Aircraft Propeller Vibration

Propeller Vibration

• Aircraft propellers are made to be strong and durable, capable of absorbing the stresses of flight.
• Propeller blades range in weight from a few kilograms to hundreds of kilograms depending on the model.
• Aerodynamic loads and rotational speeds in flight make the propeller prone to vibration if there is even a slight imbalance.
• Excessive vibration can originate from the aircraft's engine, propeller, spinner, or a combination of all three.
• The engine's vibration isolators are designed to filter out most vibration, but they don't eliminate all of it, so excessive vibration can lead to several problems.
• Out-of-balance propellers vibrate, causing wear on the engine's vibration isolators.
• Excessive shaking from vibration can cause cracks in the airframe, cowl, spinner, or spinner bulkhead.
• Vibration can cause cracked or loose exhaust connections.
• Makers of exhausts require propeller dynamic balancing for exhaust systems to qualify for an extended warranty.
• Vibration can cause premature engine wear, pilot stress, and fatigue.
• The frequency of a vibration gives a clue as to whether it is caused by the propeller or the engine.
• Vibrations are counted by the number of vibrations per revolution of the propeller. These are referred to as "two-per", "half-per", etc.
• A one-per vibration occurs every revolution, indicating a problem with the propeller or, rarely, the engine crankshaft.
• A half-per vibration occurs every other revolution, commonly caused by a cylinder malfunction.
• A small vibration at a frequency greater than once per revolution usually indicates bearing wear or an accessory malfunction, such as the alternator.
• Anything with mass vibrates at a certain frequency based on its material and structure when struck or excited by an outside force.
• Vibration analysis requires understanding that engine and engine mounts can cause the vibration.
• Vibration from the propeller is caused by a mass imbalance when the center of gravity of the propeller is not in the same location as the center of rotation.
• Mass imbalance is usually from material removal to repair nicks on blades or differing degrees of blade erosion.
• Mass imbalance can be remedied by balancing the propeller and checking for correct blade track and indexing.
• Compression imbalances or a cylinder with excessively low compression can cause vibration
• Crankshaft counterweight wear can also cause vibration
• Worn engine vibration isolators permit excessive vibration, allowing the front of the engine to sag downward
• Cracked engine mounts can cause significant vibration
• Spinners with heavy spots due to manufacturing defects or repairs can produce slight imbalances that result in vibration.
• Heavy spots in spinners can be detected by laying the spinner on a flat table and slightly rolling it to observe where it comes to rest.
• The forward tip of the spinner should be aligned with the centre of rotation of the propeller and if the nose wobbles re-align the spinner by loosening the mounting screws and re-tightening.

Propeller Tracking

• Propeller track refers to the path traced by a blade segment during one rotation.
• Variation in the prop track leads to differences in the angle of attack creating different thrust & resulting vibration.
• Propeller track should be checked on every annual and 100-hour inspection as well as whenever vibration is an issue.
• The wheels should be chocked to stop movement of the aeroplane
• Place a board under the propeller so the blade tip almost touches
• Mark the board at the tip of the propeller and rotate the propeller until the next blade is in the proximity of the mark.
• Out-of-track amount varies based on the propeller manufacturer's specifications
• A light aeroplane has max allowance for a metal propeller of 1.6 mm (1/16")
• The maximum allowance for a mm wood propeller of 3.2 mm (1/8")
• Slight out-of-track conditions for metal or wood fixed-pitch propellers can be fixed using thin metal shims between the propeller and the crankshaft flange.
• Out-of-track conditions on constant-speed propellers should be inspected by a certified professional.

