11.1 Theory of Flight

• An aircraft's motion is characterized by its rotation about three axes: lateral, longitudinal, and vertical.
• Movements about these axes are pitch, roll, and yaw, respectively.

Lateral Axis (Pitch Axis)

• The lateral axis runs horizontally from wing-tip to wing-tip.
• Pitch motion occurs with rotation around this axis, controlled by elevators.
• Elevators change the aircraft's angle of attack, moving the nose up or down

Longitudinal Axis (Roll Axis)

• The longitudinal axis runs from the nose to the tail.
• Roll motion occurs with rotation around this axis, controlled by ailerons
• Ailerons create differential lift between wings, tilting the aircraft to bank.

Vertical Axis (Yaw Axis)

• The vertical axis runs vertically through the aircraft's center of gravity.
• Yaw motion occurs with rotation around this axis, controlled by the rudder.
• The rudder varies airflow over vertical surfaces, turning the aircraft left or right.
• Pilots manipulate ailerons, elevators, and the rudder to achieve the desired orientation and trajectory in flight.

Primary Flight Controls

• Primary flight controls direct the aircraft about the three axes.
• Ailerons control roll.
• Elevators control pitch.
• Rudder controls yaw.
• Roll spoilers or speed brakes can be added to large commercial aircraft to improve control surface efficiency.

Roll Control (Ailerons and Spoilers)

• Ailerons are on the outboard trailing edge of the wings.
• Moving the control stick right raises the right aileron and lowers the left aileron.
• Upward deflection of the right aileron reduces wing camber, decreasing lift.
• Downward deflection of the left aileron increases camber, increasing lift.
• Differential lift causes the aircraft to roll right and vice versa for a left turn.
• Force on a control surface is a product of kinetic energy and surface area.
• Dynamic energy is expressed as: Energy = 0.5 x ρ x V^2 (ρ = air density, V = airspeed).
• Energy in the air is proportional to the square of airspeed, so doubling airspeed quadruples pressure on the control surface.
• Some larger aircraft have inboard and outboard ailerons to combat high-speed forces.
• Inboard and outboard ailerons deflect during takeoff, landing, and low speeds for maximum leverage.
• Outboard ailerons are locked out as speed increases, and roll is controlled by roll spoilers and smaller inboard ailerons.

Spoilers

• Spoilers supplement ailerons by reducing lift on the wing with the upgoing aileron.
• Roll spoilers counteract lift-induced drag that causes adverse yaw.
• Their differential operation is linked to the aileron control system.
• Roll spoilers help execute accurate turns and reduce the need for large aileron deflections at high speeds.

Pitch Control (Elevators, Stabilators, Variable Incidence Stabilisers, and Canards)

• Elevators are on the trailing edge of the horizontal stabilizer.
• Used for longitudinal or pitch control around the lateral axis, furthest from the center of gravity for greater leverage.
• Elevators move collectively up or down and create nose-up or nose-down motions respectively.
• Pushing the control column forward pitches the nose down, pulling it rearward pitches the nose up.

Stabilators

• Stabilators combine the functions of elevators and a horizontal stabilizer.
• Called all-moving tails or slab tails, they rotate around their horizontal axis to control pitch.
• The entire horizontal tail surface responds to pilot inputs, unlike the elevator control on a fixed stabilizer.
• Often installed in light general aviation aircraft and equipped with an anti-balance tab.
• Commonly found on high-speed military combat aircraft for rapid maneuverability.
• They circumvent normal shock wave formation near the elevator hinge line which can render the control surface ineffective during transonic flight.

Variable Incidence Stabilisers

• A variable incidence stabilizer or Trimmable Horizontal Stabilizer (THS) is for pitch trimming only.
• Reduces drag due to less requirement for elevator deflection.
• Increasing the angle of attack of the THS results in a nose-down attitude and vice versa.
• Adjustments include passenger and freight distribution, fuel consumption, and flap and engine settings during flight.
• Controlled via a screw jack mechanism and trim wheels or electrical servo motors.
• Movement is instinctive: forward for nose-down, rearward for nose-up.

Canards

• Canards are horizontal control surfaces at the front of the aircraft ahead of the main wings.
• Used primarily for pitch control, adjusting the aircraft's nose-up or nose-down motion.
• Canards improve stability by providing a nose-down force, enhancing stall resistance and maneuverability.
• Canards stall first as the aircraft approaches a stall, inducing a nose-down pitching moment for recovery.
• Beneficial in high angle of attack maneuvers, as they maintain control authority.
• Canards add redundancy to the flight control system, providing partial control in case of main wing or tail elevator failure.
• They contribute to lift generation and affect trim, the balance between pitch and lift.

