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Advanced Aerodynamics, Performance And Systems Knowledge Flashcards

1
Q

What factors affect hydroplaning speed?

A
  1. Tyre pressure - dominant factor
  2. Wheel loading
  3. Water film thickness
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2
Q

How would you control dutch roll in flight?

A

Apply opposite aileron to the up going wing.

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3
Q

What is the formula for rate one turn AoB at a given TAS?

A

AoB = (TAS/10)+7

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4
Q

How many meters per nautical mile?

How many feet per nautical mile?

A

1850m

6080’.

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5
Q

Does a forward/rear movement of CoG increase/decrease stall speed?

A

A forward movement increases stall speed and vice versa.

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6
Q

What is the angle of incidence?

A

The angle between the chord line and the aircraft’s longitudinal datum.

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7
Q

What is the formula for lift?

A

Lift=1/2RV2SCL

1/2R=half the value of density
V2=velocity squared
S=wing plan area
Cl=the coefficient of lift - this is the lifting ability of the wing. It depends on both the shape of the wing and AoA.

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8
Q

What are vortex generators for?

A

They re-energise the boundary layer, delaying separation. This will lead to:

  • Increased achievable speeds,
  • Improve initial buffet boundaries,
  • Improve aircraft controllability,
  • Reduce vibrations from boundary layer separation.
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9
Q

Describe how the centre of pressure moves

A
  1. Forward with increasing AoA.

2. Rearwards with decreasing AoA.

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10
Q

Which part of the wing normally stalls first?

A 
The wingroot (with proper washout).
The reason for this is so that you still have roll control and so that the nose pitches down on a swept wing aircraft. This is a stable movement.

Earlier aircraft without enough washout stalled at the tips first which pitched the aircraft up further increasing the angle of attack and drag. Aircraft with T tails suffered lack of tailplane and elevator effectiveness because the tail was in the path of the disturbed air coming off the wing. These aircraft became superstalled or deepstalled, and some could not be recovered. Thus stickshakers and stickpushers were developed for aircraft with unacceptable stall characteristics.

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11
Q

How do you calculate lead distance for turning onto an arc?

A

Standard rate turn radius = 1% of TAS / 2.

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12
Q

What is a chord line?

A

A straight line from the leading edge to the trailing edge.

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13
Q

What are the advantages and disadvantages of wing mounted engines?

A

Advantages:

Engines provide bending relief thus reducing wing structure weight.

Intake efficiency is not compromised except perhaps in reverse.

The wing profile is not compromised.

At high angles of attack the engine pylons tend to act as fences, controlling spanwise flow.

Interference drag is low.

Thrust reverser design is not compromised.

Engine accessibility is good.

Less containment devices needed in the event of failure.

Disadvantages:

More yaw following engine failure. Bigger rudders and fins required.

Reduced ground clearance.

A low thrust line can cause pitch up with power and pitch down with reducing power.

FOD damage is higher.

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14
Q

How can dutch roll be reduced through the design of an aircraft?

A

For the same amount of sweep back you can enlarge the fin and rudder. This would give more directional stability but going too far can impact lateral stability leading to spiral instability.

Reduce sweep back.

Utilise yaw dampers.

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15
Q

How can you calculate the lead radial?

A

Turn radius x radials per nm

So if a 12nm arc doing 150kts, 2.5/3=.8 x 5 = 4 radials.

15nm arc doing 180kts, 3/3=1 x 4 = 4 radials.

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16
Q

How does a yaw damper work?

A

Sense changes in yaw and provides rudder input to oppose the yaw.

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17
Q

What are the advantages of a swept wing?

A
  • It allows a high mach number cruise speed due to it’s lower drag. This is because the swept wing is only sensitive to the component of airflow velocity across the chord of the wing. The apparent airspeed across the chord is less than the real airspeed. This means that wing can be flown to a higher speed before the critical mach number is reached.
  • Obviously a thin wing is also required so as reduce the camber and so reduce the acceleration of air over the upper surface.
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18
Q

What is the effect of weight on rate of descent?

A

Increasing weight will increase RoD.

A heavier aircraft will fly at a higher EAS for a given AoA so its RoD will be increased.

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19
Q

What is specific fuel consumption?

A

Quantity of fuel consumed per hour divided by the thrust of the engine.

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20
Q

What are the disadvantages of a swept wing?

A

It can be subject to tip stalling which due to the wing tips being behind the center of gravity causing the aircraft to pitch up.
This has largely been fixed by washout (reducing the incidence at the wingtips so the wingroot stalls first).

