Flight Flashcards
What is lift
the force that is perpendicular to the direction of fluid flow
What happens when an air flow is superimposed with a rotating movement of air around a cylinder?
air will be induced to move with a no slip condition. you will create a region of high/low pressure.
state the direction of the lift force for (air direction, rotation direction)
a. to the right, clockwise
b. to the right, anti-clockwise
c. to the left, clockwise
d. to the left, anti-clockwise
a. upwards
b. downwards
c. upwards
d. downwards
Explain the magnus effect using Newton’s 3rd law
-air is directed downwards and since momentum needs to be conserved the cylinder is propelled upwards
Explain the magnus effect using Bernoullis
you would impart a greater velocity on one end ( causing greater dynamic and lower static pressure) ; the end which has a slower velocity (have a smaller dynamic and greater static pressure). the pressure gradient (want to move from greater to lower pressure)
How do aerofoils assist in flight
- the air passing through the top surface travels faster than the air passing through the bottom surface
- by Bernoulli the air on top has greater dynamic pressure than the air at the bottom ( so the air at the bottom has a higher static pressure and there is lower pressure below the wing) this results in lift
How is the aerofoil related to a bound vortex
It has the same air flow patterns without actually rotating
for a positive angle of attack must the relative wind be above or below the chord line
must be below the chord line for a positive angle of attack
Draw the leading edge, chord, trailing edge and a positive angle of attack
L 16 slide 22
what direction does the wing tilt the air at a positive angle of attack, what does this lead to?
air is tilted forwards and downwards which causes the wing to be pushed backwards (drag) and upwards ( lift)
What are 4 forces acting on a flying animal
weight, lift
thrust, drag
What happens when lift> weight
lift < weight
lift =weight
bird accelerates upwards, bird accelerates downwards, bird remains at height
What happens when
thrust > drag
thrust < drag
thrust = drag
thrust > drag –>accelerates forward
thrust < drag –>deccelerates ( backward)
thrust = drag –>constant speed
how is lift and drag affected by velocity
proportionate to v^2
How would you normalize a lift force vs angle of attack graph
Plot coefficient of lift ( Cl = F_L / 1/2pv^2 * A )vs angle of attack
How does the lift vs angle of attack graph differ between a symmetric and asymmetric aerofoil (DRAW IT OUT)
slide 11 L 17
Why is the lift produced from a asymmetric aerofoil typically greater than that produced by a symmetric aerofoil
the air traveling at the top is moving at a greater speed than that at the bottom so the static pressure is even lower , greater pressure difference = more lift
How is the optimal angle of attack chosen
It is the the angle that corresponds to the highest lift to drag ratio.
What is a polar plot.
It is a lift vs drag graph that is used to find the optimum angle of attack ( go from y axis to the first pt that touches the curve)
What are the factors that can be used to increase the lift
- increase the velocity
- increase the planform area of the wings ( but also increases drag)
- increase the angle of attack
What is stalling
It is the angle of attack where the lift force can no longer increase but starts to decrease, I think it’s cause you introduce an early boundary layer separation
What is the lift force perpendicular to and the drag force parallel to
The lift and drag force are perpendicular and parallel to the RELATIVE WIND the object experiences
Why would we change the pitch angle
To get a positive angle of attack, especially when we have the propeller moving forward and hits upon oncoming wind ( relative wind will be at an angle)
What is a consequence when we have a large pitch angle
the torque opposing the blade motion increases
