AERODYNAMICS ONE EXAM BANK Flashcards
1
Q
Why do we have ISA?
A
Allows for accurate comparisons of a/c performance and pressure, temp and density variations around the globe
2
Q
ISA Figures? (Incl tropopause)
A
- MSL Temp: 15deg C
- MSL Pressure: 1013.25hpa
- MSL Density: 1.225kg/m3
- Temp Lapse rate: 1.98deg C/1000ft
- Pressure lapse rate: 1hPa/30ft (blw 10,000ft)
- Tropopause: 36,090ft ave
- Tropopause temp: -56.5 deg C
3
Q
Effect of altitude on density?
A
In the atmosphere, the rapid drop in px as altitude increases has the dominant effect on density (decrease) (in comparison to increase in px due to temp decrease).
Therefore, density reduces with increasing alt.
4
Q
Airspeeds?
A
IAS: Indicated (ASI reading)
CAS: Calibrated (IAS corrected for px & instrument errors)
EAS: Equivalent (CAS corrected for compressibility of air)
TAS: True (EAS corrected for density)
G/S: Ground speed, TAS corrected for HWC/TWC.
5
Q
Why is TAS & EAS important?
A
TAS: sig bc it gives a measure of the speed of a body relative to the undisturbed air.
EAS: Is how many particles you are moving through. sig bc the aerodynamic forces acting on the a/c are directly proportional to the dynamic px (and thus EAS)
6
Q
Caveat wrt Bernoulli’s theorem?
A
Assumes fluid is ideal & only applies to an incompressible fluid with no viscosity
We can assume air is ideal below 0.4M (subsonic)
7
Q
Describe Bernoulli’s Theorem?
A
In an ideal fluid with a steady streamline flow, the sum of the energies present remains constant.
Potential energy & heat energy are insig, therefore;
Px energy + Kinetic energy = constant.
Theorem can be used to describe changes in velocity & px of air as it flows over an aerofoil resulting in lift.
8
Q
Describe the px distribution over the aerofoil using Bernoulli’s theorem?
Diagram?
A
Bernoulli’s: As speed of flow (KE) increases, static px decreases.
Airflow is accelerated over upper SFC of the aerofoil, as it has to travel further in the same amount of time to reach the trailing edge.
Increase in velocity = decrease in px. Results in px differential b/t upper & lower SFC (low above, high below), results in lift.
9
Q
Define free stream flow/relative airflow
A
Air in a region where px, temp and relative velocity are unaffected by the passage of the a/c through it.
10
Q
Define total reaction
A
The resultant of all the aerodynamic forces acting on the wing or the aerofoil section
11
Q
Define Lift
A
The component of TR which is perpendicular to the RAF/flight path
12
Q
Define Total drag
A
The sum of all components of the aerodynamic forces, which act parallel and opposite to the direction of flight
13
Q
Define Centre of Pressure
A
The point, usually on the chord line through which the TR is considered to act
(Lift & Drag vectors act through CoP)
14
Q
Define chord line
A
A straight line joining the centers of curvature of LE & TEs of an aerofoil
15
Q
Define Chord
A
Dist b/t LE & TEs, measured along the chord line
16
Q
Define angle of attack
A
The angle b/t the chord line and the flight path (or RAF)
17
Q
Define camber line
A
Line joining LE & TEs of an aerofoil, equidistant from upper & lower SFCs
18
Q
Angle of incidence?
A
The angle at which an aerofoil is attached to the fuselage. Angle b/t the mean chord line and the longitudinal fuselage datum
19
Q
Washout?
A
The decrease of Angle of incidence b/t wing root and wingtip
20
Q
Thickness/chord ratio?
A
The maximum thickness or depth of a wing section expressed as a percentage of the chord length
21
Q
Wing area?
A
The area of a wing projected on a plane perpendicular to the normal axis
22
Q
Wing loading?
A
The weight per unit of area of the wing = weight/wing area
23
Q
Streamline?
A
The path traced by a particle in a steady fluid flow
24
Q
Draw an aerofoil diagram w 12 components
A
- Upper/lower SFC
- LE & TE
- LE radius
- Chord line
- Chord
- AoA
- RAF
- CoP
- Lift/drag/TR
- Mean Camber line
- Location of max thickness
- Location on max camber
25
Lift equation? (Drag?)