It has a higher stall speed because the sweep reduces the lift in the same way it reduces the drag. This means that advanced and complicated flap and leading edge devices
are required to reduce the airspeeds for takeoff and landing.

It requires a higher angle of attack to produce the same amount of lift as an unswept wing. This produces high nose up attitudes for takeoff (possibility of tailstrikes) and landing. Also means that the profile drag is more – higher thrust required on approach and landing.

It has poor oscillatory stability. It has marked roll with yaw due to the reduced sweepback on the advancing wing producing more lift and also the increased projected
span. This leads to Dutch Roll.

On swept wing aircraft with podded engines far out on the wing, there is an increased possibility of scrapping them on the runway on takeoff or landing if the aircraft is rolled significantly. This is because the outer part of the wing is behind the main gear which is the pivot for the manoeuvre.

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21
Q

Describe wing tip vortices

A

Created by span-wise airflow over the upper and lower surfaces of an aerofoil that meet at the wing tips creating turbulence and therefore increasing induced drag.

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22
Q

What is the formula for required rate of descent on a 3° glide-path?

A

ROD = 1/2GS x 10

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23
Q

What is the formula for Mach Number and how to IAS/TAS and Mach number inter-relate in a climb?

A

Mach No = TAS/LSS

As you climb, constant IAS = increasing TAS and increasing Mach.
constant TAS = decreasing IAS, increasing Mach.
Constant Mach = decreasing IAS, decreasing TAS (provided temp is decreasing)

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24
Q

How would you recover from an Mmo overspeed?

A
  • Speedbrake
  • roll wings level and raise the nose carefully
  • small amounts of stab trim
  • carefully reduce thrust noting thrust line.
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25
Q

What is Horne’s formula?

A

A simple hydroplaning formula - GS (kts) = 9 x squareroot of tyre pressure in PSI.

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26
Q

What is the formula for Total Air Temperature?

A

TAT=SAT+Ram rise.

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27
Q

How do you calculate headwind component?

A 
Headwind  = wind speed x cos(wind angle)
Cos10=1
Cos20=0.9
Cos30=0.9
Cos40=0.8
Cos50=0.6
Cos60=0.5
Cos70=0.3
Cos80=0.2
Cos90=0
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28
Q

How do you fly for best endurance?

A

You have to stay airborne as long as possible on a given amount of fuel. The lowest fuel consumption rate is needed.

Minimum drag and hence minimum thrust is required.

Altitude improves endurance as turbine engines are more efficient at the higher RPMs necessary to maintain the minimum thrust at altitude.

29
Q

How does an aft CoG affect stability?

A

Reduces longitudinal and directional stability.

30
Q

What will selecting a higher flap setting do to V2 for take off?

A

It will reduce V2.

31
Q

Will an ASI under/over read with an increase/decrease in altitude from when the static port became blocked?

A

ASI will under read with an increase in altitude as the static balancing pressure will be higher than it should be.

32
Q

What is a Mach Trimmer?

A

A device fitted to some jet aircraft which trims the stabiliser up at mach numbers exceeding MCrit. It is used because some aircraft experience either lack of elevator effectiveness or very heavy elevator forces at high mach numbers above MCrit.

33
Q

What are the advantages of a fan engine?

A
  1. Smaller engine size.
  2. Better propulsive efficiency.
  3. Better specific fuel consumption.
  4. Reduction in engine noise.
  5. Contaminations are centrifugally discharged through the bypass duct, therefore protecting the main engine core from damage and even a flame out from water contamination.
34
Q

How will a blocked static system impact the altimeter and VSI?

A

Altimeter will be frozen at the altitude where the blockage occurred.
VSI will be frozen on zero for a total blockage. A partial blockage will decrease the rate.

35
Q

What is angle of attack?

A

The angle between the chord line and the relative airflow.

36
Q

How do designers increase the Critical Mach Number of an aircraft?

A

Number of ways:

Lift / Drag Formula: ½ Rho V2 S CL ½Rho V2 S CD

Low Wing Area: A larger wing will have increased drag but better lift.

Aspect Ratio: High aspect ratio causes less induced drag but can be a problem for the structural people. I.e. High bending forces involved and also a very thick wing root to support the long wing. A thick wing has a lower critical mach number so it is a tradeoff in many respects.

Sweep: To little sweep causes a high drag rise at a low mach number. Too much sweep causes poor oscillatory stability and a tendency for the tips to stall. Also as fuel burns off there is a large center of gravity change in a
highly swept wing.