Propeller Balancing

• Two most important types of balance.
  • Dynamic Balance
  • Static Balance
• Dynamic imbalance is classified into
  • Mass Imbalance
  • Aerodynamic Imbalance
• Static balance is a scenario when the centre of gravity lies on the "axis of rotation"
• A body rotates about a fixed point and is static balance when "it stops at a random position" when rotated.
• Static balance is rectified at propeller repair shops
• Propeller to be set on a mandrel and placed across level knife edges
• The measurement is to be taken in the vertical & horizontal positions.
• Horizontal imbalance (propeller to vertical)-Fixed by adding "solder to the metal"
• Vertical imbalance when a rotates to a horizontal- Fixed by attaching a "brass weight" with countersunk screws"
• Fixed pitch metal propellers are balanced in a repair shop by taking metal from the heavy side and refinishing
• Constant-speed propellers can be balanced with a lead washer on a balancing stud, inside the hollow blade shank.
• Unbalance can be corrected with lead wool inside the hollow shanks of bolts that fasten the halves of the propeller barrels together.
• This type of balancing can only be undertaken by a licenced repair shop
• Dynamic balance occurs where couples are set up by centrifugal forces i.e "Algebraic sum at any plane is zero"
• Dynamic balance is the most effective when the propeller is fitted to the engine.
• Dynamic prop balancing needs to be done after engine/propeller overhaul as well as medium time
• Dynamic prop balancing needs to done when the prop is removed/reinstalled.
• Essential tools for helicopter balance are the "aircraft balancers and analysers"
• TEC Aviation ACES ProBalancer is a device that has the capacity to measure vibration"
• TEC Aviation ACES ProBalancer shows the quantity/location to balance the propellers.

Dynamic Balance Procedure

• Dynamic balance procedure requires two sensors are attached to the engine:
  • accelerometer
  • optical position and RPM sensor (phototach)
• Two main readings are required: the amplitude and the phase
• Vibration transducers, such as velocity or acceleration transducers, are needed to obtain the vibration amplitude,
• Piezoelectric crystals in these devices produce voltage when subjected to vibration with the voltage output correlating to the vibration.
• One transducer is mounted at the front as close to the propeller, the other (check transducer) is placed at the rear of the engine.
• Dual sensors distinguish between propeller and crankshaft vibrations since both operate at the same RPM.
• Maximizing sensor proximity to the imbalance source amplifies amplitude readings.
• The phototach, is mounted behind of the propeller and emits a light beam towards the propeller's rear for triggering.
• Reflective tape positioned on one of the propeller blades aligns with the phototach.
• The analyzers indicates phase and amplitude, and the correlated data of the phototach and the front sensor is used to calculate a balance solution.
• Post solution a trial weight, like a surface area washer, to be placed under the spinner retaining screws.
• Once vibration is acceptable the weights are put into the gear ring, with spinner bulkheads pre drilled facilitating setup.
• Weights are added permanently with the aircraft running verifying the balance when completed.

Vibration Spectrum Survey

• Any propeller balance will conclude with a vibration spectrum survey
• It's confirmed one-per-revolution vibration that is induced by propeller imbalance is diminished with no aerodynamic issues.
• A deeper overview of the rotating parts finds what produces the vibration
• The spectral survey highlights frequency levels and its vibrations (RMP, CMP, or Hertz)
• Vibration levels occur with moving parts which have a degree of freedom
• Components all potentially share like/identical frequencies that make the troubleshooting more in depth.
• Data for sensors is gathered using the spectrum function digital/balance analysers.
• Spectral readings are digital representations or "signatures" of data when captured that need interpretation.

Harmonics

• Spikes from a rotating component/derivatives are multiples on the signature graph.
• Multiples from RPM fundamentals are harmonies/orders

Half Orders

• Harmonics is abnormal and a result of many problems.
• Damper mounts can increase vibration, also check mount integrity.
• Half order can also indicate components malfunction.
• Ailments that can show high half order are compression losses, fuel mixture, induction/valve lift issues or spark timing.
• Injection problems/bad magnets also need investigation, coupled with too much combustion.
• 2.5mm & 7.5mm is normal 0.1-0.3"
• 7.5mm or 0.3" is too high and could show combustion issues.

First Order

• Normally show a mass imbalance within either the prop or a worn crank.
• To determine whether to test the propel cranks, 2 vibration sensors can be used.
• Any sensor at the rear must correspond with findings that are greater than the front sensor.

Second Order

• Readings "are not so easily defined"
• Inherent two per level, mainly with 2 blade props
• Could indicate bad rod or piston (contact engine manufacturer)
• 3/4 blade issues will show 3-4 times the per rev (Check the rod or piston area)
• Helis have dampers to assist to limit vibrations (Cannot adjust weights)

Aerodynamic Balance: General

• No two blades are manufactured to 100% accuracy
• Minor blade variations differ from the theoretical value for torque.
• Forces on the blades that result/don't result are periodic vibrations that means "each blade equals"
• When a blade applies more force vibration rises when it attempts to bend.
• Vibration does not get worse when throttle increases
• Thrust & Torque are not equal with a different torque force
• The result makes the forces imbalanced, creating force differences.

