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xxxFOF > Chap D: Elements of Aircraft Performance > Flashcards

Chap D: Elements of Aircraft Performance Flashcards

1
Q

why cant aircraft fly above the troposphere?

A

1) Stratosphere has ozone layer, need ozone filter to fly in it

2) higher altitude means lower air pressure, leading to larger pressure difference between cabin and atmosphere (stronger fuselage)

3) higher altitude means higher speed, might reach critical mach number inducing high level of drag (shock waves)

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2
Q

4 forces in level flight and where do those act on

A

1) Lift - acts upwards, through CP
2) Weight - acts downwards through CG
3) Thrust - acts forwards
4) Drag - acts backwards

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3
Q

what does higher AOA result in for lift?

A

higher AOA means higher CL (from CL - AOA graph)

higher CL means more lift (direct proportional)

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4
Q

what are the conditions for equilibrium?

A

ΣFy = 0 (L = W)
ΣFx = 0 (T = D)
ΣM = 0 (nose up moments = nose down moments)

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5
Q

which point is infront of each other? CG or CP and why?

A

CG is infront to ensure nose-down pitch in engine failure

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6
Q

why is it difficult to balance the forces in flight?

A

lift acts through CP, changes with AOA
weight acts through CG, changes are fuel drain

effective thrust line affected by engine position and AOA
drag line determined by aircraft configuration

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7
Q

what is the use of horizontal stabiliser?

A

nose up pitching moment due to T & D is less than nose down pitching moment due to L & W

horizontal stabiliser is use to make up for the missing nose up pitching moment

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8
Q

loads on horizontal stabiliser

A

ΣFy = 0 [ L = W + LT]
ΣFx = 0 [ T = D]
ΣM = 0

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9
Q

what is the Coanda Effect?

A

airflow over upper surface of wing will bend with the curvature of the surface which the flow will leave the wing ‘shooting downwards’

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10
Q

why is downwash bad?

A

as it will reduce the AOA of the tail if immersed in it. (lesser AOA means lesser lift)

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11
Q

how much is downwash relative to AOA?

A

downwash is half of AOA

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12
Q

how to minimise downwash effects on tail?

A

placing tail plane as far away and as high enough compared to main wing

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13
Q

how does tail plane work?

A

it produces downward lift and hence nose up moments

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14
Q

is canard rigged higher or lower incidence angle of the wing? and why?

A

higher (larger force due to small moment arm from CG)

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15
Q

what can canard configuration act as well?

A

stall prevention device (loses lift and counter-balancing moment)

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16
Q

why is canard not widely used?

A

impact of airflow over the wing and airflow entering the engine

mess up airflow going over the wing and into the engine causing performance reduction

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17
Q

how does tailess aircraft work?

A

flaps or elevators act ailerons

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18
Q

why is fixed tail plane bad?

A

fixed incidence angle, weight is reduced as time goes so lesser lift is needed which results in lesser moment for tail to balance

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19
Q

how does adjustable tail-plane work?

A

has adjustable trailing edge devices to control pitch and trim during flight
can change camber as well

helps to keep fuselage parallel to flight path (minimise drag)

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20
Q

what is trimming?

A

keeping resultant pitching moment zero

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21
Q

where is adjustable tail-plane most common in?

A

low speed aircraft

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22
Q

types of tail plane

A

1) fixed tail plane
2) adjustable tail plane
3) slab tail plane
4) all flying tail plane

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23
Q

how does L/D ratio affect tail load?

A

the higher the L/D ratio, the larger the balancing tail load needs to be, meaning larger horizontal stabiliser

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24
Q

difference between AOA and incidence angle

A

AOA: angle between relative airflow and wing chord line

incidence angle: angle between wing chord line and fuselage centreline

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25
Q

what is incidence angle?

A

best lift to drag ratio angle of attack

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26
Q

define stall speed

A

minimum speed to sustain level flight

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27
Q

how to find velocity using lift formula?

A

W = L = 1/2 ρ S CL V2

rearrange formula to find V

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28
Q

difference between stalling speed and stalling angle

A

stalling speed: lowest speed to sustain level flight

stalling angle: AOA at which CL is maximum

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29
Q

airspeed wrt to CL

A

as airspeed increases, CL decreases exponentially

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30
Q

Dp & Di wrt to V during cruise?

