8.2 Aerodynamics Flashcards
Air in the subsonic region is?
Incompressible
Air streamlines are?
Parallel to each other
The continuity equation states?
The continuity equation states that the speed of the airflow is inversely proportional to the area of the
cross-section of the tube as long as the
density remains constant
As the area cross section is halved the speed?
Doubles
When the diameter of pipe increases and the speed decreases it is known as what type of outlet?
Diffuser outlet
When the diameter of a pipe decreases and the speed increases it is known as what type of outlet?
Jet outlet
What is Bernoulli’s principle?
Bernoulli’s principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure
How is dynamic pressure calculated?
The total pressure minus the static pressure
What is the stagnation point?
The point on the wing where speed is zero, and static pressure equals total pressure
Which pressure is known as the pressure with potential energy?
Static pressure
Which pressure is known as the pressure with kinetic energy?
Dynamic pressure
On an aerofoil which surface has unchanged static pressure?
Lower surface
On an aerofoil which surface has low static pressure and high dynamic pressure?
Upper surface
What is upwash?
Some of the streamlines approaching the profile in a low position slope upwards in front of the wing and pass the aerofoil on the upper surface
What is downwash?
Looking at the trailing edge, you see that some of the streamlines of the upper surface flow downwards when leaving the profile
What is the magnus effect?
As a cylinder rotates, circulatory flow creates higher speed on the upper surface and lower speed on the lower surface, generating lift
Why does the air near the trailing edge tend to flow to upwards?
Because the upper surface has low static pressure due to the increased speed of flow
What is the profile of a wing?
The cross section
What is the chord line?
The chord line is a straight line connecting the leading edge and the trailing edge
What is the mean camber line?
The mean camber line is a line drawn halfway between the upper and the lower surfaces of the profile. The shape of the mean camber line is very important in determining the aerodynamic characteristics of a profile
At what points does the mean camber line start and end on a wing?
The start and end points of the mean camber line are the same as the end points of the chord line
What is the camber?
The camber of the profile is the displacement of the mean camber line from the chord line
What defines the shape of the mean camber line?
The maximum camber and the location or the maximum camber
The maximum camber and the location or the maximum camber use what quantities?
These quantities are expressed as a fraction or a percentage of the basic chord dimension
What is the fineness ration?
The maximum thickness as a fraction is also known as the fineness ratio
The maximum thickness of a profile is defined as?
A fraction or a percentage of the chord
The flight path velocity is?
The speed of the aircraft in a certain direction through the air
The relative wind is?
The speed and direction of the air acting on the aircraft which is passing through it
The relative wind is opposite in?
Direction to the flight path velocity
What is the angle of attack?
The angle between the chord line of the profile and the relative wind
Angle of attack is denoted by?
It is denoted by α (alpha)
The angle of incidence is?
The angle between the chord line of the profile and the
longitudinal axis of the aircraft
Angle of incidence is denoted by?
It is denoted by 𝛄 (gamma)
What are the rules when it comes to vortexes?
They always form in pairs,
They rotate counter clockwise,
On an aerofoil where are the points of higher than ambient static pressure?
At the front of the leading edge and at the trailing edge
What are the two components of aerodynamic force?
Lift and drag
What is the aerodynamic force?
The aerodynamic force is the resultant of all forces on a profile in an airflow acting on the centre of pressure
What is the centre of pressure?
The centre of pressure is the point on which all pressures and all forces act
Where is the centre of pressure typically on an aerofoil?
This point is located where the cord of a profile intersects with the resultant of the aerodynamic forces of lift and drag
What are the variables that when combined dictate the forces of of lift and drag?
The dynamic pressure
The surface area of the profile
The shape of the profile
The angle of attack
do questions on page 58 to
81 onwards
What are three main categories of drag?
Induced drag
Parasite drag
Compressible drag
What is total aircraft drag comprised of?
The sum induced drag parasite drag compressible drag
What type of drag is caused by lift?
Induced drag
Which type of drag can be caused by form, friction and interference?
Parasite drag
Which type of drag is caused by shockwaves?
