PROPELLERS

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PROPELLERS

• PROPS ARE FOR BOATS

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• Propellers are used to harness the power of the
  engine and turn it into useful thrust. BHP to
  THP.
• Each propeller blade is a small airfoil which
  produces lift (thrust) as it moves through the
  air.
• This thrust pushes (pusher propeller) or pulls
  (tractor propeller) the aircraft through the air.
• THP is a product of BHP and propeller efficiency.
• No propeller is 100 efficient.

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EFFECTIVE AND GEOMETRIC PITCH

• Geometric pitch is the theoretical distance a
  propeller should travel forward in one
  revolution.
• Effective pitch is the actual distance a
  propeller travels forward in one revolution.
• Propeller slip is the difference between
  geometric and effective pitch.

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PROPELLER THRUST

• A propeller creates thrust by accelerating a mass
  of air rearward.
• The velocity change and volume of air determine
  thrust.
• A larger diameter propeller will produce the same
  thrust with a smaller velocity change.
• This makes a propeller with a larger diameter
  more efficient.

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LIMITATIONS

• Several limitations affect propeller diameter
• Stress effects on the engine.
• Ground clearance.
• Blade strength.
• Tip speed. (tip speed is the product of rpm and
  distance of tip to hub).

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TIP SPEED

• As propeller tip speed approaches the speed of
  sound, efficiency is lost due to compressibility
  effect of the air.
• The noise associated with high tip speeds is also
  uncomfortable.
• High performance engines must incorporate
  reduction gearing to limit prop rpm to an
  effective range.

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RELATIVE WIND

• The angle of attack on the propeller blade has a
  great effect on the efficiency of the propeller.
• The angle of attack of a fixed pitch propeller
  changes as the forward speed of the aircraft
  changes.
• The most efficient angle of attack ranges between
  2 and 4.
• In order to maintain propeller efficiency
  throughout a greater range of airspeeds we can
  utilize a variable pitch propeller (constant
  speed).

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PROPELLER TERMINOLOGY

• Blade angle The angle between the chord line of
  the blade and the plane of rotation.
• Pitch The angle between the face of the blade
  and the plane of rotation.
• NOTE Pitch and blade angle are usually used
  interchangeably.
• Hub the point of attachment for the individual
  blades.
• Dome stores oil pressure from the governor to
  control blade angle.
• Spinner aerodynamic shield for the propeller
  hub.

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FORCES ACTING ON PROPELLERS

• Centrifugal forces the rotating propeller blades
  away from the hub.
• Torque bending bends the propeller blades
  opposite to the direction of rotation.
• Thrust bending bends the propeller blades
  forward.
• Aerodynamic twisting forces the individual
  blades to rotate to a lower blade angle.
• Centrifugal twisting forces the individual
  blades to rotate to a lower blade angle.

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Centrifugal Twisting Force
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TYPES OF PROPELLERS

• Propellers can be made from a variety of
  materials wood, aluminum alloy, fiberglass,
  carbon fiber, composites.
• It must be durable and flexible to withstand the
  forces applied during normal operations.
• Propellers are designed with different blade
  shapes and configurations.
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TYPES OF PROPELLERS

• Fixed-pitch the blade angle cannot be changed.
  The angle will be fixed at an angle that suits
  the type of aircraft. (take-off performance,
  cruise performance, or an all-rounder). These
  propellers are usually one piece.
• Ground-adjustable the pitch can be changed
  manually on the ground.
• Controllable-pitch blade angle may be manually
  adjusted in flight.

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TYPES OF PROPELLERS

• Constant-speed incorporates propeller governors
  which automatically adjust blade angle to
  maintain a selected rpm.
• Reverse-pitch the blade angle is able to
  continue past full fine into reverse blade angle
  to create reverse thrust.
• Feathering the blade angle is able to continue
  past full course into a position which prevents
  windmilling reducing drag.

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Propfan Antonov An-70
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PROPELLER GOVERNORS

• Constant-speed propellers incorporate governors
  to control blade angle automatically.
• Blade angle is controlled by a combination of oil
  pressure, springs, counterweights, and
  aerodynamic forces.
• The propeller governor boosts engine oil pressure
  to higher values, and controls flow of this oil
  into and out of the propeller hub to change blade
  angle.

