Flow over Finite Wings Flashcards
What are the 4 sources of drag?
Pressure - comes from the presence of boundary layer (seperation)
Skin Friction - comes from the shear force from viscous boundary layer
Wave - come from pressure forces produced by shock waves
Induced - comes from vortex induced velocity and generation of lift.
What direction do vortices flow around wing tip?
Upwards onto the top surface of the wing
What does the downwash do to the free stream velocity?
It changes the angle slightly of the free stream velocity which affects the lift of the wing. Decreases effective angle of attack.
What is profile drag?
The combination of the 2D drags. Skin friction, wave and pressure drag.
What law gives the velocity induced by a 3D vortex?
The Biot-Sawart law, applicable for incomepressible inviscid flow
What is a votex filmament?
can be visulised as a thin tube which has constant circulation. It’s normally curved.
What are the Helmholtz vortex theorems?
That the magnitude of circulation is constant (constant vortex strength)
That the votex may never end inside the fluid, it must reach infinity, end at a solid boundary or form a closed loop.
Why do vorticies deplete after time in reality?
Becasue of air viscosity
What simple model is used to represent vorticity around a wing?
A horseshoe vortex
What are the three components to a horseshoe vortex?
A singular bound vortex spanning the length of the wings. Two free trailing vortices from the wing tips.
What are the drawbacks of using a single horseshoe vortex?
It predicts infinite velocity at wing tips which is unrealistic.
It also cannot predict wing lift and drag ( predicts constant lift over wingspan)
What is the local lift on a wing strongly affected by?
The neighboring lift generation
What is the lifting line model?
Several horseshoe models superimposed to create a lifting line (the wing) with different circulations (and so lifts) along it.
Who developed the lifting line model and theory?
Prandlt
What does the lifting line theory allow us to do?
Find the circulation at any point along the lifting line and this opens up the aerodynamic characteristics of the wing.
What type of wing produces the least induced drag?
Eliptical (spitfire)
What are the negatives of elliptical wings?
Trapezoidal wings are easier to built and can have similar efficiency at the right taper ratio.
Also the entire wing stalls at once causing loss of control.
What is the effect of increasing Aspect Ratio
Less induced drag, larger lift slope.
What value for taper ratio provides an efficiency closest to an elliptical wing?
0.3
What wing type is Prandlt’s lifting line theory good for?
mid-high aspect ratio straight wings
What wing type is Prandlt’s lifting line theory bad for?
low AR wings. Swept wings. Delta wings.
What is geometric twist?
When the tip angle of attack is different to the root. Washin (higher at tip) vs washout (lower at root)
What is aerodynamic twist?
When the aerofoil shape changes along the span
How do winglets work?
They prevent vortices from forming at the wingtips by blocking the flow path. This smooths the pressure discontinuity so less energy is wasted.
How do delta wing planes fly at low speed?
They have a high angle of attack, this creates vortices that reenergises the flow and makes it reattach. Preventing stall.
How can you reduce the induced drag?
Increase AR, Geometric and Aerodynamic twist
