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
True of False, Lift increases with the square of speed
True
Lift increases as the square of the speed.
If the speed doubles, the value of lift will be four times greater
True of False
Induced drag is inversely proportional to the square of speed
False - directly proportional
As the speed of square increases, lift increases as per lift formula.
Induced drag increases as lift increases.
What does the coefficient of lift tell us and what does it depend on
The coefficient of lift may be considered as the lifting effectiveness for a particular wing.
The coefficient depends upon the wing geometry;
that is the section shape planform and angle of attack.
Show with the aid of a graph what happens to CL with an increasing angle of attack (AOA)
CL = Coefficient of Lift
What is the ‘‘critical angle’’?
At a certain AOA which is called the critical angle, the airflow will no longer follow the contours of the wing and will break away from the upper surface.
At this moment the lift falls off rapidly and the aircraft will no longer be supported in flight. This condition is known as stall
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 aspect ratio’s of the following:
Gliders
Commerical aircraft
Military jets
Gliders = 40
Commercial aircraft = 5 - 8
Military aircraft = 3.5
What are the advantages of increasing the aspect ratio of a wing?
Decrease induced drag
Increase Lift
Where would you expect to see a high aspect ratio wing being utilised?
Gliders and Sail planes
What is the formula for calculating the Aspect Ratio of a rectangular wing?
How does a change in density effect lift?
When the density is low, the lift will also be comparatively lower. If an aircraft flies at a certain angle of attack at sea level and then flies at the same angle of attack at a higher altitude where density is less, the aircraft must be flown faster
The airspeed must increase as the density decreases in order to maintain the aircraft at the same angle of attack in level flight
For subsonic airfoils in use today what would be the most forward position of CP?
For subsonic airfoils in common use today, the most forward position of the centre of pressure is located at between 25 and 30 percent of the chord.
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 is the CP (Centre of Pressure)
The point of intersection of the resultant force line with the chord line of the airfoil is called the centre of pressure (CP)
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
Why would you wash out a wing?
The purpose of washout is to improve the stability of the aircraft as it approaches a stall condition.
The section of the wing near the fuselage will stall before the outer section, thus enabling the pilot to maintain good control and reducing the tendency of the aircraft to ‘‘fall off’’ on one wing.
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
True or False
High Speed aircraft have high fineness ratios
True
'’The higher the speed, the greater should be the fineness ratio’’
Define Stall
Stall is a condition in flight which occurs when LIFT produced by an airfoil can no longer maintain flight
When an aircraft is in flight, there are a number of flight conditions that might lead to a stall.
What are they?
Excessive angle of attack (AOA)
lack of airspeed
State the 2 different components of drag?
Induced drag - drag due to lift
Parasite drag - drag due to the resistance of the air
When are the components of drag (induced and parasite) at their greatest values?
Parasite drag at high airspeeds
Induced drag at low airspeeds ( at low airspeeds, you hve to produce more lift to stay airborne, this increased lift increases induced drag)
What methods are employed to reduce the following types of parasite drag:
Form drag
Interference drag
Skin friction
Form Drag: Shaping aircraft so that the flow of air past them is as smooth as possible ie streamlining of all exposed parts.
Interference drag: Design the transition surfaces between two bodies (engine and wing) without sharp edges e.g the connection between the wing and fuselage is designed as a fillet.
Skin drag: Smooth paint etc
What are the 3 types of parasite drag?
- Form Drag
- Interference Drag
- Skin Friction
Explain Form drag
This is the portion of the resistance which is due to the fact that when a viscous fluid flows past a solid object, vortices are formed and we no longer get a smooth streamlined flow.
To reduce form drag, all parts on the aircraft exposed to the air need to be as smooth as possible.
Explain interference drag
Samples of areas affected by interference drag are;
wing to fuselage attachment, or engine nacelles to wing connection.
To reduce this drag it is necessary to design the transition surfaces between two bodies without sharp edges e.g. the connection between the fuselage is designed as a fillet.
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.
What is coefficient of drag?
A unitless quantity that is used to quantify the drag or resistance of an object moving through a fluid.
High value = high drag
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.
What is Rime Ice?
Rime ice is rough ice that freezes instantly.
What is glaze ice?
Glaze ice is clear ice that doesn’t freeze instantly,
it creeps in and can freeze/damage bearings
How does ice affect the performance of aircraft?
Reduces the stalling angle (16^ to 12^)
Increases stalling speed ( 80 Knts to 120 Knts)
