Lift

Question 1

Lift formula

Answer 1

Lift = CL x 1/2pV^2 x S

Question 2

Factors effecting lift

Answer 2

• Freestream air density
• Freestream velocity
• Size of wing (planform area)
• Shape of wing (planform + section)
• Condition of surface
• Angle of attack

Question 3

3 basic functions of the lift formula

Answer 3

• CL (coefficient of lift)
• 1/2pV^2 (dynamic energy)
• S (size of wing)

Question 4

What does CL provide a measure of, and what does its value depend on?

Answer 4

The lifting capability of a given wing at AoA, its value depends on the shape of the wing and the condition of the surface

Question 5

Zero lift angle

Answer 5

Is the angle of attack where no lift is produced (so CL = 0), since a GP aerofoil is cambered, CL will have a small value at 0 degrees angle of attack so the zero-lift angle occurs at a small negative angle of attack

Question 6

Where does CL max occur on a typical CL curve

Answer 6

Occurs at the stalling angle of attack

Question 7

What does a high CL max indicate?

Answer 7

That it has a greater lifting capacity enabling it to fly at a slower speed without stalling and have increased manoeuvrability

Question 8

Effect of camber on CL curve

Answer 8

As camber increases the CL also increases over all normal operating angle of attack

Question 9

Effect of surface roughness on CL curve

Answer 9

Any roughness (including contamination) reduces CL and can lead to early stall onset

Question 10

Components of 3D flow over a wing

Answer 10

• Vertical
• Chordwise
• Spanwise (from wing roots to wing tip)

Question 11

Spanwise pressure distribution

Answer 11

The greatest pressure distribution is towards the roots, the total pressure distribution induces an outward spanwise flow under the wing and an inward flow over the upper surface, as a result vortices are formed and are shed from the wingtips and trailing edges of the wings

Question 12

Formation of wingtip vortices

Answer 12

Are the major effect of the air spilling from high pressure to low pressure around the wingtips, the core of each vortex spins at a high-speed dragging more air from its surrounding with it and growing as it extends back from the wingtip

Question 13

Formation of trailing edge vortices

Answer 13

Are the result of airflow meeting at the trailing edge at slightly different angles, they are less pronounced/stable than wingtip vortices

Question 14

Induced downwash from wingtip vortices

Answer 14

• Wingtip vortices produce a downwash behind the wing
• The induced downwash effects the overall average angle of the airflow over the wing
• Increased downwash reduces the effective angle of attack
• This airflow which is what the aerofoil reacts to is called the effective relative airflow
• The geometric AoA (between the remote RAF and the chord line) is reduced by the downwash angle to what is called the effective angle of attack

Question 15

Aspect ratio

Answer 15

Is the ratio of the wingspan to the chord of the wing
- AR = span^2 /gross wing area (gross wing area includes the fuselage area of the wing)

Question 16

Effect of AR on CL

Answer 16

As AR decreases CL increases which results in an increase in the geometric staling angle (Note: this is for a wing of the same section as well as wing area)