Propeller System

Forces Acting on the Propeller:

• Centrifugal Force: the outward force experienced when the propeller spins
• Thrust Bending Force: the forward force experienced by the propeller, causing the propeller to bend forward
• Torque Bending Force: the force experienced by the propeller due to rotation, causing the blades to bend opposite direction of rotation
• Aerodynamic Twisting Moment: twisting moment which attempts to increase blade angle from the center of pressure
• Centrifugal Twisting Moment: twisting moment which attempts to decrease the blade angle
• Vibration: harmonics aerodynamic forces

Constant-speed propeller controls:

1. Throttle Lever:

   • The throttle enables the pilot to control engine power output (registered on the manifold pressure gauge)
     • Advancing the throttle results in the engine spinning faster
     • Retarding the throttle results in the engine spinning slower
   • The engine crankshaft is attached to and therefore spins the propeller
   • A governor must then act as the transmission, adjusting the propeller's pitch to maintain engine RPM
     • If the engine spins faster, the governor increases propeller pitch
     • If the engine spins slower, the governor decreases propeller pitch
   • For example, after setting the desired RPM during cruising flight, an increase in airspeed or decrease in propeller load will cause the propeller blade angle to increase as necessary to maintain the selected RPM

2. Propeller Lever:

   • Propeller lever provides manual control propeller pitch (registered on the tachometer)
     • Advancing the propeller lever results in a lower propeller pitch
     • Retarding the propeller lever results in a higher propeller pitch
   • This change in pitch adjust the desired engine RPM
     • If the propeller pitch decreases, the engine RPM increases
     • If the propeller pitch increases, the engine RPM decreases
   • Once set, the governor automatically adjusts the propeller blade angle as necessary to maintain the selected RPM
   • This registers as pressure on the manifold
   • The gauge measures the absolute pressure of the fuel/air mixture inside the intake manifold and is more correctly a measure of Manifold Absolute Pressure (MAP)

Propeller Governor Operation:

• When an engine is running at constant speed, the torque (power) exerted by the engine at the propeller shaft must equal the opposing load provided by the resistance of the air
• The r.p.m. is controlled by regulating the torque absorbed by the propeller-in other words by increasing or decreasing the resistance offered by the air to the propeller
• In the case of a fixed-pitch propeller, the torque absorbed by the propeller is a function of speed, or r.p.m.
• If the power output of the engine is changed, the engine will accelerate or decelerate until an r.p.m. is reached at which the power delivered is equal to the power absorbed
• In the case of a constant-speed propeller, the power absorbed is independent of the r.p.m., for by varying the pitch of the blades, the air resistance and hence the torque or load, can be changed without reference to propeller speed
• This is accomplished with a constant-speed propeller by means of a governor
• The governor, in most cases, is geared to the engine crankshaft and thus is sensitive to changes in engine r.p.m.

• Propeller Governor Operation:

  • As long as the propeller blade angle is within the constant-speed range and not against either pitch stop, a constant engine RPM will be maintained
    • If the propeller blades contact a pitch stop, the engine RPM will increase or decrease as appropriate with changes in airspeed and propeller load, as there is no more room to travel
    • For example, once a specific RPM has been selected, if aircraft speed decreases enough to rotate the propeller blades until they contact the low pitch stop, any further decrease in airspeed will cause engine RPM to decrease the same way as if a fixed-pitch propeller were installed
  • RPM changes according to the speed and AoA of the airplane; varying directly

  • Governor Mechanics: Propeller Lever Movement:

    • Propeller Lever Retarded:

      • Flyweights move out
      • Speeder spring tension decreases
      • Pilot valve moves upward
      • Oil flows out of the propeller hub
      • Propeller pitch increases

    • Propeller Lever Advanced:

      • Flyweights move in
      • Speeder spring tension increases
      • Pilot valve moves downward
      • Oil flows into the propeller hub
      • Propeller pitch decreases
• Airspeeds:
  • Increase in airspeed (pitch down):
    • The propeller will speed up
    • The flyweights will fly outward
    • The pilot valve moves upward
    • Oil flows into the propeller hub
    • The propeller pitch increases
  • Decrease in airspeed (pitch up):
    • The propeller will slow down
    • The flyweights will fly inward
    • The pilot valve moves down
    • Oil flows out of the propeller hub
    • The propeller pitch decreases

Propeller System Checks:

