top 3 causes of aircraft vibration

Top 3 Vibration Causes

VIBRATION• Effects• Causes• Terminology• Detection• Correction• Examples• Tips

Vibration EffectsBearing Wear

Vibration causes increased maintenance• Accelerated component failure• Engine / motor failure

Bearing life reduced by third power10x increase in load = 1000x decrease in life

Sliding Wear

Generally varies linear with load

@1 IPS (Inch per Second variation), 7800 RPM, engine experiences 2G oscillation, variation of .001 in.

Industry standard is vibration less than 0.1 IPS; 10x reduction in vibration is 10x reduction in wear rate.

Vibration Effects

Cowling cracks

Vibration Effects

Exhaust Cracks

Vibration Effects - Pilot Comfort

Cause: 1) Mass Imbalance

Center of Rotation vs. Center of Gravity

Why?• Manufacturing defects• Installation error• Propeller damage• Moisture / corrosion• Maintenance / paint / repair

Cause: 1) Mass Imbalance

Cause: 1) Mass Imbalance

Center of Rotation vs. Center of Gravity

• Manufacturing defects• Installation error• Propeller damage• Moisture / corrosion• Maintenance / paint / repair

Cause: 1) Mass Imbalance

Center of Rotation vs. Center of Gravity

• Manufacturing defects• Installation error• Propeller damage• Moisture / corrosion• Maintenance / paint / repair

Cause: 1) Mass ImbalanceCenter of Rotation vs. Center of Gravity

• Manufacturing defects• Installation error• Propeller damage• Moisture / corrosion• Maintenance / paint / repair

Cause: 1) Mass ImbalanceCenter of Rotation vs. Center of Gravity

• Manufacturing defects• Installation error• Propeller damage• Moisture / corrosion• Maintenance / paint / repair

Cause: 2) Aerodynamic Imbalance

Blade track

Cause: 2) Aerodynamic ImbalanceBlade track

Cause: 2) Aerodynamic Imbalance

Blade pitch

Cause: 2) Aerodynamic Imbalance

Blade pitch

Cause: 2) Aerodynamic Imbalance

Blade shape

Cause: 3) It’s Complicated

• Compression• Spark• Fuel flow• Worn bearings • Intake / Exhaust variances• Vibration isolators• Torsional absorbers• Gearbox• Aerodynamic interaction

TerminologyIPS – Inches per Second (Velocity)

1 per – Rotates at Prop / Rotor RPM (Once Per Revolution)½ per – Every other rotation, 4 - stroke2 per – Twice per rotation (Two Blade)3 per – Three per rotation (Three Blade)

Vibration Correction – 1 PerInstall Equipment

Measure the 1-per Vibration

• Warm up the engine

• User averaging to optimize signal

• 2090RPM

• 0.88IPS @ 258

Vibration Correction – 1 Per

Realign blade with phototach

Vibration Correction – 1 Per

Add / Remove Weight

• Leave prop clocked

• 258 degrees in the direction of prop rotation

• 50 grams per IPS starting point

• Remove weight at 258 or add at 78

• Rerun, usually takes 2-3 cycles

Vibration Correction – 1 Per

Final Reading

Vibration Correction – 1 Per

Vibration Correction - TroubleIf it doesn’t work, something is typically wrong

Phase angle changing

Vibration Correction - TroubleIf it doesn’t work, something is typically wrong

New IPS and angle each run

Vibration Correction – Advanced

DynaVibe GX Balancer and Analyzer

• AutoBalance

• TruePhase Measurement

• Spectrum Analysis

• TotalVibe

• Reporting

Vibration Correction – Advanced

AutoBalance

Setup Screen

• Owner / Engine Info• Blade Location• Direction of Rotation• Phototach Location• Accelerometer Location• Weight Addition Locations

Vibration Correction – Advanced

AutoBalance

• Polar Chart• RPM• IPS and phase

• Generates Split weight mass amounts and location

Vibration Correction – Advanced

Spectrum

Long-Ez

1-per

2-per

2.6 – per

Vibration Correction – Advanced

Spectrum

Long-Ez

½ per - rough

1-per

2-per

Vibration Correction – AdvancedSpectrum

½- VW - started at 0.7 IPS

Vibration Correction – AdvancedSpectrum

Kitfox IIIVery large 3 perRough engine shows 1.5 per

crank

½ crank

Vibration Correction – AdvancedSpectrum Crank

1.5 Crank

½ Crank

Left engine has ½ per crank vibration, bad cylinder

Right engine has a 1.5 per crank, bad mag

Vibration Correction – AdvancedSpectrum

Vibration Correction – AdvancedTotal Vibe

Aircraft Resonance

Magnitude vs. RPM

Rotax 912 Setup

Mount two DynaVibe accelerometers on the engine.

Rotax 912 @ gearbox

Gearbox Vibe spectrum shows a slight propeller imbalance (1per < 0.2 IPS).

Gearbox Vibe spectrum shows a ½ per crank (1.2per of 0.3IPS) indicating carbs need balanced.

Rotax 912 @ Right Carb

Analysis reveals a “2-per” vibration.

Additionally there is a 2.43 per vibration caused by the front-back variation in airflow.

These two frequencies together cause a low frequency 7hz rumble that shakes the airplane. (beat frequency)

Rotax 912 as RPM changes

Tips• FAA Advisory Circular 20-37E

• Verify propeller bolt torque on wooden propellers every 25 hours at oil change, or when changing environments

• 1.20 IPS requires static rebalance

• 0.003 inch crank run out can cause 0.6 IPS

• Typical Problems:Fiberglass back platesUnbalanced spinnersBlade Pitch

Vibration Correction – Helicopter

Vibration Correction – Helicopter

Equipment Costs• DynaVibe Classic– RPM, 1 Per IPS & Angle– $1495

• DynaVibe GX2– AutoBalance, Spectra, TotalVibe– 2 Accelerometers at the same time – DynaTrack, Strobe, Highspeed– $3995