Dynamics and Flight Stability

7/30/2019 Dynamics and Flight Stability

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8.4 FLIGHT STABILITY AND

DYNAMICS

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AXES OF AN AIRCRAFT

Aircraft is completely free to move in any direction

Manoeuvre dive, climb, turn and roll, or perform

combinations of these.

Whenever an aircraft changes its attitude in flight, it must turn

about one or all of these axes.

Axes imaginary lines passing through the centre of the

aircraft.

AXES ON AIRCRAFT


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AXES OF AN AIRCRAFT

AXES OF AN AIRCRAFT


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Longitudinal Axis

o Lengthwise from nose

to tail through center of

gravity

o Rotation about this axisis called roll

o Rolling is produced by

movement ofailerons

AXES OF AN AIRCRAFT


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Lateral Axis

o Spanwise from wingtip

to wingtip through

center of gravity

o Rotation about this axisis calledpitch (nose up

or nose down)

o Pitching is produced by

movement of the

elevators

AXES OF AN AIRCRAFT


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Normal or Vertical Axis

o Passes from top to

bottom of the aircraft

through center of gravity

o Right angle tolongitudinal and lateral

axis

o Rotation about this axis is

called yawo Yawing is produced by

movement of the rudder

AXES OF AN AIRCRAFT


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STABILITY

o Aircraft characteristic to fly (hands off) in a straight

and level flight path

o To maintain a uniform flight path and recover from

the various upsetting forces, such as, local air gustsor air density changes that cause deflections from

the intended flight path

o

Aircraft ability to return to original position afterbeing disturbed from its flight path

o Changes are corrected automatically relieving the

pilot from the task of correcting these deviations

STABILITY


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Longitudinal Stability

Stability about lateral axismotion in pitch

Longitudinally stable aircraftdoes not tend to put its nosedown and dive or lift its nose

and stall The aircraft has a tendency to

keep a constant angle of attack

Longitudinal Stabilitymaintained by the horizontalstabilizer

By correcting nose up or downmoment will return theaircraft to level flight.

STABILITY


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Lateral Stability

Stability about longitudinal axis rolling motion

Laterally stable aircraft tend to return to the

original attitude from rolling motion

Lateral stability is maintained by the wing

(design)

a. Dihedral the upward inclination of the wings

from their point of attachmentb. Sweepback wing leading edges are inclined

backwards from their points of attachment

STABILITY


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Lateral Stability

STABILITY

Dihedral Sweepback


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Directional Stability

Stability about the vertical axis

Directionally stable aircraft tends to remain onits course in straight and level flight

Directional stability is maintained by keel

surface of the vertical stabilizer

Sweptbackwings also aid in directional

stability (frontal area)

STABILITY


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Directional Stability

STABILITY


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Types of stability and motion

Stability Axes Motion about the Axis

Longitudinal Lateral Pitch

Lateral Longitudinal Roll

Directional Normal Yaw


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CONTROL IN FLIGHT

Different control surfaces used to provide

aircraft control about each of the three axes

Movement of the control surface will change

the airflow over the aircrafts surface

disturbed the balanced forces

Aircraft controls are designed to be instinctive

CONTROL IN FLIGHT


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Control surfaces movement


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Lateral Control

Controlling the aircraft about its longitudinal axis (rolling

motion)

Provided by the ailerons

Rolling motion produce by increasing lift on one wing and

reduce lift on the opposite wing

Ailerons

Hinged to the trailing edge towards the wingtips and

form part of a wing

Operated from the cockpit by mean of a control wheel

or control stickorjoystick

CONTROL IN FLIGHT


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Lateral Control

Sideways movement of the pilots control stick will cause the

aileron on one wing to move upwards and, simultaneously,

the aileron on the other wing to move downwards

The unequal wing lift on each side of the aircraft produces a

roll

CONTROL IN FLIGHT


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Lateral Control

For aircraft to roll one aileron deflected upward and one

downward

Lowered aileron lift increase + drag also increase (aileron

drag or adverse yaw)

The increased drag tries to turn the aircraft in the direction

opposite to that desired

Frise aileron or differential ailerons travel system used to

overcome the problem of aileron drag

CONTROL IN FLIGHT


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Aileron Drag/Adverse Yaw

CONTROL IN FLIGHT

Frise aileronDifferential ailerons travel


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Longitudinal Control

Controlling the aircraft about the lateral axis (pitchingmotion)

Provided by elevators

Elevators are hinged to the trailing edge of the

horizontal stabilizerPitching motion

Forward control column elevators moves down givingthe tailplane a positive camber thereby increasing its lifton the tail nose pitch down (dive)

Backward control column elevators moves up givingthe tailplane a reverse camber, producing negative lifton the tail nose pitch up (climb)

CONTROL IN FLIGHT


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Longitudinal Control

CONTROL IN FLIGHT


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Directional Control

Involves rotation about the normal axis (yawingmotion)

Controlled by rudder which is hinged to the trailingedge of the vertical stabilizer (Fin)

Movement of rudder is by a pair of rudder pedalslocated in the cockpit

Yawing motion Yaw to the left move the left pedal forward, rudder is

moved to the left and the nose will turn to the leftabout normal axis.

The opposite effect is obtained from the forwardmovement of the pilots right foot.

