8.2 Aerodynamic Principles Flashcards
Where is the air incompressible?
In the subsonic region
What is the continuity equation?
A1 V1 = A2 V2
What does the continuity equation state?
The speed of airflow is inversely proportional to the area of the cross-section of the tube as long as the density remains the same
What are the terms used for:
- tube diameter increasing and speed decreases
- tube diameter decreasing and speed increases
- diffuser outlet
- jet outlet
What does Bernoulli’s principle state?
The total pressure is equal to the sum of the static pressure and the dynamic pressure
The total pressure remains constant
What is IAS?
Indicated air speed
What is the point of stagnation?
The speed of the airflow = zero
The static pressure is equal to the total pressure
What is the Magnus effect?
The mechanically induced circulation around an object
What is the rule of vortices?
They always form in pairs and rotate counter-clockwise
What is up-wash and down-wash?
Streamlines approaching or leaving the wing flow upwards or downwards
As the speed increases, what happens to the static and dynamic pressure?
Static - decreases
Dynamic - increases
What is a profile?
A cross-section of a wing
What is the chord line?
A straight line that connects the leading edge to the trailing edge
What is the mean camber line?
A line drawn half-way between the upper and the lower surfaces of the profile
What is the camber?
The displacement between the mean camber line and the chord line
What is the maximum camber of a low-speed profile?
Maximum camber of 5% located 45% aft of the leading edge
What is the maximum thickness of a low-speed profile?
Maximum thickness of 18% located 30% aft of the leading edge
What is the maximum thickness (as a fraction) of a profile also known as?
The fineness ratio
What is the flight path velocity?
The speed of the aircraft in a certain direction through the air
What is the relative wind?
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 (a - alpha)
What is the angle of incidence?
The angle between the chord line of the profile and the longitudinal axis of the aircraft (y - gamma)
What is the equation for taper ratio?
n (lambda) = Ct (tip) / Cr (root)
How is the sweep angle measured?
The angle between 25% from the leading edge and the perpendicular line, both measured from the root chord
Positive (backwards)
Negative (forwards)
What is the positive and negative Dihedral angle?
Positive dihedral - wings upwards
Negative dihedral - wings downwards
What is the aerodynamic force?
The resultant of all forces on a profile in an airflow acting on the centre of pressure
What are the two components that make up aerodynamic forces?
Lift - perpendicular to relative wind
Drag - parallel to relative wind
What is the equation of lift/drag (Theoretical lift/drag)?
Lift/Drag Force = Dynamic Pressure x Area
F = 1/2PV^2 x A
What is the centre of pressure?
The point at which all pressures and forces act
What are the 4 things that affect lift and drag?
- Dynamic Pressure
- Surface Area
- Shape of Profile
- Angle of Attack
How is the Coefficient of lift (CL) calculated?
Measured lift
——————
Theoretical lift
It is not possible to calculate actual lift. What is used to measure this?
A wind tunnel with two scales:
- horizontal = drag
- vertical = lift
How is the Coefficient of drag (CD) calculated?
Measured drag
———————
Theoretical drag
If the angle of attack increases above amax (alpha max) what happens?
The airflow cannot follow the upper surface of the profile and an airflow separation called a “stall” occurs
If a profile has a larger camber, what increases?
The Coefficient of lift at a smaller angle of attack
If a profile has a larger thickness, what increases?
- higher max coefficient of lift
- higher amax (alpha max)
- higher angle of attack
What are the disadvantages of creating a profile with a larger camber or thickness?
- more drag
- low critical Mach number
How is the angle for the best glide ratio calculated?
Using a graph, drawing a tangent from the intersection of the axis to the graph
What is the same as the ratio of lift and drag?
The ratio of lift coefficient to the drag coefficient
What is the glide ratio?
The best possible lift for the least possible drag
What are the three categories of drag?
- induced
- parasite
- compressible
What is induced drag?
Drag on the wing caused by the lift
What is parasite drag?
Drag not related to the lift. Can be due to distribution of pressure, friction or aerodynamic interference.
What is compressible drag?
Drag caused by shockwaves on an aircraft approaching the speed of sound
What is drag?
It is the aerodynamic force which is parallel to the relative wind. It is opposite force to thrust
What is the turbulence at the wing tips called?
