Lift

Question 1

State the LIFT equation.

Answer 1

Lift = C_L x [(1/2)pV²] x S

Question 2

Considering the lift equation, lift is dependant on what 4 factors?

Answer 2

1. Free stream velocity (TAS - V^2); 2. Air Density (p); 3. Wing Area (S); and 4. Co-efficient of Lift (CL)

Question 3

What are the factors affecting the Co-efficient of Lift?

Answer 3

Co-efficient of Lift depends on: 1. AoA; 2. Shape of the aerofoil (Camber, Thickness); 3. Wing Planform (AR, Sweep); 4. Boundary Layer Control (e.g. Slat-slot, suction); and 5. Condition of the wing (clean, rough)

Question 4

What are the three lift generation theories?

Answer 4

1. Pressure distribution theory; 2. Reaction or changes in momentum theory; 3. Circulation and vortex lift theory.

Question 5

What is the Magnus effect?

Answer 5

Lift generated by the rotation of a cylindrical (or spherical) object. The rotation generates an increase in local velocity on one side and hence a reduction in pressure which generates a lift force (backspin etc).

Question 6

If you doubled the TAS, the lift generated would be: a) The same; b) 3x greater; c) 4x greater; or d) 2x greater

Answer 6

c) It would be 4x greater due to velocity being squared

Question 7

At what AoA is the Co-efficient of Lift at its maximum? a) 4 degrees AoA; b) Stall AoA; c) 16 degrees AoA; or d) 0 degrees AoA

Answer 7

b)

Question 8

For a symmetrical aerofoil, at what AoA does the wing produce zero lift and min drag?

Answer 8

0 degrees AoA

Question 9

For a cambered aerofoil, at what AoA does the wing produce zero lift?

Answer 9

At some AoA less than 0 degrees

Question 10

In this diagram, the blue line is a standard aerofoil. What modification has been made to the aerofoil on the green line, and on the red line.

Answer 10

Green line: Flaps change camber

Red Line: Slats-slots energise the airflow and prevent separation occuring until later.

Question 11

What effect does the LE radius of a wing have on the Coefficient of lift?

Answer 11

A smaller LE radius will cause the stall to be more abrupt and sudden. Large LE radius will be longer and gradual.

Question 12

What effect does AR and Sweep have on C_L?

Answer 12

As sweep is increased and AR reduced (fighter jet style) from an infinite AR (gliders are as close as possible to this) the wing will become less efficient.

With high sweep and low AR:

1. C_L max reduced
2. Stall AoA increase
3. Stall speed increase

Question 13

What effect does roughness (say icing) have on the C_L?

Answer 13

It reduces the stalling angle and hence the max C_L for that aerofoil.

Question 14

How does the Centre of Pressure move with an INCREASE in AoA?

Answer 14

The CP will move toward the LE as AoA increases up until the point of stall.

Question 15

How does the centre of pressure move after the point of stall which aids stall recovery?

Answer 15

The CP will move rearward toward the TE which generates a nose down pitching moment which aids in stall recovery by decreasing the AoA.