Chap C: Aerofoil & other aerodynamic shapes Flashcards
why is curved surface better than flat surface?
1) air flow smoother
2) more lift (compared to drag)
3) greater depth/thickness (more space for fuel & structural strength)
which flow speed is faster, above or below the aerofoil?
above the aerofoil
how does static pressure change when air flows on aerofoil and how does it affect the total pressure?
upper surface: static pressure decreases
lower surface: static pressure increases
Ptotal is constant, so when Pstatic decreases, Pdyn increases
what is total reaction force resolved into?
lift (perpendicular to direction of airflow)
drag (parallel to direction of airflow)
when aircraft is in straight and level flight, lift is?
vertical
(lift is horizontal during vertical climb)
pressure coefficient formula & signs of Cp when pointing in and out
(Plocal - Pfree stream) / Pdynamic of free stream
Cp -ve when pointing out
Cp +ve when pointing in
how does streamlines change when above the aerofoil?
bunched together indicating increased in flow velocity
force of wing formula using pressure differential
F = pressure differential x area
define pitching moment
a turning effect tending to raise or lower its nose results when the centres of gravity and pressure do no coincide
how does change in centre of pressure (CP) affect the nose wrt CG
if CP is forward of CG: nose pitch up
if CP is aft of CG: nose pitch down
if CP & CG same point: net forces cancel each other
define angle of attack
α, angle between chord line and relative wind
what is chord line?
line joining leading edge to trailing edge
what is stalling angle?
stalling angle is the AOA for maximum lift
important factors in stalling
1) suddenness & magnitude of lift loss
2) increase in pitching moment
what is the difference between stall speed and stall angle?
stall speed: minimum speed to maintain weight level flight
stall angle: max angle of wing to reach max lift
list the 3 types of drag
1) form/pressure drag
2) skin friction
3) induced drag
how is drag relative to AOA?
drag increases rapidly as AOA increases
how does lift change when AOA changes?
as AOA increases, lift also increases
best lift drag ratio is called
rigger’s angle / angle of incidence
what is best lift drag ratio?
when lift is the max and drag is min
what happens when we pitch moment about Leading Edge (LE) and Trailing Edge (TE)?
about LE: nose down, AOA decreases
about TE: nose up, AOA increases
effects of AOA on airflow
separation bubbles forms at trailing edge at no AOA
separation point moves forward as AOA increases
how does CP change when AOA changes?
Cp moves forward of aerofoil when AOA increases up till stalling
define aerodynamic centre
point on chord about which pitching moment is nearly constant at all AOAs
where is aerodynamic centre usually at?
25% chord length measured from leading edge
which is fixed, aerodynamic centre or CP?
AC is fixed,
CP moves forward as AOA increases up till stalling
NACA naming aerofoil
ABC
A - thickness of max camber as %tage of chord
B - location of max camber in 10th of chord
C - max thickness of aerofoil in %tage of chord
NACA 4412
max camber of 4% (A), located 40% (B) of leading-edge, max thickness of 12% (C).
wing span formula
b = 2s
b - wing span
s - semi span
fineness and thickness ratio formula
fineness ratio = c / tmax
thickness ratio = tmax / c
c - chord
tmax - max thickness
FR no dimensions
TR in %tage
fineness ratio wrt to frontal profile
increase in FR, decrease in frontal profile
what does thickness ratio affect?
maximum lift, critical Mach number
define angle of incidence
angle which an aircraft wing is mounted relative to its fuselage (also same as rigger’s angle)
possible lift & drag variables
1) shape
2) flow velocity
3) fluid density
4) AOA
5) viscosity (Re)
for lift: plan area
for drag: frontal area
life coefficient relative to AOA
same as lift vs AOA graph
CL increases linearly with AOA
why use plan area to calculate drag?
as frontal area change is much larger & harder to estimate
Pitching moment coefficient formula
use lift coefficient formula and change L to M and times 1/c, where c is chord length
pressure coefficient formula simplified
Cp = 1 - (Vlocal / V)2
what causes skin friction?
1) drag due to boundary layer (shear stress on the surface)
2) total effect taken over entire surface
3) higher in turbulent boundary layer
what cause induced drag?
aka trailing vortex, due to pressure differential between upper & lower surface of wing
exists in inviscid fluids as is not viscosity related
what does induced drag affect?
loss of lift at wing tips
greatest at high AOAs
direct proportional to lift2
inverse proper. to speed or b2
2 important factors during level flight/cruise
1) Lift = Weight
2) Thrust = Drag
induced drag relative to dynamic pressure and velocity
Di is inversely proportional to Pdyn & velocity
what do vortices create?
upwash away from the plane
downwash within the wing span
why do birds/aircraft fly in delta formation?
as the following bird/aircraft will have easier flight due to upwash from the leading bird/ac
higher lift, less drag
what is parasite drag?
drag due to non-lifting surfaces combined
what consists of profile drag and what does it depend on?
skin friction and parasite drag,
it depends on frontal area, density@alt and velocity
how to minimise induced drag?
1) increase aspect ratio (longer span/ shorted chord)
2) lower AOA
3) higher speed (requires less CL for the same lift
best theoretical planform for least induced drag
elliptical wings
what is winglet used for?
increase aspect ratio without substantially increasing wing span
what do winglets do?
reduce mixing of airflow between upper and lower surfaces of wing, therefore reducing induced drag
what are high-lift devices used for?
reduce take-off/landing speeds and distances
how does suction increase lift?
suction sucks in the sluggish flow thereby delaying flow separation
(not practical as it may be clogged)
how does blowing increase lift?
high-pressure air blown out energises sluggish air flow and delays separation
(more practical than suction but complicated piping)
how does slots/slats increase lift?
as it opens up, high pressure air is forced from lower to upper surface energies sluggish boundary layer near trailing edge,
increases stall angle and CLmax
how does flaps increase lift?
increases camber, which increases lift, which improves take-off performance and generate more lift at lower airspeed
list high lift devices
1) boundary layer control (suction & blowing)
2) camber change
3) increase in area
what do spoilers do?
mess up airflow to increase drag
what do speed brakes do?
enable optimum descent without decreasing thrust or slowing down the aircraft quickly when landing
what do lift dumpers do?
destroy lift to create more drag
flaps at 90 degrees and spoilers double up as lift dumpers
what do wing fences do?
control airflow, provide better slow speed handling and stall characteristics
why is it better to compare lift & drag coefficient rather than lift and drag?
as the coefficients do not have dimensions, it does not depend of wing area and Pdyn
how can flow separation be delayed?
by reducing AOA
what will increase lift but lower stalling angle?
increasing aerofoil camber
induced drag is resistance caused by?
trailing vortices at wing tips
MAC formula
b / AR
difference between symmetric and non-symmetric airfoil graphs
L or CL: Symmetric starts from origin
non-symmetric starts above
Drag: both starts above origin
CL/CD: symmetric starts from origin
non-symmetric starts above origin
