Aerodynamics & Performance

Introduction:

  • Aerodynamics is the branch of dynamics dealing with the motion of air and other gases, which gives us the performance we need to fly
  • It is essential to understand basic aerodynamic concepts to understand the operation of the components and subcomponents of an aircraft
  • It can be associated with the forces acting on an object in motion through the air or with an object that is stationary in a current of air
  • Several factors affect aircraft performance, including the atmosphere, aerodynamics, and aircraft icing
  • Pilots need an understanding of these factors for a sound basis for prediction of aircraft response to control inputs, which come in the form of performance

Lift and Basic Aerodynamics:

  • The Four Forces
    Pilot's Handbook of Aeronautical Knowledge,
    The Four Forces
  • Lift/Drag Performance Curve
    Drag vs. Speed
  • Aircraft Axis
    Axis of an Airplane
  • Aircraft Components and Structure
  • Four forces act upon an aircraft, making up what we call the Principles of Flight
  • Understanding how these forces are created and, more importantly, impact each other allows pilots to know how they are manipulated to control an aircraft in flight
  • These principle forces are thrust, drag, weight, and lift: [Figure 1]

    • Thrust:

      • Thrust is the forward force produced by the powerplant/propeller
      • It opposes or overcomes the force of drag

    • Drag:

      • Drag is a rearward, retarding force and is caused by disruption of airflow by the wing, fuselage, and other protruding objects
      • Drag opposes thrust and acts rearward parallel to the relative wind

    • Weight:

      • Weight is the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage
      • Weight pulls the aircraft downward because of the force of gravity

    • Lift:

      • Lift opposes the downward force of weight, is produced by the dynamic effect of the air acting on the wing, and acts perpendicular to the flight path through the wing's center of lift (CL)
    • The Four Forces
      Pilot's Handbook of Aeronautical Knowledge,
      The Four Forces
  • All aircraft are designed with different handling characteristics in mind, which determine aircraft stability
  • An aircraft moves in three dimensions and is controlled by moving it about one or more of its axes:
    • The longitudinal, or roll, axis extends through the aircraft from nose to tail, with the line passing through the CG
    • The lateral or pitch axis extends across the aircraft on a line through the wing tips, again passing through the CG
    • The vertical, or yaw, axis passes through the aircraft vertically, intersecting the CG
    • All control movements cause the aircraft to move around one or more of these axes and allow for the control of the aircraft in flight [Figure 3]
  • Aircraft performance designs are dependent upon operating within weight and balance limitations
  • NASA - Aerodynamics Index
  • Lift/Drag Performance Curve
    Drag vs. Speed
  • Aircraft Axis
    Axis of an Airplane

Aircraft Performance:

Performance Calculations Versus Reality:

  • Performance is calculated with charts whcih are based on expected performance under a set of conditions
    • Said differently, POH numbers require POH technique
  • The conditions present at time of calculating will always differ from the charts and graphs to a degree, sometimes a large degree
    • Factors to consider include mechanical, environmental, skill, geographic location or altitude, etc.
  • It is important to use considervative numbers
    • Consider always rounding up, not interpolating but using the higher number, etc.
    • Realize obstacles like trees may not be 50' and therefore charts wholly inadequate to calculate by the numbers
  • Consider adding buffers of 50-100% to anything calculated, and to monitor performance while in flight
    • If numbers were overly conservative, they can be adjusted for subsequent flights

Conclusion:


References: