Ch 6 - Intro To Lift

Question 1

Lift

Answer 1

A component of the Total Reaction produced by the Aerofoil.

Lift is perpendicular to the relative airflow

Drag is parallel and in the same direction as the RAF

Lift is created by mass flow over the wing (influenced by the angle of attack, camber and thickness of wing design)

Question 2

Aerodynamic Force Coefficient equation

Answer 2

Aerodynamic force = q S Cf

Where q = dynamic Pressure of airflow (KE)
S = surface area of the Aerofoil
Cf = Coefficient of Aerodynamic Force

F = 1/2 rho V^2 S Cf

As q = 1/2 rho v^2

Question 3

Lift Equation

Answer 3

L = q S Cl

q = Dynamic Pressure of airflow (KE)
S = Surface area of the Aerofoil
Cl = Coefficient of lift (dimensionless number used to balance equation)

L = 1/2 rho V^2 S Cl

L is proportional IAS^2 S Cl

In level flight, lift force = weight force (balanced)

Question 4

Coefficient of Lift

Answer 4

Dimensionless
Natural lifting ability of an Aerofoil for a given airflow speed and Aerofoil surface area

What impacts the coefficient:

• AoA (angle of attack)
• Thickness of the Aerofoil
• Camber of the Aerofoil (lift augmentation devices)

Cl = The lift produced per unit area and unit dynamic pressure. It measures the ability of a wing to produce lift

Question 5

AoA on a Symmetrical Wing

Answer 5

• As the pressure difference, which is perpendicular to the airflow, increases, so too must Cl
• At 0 degrees, Cl = 0 and the Amount of lift = 0
• As the angle of attack increases so does the coefficient of lift
• CoP is always 25% down the length of the wing (sub supersonic)

Question 6

Effect of Aerofoil Thickness on Cl

Answer 6

As thickness increases, the Cl MAX increases (lifting ability great for a thicker aerofoil)

• Stalling angle of attack increases
• Rate of Cl increase with the with angle of attack is the same

Thinner designs are better for high speed flights as there is less drag, but they have to perform a faster landing

Question 7

Effect of Camber

Answer 7

A more cambered wing will have:

• More lift for a given surface area
• A higher Cl MAX
• Maybe a slightly increases Stalling Angle of Attack

Question 8

Effect of Trailing Edge Flat on Cl

Answer 8

A trailing edge flap increases the camber of the aerofoil for a given airspeed

• Increases the Cl
• But also increases the Cd

Lift to drag ratio decreases

• Increases Cl MAX
• Means you can land slower
• Stalling AoA decreases

Question 9

AoA on a Cambered wing

Answer 9

Cl increases with a cambered wing for a given surface area and airflow speed

Question 10

Lift in Level Flight

Answer 10

Lift force equal and opposite to the weight force

Question 11

Cl and AoA for Aerofoils

Answer 11

High Speed Aerofoils:

• Thin
• Sharp
• Less Cambered
• Lower Cl MAX and a lower critical AoA

Slow Speed Aerofoils:

• Increased thickness
• Increased Camber
• Rounder leading edges
• Higher Cl MAX and higher critical AoA

Question 12

The 3 Step Approach to answering

Answer 12

1 - Is the answer going to be higher or lower

2 - Equation

3 - Is speed a variable ?

Yes - 2 speeds ? (Yes)^2
          Square Root(No) - looking for a speed

Question 13

Load Factor

Answer 13

LF = L/W for straight and level flight

In a turn:

LF = 1/cos (angle of bank AoB)

Question 14

Stall Speed in a turn

Answer 14

= Stalling speed at 1g x the square root of the Load Factor in a turn

Higher than straight and level stall speed

Question 15

Lift Collapse

Answer 15

When the coefficient of lift can no longer keep up with the strain being put on it