Yaw Control

• The rudder is hinged to the rear of the vertical stabilizer for directional or yaw control.
• A left-deflected rudder creates a nose-left motion, and a right-deflected rudder creates a nose-right motion.
• Controlled through foot-operated rudder pedals and is instinctive, right for right, and left for left.
• The rudder limiting system restricts rudder deflection as airspeed increases to prevent structural failure.
• Pilot rudder pedal deflection is not restricted, but rudder deflection is limited.
• Rudder travel limiter is controlled by the Feel and Limitation Computers (FLC) to maintain yaw control and limit lateral loads.
• Yaw and roll movements create secondary effects on each other.
• Yaw causes one wing to advance, increasing lift and rolling the aircraft in the yaw direction.
• Roll causes sideslip, resulting in force on the vertical stabilizer and yaw in the roll direction which is addressed by rudder input.
• The rudder is used independently to correct crosswind or single-engine failure.
• Adverse yaw occurs in light aircraft, where the down-going aileron increases lift and lift-induced drag, yawing the aircraft opposite to the roll direction.
• Adverse yaw is addressed by design features like frise ailerons or differential ailerons.

Frise Ailerons

• Frise ailerons have a specially contoured leading edge.
• When deflected up, the leading edge protrudes into the airflow, increasing profile drag to balance lift-induced drag.

Differential Ailerons

• Differential ailerons are rigged so the up-going aileron deflects more than the down-going aileron.
• Creates a drag imbalance that counters adverse yaw.
• High vertical stabilizers can cause adverse roll, where rudder deflection creates a rolling moment opposite to rudder deflection.
• An aileron-rudder interconnect system helps overcome adverse roll.
• Swept-wing aircraft are prone to yaw and roll instability, causing Dutch roll.
• Yaw damping systems correct uncommanded rudder oscillations.

Elevons

• Elevons combine ailerons and elevators, found on delta-wing aircraft.
• No horizontal stabilizer exists. The control surfaces for pitch and roll are combined and located at the trailing edge of the wing.
• When a pitch input is commanded, the elevons deflect up and down collectively.
• When a roll input is commanded, the elevons move in opposing directions.
• Elevons may move differentially in the same direction in response to pitch and roll combined input.

Ruddervators

• Ruddervators combine a rudder and an elevator.
• Ruddervators can be found on V-tailed aircraft where the horizontal and vertical stabilizers are combined into two angled stabilizers.
• These each have a control surface hinged to the trailing edge.
• Ruddervators mimic conventional control surfaces using a complex actuation system.
• Yaw to the left is from moving the pedals left, deflecting the left ruddervator down and left, and the right ruddervator up and left; the opposite deflects to the right.
• Pitch nose up is from moving the control column or stick back, deflecting the left ruddervator up and right, and the right ruddervator up and left; the opposite pitches nose down.

High-Lift Device: Slots

• Slots are fixed ducts on the wing's leading edge.
• Slots allow high-pressure air from below to re-energize the boundary layer atop the wing.
• This prevents separation, stagnation, and stall at high angles of attack.
• Allow slower flight speeds and higher angles of attack.
• Slots do not increase lift directly, but re-energize the boundary layer to increase the stall angle.
• Can be positioned forward of the ailerons to preserve aileron function if the wing stalls.
• A disadvantage of slots is that they produce a fair amount of drag.

High-Lift Devices: Slats

• Slats serve the same purpose as slots in a comparable way. The difference is that slats are movable and are retracted forming the leading edge when not needed.
• Slats are movable, forming the leading edge when retracted.
• When deployed they form a slot, increasing wing camber and overall lift.
• Improves lift by delaying stall to a higher angle of attack, acting as a stall protection device.
• Slats are selected as a part of a flap configuration, or they can be selected independently of the flaps with a switch in the cockpit or flight deck.

High-Lift Devices: Flaps

• Flaps increase lift during low speeds like takeoff and landing and produce significant drag for landing.
• Extending the trailing edge of the wing increases the camber to raise the lift coefficient. It also, in some cases, increases the surface area of the wing depending on the flap design.
• As flaps deploy, aerodynamic forces move the wing’s center of pressure rearwards, pitching the nose down. Retracting the flaps pitches the nose up.

