2D / 3D Flow Flashcards
What is skin friction
Friction cause by the viscosity of air
What is a free stream flow
The airflow not affected by the aircrafts surface
What happens to the airflow in the boundary layer
Slowed down by aircrafts surface
After what height does the boundary layer not take effect anymore
Once speed is over 99% of the free stream flow speed
What is the speed of the bottom layer of the boundary layer
0 m/s
Why does smooth laminar flow change to turbulent flow
Because skin friction causes the laminar layer to slow down and lose energy which causes the air to tumble over itself
What is the name of the point in which laminar flow changes into turbulent flow
Transition point
What is the rate of speed increase through the boundary layer with turbulent flow
Rapid increase
How easy is it for laminar flow to detach from the wing
Very easy
How easy is it for turbulent flow to be detached from the wing
Very difficult
What boundary layer produces the most friction; laminar or turbulent?
Turbulent
As the curve of the airflow increases what happens to the transition point
Transition point moves forward
What does a low Reynolds number mean? (Anything below 500,000)
Laminar flow : late transition point
What does a high Reynolds number mean? (Anything greater than 10,000,000)
Turbulent flow : early transition point
What happens With the stagnation points with a increased AOA
Pressure gradient increases therefor stagnation points increase
At slow speeds where is the transition point in relation to leading edge / training edge
Closer to leading edge
At high speeds where is the transition point in relation to leading edge/ trailing edge
Further back towards the trailing edge
What does an adverse pressure gradient do
Try and push air round the trailing edge and against the airflow
Where is the point of maximum thickness usually on a low speed aerofoil
30% of the chord
Where is the point of maximum thickness usually in a modern airliner
50-60% of the chord
What affect does AOA have on the pressure gradient
Higher AOA means higher pressure gradient
What is the separation point
The point at which the turbulent air loses enough kinetic energy so that it is overcome by the adverse pressure gradient resulting in separation as the boundary layer rides above the adverse pressure gradient flow
What are the effects of the separation point
Air behind separation point is turbulent
Consequently large reduction in life and large increase in drag
What is the taper ratio
Top chord
—
What is the aspect ratio
Wingspan^2/ area
Or
Span/chord
What is the sweep angle
The angle from the lateral axis to the centre of pressure
737 = 25° 747= 35°
What is the mean aerodynamic chord (MAC)
The chord of an imaginary rectangular wing with similar longitudinal stability properties to the actual wing
What is the rigging angle
The angle between the wing root and the longitudinal axis
What is washout
Where the angle of incidence decreases along wing root
What is the angle of incidence
The angle between the chord line and the aircrafts longitudinal axis
What is a dihedral angle
The angle between the wingtip and the lateral axis
When the wingtip is higher than the root
What is the anhedral angle
The angle between the wing tip and lateral axis
when wingtip is lower than the root
How is a wing tip vortex created
When the high pressure air from beneath the wing tries to “flow” to the low pressure area on top of the wing but because the wing is moving, by the time the air reaches the top of the wing, it’s already not there, meaning the air has no where to go causing vortex
How does speed effect span wise flow
The slower you go the greater the spanwise flow
When does tip vortex decrease?
If:
Aircraft speed increases
Wing aspect ratio increases
Amount of lift being produced decreases
What are the effects of a tip trailing vortex
Downwash
Drag
How are trailing edge vortices created
When lower wing air going towards wing tip meets over wing air going towards wing root
At trailing edge these are going different directions but collide causing a vortex
How does vortices affect upwash and downwash in a 3D flow
Upwash decreases and downwash increases (compared to a 2d flow)
What is effective airflow
RAF adjusted for downwash
What is the induced angle of attack
The angle between relative air flow and effective airflow
In 3D flow what is induced drag
The component of the vector parallel to the relative airflow
What is the angle of attack in a 3D flow
The angle between the aircrafts longitudinal axis and the relative airflow
How does angle of attack effect lift to induced drag proportion
Lower angle of attack = smaller vortices = lower effective angle of attack = larger induced angle of attack = lower lift to drag proportion
How does speed affect the effective angle of attack
Bigger when faster
How does being closer to the tip affect the EAF
EAF moves closer to chord line as downwash is greater
What is dynamic pressure equal to?
Half density x true airspeed^2
What forces predominate in w laminar flow
Viscous forced predominate