What has the largest effect of Lift/drag?
CL: Coefficient of lift
p: Air density
V2: Free stream velocity (TAS)
S: WIng area
(Drag is same, but Cd)
Velocity, as it is squared
26
Factors affecting lift? (8)
- Free stream velocity
- Air density
- Wing area
- Wing shape in section & planform
- AoA
- Condition of SFC (esp LE as this affects the sep pt)
- Viscosity of air
- Speed of sound
27
Factors affecting coefficient of lift/drag (5)
- AoA
- Shape of wing section & planform
- Condition of wing SFC
- Reynolds #
- Speed of sound
28
Axes of rotation through CoG?
- Vertical (normal) axis through CoG: Yaw (nose L/R) (rudder)
- Lateral axis wingtip-wingtip: Pitch (nose up/down) (elevator)
- Longitudinal axis nose to tail: Roll (ailerons)
29
Define aspect ratio?
The ratio of span/chord OR span2/wing area.
Affects the degree to which induced downwash influences the wings characteristics.
High AR - gliders: less affect
Low AR - Fighter: more effect (reduces wings 'effective' AoA)
30
Describe how T6 ailerons work
Differential ailerons.
Up going aileron deflects at a larger angle compared to down-going aileron.
This reduces the differences in drag & helps eliminate adverse yaw.
(e.g. 20deg up - 11deg down)
31
Define the boundary layer?
- The layer of air extending from SFC to the point where no dragging effect is discernible. (wing drags air particles with it)
- The nature of the boundary layer is a controlling factor in drag, maximum lift & stall characteristics of the wing.
- The region of flow in which the speed is less than 99% of the RAF. And usually exists in two forms; laminar & turbulent.
32
Transition point?
Point on the aerofoil at which laminar flow changes to turbulent flow.
33
What is adverse px gradient?
Over the wing SFC, the px reaches a minimum at the point of maximum thickness (has to move fastest at this point). Px increases from here towards the TE.
Px always flows high to low, due to this the px gradient actually opposes the airflow over the wing = Adverse px gradient
34
How does adverse px gradient affect the separation pt?
APG opposes the flow & therefore decreases kinetic energy of the boundary layer until it reaches zero velocity (relative to the wing). Detachment of the airflow at a particular point due to this is known as the separation pt.
35
What occurs beyond the separation pt?
Separation of the airflow from the aerofoil & resultant Turbulent wake
36
What is the stagnation pt?
The point on the wing where the velocity of the air particles is said to be zero. (Creates an area of relatively higher px on LE)
This will lie somewhere on the LE where the flow divides to go over upper & lower SFCs.
Will move with changes in AoA (down as AoA increases & vice versa).
37
Effect of zero camber on lift & CoP movement?
Where is the separation point?
A symmetrical aerofoil produces no lift at zero AoA because it has no px differential above & below.
With a symmetrical section there is virtually no CoP movement at subsonic speeds.
At higher AoA the sep pt moves down, effectively creating camber.
38
Effect of positive camber on lift at zero AoA & CoP movement?
Positively cambered aerofoil will produce lift at zero AoA b/c the airflow attains higher velocity over the upper SFC creating px differential & lift.
With a cambered aerofoil the CoP movement over the normal working range of angles of attack is b/t 20-30% of the chord aft of the leading edge.
39
How does a higher T/C ratio affect movement of the CoP?
There will be greater movement of the CoP over the working ranges of AoA, by effectively creating camber as upper flow has to go further than lower flow.
40
8 factors affecting takeoff distance
- Wind
- SFC Condxn
- Slope
- Altitude
- Temp
- A/C weight
- Config
- Engine power
41
**Effect of increase in alt on turn radius?
Turn radius also increases, turn rate decreases (with constant IAS)
Due to higher TAS at higher alt (for a constant EAS), momentum of a/c will be higher which results in greater turn radius/lesser turn rate.
42
G at 60deg AoB?
2G (max rate turn)
43
4 Factors affecting turn performance?
(FAST)
Alt & Speed
Thrust
Flap
44
Effects of banking on lift?
What must we do to prevent descent (lift only)?
For a/c to turn, centripetal force is rqd to pull a/c towards centre of turn.
A/C banked (lift vector inclined) > AoA kept constant > Vertical component of lift becomes too small to balance wgt > a/c starts to descend.
