Aircraft Performance and Atmospheric Conditions

Physics of the Atmosphere

• Learning objective is to interpret ISA conditions for temperature, pressure, humidity and density at sea level, and identify basic changes in atmosphere with altitude (Level 2).
• The atmosphere is a layer or set of layers of gases surrounding a planet, where weather and climatic conditions are generated.
• The atmosphere is composed of 78% nitrogen, 21% oxygen, and 1% other gases like carbon dioxide.
• Physical properties of gases like pressure, density, and temperature vary within the atmosphere, affecting aircraft performance.
• Changes in atmospheric densities affect aircraft performance and forces like lift, drag, and engine power, due to atmospheric pressure, temperature, and humidity variations.

Atmospheric Regions

• The atmosphere is classified into regions based on temperature variation with altitude: troposphere, tropopause, stratosphere, mesosphere, and thermosphere (ionosphere).
• Aircraft fly in the troposphere and the lowest part of the stratosphere.
• The troposphere is where we live and most aircraft fly, extending from the surface to the tropopause.
• The troposphere contains water vapor, which causes clouds and weather
• Within the troposphere, the temperature drops approximately 2°C for each 1000 ft increase in altitude, known as the lapse rate.
• The tropopause is the point in the atmosphere where the temperature is consistent regardless of altitude.
• The tropopause temperature measures around -57 °C.
• The tropopause occurs at approximately 20,000 ft over the poles and 60,000 ft above the equator.
• International Standard Atmosphere (ISA) assumes the average height of the tropopause is 36,000 ft.
• The stratosphere is the atmospheric layer extending above the tropopause, with no water vapor or weather.

Atmospheric Conditions

• The weight of air above any surface causes pressure on that surface.
• Average pressure at sea level is 14.7 PSI, 29.92 inches of mercury (Hg), 760 mm of Hg, or 1013.25 millibars/hecto Pascal.
• Mercury barometers measure atmospheric pressure using an upside-down tube filled with mercury in a mercury-filled vessel.
• At sea level, the height of the mercury in the tube measures standard day condition at 29.92 in. Hg or 1013.25 mbar.
• Density is mass per unit of volume and a property of air which makes all flight possible.
• Lower air density makes flight more difficult.
• Air at high altitudes (low pressure) is less dense than at low altitudes (high pressure).
• The higher we ascend in the atmosphere, the lower the weight of the atmosphere above us, therefore the pressure will decrease.
• Density increases as pressure increases and temperature decreases.

Air Density

• Three factors affecting air density are pressure, temperature and altitude.
• As atmospheric pressure decreases, air density decreases.
• As temperature increases, density decreases due to the volume of air expanding.
• As altitude increases, air temperature and pressure decrease.
• The decrease in air pressure has a greater effect on air density than the decrease in temperature.
• Air becomes less dense with increasing altitude.
• There is a gradual decrease in temperature when ascending in the atmosphere; in the troposphere, temp drops at the lapse rate of 2 °C for every 1000 feet increase in altitude.
• The rate of temperature decrease does not alter until about 36 000 ft where Tropopause is reached
• Standard air viscosity is important in aerodynamics because air tends to stick to any surface over which it flows, slowing down the motion of the air.

Humidity

• Humidity is the condition of moisture or dampness due to the amount of water vapour present in the air, depending on temperature.
• With small water vapor proportion, the air is said to be dry; with significant proportion, the air is described as humid
• The higher the temperature of air affects the more water vapor it can absorb.
• On a humid day, air is less dense due to water vapour displacing some of the dry air; water vapour weighs approximately 5/8ths as much as an equal volume of perfectly dry air.
• Absolute humidity refers to the actual amount of water vapour in a mixture of air and water; the higher the air temperature, the more water vapour the air can hold.
• Relative humidity relates to the the amount of moisture in the air to the amount that would be present if the air were saturated.
• Relative humidity has a dramatic effect on aircraft performance because of its effect on air density.
• Note: Air is most dense when it is perfectly dry.
• For practical application in aviation, temperature and dew point are used more often than relative humidity to measure the amount of water vapor in the air.
• Dew point is the temperature to which the air must be lowered to become saturated and condense water vapour out to liquid water.

