Stability and Control

1
Stability and Control
2
Stability

• Planes and Axes
• There are terms to describe the movement of an
  aircraft in 3 dimensions.
• This is movement of the aircraft relative to
  itself- changes of it attitude- rather than it
  travelling through the air.

3
3 Planes of Movement

• There are three planes in which an aircraft
  moves.
• Moves in the pitching plane (i.e. pitches) about
  its lateral axis
• Moves in the rolling plane (i.e. rolls) about its
  longitudinal axis
• Moves in the yawing plane (i.e. yaws ) about its
  normal axis

4
Why Stability?

• If an aircraft experiences turbulence, a
  well-designed aircraft will tend to go back to
  level flight of its own accord, without a pilot
  having to make continual adjustments.
• This property is called stability and to
  understand it we need to consider stability in
  each of the 3 planes.

5
Stability in the Pitching Plane (Longitudinal
Stability)
If some disturbance jolts the aircraft into a
tail-down attitude, the tail plane momentarily
has an angle of attack to the oncoming air
consequently it produces lift which levers the
aircraft back to level position
6
Stability in the Pitching Plane (Longitudinal
Stability)
If some disturbance jolts the aircraft into a
nose-down attitude, the tail plane produces a
downward force which levers the aircraft back
into a level position. In this position the tail
plane has no upward and downward force upon it
7
Stability in the Rolling Plane (Lateral Stability)

• Most aircraft have wings which are set into the
  fuselage at a slight upward angle to the
  horizontal called the dihedral angle
• This makes the aircraft laterally stable

8

• If the wing drops in turbulent air, the lift
  force is no longer vertical and it no longer
  opposes the weight fully.
• Consequently, the aircraft begins to slip
  sideways, down towards the lower wing, and a side
  wind strikes it.

Side wind
9

• The lower wing, because of its dihedral angle,
  meets the side-wind at an angle- an angle of
  attack- which is greater than that of the upper
  wing.
• The lower wing therefore produces more lift than
  the upper wing and rolls the aircraft back until
  the wings are level
• Another reason is that the upper wing is shielded
  by the fuselage from some of the side-wind
  (airflow is slower)

Side-wind
10
High wing aircraft

• Another stable design is the high wing aircraft,
  where the centre of gravity is well below the
  wing here, the pendulum effect of the weight of
  the aircraft gives lateral stability

11
Anhedral angle

• Anhedral wings work on the same principle as
  dihedral, but with the opposite effect, creating
  lateral instability. This is the designers way
  of reducing the excessive lateral stability which
  can be encountered with swept-back wings.

12
Stability in the Yawing Plane (Directional
Stability)

• If an aircraft is made to yaw to one side by an
  air disturbance, the side-wind blowing on its
  fuselage and fin surfaces creates a sideways
  force which, on areas to the rear of the centre
  of gravity, will tend to yaw the aircraft back to
  its original heading, just like a weather cock.

13

• Of course, the sideways force on areas ahead of
  the centre of gravity will have the opposite (an
  unwanted) effect which is why most aircraft have
  a fin, placed as far back as possible, to
  increase the weathercock effect and ensure
  directional stability.

14
How much Stability?

• High stability can be extremely tiring as the
  pilot constantly has to overcome the stabilising
  forces.
• Conversely, too little stability will mean that
  the pilot has to make continual corrections to
  keep the aircraft on the chosen flight path.
• The designer must strike a balance
• The amount of stability built into an aircraft
  depends on its role.
• Higher for large aircraft, and least for fighters
  where good manoeuvrability is required

15
The Pilots Controls

• The three main controls are elevators, ailerons
  and rudder
• Using these controls the pilot can make the
  aircraft
• A) Pitch Where the nose of the aircraft rises
  (pitching up) or falls (pitching down)
• B) Roll When one wing rises while the other
  falls. Right wing down is rolling right.
• C) Yaw- When the nose of the aircraft moves left
  (left yaw) or right (right yaw)
• These are all relative to the pilot and not the
  horizon!!

16
Which way is up?

• If the nose pitches towards the pilots head it
  is pitching up, and if it pitched towards the
  pilots feet it is pitching down. Similarly,
  when we say left yaw or right yaw, left
  roll or right roll, we are always referring to
  the pilots left and the pilots right,
  regardless of the attitude of the aircraft.

17
How The Pilot Uses The ControlsUse of Elevators

• Elevators make the aircrafts nose pitch
  nose-down or nose-up.
• The located on the tailplane and are hinged to
  the trailing edge where they have the most
  leverage.

18
How The Pilot Uses The ControlsUse of Elevators

• It is linked to the pilots control column
  (usually called the stick)
• Moving it forwards lowers the elevators and this
  levers the tail up/nose down about its lateral
  axis.

Elevators move up and down
19
Effect of Elevators

• In straight and level flight, the aircraft would
  dive

20
Effect of Elevators

• In a vertical climb, the aircraft would turn
  towards the level position.

