Boundary Layer and Stall in Aerodynamics

Boundary Layer

• A thin layer of air at the surface of an airfoil where velocities are reduced due to skin friction.
• Velocity changes from zero at the surface to the free stream value away from the surface.
• Air can flow smoothly (laminar) or in a random pattern (turbulent) depending on air viscosity and surface roughness.

Stall

• Occurs when smooth airflow over the wing's surface breaks away and becomes turbulent.
• Associated with high angle of attack flight conditions or shock waves that form when aircraft speed approaches its critical Mach number.
• Shock waves cause air separation at the upper surface of the wing, leading to buffeting of the aircraft's controls.

Vortex Generators

• Short, low aspect ratio aerofoils arranged in pairs and extended into and above the boundary layer.
• Attached to the aerofoil surface by adhesive, they induce high-energy air from the free stream airflow into the boundary layer.
• Energize the air to prevent air separation, and can be fitted in various ways and locations.

Stall Strips

• Inverted triangles used to break the airflow over the root section to create stall at high angle of attack.
• Fitted on the leading edge at the wing root, ensuring wing root stall before the wing tip at high angle of attack.
• Turbulence created may strike the tailplane, resulting in vibration warning to the pilot of an approaching stall.

Riblet Films

• Microgrooves with V-shaped cross-sections that reduce skin friction and prolong laminar flow over the wing.
• Force turbulent air through the narrow passage of the grooves, increasing air velocity to flow as laminar airflow.

Wing Fences

• Fitted about mid-span on the leading edge of the wing and extending rearward.
• Designed to control the spanwise flow of the boundary layer over the top of the wing.
• Straighten the airflow over the ailerons, improving their effectiveness, and prevent air spillage from beneath the wing to the top.

Saw Tooth or Notch Leading Edge

• A small increase in wing chord on the outer portion of the wing.
• Forms an 'invisible wall' of high-velocity air that flows over the wing and straightens the spanwise flow.
• Similar in function to the wing fence but removes the extra drag and weight penalty.

Winglets

• Give better airflow control and reduce drag produced by the wing by using the up-flow from below the wing to produce a forward thrust.
• Work best at higher speeds, but add weight to the aircraft and increase parasitic or profile drag.
• Compensated by the large reduction in induced drag at the wingtip, resulting in a significant fuel saving.

Primary Flight Control

Elevator

• Located on the tail section of an aircraft, controlling pitch (up and down movement)
• Moves the aircraft's nose up or down to change altitude
• Controlled by the control yoke or stick

Rudder

• Located on the tail section of an aircraft, controlling yaw (left and right movement)
• Moves the aircraft's nose left or right to change direction
• Controlled by the rudder pedals

Control Surfaces

• Consist of ailerons, elevators, and rudder
• Create aerodynamic forces by moving parts on the aircraft
• Control aircraft's orientation and direction
• Shaped to produce a difference in air pressure above and below the surface

Flight Envelope

• Defines the range of speeds and altitudes within which an aircraft can operate safely
• Determined by the aircraft's design and performance characteristics
• Includes limits on speed, altitude, and maneuverability
• Essential to operate within the flight envelope to avoid loss of control or structural damage

Aileron

• Located on the wings of an aircraft, controlling roll (banking left and right)
• Moves the aircraft's wings up or down to change direction
• Controlled by the control yoke or stick
• Used to maintain balance and stability during turns