EASA Module 8: Basic Aerodynamics and Atmosphere

• EASA Module 8 covers basic aerodynamics, including the atmosphere, airflow, aerofoils, drag, high lift devices, and propellers.

Atmosphere

• The atmosphere is a mixture of gases surrounding the Earth, primarily nitrogen and oxygen.
• Atmospheric pressure decreases with altitude. At sea level, standard pressure is 1013.25 hPa or 29.92 inches of mercury (in Hg).
• Temperature generally decreases with altitude in the troposphere (the lowest layer), at a rate of approximately 2°C per 1000 feet.
• Density also decreases with altitude. Air density is a critical factor in aircraft performance.
• Humidity is the amount of water vapor in the air. Higher humidity decreases air density.
• The International Standard Atmosphere (ISA) defines standard sea level conditions as 15°C and 1013.25 hPa.
• Aircraft performance charts are often based on ISA conditions, and corrections must be made for deviations.

Airflow

• Airflow around an aircraft is governed by Bernoulli's principle and Newton's laws of motion.
• Bernoulli's principle states that as the speed of a fluid (air) increases, the pressure decreases.
• Continuity equation explains that in a closed system, mass flow rate remains constant; thus, if the area decreases, the velocity must increase.
• Streamlines represent the path of air particles in a moving fluid.
• Laminar flow is smooth, orderly airflow and produces less drag.
• Turbulent flow is irregular and chaotic airflow, increasing drag.
• The boundary layer is the thin layer of air close to the surface of the aircraft.
• The boundary layer can be laminar or turbulent.
• Adverse pressure gradient happens when the pressure increases in the direction of the flow, leading to boundary layer separation.

Aerofoils

• An aerofoil is a streamlined shape designed to produce lift when moving through the air.
• The leading edge is the front of the aerofoil.
• The trailing edge is the rear of the aerofoil.
• The chord line is an imaginary straight line from the leading edge to the trailing edge.
• The camber is the curvature of the aerofoil's upper and lower surfaces.
• The mean camber line is the line equidistant from the upper and lower surfaces.
• Angle of attack (AOA) is the angle between the chord line and the relative airflow.
• Lift is the force acting perpendicular to the relative airflow.
• Lift is primarily generated by the pressure difference between the upper and lower surfaces of the aerofoil; Lower pressure above, higher pressure below.
• The stagnation point is where the airflow divides and comes to rest on the leading edge.
• The coefficient of lift (Cl) is a dimensionless quantity that represents the lift-generating capability of an aerofoil.
• Stalling occurs when the angle of attack is too high, causing the airflow to separate from the upper surface and resulting in a loss of lift.
• The critical angle of attack is the angle at which the stall occurs.
• The stall speed is the minimum speed at which an aircraft can maintain lift.

Drag

• Drag is the force that opposes the motion of an aircraft through the air.
• Parasite drag is the sum of form drag, skin friction drag, and interference drag.
• Form drag is caused by the shape of the object obstructing airflow.
• Skin friction drag is caused by the friction of the air against the surface of the aircraft.
• Interference drag is caused by the intersection of airflow streams around different parts of the aircraft.
• Induced drag is caused by the production of lift.
• Vortices are formed at the wingtips as air spills from the high-pressure area below the wing to the low-pressure area above the wing.
• Wingtip vortices create downwash, which increases induced drag.
• Total drag is the sum of parasite drag and induced drag.
• The drag curve shows the relationship between airspeed and drag.
• L/D ratio is the ratio of lift to drag.
• The maximum L/D ratio represents the most efficient angle of attack, minimizing drag for a given lift.

High Lift Devices

• High lift devices are used to increase lift at lower speeds, such as during takeoff and landing.
• Flaps are hinged surfaces on the trailing edge of the wing that increase the wing's camber and/or area.
• Slats are movable surfaces on the leading edge of the wing that create a slot, allowing high-energy air to flow over the wing.
• Slots are fixed openings in the leading edge of the wing that allow high-energy air to flow over the wing.
• Spoilers are surfaces on the upper surface of the wing that disrupt airflow and reduce lift.
• Leading edge flaps are similar to slats, increasing the camber and providing a slot effect.
• Boundary layer control systems, such as blowing or suction, can be used to delay boundary layer separation and increase lift.

Propellers

• A propeller is a rotating airfoil that generates thrust.
• Propeller blades are twisted to maintain a relatively constant angle of attack along their length.
• Blade angle is the angle between the chord line of the propeller blade and the plane of rotation.
• Propeller pitch is the theoretical distance a propeller advances in one revolution.
• Geometric pitch is the theoretical distance.
• Effective pitch is the actual distance the propeller advances.
• Propeller slip is the difference between geometric pitch and effective pitch.
• Thrust is the force produced by the propeller, pushing the aircraft forward.
• Torque is the rotational force applied to the propeller shaft.
• Propeller efficiency is the ratio of thrust power to brake horsepower.
• Fixed-pitch propellers have a constant blade angle.
• Constant-speed propellers automatically adjust the blade angle to maintain a constant engine RPM.
• Feathering propellers can be rotated to align with the airflow, minimizing drag when the engine is not operating.
• Reverse pitch propellers can be used to create reverse thrust for braking.
• Propeller blade angle is usually measured in degrees, while propeller pitch is measured in inches.

EASA Module 8 covers fundamental aerodynamics, focusing on the atmosphere and airflow. Key topics include atmospheric pressure, temperature, density, and humidity. Understanding the International Standard Atmosphere (ISA) and its impact on aircraft performance is crucial.