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THEORY OF FLIGHT - TOPICS
• The Sections of an Airplane
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Control Surfaces
Aerofoils
Definitions
The Four Forces of Flight
The Three Axes of Flight
Stability
Stalls, Spins, Spiral Dives, Load Factor
Pitot/Static System
THE AIRPLANE
Aeroplane: A power driven, heavier than air,
aircraft deriving its lift in flight from
aerodynamic reactions on surfaces that remain
fixed under given conditions of flight.
Aircraft: Any machine capable of deriving
support in the atmosphere from reactions with
the air.
An aeroplane is an aircraft, but an aircraft is
not always an aeroplane.
Other Aircraft: Airship, Helicopter, Balloon,
Glider, Ultralight.
PARTS OF AN AIRPLANE
The essential components of an airplane are:
•The fuselage or body
•The wings or lifting surfaces
•The Empennage (tail) or canard
•The propulsion system, ie. Engines
•The undercarriage or landing gear
PARTS OF AN AIRPLANE
PARTS OF AN AIRPLANE
The Fuselage
•Central body
•Accommodates
crew, passengers
and cargo
•Part which the other
components attach
to
Construction Types
•Truss Type
•
•
Longerons, N girders,
warren trusses for a
frame
Fabric, metal or
composite materials for
the covering.
•Monocoque
•
•
Stringers, formers,
bulkheads for carrying
the loads.
Stressed skin to carry
some of the loads too.
•Semi-monocoque
•
When stiffeners are
provided to carry some
of the load.
PARTS OF AN AIRPLANE
The Empennage
•Horizontal
stabilizer
•Elevator
•Fin
•Rudder
Unconventional
Parts
• Canard
• Stabilator
PARTS OF AN AIRPLANE
Wings
Configurations:
•Monoplane
•Biplane
Positioning
•High Wing
•Mid Wing
•Low Wing
Internal Construction
•Spars
•Ribs
•Compression Struts
•Drag / Anti-drag Wires
External Construction
•Ailerons
•Flaps
•Struts
•Engine Cowl
Definitions
Wing Span: Maximum distance from wing tip to wing tip.
Leading Edge: Front edge of the wing.
Trailing Edge: Rear edge of the wing.
Chord: An imaginary straight line joining the leading edge and trailing edge
PARTS OF AN AIRPLANE
Aerofoil Designs
There are many different types of designs, each
for a specific purpose.
Basically an aircraft designed for slow speeds
will have a thick wing. A thin wing is best for high
speeds. A wide but narrow wing is best for
gliders.
PARTS OF AN AIRPLANE
Types of Aerofoils
Conventional: thick for better structure and
lower weight for better stall characteristics. The
thickest part is at 25% chord.
Laminar flow: usually thin. Originally designed to
fly faster. The leading edge is more pointed and
its lower and upper surfaces are nearly
symmetrical. The thickest part is at 50% chord.
PARTS OF AN AIRPLANE
Aspect Ratio = Wing Span
Average Chord
PARTS OF AN AIRPLANE
Landing Gear
Function:
•Absorb shock of
landing
•Support the weight
of the aircraft
•Enable aircraft to
move around on the
ground
Types
•Fixed
•Retractable
Configuration
•Tricycle
•Tail dragger
PARTS OF AN AIRPLANE
Propulsion system
General aviation aircraft today use a gasoline
powered, air cooled, internal combustion engine
which drives a 2 or 3 bladed propeller.
QUESTIONS?
The Airplane
Parts of the Airplane
Fuselage
Empennage
Wings
Propulsion system
Landing gear
How do we classify a 2
wing aircraft?
What do we call the
configuration of
landing gear which has
three wheels, 2 under
the wings and one
under the nose?
What is the chord line?
What is the wing span?
THE FOUR FORCES
The four forces:
1.Lift
2.Weight
3.Thrust
4.Drag
THE FOUR FORCES
Chord Line: Straight line running from the
leading edge to the trailing edge of the wing.
Angle of Attack: The angle formed between the
chord line and the relative airflow.
Camber: The curvature of the upper or lower
surface of the wing.
THE FOUR FORCES
Relative Airflow: the direction in which the air is
moving relative to the chord of the wing. The
flight path and relative airflow are always
parallel but opposite.
Center of Pressure: the point on the wing where
all lift acts through.
