Aerodynamics Flashcards
1
Q
What is an aerofoil?
A
A body that gives a large lift force compared with its drag when in a moving airstream.
2
Q
What is an aerofoil chord line?
A
A line from the leading edge to the trailing edge of an aerofoil.
3
Q
What is a mean camber line?
A
A line of equidistance from the leading edge to the trailing edge of an aerofoil.
4
Q
What is the angle of incidence?
A
The angle between the chord line and a longitudinal datum on the aircraft.
5
Q
What is the angle of attack?
A
The angle between the chord line and the relative airflow.
6
Q
What is wing washout?
A
Where there is a decrease in the angle at the chord of the wing route to the wingtip.
7
Q
What is dihedral?
A
An upwards inclination of the wing.
8
Q
What is anhedral?
A
A downwards inclination of the wing.
9
Q
What is lift?
A
Lift is the force generated by an aerofoil due to pressure differences above and below when passed through moving air.
10
Q
What is coefficient of lift?
A
This is the lifting ability of a wing and is dependent on the surface area of the wing and the chord as well as the angle of attack.
11
Q
Describe the CoP
A
CoP is the point where the wings lifting force is produced and is at right angles to the relative air flow.
12
Q
What happens to the CoP as you increase the AoA?
A
It moves towards the leading edge of the wing.
13
Q
Describe aspect ratio?
A
AR is the ratio of the wingspan to the geometric chord.
14
Q
During what phase of flight is lift the greatest?
A
Generally during take off due to optimised AoA at this stage of flight. It is affected by: Flaps Speed of the airflow over the wing Angle of attack Air Density
15
Q
What is a direct lift control?
A
Elevator / Stabilator which creates an upwards or downwards balancing force determining the attitude of the aircraft about its lateral axis.
16
Q
What are high lift devices? (3)
A
Trailing edge flaps (Fowler Flaps - increase area of the wing) Leading edge flaps (Krueger Flaps - Create a longer wing chord line / camber / area) Slats and slots - Used to re-energise the boundary layer and delay separation
17
Q
What are the different types of Drag?
A
Parasite: [Skin / Pressure (form)] known as profile drag / Interference drag. Induced: Generated by lift and is associated with high AoA and wing tip vortices.
18
Q
Describe the drag curve for a piston / propellor aircraft? (Take this as a straight wing)
A
Well defined steep parasite drag curve as it’s not designed for high speeds. Well defined steep induced drag curve as it has a high CL at lower airspeeds. Well defined Vmd (stable / unstable speeds) with Lower stalling speed than a jet.
19
Q
Describe the drag curve for a jet aircraft? (Take this as a swept wing)
A
Shallower parasite drag curve as it’s designed for higher speeds. Shallower induced drag curve as generates a lower CL at lower airspeeds. Flatter Vmd (stable / unstable speeds) with high stalling speed than a straight wing aircraft.
20
Q
What are high drag devices?
A
Trailing edge flap (Drag flap) Spoilers: Flight detent = Speed brake / Ground = lift dumpers Landing gear
21
Q
What causes wingtip vortices?
A
Spanwise flow of air on the upper and lower wing surface meeting at the wingtips as turbulence and causing induced drag. This is especially true on a swept wing aircraft.
22
Q
What is the affect of spanwise flow over a wing?
A
Increased wingtip vortices Reduced aileron efficiency Reverse spanwise flow contributes to wingtip stall
23
Q
What are the affects of wingtip vortices?
A
Induced drag / loss of energy Turbulence which may affect the safety of other aircraft up to 1000 feet below you and up to 9 nm behind you The downwash may affect the longitudinal stability of the aircraft
24
Q
How to you prevent spanwise flow on a wing, especially a swept wing?
A
The use of wing fences and vortex generators.
25
What are the purpose of vortex generators?
To re-energise the boundary layer and delay separation in order to maximise the effectiveness of the control surfaces (especially the ailerons).
26
What are the purpose of wing fences?
These help prevent reverse spanwise flow on the upper surface from reaching the wingtip and therefore reducing the chances of a wing tip stall.
27
What are winglets and how do they work?
