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Load Factor, Va, Vortices

Objective​

To understand how load factor is induced on an airframe, and how to avoid an excessive amount. Also the effect of wingtip vortices on performance and their dangers when operating near other aircraft.

Motivation​

Gives students an intuitive understand of the causes that induce load factor on airframe and their corresponding dangers, so they can recognize them in flight. Also give the students a conceptualization of wing tip vortices, so they can better visualize vortices from their own airplane and others.

Timing​

45 minutes

Format​

Elements​

Load Factor​

  • Load factor is the acceleration of the airplane in the downward direction
    • The "positive" G's of the airplane
    • In straight-and-level flight it is one G
  • Increase in load factor puts stress on the airframe
  • Categories of airplanes have different load factor limits, as required by the FARs:
    • Normal category: 3.8 to -1.52 G's
    • Utility category (spins/aerobatic): 4.4 to 1.76G
      • Sometimes different weight or CG limits apply
    • Aerobatic: 6 to -3 G's
    • Load factor limit are often placarded in the airplanes (in new airplanes)
  • Gusts, turns, and climbs can all induce load on the airplane
  • Scenario where a sudden updraft hits an airplane
    • AoA suddenly increases, upward force is increased, load factor increases
    • The magnitude of this force is proportional to:
      • the increase in AoA, and
      • the speed the aircraft was traveling
  • Design maneuvering speed: Va
    • The speed at which the airplane will exceed the critical AoA (and stall) before than AoA can induce a destructive load factor
    • This is good for a full deflection of an single control surface
    • Flight in rough should be done at or below Va
  • VG diagram
    • Remember, force from lift increases with the square of the airspeed
    • The is why the accelerate stall line is curved
    • Where the accelerated stall line meets the 4.4g mark is maneuvering speed
  • Va and weight
    • For a given airspeed:
      • A lighter airplane will require less AoA in S&L flight
      • A heavier airplane will require more AoA in S&L flight
    • This means the heavier airplane is closer to the critical angle of attack, and has less room to increase

Wingtip Vortices​

  • Pressure difference between top and bottom of wing combine at the wingtip
  • This causes a rotation of air outward and upward from the bottom to the top of the wing
  • Vortexes are greatest when the airplane is: Heavy, slow, and clean

Wake Turbulence Avoidance​

  • When you fly into another airplanes vortices it's called wake turbulence
  • Visualize the vortices about 500' below the airplane and behind it
  • Vortices tend to stay on the ground and move with the wing
  • Light quartering tailwinds cause the greatest threat
  • Large aircraft departs from parallel runway: delay departure
  • Large aircraft departs from the same runway: wait at least 2 minutes
  • Departing behind a large aircraft
    • Note the aircraft's rotation point
    • Rotate prior and climb away from the flight path, turning into the wind if needed
    • ATC will provide a minimum of 3 minutes of delay when departing behind a large airplane
  • Keep at least 1000' below another large airplane in flight

References​