M8.2 Flashcards
Why is the air in the subsonic region incompressible?
Because the flying body does not reach the speeds necessary
What is the continuity equation?
V1A1=V2A2
What is a jet outlet?
When the diameter decreases and the speed increases
What is a diffuser jet?
When the diameter increase the speed decreases
What is Bernoulli’s principle?
The total presser is always the sum of static pressure and dynamic pressure
What is the point of stagnation?
The speed of airflow falls to zero and the static pressure equals the total pressure.
What happens in the Venturi tube?
The velocity increases and pressure decreases as the cross sectional are decreases
What is up wash?
When some streamlines approaching the edge in the low position slope upwards and pass on top of the aero foil
What is down wash?
Some streamlines of the upper surface flow downwards when leaving the aero foil
What is the Magnus effect?
Mechanically induced circulation
What is a profile?
The cross-section of a wing
Sometimes call an aerofoil section
What is the chord line?
This is a straight line connection the leading edge and the trailing edge
What is the mean chamber line?
Is a line drawn halfway between the upper and lower surfaces of the profile
What is the chamber of the profile?
It is the displacement of the mean camber line from the chord line
What is the fineness ratio?
The maximum thickness of a profile is defined as a fraction or a percentage of the chord
What is the flight path velocity?
It is the speed of the aircraft in a certain direction through the air
What is the relative wind?
It is the speed and direction of the air acting on the aircraft which is passing through it
What is the angle of attack?
The angle between the chord line of the profile and the relative wind
(Signified by alpha a)
What is the angle of incidence?
It is the angle between the chord line of the profile and the longitudinal axis of the aircraft
(Signified by gamma Y)
How do you work out the wing area?
Wing span x the wing chord
if the wings are tapered then take an average wing chord
What is the aspect ratio?
It is the wing span divided by the average chord
What is the taper ration?
It is the chord tip divided by the chord root
What is the average chord?
It is the geomatics average of all the chords
Does a higher or lower aspect ratio cause more drag?
Lower aspect ratio has more drag
What is the sweep angle?
It is the angle of the wings taken from 25% of the wing
What is the dihedral angle?
It is the angle between the wing and the horizontal line of the aircraft
What is a positive and negative dihedral?
-Positive is when the wings point upwards -negative is when the wings point downwards
What is the finesses ratio?
The ratio between thickens and length of a profile
What is centre pressure?
This is where all forces on a profile connect
What four things affect lift and drag?
- dynamic pressure
- surface area
- shape of profile
- angle of attack
Why is calculating lift difficult?
The profile has different pressure due to different aircraft area
(Must use a wind tunnel)
What is interference drag?
Is the turbulence in the airflow caused by sharp corners which comes from parts being added together
(Can be reduced by fairings)
Where does compressible drag occur?
Transonic and supersonic
How is compressible drag caused?
By shock waves on an aircraft approaching the speed of sound
When is static pressure at the highest point
Point of stagnation where the air comes to a stop
At what points do we have maximum air velocity and minimum static pressure
Point near maximum thickness of the profile
What is aerodynamic force
The resultant of all forces on a profile in an airflow acting on the centre of pressure
What four factors does lift and drag depend on:
- Dynamic Pressure
- Surface Area
- Shape of profile
- Angle of attack
What are the four wing types?
- elliptical
- rectangular
- tapered
- swept
What are the two types of wing design?
- geometrically twisted wing
- aerodynamically twisted wing
What is the difference between the two types of wing designs?
Geometrically twisted:
-the camber is constant across the span
-angle of incidence is greater at the root than tip
-chord lines are not parallel
Aerodynamically twisted:
-the camber is greater at the root than the tip
-angle of incidence is constant across the span
-chord is parallel
What does Mean aerodynamic chord (MAC) mean?
It is the chord line through the centre of its lift
How do you work out theoretical lift?
0.5 x density x velocity^2 x area
How do you work out the coefficient lift?
Measured lift/Theoretical lift
same for coefficient drag
What is alpha max?
The maximum angle to get the most lift before stall is created
What will a high camber produce?
It will have a much higher coefficient of lift at the zero angle of attack
What is a polar diagram (Lilienthal Diagram)?
It is a diagram that compares the coefficient of lift and coefficient of drag to give information about the performance of the profiles
What are the three main types of drag?
- Induced drag (Lift)
- Parasite drag (Form, Friction, Interference)
- Compressible drag (Shockwaves)
What is induced drag
Drag on the wing caused by lift
What is parasite drag?
- drag caused by the distribution of pressue
- drag caused by skin friction
- aerodynamic interference
What is compressible drag?
Drag caused by shock waves on an aircraft approaching the speed of sound
What causes wing tip vortex’s?
Wing tip vortex’s are caused by turbulence at the wing tips
How do wing tip vortex’s affect the aircraft?
The turbulence absorbs energy and increases drag
What type of drag are wing tip vortex’s?
Induced drag
What direction do the streamlines go when they go over the wing profile?
They go over and then head inwards towards the fuselage
Outward if under wing profile
What is used to reduce wing tip vortex’s?
Winglets or a wing tip fence
What is a bound vortex?
It is the circulation of up-wash and down-wash
What is trapped (retarded) air called?
The boundary layer
What does the boundary layer do to the streamlines?
It slows down the layer above it which slows the layer above that and so on until the free stream velocity is restored
What are the two basic boundary layers?
- laminar boundary layer
- turbulent boundary layer
What happens to the layers of air in the laminar boundary layers and the turbulent boundary layer?
- In the laminar boundary layer they do not move from layer to layer
- in the turbulent boundary layer they move from layer to layer (produces an exchange of energy)
Why is there a slot in a profile?
It transfers air with high energy from the lower side to the upper side that gives trapped air energy to reduce air separation
How much more drag is in the turbulent boundary layer compared to the laminar boundary layer?
3 times the amount
What is the laminar boundary layer?
It is the immediate downstream of the leading edge
What is the turbulent boundary layer?
The immediate downstream of the laminar boundary layer
What is total drag?
Induced drag (Lift) and Parasite drag (Form, Friction, Interference)
What happens if the centre of lift is equal to the centre of gravity?
The aircraft will have a steady level flight
What happens when the centre of lift is in front of the centre of gravity?
The aircraft will point upwards
What happens when the centre of lift is behind of the centre of gravity?
The aircraft will point downwards
What happens when the root of the wing stalls?
The centre of lift on the wing heads to the wing tips which puts it behind of the centre of gravity this puts the nose down. A pilot input is not required
What happens when the tips of the wing stalls?
The centre of lift on the wing heads to the root which puts it in front of the centre of gravity this puts the nose up. A pilot input is required
What type of stall is the most dangerous?
Wing tip stall
What does a small aircraft use to prevent wing tip stalling?
A stall strip (it also disturbs lift)
What does a large aircraft use to prevent wing tip stalling?
Slats
What is the job of a wing fence or saw-tooth wings?
They keep air particles going in a straight line by reducing the effects of span-wise flow (when air flows on the wing and towards the span)
What is a vortex generator?
It is a small blip on the wing that is used to improve boundary layer control
