
It’s created due to pressure differentials between upper and lower wing on the wing tip, since the lower pressure moves towards the higher
What does the intensity of the tip vortex depend on?
Two factors:
The pressure differential - Higher equals greater vortex
The amount of time the driving force is given to operate on the air mass
When will tip vortex intensity be reduced?
Aircraft speed increase
Wing aspect ratio increase
The amount of lift being produced reduces
What does a tip vortex produce?
Downwash
Drag - Energy is used to produce a vortex
What does the spanwise flow produce on the trailing edge and results in drag?
Trailing edge vortices
Where is upwash and downwash located around the wing?
Upwash in front and downwash behind
What is the definition on an effective wing?
Produces the most lift with the least amount of drag
What do you increase in order to have more lift and produce the least amount of drag?
The aspect ratio
What is the disadvantages to a higher aspect ratio?
Where is the most and least lift generated on an eliptical wing?
Most at wing root and least at wing tip
Where is the most and least lift generated on an rectangular wing?
Most at wing root and least at wing tip
Where is the most and least lift generated on an tapered wing?
Most at midspan, then wing root and least at wing tip
Where is the most and least lift generated on an tapered and sweep back wing?
Most close to the wing tip less towards the wing root and least at the wing tip
What is washout?
Washout is a characteristic of aircraft wing design which deliberately reduces the lift distribution across the span of an aircraft’s wing. The wing is designed so that the angle of incidence is greater at the wing roots and decreases across the span, becoming lowest at the wing tip
How is washout made?
By making a camber change so lift is gradually reduced from the root to the wing tip
What is wing loading?
The weight of the aeroplane divided by the wing area
What does a high wing loading produce?
More intense vortices
