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Chap C: Aerofoil & other aerodynamic shapes Flashcards

1
Q

why is curved surface better than flat surface?

A

1) air flow smoother
2) more lift (compared to drag)
3) greater depth/thickness (more space for fuel & structural strength)

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2
Q

which flow speed is faster, above or below the aerofoil?

A

above the aerofoil

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3
Q

how does static pressure change when air flows on aerofoil and how does it affect the total pressure?

A

upper surface: static pressure decreases

lower surface: static pressure increases

Ptotal is constant, so when Pstatic decreases, Pdyn increases

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4
Q

what is total reaction force resolved into?

A

lift (perpendicular to direction of airflow)
drag (parallel to direction of airflow)

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5
Q

when aircraft is in straight and level flight, lift is?

A

vertical

(lift is horizontal during vertical climb)

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6
Q

pressure coefficient formula & signs of Cp when pointing in and out

A

(Plocal - Pfree stream) / Pdynamic of free stream

Cp -ve when pointing out
Cp +ve when pointing in

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7
Q

how does streamlines change when above the aerofoil?

A

bunched together indicating increased in flow velocity

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8
Q

force of wing formula using pressure differential

A

F = pressure differential x area

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9
Q

define pitching moment

A

a turning effect tending to raise or lower its nose results when the centres of gravity and pressure do no coincide

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10
Q

how does change in centre of pressure (CP) affect the nose wrt CG

A

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

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11
Q

define angle of attack

A

α, angle between chord line and relative wind

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12
Q

what is chord line?

A

line joining leading edge to trailing edge

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13
Q

what is stalling angle?

A

stalling angle is the AOA for maximum lift

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14
Q

important factors in stalling

A

1) suddenness & magnitude of lift loss
2) increase in pitching moment

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15
Q

what is the difference between stall speed and stall angle?

A

stall speed: minimum speed to maintain weight level flight

stall angle: max angle of wing to reach max lift

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16
Q

list the 3 types of drag

A

1) form/pressure drag
2) skin friction
3) induced drag

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17
Q

how is drag relative to AOA?

A

drag increases rapidly as AOA increases

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18
Q

how does lift change when AOA changes?

A

as AOA increases, lift also increases

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19
Q

best lift drag ratio is called

A

rigger’s angle / angle of incidence

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20
Q

what is best lift drag ratio?

A

when lift is the max and drag is min

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21
Q

what happens when we pitch moment about Leading Edge (LE) and Trailing Edge (TE)?

A

about LE: nose down, AOA decreases

about TE: nose up, AOA increases

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22
Q

effects of AOA on airflow

A

separation bubbles forms at trailing edge at no AOA

separation point moves forward as AOA increases

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23
Q

how does CP change when AOA changes?

A

Cp moves forward of aerofoil when AOA increases up till stalling

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24
Q

define aerodynamic centre

A

point on chord about which pitching moment is nearly constant at all AOAs

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25
where is aerodynamic centre usually at?
25% chord length measured from leading edge
26
which is fixed, aerodynamic centre or CP?
AC is fixed, CP moves forward as AOA increases up till stalling
27
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).
28
wing span formula
b = 2s b - wing span s - semi span
29
fineness and thickness ratio formula
fineness ratio = c / tmax thickness ratio = tmax / c c - chord tmax - max thickness FR no dimensions TR in %tage
30
fineness ratio wrt to frontal profile
increase in FR, decrease in frontal profile
31
what does thickness ratio affect?
maximum lift, critical Mach number
32
define angle of incidence
angle which an aircraft wing is mounted relative to its fuselage (also same as rigger's angle)
33
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
34
life coefficient relative to AOA
same as lift vs AOA graph CL increases linearly with AOA
35
why use plan area to calculate drag?
as frontal area change is much larger & harder to estimate
36
Pitching moment coefficient formula
use lift coefficient formula and change L to M and times 1/c, where c is chord length
37
pressure coefficient formula simplified
Cp = 1 - (Vlocal / V)2
38
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
39
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
40
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
41
2 important factors during level flight/cruise
1) Lift = Weight 2) Thrust = Drag
42
induced drag relative to dynamic pressure and velocity
Di is inversely proportional to Pdyn & velocity
43
what do vortices create?
upwash away from the plane downwash within the wing span
44
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
45
what is parasite drag?
drag due to non-lifting surfaces combined
46
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
47
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
48
best theoretical planform for least induced drag
elliptical wings
49
what is winglet used for?
increase aspect ratio without substantially increasing wing span
50
what do winglets do?
reduce mixing of airflow between upper and lower surfaces of wing, therefore reducing induced drag
51
what are high-lift devices used for?
reduce take-off/landing speeds and distances
52
how does suction increase lift?
suction sucks in the sluggish flow thereby delaying flow separation (not practical as it may be clogged)
53
how does blowing increase lift?
high-pressure air blown out energises sluggish air flow and delays separation (more practical than suction but complicated piping)
54
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
55
how does flaps increase lift?
increases camber, which increases lift, which improves take-off performance and generate more lift at lower airspeed
56
list high lift devices
1) boundary layer control (suction & blowing) 2) camber change 3) increase in area
57
what do spoilers do?
mess up airflow to increase drag
58
what do speed brakes do?
enable optimum descent without decreasing thrust or slowing down the aircraft quickly when landing
59
what do lift dumpers do?
destroy lift to create more drag flaps at 90 degrees and spoilers double up as lift dumpers
60
what do wing fences do?
control airflow, provide better slow speed handling and stall characteristics
61
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
62
how can flow separation be delayed?
by reducing AOA
63
what will increase lift but lower stalling angle?
increasing aerofoil camber
64
induced drag is resistance caused by?
trailing vortices at wing tips
65
MAC formula
b / AR
66
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

xxxFOF (6 decks)

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