8.2- Aerodynamics Flashcards
What is the dynamic pressure proportional to?
Density and square of velocity.
What is the formula for Kinetic energy?
Half p x V squared
p= air density
How is kinetic energy passed onto the tube material?
Usually as elastic potential energy.
What is streamline or free stream flow?
Where the particles of a fluid move in an orderly manner and maintain relative positions.
What is laminar flow?
Smooth regular airflow patterns around an object
When does turbulent flow occur?
When air is disturbed and separates from the surface.
What is the result of turbulent flow?
Formation of swirling eddies in the bodies wake.
Is the air compressible in the subsonic region?
No
What does the continuity equation state?
Speed of the airflow is inversely proportional to the area of the cross section of the tube.
What is a diffuser outlet?
Used when the diameter increases and the speed decreases.
In Bernoulli’s principle when the valve is closed what is the total pressure?
Equal to the static.
In Bernoulli’s principle does the total pressure ever change?
No
Where is actual static pressure measured?
Static port
Because there is only a total pressure pitot and static how is dynamic measured?
Total - static = dynamic
How much does the dynamic pressure increase when the speed increases 4 times?
16 times
How is the dynamic pressure indicated to the pilot?
As indicated air speed (IAS)
What is the point of stagnation?
The point where the airflow falls to zero and the static pressure equals the total pressure.
If there’s no dynamic pressure what else is there not?
Flow
In a venturi tube where is the airflow the fastest?
The narrowest point.
What is the boundary layer?
The layer of air immediately in the area of the surface.
What is the no slip condition?
Where the air on the skin is stationery due to friction.
What happens in the air layers between the no slip and free stream?
Each layer shears slightly more than the last.
What happens to the boundary layer as the aircraft velocity is increased?
It becomes thinner.
What happens if the air viscosity is decreased or density is increased?
The boundary layer becomes thinner.
What is the laminar flow?
When the particles all flow parallel to one another in layers.
Why is laminar flow desirable?
High lift and low skin friction.
Why is laminar flow low drag?
Low shear stress of the layers of air.
How is it easier to achieve a laminar flow?
With a positive pressure gradient from the leading edge.
The pressure gradient is negative from where to where.
From the thickest part of the wing to the trailing edge.
How does wing stall happen?
When the energy in the flow is insufficient, it detaches from the surface and the flow stops and starts to reverse.
What does an increase in angle of attack do to the flow?
Airflow becomes more turbulent.
What is relative airflow?
Direction of airflow with respect to the object moving through it.
What is the stagnation point?
Region of space around the aircraft where the velocity of the relative airflow is zero.
What does the stagnation point result in?
Local static pressure.
What is a common stagnation point?
Leading edges of the wing
In straight and level flight the laminar boundary is located where?
Immediately downstream of the leading edge.
What is the transition point?
Where the boundary layer changes from laminar to turbulent which is near the maximum point of thickness.
If the angle of attack increases what happens to the transition point?
Moves closer to the leading edge and the adverse pressure gradient becomes stronger. This moves the separation point forward.
How does the wing get a thrust effect?
Flow of the air is assisted by its movement by flowing from a high to low pressure.
What happens when the flow seperates?
Lift is reduced and pressure drag increases.
Are turbulent boundary layers more energetic then laminar?
Yes
Is flow separation more or less likely in a turbulent boundary layer?
Less likely as it is more resistive to the separation.
How is flow separation felt in light aircraft?
Buffeting with the control surfaces going light
In heavier aircraft how is the pilot warned of flow separation?
Stall warning devices.
What are the advantages of laminar flow?
Less surface friction giving lower fuel use etc.
What is the issue with laminar flow?
It is difficult to achieve at high speed and high angles of attack.
How is wing stall avoided in the design of commercial aircraft?
Deliberately cause turbulent flow over the rear part of the wing.
What is upwash?
The oncoming air flowing onto the upper surface due to the lower pressure.
What is the suction peak?
The large area above the wing where the accelerated flow around the leading edge sucks in the surrounding air.
What is downwash?
After passing over the wing the air returns to its normal state.
What is the bound vortex?
Circulation around the profile causes upwash on the LE and a downwash on the TE.
What do the gutters above the doors represent?
The upwash and downwash of the vortices.
What does the downwash do?
alter the flow direction and speed in the vicinity of the wing and tail surfaces.
What is induced drag?
Increase in angle of attack tips the total reaction force aft increasing in drag as well as lift.
What is induced alpha?