Winglets are aerodynamic efficient surfaces that are placed at the end of the wing in order to help prevent the mixing of the spanwise flow which causes induced drag. Often the upper and lower spanwise flows are dispensed at different points.
28
What limits an aircrafts structural weight?
The force generated to balance weight is lift. The direct limit of weight is the amount of lift that can be generated by the aircraft. Design Speed Power Airspeed
29
What are the affects of excess aircraft weight? (5)
Performance: LDR / TORR increased Range / Speed / ROC & ROD decreased Stall speeds increased Manoeuvrability reduced Safety Margins decreased Wear on Tyres and brakes increased
30
Describe the CoG?
CoG is the point at which all of the aircrafts weight acts through.
31
Describe a component arm?
This is the distance from a datum that forms a moment arm. Moment = Force x Distance. If nose up then it's positive If nose down then it's negative
32
How is the lift weight moment balanced?
When The lift weight moment is not perfectly balanced, extra forces are provided by the horizontal tailplane to centre the pitching moment.
33
Describe the centre of gravity range?
This related to the aircrafts most forward and most rearward CoG position along the aircrafts longitudinal axis that the aircraft is permitted to fly. This is so the horizontal tailplane can generate enough force to balance the aircrafts lift-weight couple.
34
Why does forward CoG need to be kept within limits? (3)
To ensure the aircraft is not too nose heavy and to ensure the tailplane has enough turning moment to overcome the natural longitudinal stability. To ensure that there is enough controllability without high stick forces at low speeds (rotating / flaring). To ensure minimum horizontal tailplane deflection in order to reduce drag and maintain performance.
35
Why does aft CoG need to be kept within limits? (3)
To ensure the aircraft is not too "tail heavy" and to ensure the aircraft remains longitudinally stable. To ensure adequate stick force is felt through the control column when pitching. To ensure minimum horizontal tailplane deflection in order to reduce drag and maintain performance.
36
What are the affects of a CoG being outside its forward limit? (4)
Longitudinal stability is increased which will reduce controllability of the aircraft A large balancing download from the horizontal stabiliser which will: Increase stall speed Increase drag Reduce performance Reduce range Less elevator trim available In flight minimum speeds are restricted due to the high AoA that is required at lower airspeeds
37
What are the affects of a CoG being outside its aft limit? (5)
Longitudinal stability is reduced which will mean the aircraft is too "tail heavy" Over controllable which means the airframe could be overstressed A large balancing upload from the horizontal stabiliser which will: Reduce stall speed Reduce drag Increase performance Increase range Less elevator trim available In flight minimum speeds are restricted due to the high AoA that is required at lower airspeeds
38
Why does a forward CoG increase the stall speed of the aircraft?
Having a forward CoG means that the horizontal tailplane will have to generate a higher downforce. This higher downforce increases the aircrafts effective weight. As weight is a function of the stall speed, the heavier the aircraft the higher the stall speed.
39
Why does a jet aircraft have a large CoG range?
Typically as a large jet aircraft burns a lot of fuel the CoG will change during flight. In order to compensate for that it has a large horizontal tailplane in order to generate the necessary forces required to maintain longitudinal stability.
40
What causes CoG movement? (3)
Fuel burn - Most common, also because wings are swept it changes the weight distribution Passenger movement High speed - At higher speeds the aircraft adopts a greater nose down attitude.
41
How does weight affect the aircrafts flight profile descent point?
The heavier the aircraft, the earlier its required descent point. The heavier the aircraft the shallower it's rate of descent needs to be and therefore it will be covering a greater distance over the ground. Momentum = mass x velocity.
42
How would you design a high speed aircraft wing?
Swept Thin Minimal camber
43
How does swept wing increase M crit?
Since the wing is responsive only to the velocity vector normal to the leading edge, the effective chordwise velocity is reduced. Thus airspeed can be increased before the effective chordwise component becomes sonic.
44
What is the ideal fineness ratio?
About 3:1
45
How would you optimise the lift design on a high wing? (3)
Trailing edge flaps (Fowler Flaps - increase area of the wing) Leading edge flaps (Krueger Flaps - Create a longer wing chord line / camber / area) Slats and slots - Used to re-energise the boundary layer and delay separation
46
Advantages of a swept wing? (3)
As it provides less lift, ironically it is less responsive to turbulence / updrafts / downdraft. Higher Mach cruise speeds. Reduced form drag due to reduced camber and thickness. of the wing.
47
Disadvantages of a swept back wing? (6)
Increase stall speed / Lower CL Max More violent Mach tuck characteristics Reduced lift qualities Tip stall characteristics Possibility of super stall - needs stick pusher Speed instability below VMD
48
Where does a swept wing stall first? What effect does this have?
Wing tip stalls first and therefore the CoP moves forward and more inboard which causes a nose up pitching moment. This effect is reduced by: Washout or twist Greater camber at the tip, increasing airflow at the tip
49
What is speed stability?
Speed is said to be stable if the aircraft after being disturbed from its trimmed state, returns back to its original speed
50
What is Mach number?
True Airspeed indication given in relation to the local speed of sound. M = TAS / LSS LSS = 38.94 x √ T+ 273
51
What is the critical Mach number, Mcrit?
Where speed at which the the airflow over an aerofoil first becomes sonic.
52
What are the characteristics of reaching Mcrit?
Mach buffet - caused by shockwaves on the upper surface of the wing Increase in drag - caused by the breakdown of airflow Nose down change in attitude as the CoP moves rearwards on a swept aircraft (Mach Tuck) A possible loss of control
53
What are the changes of the CoP as an aircraft goes past Mcrit?
CoP initially moves rearwards slightly, then forwards and then rearwards.
54
What is Mach tuck?
Mach tuck is what happens when the aircraft exceed Mcrit and the CoP moves behind the CoG and causes a nose down pitch moment.
55
What is a Mach trimmer and what is it used for?
a Mach trimmer causes a proportionately upward movement of the elevator in order to prevent Mach tuck above an aircrafts Mcrit number.
56
What are the effects of compressability?
The effects of compressability is the effect of air being compressed onto a surface at right angles to the relative airflow. This increases density and the dynamic pressure then rises above its expected value.
It can cause compressibility error on dynamic pressure reading flight instruments such as ASI.
Compressed air is experienced on the leading edge of the wing and causes a shock wave at Mcrit.
57
Explain maneuovrability margins / envelope
An aircraft must observe certain speed limitations in order for safe operation. These speeds consider:
Strength of aircraft structure
Stiffness of the aircraft structure
Adequate control of the aircraft
EASA regulations give limit load factors for transport jet of 2.5 and -1.0.
Va - Design maneouvring speed is the speed at which sudden full elevator deflection can be made without exceeding the design limit load factor.
Vc - A speed set by the designer that allows for speed upsets in the cruise
Vd - Design dive speed which allows for a shallow dive from Vc until recovery
58
What is coffin corner?
Coffin corner occurs at the aerodynamic ceiling of the aircraft where low speed stall buffet and the onset of high speed Mach buffet occur.
59
Why does an aircraft stall?
An aircraft stalls when the boundary layer over the upper surface of the wing, which creates lift, breaks away. This is caused by the aircraft exceeding its critical angle of attack, which can be at any airspeed.
60
What properties effect an aircraft stall speed? (6)
Weight
Load factor
CoG position
Configuration
Altitude
Wing design
61
How does stall speed vary with weight?
The higher the weight the more lift that is required to balance this.
As the stall occurs at a constant AoA we can only increase the lift by increasing the speed. Therefore the stall speed will increase in proportion to the square root of the aircrafts weight.
62
What wing design area delay the breakup of the airflow (stall)?
Wing slots are the main design feature which is used to re-energise the boundary layer of the wing and delay separation. These are used to achieve a higher CL and lower speed at the stall AoA.
Lower Angle of Incidence and greater camber for a particular wing section (wing tip for example).
63
What changes the aircrafts angle of attack at the stall?
The movement of the CoP point at the stall causes a change in the aircrafts angle of attack.
Normally a swept wing aircraft the CoP moves rearwards as the wing is designed to induce the stall at the wing root, which provides a stable nose down pitching moment upon the stall.
64
What happens to the stall speed at very high altitudes and why?
Stall speed increases at very high altitudes due to:
Mach number compressibility effect on the wing disturbs the pressure patter and increases the effective weight on the wing resulting in a higher EAS stall speed.