International Standard Atmosphere

• (ICAO) administers the International Standard Atmosphere (ISA).
• Varying atmospheric conditions cause significant changes in an aircrafts performance.
• Air temperature, pressure, and density vary from place to place and day to day leading to the need to develop a standard set of conditions aircraft performance can be measured against & compared to via graphs/charts.
• At mean sea level, Temperature = 15°C, Pressure = 1013.25 hPa (mb), and Density = 1.225 kg/m.
• From MSL to 11 km, temperature decreases by 1.98°C per 1000 ft.
• From 11 km to 20 km, temperature remains constant at -56.5°C.
• From 20 km to 32 km, temperature rises by 0.3°C per 1000 ft.
• Values in the ISA Reference Values table are referred to as ISA Standard Day.
• Lapse rate and Sea level temp are used to calculate air density from pressure and temperature. -Lapse rate: 2 °C/1000 feet. -Tropopause height: 36,000 feet. -Sea level pressure: 1013.25hPa = 29.92 in Hg = 14.7 psi. -Sea level temperature: 15 °C. -Gravity (g): 32.174 ft/sec² = 9.81 m/s².
• Pressure altitude is indicated altitude when an altimeter is set to 29.92 in. Hg (1013 hPa) and is used in aircraft performance calculations and high-altitude flight.
• High Density Altitude means Decreased Performance.
• Density altitude, an indicator of aircraft performance, the published performance criteria (POH) generally is based on standard atmospheric conditions at sea level.
• An aircraft will not perform according to book numbers unless conditions are the same as used to develop published performance criteria.

Aerodynamics

• Describe airflow characteristics as air flows around various shapes (Level 2).
• Explain the meaning of the terms laminar flow, turbulent flow, boundary layer, free stream flow and stagnation as it relates to airflow (Level 2).
• Explain relative airflow, up wash, downwash, vortices and how vortices are formed (Level 2).
• Explain the term camber and calculate the mean camber line on a given aerofoil (Level 2).
• Explain the term chord and identify a chord line on a given aerofoil (Level 2).
• Explain the terms fineness ratio, angle of attack and centre of pressure (Level 2).
• Explain the term resultant force with respect to lift (Level 2).
• The movement of air relative to the aircraft (or aerofoil) is relative airflow.
• If a wing moves forward horizontally, the relative airflow moves backward horizontally, parallel to and opposite the aircraft flight path but does not depend on aircraft’s flight attitude or direction and speed of the wind.
• Upwash is an area in front of the leading edge of an aerofoil where the airflow tends to move upwards.
• Downwash is an area behind the trailing edge of an aerofoil where the airflow tends to move downwards.
• Eddies or vortices that rotate clockwise (viewed from the rer) from left wing and anticlockwise from the right wing, are caused by the airflow over the top surface of a wing meets with that of the airflow over lowe surface to the trailing edge.
• Vortices on one side tend to join up and form one large vortex at each wing tip, called wing-tip vortices occurring continuously while an aeroplane is flying.
• Wake turbulence involves disturbance in the atmosphere forms behind an aircraft when it passes through the air.
• Viscosity of air is important in aerodynamics because air tends to stick to any surface over which it flows, slowing down the motion of the air.

Terms and Principles Related to Airflow

• Technique used in wind tunnels to introduce smoke in front of the aerofoil that is being tested, spreading out into parallel lines spreading lines called streamlines that visualize the airflow over the aerofoil.
• Laminar flow is the lines continuing smoothly over and past the airflow, creating very little drag and the the aerofoil,
• Chaotic and turbulent streamlines indicate the aerofoil is creating more drag.
• Lines which show the direction of the flow are called streamlinesand a body shaped to produce the least possible resistance is called a streamline shape.
• Boundary Layer relates to the very thin layer of air lying over the surface of the wing (and every surface of the aeroplane) beginning at the stagnation area, and it will originate at the leading edge where the air is brought to rest.
• Air viscosity causes this air layer tends to adhere to the wing, varying from aero in the surface of the aerofoil to the velocity of the free stream at the edge of the boundary layer
• The transition point is point on the wing at which the boundary layer changes from laminar to turbulent flow (moving forward as speed increases), skin friction also increases
• Separation points are the points on the wing at which the boundary layers break away from surface.
• Bernoulli's principle helps explain that an aircraft can achieve lift shape of its wings.
• A practical application of Bernoulli's principle is the venturi tube with air inlet a throat (constricted point) and an oulet increases in diameter toward the rear.
• Air flow over the top of the wing is faster while air flow underneath is slower.
• Fast-moving air exhibits low air pressure, while slow-moving air exhibits high air pressure.
• The higher pressure underneath the wings pushes the aircraft up through the lower air pressure.