The angle between the effective airflow and relative airflow.
The stronger the vortices the….?
Stronger the induced drag.
How are vortices started?
The pressure differential between upper and lower surface aft of the wing/.
What is the chord line?
Straight line connecting the leading edge and trailing edge.
What is the chord?
The distance the chord line covers from LE to TE.
What is the mean camber line?
A line drawn equidistant from the upper and lower surfaces.
What is the camber of the profile?
The displacement between the mean camber line and the chord line.
How is the maximum camber expressed?
As a percentage of the chord.
When the camber line is above the chord line what is it said to have?
Positive camber
What does a typical low speed profile have as its maximum camber?
5% located aft 45% of the leading edge.
What is the fineness ratio?
Maximum thickness or depth of a section expressed as a percentage of the chord.
What is the fraction?
Maximum thickness of the profile
What maximum thickness does a typical low speed profile have?
Maximum 18%, 30% after the leading edge.
What is the aspect ratio of the aerofoil?
Ratio between the length and average width of the surface.
What is the aspect ratio of the wing?
Ratio of wing length to the average chord.
What is the formula for the aspect ratio of a tapered wing?
Wing span squared divided by the wing area.
Is the aspect ratio inversely or directly proportional to the induced drag?
Inversely
What kind of aspect ratios do gliders have?
High
What is induced drag affected by?
Aspect ratio
Wing tip design
Aircraft speed
What is wash in?
The angle of incidence that is greater toward the wing tip.
How is wash in wash out acheived?
Geometrical twisting of the wing.
Where is the angle incidence greater in a geometrically twisted wing?
Root
On an aerodynamically twisted wing what happens to the angle of incidence?
It is constant across the wing.
What is the mean aerodynamic chord?
Average chord length of a tapered or swept wing.
How is the mean aerodynamic chord found?
At the root draw two extension lines that equal the tip. Do the opposite at the tip, from each corner cross the lines over. Where they cross is where the MAC is.
What is the angle of attack?
Angle between the chord line of the profile and the relative wind.
What is the angle of attack often referred to as?
Alpha
What is the angle of incidence?
Angle between the chord line and the longitudinal axis of the aircraft
What is the angle of incidence denoted by?
Gamma (y)
Is the angle of incidence fixed or variable for the wing?
Fixed
What is the centre of pressure?
Is the point on the chord line where the total sum of a pressure field act on a body… otherwise known as the centre of lift.
What can the total reaction be broken down into?
Lift and drag
At normal cruising speeds with a small positive angle of attack where is the CP positioned?
25% back along the chord line.
If the angle of attack increases what happens to the centre of pressure?
Moves forward toward the LE.
For most aerofoil designs what is considered the max AoA before flow separation?
15-18%
What is the stalling angle?
The angle where flow separation occurs and lift can no longer be sustained.
What are the four different wing shapes?
Elliptical
Rectangular
Tapered
Swept
What is the reason for different stall characteristics of each wing shape?
The down wash behind the wing changes the local angle of attack.
An elliptical wing has a constant down wash, where does it stall?
The entire wing
What part of a rectangular wing is the last to stall?
The tip.
On a tapered wing what is the last part to stall?
Root
Where does a swept wing stall first?
Tip
What is the issue with stalling at the tip first?
Implications for the lateral control and stability of the aircraft.
What is the sweep angle?
The angle between the line of 25% chords and a line perpendicular to the root chord.
What way does a positive sweep go?
Backwards
What happens the more swept back a wing is?
The more stable it is on the roll axis.
What is the critical mach number?
Speed at which the shockwave begins to appear.
What is a key benefit of swept back wing in terms of the airflow and mach number?
Reducing the amount of airflow flowing parallel to the chord line, there is a reduction in the acceleration and therefore an equivalent reduction in the critical mach number.
What are some of the negatives of swept wing?
They tend to be heavier and bigger as they produce less lift
Has a poorer ratio of lift to drag
What is a positive dihedral wing?
A wing where the tip is higher than the root.
What does a dihedral angle increase?
Roll stability
What is anhedral?
Negative dihedral
What is the benefit of anhedral?
Helps to reduce the amount of lift the wing can generate at high angles of attack.
What is total aircraft drag a sum of?
Induced
Parasite
Compressible
What is parasite drag?
Caused by the distribution of pressure (form drag) skin friction and aerodynamic interference.
What is compressible drag?
Caused by the shock waves on an aircraft approaching the speed of sound.
