Lift

Question 1

What is lift?

Answer 1

Lift is the phenomenon generated by an aerofoil due to pressure differences above and below the aerofoil.

Question 2

How is lift created?

Answer 2

Due to the cambered surface on the top of an aerofoil and the flat underside, the air, through which the aerofoil passes, has to travel further and faster over the top. This creates a slightly lesser pressure of the air on top compared to the bottom. Higher pressure air always tends to travel toward lower pressure air creating a lifting force on the aerofoil.

Question 3

What is the lift formula?

Answer 3

L = Cl x 1/2& x V2 x S

Question 4

What is coefficient of lift?

Answer 4

Coefficient of lift is the lifting ability of a particular wing. It depends on both the shape of the wing section and the AoA.

Question 5

Describe centre of pressure (CoP)!

Answer 5

The CoP is represented as a single point acting on the wing chord line at a right angle to the relative airflow, through which the wing’s lifting force is produced. The position of the CoP is not fixed but moves along the chord line as AoA is changes and therefore the pressure distribution around the aerofoil. In normal flight the higher the AoA the further fwd the CoP and vice versa.

Question 6

Does lift and weight act through the same point on an aircraft?

Answer 6

No, lift and weight do not act through the same point. Lift acts through the CoP and weight through the CoG. Their slight offset can cause an up or down pitching moment.

Question 7

Describe aspect ratio!

Answer 7

The aspect ratio of an aerofoil is the relationship of the wing span to the wing chord. A higher aspect ratio wing (gliders) will produce high lift and a low aspect ratio wing (fighter aircraft) will produce less lift but high speed.

Question 8

During what phase of flight is lift the greatest?

Answer 8

Normally during take-off as configuration (flaps, slats,..), speed of airflow over wing, AoA, and air density contribute towards the high lift capability of the aerofoil. In unusual circumstances the highest lift is produced just before the point of stall of the aerofoil, as it is at this point where the highest Cl is produced for a given AoA.

Question 9

What is direct lift control?

Answer 9

The amount of lift can be directly controlled by the operation of the elevator/stabilizer. Through these surfaces the AoA of the main wing can be controlled.

Question 10

What are high lift devices?

Answer 10

• Trailing edge flaps (Fowler flaps)
• Leading edge flaps (Krueger flaps/slats)
• Slots