21
How The Pilot Uses The ControlsUse of Elevators

• Moving it backwards of course, has the opposite
  effect.
• Stick Back, nose up
• But always remember that up and down are
  measured solely in relation to the pilot, and not
  to the world outside the aircraft.

Elevators move up and down
22
The Rolling Plane Use of Ailerons

• To roll the aircraft, the pilot uses the two
  moveable parts of the wings called the ailerons.
• They are linked to the stick and are located near
  the wing tips, where they have the most leverage
  about the centre of gravity

23

• By moving the stick left, the pilot raises the
  left aileron and depressed the right.
• The left aileron thus has a reduced angle of
  attack and less lift.
• This rolls the aircraft to the left about its
  longitudinal axis

Remember Stick to the right, roll to the
right Stick to the left, roll to the left
24
The Yawing PlaneUse of Rudder

• To move the aircraft in the yawing plane, the
  pilot uses the rudder, which is linked to the
  rudder pedals in the cockpit.
• The rudder is a single control surface which is
  hinged to the trailing edge of the fin.

25
The Yawing PlaneUse of Rudder

• The pilots feet rest on the rudder pedals during
  normal flight.
• To yaw to the left, the pilot pushes the left
  pedal forward, which makes the rudder move out to
  the left.

26

• This produces a sideways force to the right.
• The tail is pushed sideways to the right, and the
  nose, of course to the left the aircraft is now
  yawing to the left about its normal axis

Left rudder, yaw to the left Right rudder, yaw to
the right
27
What is Trimming?

• Trimming tabs are used to trim out (or cancel
  out) the forces on the stick or rudder which are
  created by alteration in power, speed or attitude
  or changes to the weight of the aircraft when
  fuel is used up, bombs dropped or ammunition
  fired.
• No pilot could fly accurately or safely for long
  without some help from Trimming Tabs.

28
Where are Trimming Tabs?

• They are hinged in the trailing edges of the
  elevators, ailerons and rudder
• They can be moved at an angle to those surfaces
  by separate controls in the cockpit

29
How do Trimming Tabs Work?

• If an aircraft has become nose heavy, a pilot
  would have to maintain steady backwards pressure
  on the stick.
• To trim out the constant backward force on the
  stick, the pilot operates the elevator trim
  control so as to depress the elevator trimming
  tabs downward into the air flow
• This produces lift which helps to hold the
  elevator up at the required angle
• This removes the need for a continuous force on
  the control column.

Holds Elevator up
Trim tab down
Aileron and rudder trimming tabs Operate on the
same principle
The aircraft is said to be trimmed
longitudinally and will fly hands off
30
Location of flaps

• For the sake of safety, an aircrafts wing should
  be designed so that the aircraft can make its
  approach to land at a controlled slow speed, and
  along a moderately steep approach path (so that
  they can see over the nose)
• A wing is designed largely for its main task, and
  flaps are used on the approach and landing.

31

• Flaps are hinged surfaces along the trailing
  edges of the wings, in board of the ailerons.
• The flaps are operated in stages by the pilot.
• In the unused or up position they lie flush
  with the wing surface and form part of the wing
• In the fully down position they are at a angle of
  90 or so to the wings surfaces

32
Types of flaps

• The many types of flaps all increase the
  effective camber of the wing and hence its lift
• In addition they will also increase drag

33

• For most flaps there is a huge gain in lift when
  they are lowered to angles of 30 to 60. For
  these angles drag increases only moderately.
• It is now possible to use slower, safer approach
  and landing speeds. However, because of very
  little drag being produced, the approach angle is
  scarcely affected

34

• If 90 of flap are selected this gives a
  tremendous increase in drag which in turn means
  that the pilot must lower the nose considerably
  to maintain the approach speed- in other words-
  the pilot has a much steeper approach angle and a
  better forward view.
• 15 of flap will also improve the lift at
  take-off speeds resulting in shortened take-off
  runs

35
Effect of slats on airflow

• Slat improve handling at low speeds.
• They are small aerofoils (shaped so as to develop
  lift), positioned along the leading edge of each
  wing

36
Effect of slats on airflow

• When slats are not needed they are held in the
  closed position by springs.
• At high angles of attack the slats open
  automatically and improve the airflow over the
  wing, with some beneficial effects.
• Instead of stalling at 15 or so, the wing can
  reach an angle of as much as 25 before stalling,
  and the stalling speed is reduced.
• They do, however, cause extra drag which is
  unwanted at high operating speeds- they then
  close automatically.

37
Inter-connected slat and flap

• To provide maximum lift, so as to take off in the
  shortest possible distance, the leading edge
  slats are opened fully, and the flaps are
  extended to about 20

Once the aircraft is airborne, the slats and
flaps are closed
38
Questions
39
(No Transcript)
40
Answers