8.2 - Aerodynamics Flashcards
State Bernoulli’s Principle?
Bernoulli’s principle states that when energy is neither added nor subtracted from a fluid in motion, the potential energy, (static pressure), will decrease when the kinetic energy, velocity (dynamic pressure) is increased.
(Venturi)
What happens to Pressure in the narrow point of a venturi?
What happens to dynamic pressure in the divergent portion of a venturi?
Potential energy = static pressure
Dynamic pressure = kinetic energy (velocity of fluid)
The dynamic pressure (velocity of the fluid) increases
What is drag?
The force which opposes motion through the air. It acts parallel to and in the opposite direction to the flight path
What is laminar boundary layer?
Laminar flow is an orderly motion in which successive strata of air particles slide past each other in much the same way as the action of a pack of cards when thrown along a flat surface.
What is turbulent boundary layer?
As the flow continues back from the leading edge, friction forces in the boundary layer continue to dissipate the energy of the airflow, slowing it down.
The laminar boundary layer increases in thickness with increased distance from the leading edge.
Some distance back from the leading edge, the laminar flow begins an oscillatory disturbance which is unstable.
The point at which the laminar boundary layer changes to turbulent is called?
Transition point
Where would you find the stagnation point on a wing and how is it formed?
The stagnation point is the point at which the air is brought to rest by the leading edge of the wing.
It is also the point at which the boundary layer starts.
The point where the boundary layer breaks away from the wing is called?
Separation point
Explain relative airflow
Relative airflowis the direction of the airflow with respect to an airfoil.
Flight path and relative airflow are parallel but travel in opposite directions.
Explain upwash
The diagram shows that, in advance of the wing, the streamlines of the air curve towards the top surface. Upwash, as this is called, is an inherent feature of any surface which is producing lift and exists because air always tends to flow towards an area of low pressure. The deeper the low-pressure region, the greater the amount of upwash.
Explain Downwash
The aircraft is propelled through the air by its powerplant and the air passes over the lift producing surfaces. It is this downward deflection or downwashing of the air which has an opposing effect, of that pushing upward on the aircraft.
Downwash is a form of induced drag.
Induced drag comes as a result of lift.
How are wing tip vortices formed?
(What direction do they flow in when viewed from the rear? ie CW or CCW)
When two airflows, from the top and bottom surfaces meet at the trailing edge they are flowing at an angle to each other and cause eddies or vortices which rotate clockwise (viewed from the rear) from the left wing, and anti-clockwise from the right wing.
All the vortices on one side tend to join up and form one large vortex, which is shed from each wing tip. These are called Wing Tip Vortices.
What type of drag do they produce?
(Wing tip vortices)
Induced Drag
What is the purpose of a winglet and wing fence?
To minimise the induced drag caused by wing tip vortices
Define Angle of Attack? (AOA)
The acute angle formed between the chord line of the wing and the direction of the relative air flow.
Define Angle of Incidence
Angle of incidence is the acute angle formed between the chord of the wing and a line drawn parallel to the longitudinal axis of the aircraft.
What is a chord line?
An imaginary line drawn from the centre of the leading edge to the centre of the trailing edge through the cross section of an airfoil.
Define wing span
Wing span is the measurement from wing tip to wing tip and includes the width of the fuselage
What is Camber?
The curvature of the upper and lower surfaces of an airfoil from the leading edge to the trailing edge
Explain how lift is generated on an airfoil.
Almost any shape will generate lift if is it either cambered or inclined to the flow direction.
If the flow around any inclined or cambered section is studied it will be found that the air always goes faster over the upper surface. Which is longer than the lower surface.
Bernoulli’s principle tells us that this increases airspeed over the top is accompanied by a decrease in pressure. Meanwhile the pressure underneath the airfoil is greater creating lift.
Explain Aerodynamic Resultant and Centre of Pressure (Cp)
Aerodynamic Resultant is the vector sum of the magnitude and direction of all the lift forces produced by aerofoil.
Centre of Pressure is a theoretical point through which the aerodynamic resultant is said to act.
What is the lift formula?
What factors affect the lift produced by a wing?
(PAADS)
Planform of the wing
Area of the wing ( wing area)
Angle of Attack
Air Density
Speed of the relative airflow
Show with the aid of a graph what happens to CL with an increasing angle of attack (AOA)
CL = Coefficient of Lift
Show with the aid of a diagram the pressure envelope of an airfoil with a decreasing and increaing AOA (Angle of Attack)
What is ‘‘Aspect Ratio’’?
Note: Subject to certain limitations such as its own weight, the most efficient airfoil has the greatest aspect ratio
What is the formula for calculating the Aspect Ratio of a rectangular wing?
What happens to the CP as angle of attack is increased?
Throughout most of the flight range, the centre of pressure moves forwards with increasing angle of attack up to the critical or stalling angle (16^) when the CP will move rearwards very rapidly with any further increase in angle of attack
What happens to CP once you reach CP max?
The CP will move rearwards very rapidly with any further increase in angle of attack.
What is Washout?
Many aircrafts are designed with a greater angle of incidence at the root of the wing than at the tip;
This characteristic is called Washout
What is the finess ratio?
fineness ratio = a/b
length / breadth
What does it mean when an aircraft has a high fineness ratio?
Higher the air speed
How do you calculate fineness ratio and what is the best results?
Best results: 4 / 1
Fineness ratio = a / b
Explain what the lift drag ratio is and how it is effected by angle of attack
This is an efficiency curve and shows us the angle of attack at which we will have the highest ratio of lift to drag.
Occurs at lower angle of attacks as induced drag varies directly with the lift generated.
Why does induced drag decrease with the square of speed?
Induced drag is the drag as a result of lift.
As the speed increases, a lower AOA is required to produce lift and so induced drag decreases.
However, with increasing airspeed, parasite drag increases.
Briefly explain how the centre of pressure affects pitch.
If the centre of pressure is before the centre of gravity, the aircraft will pitch nose up.
If the centre of pressure is after the centre of gravity, the aircraft will pitch nose down.
