1.
8.4 FLIGHT STABILITY AND
DYNAMICS
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2.
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

3.
AXES OF AN AIRCRAFT
AXES OF AN AIRCRAFT

4.
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

5.
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

6.
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

7.
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
STABILITY

8.
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

9.
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 attachment
b. Sweepback – wing leading edges are inclined
backwards from their points of attachment
STABILITY

10.
Lateral Stability
Dihedral Sweepback
STABILITY

11.
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

12.
Directional Stability
STABILITY

13.
Types of stability and motion
Stability Axes Motion about the Axis
Longitudinal Lateral Pitch
Lateral Longitudinal Roll
Directional Normal Yaw

14.
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

15.
Control surfaces movement

16.
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

17.
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

18.
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

19.
Aileron Drag/Adverse Yaw
Differential ailerons travel Frise aileron
CONTROL IN FLIGHT

20.
Longitudinal Control
 Controlling the aircraft about the lateral axis (pitching
motion)
 Provided by elevators
 Elevators are hinged to the trailing edge of the
horizontal stabilizer
 Pitching 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

21.
Longitudinal Control
CONTROL IN FLIGHT

22.
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 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 left
about normal axis.
– The opposite effect is obtained from the forward
movement of the pilot’s right foot.
CONTROL IN FLIGHT

23.
Directional Control

24.
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 group
ii. secondary group
iii. auxiliary group
FLIGHT CONTROL SURFACES

25.
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.
FLIGHT CONTROL SURFACES

26.
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 tab
iv. Spring tab
FLIGHT CONTROL SURFACES

27.
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
flight
ii. Controllable trim tabs – Can be controlled in flight by
pilots (control by mechanical linkage or electric motor)
FLIGHT CONTROL SURFACES

28.
Trim Tabs System
Fixed trim tab Controllable trim tab
FLIGHT CONTROL SURFACES

29.
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
FLIGHT CONTROL SURFACES

30.
Elevator Trim
Rudder Trim
Aileron Trim
FLIGHT CONTROL SURFACES

31.
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.
FLIGHT CONTROL SURFACES

32.
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..
FLIGHT CONTROL SURFACES

33.
Aileron Trim Controls
FLIGHT CONTROL SURFACES

34.
Elevator Trim Controls
FLIGHT CONTROL SURFACES

35.
Rudder Trim Controls
FLIGHT CONTROL SURFACES

36.
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
FLIGHT CONTROL SURFACES

37.
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 theFLIGHT CONTROL SURFACES
control surface to move

38.
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.
FLIGHT CONTROL SURFACES

39.
Auxiliary Group
 This group of flight control surfaces include:-
i. wing flaps
ii. spoilers
iii. speed brakes
iv. leading edge flaps
v. 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

40.
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
FLIGHT CONTROL SURFACES

41.
Flaps
FLIGHT CONTROL SURFACES

42.
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]

43.
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
FLIGHT CONTROL SURFACES [Auxiliary
Group]

44.
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
FLIGHT CONTROL SURFACES [Auxiliary
Group]

45.
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.
FLIGHT CONTROL SURFACES [Auxiliary
Group]

46.
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
FLIGHT CONTROL SURFACES [Auxiliary
Group]

47.
Slotted Flap
FLIGHT CONTROL SURFACES [Auxiliary
Group]

48.
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
FLIGHT CONTROL SURFACES [Auxiliary
Group]

49.
Slotted Fowler Flap
FLIGHT CONTROL SURFACES [Auxiliary
Group]

50.
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
FLIGHT CONTROL SURFACES [Auxiliary
Group]

51.
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
FLIGHT CONTROL SURFACES [Auxiliary
Group]

52.
Slats
FLIGHT CONTROL SURFACES [Auxiliary
Group]

53.
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
FLIGHT CONTROL SURFACES [Auxiliary
Group]

54.
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
FLIGHT CONTROL SURFACES [Auxiliary
Group]

55.
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
FLIGHT CONTROL SURFACES [Auxiliary
Group]

56.
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.
DUAL PURPOSE CONTROLS

57.
DUAL PURPOSE CONTROLS
DUAL PURPOSE CONTROLS

58.
Elevons
o 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
DUAL PURPOSE CONTROLS

59.
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 DUAL PURPOSE CONTROLS

60.
Tailerons
o 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, th
e other side down DUAL PURPOSE CONTROLS