EASA PART-66 MODULE 8.4 : FLIGHT STABILITY AND DYNAMICS

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

AXES OF AN AIRCRAFT

AXES OF AN AIRCRAFT

Longitudinal Axis

o Lengthwise from nose to tail through center of gravity

o Rotation about this axis is called roll

o Rolling is produced by movement of ailerons

AXES OF AN AIRCRAFT

Lateral Axis

o Spanwise from wingtip to wingtip through center of gravity

o Rotation about this axis is called pitch (nose up or nose down)

o Pitching is produced by movement of the elevators

AXES OF AN AIRCRAFT

Normal or Vertical Axis

o Passes from top to bottom of the aircraft through center of gravity

o Right angle to longitudinal and lateral axis

o Rotation about this axis is called yaw

o Yawing is produced by movement of the rudder

AXES OF AN AIRCRAFT

STABILITY

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 gusts or air density changes that cause deflections from the intended flight path

o Aircraft ability to return to original position after being disturbed from its flight path

o Changes are corrected automatically relieving the pilot from the task of correcting these deviations

Longitudinal Stability Stability about lateral axis

motion in pitch Longitudinally stable aircraft

does not tend to put its nose down and dive or lift its nose and stall

The aircraft has a tendency to keep a constant angle of attack

Longitudinal Stability maintained by the horizontal stabilizer

By correcting nose up or down moment will return the aircraft to level flight.

STABILITY

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

Lateral Stability

STABILITY

Dihedral Sweepback

Directional Stability

Stability about the vertical axis Directionally stable aircraft tends to remain on

its course in straight and level flight Directional stability is maintained by keel

surface of the vertical stabilizer Sweptback wings also aid in directional

stability (frontal area)

STABILITY

Directional Stability

STABILITY

Types of stability and motion

Stability Axes Motion about the Axis Longitudinal Lateral Pitch Lateral Longitudinal Roll Directional Normal Yaw

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 aircraft’s surface disturbed the balanced forces

Aircraft controls are designed to be instinctive

CONTROL IN FLIGHT

Control surfaces movement

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 stick or joystick

CONTROL IN FLIGHT

Lateral Control Sideways movement of the pilot’s 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

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

Aileron Drag/Adverse Yaw

CONTROL IN FLIGHT

Frise aileronDifferential ailerons travel

Longitudinal ControlControlling the aircraft about the lateral axis (pitching

motion)Provided by elevatorsElevators are hinged to the trailing edge of the horizontal

stabilizerPitching motion

– Forward control column elevators moves down giving the tailplane a positive camber thereby increasing its lift on the tail nose pitch down (dive)

– Backward control column elevators moves up giving the tailplane a reverse camber, producing negative lift on the tail nose pitch up (climb)

CONTROL IN FLIGHT

Longitudinal Control

CONTROL IN FLIGHT

Directional Control

Involves rotation about the normal axis (yawing motion)Controlled by rudder which is hinged to the trailing edge

of the vertical stabilizer (Fin)Movement of rudder is by a pair of rudder pedals located

in the cockpitYawing motion

– Yaw to the left move the left pedal forward, rudder is moved to the left and the nose will turn to the left about normal axis.

– The opposite effect is obtained from the forward movement of the pilot’s right foot.

CONTROL IN FLIGHT

Directional Control

FLIGHT CONTROL SURFACES

Movable airfoils designed to change the attitude of the aircraft about its three axes during flight

Divided into three groups:- i. primary groupii. secondary groupiii. auxiliary group


Primary Group

i. Ailerons hinged horizontally at the outboard trailing edge of each wing

ii. Elevators hinged horizontally at the rear of each horizontal stabiliser

iii. Rudder hinged vertically at the rear of the vertical stabiliser

The ailerons and elevators are operated from the cockpit by a control stick or by a control wheel or by a joy stick.

The rudder is operated by foot pedals.


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 reasonsi. Trim tabii. Balance tabiii. Servo tabiv. Spring tab


Trim Tabs System

To trim out any unbalanced condition exist during flight, without applying any pressure on the primary controls

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 before

flightii. Controllable trim tabs – Can be controlled in flight by

pilots (control by mechanical linkage or electric motor)


Trim Tabs System

Fixed trim tab Controllable trim tab


Trim Control Trim tab(aileron, rudder, elevator) can be controlled

manually or electrically Manual – control knob or wheel located on the

centre console Electrical – by a thumb switch located on the control

column for aileron and elevator rudder trim switch located on the centre console adjacent to the rudder trim wheel

During operation, the tabs will always moved in the opposite direction from the primary control surfaces



Elevator Trim

Rudder Trim

Aileron Trim

Manual Control• To lower the right wing of the airplane and raise the left, the

aileron 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.


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 move

the elevator tab and moving the switch to the left or

right will move the aileron tab..