Compressibility error correction that forms part of the difference between the IAS and EAS (which is IAS corrected for PIM and compressability) due to mach number, resulting in a higher IAS stall speed.
65
What is a super stall?
Where the aircraft has a nose up pitching tendancy.
An ineffective tailplane due to the wing wake which now becomes low energy distrubed turbulent air.
66
What systems protect against a stall? (2)
Stall warners and stick pushers.
Stall warners are either artificial audio warning and or stick shaker, which is usually activated 5% or 5kts greater that the stall speed.
Stick pushers are usually fitted to aircraft with superstall tendancies and will activate after the stall warner, usually 2% or 2 kts whichever greater above the stall speed, giving and automatic nose down command.
67
What is Dutch Roll?
Dutch roll is a combination of yawing and rolling and is associated with swept wing aeroplanes.
It is where the outer wing moves faster and faces straight on to the relative airflow. This causes this wing to generate more lift, whilst at the same time the inner wing will travel slower and in effect cause the wing to become more swept and reduces the lift and a noticeable bank. The same thing then happens again but in the opposite dorection and this phenominon is knows as Dutch roll.
68
How do you recover from Dutch roll?
To dampen the oscilation, only aileron input (opposite to the roll) will recover from the Dutch roll.
Although it is the yawing motion that is the root cause of dutch roll, usually the pilots rudder input would worsen the situation as the yawing motion happens extremely quickly, whereby the roll is usually long enough for the pilot to be able to correct.
A reduction in altitude and an increase in CAS can ease the problem.
69
What prevents Dutch roll?
Yaw dampers increase the effectiveness of the rudder which prevents Dutch roll.
70
What is directional stability?
It is the aircrafts ability to regain direction (heading) after it has been directionally disturbed.
The size and shape of the fin coupled with the moment arm are major contributors to directional static stability.
71
What is spiral stability?
This is the ability for an aircraft that is in a properly coordinated banked turn to return to its laterally level flight attitude.
72
What is lateral stability? What design features contribute to positive lateral stability? (4)
Lateral stability is the ability of an aircraft to return back to its laterally level position when ailerons are released in a sideslip.
Dihedral
Swept wing
High wing
High fin
73
What is longitudinal stability? What are the major contributors? (2)
Longitudinal stability is the ability for an aircraft to return to a stable pitch poisition about its lateral axis after a disturbance.
CoG
Tailplane (moment arm)
74
Describe stability at high altitudes?
Longitudinal, lateral, directional and oscillatory stabilty in general are reduced at high altitudes in terms of dynamic stability.
However, spiral dynamic stability increases as the wings become more dominant that the fin at altitude.
75
What are the four flying quality penalties at high altitude?
More speed restrictions
Less maneouvrability
Reduced aerodynamic damping
Reduced stability
76
What are the primary flight controls?
Elevators - Pitch
Rudder - Yaw
Ailerons - Roll
Roll control spoilers - Roll
77
What is the elevator? How does it work?
An elevator is a hinged control surface at the rear of the horizontal stabiliser. It is controlled by pulling back on the control collumn.
When pulled back, the elevator control surface moves upwards, thus creating a downwards force from the horizontal stabiliser. This causes the aircraft to pitch up about its lateral axis.
78
What are ailerons? How do they work?
Ailerons are the control surfaces that control the roll of the aircraft and are controlled by moving the controls left or right.
The aileron on the up going wing, moves down which increases the lift on that wing.
The aileron on the downgoing wing moves up which causes a reduction in lift on that wing.
The net effect is that the aircraft rolls about its longitudinal axis.
79
What is a rudder? How does it work?
The rudder is a control surface that is on the rear of the fin of the aircraft and it is controlled by moving rudder pedals left and right.
The rudder controls the yaw of the aircraft about its "normal" axis.
80
How does the effectiveness of the control surfaces vary with speed?
The effectiveness of the control surfaces increases with speed, thus at lower airspeeds a larger movement is required and at higher airspeeds a smaller movement is required to deliver the same control force.
81
What is elevator reversal?
Elivator reversal occurs at high speeds when the forces are great enough to cause a twisting moment on the defected elevator surface to either a neutral or opposite position.