Aerofoil

• Is the term used to describe the characteristic shape of the cross-section of an aircraft wing whose purpose is to generate lift.
• The chord of the is a straight line joining the leading edge to the trailing edge and is used as an arbitrary reference line when measuring the angular position of the wing in relation to the airflow.
• Camber is curvature of aerofoil section from leading/trailing and the amount of camber is expressed as the ratio of the maximum departure of the curve to chord length.
• Upper camber refers to the curve on the upper surface of an aerofoil; lower camber refers to the curve of the lower surface.
• Mean camber is a line that forms the equal distance between the upper and lower surface. Camber is positive/negative when the departure is upwards from the chordline or downward.
• When upper and lower cambers of an aerofoil are the same, it is said to be symmetrical.
• Maximum camber is the maximum distance between the chord line and the mean camber line.
• Fineness ratio measures aerofoil thickness as the ratio of length to breadth.
• Angle of attack is is the angle between the aerofoilis chord line and free-stream flow. It should be increased to boost lift, but if it's above stalling angle of attack lift will very rapidly drop to zero again and the aerofoil is said to have stalled.
• Angle of incidence is the acute angle which the wing cord makes with the longitudinal axis of the aircraft where the is attached is fixed in manufacture and does not change.
• Symmetrical Aerofoils have the same shape on both sides of its centre line while nonsymmetrical dont.
• The total sum pressure differences between the top and bottom surfaces produces reaction which acts at point called the Centre of Pressure.
• In most aerofoils, CoP position is generally located at the 25% chord.

Aerodynamics II

• Induced drag and Parasite drag
• Common wing shapes and their use
• Terms aspect ratio, wash in and wash out
• The terms thrust and weight
• The role of wing angle of attack, wing shape and the lift coefficient in the generation of lift
• The role of wing shape and drag coefficient in the generation of drag
• The aerodynamic polar curve and how it may be used
• Aerodynamic stall and the resultant lift immediately before/after the point of stall Aerofoil contamination aerodynamic effects (ice, snow, frost)
• Drag is caused by aircraft surface that deflects, interferes with smooth airflow around the aeroplane.
• You increase airspeed and angle of attack, you increase drag/lift, then drag works in flight opposition, opposes forward (thrust) reduces plane speed.
• Induced drag: Drag due to lift.
• Parasite drag: Drag due to air viscosity.
• Induced drag is caused by wings lifting related to the wing’s angle.
• With higher the angle, induced drag raises.
• Air over wing flows inward: Decreased pressure over surface is less than the wingtip’s outside pressure.
• Form drag involves result from aerodynamic resistance/motion (shape of plane)
• Skin friction drag is due to the friction of aircraft surface even if smooth or microscopic.
• Interference drag: from varied-currents, air to meet/interact over an aeroplane.
• Airflow above and lowers from wing combines/has vortices. (from rear, left-wing side vortexes clockwise, from the right-wing, vortexes anti-clockwise).
• A stall is caused by separation of airflow (from wingis upper surface.) = rapid reduction in lift. Stall starts when max amount lift has beend eveloped)
• Amount of lift depends on density, air speed round air, plane is lift-surface shape top side curved air speeds up, air pressure becomes less.
• More lifting angle, more more push power for wing-top (less top-air.)
• At constant speed and level flight, lift equals weight/thrust equals drag (aerodynamic forces).
• The wingis cross-section is Aerofoil (wing design divides air lower high-pressure, higher upper low-pressure area.s)

Lift Forces and More

• Lift is reduced by the wing and can increase due to temperature, aspect ratio etc
• Wing area, aerofoil shape, air relativespeed to surfaces, is relative angle (affect lift).
• Total Drag Force D = Induced, Form, Skin Friction, Interference.
• Lift-Coefficient (CL) measures lifts relative to dynamic pressure, this in turn is affected by the angle of attack and has a formula that shows LIFT = CL * (½ * p * V2 * s), this formula can be manipulated to determine Drag Coefficient.
• Aerodynamics curve graph, is the curve that occurs when an aircraft is about a 4º angle of attac and that ratio is known as the optimum lift-to-drag (L/D) ratio.

