Flight mechanics is all about understanding the forces that keep planes in the air. Lift, drag, thrust, and weight work together to make flight possible. Mastering these concepts is crucial for any aspiring aerospace engineer.

Aerodynamics principles like Bernoulli's principle and angle of attack explain how wings generate lift. Newton's laws apply to aviation too, helping us understand how planes move and respond to forces in flight.

Fundamentals of Flight Mechanics

Forces in aircraft flight

• Lift
  • Upward force generated by the difference in air pressure above and below the wings
  • Depends on factors such as airspeed, angle of attack, and wing shape (airfoil)
  • Increases with higher airspeed and angle of attack (up to the critical angle)
• Drag
  • Force acting opposite to the direction of motion, resisting the aircraft's forward movement
  • Consists of parasite drag (form drag and skin friction) and induced drag (due to lift generation)
  • Parasite drag increases with airspeed, while induced drag decreases with airspeed
• Thrust
  • Forward force generated by the aircraft's propulsion system (jet engines or propellers)
  • Overcomes drag to maintain or increase airspeed
  • Can be adjusted by the pilot to control the aircraft's speed and climb/descent rate
• Weight
  • Downward force due to the aircraft's mass and gravity
  • Acts through the center of gravity (CG)
  • Balances lift in steady, level flight

Principles of aerodynamics

• Bernoulli's principle
  • As airspeed increases, pressure decreases, creating a pressure difference between the upper and lower surfaces of the wing
  • Faster-moving air above the wing generates lower pressure compared to slower-moving air below the wing
• Angle of attack (AoA)
  • The angle between the wing's chord line and the oncoming airflow
  • Increasing AoA increases lift up to the critical angle, beyond which stall occurs (airflow separates from the wing)
  • Higher AoA also increases induced drag
• Airfoil shape
  • Curved upper surface and flatter lower surface creates a pressure difference, generating lift
  • Leading edge (front) and trailing edge (rear) design affects stall characteristics and drag
• Boundary layer
  • Thin layer of air near the wing's surface affected by viscosity
  • Laminar flow (smooth) transitions to turbulent flow (chaotic) as air moves along the wing
  • Turbulent flow increases skin friction drag but helps prevent flow separation (stall)

Newton's laws in aviation

1. First law (inertia)

   • An aircraft at rest stays at rest, and an aircraft in motion stays in motion with constant velocity, unless acted upon by an external force (thrust, drag, lift, or weight)
   • Tendency to resist changes in motion

2. Second law ($F = ma$)

   • The net force on an aircraft equals its mass times its acceleration
   • Applies to aircraft motion, considering the four forces acting on them (thrust, drag, lift, and weight)
   • Determines the aircraft's acceleration and velocity changes

3. Third law (action-reaction)

   • For every action, there is an equal and opposite reaction
   • Evident in the generation of lift (wing pushes air down, air pushes wing up) and thrust (engine pushes air back, air pushes engine forward)
   • Helps explain the forces acting on an aircraft during flight

Flight Performance Factors

Airspeed, altitude and density relationships

• Air density
  • Decreases with increasing altitude due to lower air pressure and temperature
  • Affects lift generation (lower density reduces lift), drag (lower density reduces drag), and engine performance (lower density reduces thrust)
  • Density altitude is the altitude in the standard atmosphere corresponding to a particular air density, affecting aircraft performance (takeoff, climb, and landing)
• True airspeed (TAS)
  • The actual speed of the aircraft relative to the air, increases with altitude for a given indicated airspeed (IAS) due to lower air density
  • Important for navigation, fuel planning, and aircraft performance calculations
• Indicated airspeed (IAS)
  • The speed shown on the airspeed indicator, depends on the dynamic pressure sensed by the pitot-static system
  • Used for monitoring aircraft speed limits (never exceed speed, stall speed) and maneuvers
• Mach number
  • The ratio of the aircraft's speed to the local speed of sound, important for high-speed flight
  • Critical Mach number is the point at which local airflow reaches the speed of sound, causing shock waves and increased drag (transonic and supersonic flight)
  • Mach number affects aircraft stability, control, and structural integrity at high speeds