Factors and Correction

• Thrust and Torque are perpendicular to each other.
• Difficult to apply a single correction to these forces so the balances are therefore for either one force
• Aerodynamic correction factor ACF with either a note to correct either T thrust or Q torque
• A single prop needs to have balance in place with a factor with corrections all for the same factor.
• If performance is sub 5 mmps/0.2 ips and complaints still arise then aero issues could still be the problem.
• Detect with imbalance that comes from reflector tap on blade tips or aircraft.
• During RPM observation look for an alternate light course or blade angle with a light for viewing.
• If they are all the same then the aero is present and is correctable to verify.

Blade Indexing

• Aero balancing is by aligning baseline setting of each blade to other props.
• Known as blade indexing with 2 steps dependent on the types of propellers.
• Amount has to be added or subtracted that is represented in degrees and marked on the blade.
• Vernier adjusts each root to make this happen with correction factor marking
• Engage with outer periphery serrations with gear segment serrations and blade indexes
• 2 degrees serration blade gear segment, blade bush and factors marked
• Imagine application at 12.4ft so disengage from the index to coarse & turn 4' into coarse + re-engage.

Blade Damage Assessment: General Procedures

• Reference must be given to the procedures and limitations of the proper maintenance manual before carrying out any repairs to propeller blades.
• The following procedures and limitations are provided for general guidance only:
• Repairable blade damage is classified as follows:
  • Superficial Damage
  • Damage to blade tips
• Superficial damage can be blended by using a scraper, smooth file, or fine abrasive paper.
• Damage at the tips of the blades, cropping to a maximum of 25 mm (1 in) is allowed.
• Once completed the repair of the blade must be repainted to avoid corrosion.
• It is not possible to carry out repairs to:
  • Folded steel or composite blades
  • On the shank on any propeller
• The repairs carried out are to adhere to the manufacturer's instructions.
• Wood propellers can be repaired with sawdust wood filler and a water proof glue.

Aluminum blade repair

• The damage and size must be assessed and is to be in line with all rules.
• Make smooth and follow contour area after the repair and file/cloth repairs.
• Any cracks must be x-rayed with a magnifier in the area.
• Surface must be clean and must also have instructions as per the manufacture

Blade cropping

• Judge what alloy has to be taken to go against limit
• Log and check previous documents
• Test if the others have balance requirements.
• Fabricate the template based on area that's good
• Cut off damage with re-establishments
• Follow finish to re-establishments
• Document each metal and location.

General Repair Procedures

• Wooden propellers are very sensitive to damage to the material in which they are made.
• Frequently inspected for breaks, cracks, scores, delamination security and nicks.
• Minor defects of the surface finish can be restored.
• Any wood damage is based on proper process with manufacturer.
• Maintain to inspection per specifications with airframe
• Damaged parts like blades or boss can be the next problem for repair

Wood propellers inspections

• bolt holes
• boss faces
• centre bore
• mounting hub/fasteners
• cones checked
• Contact of hub/cone checked with engineers' blue(80% contact normally)

Wood propeller defects and repairs

• Depth/area should not exceed the aircraft manual
• Indent with small cracks
• Deep cuts
• Tips/leading edges
• metal sheaths to prevent damage in wood or repairs
• If followed well, balance needs to be taken/checked

Metal Fixed-Pitch Propellers

• Aluminum ally propeller blades have potential limitations.
• Sharp scores or indent are cause of stress, especially where they form as part of the blade.
• corrosion, cuts and dents always looked on
• Failures with decals and attachments must be looked on with the stickers where attention is required before taking inventory during operation.
• metal blades need to be pulled off during either repairs/condition checks (check before).
• Cracks have to be visually checked with non-destructive methods

Metal Variable-Pitch Propellers

• Inspect with the utilization of metal blades that do not follow the fatigue life.
• Inspection & Repair has be manufacturer certified/approved.
• Utilize and maintain alloy blades in accordance with manufacturers' orders.

Periodic Maintenance for All Propellers

• All visible parts must be free from debris and clear with components and lines for inspections
• Blades have to be free from abrasion, corrosion and nicks.
• Blade routes and hubs/spinners have leak inspections that can be found.
• The PCU and Pipes are inspected to leak.
• Contacts/Spring rings must be inspected for condition/wear.