Taper: This is the ratio of root chord to the wingtip chord. Optimum is about 2 ½ to 1. Each section of the wing will produce the correct proportion of lift. If it is too small then the wing will be heavier from a structural point of view. If too large then high local coefficients of lift are produced which tends to make the tips stall first or the wing to suffer bad stalling characteristics.

Thickness / Chord Ratio: A thin wing is required for high mach numbers. A thick wing is required for structural strength, accommodating fuel, landing gear, flaps, and also to lower the stalling speeds.

37
Q

Below what speed does compressibility have little effect?

A

300kts

38
Q

How is stability affected by high altitude/high speed flight?

A

Aerodynamic damping is reduced at high altitude. There is less restoring force when a displacement happens. ½ Rho V2 is the reason why (less lift). Less air density at high altitude and the V2 forces are not high as V2 is based on IAS.

Directional control can be affected. If right rudder is applied it can accelerate the left wing to it’s critical mach number which thus loses lift and has increased drag so the aircraft yaws to the left and rolls left. This means that spiral stability is increased. The aircraft won’t enter a spiral dive.

Lateral control can be affected as normally the outboard ailerons are locked out due to wing twist that they cause leading to an opposite roll. Thus only the inboard
ailerons are available and possibly the differential spoilers. Oscillatory stability is reduced due to less roll control.

Longitudinal stability is reduced. (See Mach Tuck, Mach trimmers)

Note: spoilers are normally locked out as well due to the high drag penalties associated with their use at high speed.

As altitude increases, true air speed increases for the given equivalent air speed, resulting in decreased aerodynamic forces. Thus, at higher altitudes the pilot must apply greater opposite control movements to arrest rotation.

39
Q

What are the advantages of a wide chord fan engine?

A
  1. Better fuel economy.
  2. More thrust.
  3. Less weight.
  4. Less noise.
40
Q

How does the speed range between low speed buffet and high speed mach buffet change with altitude and weight?

A
  • It reduces with altitude,
  • It reduces with weight as stall speeds increase even though Vmo/Mmo remain the same.
  • High speed buffet may be encountered at high altitude before Mmo with turbulence or manoeuvring.
41
Q

What are some effects of wing tip vortices?

A
  1. Potentially reduces aileron efficiency.
  2. Reversed span-wise flow at the outside edge of the upper surface can contribute to wing-tip stall.
  3. Increases induced drag.
  4. Creating an aviation hazard.
  5. Can potentially affect the tailplane RAF.
42
Q

Describe Aspect Ratio

A

The ratio of an aerofoil’s span to its chord.

  1. High ratio = high lift (gliders)
  2. Low ratio = low lift but capable of higher speeds.
43
Q

What are the effefct of excessive aircraft weight?

A
  1. Performance reduced:
    - Takeoff and landing performance,
    - Rate of climb and ceiling,
    - Range and endurance,
    - Maximum speed.
  2. Stall speed increased.
  3. Manoeuvrability reduced.
  4. Wear on brakes and tyres increased.
  5. Structural safety margins reduced.
44
Q

What is the effect of weight on glide range?

A

Glide range does not vary with weight in still wind provided that the aircraft is flown at its optimal AoA and speed (VIMD) for that weight.

Glide range is proportional to the lift/drag ratio which does not vary with weight. Weight will affect time to descend, but not distance. So with a head wind, you can get further by being heavier but with a tail wind it is better to be lighter.

45
Q

What do wing fences do?

A

They reduce span-wise flow.

46
Q

What produces the maximum glide range?

A

The maximum lift/drag ratio. This is obtained by the aircraft being flown at its optimal AoA and corresponding min drag speed (VIMD).

47
Q

What is the equivalent braking coefficient for a hydroplaning tyre?

A

Equivalent to icy or slippery runway. Approximately 20% of a dry runway braking coefficient.

48
Q

How does a glove fairing at the wing root affect Mcr?

A

It increases it by delaying the formation of the shock wave and decreases drag in the cruise speed range.

49
Q

Why should a mach meter be used rather than an ASI above FL270?

A

The IAS at which sections of the aircraft reach their local speed of sound changes with pressure. Therefore flight at a mach number will ensure operations below Mcrit.

50
Q

What are the forces acting on an aircraft in flight?

A
  1. Thrust
  2. Drag
  3. Lift
  4. Weight

In a banked turn, weight is a constant, but lift is lost due to tilting the lift vector. Therefore, to maintain altitude in a banked turn, the vertical component of lift must be made to equal the weight by increasing speed and/or increasing AoA.