THE FOUR FORCES
Laminar Airflow: The smooth streamlined airflow
over the wing.
Turbulent Airflow: Airflow over the wing which is no
longer smooth, but has become chaotic.
Transition Point: The point where the laminar airflow
changes to turbulent airflow.
Boundary Layer: the very thin layer of air that stick
to the wings due to skin friction. This layer of air
consists of the laminar and turbulent airflow.
Separation Point: the point at which the boundary
layer pulls away from the wing.
Mean Chord: the average chord of the wing.
THE FOUR FORCES
Angle of Incidence: the angle at which the wing is
permanently inclined to the longitudinal axis.
Wash in/Wash out: Wings that are slightly
twisted, causing the wing root to have a higher
angle of attack. The wing root will stall first.
Spoilers/Dive brakes: devices fitted into the wing
to increase drag and decrease lift. Spoilers are
on top, dive brakes are on the bottom.
THE FOUR FORCES
Wing Fences: fin like surfaces on the upper part
of the wing to control airflow, providing better
slow speed handling and stall characteristics.
Slats: auxiliary aerofoils fitted to the leading edge
of the wing to improve lateral control. Slats pull
out from wing at high angles off attack. Pushes
the turbulent airflow further back.
Slots: are air passage ways built into the wing on
the leading edge. At high angles of attack the air
flows through the holes and smoothing out the
turbulent airflow.
PARTS OF AN AIRPLANE
Slots
Slats
THE FOUR FORCES
Lift
The upward force which sustains the aircraft in
flight.
Lift acts through the center of pressure,
perpendicular to the relative wind or flight path,
regardless of the angle of attack.
THE FOUR FORCES
Newton’s Three Laws of Motion:
1.An object when in motion tends to remain in
motion.
2.An external force must be applied to alter the
state of motion.
3.For every action there is an equal but opposite
reaction.
THE FOUR FORCES
Bernoulli’s Principle: The total energy in any
system remains constant. If one element
increases, the other element must decrease to
counterbalance it.
Air flowing over the upper surface of a wing will
accelerate to catch up to the air flowing under
the wing. As the air speeds up, the pressure will
decrease. On the bottom of the wing the
pressure will increase.
The differential in pressure between the upper
and lower surfaces of the wing is what causes
the upward force, known as lift.
THE FOUR FORCES
Downwash: the flow of air downward towards the
trailing edge of the wing. The airflow passing
under the wing is deflected downward by the
bottom surface of the wing. The wing receives
an upward force, therefore downwash
contributes to lift.
THE FOUR FORCES
Bernoulli’s Principle
THE FOUR FORCES
Bernoulli vs. Newton
Bernoulli: The total energy in a system remains
constant. As air velocity increases, pressure
decreases.
Newton: For every action there is an equal and
opposite reaction.
THE FOUR FORCES
Weight
The downward force due to gravity, directly
opposed to lift.
Weight acts through the center of gravity.
THE FOUR FORCES
Thrust
The force produced by the propeller.
Air is pushed backwards, causing thrust in the
opposite direction.
Thrust can be moving a large mass of air
backwards at a slow speed, such as a propeller, or
a small mass of air backward at a high speed,
such as with a turbine engine.
THE FOUR FORCES
Drag
The resistance of forward motion.
For an aircraft to maintain steady flight there
needs to be enough lift to overcome weight
and enough thrust to overcome drag.
THE FOUR FORCES
Drag
Total aircraft drag
Parasite drag
Induced drag
Interference drag
Profile drag
Form drag
Skin friction
THE FOUR FORCES
Total Drag: the resistance the aircraft experiences
when moving through the air. There are 2 major
types of drag.
Induced Drag: Drag caused by the parts of the
aircraft which contribute to lift.
Parasite Drag: Drag caused by the parts of the
aircraft which do not contribute to lift. Parasite
Drag can be broken down into 2 types of drag.
THE FOUR FORCES
Induced Drag:
The higher pressure on the lower surface of the
wing will cause the air to flow outwards (high
pressure wants to push away)
The lower pressure on the upper surface will
cause the air to flow inwards (lower pressure
wants to gather together)
Nature needs balance: higher pressure air will
flow to areas of low pressure to equalize.
Airflow moving outwards from the bottom of the
wing will curl up over the wingtip. This drag is
commonly called wing tip vortices.