A

Dp increases as V increases
Di decreases as V increases

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31
Q

when does Dp = Di occur?

A

when V is at min. drag speed

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32
Q

resultant total drag minimum occurs when? and what does it mean?

A

when Di = Dp

minimum drag speed leading to lesser fuel consumption

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33
Q

how does Di change when H changes? and how does air density affect Di?

A

as altitude increases, Di increases (VTAS remains constant)

Di is inversely proportional to density

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34
Q

how is Dp relative to frontal area?

A

as frontal area increases, Dp increases

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35
Q

how does altitude affect Dp?

A

as H increases, Dp decreases

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36
Q

how does ice/frost affect the aircraft?

A

formation of ice/frost may change wing profile, leading to Vstall changes

may create unintended roll

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37
Q

why is icing a hazard to flight?

A

changes aerofoil section profile, destroys smooth flow of air

ice may break mid-flight and strike engine intake/propeller

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38
Q

how does ice formation affect stall angle?

A

stall angle decreases

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39
Q

what is de-icing?

A

procedure where frost/ice/snow is removed in order to provide clean surface

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40
Q

difference between de-icing and anti-icing

A

de-icing: reactive, breaking up after allowing ice to form

anti-icing: proactive, preventing ice from forming

41
Q

how to implement anti-icing

A

heating or chemical fluids to prevent ice from forming

42
Q

how does snow/ice on runway affect aircraft performance?

A

snow/ice makes runway smoother, leading to less rolling friction,

a/c needs longer takeoff and landing distance

43
Q

definition of maximum speed

A

Vmax is the speed which there is zero excess thrust available

44
Q

definition of minimum speed

A

Vmin is approx = Vstal (based on aerodynamics)

45
Q

common aircraft propulsion types

A

1) reciprocating engine
2) turbojet
3) turbofan
4) turboprop

46
Q

when is turbojet good at?

A

high flight speed (higher thrust and efficiency)

47
Q

thrust derivation from Newton 2nd law

A

F = ma + ṁV

where, ma is for solid body, ṁV is for fluid motion

F = T = ṁV (ma = 0, no solid body in propulsion)

ṁ - mass flow rate
V - flow speed

also can be written as T = ρAV2 or T = ṁ∆v

48
Q

how to increase thrust by formula?

A

T = ṁV

increasing ṁ or V or both

49
Q

propeller efficiency relative to H

A

as H increases, density decreases, so propeller eff. decreases

50
Q

KE wasted formula

A

KElost = 1/2 (Vj - V)2

Vj - jet exhaust speed
V - speed of air coming in to engine

51
Q

how does jet engine perform better at high altitude?

A

as H increases, temp decreases, making it cooler to fly in (can help with cooling of engine)

52
Q

Power available formula during cruise

A

PA = TVTAS = DV = PR

53
Q

propulsion efficiency formula

A

ηp = (useful power avail.)/(total power generated)

ηp = (2V)/(V - Vj)

54
Q

how does propeller aircraft create thrust?

A

engine transmit torque to propeller

propeller converts torque to thrust

55
Q

why is propeller blade twisted?

A

to meet relative wind at most efficient AOA

56
Q

jet engine working principle

A

75% of energy used for compression of air

25% left used for exhaust gas (thrust)

57
Q

jet engine points

A

drag same regardless of altitude (can fly faster)
T due to small ṁ with large ∆v

58
Q

how is turbulent air good for jet engine?

A

turbulent air promotes mixtures of fuel-air

better burning efficiency

59
Q

turboprop points

A

fast acceleration to take-off
95% of thrust developed by propeller, remaining 5% by jet exhuast

60
Q

turbofan points

A

combining high thrust from turbojet and high efficiency from propeller

large volume of air bypass combustors

fan bypass air can be used to cool down jet core

61
Q

efficiencies of turboprop/fan/jet

A

propeller good for low speed
jet good for high speed

turbofan combines both

62
Q

how is independent concentric spinning spool useful?

A

allows for independent control of spool, leading to better efficiency

63
Q

definition of range

A

maximum distance an aircraft can fly for a given quantity of fuel (usually assumed full tank)

64
Q

what does range depends on?

