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xxxFOF > Chap E: Stability and Control > Flashcards

Chap E: Stability and Control Flashcards

1
Q

definition of stability

A

inherent ability of an object to develop a system that will tend to return to its original or undisturbed state after a disturbance

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2
Q

what is a stable aircraft?

A

tends to return to its original attitude and speed with no pilot effort on being disturbed

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3
Q

aircraft stability types

A

1) static stability
2) dynamic stability

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4
Q

definition of static stability

A

initial tendency of a/c to return to its original position when disturbed, motion may or may not return to its original position

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5
Q

does static stability guarantee an original position over time? why or why not?

A

no, as if the initial tendency is too large, it may not be able to return to its original flight state (too big of a change)

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6
Q

define positive static stability

A

initial tendency of a system to return to the original state of equilibrium after being disturbed

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7
Q

define neutral static stability

A

initial tendency of a system to remain in the new state after its equilibrium has been disturbed

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8
Q

define negative static stability

A

initial tendency of a system to go further away from the original state of equilibrium after being disturbed

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9
Q

what is dynamic stability?

A

aircraft response over time when disturbed

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10
Q

what does a dynamically stable aircraft mean?

A

a/c will return to its original state after a period of time

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11
Q

aircraft with positive dynamic stability means?

A

oscillations damp out over time and a/c returns to the original flight path

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12
Q

negative dynamic stability meaning

A

each oscillation causes a/c to stray further away from the original state

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13
Q

deadbeat stability meaning

A

when there is no tendency to oscillate after initial disturbance

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14
Q

when is it more desirable to have neutral or negative stability?

A

in fighter jets, as they require high manoeuvrability and having high stability will make it more difficult to control

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15
Q

definition of stick-free

A

control stick is not held fixed such that control surfaces can rotate freely during response motion

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16
Q

definition of stick-fixed

A

control stick held fixed in response motion (control surfaces not being able to deflect/rotate freely)

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17
Q

power-on and power-off meaning

A

whether the engine is developing thrust

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18
Q

definition of longitudinal stability

A

stability of an aircraft in pitching

or

stability about the lateral axis

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19
Q

names of the axis

A

longitudinal axis: x
lateral axis: y
vertical axis: z

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20
Q

definition of lateral stability

A

aircraft’s resistance to roll

or stability about longitudinal axis

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21
Q

definition of directional stability

A

aircraft’s resistance to yaw

or stability about vertical axis

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22
Q

factors affecting longitudinal stability

A
  • position of CG
  • location of tail plane (area, AR, riggers angle)
  • tail/nose heavy (tail is more unstable)
  • pitching moment
  • thrust line
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23
Q

tail plane action from change in AOA, effect on speed

A

if AOA is 0: airflow around tail is evenly distributed

if AOA increases: more airflow at upper surface (lesser speed)

if AOA decreases: more airflow at lower surface (more speed)

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24
Q

what is longitudinal dihedral?

A

difference in incidence angle between main and tail planes

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25
Q

why is swept back wings more stable than rectangle wingss?

A

Cp is further aft, means longer moment arm, leading increase nose down pitching moment

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26
Q

where does the thrust line need to be relative to CG to make aircraft stable?

A

thurst line above CG

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27
Q

what is a phugoid?

A

long period of pitching oscillation of aircraft, change in speed and height

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28
Q

what is short period mode?

A

motion is damped out within a short time

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29
Q

difference between phugoid and short period oscillation

A

phugoid: long oscillations (> 30secs)
short period: short oscillations (< 6secs)

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30
Q

parameters for lateral stability

A

1) dihedral angle
2) high wing (thus low CG)
3) sweepback
4) fin area

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31
Q

what is dihedral angle?

A

angle between each wing plane and the horizontal

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32
Q

how does a dihedral wing help with stability in a sideslip?

A

upgoing wing is shield by fuselage, also causing airflow to rush downwards, reuducing AOA thus lift on the upgoing side

downgoing side will have more lift than upgoing side, creating a restoring rolling moment

33
Q

how does high wing and low CG help with lateral stability?

A

air will rush up to the downgoing side of wing, increasing AOA, creating more lift

whereas air will rush down to the upgoing side of wing, decrasing AOA, lesser lift

34
Q

what capability does the high wing configuration have?

A

self roll correcting (dihedral effect)

35
Q

how to counter excessive lateral stability?

A

implementing anhedral (negative dihedral)

36
Q

how does lateral stability work in a sideslip?

A

lift on lower/ downside going wing is higher than the raised upside going wing as upside going wing is shielded from relative airflow by fuselage

difference in lift causes the a/c to restore roll

37
Q

what does anhedral do to stability?

A

reduces lateral stability, increases control

38
Q

what does sweepback do?

A

improve directional stability, delay shock wave formation at transonic speed

it is beneficial to all stabilities

39
Q

how does sweepback help with lateral stabiltiy?

