8.2 aerodynamics Flashcards
kinetic energy =
KE = 1/2 p x V2. p = local air density V = is speed in m/s. KE = joules
principle of continuity
energy and mass can neither be created or destroyed. they can only be changed from one form to another
smooth regular airflow patterns around an object are called
laminar flow
turbulent airflow is
when air is disturbed and separates from the surface of a moving body .
a flying body does not attain the speeds necessary to compess air in what region
subsonic region
the speed of airflow is inversely proportional to the area of the cross-section of the tube if the density remains constant.
V1 x A1 = V2 x A2
Principal of continuity
diffuser outlet means
when the diameter increases and the speed decreases
jet outlet means
when the diameter decreases and speed increases
in bernoulis principle using a valve what happens to the pressures
as valve is opened, static press decreases whilst dynamic press increases. the total press is always unchanged.
dynamic pressure =
total pressure - static pressure
dynamic pressure expressed mathematically is
q = 1/2 p x V2
dynamic pressure increases sixteen times if speed increases by how much
4 times
dynamic pressure is indicated to the pilot as
the Indicated Air Speed (IAS)
at the point of stagnation…
the speed of the airflow falls to zero and static press equals total press.if there is no dynamic press, there is no flow
the difference in static pressure acts on the surface to create what force
lift
the boundary layer is
the layer of fluid in the immediate area of a surface
no slip condition is
when air flows over a wing, the fluid at the surface are stationary, as molecules at the surface are brought to rest by friction
after the boundary layer what happens
layers bind to each other but shear slightly relative to the layer beneath them until a layer reaches the velocity of the free stream
the fixed boundary is
where the layer close to the skin of the wing has zero velocity
the moving boundary is
the point where the air has met free air stream velocity
the region between the fixed boundary and moving boundary is called the
boundary layer
if the aircraft velocity is increased what happens to the boundary layer
it becomes thinner
if fluid viscosity is decreased or fluid density is increased what happens to the boundary layer
it becomes thinner
laminar flow is
smooth flow parallel to one another. stream lines not crossing eachother.
what can affect laminar flow
rivets and other protruding objects
laminar flow over a wing is desirable because
it gives high lift and low skin friction drag
at sufficinent distance where air is unaffected by the aircraft moving through it, it is considered to be what
free stream velocity
relative airflow is the direction of the airflow with respect to the object moving through it
in level flight - the aircraft is flying directly into the wind
nose up attitude - the wind would appear to come from below.
a region of space around the aircraft where the velocity of the relative airflow is zero
stagnation point
common location for stagnation points on an aircraft is
leading edges on wings and it results in a local increase in static pressure compared to the surrounding fluid.
if the angle of attack is increased, what happens to the transition point
it moves closer to the leading edge and the adverse pressure gradient becomes stronger.
advantages of laminar flow
low fuel usage
greater range
faster top speed
glide further
disadvantage of laminar flow
due to lower kinetic energy contained, its easier for flow separation to occur.
this results in easier stalling at lower AOA
upwash is
as air flows towards a wing, it will be turned towards the low-pressure region of the upper surface of the aerofoil.
downwash is
as the air flows back over the aerofoil it will tend to slow down and returns to its original state
vortices are
circular patterns of rotating air formed behind a wing or control surface as it generates lift or force from the pressure differential
vortices rotate which way
in a conventional view from behind
clockwise - left wing tip
anti-clockwise - right wing tip
camber is
mean camber line is a line drawn of equal distance between the upper and lower surfaces of the profile.
chord is
chord line is a straight line connecting the leading edge and trailing edge. the chord is the distance between leading and trailing edges.
mean aerodynamic chord (MAC) is
average chord length of a tapered or swept wing
profile (parasite drag) is
not related to the lift.
caused by distribution of pressure(form drag).
skin friction(friction drag) or
aerodynamic interference (interference drag)
induced drag is and is affected by
the drag on the wing caused by the lift.
.aspect ratio
.the wing tip design
.the aircraft speed
centre of pressure
AKA centre of lift is the point on the chord line where the total sum of a pressure field acts on a body.
angle of attack (AOA)
the angle between the chord line of the profile and the relative wind. often referred to as a (alpha).
wash in and wash out is
“wash in” refers to an angle of incidence that it greater toward the wing tips.