Plain Flap

• The plain flap is an integral part of the wing. Actuation downwards increases lift around 50 - 55%.
• Causes a lot of drag and produces a marked nose-down pitching moment due to the rearward movement of the center of pressure.

Split Flap

• The split flap panel is flush with the lower surface of the wing. When lowered, it alters the underwing profile increases the camber. Actuation increases the lift by around 60 - 65%.
• It does not alter the surface area or the shape of the top of the wing. So, it avoids a boundary layer separation over the upper surface but causes more drag than a plain flap.

Slotted Flap

• Slotted flaps have slots near the trailing edge of the wing when extended. The slots allow air from the lower side of the wing to flow to the upper side and re-energize the boundary layer. An extension of the flap increases the camber but not the wing area. Actuation increases the lift by around 65 - 70%.
• There is not as much drag created with this design in comparison to previously discussed designs.

Fowler Flap

• The Fowler flap forms part of the underside of the trailing edge of the wing. Actuation extends rearwards first and then downwards along a flap track. Actuation lifts more than slotted flaps, up to 95%.
• Causes a nose-down pitch moment due to the rearward movement of the center of pressure.

Slotted Fowler Flap

• The slotted Fowler flap creates a gap between the trailing edge of the wing and the leading edge of the flap. This allows air from the high-pressure region under the wing to flow to the upper surface of the flap and re-energise the boundary layer and delays separation. This design primarily increases the area of the wing and then the camber, increasing lift and giving the lowest drag penalty possible.

Leading Edge Flaps

• Heavy aircraft often have leading edge flaps.
• Use with trailing edge flaps expands wing camber and lift.
• Designs of leading edge flaps essentially provide the same effect.
• Activation of the trailing edge flaps automatically deploys the leading edge flaps, and extends the camber of the wing.

Leading Edge Droop

• A leading edge droop is a device on the leading edge designed to improve airflow over the wings at high angles of attack. On selection, the entire leading edge section rotates downwards. This increases camber and therefore lift.
• Improves airflow over the wings at high angles of attack.

Krueger Flaps

• Krueger flaps are lift enhancement devices fitted to the leading edge of a wing.
• Rotation of a portion of the lower wing out in front of the main wing leading edge.
• Krueger flaps, hinged at their foremost position that, once deployed, actually become their trailing edges, hinge forwards from the under the surface of the wing, increasing the wing camber and the maximum coefficient of lift.
• Creates a much more pronounced blunt leading edge on the wing, helping to achieve better low-speed handling.

Flaperons

• Flaperons are ailerons that act as a flap.
• During takeoff and landing, they droop with the flaps but can still move up and down to provide aileron control.
• Deflect collectively as flaps and differentially as ailerons.
• Many flaperon designs mount the control surfaces away from the wing for undisturbed airflow.

Drag Devices

• Drag-inducing devices increase drag to slow the aircraft or dump lift.
• Spoilers can also be used with the ailerons to roll.
• Air brakes raise drag without changing lift, whereas spoilers decrease the lift-to-drag ratio and require a higher angle of attack, resulting in higher stall speed.
• Spoilers are hinged panels on the upper surface of the wing.

Flight Spoilers

• Spoilers serve as speed brakes to augment a controlled descent.
• They operate differentially to assist in lateral control and stability.
• They dump lift on landing.

Ground Spoilers

• On spoiler-equipped aircraft, some of the spoiler panels have a flight spoiler function which is often referred to as "speed brakes".
• All spoiler panels are extended to their maximum angle upon landing or during a rejected take-off.
• The primary purpose is to maximise wheel brake efficiency by 'spoiling' or dumping the lift generated by the wing.

Roll Spoilers

• Roll spoilers are used in conjunction with ailerons to enhance roll control.
• The roll spoilers are often called 'spoilerons'.
• Beneficial for their opposition to adverse yaw.
• Deployment decreases lift on one wing to initiate roll.
• Often are outboard or mid-span spoilers.
• Provide an advantage over ailerons for high-speed flight.

Speed Brakes

• Speed brakes are high-drag devices fitted to military and some commercial aircraft.
• Usually fuselage-mounted panels that extend into the airstream to create drag.
• Used during final approach and after landing.
• Positioned where the aircraft structure is strong enough to withstand heavy air-loads.

