Aeroplanlære - Kap 4 (Drag) Flashcards
When moving through a fluid (Air) we encounter a resistance. What is this called?
Drag
In two dimensional flow which 2 forms of drag do we have?
Skin friction drag
Form drag
What causes form drag?
Caused by the difference in pressure in front and behind the aerofoil
What causes skin friction drag?
Caused by shear forces in the boundary layer due to the air’s viscosity
What are form drag and skin friction drag together known as?
Profile drag
Does profile drag produce lift?
No
What is the equation for form drag?
What happens to the air molecules when they hit the aerofoil?
What is the boundary layer?
The layer from the aerofoil extending out to 99% of the free stream flow
Where does skin friction drag take place?
Within the boundary layer
When is the most energy produced in this airflow?
When the airflow becomes turbulent
Is the laminar boundary layer higher or lower than the turbulent boundary layer?
The laminar is lower than the turbulent boundary layer
Does laminar flow produce more or less skin fraction than turbulent air?
The laminar flow produce less and possesses less kinetic energy than turbulent
Does airflow speed increase or decrease when getting further from the surface in a laminar boundary layer?
It increases the further from the surface
Does airflow speed increase or decrease when getting further from the surface in a turbulent boundary layer?
It increases the further from the surface but less than a laminar boundary layer
In which way does turbulent airflow move?
It has flows in many directions
Why do turbulent airflow in the turbulent boundary layer create more skin friction drag?
What is the point where the laminar boundary layer switches too a turbulent boundary layer?
Transition point
What is the equation for reynolds number?
What does Reynolds number tell us?
Where is the lowest surface pressure located on an aerofoil?
Approximately at the point of maximum thickness
From the leading edge too the lowest surface pressure, which way is the pressure gradient?
The same way as the direction of the flow
What is the adverse pressure gradient?
When the high pressure from under the wing seeks towards the top of the wing from trailing edge towards leading edge
What is the point where turbulent airflow separates from the aerofoil?
Separation point
* Caused by low speed and the adverse pressure gradient
Will profile drag occur if there is no lift being generated
Yes
What causes induced drag to be produced?
It is caused by the vortices produced whenever lift is produced
When is interference drag produced?
Which categories of aircraft drag is there and how are they connected?
Which factors affect form drag?
Which factors affect skin friction drag?
Which factors affect interference drag?
When the airflow across one component of an aircraft is forced to mix with the airflow across an adjacent or proximal component
Which form of drag increases when airspeed increase?
Parasite drag
When using flaps and gears which form of drag increase?
Parasite drag (Greater frontal area)
What is the equation for parasite drag?
What causes changes in the coefficient of parasite drag?
Which form of drag decreases when airspeed increase?
Induced drag
Does induced drag increase or decrease with higher weight?
Increase
Does a glider have a higher induced drag than short winged aircraft?
No since induced drag is inversely proportional with aspect ratio and a glider has a high wing aspect ratio, therefore less vortices
What is the equation for induced drag?
What is downwash and the coefficient for induced drag proportional and inversely proportional too?
An aircraft is flying straight and level and doubles it’s airspeed. What happens too lift and induced drag too keep flying straight and level?
Lift must be reduced to 1/4
Induced drag is equal to lift^2 so that must be 1/4 x 1/4 = 1/16
What is total drag?
The sum of parasite drag and induced drag
What is Vmd?
Velocity minimum drag
When is speed stable and unstable?
Unstable behind velocity minimum drag and stable above
What happens to the drag curve when increasing altitude?
It moves right and thereby Vmd is higher
What happens to the drag curve when increasing weight?
It moves right and up and thereby Vmd is higher
What happens to the drag curve when changing the configuration to a dirty configuration?
It moves left and up and thereby Vmd is lower
What is the equation for total drag?
How does the coefficient for parasite drag change with the coefficient for lift?
It has a constant stable drag until Clmax but with a slight increase at the end
How does the coefficient for induced drag change with the coefficient for lift?
Lift and drag increase slightly until Clmax and then lift decrease after
Explain where the different drag coefficients are seen on the drag polar
The coefficient for parasite drag is seen first thereafter it’s a combination
How do you find the greatest ratio between the coefficient for drag and the coefficient for lift on the total drag curve?
By drawing a tangent line from zero to the graph
What is not shown on the lift drag ratio graph?
Blue line = The coefficient for drag
Light blue line = The coefficient for lift
Orange line = The coefficient for lift / The coefficient for drag
X axis = Angle of attack
