What's the Difference Between a Fixed Pitch and Variable Pitch ...

In earlier posts we’ve covered how different types of propellers – such as fixed pitch propellers and variable pitch propellers – operate. But today we’re going to break down the differences in a single post.

If you want to learn more, please visit our website.

As the names imply, the core difference between the two prop designs involves the orientation of the blades relative to the propeller hub. Pitch affects the way a propeller blade will slice through the air. A low (fine) pitch positions the blade of the propeller in a slightly more vertical position from ground level. This configuration generates more thrust and is ideal for take-off and ascent. At cruising altitude, a more horizontal, high (coarse) propeller pitch allows the prop to move through the air more efficiently. This conserves fuel and reduces strain on the engine.

One hundred years ago, when early aviation engineering was still in its infancy, fixed pitch propellers were your only option. A fixed pitch propeller has its pitch determined at the factory when the propeller is made. Its blade orientation was tested and built for a specific airplane to offer the optimal blend of performance and efficiency. Fixed pitch propellers were the norm until about World War II, when both pilots and aeronautical engineers began pushing aircraft to higher performance levels.

Here’s a quick video breakdown on fixed pitch propellers:

Goto Rayi to know more.

Though born out of wartime, the benefits of variable pitch propellers made them appealing to commercial aircraft designers. In a variable pitch propeller, the blade pitch can be manipulated directly by the pilot in the form of cockpit controls, or automatically based on the speed of the engine. On aircraft with automatic control, blade pitch is continually adjusted as the propeller adapts to altitude, engine RPM, and other parameters. The blade pitch is typically regulated via a hydraulic or electrical system. Varying pitch allows the propeller to maintain the ideal blade orientation for the exact flying conditions.

For a great rundown on variable pitch propellers and how they operate, check this out:

So, there you have it: the difference between fixed pitch and variable pitch propellers is ultimately how much control you have over the operational efficiency of your aircraft.

For more information, please visit ground adjustable pitch propeller.

• Aircraft are equipped with different types of propellers:
  • Fixed pitch
  • Adjustable pitch
  • Constant-Speed

• Fixed-Pitch Propeller:

  • A fixed-pitch propeller design with fixed blade angles set by the manufacturer, which cannot be changed
  • Advantages:
    • Low weight
    • Simplicity
    • Low cost are needed
  • Disadvantages:
    • Increases/decreases RPM with speed
  • There are two (sort of three) types of fixed-pitch propellers:

    1. Optimal Climb Fixed-Pitch Propeller:

       • Has a lower pitch, therefore less drag
       • Less drag results in higher RPM and more horsepower capability, which increases performance during takeoffs and climbs, but decreases performance during cruising flight

    2. Optimal Cruise Fixed-Pitch Propeller:

       • The cruise propeller has a higher pitch, therefore more drag
       • More drag results in lower RPM and less horsepower capability, which decreases performance during takeoffs and climbs, but increases efficiency during cruising flight

    3. Combination/Compromise Fixed-Pitch Propeller:

       • Some propeller designs may compromise between cruise and climb
  • Since a fixed-pitch propeller achieves the best efficiency only at a given combination of airspeed and RPM, the pitch setting is ideal for neither cruise nor climb and must compromise between the two
  • The propeller is usually mounted on a shaft, which may be an extension of the engine crankshaft
    • In this case, the RPM of the propeller would be the same as the crankshaft RPM
  • On some engines, the propeller is mounted on a shaft geared to the engine crankshaft
    • In this type, the RPM of the propeller is different than that of the engine

• Adjustable (controllable)-Pitch Propeller:

  • The adjustable-pitch propeller was the forerunner of the constant-speed propeller
  • It is a propeller with blades whose pitch can be adjusted on the ground with the engine not running, but which cannot be adjusted in flight
    • It is also referred to as a ground adjustable propeller
  • The first adjustable-pitch propeller systems provided only two pitch settings: low and high
  • Today, most adjustable-pitch propeller systems are capable of a range of pitch settings

• Constant-Speed Propeller:

  • A constant-speed propeller is a controllable-pitch propeller whose pitch is automatically varied in flight by a governor maintaining constant RPM despite varying air loads
    • The most common type of adjustable-pitch propeller
    • Also referred to as a variable-pitch or controllable-pitch propeller
  • Using oil and nitrogen pressure, the governer controls the pitch angle of the propeller to maintain the desired RPM
  • Advantages
    • It converts a high percentage of brake horsepower (BHP) into thrust horsepower (THP) over a wide range of RPM and airspeed combinations
    • More efficient than other propellers because it allows selection of the most efficient engine RPM for the given conditions
  • Disadvantages:
    • Heavier, more complex.

  • 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)
  • AoA Changes Based On:
    • Blade Angle
    • Blade Speed
    • Velocity of Air

• A governor is a mechanical device within a constant-speed propeller that maintains propeller speed
  • It does so by sensing the speed of a propeller and through mechanical feedback, adjusting pitch
• Constantly works to balance optimum RPM vs. manifold pressure
• The pilot controls the engine r.p.m. indirectly by means of a propeller control in the cockpit, which is connected to the governor
• As conditions change, the governor moves a piston to maintain engine RPM [Figure 4]
• When a governor is employed, engine oil is used and the oil pressure is usually boosted by a pump, which is integrated with the governor
  • Generally, the oil pressure used for pitch change comes directly from the engine lubricating system

• Counterweights:

  • Increases blade angle
  • Overcome aerodynamic twisting force (low-pitch)

• Compressed N2 charge:

  • Increases blade angle

• 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.
  • 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

• 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