Boundary Layer Control

• Air flowing over a swept wing splits in two directions causing the boundary layer to thicken towards the wing tip and making it prone to separation with an increased angle of attack.

Wing Fence

• Flat plates fixed to the upper surface or wrap around the leading edge to reduce spanwise flow and wing tip stall.
• They obstruct spanwise flow and redirect it along the chord, delaying boundary layer separation and stall.

Saw Tooth Leading Edges

• An extension of the leading edge or a zig-zag cut-out to reduce wing-tip stall.
• A cut-out creates a vortex that limits boundary layer outflow, redirecting spanwise flow and reinvigorating airflow.

Boundary Layer Control Using Vortex Generators

• Vortex generators are small aerofoil sections that are placed vertically on the surface of a large wing.
• Re-energizes a slow-moving boundary layer
• Produces lift that acts sideways, causing a vortex that draws high-energy air from outside the boundary layer
• Often installed in opposing pairs at a high angle of attack.

Stall Wedges

• Stall wedges are leading edge devices positioned on the inboard section of a tapered wing.
• Stall wedges encourage the root to stall first.

Introduction to Trim Tabs

• Trim tabs are used during flight.
• The trim tabs adjust the aerodynamic forces on the control surfaces to maintain the set attitude without control input.
• They compensate for imbalances because of variations in the centre of gravity.

Trim Tabs

• Small control surfaces on the trailing edge of a primary flight control surface.
• Fixed trim tabs can only be adjusted on the ground.
• Adjustable trim tabs are moved by the pilot using a trim wheel or switch.
• The trim tab is deflected in the direction that opposes that of the flight control surface deflection.
• Deflection repositions the control surface to a new neutral position, and it could limit the range of movement of the control surface.

Balance Tab

• Fitted at the trailing edge, the control surface has a balance tab fitted at its trailing edge.
• A balance tab is connected to the control surface by a fixed rod so that when the control surface is deflected, the tab automatically moves in the opposite direction.

Anti-Balance Tab

• An anti-balance tab is on the trailing edge of a control surface
• The tab is connected so that it moves in the same direction as the control surface, giving resistance to movement.

Servo Tabs

• Used on larger aircraft with larger control surfaces.
• The servo tab moves in the opposite direction to the flight control and is operated directly by the pilot.
• Pilot operates the servo tab, and the force produced operates the control surface.

Spring Tabs

• A spring tab is like a servo tab and only works when the force on a flight control surface reaches a certain value, from pilot input.

Mass Balance

• Mass balance is concerned with the position of the Centre of Gravity (CG) and Centre of Pressure (CP) in relation to each other and the pivot or hinge point of a control surface.

Mass Balance Flutter

• Mass balance flutter, "Torsional Aileron Flutter," affect primary flying controls.
• With the aileron displaced downwards, it exerts lifting force on the aileron hinge. The wing twists causing inertial lag and oscillation.

Control Surface Bias

• Control surface bias is a configuration applied to aircraft control surfaces to achieve optimal flight characteristics during flight.
• A small offset or deflection is applied to control surfaces, creating a continuous aerodynamic force.

Aerodynamic Balance

• Aerodynamic balance is concerned with the effort required to move a control surface.
• Forces act through the Centre of Pressures located about one-third of the chord length.

Horn Balance

• Horn is forward of the hinge line and moves during deflection, aiding deflection.

Inset Hinges

• Inset hinge on this control surface is located behind the leading edge and creates the same purpose of the horn.

Balance Panels

• A plate connected to the leading edge of the control surface which allows outside static pressure via slots.
• Pressure difference assists deflection.

Speed of Sound

• The speed of sound is the rate at which pressure waves travel through a medium.
• Sound requires a medium to propagate, relying on particle interaction to transmit vibrations.
• The speed of sound at sea level (15 °C) is 340 m/s (760 mph); at 36,000 ft (-56.5 °C), it's 295 m/s (660 mph).

Mach Number

• Mach number is the ratio of an object's velocity (V) to the speed of sound (a) in the same medium V/a.
• Expressed as: M = V/a, where M is the Mach number, V is the object's velocity, and a is the speed of sound
• A dimensionless quantity.

High Speed Flight

• As an aircraft approaches the speed of sound, it experiences compressibility effects.
• Compressibility effects manifest as shock wave formation, drag, reduction in lift, buffeting, and trim and control issues.

Subsonic, Transonic, and Supersonic Flight

• Three speed regions in which aircraft can operate are: subsonic, transonic, and supersonic

Subsonic flight

• Subsonic flight is below the speed of sound.