Compressible drag
How is induced drag created?
High pressure flow under the wing spills over the wing tips mixing with the low pressure air on the top of the wing, this creates energy absorbing vortices creating drag
Induced drag is lower with what type of lift distribution?
Elliptical lift distribution
As well as wingtip vortices, what other vortex is present on a wing?
Bound vortex
What is a bound vortex?
A bound vortex is a vortex that effects airflow over the wing as well as the air that in front of the wing
What are the effects of the bound vortex?
Increased drag as well as less lift created due to the reduced effective angle of attack
What is the induced alpha?
The angle between the effective airflow and the relative airflow
With a strong downwash of the bound vortex, what is the effect on the tailplane?
Due to the deflected flow in a downwards direction the effective angle of attack is reduced
Which type of aspect ratio has the least amount of tip vortex and therefore induced drag?
High aspect ratio wings
When are wing tip vortexes smallest?
In cruise, with a low angle of attack when the pressure difference between the upper and lower surfaces is small
When are wing tip vortexes largest?
With a high angle of attack with the largest pressure difference between the upper and lower surface
What is form drag?
Created by the pressure distribution on a body, namely the lack of symmetrical flow patterns creating different pressure areas, the difference in pressure causes form drag
How do you reduce form drag?
By streamlining
What is form drag on a wing called?
Wing drag or profile drag
What is the relationship between form drag and friction drag on a profile?
A profile with a low form drag has a high friction drag and a profile with a high form drag has a low friction drag
What is the relationship between length to diameter of a profile?
The profile with a higher length to diameter ratio has the lowest form drag and a profile with the lower length to diameter ratio of one has the highest form
drag
What is friction drag?
Caused by the first layer of airstream on the wing getting caught in the rough surface, slowing to zero, this has the effect of retarding the airstreams above it, this continues until freestream velocity is restored
What are the two types of boundary layers?
Turbulent and laminar
Where is the laminar flow boundary located?
Just downstream of the leading edge
Where is the turbulent boundary layer located?
Downstream of the laminar boundary layer
In the turbulent boundary layer what happens to the layers of airstreams?
The travel from one layer to another, exchanging energy
Which boundary layer is thickest?
The turbulent boundary layer
How much more friction drag does the turbulent boundary layer create compared to the laminar layer?
About three time more
Which boundary layer has less tendency for flow separation?
The turbulent because of higher kinetic energy close to the surface
How do air particles flow as it passes across a wing?
An air particle around a profile moves from a high
pressure area to a low pressure area and then back to a high pressure area again
What can be used to prevent flow separation?
A slot near the training edge
How does a slot work?
The slot transfers air with high energy from the lower side to the upper side of the profile and this gives the stationary air particle the energy it needs to move to the high pressure area at the trailing edge
What is a laminar wing?
A laminar wing is one which has low friction drag because it has a larger laminar region and a larger delayed transition to the turbulent boundary layer region
What is interference drag?
Interference drag is the turbulence in the airflow caused by the sharp corners which result from when components are joined together or placed in close
proximity
How can interference drag be reduced?
By use of fairings
When does compressible drag occur?
Compressible drag only occurs in transonic and supersonic flight
What causes compressible drag?
It is caused by the shock waves on an aircraft approaching the speed of sound. Sometimes, it is called wave drag
Which speed region do you find a mix of subsonic and supersonic airflow?
Transonic
How does compressible drag get created?
As the boundary layer pass through a shockwave it thickens and a flow separation occurs, this flow separation is the cause of the additional drag
What two components make up total drag?
Induced and parasite drag
With increasing speed, what is the relation ship of speed, total drag, parasite drag and induced drag
Total drag is very high at low speeds because of the high induced drag.
It then decreases to a minimum at an intermediate speed, and then increases again because of the increasing parasite drag
What is the wing area?
The wing area is the projection of the outline on the plane of the chord. It includes the area of the fuselage which is between the wings
What is the wing area formula?
wing area (S) which equals the wing span (b) multiplied by the chord of the wing (c)
How is wing tip chord denoted?