A

1) engine performance
2) type of powerplant
3) fuel consumption
4) amount of fuel carried
5) L/D ratio

65
Q

range equation

A

R = (V/ct) * (L/D) * ln(W0/W1)

ct - thrust specific fuel consumption
W0 - gross weight of a/c
W1 - empty weight of a/c

66
Q

how to achieve max range?

A
  • carry as much fuel as possible
  • fly at L/D max (D is minimum)
67
Q

definition of endurance

A

maximum duration an a/c may stay airborne for a given quantity of fuel

68
Q

endurance equation

A

E = (1/ct) * (L/D) * ln(W0/W1)

ct - thrust specific fuel consumption
W0 - gross weight of a/c
W1 - empty weight of a/c

69
Q

conditions for range and endurance depends on?

A

1) weight (also amt of fuel carried)
2) altitude
3) thrust provided type

70
Q

how to obtain max range & endurance for jet aircraft?

A
  • fly at max:
    for range: CL1/2/CD
    for endurance: max L/D ratio
  • fly at high altitude (for range)
  • lowest ct
  • highest possible gross weight to empty ratio (as much fuel as possible)
71
Q

definition of maximum endurance speed

A

speed when power required is least

72
Q

definition of maximum range speed

A

speed for which thrust required is least

73
Q

explain ground effect

A

when a/c flies close to the ground, CDi from wing tip vortices is reduced due to the reduction in induced downwash angle

74
Q

benefit of ground effect

A

induced drag is significantly reduced

75
Q

ground effect relative to wing’s height above ground

A

GE tends to 1 when further away from ground

76
Q

rolling friction formula

A

R = µr * NormalForce

77
Q

why is AOA for take-off significantly less than stalling angle?

A

1) reduce both Di and Dp by reducing lift
2) avoid stall due to over rotation during LO
3) avoid tail striking ground (too steep)

78
Q

landed speed definition

A

stalling speed for touching down

79
Q

why does power in propeller decrease with altitude?

A

higher altitude has lower density, lower mass flow

80
Q

Lift and Thrust forces formula during climb

A

L = Wcosy

T = D + Wsiny

81
Q

speed for best rate of climb is when?

A

when there is greatest reserve of power

82
Q

rate of climb & climb angle symbol

A

Vc (rate of climb)
γ (climb angle)

83
Q

how does weight affect Vstall, drag, and Prequired?

A

weight increase means:
Vstall increase
Drag increase
Prequired increase

84
Q

definition of absolute ceiling

A

height when there is no power reserve available for climb

85
Q

definition of service ceiling

A

height where climb velocity falls below 30m/min

86
Q

how many degrees of freedom does an aircraft have?

A

6
3 translational, 3 rotational

87
Q

definition of load factor

A

ratio of wing loading during a manoeuvre to that for straight and level flight

(lift to weight ratio during manoeuvre)

88
Q

what is a coordinated bank turn?

A

a turn where a/c does not lose altitude or speed

89
Q

why is inverted flight dangerous?

A
  • a/c may not be able to upright itself
  • engine may shut down due to fuel supply interrupted
90
Q

how does glider generate velocity for flight?

A

by trading PE for KE

it loses altitude to gain speed

91
Q

types of rising air to increase glider flight time

A

thermals - rising columns of hot air
ridge lift - wind deflected upwards
wave lift

92
Q

what are thermals?

A

columns of rising air created by heating of surface by the sun

93
Q

what is ridge lift

A

wind blowing against mountains, hills

94
Q

endurance for a glider

A

AOA that is max L/D ratio

95
Q

methods to lower stalling speed of aircraft

A

using lift devices (slots, slats, flaps) by changing shape & area of aerofoil

96
Q

when do we need to lower stalling speed?

A

during take-off and landing, as the aircraft is at a high AOA

97
Q

why do aircraft with canard wings less likely to stall?

A

higher angle of incidence than the main wing, meaning higher AOA, it will stall before the main wing, removing the counter-balancing moment created by it

98
Q

why is lfit due to wing slightly higher than weight during cruise?

A

as L = W + LT
lift created by horizontal stabiliser is downwards along with weight

99
Q

AOA wrt airspeed graph

A

as airspeed increases, AOA decreases exponentially

xxxFOF (6 decks)

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