82
What is adverse yaw?
This is where the upgoing aileron increases induced drag and gives a subsequent yawing effect. It can be removed with the use of differential aileron (upgoing aileron moves through a greater deflection than the down going one) or frise ailerons (the leading edge of the upgoing aileron petrudes beneathe the wing thus causing parasite drag to balance out the induced drag from the downgoing aileron).
83
What is aileron reversal?
This occurs at high speeds where enough force is generated to twist the wing thus decreasing the angle of incidence and can cause an adverse rolling motion.
84
What are spoilers and how do they work?
Spoilers consist of openning pannels on the surface of the wing and have the effect of increasing the drag and reducing the lift.
For roll control:
Spoilers are raised on one wing and not the other and work with the ailerons. They are very effective, however cause an overall loss of lift.
As air speed brakes:
The spoilers are raised on both wings to a flight detent position and is used to slow down the speed of the aircraft. Usually buffetting is felt when they are in operation.
As ground lift dumpers:
The spoilers are raised on both wings on the ground to the ground detent position which decreases the lift over the wing making braking more effective.
85
What are the purposes of spoilers? (3)
Roll control
Speed brakes
Ground lift dumpers
86
Describe differential and non differential spoilers?
Non differential:
When spoilers are deloyed as speed break, when a roll command is given the spoilers on one side will extend more that the other side.
Differential:
When spoilers are deloyed as speed break, when a roll command is given the spoilers on one side will extend and the spoilers on the other side will retract.
87
Give 6 reasons for spoilers:
Ailerons are limited in size and therefore effectiveness
On a thin swept wing ailerons that are too large will experience wing twisting
Ailerons at high speed tend to lose effectivness due to spanwise flow accross the aileron
High speed swept wing can cause a strong rolling moment known as adverse rolling moment with yaw
Because the aircraft has low drag and the engines have a slow response time there is need for the ability to lose height and speed quickly
On landing or RTO there is a need to dump lift in order to make braking more effective
88
What limits the use of spoilers and why do spoilers blow back?
Principally spoilers are limited by very high speeds which cause them to blow back to almost their retracted position.
89
How is spoiler blowback prevented?
By operating them within the normal operating speed ranges of the aircraft.
90
What do leading edge slats do?
Leading edge slats reduce stall speed by increasing the surface area and the camber of the wing which increases CL.
91
What are Krueger flaps?
Krueger flaps are leading edge devices which increase the camber of the wing and the surfrace area therefore increasing CL.
92
What are fowler flaps?
Fowler flaps (usually triple slotted) are trailing edge devices which initially extend to increase the surface area of the wing and increase CL and reduce the stall speed. When fully extended they create drag.
93
What is the primary use of flaps on a jet aircraft?
The primary use of flaps on a jet aircraft is to reduce take off and landing distances by increasing the camber of the wing and the area, increasing the CL and reducing the stall speed.
94
What are the effects of extending flaps in flight? (4)
Generally they will cause a change in pitching moment.
If the CoP is in front of the CoG then a nose up pitch will occur
If the CoP is behind the CoG then a nose down pitch will occur
The flaps will increase the amount of downwash which will reduce the AoA on the tailplane and cause a nose up moment
The increase in drag will cause a nose up or down pitching moment depending whether they are above or below the lateral axis
95
What is the effect of rasing flaps in flight? (2)
A loss of lift will occur if not compensated for by:
increase in speed
change of attitude
96
How do flaps affect take off ground run?
Taking off with flaps increases the CL and therefore reduces the stall speed and the rotation speed and V2.
The drag incrememnt however increases when the aircraft is in flight which reduces the climb performance of the aircraft.
97
What are the purposes of yaw and roll dampers and how do they work?
They are used to prevent and dampen out Dutch roll.
The yaw damper is a rate gyro system that is sensitive to changes in yaw, which applies opposite rudder before Dutch roll can occur.
The roll damper is a supplement to the yaw damper for the Dutch roll and dampens out the Dutch roll once it has been established.
The roll damper is also used in order to provide sprial stability.
98
Describe parallel yaw dampers?
Parallel yaw dampers apply rudder control through the same control run as the pilot and moves the rudder pedals, which can increase the rudder loads felt by the pilot. This makes matters worst in the event of an engine failure on take off.