Transonic flight

• Transonic flight includes a combination of subsonic and supersonic airflow, creating a shock wave that results in the center of lift to shift rearward, resulting in a nose-down pitch tendency.

Supersonic Flight

• Supersonic is all speeds around the aircraft are higher than the speed of sound.

Critical Mach Number

• The critical Mach number of an aircraft is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it.
• When an aircraft approaches MCrit, it encounters compressibility effects, and shock waves begin to form
• The aircraft can be flying below the speed of sound but areas around the wing and other curved sections experience localised supersonic flow.

Compressibility Buffet

• Undesirable aerodynamic effects felt when an aircraft enters the transonic range.
• A pressure concentration forms as a shock wave. A sudden rise in drag, a reduction in lift, buffeting, uncommanded changes in trim and stability, and control problems are experienced.

Shock Wave

• Shock wave forms when aircraft reaches the speed of sound, resulting in pressure waves, which is sound energy, compress and accumulate, forming a bow shock wave.
• Aircraft overtakes pressure waves above the speed of sound, radiating behind it.
• The air ahead of the wave will be compressed and traveling at or above the speed of sound.
• As the air passes through the shock wave, it decelerates to a subsonic speed (below Mach 1) resulting in an increase in pressure, energy dissipation, raised temperature,

Boundary Layers

• Behind the shock waves thickens, becomes turbulent, losing kinetic energy, causing flow separation.
• Causes a turbulent wake resulting in buffeting, reduced effectiveness of the controls, and loss of stability, and a dramatic increase in drag called 'shock' or "wave" drag.

Oblique Shock Waves

• Bow wave at the leading edge of a wing traps a section of air on the leading edge, limiting aircraft speed.
• Supersonic aerofoil sections have a sharp or pointed leading edge, allowing the bow wave to attach and form an oblique shock wave sloping backwards.
• Airflow forward of the oblique shock wave is above March 1.

Flat Plate Profiles

• The airflow above and below the surface passes through expansion waves and oblique shock waves.
• The airflow over the upper surface passes through an expansion wave at the leading edge and then an oblique shock wave at the trailing edge.
• The airflow under the plate passes through an oblique shock wave at the leading edge and then an expansion wave at the trailing edge.

Wave Drag

• Portion of total drag caused by shock waves.
• Shock waves turn useful energy into heat energy.
• The Ways to reduce wave drag is use vortex generators or apply the area rule.

Vortex Generator

• Transfers energy from the free air stream to the boundary layer.
• Reduces the flow separation.
• Produces an oblique shock wave inside the supersonic airflow

Area Rule

• The Area Rule, sometimes referred to as the Transonic Area Rule, is a design procedure used to reduce an aircraft's drag at transonic speeds which occur between about Mach 0.75 and 1.2.
• Longitudinal cross-sectional area distribution is the same, independent of how the area is distributed laterally.
• Maintaining this perfect profile is impracticable, narrow the fuselage for wing placements.

Aerodynamic Heating

• Heating of the airframe caused by the friction of the air. Significant when the aircraft is flying supersonically.
• Reduction in velocity and temperature occurs, with the greatest reduction in velocity and increase in temperature occurring at the various stagnation points on the aircraft.

Factors Affecting Airflow in Engine Intakes of High-speed Aircraft

• Gas turbine engine compressors cannot accept axial velocities of much more than Mach 0.4.
• For a supersonic aircraft, the air entering the compressor section of its engine must be slowed to subsonic velocity, with the least possible waste of energy.
• The intake design must slow the air with the weakest possible series or combination of shock waves to minimise the energy losses caused by temperature increases.

Normal Shock Diffuser Inlet

• A normal shock diffuser inlet employs a single normal shock wave at the inlet to slow the air to subsonic velocity.

Single and Multiple Oblique Shock Inlet

• This design forms an external oblique shock wave to slow the supersonic airflow before the normal shock occurs.
• A multiple oblique shock inlet uses a series of very weak oblique shock waves to gradually slow the supersonic airflow before the normal shock occurs.

Variable Supersonic Inlet

• Variable Supersonic Inlets are equipped with actuator-operated panels that can be varied to suit different conditions.

Effects of Sweepback on Critical Mach Number

• Sweepback increases the critical Mach number.
• Wings are designed to carry weight and house fuel tanks, this is achieved by sweeping the wing.
• Increase strength.