THE FOUR FORCES
THE FOUR FORCES
Interference Drag: A parasite drag. Drag caused
by the joining of two or more parts. Solution:
Streamline parts.
Profile Drag: A parasite drag which is further
broken down into 2 types.
Form Drag: A profile drag which is drag caused by
the shape of the aircraft.
Skin Friction: A profile drag caused by the
tendency of air flowing over a surface to stick to
it.
THE FOUR FORCES
Parasite drag
INCREASES with
velocity.
Induced drag
DECREASES with
velocity.
THE FOUR FORCES
Reducing Parasite Drag:
Form:
•Streamlining
•Reduce frontal area of a/c (ie. Radial engines)
•Retractable landing gear
Skin Friction:
•Clean aircraft
•Wax
•Flush rivets
Interference Drag
•Landing gear fairings
•Streamlined design (rounded fuselage)
THE FOUR FORCES
Reducing Induced Drag
•High aspect ratio wings (ratio of the span to
the mean chord)
Aspect ratio = induced drag
•Winglets
•Ground effect
THE FOUR FORCES
Equilibrium
When two forces
are equal but
opposite.
A Steady state of
motion.
Couples
When two forces
are equal and
opposite, but
parallel.
A couple will cause
a turning motion
about an axis.
THE FOUR FORCES
Lift and Weight
When lift and
weight are equal,
but opposite, the
aircraft will be in a
state of equilibrium.
Lift > Weight will
cause the aircraft
to climb.
Lift<Weight will
cause the aircraft
to descend.
Thrust and Drag
When equal but
opposite, they will
be in a state of
equilibrium.
Thrust>Drag will
cause the aircraft
to accelerate.
Thrust<Drag will
cause the aircraft
to decelerate.
THE FOUR FORCES
Weight and Lift
If weight is ahead of
lift, the couple will
cause the nose to
turn down.
If lift is ahead of
weight, the couple
will cause the nose to
turn up.
Thrust and Drag
If drag is above
thrust, the nose will
turn up.
If thrust is above
drag, the nose will
turn down.
THE THREE AXES
THE THREE AXES
Axis (About)
Movement
Control Surface
Control Input
Longitudinal
Roll
Ailerons
Stick/Control
Column
(left/right)
Lateral
Pitch
Elevator
Stick/Control
Column
(aft/forward)
Normal (Vertical)
Yaw
Rudder
Rudder Pedals
THE THREE AXES
Longitudinal Axis:
The imaginary line which runs from the nose to
the tail.
Movement ABOUT this axis is known as ROLL
and is produced and controlled by the
AILERONS.
Movement OF this axis is known as PITCH and is
produced and controlled by the ELEVATOR.
THE THREE AXES
Lateral Axis:
An imaginary line running from wing tip to wing
tip.
Movement ABOUT this axis is known as PITCH
and is produced and controlled by the ELEVATOR.
Movement OF this axis is known as ROLL and is
produced and controlled by the AILERONS.
THE THREE AXES
Vertical or Normal Axis:
An imaginary line running vertically through the
center of gravity.
Movement ABOUT this axis is known as YAW and
is CONTROLLED by the RUDDER.
MOVEMENTS AND ATTITUDES
Movements are
related to the
aircraft.
Attitudes are
related to the
horizon.
Movements are produced and
controlled to achieve an attitude.
MOVEMENTS
Roll: movement about the longitudinal axis.
Achieved through the lateral movement of the
control column. When the column is moved left
the left aileron will move up, while the right aileron
will move down.
The up-going aileron will cause a lower angle of
attack, and therefore less lift. The down-going
aileron will create a larger angle of attack, and
therefor more lift. The aircraft will roll to the upgoing aileron.
Aileron Drag: in rolling the wings, the down-going
aileron increases the lift on the outside wing,
increasing induced drag. The result is a slight turn
in the opposite direction.
MOVEMENTS
Roll
MOVEMENTS
Pitch: movement about the lateral axis.
Achieved through the aft/forward movement of
the control column. When the column is moved
aft the elevator will move up, causing the tail to
be pushed down and the nose pushed up. The
opposite holds true if the column is moved
forward.
MOVEMENTS
Pitch
MOVEMENTS
Yaw: movement about the normal/vertical axis.
Controlled through the movement of the
rudder.
We do NOT produce yaw, we just control yaw!