Aileron Trim Controls


Elevator Trim Controls


Rudder Trim Controls


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

pilot’s 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


Servo Tabs System• Help in moving large

primary control surfaces (similar to balance tab but differs in operation)

• Pilot input from the cockpit moves the tab, and the tab in turn develops forces which move the primary control surface

• A movement of the tab down will cause the control surface to move up and vice versa


Spring Tabs System At high speed , the control

surfaces become increasingly difficult to move due to aerodynamic loads

The spring tab helps to overcome this problem

At low speed the spring tab remains in a neutral position, inline with the control surface.

Only at high speed, where the aerodynamic load is great, the tab functions as an aid in moving the primary control surface.


Auxiliary Group This group of flight control surfaces include:-

i. wing flapsii. spoilersiii. speed brakesiv. leading edge flapsv. slots and slats

May be divided into two sub-groups; Those whose primary purpose is lift augmenting e.g. flaps, slots

and slats those whose primary purpose is lift decreasing e.g. speed

brakes and spoilers FLIGHT CONTROL SURFACES

Flaps

High lift device hinged on the inboard trailing edge of the wing

Controlled from the cockpit, and when not in use fits smoothly into the lower surface of each wing

Flaps increases the camber of a wing and therefore the lift of the wing, making it possible for the speed of the aircraft to be decreased without stalling

Flaps are primarily used during take-off and landing


Flaps


Plain flaps

• Retracted to form a complete section of the wing trailing edge

• When in use it is hinged downwards

FLIGHT CONTROL SURFACES [Auxiliary Group]

Split flap

• This flap is hinged at the lower part of the wing trailing edge.

• When lowered, the wing top surface is unchanged, thus eliminating the airflow break-away like what occurring over the top of the plain flap when lowering


Zap Flap

• Similar to the split flap except that the flap hinge travels rearward when lowered

• Increases wing effective area as well as its camber without changing the shape of the top surface

• Like the split flap there is little risk of flow separation on top of the wing


Fowler Flap

• The fowler is similar to the split flap but, when in use, it is moved rearwards and downwards on tracks.

• This action will increase the wing camber and also the wing area to give additional lift.


Slotted Flap• A gap or slot formed between

the flap and the wing structure• Air will flow from the wing lower

surface, through the gap and over the top of the flap

• This airflow will maintain lift by speeding up as it passes through the slot and remaining in contact with the flat top surface, even at large flap angles

• Without the slot the upper surface airflow would break away


Slotted Flap


Slotted Fowler Flap

• A Fowler flap with slot• Multi-slotted on improved

design• Increase camber and area• The breakaway of the

airflow from the flap upper surface can be delayed until even greater angles of flap depression by providing two or more slots


Slotted Fowler Flap


Leading edge flap

• Referred as Krueger’s Flap• To increase lift at low speed• Increase camber increase

lift• Leading and trailing edge

flaps are normally coupled to operate together

• May be lowered automatically when the aircraft’s speed falls to near the stalling speed


Slats• For low speed operation

other than take-off or landing• A small, highly-cambered

airfoils fitted to the wing leading edges

• May be fixed open, or controlled to operate alone or jointly with the flaps

• Some aircraft have slats which open automatically when the wing angle of attack exceeds a predetermined value


Slats


Slot

• Is a series of suitably shaped apertures built into the wing structure near the wing tips

• It increase the stalling angle by guiding and accelerating air from below the wing and discharging it over the upper surface in the normal way


Airbrakes/Speed brakes• Movable panels forming part of

the contour of the wings or fuselage

• Deflected into the airflow by hydraulic actuators to give a rapid reduction in speed when is required.

• Used to control speed during descent and landing approach

• Installed on the strongest airframe structure able to accept the braking loads and also where the braking drag does not effect the aircraft stability


Spoilers

• Are plates fitted to the upper surface of the wing and usually deflected upward by hydraulic actuators

• The purpose is to disturb the smooth airflow across the top of the wing, thereby increasing drag and decreased lift on that aircraft


DUAL PURPOSE CONTROLS

The design of some aircraft makes it impossible to mount the conventional aileron, elevator and rudder control surfaces in their normal positions.

An example of this is a delta wing type aircraft. This has no separate tailplane, and the elevators have

to be mounted on the wing trailing edges. This presents a space problem because the wings

already house the ailerons and flaps. The solution in this case is to use one set of control

surfaces to perform the function of both.




Elevonso Use to perform the

function of both

elevators and ailerons

o The surfaces are moved

in the same direction to

serve as elevators and

in opposite directions to

act as aileronsDUAL PURPOSE CONTROLS

Ruddervators o Prevent hot exhaust

gases from the turbo-jet engine 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 stability and control both longitudinally and directionally


Taileronso On some high speed

aircraft it is often necessary to have flaps which occupy the entire trailing edges of the wings, leaving no space for the ailerons.

o Controllable tailplane move separately.

o Pitch angling both sides either up, or down, together

o Roll angling one side up and, simultaneously, the other side down

o Roll regulate lift on taiplane


EASA PART-66 MODULE 8.4 : FLIGHT STABILITY AND DYNAMICS

Education

longitudinal control

lateral stability stability

directional control

climb control

control wheelor control

joystick control

instinctive control

aroll control