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xxxFOF > Chap E: Stability and Control > Flashcards

Chap E: Stability and Control Flashcards

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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
why is swept back wings more stable than rectangle wingss?
Cp is further aft, means longer moment arm, leading increase nose down pitching moment
26
where does the thrust line need to be relative to CG to make aircraft stable?
thurst line above CG
27
what is a phugoid?
long period of pitching oscillation of aircraft, change in speed and height
28
what is short period mode?
motion is damped out within a short time
29
difference between phugoid and short period oscillation
phugoid: long oscillations (> 30secs) short period: short oscillations (< 6secs)
30
parameters for lateral stability
1) dihedral angle 2) high wing (thus low CG) 3) sweepback 4) fin area
31
what is dihedral angle?
angle between each wing plane and the horizontal
32
how does a dihedral wing help with stability in a sideslip?
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
how does high wing and low CG help with lateral stability?
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
what capability does the high wing configuration have?
self roll correcting (dihedral effect)
35
how to counter excessive lateral stability?
implementing anhedral (negative dihedral)
36
how does lateral stability work in a sideslip?
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
what does anhedral do to stability?
reduces lateral stability, increases control
38
what does sweepback do?
improve directional stability, delay shock wave formation at transonic speed it is beneficial to all stabilities
39
how does sweepback help with lateral stabiltiy?
down going wing: higher AR, more lift up going wing: lesser AR, less lift
40
side surface area relative to lateral stability
high side surface area above CG tends to create restoring rolling moments
41
parameters for directional stability
- size & location of vertical stabiliser (side surface area) - sweepback
42
what is the fin area required for directional stability?
dependent on net sum of all restoring moments associated with side surface area forward and aft of CG
43
how does ventral fin help with directional stability?
reduces effect of dutch roll
44
how does sweepback help with directional stability?
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
what is adverse yaw?
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
what is sideslip
when a faster moving outer wing produces more lfit than the inner wing causing a roll in the direction of yaw
47
definition of dutch roll
back and forth motion resulting from dihedral attempting to return wing to level but overshoots and enters a sideslip in the opposite direction
48
what happens when yaw correction is more than roll correction
heading change flat spin - a/c rotate horizontally while dropping spiral dive
49
how does spiral instability occur?
when aircraft have excess directional stability than lateral stability
50
what is spin mode?
aggravated stall, high AOA and rate of descent, low airspeed
51
what causes spin mode?
rotation of aircraft about its CG where the aircraft follows a downward screw path
52
definition of controllability
ability of aircraft to respond to control surface displacement and achieve desired condition of flight
53
list of primary controls
- longitudinal (pitch) by elevators - lateral (roll) by ailerons - directional (yaw) by rudder
54
how does elevator control pitch?
up elevator increases downward lift on tail, moving it down --> nose pitch up increases AOA --> more lift = climb
55
how does aileron control roll?
more lift on left wing and less lift on right wing causes aircraft to roll right
56
how does rudder control yaw?
deflecting rudder to the right creates a leftward lateral force that moves the fin to the left hence the aircraft to the left
57
power and effectiveness of control can be determined by?
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
dual flight action controls
elevon - elevator + aileron flaperon - flap + aileron ruddervator - rudder + elevator
59
what does elevon control?
pitch and roll control
60
what does flaperon control?
CL and roll control
61
what does ruddervator control?
yaw and pitch control
62
hinge moment definition
forces generated by the control surfaces
63
why is balanced control needed?
hinge moments is noticeable at high speed due to high dynamic pressure
64
methods to achieve balanced control
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
how to attain aerodynamic balance using hinge line?
hinge line may be located along the CP of the control surface or just forward of it to reduce hinge moments
66
how to attain aerodynamic balance?
inset hinge horn balance internal balance tabs
67
how to attain aerodynamic balance using horn balance?
horn balance reduces hinge moments
68
what are balance tabs?
small movable surface fitted to trailing edge of main control surface to help move control surface
69
what is trim?
balancing an aircraft in flight around its CG
70
what are trim tabs?
help stabilise the aircraft and free pilot from constantly adjusting the controls
71
control problems at low speed
reduced aerodynamic forces near stall angle may be immersed in separated flow
72
what may happen if controls are deployed at low speeds?
induce stall leading to control reversal
73
what may be used to improve low speed control
slots used with down going aileron to delay stalling; spoilers used with up going aileron to reduce lift
74
dangers of executing a bank turn at low speed
control reversal may happen
75
how can spoilers be used at high AOA to help with lateral stability?
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
difference between balance and trim?
trim - balancing of aircraft in flight around its CG balance - reduction of hinge moments
77
why is root stall more desirable than wing tip stall?
aircraft less likely to roll, due to difference in moments generated by the lift ailerons at the wing tips are still effective
78
how does Dutch roll occur?
when roll correction is more than yaw correction

xxxFOF (6 decks)

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