MOVEMENTS
Yaw
LEFT TURNING TENDENCIES
Adverse Yaw/Aileron Drag:
In a roll the aircraft has a tendency to initially roll
in the opposite direction of the turn. The downgoing aileron has an increased camber, causing
more induced drag. The airplane skids outward
on the turn.
Controlled by applying rudder in the direction of
the turn.
LEFT TURNING TENDENCIES
Asymmetric Thrust:
At high angles of attack, the down-going
propeller blade meets the relative airflow at a
higher angle of attack, creating more lift on the
right side, therefore the aircraft will yaw to the
left.
Corrected by the application of right rudder.
Propellers turn clockwise from the view of the
pilot.
LEFT TURNING TENDENCIES
Asymmetric Thrust
LEFT TURNING TENDENCIES
Gyroscopic Procession:
A propeller acts like a gyro. Any force applied to a
spinning gyro will act 90 degrees to the direction
of the rotation. Raising the aircraft tail will apply
a force to the top of the propeller arc, resulting in
the force applied to the right of the arc, causing a
yaw to the left.
LEFT TURNING TENDENCIES
Gyroscopic Procession
LEFT TURNING TENDENCIES
Torque: As the propeller spins clockwise the
result is the aircraft rotating counter-clockwise.
LEFT TURNING TENDENCIES
Slip stream: Airflow from the rotating propeller
cork-screws around the aircraft hitting the fin on
the left side pushing the tail right and nose left.
ATTITUDES
The rolling movement produced and controlled
by the ailerons will achieve a BANKED ATTITUDE.
The pitching movement produced and controlled
by the elevator will achieve a PITCH ATTITUDE.
STABILITY
The tendency of an aircraft in flight to remain in
its trimmed attitude and return to this attitude if
disturbed without help from the pilot.
Static Stability: The INITIAL tendency of the
aircraft to return.
Dynamic Stability: The OVERALL tendency of the
aircraft to return.
STABILITY
Types of Stability
Positive: Tendency of aircraft to return to
equilibrium.
Neutral: New equilibrium reached at any point of
displacement. (Doesn’t return to original
position)
Negative: Tendency to continue in direction of
displacement.
STABILITY
Longitudinal Stability: Stability AROUND the lateral
axis – PITCH stability
Longitudinal Stable: Tend to return to its trimmed
Angle of Attack
Longitudinal Unstable: Tendency to climb or dive
STABILITY
Factors Influencing Longitudinal Stability:
•Horizontal Stabilizer
•
•
The larger the size the more stable the aircraft.
Being positioned at the far end of the lever arm cause a more
stable aircraft.
•Center of Gravity
•
•
•
Loading within the Center of Gravity will be stable
Too far forward will cause more of a tail down force and therefore
a greater apparent weight, and a higher stalling speed. Could lead
to the inability to pull out of a dive. The nose would be too heavy
and the take-off distance would be longer.
Too far aft will be more dangerous than too far forward. Very
unstable in pitch regardless of speed. Elevators may be
ineffective in stall and spin recoveries.
STABILITY
Lateral Stability: Stability AROUND the
longitudinal axis – ROLL Stability
Lateral Stability is achieved through
•Dihedral
•Keel Effect
•Sweepback
STABILITY
Dihedral
The angle that each wing makes with the
horizontal.
A displaced wing will drop, and the resulting
unbalanced force will cause a sideslip, which
will put the lower wing at a higher angle of
attack. The lower wing will then produce more
lift and the wings will return back level.
STABILITY
Keel Effect
Most high wing aircraft are laterally stable
because of their position of the wings to the
body. The weight is therefore low.
When the airplane is disturbed and one wing is
low, the low weight will act like a pendulum and
the aircraft will return to its original position.
STABILITY
Sweepback
A sweptback wing is one which has a rearward
sloping leading edge.
When a disturbance causes a slip or a dropped
wing, the lower wing will have more of its leading
edge presented to the relative airflow, resulting
in that wing being pushed back to its original
position.
STABILITY
Directional Stability: Stability AROUND the normal
or vertical axis.
Stability is achieved through the fin and rudder.
An airplane has the tendency to fly head on into
wind, like a weather vein. If the airplane yaws
away from its course, the airflow will strike the
side of the fin or rudder from the side and for it
back to its original position.
CLIMBING
The ability to climb is dependant upon the ability
to produce thrust. Lift always acts perpendicular
to the relative airflow.
