Lift & Drag Flashcards
Definition: Lift
Force perpendicular to direction of travel and must exceed weight for aircraft to fly
Equation for lift
L = 1/2 . ρ . V^2 . S . Cl
Equation for drag
D = 1/2 . ρ . V^2 . S . Cd
What is Cl
Coefficient of lift - measurement of the effectiveness of the wing at generating lift (depends on airfoil and angle of attack)
Difference between streamlines above and below the wing
Above - streamlines forced closed together indicating faster airflow
Below - streamlines more widely spaced apart
Bernoulli’s principle - Equation
(1/2 . ρ . V^2) + ρ = ρo
^dynamic ^static ^ total pressure - constant
As airspeed increases, dynamic pressure increases, thus static pressure decreases
Why is lift generated
With increased dynamic pressure above the wing, this means that static pressure below the wing is greater than that above. This relative difference will generate lift.
Why does airflow around a wing behave like this
The wing behaves like a vortex (spinning object) which leads to it generating trailing and starting vortices
How does the lift coefficient vary with angle of attack
- For small angle of attack Cl increases directly proportionally with it
- Sudden drop in Cl when stall is reached
- then Cl increases slightly again as angle of attack continues to increase
How does changing the camber value effect a graph plotting Cl and angle of attack
Translates it along y direction (Cl values)
At what point on a Cl v. angle of attack graph does it represent stall
Maximum point
What is the boundary layer
A region (only a few cm) above the wing where the air is slowed down as it tries to “stick” to the wing. This is because of the viscosity of air.
Equation linking Cl and Cd
Cd = a + b(Cl^2)
Therefore when there is no lift Cd = a
What are three different types of parasitic drag
- form drag
- skin friction
- interference
Describe form drag
due to adverse pressure gradients which lead to low static pressure at the rear of an object. Higher for blunt objects