A

down going wing: higher AR, more lift
up going wing: lesser AR, less lift

40
Q

side surface area relative to lateral stability

A

high side surface area above CG tends to create restoring rolling moments

41
Q

parameters for directional stability

A
  • size & location of vertical stabiliser (side surface area)
  • sweepback
42
Q

what is the fin area required for directional stability?

A

dependent on net sum of all restoring moments associated with side surface area forward and aft of CG

43
Q

how does ventral fin help with directional stability?

A

reduces effect of dutch roll

44
Q

how does sweepback help with directional stability?

A

more drag on the side that is closer to the front has a higher frontal area which means higher drag than the opposite side

45
Q

what is adverse yaw?

A

side effect caused by a rolling motion

turn right means left aileron down, right aileron up, more lift on the left side than the right

more lift means higher Di means Dleft wing > Dright wing, causing aircraft to pull to yaw to the left

46
Q

what is sideslip

A

when a faster moving outer wing produces more lfit than the inner wing causing a roll in the direction of yaw

47
Q

definition of dutch roll

A

back and forth motion resulting from dihedral attempting to return wing to level but overshoots and enters a sideslip in the opposite direction

48
Q

what happens when yaw correction is more than roll correction

A

heading change

flat spin - a/c rotate horizontally while dropping

spiral dive

49
Q

how does spiral instability occur?

A

when aircraft have excess directional stability than lateral stability

50
Q

what is spin mode?

A

aggravated stall, high AOA and rate of descent, low airspeed

51
Q

what causes spin mode?

A

rotation of aircraft about its CG where the aircraft follows a downward screw path

52
Q

definition of controllability

A

ability of aircraft to respond to control surface displacement and achieve desired condition of flight

53
Q

list of primary controls

A
  • longitudinal (pitch) by elevators
  • lateral (roll) by ailerons
  • directional (yaw) by rudder
54
Q

how does elevator control pitch?

A

up elevator increases downward lift on tail, moving it down –> nose pitch up increases AOA –> more lift = climb

55
Q

how does aileron control roll?

A

more lift on left wing and less lift on right wing causes aircraft to roll right

56
Q

how does rudder control yaw?

A

deflecting rudder to the right creates a leftward lateral force that moves the fin to the left hence the aircraft to the left

57
Q

power and effectiveness of control can be determined by?

A

1) size & shape of control surfaces - area, AR
2) deflection angle - changes overall camber
3) equivalent airspeed - force on surfaces
4) moment arm - moment proportional to distance

58
Q

dual flight action controls

A

elevon - elevator + aileron
flaperon - flap + aileron
ruddervator - rudder + elevator

59
Q

what does elevon control?

A

pitch and roll control

60
Q

what does flaperon control?

A

CL and roll control

61
Q

what does ruddervator control?

A

yaw and pitch control

62
Q

hinge moment definition

A

forces generated by the control surfaces

63
Q

why is balanced control needed?

A

hinge moments is noticeable at high speed due to high dynamic pressure

64
Q

methods to achieve balanced control

A

mass balance (locate CG of control surface on or close to the hinge line)
aerodynamic balance (locate CP of control surface on or close to the hinge line)

65
Q

how to attain aerodynamic balance using hinge line?

A

hinge line may be located along the CP of the control surface or just forward of it to reduce hinge moments

66
Q

how to attain aerodynamic balance?

A

inset hinge
horn balance
internal balance
tabs

67
Q

how to attain aerodynamic balance using horn balance?

A

horn balance reduces hinge moments

68
Q

what are balance tabs?

A

small movable surface fitted to trailing edge of main control surface to help move control surface

69
Q

what is trim?

A

balancing an aircraft in flight around its CG

70
Q

what are trim tabs?

A

help stabilise the aircraft and free pilot from constantly adjusting the controls

71
Q

control problems at low speed

A

reduced aerodynamic forces
near stall angle
may be immersed in separated flow

72
Q

what may happen if controls are deployed at low speeds?

A

induce stall leading to control reversal

73
Q

what may be used to improve low speed control

A

slots used with down going aileron to delay stalling;
spoilers used with up going aileron to reduce lift

74
Q

dangers of executing a bank turn at low speed

A

control reversal may happen

75
Q

how can spoilers be used at high AOA to help with lateral stability?

A

deployment of spoiler on the up-deflecting aileron (non-stalled wing) can reduce lift on that side which balances out the lift between the wing

76
Q

difference between balance and trim?

A

trim - balancing of aircraft in flight around its CG

balance - reduction of hinge moments

77
Q

why is root stall more desirable than wing tip stall?

A

aircraft less likely to roll, due to difference in moments generated by the lift

ailerons at the wing tips are still effective

78
Q

how does Dutch roll occur?

A

when roll correction is more than yaw correction

xxxFOF (6 decks)

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