99
Describe a series yaw damper?
A series yaw damper does not match the pilots inputs on the rudder which means the rudder forces are not increased should there be an engine failure.
100
What is a stabiliser / variable-incidence tailplane?
This is an all moving horizontal tailplane control surface.
Usually an all moving tailplane is called a stabiliser when it is soley responsible for longitudinal balancing and it has a separate elevator with its own controls for pitch.
A variable incidence tailplane is one which is all moving and does not have an elevator.
101
What is the purpose of a stabiliser?
To provide longitudinal balancing force to the aircraft, thus the elevator range is not compromised and remains the sole control for the pitch of the aircraft.
102
What are the four reasons for a variable-incidence tailplane/ stabiliser especially for a jet aircraft?
to provide a balancing force for a large CoG range
to provide a balancing force for a large speed range
to cope with large trim changes (config etc)
to reduce elevator trim drag to a minimum
103
Describe the effects of a stuck stabiliser
The longitudinal balancing is degraded due to a backup elevator providing the balaning force. This means:
Heavy stick force for the pilot
Pilot inputs are slower as the elevator surface is much smaller than the stabiliser.
Pitch control capability is reduced
104
What is the best CoG position for a stuck stabiliser and why?
Aft CoG because this increases the maneouvrability of the aircraft.
105
What is the required action with a stuck stabiliser? (6)
Maintain speed at which jam occurred - this will be a trimmed position.
Divert to a nearby airfield so the CoG does not change excessively over a prolonged flight
Move the CoG to an aft position
Reduce speed as late as possible
Plan for a long final - Gear and flaps down early
Use a reduced flap setting for landing - This reduces the landing flare
106
What is the required action for trim runAway? (4)
Hold control column firmly
Diengage autopilot
Stab trim switched to cut out
Grab and hold the trim wheel
107
Describe the effects of a failure / reduction in elevator feels?
Artificial feel (Q feel) meet the requirement of progressive feel on powered controls, especially the elevator in order to prevent over stressing the aircraft.
When this sytem fails, the feel on the elevator is significantly reduced and the controls must be moved slowly and and smoothly with great care.
Turbulence should be avoided.
108
What is the best CoG position when the Q feel system has failed?
Forward CoG which will increase the stability of the aircraft and reduces the effectiveness of the control
109
Describe the air loads on a control surface?
When a control surface is deflected it increases the dynamic pressure around the control surface.
When multiplied by the control surface arm it will give the size of the moment trying to rotate the control back to its neutral position. This is known as a **hinge moment**.
110
What is a tab surface and what can it be used for? (3)
A tab surface is an extension of a control surface which can be used for the following:
Trimming
Control balancing
Servo operation of the control (moves the tab which moves the surface)
111
What is a balance tab?
A balance tab is an extension of a control surface which moves in the opposite direction.
This reduced the moment arm (and the control effectiveness) as we well as the stick force to a manageable level for the pilot.
112
What is a hinge/horn balance?
A hinge or horn balance is an extension of the surface area which petrudes in the opposite direction in order to create drage in the opposite direction which reduces the stick force required to move the surface. This reduces the control surface hinge moment.
113
What is a mass balance and what is it used for?
A mass balance is a weight that is placed forward of the hinge line of the control surface which assists in reducing stick force and also prevents control surface flutter.
114
Why does a powered control surface need an artificial feel system?
The reason for this is due to the fact that feedback is not given to the pilot on powered controls. This helps keep the pilot in the loop and prevents any overstressing of the control surface.
115
How does an artificial feel (Q feel) system work?
A device that is sensitive to dynamic pressure is used whereby pitot pressure is fed into one side of the chamber and static pressure is fed into the other side. This moves the diaphram which is influenced by dynamic pressure and causes regulated hydraulic pressure to provide resistance to the pilots inputs.
116
What are the inputs to the Q feel system?
Static and Pitot pressure
Control surface angle of deflection (Power Flying Control Unit - PCFU)
117
What are active controls?
A surface that moves automatically in response to a non direct input.
For example, slats on 737-300 extend when they sense that trailing edge flaps have been extended to 1-5 position.
118