Therefore the vertical component of lift must
increase to balance the vertical component of
thrust.
CLIMBING
Best Rate of Climb
The rate of climb which will gain the most
altitude in the least time. This climb is normally
used on take-off.
Best Angle of Climb
The angle that will gain the most altitude in
the least distance. Climb used when climbing
over obstacles.
Normal Climb
The rate of climb which should be used for
any prolonged cruise climb.
GLIDING
In gliding there is NO power from the engine and
the aircraft is under influence of gravity. Of the
four forces thrust is now absent and a state of
equilibrium must be maintained by lift, drag and
weight only.
GLIDING
Best Glide Speed for Range
The speed at which the aircraft will glide
the furthest distance for altitude lost.
Best Glide Speed for Endurance
The speed at which an aircraft will glide
the greatest amount of time for altitude lost.
TURNS
TURNS
The force of lift acts 90º to the wing span. In a
turn this force of lift is inclined away from the
vertical. Therefore the vertical forces are no
longer in a state of equilibrium.
The airplane will descend unless the angle of
attack is increased to produce more lift.
TURNS
In a turn the lift force has two components.
•Vertical: opposes weight (lift)
•Horizontal: makes the airplane turn. This
horizontal force is known as Centripetal force
(toward the turn)
TURNS
Steeper the angle
of bank:
•The greater the
rate of turn
•The less the radius
of turn
•The higher the
stalling speed; and
•The greater the
loading
Higher airspeed in
a turn:
•Slower rate of turn;
and
•Larger radius of
turn
STALLS
A stall is the result of the inability of a wing to
produce lift to counteract the weight of the
aircraft.
A smooth laminar airflow is needed to produce
lift. The stall occurs where the angle of attack is
increased up to the point where the laminar flow
is unable to follow the curvature of the upper
surface.
STALLS
The airflow separates from the
wing, becoming turbulent,
resulting in a loss of lift.
As the angle of attack
increases, the center of
pressure moves forward on the
wing until the CRITICAL ANGLE
OF ATTACK is reached.
At this point the C of P moves
abruptly backwards on the
wing, which is now stalled.
STALLS
Factors Affecting the Stall
Weight: greater weight = higher angle of attack =
higher stalling speed.
C of G: forward C of G = higher angle of attack =
higher stalling speed
Turbulence: higher stalling speed because an upward
vertical gust could cause the aircraft to exceed its
critical angle
Turns: an increase in bank = increased load factor =
higher stalling speed
Flaps: increases lifting potential = lower stalling speed
Aircraft Condition: contaminated wings will increase
drag = higher stalling speed
STALLS
An aircraft will stall if the critical angle of attack
is exceeded.
It will stall at any airspeed if the critical angle of
attack is exceeded.
It will stall at any attitude if the critical angle of
attack is exceeded.
It will stall at the same indicated airspeed
regardless of altitude.
SPINNING
The auto-rotation developed after an aggravated
stall. If a stalled plane drops a wing, or if rudder is
applied to produce yaw, the down-going wing will
have a greater angle of attack to the relative
airflow, will receive less lift and drop more rapidly.
Drag on the down-going wing increases sharply,
increasing the AoA on this wing, stalling it further
and further.
The nose drops and auto-rotation sets in.
SPINNING
SPIRAL DIVE
A spiral dive is a very steep descending turn
where the aircraft is in an excessively nose
down attitude and at an excessively increasing
rate of descent.
Characterized by
•Excessive angle of bank
•Rapidly increasing airspeed
•Rapidly increasing rate of descent
SPIRAL DIVES VS. SPINS
A spiral dive resembles a spin, but it is extremely
important the recognize the difference between
them.
In a spin the airspeed is constant and low (at or
near the stall speed)
In a spiral dive the airspeed is increasing rapidly.
LOAD FACTOR
Dead Load: the weight of the aircraft standing on
the ground.
Live Load: the change in dead load due to
acceleration or turns.
Load Factor: the ratio of the actual load acting on
the wings to the gross weight of the aircraft. Live
load divided by the dead load.
LOAD FACTOR
Why are load factors important?
•A Dangerous overload is possible
•An increase in load factor will result in an
increase in the stalling speed
LOAD FACTORS
We express load factors in ‘G’s.
•When on the ground or in straight and level unaccelerated flight, you are said to be under the
force of 1G or one time the force of gravity.