Ct, is the chord at the wing tip
How is wing root chord denoted?
The root chord, Cr, is the chord at the wing centreline
What is the wing taper ratio formula?
The taper ratio, λ (lambda), is the ratio of the tip chord to the root chord:
λ = Ct/Cr
What is the wing average chord?
The average chord (c) is the geometric average of all the chords:
c = (Ct + Cr)/2
How is wing area calculated?
The wing area (A) is the average chord multiplied by the wing span
What is the problem with a wing that has to much of a taper ratio?
The risk of a tip stall is very high
What is the benefit of a wing with a high taper ratio?
High local stiffness at the root
With a rectangular or un-tapered wing what are the vortexes produced like?
produces a much larger vortex than the
tapered section. This is because the longer the tip chord, the greater the spillage of air from the lower surface onto the upper surface, and the larger the wing tip vortex
What are the two trade-offs for taper ratio?
The main trade-offs are between structural merit and adequate resistance to tip stall
What is the formula for aspect ratio?
The aspect ratio (AR) is the wing span (b) divided by the average chord (c)
With regards to wing sweep which direction does a wing sweep with positive sweep?
Backwards
With regards to wing sweep which direction does a wing sweep with negative sweep?
Forwards
What is the relationship between sweep angle and roll axis stability?
The more swept-back a wing is the more stable the aeroplane becomes on the roll axis
How does span wise flow on a swept wing effect stability in roll?
The span-wise flow (flow component along the chord; does not produce lift) on a sweptback wing is directed outwards and it increases towards the tip. Side-slip caused by banking decreases span-wise flow on the
lower wing and increases it on the higher one and the lift difference tries to level the wings
What can occur is the wing is sufficiently swept back when flying supersonic?
If the angle of sweep is sufficient, the normal component of velocity can be slower than the speed of sound even when the aircraft is flying faster than the speed of sound
How does the sweep angle depend on speed?
The normal component of velocity is roughly equal to the relative air speed multiplied by the cosine of the angle of sweep. It follows that the amount of
sweep required increases with increasing aircraft maximum speed
What is the critical Mach number?
The speed where the shock wave begins to appear (or where the airflow reaches the speed of sound) is called the Critical Mach number
Disadvantages of swept wings?
Swept wings reduce the amount of lift produced for a given flight speed, wing area and angle of attack
Larger and heavier wings
Poorer ratio of lift to drag
Advantages of swept wings?
Swept wings only show advantage for aircraft designed to fly close to or above the speed of sound. For low speed aircraft, they have positive disadvantage, as outlined above, and it would be a mistake to introduce wing sweep for purely aesthetic reasons
Wing sweep is also used as a means of providing stability in tailless designs
What are the benefits of dihedral?
Increased stability in roll
What are the benefits of an-hedral?
The wing tips are lower than the wing base. This angle increases the roll performance. Anhedral wings increase manoeuvrability of the aircraft and are
commonly used in fighter aircraft
What occurs with the matching vortexes with one at the leading edge and one at the trailing edge?
The matching vortex to that at the trailing edge is the circulating vortex, which reinforces the airflow on the top of the aerofoil, making it faster
What is the formula for theoretical lift?
Dynamic pressure multiplied by surface area
How is dynamic pressure calculated?
Dynamic pressure is half the air density multiplied by the velocity squared
What is the full formula for theoretical lift?
½ x ρ x V2 x A
How is actual lift measured?
In a wind tunnel, with a universal joint and two scales, vertical to measure lift and horizontal to measure drag
What is the purpose of coefficient of lift?
A coefficient of lift (CL) is introduced to the lift equation to account for the difference between the measured lift and the theoretical lift
What two components make up the coefficient of lift?
The coefficient of lift is the measured lift divided by the theoretical lift
What is the lift equation taking into account the coefficient of lift?
The lift equation is now the coefficient of lift multiplied by the dynamic pressure multiplied by the surface area
What is the purpose of the coefficient of drag?
The coefficient of drag (CD) is introduced to the drag
equation to account for the difference between measured drag and theoretical drag
How is the coefficient of drag calculated?
