Flow over Finite Wings

Question 1

What are the 4 sources of drag?

Answer 1

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.

Question 2

What direction do vortices flow around wing tip?

Answer 2

Upwards onto the top surface of the wing

Question 3

What does the downwash do to the free stream velocity?

Answer 3

It changes the angle slightly of the free stream velocity which affects the lift of the wing. Decreases effective angle of attack.

Question 4

What is profile drag?

Answer 4

The combination of the 2D drags. Skin friction, wave and pressure drag.

Question 5

What law gives the velocity induced by a 3D vortex?

Answer 5

The Biot-Sawart law, applicable for incomepressible inviscid flow

Question 6

What is a votex filmament?

Answer 6

can be visulised as a thin tube which has constant circulation. It’s normally curved.

Question 7

What are the Helmholtz vortex theorems?

Answer 7

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.

Question 8

Why do vorticies deplete after time in reality?

Answer 8

Becasue of air viscosity

Question 9

What simple model is used to represent vorticity around a wing?

Answer 9

A horseshoe vortex

Question 10

What are the three components to a horseshoe vortex?

Answer 10

A singular bound vortex spanning the length of the wings. Two free trailing vortices from the wing tips.

Question 11

What are the drawbacks of using a single horseshoe vortex?

Answer 11

It predicts infinite velocity at wing tips which is unrealistic.
It also cannot predict wing lift and drag ( predicts constant lift over wingspan)

Question 12

What is the local lift on a wing strongly affected by?

Answer 12

The neighboring lift generation

Question 13

What is the lifting line model?

Answer 13

Several horseshoe models superimposed to create a lifting line (the wing) with different circulations (and so lifts) along it.

Question 14

Who developed the lifting line model and theory?

Answer 14

Prandlt

Question 15

What does the lifting line theory allow us to do?

Answer 15

Find the circulation at any point along the lifting line and this opens up the aerodynamic characteristics of the wing.

Question 16

What type of wing produces the least induced drag?

Answer 16

Eliptical (spitfire)

Question 17

What are the negatives of elliptical wings?

Answer 17

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.

Question 18

What is the effect of increasing Aspect Ratio

Answer 18

Less induced drag, larger lift slope.

Question 19

What value for taper ratio provides an efficiency closest to an elliptical wing?

Answer 19

0.3

Question 20

What wing type is Prandlt’s lifting line theory good for?

Answer 20

mid-high aspect ratio straight wings

Question 21

What wing type is Prandlt’s lifting line theory bad for?

Answer 21

low AR wings. Swept wings. Delta wings.

Question 22

What is geometric twist?

Answer 22

When the tip angle of attack is different to the root. Washin (higher at tip) vs washout (lower at root)

Question 23

What is aerodynamic twist?

Answer 23

When the aerofoil shape changes along the span

Question 24

How do winglets work?

Answer 24

They prevent vortices from forming at the wingtips by blocking the flow path. This smooths the pressure discontinuity so less energy is wasted.

Question 25

How do delta wing planes fly at low speed?

Answer 25

They have a high angle of attack, this creates vortices that reenergises the flow and makes it reattach. Preventing stall.

Question 26

How can you reduce the induced drag?

Answer 26

Increase AR, Geometric and Aerodynamic twist