CONTROL IN FLIGHT


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Directional Control


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Primary Group

i. Ailerons hinged horizontally at the outboard trailingedge of each wing

ii. Elevators hinged horizontally at the rear of eachhorizontal stabiliser

iii. Rudderhinged vertically at the rear of the verticalstabiliser

The ailerons and elevators are operated from the

cockpit by a control stick or by a control wheel or by ajoy stick.

The rudder is operated by foot pedals.

FLIGHT CONTROL SURFACES


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Secondary Group

Tabs small auxiliary control surfaces hinged at

the trailing edge of a main flying control surfaces

Various types of tab and fitted for various reasons

i. Trim tab

ii. Balance tab

iii. Servo tabiv. Spring tab



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Trim Tabs System

To trim out any unbalanced condition exist duringflight, without applying any pressure on the primarycontrols

Each trim tab is hinged to its parent primary control

surface, but is operated by an independent control

Trim Tab can be sub divided into two types:

i. Fixed trim tabs Only adjustable on ground beforeflight

ii. Controllable trim tabs Can be controlled in flight bypilots (control by mechanical linkage or electric motor)



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Trim Tabs System

Fixed trim tab Controllable trim tab



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Elevator Trim

Rudder Trim

Aileron Trim


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Manual Control

To lower the right wing of the airplane and raise the left, theaileron tab control wheel is moved to the right and the

reverse direction is used to lower the left wing.

To trim the nose up, the elevator tab control wheel is moved

rearward, and to lower the nose, the wheel is moved forward.

To yaw to the left, the rudder tab control wheel is moved to

the left and to yaw to the right, the control wheel is moved to

the right.



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Electrical Trim Controls

Electrically operated systems are controlled by

switches located at the top of the control column.

These switches are moved forward or aft, to movethe elevator tab and moving the switch to the left or

right will move the aileron tab..



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Aileron Trim Controls



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Elevator Trim Controls



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Rudder Trim Controls



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Balance Tabs System Assist pilot in moving the control surface (reduce

pilots effort large control surface)

Control rod cause the tab to move in the opposite

direction to the movement of the primary control

surface aerodynamic forces acting on the tab,assist in moving the main control surface



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Spring Tabs System

At high speed , the controlsurfaces become increasinglydifficult to move due toaerodynamic loads

The spring tab helps to

overcome this problem At low speed the spring tabremains in a neutral position,inline with the control surface.

Only at high speed, where theaerodynamic load is great, the

tab functions as an aid inmoving the primary controlsurface.



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Flaps

High lift device hinged on the inboard trailingedge of the wing

Controlled from the cockpit, and when not in use

fits smoothly into the lower surface of each wingFlaps increases the camberof a wing and

therefore the liftof the wing, making it possiblefor the speed of the aircraft to be decreased

without stallingFlaps are primarily used during take-off and

landing



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Flaps



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Split flap

This flap is hinged at thelower part of the wingtrailing edge.

When lowered, the wingtop surface is unchanged,thus eliminating theairflow break-away like

what occurring over thetop of the plain flap whenlowering

FLIGHT CONTROL SURFACES [Auxiliary

Group]


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Zap Flap

Similar to the split flapexcept that theflap hingetravels rearward whenlowered

Increases wing effectivearea as well as its camberwithout changing theshape of the top surface

Like the split flap there is

little risk of flowseparation on top of thewing


Group]


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Fowler Flap

The fowler is similar tothe split flap but, whenin use, it is movedrearwards and

downwards on tracks.

This action will increasethe wing camber and

also the wing area togive additional lift.


Group]


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Slotted Flap

A gap or slot formed betweenthe flap and the wing structure

Air will flow from the winglower surface, through the gapand over the top of the flap

This airflow will maintain liftby speeding up as it passesthrough the slot andremaining in contact with theflat top surface, even at largeflap angles

Without the slot the uppersurface airflow would breakaway


Group]


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Slotted Flap


Group]


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Slats

For low speed operationother than take-off orlanding

A small, highly-camberedairfoils fitted to the wing

leading edges May be fixed open, or

controlled to operate aloneor jointly with the flaps

Some aircraft have slats

which open automaticallywhen the wing angle ofattack exceeds apredetermined value


Group]


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Slats


Group]


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Slot

Is a series of suitablyshaped apertures builtinto the wing structurenear the wing tips

It increase the stallingangle by guiding andaccelerating air frombelow the wing and

discharging it over theupper surface in thenormal way


Group]


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Airbrakes/Speed brakes

Movable panels forming partof the contour of the wings orfuselage

Deflected into the airflow byhydraulic actuators to give arapid reduction in speedwhenis required.

Used to control speed duringdescent and landing approach

Installed on the strongestairframe structure able to

accept the braking loads andalso where the braking dragdoes not effect the aircraftstability


Group]


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DUAL PURPOSE CONTROLS

The design of some aircraft makes it impossibleto mount the conventional aileron, elevator andrudder control surfaces in their normal positions.

An example of this is a delta wing type aircraft.

This has no separate tailplane, and the elevatorshave to be mounted on the wing trailing edges.

This presents a space problem because the wingsalready house the ailerons and flaps.

The solution in this case is to use one set ofcontrol surfaces to perform the function of both.

DUAL PURPOSE CONTROLS


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Ruddervatorso Prevent hot exhaust

gases from the turbo-jetengine playing on the

tail unit surfaces, and

for other design

considerations, some

light aircraft have

tailplanes with very

pronounced dihedral

angles

o V tailplane with its

hinged aft control

surfaces provides DUAL PURPOSE CONTROLS


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Dynamics and Flight Stability