The coefficient of drag is the measured drag divided by the theoretical drag
What is the drag equation?
The drag equation is the coefficient of drag multiplied by the dynamic pressure multiplied by the surface area
What is the coefficient of lift a function of?
Angle of attack and the shape of the profile
With a change in angle of attack, what else changes?
The measured lift and the coefficient of lift
With an ever increasing angle of attack, what happens to the coefficient of lift?
The coefficient of lift will increases until it reaches alpha max, which is the point of maximum coefficient of lift and angle of attack, it will then begin to decrease as the air cannot follow the upper surface and the wing stalls
With two profiles, with the same camber, but with one being thicker, what is the effect of the thicker profile?
The thicker profile has the same coefficient of lift at lower angles of attack but a higher coefficient of lift when the angle of attack increases above approximately ten degrees.
You can also see that the thicker profile has a higher maximum coefficient of lift and a higher alpha max
What is the effect of a profile with high camber?
High camber allows the profile to have a high coefficient of lift at zero angle of attack, the profile will also typically have a higher maximum coefficient of lift but a lower maximum alpha max
What is the advantage for a profile with a high maximum lift coefficient?
The aircraft will be able to fly slowly
What is the disadvantages for a profile with a high coefficient of lift?
The disadvantages are that the thickness and camber necessary for profiles with a high maximum lift coefficient may produce high drag and a low critical
Mach number
What is the effect of ice formation on a wing?
The build-up of frost, snow or ice increases the weight and shape of the object on which it is formed.
Upper surface frost, especially leading edge ice formation, reduces the maximum coefficient of lift and the maximum angle of attack
How does angle of attack effect the coefficient of drag?
At lower angles of attack the coefficient of drag is low
and small changes in the angle of attack produce only slight changes in the coefficient of drag.
At higher angles of attack the coefficient of drag is much greater and small changes in the angle of attack produce significant changes in the coefficient of
drag. It can be seen that a stall produces a large increase in drag
What diagram can provide information about a profile?
The polar diagram (Lilienthal)
How does a polar diagram give us information about a profile?
The polar curve shows how the lift and drag coefficients can be combined to give us information about the performance of profiles by plotting the
coefficient of lift against the coefficient of drag for each angle of attack
Using the polar diagram what else can be said about the aircraft?
If you draw the diagram with the coefficients of the aircraft, it can also tell you about the layout of a complete aircraft.
A higher glide ratio means a lower drag at a given lift. This results in a lower installed engine thrust to overcome the drag, and this means lower weight,
lower fuel consumption, higher payload or longer range
When gliding what compensates for the lack of thrust
The potential energy of the lost height substitutes the thrust of the engines and compensates the drag
What are the four forces to take into account when discussing a glide?
The weight of the aircraft acts vertical to the ground and the drag acts parallel to the glide path. The third force is the lift perpendicular to the glide-path, and
fourth, the resultant of lift and drag
An aircraft with high efficiency will have what effect on glide angle and drag?
The smaller the glide angle, the smaller the drag vector, the smaller the necessary
thrust, and the higher the efficiency of the aircraft
What is the variation of the polar diagram?
The lift to drag diagram
How is the lift to drag diagram different to the polar diagram?
The ratio of the lift to the drag is plotted against the angle of attack
On the lift to drag diagram what is the the same as in the polar diagram?
The lift-drag ratio diagram shows the maximum lift drag ratio, which is the same as the best glide ratio in the polar diagram
When an aircraft enters a climb, what additional component must be counteracted with thrust?
In order to maintain a constant speed in the climb, the thrust of the aircraft must now not only equal the drag, but also the rearwards acting weight W Sin θ
With steeper rates of climb what must be done to maintain speed?
The steeper the climb, the greater the component W Sin θ hence the need for additional thrust to maintain constant airspeed
As well as weight acting parallel to drag, what other factor requires more thrust?
As well as climb increasing the drag force, the amount of lift generated (signified by W Cos θ) will also be less than the aircraft weight. The pilot must therefore increase the thrust in order to provide the necessary lift force to climb the aircraft