Therefore the higher the AoB, the higher the AoA rqd to keep a/c level in the turn. (vertical component of lift is then large enough to maintain lvl flt, horizontal component is large enough to produce CF rqd to turn a/c)
45
Effect of turn on power?
Increase in lift required in turn,
subsequent increase in drag
More power rqd in turn to maintain constant airspeed
46
How to decrease wing loading?
Load factor = L/W
By decreasing the overall weight and by increasing wing area.
47
Factors affecting flight ctrl effectiveness?
- Size & shape of ctrl
- Deflection angle (variable depending on pilot input)
- EAS^2
- Moment arm (dist from CoG)
48
What is adverse ailerpn yaw?
Undesirable yaw that causes the nose of the a/c to yaw out of the turn due to the drag created by the up-going wing.
49
Four methods of reducing adverse yaw?
(Short description for each)
- Differential ailerons
- Frieze Ailerons
- Coupling of ctrls
- Spoilers
50
Damping in roll effect?
When a/c rolls, down going wing AoA increases, up going decreases.
Lift is therefore increased on down going & decreased on up going.
New rolling moment produced opposes the initial disturbance & the direction of the initial roll (stable)
51
How does altitude affect damping in roll effect?
^ alt = increase in roll rate, due to decrease in roll damping (caused by increase in TAS for a given EAS with alt)
52
Define Stall
An a/c stalls when the sep point in the boundary layer moves forward and the flow over the upper SFC of the wing breaks down with associated loss of lift.
53
Basic stall speed?
The airspeed below which a clean a/c of maximum weight, with the throttles closed, can no longer maintain S&L flight due to exceedance of the critical angle
54
**Incipient stall symptoms? (4)
- Low & decreasing airspeed
- High nose attitude
- Reduced ctrl response & feel
- Light buffet (caused by turbulent separated flow buffeting tailplane)
55
**Fully developed stall symptoms (4)
- Heavy buffet (caused by airflow over wing collapsing & flowing back over tailplane)
- Nose down pitch (caused by CoP moving rapidly aft & abrupt loss of lift at the stall)
- (Often high) Rate of descent developing
- Poss wing drop (caused by one wing stalling before the other).
56
Stick shaker in T6?
felt 5kts prior to stall
Buffet 3kts prior to stall
57
4 ways to reduce stall speed?
- Deploy flaps
- Increase power
- Decrease weight
- Slipstream
58
Effect of weight on stall speed?
If weight is increased, lift rqd to maintain S&L also increases.
If AoA already at crit angle, the additional lift rqd can only be produced by flying faster
^W = ^Vs
59
How does deploying flaps reduce stall speed?
CL max is increased with use of flaps/slats due increase in wing area.
Stall speed is lowered.
60
How does increasing power reduce stall speed?
If power is applied at the stall, the nose high attitude gives a vertical component of thrust, which assists in supporting the weight, hence less lift is rqd from the wings, reducing Vs
61
How does slipstream reduce stall speed in comparison to pwr off case?
Local increase in dynamic px from the slipstream will give more lift in comparison to power off case.
Lower airspeed is therefore able to support a/c wgt.
Decreasing Vs
62
Why does stall speed increase with increase in load factor (G)
Because to maintain altitude in a turn or maneuver, an aircraft needs to generate more lift, requiring a higher angle of attack, which in turn means the aircraft will stall at a higher airspeed
63
Formula for load factor (G)?
How does it vary?
G = Lift/weight
Varies with AoB due to angular acceleration in the direction of the turn (^ AoB = ^ in G & ^ in lift)
64
Nose wheel a/c wheel dist?
Most common today,
two main units slightly aft of CoG & smaller nose wheel to balance remaining load.
65
Why are nose wheel a/c more stable than taildraggers?
Due to pstn on CoG in relation to the axis of the main wheels and the moment that is created
66
6 Advantages of tricycle undercarriage a/c?
- Positively stable during its ground roll
- Lookout ahead is good
- Good grd ctrl using NWS
- Nil tendency to nose over under hard braking
- Jet efflux is clear of the ground
- Tail brake-chutes easily deployed
67
Max angle climb? (OCS)
Graph?
A/C gains most alt in shortest dist.
Flown at the speed which gives the max excess of thrust after opposing drag.
Piston: usually as slow as is safe above rotate.