51
Q

How to winglets work?

A

They prevent inter-mixing of the upper and lower surface span-wise flows.

52
Q

How do you calculate cross-wind component?

A 
Cross wind  = wind speed x sin(wind angle)
Sin10=0.2
Sin20=0.3
Sin30=0.5
Sin40=0.6
Sin50=0.8
Sin60=0.9
Sin70=0.9
Sin80=1
Sin90=1
53
Q

You observe the altimeter in a parked aircraft while a cold front passes the airport. What does the altimeter indicate?

A

The altimeter will over-read then under-read as the pressure decreases, then increases after the front.

54
Q

Why do jets fly as high as possible?

A
  1. Gas turbines operate most efficiently at high rpm - Best specific fuel consumption.
  2. Profile drag is the lowest - EAS reduces flying a constant mach as you climb at high altitudes.

Only at high altitudes will the reduced thrust needed to balance the reduced drag in the cruise, be achieved at high rpm.

55
Q

How will a blocked pitot tube affect an ASI in steady flight, a climb or a descent?

A

Forward flight - ASI would under-read or not read.
Climb - ASI would over-read as static balancing pressure decreases.
Descent - ASI would under-read as static balancing pressure increases.

56
Q

What is washout?

A

A decrease in the angle of incidence along the length of the aerofoil.

57
Q

How can you prevent span-wise flow on a swept wing?

A

Fences and vortex generators.

These direct the airflow perpendicular to the chord.

Vortex generators disturb the air which means it will be denser which can help control surfaces.

58
Q

Why is too much taper bad?

A

It lead to tip stalling whilst washout reduces this.

59
Q

What is Mach Tuck?

A
  • When the aircraft pitches now down on reaching Mcrit.
  • The shockwave on the upper surface upsets the lift distribution chordwise and causes the centre of lift to move rearwards.
  • Swept wings tend to experience shock waves at the root first because of the higher angle of incidence (due to washout) and the camber is thicker. This causes a loss of lift to be experienced inboard first which is forward of the centre of lift.
  • The shockwave can also cause a reduction in downwash over the tailplane.
60
Q

What is the formula for local speed of sound?

A

LSS = 38.94 * square root of temp in K.

61
Q

Define VIMD

A

Minimum Drag Speed - where profile drag and induced drag cross over.

Represents the best lift/drag ratio.

62
Q

What is the formula for max drift angle?

A

MDA=(60/TAS)xwind speed

63
Q

Describe three types of high lift devices

A
  1. Trailing edge (fowler) flaps - increase lift by changing the shape of the aerofoil (S/CL).
  2. Leading edge flaps/slats (Krueger flaps) - increase lift by creating a longer wing chord line (S/CL).
  3. Slots - prevent/delay the separation of the boundary layer (CL).
64
Q

How do you fly for best range?

A

You have to cover the most nautical ground miles per unit of fuel used. In terms of engine efficiency this is achieved at high RPM which can only be achieved at high altitudes with jet engines. In terms of airframe efficiency it is achieved at the lowest drag to airspeed / thrust / fuel consumption. At high altitudes the TAS is greater for a given IAS so more nautical miles will be covered than at low altitudes so flying high has
two advantages – better groundspeeds and better fuel efficiency.

65
Q

Define the two major types of drag and their relationship.

A

Profile Drag (zero lift drag):

  1. Form or pressure drag.
  2. Skin-friction drag.
  3. Interference drag.
    - Increases directly with airspeed.

Induced Drag (lift drag):

  1. Drag created through the production of lift by exposing more surface area to the free stream at higher AoA and increasing vortices.
    - Reduces with speed
66
Q

What airspeed is flown for max endurance?

A

Vimd.

67
Q

What is the effect of altitude on endurance?

A
  1. Wing/airframe impact - nil.
  2. Engine impact:
    - engine operates at best ratio of fuel flow to thrust when at high RPM and with low inlet temps.
    - at altitude, higher RPM is required to produce thrust required to maintain balanced flight at any speed including Vimd.
    - therefore endurance is increased with altitude.
68
Q

What is the effect of altitude on Vimd?

A

No effect.

69
Q

What is the effect of weight on Vimd?

A

Increased weight requires more thrust to maintain balanced flight because more lift is required to balance weight, hence more drag.

Because more lift is required, the aircraft must fly faster. This moves the drag/IAS curve to the right and up.

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