Theory of flight Air cadet ground school
Theory of flight - topics • The Sections of an Airplane • 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 • 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. • The Fuselage • Central body • Accommodates crew, passengers and cargo • Part which the other components attach to
Parts of an airplane • Unconventional Parts • Canard • Stabilator • The Empennage • Horizontal stabilizer • Elevator • Fin • Rudder
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 • Types • Fixed • Retractable • Configuration • Tricycle • Tail dragger • Landing Gear • Function: • Absorb shock of landing • Support the weight of the aircraft • Enable aircraft to move around on the ground
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? • 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 Airplane • Parts of the Airplane • Fuselage • Empennage • Wings • Propulsion system • Landing gear
The Four Forces • The four forces: • Lift • Weight • Thrust • 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 • Slats • Slots
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: • An object when in motion tends to remain in motion. • An external force must be applied to alter the state of motion. • 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 • 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 • Induced drag DECREASES with velocity. • Parasite drag INCREASES 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 • Couples • When two forces are equal and opposite, but parallel. • A couple will cause a turning motion about an axis. • Equilibrium • When two forces are equal but opposite. • A Steady state of motion.
The four forces • 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. • 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.
The four forces • Thrust and Drag • If drag is above thrust, the nose will turn up. • If thrust is above drag, the nose will turn down. • 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.
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 up-going 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.