• The engine is started with the propeller control in the low pitch/high r.p.m. position to reduce the load or drag of the propeller and the result is easier starting and warm-up of the engine
• During warm-up, the propeller blade changing mechanism should be operated slowly and smoothly through a full cycle to determine whether the system is operating correctly, and to circulate fresh warm oil through the propeller governor system
  • This is done by moving the propeller control to the high pitch/low r.p.m. position (full forward), allowing the r.p.m. to stabilize, and then moving the propeller control back to the low pitch takeoff position
• Cycling warm oil through the propeller cylinder circulates residual oil that could impede system operation if cold or congealed
  • Consequently, if the propeller isn't exercised before takeoff, there is a possibility that the engine may over-speed on takeoff
• Pilots generally look for an RPM drop, an oil pressure change, and a manifold pressure increase, although the POH is the guiding document on when to cycle, how often to cycle, and what to look for

Constant Speed Propeller Operation:

• An airplane equipped with a constant-speed propeller has better takeoff performance than a similarly powered airplane equipped with a fixed-pitch propeller
  • This is because with a constant-speed propeller, an airplane can develop its maximum rated horsepower (red line on the tachometer) while motionless
• An airplane with a fixed-pitch propeller, on the other hand, must accelerate down the runway to increase airspeed and aerodynamically unload the propeller so that r.p.m. and horsepower can steadily build up to their maximum
• With a constant-speed propeller, the tachometer reading should come up to within 40 r.p.m. of the red line as soon as full power is applied, and should remain there for the entire takeoff
• Excessive manifold pressure raises the cylinder compression pressure and temperatures, resulting in high stresses within the engine
• A combination of high manifold pressure and low r.p.m. can induce damaging detonation
• To avoid these situations, the following sequence should be followed when making power changes
  • When increasing power, increase the r.p.m. first, and then the manifold pressure
  • When decreasing power, decrease the manifold pressure first, and then decrease the r.p.m.
• It is a fallacy that (in non-turbocharged engines) the manifold pressure in inches of mercury (inches Hg) should never exceed r.p.m. in hundreds for cruise power settings
• Whatever the combinations of r.p.m. and manifold pressure listed in the POH/AFM charts-they have been flight tested and approved by the airframe and powerplant engineers for the respective airframe and engine manufacturer
• With a constant-speed propeller, a power descent can be made without over-speeding the engine
• The system compensates for the increased airspeed of the descent by increasing the propeller blade angles
• If the descent is too rapid, or is being made from a high altitude, the maximum blade angle limit of the blades is not sufficient to hold the r.p.m. constant
• When this occurs, the r.p.m. is responsive to any change in throttle setting
• Some pilots consider it advisable to set the propeller control for maximum r.p.m. during the approach to have full horsepower available in case of emergency
• If the governor is set for this higher r.p.m. early in the approach when the blades have not yet reached their minimum angle stops, the r.p.m. may increase to unsafe limits
• However, if the propeller control is not readjusted for the takeoff r.p.m. until the approach is almost completed, the blades will be against, or very near their minimum angle stops and there will be little if any change in r.p.m.
• In case of emergency, both throttle and propeller controls should be moved to takeoff positions
• Many pilots prefer to feel the airplane respond immediately when they give short bursts of the throttle during approach
• By making the approach under a little power and having the propeller control set at or near cruising r.p.m., this result can be obtained
• If an emergency demanding full power should arise during approach, the sudden advancing of the throttle will cause momentary over-speeding of the engine beyond the r.p.m. for which the governor is adjusted
  • This temporary increase in engine speed acts as an emergency power reserve
• Some important points to remember concerning constant-speed propeller operation are:
  • All power changes should be made smoothly and slowly to avoid over-boosting and/or over-speeding
    • A momentary propeller over-speed may occur when the throttle is advanced rapidly for takeoff
    • This is usually not serious if the rated r.p.m. is not exceeded by 10% for more than 3 seconds

Blade Angle Control:

• The higher pressure provides a quicker blade angle change
• The r.p.m. at which the propeller is to operate is adjusted in the governor head
• The pilot changes this setting by changing the position of the governor rack through the cockpit propeller control
• On some constant-speed propellers, changes in pitch are obtained by the use of an inherent centrifugal twisting moment of the blades that tends to flatten the blades toward low pitch, and oil pressure applied to a hydraulic piston connected to the propeller blades which moves them toward high pitch
• Another type of constant-speed propeller uses counterweights attached to the blade shanks in the hub
• Governor oil pressure and the blade twisting moment move the blades toward the low pitch position, and centrifugal force acting on the counterweights moves them (and the blades) toward the high pitch position
• In the first case above, governor oil pressure moves the blades toward high pitch, and in the second case, governor oil pressure and the blade twisting moment move the blades toward low pitch
• A loss of governor oil pressure, therefore, will affect each differently

