1.2 Fundamentals of Flight Mechanics

Flight mechanics explains the forces and principles that govern how aircraft fly. These concepts form the foundation of aerospace engineering, and you'll see them come up again and again in later units on aircraft design, stability, and propulsion.

Fundamentals of Flight Mechanics

Forces in aircraft flight

Four forces act on an aircraft in flight. In steady, level flight at constant speed, these forces are in balance: lift equals weight, and thrust equals drag. When any force becomes greater than its opposite, the aircraft accelerates in that direction.

• Lift
  • Upward force generated by the difference in air pressure above and below the wings
  • Depends on airspeed, angle of attack, wing area, and wing shape (airfoil)
  • Increases with higher airspeed and angle of attack, but only up to the critical angle of attack, beyond which the wing stalls and lift drops sharply
• Drag
  • Force acting opposite to the direction of motion, resisting forward movement
  • Has two main components: parasite drag (form drag + skin friction) and induced drag (a byproduct of generating lift)
  • Parasite drag increases with airspeed, while induced drag decreases with airspeed. This tradeoff means there's a specific speed where total drag is minimized
• Thrust
  • Forward force produced by the propulsion system (jet engines or propellers)
  • Must overcome drag to maintain or increase airspeed
  • The pilot adjusts thrust to control speed and climb or descent rate
• Weight
  • Downward force due to the aircraft's mass and gravity ($W = mg$)
  • Acts through the center of gravity (CG), which affects the aircraft's balance and stability
  • In steady, level flight, lift must equal weight

Principles of aerodynamics

Lift generation involves both pressure differences and the deflection of air. Two complementary explanations help describe how wings produce lift.

• Bernoulli's principle
  • As the speed of airflow increases, its static pressure decreases. The curved upper surface of a wing accelerates airflow, creating lower pressure on top compared to the higher pressure underneath.
  • This pressure difference across the wing produces an upward net force. Bernoulli's principle is one part of the lift story, but it works alongside Newton's third law (see below).
• Angle of attack (AoA)
  • The angle between the wing's chord line (a straight line from leading edge to trailing edge) and the direction of the oncoming airflow
  • Increasing AoA increases lift because the wing deflects more air downward. However, past the critical angle of attack (typically around 15–20° for most airfoils), airflow separates from the upper surface and the wing stalls, causing a sudden loss of lift.
  • Higher AoA also increases induced drag
• Airfoil shape
  • A typical airfoil has a curved upper surface (camber) and a flatter lower surface, which naturally creates a pressure difference even at small angles of attack
  • The design of the leading edge and trailing edge affects how smoothly air flows over the wing, influencing stall behavior and drag
• Boundary layer
  • A thin layer of air right next to the wing's surface where viscosity (air's internal friction) slows the flow down
  • Near the leading edge, this layer is usually laminar (smooth, orderly flow), but it transitions to turbulent (chaotic, mixed flow) as it moves toward the trailing edge
  • Turbulent flow creates more skin friction drag, but it also carries more energy, which helps the airflow stay attached to the wing longer and delays stall

Newton's laws in aviation

Newton's three laws of motion apply directly to how aircraft move and how forces interact during flight.

1. First law (inertia): An aircraft in motion stays in motion at constant velocity unless a net external force acts on it. This is why an aircraft in level, unaccelerated flight doesn't need increasing thrust; it just needs enough thrust to balance drag.

2. Second law ($F = ma$): The net force on an aircraft equals its mass times its acceleration. If thrust exceeds drag, the aircraft accelerates forward. If lift exceeds weight, the aircraft accelerates upward. The larger the aircraft's mass, the more force is needed to change its velocity.

3. Third law (action-reaction): For every action, there is an equal and opposite reaction. A wing generates lift by pushing air downward; in return, the air pushes the wing upward. Similarly, a jet engine accelerates exhaust gases backward, and the reaction force pushes the aircraft forward.

Flight Performance Factors

Airspeed, altitude, and density relationships

Aircraft performance changes significantly with altitude, mostly because air density drops as you go higher. Understanding the relationship between density, airspeed, and altitude is essential for predicting how an aircraft will perform.

• Air density
  • Decreases with increasing altitude because both air pressure and temperature drop
  • Lower density reduces lift (less air for the wing to work with), reduces drag, and reduces engine thrust (less oxygen for combustion)
  • Density altitude is the altitude in the standard atmosphere that corresponds to the actual air density you're experiencing. On a hot day at a high-elevation airport, density altitude can be much higher than the field elevation, which means longer takeoff rolls and reduced climb performance.
• True airspeed (TAS)
  • The actual speed of the aircraft relative to the surrounding air
  • For a given indicated airspeed, TAS increases with altitude because the thinner air requires the aircraft to move faster to produce the same dynamic pressure
  • Used for navigation and fuel planning
• Indicated airspeed (IAS)
  • The speed displayed on the cockpit airspeed indicator, based on dynamic pressure measured by the pitot-static system
  • Pilots rely on IAS for critical speed references like stall speed and never-exceed speed, because aerodynamic forces depend on dynamic pressure, not true airspeed
• Mach number
  • The ratio of the aircraft's speed to the local speed of sound: $M = \frac{V}{a}$, where $V$ is the aircraft's speed and $a$ is the speed of sound at that altitude
  • The critical Mach number is the flight Mach number at which airflow over some part of the aircraft first reaches the speed of sound. Beyond this point, shock waves form, drag rises sharply, and the aircraft enters the transonic regime.
  • At high Mach numbers, aircraft can experience changes in stability, control difficulties, and increased structural loads