Chap E: Stability and Control Flashcards
definition of stability
inherent ability of an object to develop a system that will tend to return to its original or undisturbed state after a disturbance
what is a stable aircraft?
tends to return to its original attitude and speed with no pilot effort on being disturbed
aircraft stability types
1) static stability
2) dynamic stability
definition of static stability
initial tendency of a/c to return to its original position when disturbed, motion may or may not return to its original position
does static stability guarantee an original position over time? why or why not?
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)
define positive static stability
initial tendency of a system to return to the original state of equilibrium after being disturbed
define neutral static stability
initial tendency of a system to remain in the new state after its equilibrium has been disturbed
define negative static stability
initial tendency of a system to go further away from the original state of equilibrium after being disturbed
what is dynamic stability?
aircraft response over time when disturbed
what does a dynamically stable aircraft mean?
a/c will return to its original state after a period of time
aircraft with positive dynamic stability means?
oscillations damp out over time and a/c returns to the original flight path
negative dynamic stability meaning
each oscillation causes a/c to stray further away from the original state
deadbeat stability meaning
when there is no tendency to oscillate after initial disturbance
when is it more desirable to have neutral or negative stability?
in fighter jets, as they require high manoeuvrability and having high stability will make it more difficult to control
definition of stick-free
control stick is not held fixed such that control surfaces can rotate freely during response motion
definition of stick-fixed
control stick held fixed in response motion (control surfaces not being able to deflect/rotate freely)
power-on and power-off meaning
whether the engine is developing thrust
definition of longitudinal stability
stability of an aircraft in pitching
or
stability about the lateral axis
names of the axis
longitudinal axis: x
lateral axis: y
vertical axis: z
definition of lateral stability
aircraft’s resistance to roll
or stability about longitudinal axis
definition of directional stability
aircraft’s resistance to yaw
or stability about vertical axis
factors affecting longitudinal stability
- position of CG
- location of tail plane (area, AR, riggers angle)
- tail/nose heavy (tail is more unstable)
- pitching moment
- thrust line
tail plane action from change in AOA, effect on speed
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)
what is longitudinal dihedral?
difference in incidence angle between main and tail planes
why is swept back wings more stable than rectangle wingss?
Cp is further aft, means longer moment arm, leading increase nose down pitching moment
where does the thrust line need to be relative to CG to make aircraft stable?
thurst line above CG
what is a phugoid?
long period of pitching oscillation of aircraft, change in speed and height
what is short period mode?
motion is damped out within a short time
difference between phugoid and short period oscillation
phugoid: long oscillations (> 30secs)
short period: short oscillations (< 6secs)
parameters for lateral stability
1) dihedral angle
2) high wing (thus low CG)
3) sweepback
4) fin area
what is dihedral angle?
angle between each wing plane and the horizontal
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
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
what capability does the high wing configuration have?
self roll correcting (dihedral effect)
how to counter excessive lateral stability?
implementing anhedral (negative dihedral)
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
what does anhedral do to stability?
reduces lateral stability, increases control
what does sweepback do?
improve directional stability, delay shock wave formation at transonic speed
it is beneficial to all stabilities
how does sweepback help with lateral stabiltiy?
down going wing: higher AR, more lift
up going wing: lesser AR, less lift
side surface area relative to lateral stability
high side surface area above CG tends to create restoring rolling moments
parameters for directional stability
- size & location of vertical stabiliser (side surface area)
- sweepback
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
how does ventral fin help with directional stability?
reduces effect of dutch roll
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
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
what is sideslip
when a faster moving outer wing produces more lfit than the inner wing causing a roll in the direction of yaw
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
what happens when yaw correction is more than roll correction
heading change
flat spin - a/c rotate horizontally while dropping
spiral dive
how does spiral instability occur?
when aircraft have excess directional stability than lateral stability
what is spin mode?
aggravated stall, high AOA and rate of descent, low airspeed
what causes spin mode?
rotation of aircraft about its CG where the aircraft follows a downward screw path
definition of controllability
ability of aircraft to respond to control surface displacement and achieve desired condition of flight
list of primary controls
- longitudinal (pitch) by elevators
- lateral (roll) by ailerons
- directional (yaw) by rudder
how does elevator control pitch?
up elevator increases downward lift on tail, moving it down –> nose pitch up increases AOA –> more lift = climb
how does aileron control roll?
more lift on left wing and less lift on right wing causes aircraft to roll right
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
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
dual flight action controls
elevon - elevator + aileron
flaperon - flap + aileron
ruddervator - rudder + elevator
what does elevon control?
pitch and roll control
what does flaperon control?
CL and roll control
what does ruddervator control?
yaw and pitch control
hinge moment definition
forces generated by the control surfaces
why is balanced control needed?
hinge moments is noticeable at high speed due to high dynamic pressure
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)
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
how to attain aerodynamic balance?
inset hinge
horn balance
internal balance
tabs
how to attain aerodynamic balance using horn balance?
horn balance reduces hinge moments
what are balance tabs?
small movable surface fitted to trailing edge of main control surface to help move control surface
what is trim?
balancing an aircraft in flight around its CG
what are trim tabs?
help stabilise the aircraft and free pilot from constantly adjusting the controls
control problems at low speed
reduced aerodynamic forces
near stall angle
may be immersed in separated flow
what may happen if controls are deployed at low speeds?
induce stall leading to control reversal
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
dangers of executing a bank turn at low speed
control reversal may happen
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
difference between balance and trim?
trim - balancing of aircraft in flight around its CG
balance - reduction of hinge moments
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
how does Dutch roll occur?
when roll correction is more than yaw correction
