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Powerplants and Propellers

Objective​

To understand the basics of a reciprocating engine and a propeller and how it produces thrust in the airplane.

Motivation​

It is important to have an intuitive understanding of how an engine operates to use the engine controls effectively and troubleshoot issues as they arise.

Format​

Timing​

1 hour

Elements​

Engines​

  • Most airplane engines are 4- or 6-cylinder, horizontally-opposed, air-cooled
  • 4-stroke engines are most common
  • 4 strokes of an engine
    • Intake
    • Compression
    • Combustion
    • Exhaust
  • Manifold pressure is the partial vacuum of the airplane "sucking in" air
  • Ignition is provided electrical, either from the electrical system or magnetos
    • Magneto are spinning magnets driven by the crank shaft of the engine which produce voltage anytime the engine is on
    • Two sets of spark plugs in each cylinder, one from each magneto
    • Magnetos can be selected independently, and will be checked before flight
  • Carburetors
    • Mechanical control of the fuel-air mixture
    • Venturi is a narrowing that accelerates the air, forcing fuel to flow out of the discharge nozzle
    • Mixture needle control mixture
    • Throttle valve controls flow of entire fuel/air mixture into the cylinders
    • Float-type carburetor: Float regulates the fuel entering the float chamber
    • Carburetor icing occurs due to the decrease in air pressure in the venturi
      • This further decreases the air temperature
      • Most likely to occur < 70° F and > 80% humidity
  • Fuel-injection
    • Uses a electronically/computer controlled unit to regulate fuel to air mixture
  • Oil
    • Wet-sump systems hold oil within the engine itself
      • An oil pump is used to circulate oil to the primary component
    • Dry-sump systems hold oil in a separate tank
  • Engine cooling
    • Air-cooling through baffles
    • Oil cooler at the front of the airplane
  • Fuel system
    • High-wing airplanes are usually gravity-fed
    • Often airplanes will have an electrical pump and an engine-driven pump
    • Fuel primer is used to draw fuel into the cylinders prior to starting
    • Fuel tanks
      • Wet wings vs wing tanks
    • Fuel selector valves
      • High-wing airplanes may have a "both" option
      • Low-wing airplanes usually do not, and they must be switched
    • Running a fuel tank dry may cause vapor lock and impurities to enter the engine
    • Fuel sumps and drains are usually present
      • At the lowest point on each tank
      • At the lowest point in the system
    • AVGAS grades
  • Turbochargers

Propellers​

  • Rotating airfoils
    • Subject to angle of attack, stalls
    • The propeller area near the hub is spinning slower than the tip
      • Because of this, propellers are "twisted" so the angle of attack varies to produce more consistent force across the propeller
  • Fixed-pitch propellers
    • Primary indication of power in the cockpit is RPM
    • Propellers are calibrated for a single speed
    • A climb prop has lower pitch, less drag: More RPM, horsepower, which increases takeoff and climb performance
    • A cruise prop has a higher pitch, more drag: Less RPM, but increases efficiency
    • Corollary to gears on a bike: Climb prop is a low gear, cruise prop is a high gear
  • Some airplanes have "adjustable" pitch propellers, which can be adjusted on the ground

Constant-Speed Propellers​

  • Constant-speed propellers vary the pitch of the propeller to maintain an given RPM
  • Effect of pitch on RPM
    • A "finer" or large pitch of a propeller increases RPM
    • A "coarser" or smaller pitch of the propeller decreases RPM
  • The propeller governor controls the pitch of the propeller to maintain a pilot-selected RPM
  • Operation
    • Engine oil is boosted into the propeller hub assembly
    • A spring is tensioned to hold the prop in the full fine or large pitch setting
    • Engine oil must work against this to coarsen the blade
      • The engine oil forces a piston which changes the angle of the blade
    • The amount of oil is governed by a set of fly-weights that are tuned to a specified RPM
      • If the prop spins faster, the flyweights move outward and more oil is forced into the blade angle piston
      • If the prop spins slower, the flyweights fall inward and less oil is allowed to enter the blade angle piston
  • When increasing power: Increase prop first, then engine power
  • When decreasing power: Decrease power first, then decrease power
  • Loss of oil pressure
    • Propeller should fail in the high pitch position
    • This may result in an overspeed condition if power is set too high

References​