•As the angle of bank increases, so does the
load factor.
•In a banked attitude of 60 = 2G's.
PITOT/STATIC SYSTEM
Pitot Tube:
Measures dynamic
and static
pressures and is
positioned to be
clear of the
slipstream and
facing the line of
flight.
Airspeed Indicator
is the only
instrument
connected to the
pitot.
Static port:
Measures static
pressure and is
vented to allow air
pressure inside the
instrument case to
equalize with the
outside air
pressure.
ASI, ALT and VSI
use static pressure.
ALTIMETER
Measures the pressure in the atmosphere, which is
the weight of the air above you at any given altitude.
This weight will change as the aircraft climbs or
descends causing the altimeter to register a change in
pressure.
The altimeter is made of a stack of aneroid capsules
each set at standard sea level. As the altitude changes
these capsules expand and contract, moving gears
and thus the hands on the altimeter face.
ALTIMETER ERRORS
Pressure: the atmospheric pressure changes
when flying from place to place. If not corrected,
the altimeter would be inaccurate.
Temperature: the altimeter is constructed to work
on the values of the ICAO standard atmosphere.
The temperature is not always this value.
Mountain Effect: when flying near mountains,
winds can be gusty and cause a drop in local
pressure. Consequently, the altimeter will not
give an accurate altitude indication.
AIRSPEED INDICATOR
Tells the pilot how fast they are travelling through
the air and not over the ground. Measures the
difference between pressures in the pitot and
static ports.
The reading on the ASI is referred to as the IAS.
TAS is the CAS corrected for ASI error due to
density and pressure.
AIRSPEED INDICATOR
The instrument is made up of an aneroid
capsule which measures the pitot pressure. The
interior of the case is sealed and the static
pressure is measured there. The changes in
dynamic pressure (pitot) cause the aneroid
capsule to expand and contract. This movement
is transmitted to a connected linkage that moves
the hand on the ASI.
AIRSPEED INDICATOR ERRORS
Density: Atmospheric density varies and as a
result this will change the accuracy of the ASI.
Position: Eddies that form as air passes over the
wing are responsible for the error.
Lag: The slowness of the working parts.
Icing: Ice formation blocking either the pitot or
static tube could give inaccurate readings.
Water: Water could block the tubes causing
inaccurate readings.
VERTICAL SPEED INDICATOR
Indicates the rate of climb and descent and is
measured in feet per minute.
Measures the change in pressure between the
capsule and the case of the instrument. The
capsule will expand and contract which is
transmitted by a linkage to the hands on the
VSI.
The instrument tends to lag 6 – 9 seconds.
MAGNETIC COMPASS
Consists of two north-seeking magnets, which
are attached to a float, which is also attached to
a compass card. This complete magnet system
is mounted on a pivot and is free to rotate. The
whole assembly is mounted within the compass
bowl, which is filled with alcohol to reduce the
weight of the compass card and the magnets.
The lubber line indicates the direction the
aircraft is heading and is in line with or parallel
to the longitudinal axis of the aircraft.
GYROSCOPES
A gyroscope is a rotor or a spinning wheel,
rotating at a high speed in a universal
mounting, called a gimbal, so its axis can be
pointed in any direction.
Gyroscopic Inertia: the tendency of a rotating
body to maintain its plane of motion.
Precession: the tendency of a rotating body,
when a force is applied perpendicular to its
plane of rotation, to turn in the direction of its
rotation 90 degrees to its axis and take up a
new plane of rotation parallel to the force
applied.
GYRO INSTRUMENTS
Heading Indicator:
An instrument designed to indicate the heading
of the airplane, and because it is steady and
accurate, to enable the pilot to steer that
heading with the least effort.
GYRO INSTRUMENTS
Attitude Indicator:
Provides the pilot with an artificial horizon as a
means of reference when the natural horizon
cannot be seen because of cloud, fog, rain or
other obstructions to visibility.
It shows the pilot the relationship between the
wings and nose of the plane and the horizon of
the earth.
GYRO INSTRUMENTS
Turn and Bank Indicator:
Aircraft indicates the direction of the turn.
Ball indicates slipping or skidding.
If the ball is opposite to the direction of turn you
are in a skidding turn.
With the ball and aircraft to the same side, the
turn is slipping.
A ball centered in a turn indicates a
coordinated turn.