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Governing Range:

• The range of possible blade angles is termed the propeller's governing range
• The governing range is defined by the limits of the propeller blade's travel between high and low blade angle pitch stops
• The blade angle range for constant-speed propellers varies from about 11.5 to 40°
• The higher the speed of the airplane, the greater the blade angle range [Figure 2]
• As long as the propeller blade angle is within the governing range and not against either pitch stop, a constant engine r.p.m. will be maintained
• However, once the propeller blade reaches its pitch-stop limit, the engine r.p.m. will increase or decrease with changes in airspeed and propeller load similar to a fixed-pitch propeller
• If it reduces pitch to low pitch stops, then any further reduction in airspeed will cause the engine r.p.m. to decrease
• Conversely, if the airspeed increases, the propeller blade angle will increase until the high pitch stop is reached then the engine r.p.m. will then begin to increase

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Tachometer:

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• In a fixed-pitch propeller, the tachometer is the indicator of engine power [Figure 2]
• Calibrated in hundreds of RPM giving a direct indication of the engine and propeller RPM
• The RPM is regulated by the throttle, which controls the fuel/air flow to the engine
• At a given altitude, the higher the tachometer reading, the higher the power output of the engine
• As altitude changes, the position of the throttle must be changed to maintain the same RPM due to lower engine output caused by a change in air density (higher/lower density altitude)
  • As altitude is increased, the throttle must be opened further to indicate the same RPM as at a lower altitude
  • As altitude is decreased, the throttle must be closed further to indicate the same RPM as at a higher altitude
• The red line on the tachometer not only indicates maximum allowable r.p.m.; it also indicates the r.p.m. required to obtain the engine's rated horsepower
• The green arc on the tachometer indicates the normal operating range
  • When developing power in this range, the engine drives the propeller
  • Below the green arc, however, it is usually the windmilling propeller that powers the engine
  • Prolonged operation below the green arc can be detrimental to the engine

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Manifold Pressure Gauge:

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• On aircraft equipped with a constant-speed propeller, power output is controlled by the throttle and indicated by a manifold pressure gauge [Figure 3]
• At a constant RPM and altitude, the amount of power produced is directly related to the fuel/air flow being delivered to the combustion chamber
• As the throttle setting is increased, more fuel and air flows to the engine and MAP increases
  • When the engine is not running, the manifold pressure gauge indicates ambient air pressure (i.e., 29.92 inches mercury (29.92 "Hg))
  • When the engine is started, the manifold pressure indication will decrease to a value less than ambient pressure (i.e., idle at 12 "Hg)
• The face of the manifold pressure gauge contains a green arc to show the normal operating range, and a red radial line to indicate the upper limit of manifold pressure
• If manifold pressure is exceeded too frequently, this stress can weaken the cylinder components and eventually cause engine failure
  • As a general rule, manifold pressure (inches) should be less than the RPM
• A pilot can avoid conditions that over-stress the cylinders by being constantly aware of the RPM, especially when increasing the manifold pressure
• When both manifold pressure and RPM need to be changed, avoid engine over-stress by making power adjustments in the proper order:
  • When power settings are being decreased, reduce manifold pressure before reducing RPM
  • If RPM is reduced before manifold pressure, manifold pressure will automatically increase, possibly exceeding the manufacturer's tolerances
  • When power settings are being increased, reverse the order-increase RPM first, then manifold pressure
  • To prevent damage to radial engines, minimize operating time at maximum RPM and manifold pressure, and avoid operation at maximum RPM and low manifold pressure. The engine and/or airframe manufacturer's recommendations should be followed to prevent severe wear, fatigue, and damage to high-performance reciprocating engines
• Engine failure or power loss is indicated on the manifold gauge as an increase in manifold pressure to a value corresponding to the ambient air pressure at the altitude where the failure occurred
• On takeoffs from low elevation airports, the manifold pressure in inches of mercury may exceed the r.p.m. This is normal in most cases. The pilot should consult the AFM/POH for limitations