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CONSTANT SPEED NON-FEATHERING

• Oil pressure is used to drive the propeller
  blades to full course blade angles.
• Aerodynamic forces acting on the individual
  blades drive the propeller blades to full fine
  blade angles.
• The prop lever adjusts tension on a speeder
  spring connected to flyweights within the
  governor.
• The flyweights are geared to the propeller, and
  sense an underspeed or overspeed.
• This change in flyweight position controls a
  pilot valve which allows oil to flow to or dump
  from the propeller hub.

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CONSTANT SPEED FEATHERING

• An important distinction to make is that a
  feathering and a non-feathering prop utilize oil
  pressure for opposite purposes.
• Oil pressure is used to drive the propeller
  blades to full fine blade angles.
• Counterweights and springs are used to drive the
  propeller blades to full course and beyond to
  feather position.

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WHY THE DIFFERENCE ?

• When dealing with a feathering propeller it makes
  sense for a loss of oil pressure to cause the
  propeller to feather.
• This way if an engine fails the propeller will
  feather and reduce drag. ( a windmilling
  propeller causes a large amount of drag).
• A non-feathering prop (single engine piston
  aircraft) utilizes aerodynamic forces to drive
  the propeller to full fine and oil pressure to
  drive the blades to full course this eliminates
  the need for counterweights and springs which
  reduces cost and weight.

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Propeller Underspeed
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Propeller Overspeed
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Propeller Onspeed
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OVERSPEED GOVERNOR

• In the event of a propeller governor failure, the
  system must be able to guard against a propeller
  overspeed.
• A propeller which reaches speeds above the rated
  rpm may cause the engine to exceed its rated
  power output.
• An overspeed governor is preset to an acceptable
  value above the normal governing range, and
  controls oil pressure to limit rpm to this value.

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AUTOFEATHER

• Turboprop aircraft incorporate the use of
  autofeather systems.
• An autofeather system senses a drop in engine
  power and automatically dumps the oil from the
  propeller hub, feathering the prop.
• These aircraft operate at higher take-off weights
  and employ large diameter props.
• If an engine were to fail just after take-off,
  the adverse drag of a non-feathered prop could be
  too much to overcome.

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UNFEATHERING ACCUMULATOR

• Starting an engine in the feathered position puts
  more strain on the starter due to increased
  rotational drag.
• An accumulator holds a hydraulic charge which is
  used to help unfeather the prop for an airstart
  attempt.

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FEATHER LATCHES

• To prevent feathering of a shutdown engine on the
  ground, spring loaded latches are incorporated.
• Centrifugal force of the rotating prop forces the
  latch to disengage allowing the propeller to
  feather.
• On the ground at low rpm the latch engages and
  prevents the prop from feathering at shutdown.

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ICE PROTECTION

• Any ice build up on the propeller blades can
  create an out of balance condition causing
  potentially damaging vibrations.
• Most modern aircraft certified for flight in
  known ice use electrically heated de-ice
  protection.
• Heating elements are attached to the propeller to
  shed any ice accumulation.
• It is important not to use this de-ice equipment
  when the engine is not running as the brushes
  will fuse to the slip ring.

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PROPELER SYNCHRONIZATION

• Propellers on multi-engine aircraft operating out
  of sync create unwanted vibration and beat.
• More sophisticated aircraft have prop sync
  systems to eliminate the condition.
• The system uses magnets to sense an out of sync
  condition and then increase or decrease slave
  prop rpm to match the master.

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PROPELLER HANDLING

• Propellers operate under high stress and
  therefore chances of failure exist.
• A thorough preflight inspection of the propeller
  is vital, to detect signs of fatigue or damage.
• Look for nicks, cracks, pitting, discoloration,
  oil streaking, and security of blades and spinner
  (gently).
• A combination tactile and visual inspection is
  best.
• If unsure of the airworthiness of a propeller ask
  an AME.

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Lightning Damage
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PROPELLER HANDLING

• When taxiing be aware of your surroundings.
• Never park a running propeller over standing
  water or debris.
• The vortex formed by propeller rotation will suck
  this contamination into the propeller causing
  pitting.
• If necessary to start an engine on a gravel
  surface sweep the area under the prop clear of
  gravel before start.

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HEADS UP !

• Always maintain situational awareness when
  working near propellers.
• If the propeller must be repositioned, move it in
  the opposite direction of normal rotation to
  avoid inadvertent firing.
• Even a stopped propeller can cause injury, and it
  is all to common for pilots to walk into them.
• We all know the consequences of stumbling into a
  moving propeller!

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Prop Strike
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