68
Max/best rate climb?
Excess power/weight (is the fastest time to climb to a height)
Point of graph which is greatest dist b/t power avlble & power required
69
What are the 4 induced effects on T/O?
Prop/engine TQ
Slipstream
Gyroscopic precession (Taildraggers only)
Asymmetric blade effect
70
Describe How prop/engine TQ effect occurs?
With prop & eng turning in the same direction, a reaction is created in the opposite direction (Newtons law).
This tends to rotate the a/c body around the longitudinal axis & applies more apparent weight on one wheel than the other.
(LH wheel on T6)
= Yaw to the left for T6 (CW rotation)
71
Describe How slipstream effect occurs?
Prop rotation causes slipstream to have a corkscrew effect, causing it to strike a side of the aft fuselage & vertical fin (LH side for T6 = LH yaw)
72
Describe How gyroscopic precession effect on T/O occurs?
Taildraggers only
Prop acts as gyro, when tail is raised, this has effect of applying force to top of prop disc.
precession results in yaw away from direction of rotation (LH for CW rotation)
73
Describe How asymmetric blade effect occurs?
- A/C in tail down attitude (increase in AoA)
- Downgoing blade has higher AoA to RAF & therefore "bites" more air in comparison to up going blade.
- Downgoing therefore produces more thrust
- Thrustline shifts right (for CW rotation)
- A/C yaws left
74
Drag Tree?
75
Define zero lift drag?
When an a/c is flying at zero lift angle of attack, the resultant of all the aerodynamic forces acts parallel and opposite to the direction of flight, as there is no lift.
Composed of;
- SFC friction drag
- Form drag
- Interference drag
76
SFC friction drag - 3 causes?
- SFC area of the a/c,
- Coefficient of the viscosity of the air,
- Rate of change of velocity across the airflow (laminar = slower, turbulent = faster).
All proportional (i.e. increase in above = increase in SFD)
77
Form drag
Drag caused by separation of the boundary layer from a SFC and the wake created by the separation.
Dependent on the shape of the object
78
Interference drag
Results due to flow interference b/t the boundary layers of parts of the a/c that join together. (e.g. wing & fuselage)
79
Lift dependent drag?
In producing lift, the a/c will produce additional drag known as LDD.
Composed of induced drag & increments of ZLD (only noticeable at high AoA)
80
Induced drag?
4 factors that influence it?
Extra drag caused by increasing AoA to maintain original climb AoA, after downwash caused by vortices reduced effective AoA.
- Planform (greatest where vortices are greatest)
- AR
- Lift & weight
- Speed
81
**Draw a diagram of the drag curve
Label & describe: Vimd, Vmp, 1.32 Vimd
Vimd: Max L/D ratio (drag is min)
Vmp: Min pwr rqd for S&L flight (min product of drag/velocity) - best endurance speed.
1.32 Vimd: Max EAS/Drag (tangent to total drag curve)
82
**Draw a diagram of the lift/drag curve, with effect of increasing weight
How does Vimd change?
Wgt ^ = LDD ^ (due increase in AoA)
83
Effect of alt on power required?
At constant wgt and EAS, drag does not change with changes in Altitude.
However Power rqd is a function of drag x TAS.
And TAS DOES change with altitude at a constant EAS.
Therefore: ^ in Alt, means ^ in PWR is rqd to fly S&L at constant EAS.
84
Power available?
Defined as the rate of doing work;
Power = thrust x velocity.
(If throttles are up, but you are stationary, you have no power output).
85
What does the minimum S&L speed require?
Requires high AoA to create appropriate amount of lift. Majority of engine power is used to overcome drag.
86
What two speeds does the minimum S&L power setting correspond to?
Lower airspeed: on the back of the drag curve
Higher airspeed: Above Vmp
87
Why is lower airspeed of the min S&L power setting undesireable?
Speed stability at the lower speed is poor, any small reduction of power will cause a decrease in airspeed due to the rapid rise in drag below that pt.
Large increases in pwr & therefore fuel will be needed to increase speed back to min S&L speed
88
Minimum power speed? (Vmp)
Caveat
The speed at which the smallest qty of drag is incurred for that weight, config & altitude.
Best endurance. (rqrs smallest power output & therefore fuel burn)
Speed stability also poor
89
Max S&L speed?