“Wash out” refers to an angle of incidence that is greater at the wing roots
fineness ratio is (thickness chord ratio)
maximum thickness of a section expressed as a percentage of the chord from the leading to trailing edge
e.g. typical low speed profile can have max thickness of 18% of chord line
30% aft of leading edge
wing shape is
.elliptical wing -constant downwash behind wing,constant local AOA and flow sep. entire wing stalls at same time
.rectangular wing -large tip vortex, large downwash at tip.lower AOA. tip last to stall
.tapered wing - downwash increases towards root tip stalls before root
.swept wing - most common, stalls at tip first dangerous implications for lateral controls and aircraft stability.
aspect ratio is
is the ratio between the length and the average width of the surface
length of wing / width of wing
the camber of the profile is
the displacement of the mean camber line from the chord line
typical low speed profile has a max camber of
5% of chord line located 45% aft of leading edge
the fuselage section through which the wing is installed is included in the wing area calculation
.
geometrically twisted wing
the camber is constant across the wing. but angle of incidence is greater at the root than at the tip.
chord lines are not parallel
aerodynamically twisted wing
-camber is greater at root than at the tip.
and angle of incidence is constant across wingspan.
-chord lines are parallel
-flow separation at root before tip
angle of incidence
the angle between the chord line of the profile and the longitudinal axis of the aircraft.
denoted by y (gamma)
what is considered the max AOA before flow separation occurs and lift can no longer be sustained. aka stalling angle
15 - 18 degrees
at normal cruising airspeeds with a small positive AOA, the CP is positioned where
approx 25% back along the chord line.
sweep angle is
the angle between the line of 25% chords and a line perpendicular to the root chord.
positive sweep = backwards
negative sweep = forwards
swept wing adv and disadv
adv. - provide stability in tailess designs. only show adv for aircraft designed to fly close to speed of sound.
dis. - reduce the amount of lift produced for a given flight speed. tends to have a poorer ratio of lift to drag than equivalent straight wing.
positive dihedral . dihedral angle is
when wingtip is higher than the root. this angle icreases roll stability aka lateral stability.
anhedral angle (negative dihedral) is
when the wing tip is lower than the root. this angle increases roll performance. often used on aircraft that require agility and manoeuvrability.
compressible drag is
caused by the shock waves on an aircraft approaching the speed of sound
what effects does speed have on lift, vortices and induced drag
low speed flight, aircraft has high AOA therefore a high lift coefficient. large pressure difference creates large wing tip vortices and therefore high induced drag.
what does form drag depend on
the frontal area of a body and the speed of the airflow
interference drag can be reduced by what
fairings e.g. engine pylons, flap tracks
compressible drag happens when
aircraft is in transonic and supersonic flight. aka wave drag
total drag =
induced drag and parasite drag combined to give total drag force on aircraft
at a constant airspeed in flight what is thrust equal to
drag
when the CG is forward of the CP what happens
natural tendency for the aircraft to want to pitch nose down vice versa
designers fixed the aft limit of CG forward of the CP to
retain flight equilibrium
in stabilised level flight, when lift force is equal to weight what happens
aircraft is in state of equilibrium
when lift becomes less than weight what happens
vertical speed will decrease
lift to drag ratio is an efficinecy paramater for what
total aircraft aerodynamics
thrust to weight ratio is an efficiency factor for what
total aircraft propulsion
force equation (Newtons second law motion of constant mass) =
F = m x a
weight equation =
W = m x g
mass equation =
m = w/g
what factors affect the generation of lift and drag
circulation imparted to the airstream (magnus effect)
lift and drag mainly depend on what
Airstream Velocity (V) which, when combined with air density (p), determines dynamic pressure.
dynamic pressure =
q = 1/2 pV2
kinetic energy =
y= 1/2mV2
aerodynamic forces are influenced by what 4 factors
the dynamic pressure
the surface area of the profile
the shape of the profile
the AOA
theoretical lift = dynamic pressure x surface area
=1/2 x p x V2 x A
it is not possible to calculate the actual lift what do we use instead.
measure it using a wind tunnel
coefficient of lift and drag are brought into the equations…
to account for the difference between the measured and theoretical values
an advantage of a high maximum lift coefficient is that
the aircraft can fly slowly
the method for evaluating wind tunnel tests was invented by who
Otto Lilienthal. diagram to find out the best glide ratio
definition of a stall
a sudden reduction of lift generated by an aerofoil when a critical AOA is reached. usually around 15%
a deep stall or super stall is a dangerous type of stall that affects what
aircraft designs with a T-tail config. and rear mounted engines. elevators become ineffective which prevents aircraft from recovering from stall.
wing tip stall..
is more dangerous than root. cenre of lift moves towards the root and forward of CG. nose up position
what is used to prevent wing tip stalling
stall strip used on smaller aircraft. mouted at leading edge. disadv is it disturbs lift
wing tip stall prevention on large aircraft/ swept wing is
slats. extend if AOA is too high.
types of ice formed
Frost
rime ice
clear ice
contamination on aerofoil can do what
cause turbulence
extra weight
loss of lift
and freezing or unbalancing of flying controls
clear ice is most dangerous. heavy, hard to see, breaks away in large lumps
.
The airflow between the streamlines is. Often compared to what
Stream tube
From the thickest point of the wing to the trailing edge, the pressure gradient is
Negative
Positive pressure gradient is found where
Near the leading edge