Aircraft Components & Design

• With aircraft the wing is important creates lift due to shape.
• Wings are at different speeds to maintain angle of attack.
• Weight reduces force for downward gravity (acts through plane gravity-center)
• Thrust is forward, powers planes with jet engine.
• Aircraft wings have wing area, straight, leading edge, etc and the designer determines the wing shape, wingspan + Wing Area,
• Wing shape variations are Rectangular (cheapest) Elliptical (efficient), Tapered/ Sweepback at very high speed
• High-lift devices will increase the lift of a wing as well as reduce the speed of stalling via flaps (hinged and movable at high speed)
• High-lift leading-edge are called slats can high (operate take-off to climb).

Wing Geometry & Design Parameters

• Aspect ratio of a wing dividing the square of the span by the wing area.
• Chord is straight from a leading/trailing edge, a wing's span is wing-tip to wing-tip and wing-area refers to the projected area.
• Mean Aerodynamic Chord (MAC) the chord (drawn center).
• An increase incidence increases with was-ins. Washes has decrease one wing (causes smaller incidence).

Load Factor

• Load factor is the ratio of exerted structure-to-structure weight
• Aircraft has an accelerometer (measures inflight factor.)
• Level-flight, load exerted (airframe) = aeroplane 1 g.
• Load limit stress with air limit increased aircraft with stall reaching lower airspeed.
• Plane stall turn during due increased factor is safety above stall that narrow margin.
• Load factor affect operational, structural, takeoff, payload
• Anhedral rigging each horizontal and is really one geometric, has angles inclinded. When rigging plane, for rigging, rigging use each horizontal.

Basic Aerodynamics of Flight (Theory of Flight)

• Learning objectives include:
  • Relationship between thrust, weigh, lift, drag, oppositions impact (Level 2)
  • Glide ratio and how achievement occurs with best lift/drag (Level 2)
  • Force relationships if straigh/level/climb/descend, constant (Level 2)
  • The state in steady/flight (Level 2).
  • Results of too force, imbalance coordination (Level 2)
  • How describe relationship with load/wing, aircraft-stall speed(Level 2)
  • Describes loads, impact aircraft (min speeds to limits) (Level 2).
  • The way load influences (limits operation to weight/forces) (Level 2).
  • Lift (augmentation) with description of methods.
  • Theory of Flight is the key for aerodynamic lift (opposes weight). Weight/lift (equal) to straight/level while lift/drag (contribute).
• Forces act in a climb with elevators to Angle (AoA) / Lift
• Air is added to the plane and weight is reduced when descend is initiated, elevator reduces
• Equilibrium when lift is equal to weight and drag is with the weight with a descent angle relative to all wind, gliding.
• Glide is ratio is the distance (related) to (height) that descending the angle trigonometry to compute is tangent with height with distance.

Static Stability

• Involves initial tendency of objects equilibrium.
• Three parts are Positive, Negative, Neutral with an Original/Attitude.

Flight Stability and Dynamics

• The aircraft can rotate around each axis, common reference = center to control the the rotation (axes.)
• Aeroplane rotations
• roll:* is on surface - rotation aileron - longitudinal, lateral- stability Aileron roll, with plane
• Pitch:* is on elevators with surface stabilizer w - lateral- longitudinal is the stability on elevator Pitch
• Yaw* is on fin on yaw surface the - vertical - a directional stability:
• Lateral on Axis to plane - known lateral/rolling tendencies is ability return this roll known stability. With sidesplits on the ability to the lateral is with to sideslip

Oscillatory Instability

• Characterizes to motion with movement combined
• Known to name if motion predominates or snaking motion ( yawing )
• The larger fin/greater dihedral create large tendency for (fin-rudde).
• Pendulum aircraft, lift-center with plane, this high, action has effect when drops when weight

Explore how atmospheric gases affect aircraft performance and the typical flight altitudes. Understand the significance of the troposphere for aviation and temperature changes with altitude. Also covers aircraft maintenance, humidity effects, angle of attack, and center of gravity impacts on flight.