Where is the spanwise flow the strongest?
The wingtip
What is the direction of the local airflow?
The average between the free stream relative airflow and the direction of the downwash.
During low speed flight why are the wing tip vortices large?
Large pressure difference between the lower and upper surface of the wing.
How is form drag reduced?
Streamlining.
What profile has the lowest form drag?
Highest length to diameter ratio.
What is the relationship between form drag and friction drag?
A form with low form drag has a high friction drag.
How can interference drag be reduced?
Fairings such as wing roots, pylons and flap tracks.
Where does compressible drag occur?
Transonic and supersonic flight.
What is compressible drag also referred to as?
Wave drag.
What happens to the boundary layer as it passes through the shockwave?
It thickens up.
What process causes compressible drag and how does it happen?
Flow separation as a result of the boundary layer thickening and an adverse pressure gradient across the shockwave.
Where is total drag at its lowest?
Intermediate speeds.
How is the direction of thrust changed?
Pivoting the engines or vectoring the exhaust gases.
What is the centre of gravity?
The point at which all the weight of the aircraft is acting.
What happens when the centre of gravity is forward of the centre of pressure?
The aircraft will want to pitch down.
What is the ratio of thrust to weight?
Thrust divided by weight
What is the lift drag ratio?
The slope angle of the resultant force when lift is created as it doesn’t act straight up but inclined backwards.
How is the lift drag ratio changed?
With angle of attack; as it increases the vector leans further back.
What is the magnus effect?
Mechanically induced circulation creating lift.
How much lift does an aerofoil produce at zero angle of attack?
zero
What theorem relates to the circulation of air around a wing?
Kutta-Joukowski
Equation for lift per metre of span?
air density (p) x airspeed (V) x circulation (K)
What is the equation for dynamic pressure?
1/2 pV squared
What is the equation for kinetic energy?
1/2 mV squared
It is not possible to calculate the actual lift so how is it done?
Wind tunnel
What do the scales in a wind tunnel measure in terms of lift measuring?
Horizontal scale to measure drag
Vertical scale to measure lift.
What is coefficient of lift?
It is used in the lift equation to account for the difference between the measured lift and the theoretical lift.
What is the equation for the coefficient of lift?
Measured lift divided by theoretical lift.
With the coefficient of lift included what is the equation for lift?
CL x dynamic pressure x surface area
What is the equation for coefficient of drag?
Measured drag divided by theoretical drag.
With the coefficient of drag included what is the equation for drag?
Cd x 1/2 x p x V squared x A
At maximum angle of attack what is the CoL?
Max coefficient of lift.
What does a higher glide ratio give?
Lower drag at given lift
In a Lilienthal diagram, the angle between the lift vector an the resultant is the same as what?
The angle of the glide path to the horizon.
What happens when the angle of glide to the horizon is smaller?
Smaller drag vector, smaller the necessary thrust and the higher the efficiency of the aircraft.
What is the maximum lift to drag ratio?
Represents the most efficient operation of the profile.
What is the typical angle at which stall starts?
15 degrees
What is deep stall?
Dangerous type of stall that effects certain types of aircraft designs.
What aircraft is most susceptible to deep stall?
Jet aircraft with a T tail and rear mounted engines.
What is affected by a deep stall?
The elevators, preventing the aircraft from the aircraft from recovering.
What happens when there is flow separation at the root of the wing?
Centre of lift moves towards the tip and also behind the centre of gravity.
What is more dangerous flow separation at the tip or the root?
Tip.
What happens when the tip stalls?
The aircraft rotates to the nose up position, AoA increases, and the stall condition gets worse.
How is wing tip stall prevented?
Stall strip, like a knife edge device on smaller aircraft.
Slats on larger aircraft.
What is the most common type of contamination?
Snow and ice formations
What is the accumulation of ice layers known as?
Ice accretion
What are the three main types ice formation?
Frost
Rime ice
Clear ice
What does upper surface ice reduce?
Maximum coefficient of lift and the critical angle of attack.
What is frost?
Forms in clear air, when parked overnight.
What is rime ice?
Formed when small, supercooled water droplets freeze on contact.
What is rime ice at ground level?
Freezing fog.
What texture is rime ice?
Brittle due to its crystalline structure.
What is clear ice?
Heavy coating of glass like ice.
Where does clear ice form?
When flying in areas with a high concentration of large, supercooled water droplets.
Why is clear ice the most dangerous?
Because it is very hard to see. An breaks in large lumps.
