Manual Propeller Pitch Control

Objectives

• Achieve an understanding of the Constant Speed System
• Achieve an understanding of Propellers and Propeller Forces
• Obtain an ability to safely and efficiently operate a Constant Speed Unit (CSU)

Propellers

• A propeller generates a force by rotating propeller blade(s)
• The propeller blades generate a lifting force (thrust) by pitching at an Angle of Attack

Propellers

• As an aircraft travels forward, the resultant relative air flow on the propeller changes angle
• This reduces the Angle of Attack of the propeller blades, producing less thrust
• By allowing a change in the pitch of the propeller, we achieve more Thrust Available at higher airspeeds

Propeller Forces

• A propeller is essentially a rotating wing
• Centrifugal Force — the force on the blades acting to pull them out of the hub, opposes bending forces
• Torque Bending Force tends to bend the propeller blades in the direction opposite that of rotation
• Thrust Bending Force thrust load that tends to bend propeller blades forward as the aircraft is pulled through the air
• Aerodynamic twisting moment (i.e. coarse pitch)
• Centrifugal twisting moment (i.e. fine pitch)

Introduction to the CSU

• A Constant Speed Unit, or Manual Propeller Pitch Control, maintains the propeller RPM at a speed selected by the pilot
• The blade angle of the propeller self-adjusts to maintain a given RPM independent of the throttle lever or manifold pressure

The Basics of a Governor

• The propeller pitch is changed hydraulically, using the same oil the engine uses
• A governor sends oil to and from the propeller hub in order to change the blade pitch

Propeller Hub

• Oil flow is controlled by the propeller governor, which is set by the propeller lever to maintain a given RPM
• The propeller lever changes the compression of the speeder spring that pushes against the flyweights
• Oil pressure into the hub increases the force on the piston and moves the blades towards a coarse pitch
• Oil draining out of the hub decreases the force on the piston and moves the blades towards a fine pitch

Speeder Spring & Flyweights

• The propeller lever sets the tension on the speeder spring, which sets the position of the flyweights
• The flyweights then set the position of the pilot valve
• The pilot valve can be fully open, partially restricted or fully closed with regard to oil flow to the propeller hub
• If the RPM on the propeller would otherwise increase with an increasing airspeed, the flyweights also move outwards (because the flyweights are directly connected to the engine crankshaft) due to centrifugal force
• This opens the pilot valve and allows more oil pressure directed to the propeller hub, moving the propeller blades to a coarser position
• The opposite is also true

Pilot Valve

• The governor pilot valve is moved up and down by the flyweights, allowing oil to flow into, or out of, the propeller hub
• Again, the amount of oil is controlled by the propellor governor

Explanation of a Propeller Governor

Fine and Coarse Pitch

• When the propeller lever is fully forward, the propeller is in fine pitch
• Fine pitch is used for takeoff, landing, go-around and ground run-ups

Fine and Coarse Pitch

• When the propeller lever is pulled backward, the pitch of the blades become coarser
• Think of it like a car
  • 80kph in second gear may be somewhere around 6000 RPM
  • However, 80kph in sixth gear is around 2000 RPM
  • When we move the propeller lever backward, we are “changing to a higher gear”

Overspeed, On Speed & Underspeed

• When the propeller is in an On Speed position, no oil flows in or out of the hub — maintaining a constant RPM
• If the RPM would otherwise decrease, to maintain RPM, the spring force becomes greater than centrifugal forces, forcing the pilot valve down. This is known as an Underspeed condition and returns the blades to a finer pitch.
• If the RPM would otherwise increase, to maintain RPM, the centrifugal forces become greater than the speeder spring force. The pilot valve lifts, allowing oil to flow into the propeller hub — moving the blades towards a coarser pitch — this is known as an Overspeed condition
• If the propeller does overspeed, reduce power/airspeed and if required, use propeller lever to control RPM
• Remember, blade angle and RPM is a function of TAS and power setting

Manifold Pressure & RPM

• Because the governor maintains a given RPM, the tachometer is no longer a reliable indication of the engine power output
• The manifold pressure gauge (MP) is now our measure of power
• Manifold pressure is generally measured in inches of mercury (inHg) from the engine inlet manifold

Manifold Pressure & RPM

• With the engine stopped, the MP gauge will read ambient atmospheric pressure
• On startup, with the propeller in fine pitch, the RPM & MP will change with the throttle — just like a fixed pitch propeller
• If the RPM is increased with the propeller lever, MP will rise & vice versa
• Carburettor & Induction Icing
  • is indicated by an unexplained MP drop and loss of performance
  • The RPM indication will remain constant

Cessna 182P Cruise Performance

Changing Power

• When decreasing power, it is important to decrease MP before decreasing RPM
• When increasing power, it is important increase RPM before increasing MP
• Just like changing down a gear to go up a hill

Engine Power/RPM

• At low RPM, the engine/propeller isn’t designed to produce large amounts of HP — forced induction engines are especially vulnerable
• Promotes engine detonation
• Remember our propeller relies on RPM to resist blade bending forces!

Airmanship/TEM/HF/LOC

Quiz on Objectives

• What are some of the forces acting on a propeller?
• By what mechanism does a propeller governer change propeller pitch?
• How are the flyweights spun and what do they in turn do within the governor?
• What happens to propeller pitch with a loss of oil pressure?

Flight Sequences

• Pre-flight inspection
  • Inspect propeller hub
  • Inspect propeller blades for any play in hub
• Engine run-up
  • Propeller cycle (x2), expecting RPM drop 200-300RPM
  • Observe the engine speed at which the CSU begins governing the propeller blade pitch angle
• Climb out
  1. Set manifold pressure
  2. Set RPM
• Level out for cruise (departure)
  1. Reduce manifold pressure
  2. Reduce RPM
  3. the manifold pressure will increase slightly after setting RPM
• Observe constant RPM with differing airspeeds
• Engine failure procedures
  • Observe glide performance with fine/coarse propeller pitch

Flight Sequences

• Enter a climb
  1. Fuel mixture full
  2. Set RPM
  3. Set manifold pressure
• Top of climb
  1. Set manifold pressure
  2. Set RPM
  3. Fuel mixture lean
• Top of descent
  1. Fuel mixture slightly richer
  2. the manifold pressure may increase due to RAM air and/or increased air density
  3. Adjust manifold pressure accordingly
• Go-around
  • “Mixture UP”
  • “Pitch UP”
  • “Power UP”
  • +RoC… “Gear UP”
  • “Flaps UP”