Wing tip vortices
What is the circulation of air (up-wash and down-wash) called around the wings causing drag?
The bound vortex
What is important about the door gutters design?
They reflect the up-wash and down-wash caused by the vortices. They are in line with the flow pattern
What are the 3 types of PARASITE drag?
- form
- friction
- interference
What is form drag?
A parasite drag caused by the pressure distribution on a body
How is form drag reduced?
Streamlining of an object
What is the relationship between form drag and friction drag?
If the form drag is low, the friction drag is high
If the form drag is high, the friction drag is low
What is the name of the area of air on top of a wing, with friction drag, causing it to retard?
The boundary layer
What are the two types of boundary layer?
- laminar boundary layer
- turbulent boundary layer
How much more friction drag does the turbulent boundary layer produce than the laminar boundary layer?
3 times more friction
Where is the laminar boundary?
Immediately downstream from the leading edge (up to the minimum static pressure/maximum thickness)
Where is the turbulent boundary?
Immediately downstream of the laminar boundary layer (starting where the static pressure is minimum/maximum thickness)
What can be introduced to a wing to prevent flow separation?
A slot
- high energy air moved from bottom of the wing up to the top through the slot
A lower profile wing is known as?
The laminar profile
How is interference drag reduced?
By using fairings
What is interference drag?
Turbulence in the airflow caused by sharp corners from when components are joined together (engines on wings etc)
- The total drag of the joined components is GREATER than the total drag of the individual components
When does compressible drag occur?
Only in transonic and supersonic flight
What is total drag?
The combination of induced drag and parasite drag
What are the four types of wing shapes?
- elliptical
- rectangular
- tapered
- swept
What type of wings stall at the tip?
- Swept
- Tapered
What type of wing does the stall start at the root?
Rectangular wings
What type of wing does the stall occur spanning across the wing?
Elliptical
What is geometric twist?
The camber is constant throughout the span but the angle of incidence is greater at the root than the tip
What is aerodynamic twist?
The camber is greater at the root than at the tip but the angle of incidence is instant across the span
Where does the total wing lift act?
It acts on the centre of lift
What is MAC?
The mean aerodynamic chord
- the chord line through the centre of lift
Where is the centre of lift located?
At 30% MAC
Where do the streamlines above and below the wings bend?
Above - towards the centre
Below - towards the wing tips
What is induced drag affected by?
The aspect ratio, aircraft speed and the wing tip design
How do you measure the theoretical lift/drag of a profile?
Dynamic pressure x area
If airflow over a wing increases in velocity, what happens to induced drag and parasite drag?
Induced drag decreases
Parasite drag increases
What is the equation for the AREA of a tapered wing?
Wing span x average chord
What boundary layer would you find the most amount of kinetic energy?
The turbulent layer
What is another name for the polar diagram?
Lilienthal diagram
What is the polar (Lilienthal) diagram used for?
To find the angle for the best glide ratio
The most efficient operation of the profile
Is the induced drag LARGER on a low aspect ratio or a high aspect ratio?
Induced drag is less on a high aspect ratio
What can be added to wing profiles to reduce induced drag?
- Winglets
- Wing tip fences
Where does an aircrafts total weight act?
The centre of gravity
What type of stall would an aircraft recover without pilot input?
A wing root stall
What type of stall is the most dangerous?
A wing tip stall as the aircraft rotates in the nose-up position
What is used on a small aircraft to prevent wing tip stall?
A stall strip (knife edge device on the leading edge of the wing root)
-disadvantage is that it disturbs the lift
What are used on larger aircraft to prevent wing tip stall?
Slats on the leading edge - giving the boundary layer more energy preventing flow separation
What is spanwise flow?
Airflow over a swept wing
- air following the leading edge
- air at right angles to the leading edge
How can spanwise flow be reduced?
- wing fences (boundary layer fences)
- saw tooth leading edge
What determines the number and location of vortex generators?
The flight test investigations
What is the purpose of a vortex generator?
It improves the boundary layer control by producing lift
How do you calculate the aspect ratio?
Wing span (b) —————— Average chord (c)
(Or the span squared over the area)
What would an average high performance sail plane’s aspect ratio be?
35
What would a jet fighter’s average aspect ratio be?
3.5
When is an aircraft in level flight?
When the CofG is in the same position as the centre of lift