Point where PWR required is equal to the max power avlble from the engine
90
Min Drag speed?
Vimd
Speed which is tangential to the power rqd curve.
Best range speed.
Operating below this speed means the effect of rising induced drag offsets the TAS advantages.
91
**Draw a graph of power rqd/available - Piston vs jet
Label the speeds
92
Two ways to climb the a/c
- Use of thrust above that rqd to maintain level flight
- Zoom climb: gain in height by loss in airspeed
93
Min radius turns?
At what AoB do we theoretically achieve min radius?
How do we actually achieve min radius (tightest) turns?
Tightest turn possible (smallest space taken up)
90deg AoB
- Max product of CL & AoB
- Minimise wing loading (W/S)
- Maximum density (Sea lvl)
94
How do we achieve max rate of turn generally?
Comparison to min radius?
3 components?
Generally, If we increase V and/or decrease radius, rate of turn will increase.
Max rate occurs at a higher speed than min radius
- Max product of CL, AoB and SPEED. (can't all be max at the same time)
- Minimise wing loading
- Maximum density (Sea lvl)
95
How does SFC area of the wing impact Min radius & max rate turns?
Increasing wing SFC for same wgt a/c will decrease wing loading - minimise radius turns & maximising rate turns.
96
How do flaps assist with lift?
Vary the camber of a wing section. Greater the camber, the greater the lift for a given AoA
97
How do slats/slots assist with lift?
Slat prolongs lift curve by delaying stall until a higher AoA, does this by;
- Flattening marked peak at the low px envelope, therefore reducing adverse px gradient & therefore delays sep pt.
Slot:
- Air that passes through slot is accelerated by venturi effect which re-energises the boundary layer (assisting against adv px grad)
98
What are slats/slots?
Small auxiliary aerofoil of highly cambered section, attached to part of LE
(slot b/t slat & wing)
99
What is boundary layer control?
Three methods?
Control of boundary layer so that it remains attached to aerofoil SFC for as long as poss.
Achieved by adding KE to lower layers;
- Suckling
- BLowing
- Vortex generators
100
How can flap assist with decreasing turn radius?
Produces more lift but also more drag but can assist with tightening turn radius (so long as within flap limiting speed/G limit)
101
Flt Ctrl SFCs: Rate controls?
Ailerons
When aileron is applied, the rolling moment is opposed by aerodynamic damping in roll.
Due to this, for a given roll deflection, a given 'rate' of roll results.
Means once desired rate of roll is achieved, ctrls can be returned to nuetral and roll will be maintained until opposite ctrl is input to bring the a/c back to nuetral.
102
Flt Ctrl SFCs: displacement controls?
Elevators & rudders
When rudder or elevator is applied, the yaw or pitch change is opposed by both aerodynamic damping & the a/c's inherent stability
(Application of the rudder/elevator) must be held to keep the pitch/yaw desired. If released, the a/c will stabilise back to nuetral
103
Airspeeds:
- Perf calcs (T/O, LDG etc)?
- Distance travelled irrespective of wind?
EAS
TAS
104
How do Frise ailerons work?
Up going aileron protrudes into the airflow below the wing, increasing drag to similar value to that produced by down going aileron
105
How does coupling of controls work to reduce adverse aileron yaw
Rudder auto-applied in direction of turn
e.g. yaw dampner
106
How do spoilers work to reduce adverse aileron yaw
Deployed on downgoing wing to increase drag
107
Where do the greatest positive pressures (lift producing vectors) occur?
At stagnation points when the flow is brought to rest (LE & TE)
108
Draw a diagram depicting the boundary layer.
Describe what each label refers to?
- Laminar: constant streamlines without turb.
- Laminar sub layer: A very thin layer that exists immediately adjacent to the SFC within the turbulent boundary layer
- Transition pt: (flashcard)
- Turbulent: chaotic, high energy motion of airflow.
- Separation pt: (flashcard)
- Adverse px gradient: (flashcard)
109
Disadvantages of tricycle undercarriage? (NOLL)
- Longer carriage rqd for prop clearance
- Nose wheel must be stronger/cushioned/ retractable = weight
- less A/D braking
- Often rqrs tail bumper
110
Components of CL? (5)
- AoA
- Shape of wing section & planform
- Cdxn of wing SFC
- Reynolds #
- Speed of sound
