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Dynamic Propeller Balancing

& Engine Vibration Analysis

Did you know 90% of all Propeller driven Aircraft can significantly benefit from Dynamic Balancing!

"Dynamic prop balance is the most cost effective thing you can do for your plane!" (Click here for Link to AOPA article)

We Specialize in Precision Dynamic Propeller Balancing. We use the Dynamic Solutions Systems MicroVib II Aircraft Analyzer, the finest dynamic balancing equipment available. All work is completed and signed off by a certified A&P mechanic (not all repair stations use certified mechanics). By having the work completed by a certified mechanic, you can be sure that we will do it right the first time!

The MicroVib II Aircraft Analyzer uses Mil spec aircraft vibration sensors and receives an annual calibration traceable to NIST. The MicroVib II allows us to perform and record both dynamic prop balancing and  engine vibration analysis.

                 

Above: Downloaded Balance History Display/Polar Plot

Run 1) 0.403 IPS     Run 2) 0.155 IPS     Run 3) 0.021 IPS

 Above: Downloaded Vibration Analysis Display

Why Should I Balance?

Recommended by Hartzell, McCauley, and most Reciprocating and Turbine engine manufacturers. We use accepted and approved balancing procedures as outlined in manufacturers maintenance manuals and FAA Advisory Circular 20-37E.

Reduce Maintenance Costs: Smoother operation means less component fatigue and longer engine/airframe life. Excess vibration will shorten the life of internal engine components, engine accessories, alternators, avionics equipment, vacuum pumps, exhaust systems, engine baffling, spinners, brackets and more.

Reduce Pilot/Passenger Fatigue: Vibration is a major element that increases wear and tear on airframes, but did you know it also increases pilot and passenger fatigue? Numerous studies have shown that vibration significantly contributes to pilot physical discomfort and fatigue. Excess Vibration  can also effect the bodies extremities, if you find your hands or feet falling asleep during flight this is a good indicator of excess airframe vibration. Less vibration means you'll arrive at your destination feeling better.


We Guarantee Results:

We typically see beginning vibration levels from 0.25 - 0.60 IPS (Inches Per Second) or more, anything above 0.20 IPS is considered out of limits and requires balancing.

The industry standard after balance limit is  0.15 IPS, this where your average balance shop will say it's "good enough".

We guarantee a level of 0.07 IPS or less after balancing, less than half the industry accepted after balance limit!

How can we get results like that you might ask, it simple; we take accurate measurements, we pay attention to detail, we take the time to do it right, and we have the best equipment available to verify the final results. Average balance job takes approximately 2 hours and 3-5 runs to accomplish.

Documentation: When the job is complete we provide you with a computer printout documenting the entire process of dynamic balance with beginning and ending levels, polar plot and  vibration spectrum analysis, and a signed log book entry placard. All records are maintained for historical purposes in both digital and paper formats for future reference.

Location & Wind Conditions:  Winds are usually not a problem unless gusts are very high (above 20 kts), the MicroVib II is able to average multiple readings eliminating the wind/gust factor that limits most other balancers to calm wind conditions.

 


Excessive Vibration Symptoms include:

  • Instrument panel shaking
  • Cracking of engine baffles
  • Generator/Alternator failures and mounts breaking
  • Exhaust system cracks
  • Spinner and Spinner bulkhead cracking
  • Feet "falling asleep" during flights
  • Repeated instrument & avionics  failures
  • Carburetor air box cracking
  • Cowling and sheet metal cracking
  • Cracked or broken engine mounts

Exp Aircraft Services Current Capabilities:

  • Fast, Accurate Dynamic Prop Balancing

  • Single and Twin engine

  • Engine Vibration Analysis

  • Vibration trend monitoring

  • Balance History Record Keeping

  • Vibration Troubleshooting

  • Tachometer Checks

       (Turbine Engines coming soon)


Vibration Range Limits

DANGER 1.25 Inches Per Second (IPS) Peak Amplitude. The propeller should be removed and a static balance performed.

VERY ROUGH 1.00 Inches Per Second (IPS) Peak Amplitude. Propeller can be dynamically balanced; however a large amount of weight will be required. A propeller static balance is recommended. Operation at this vibration level could cause damage.

ROUGH --- 0.50 Inches Per Second (IPS) Peak Amplitude. Propeller definitely requires dynamic balance. Long term operation at this vibration level could cause excessive wear.

SLIGHTLY ROUGH -- 0.25 Inches Per Second (IPS) Peak Amplitude. Dynamic balance will improve passenger comfort.

FAIR---- 0.15 Inches Per Second (IPS) Peak Amplitude. This is the maximum acceptable vibration level after dynamic balancing.

GOOD ---- 0.07 Inches Per Second (IPS) Peak Amplitude. Vibration levels below 0.07 will not be detectable by pilots or passengers.

                   

                 

                                  DSS MicroVib II Balance Kit

 

Prop Balance being accomplished on a Van's RV8A


FAQ: Dynamic Propeller Balancing

What is dynamic propeller balancing?

Dynamic Propeller Balancing is the process whereby an electronic balancer is used to measure the vibration produced by the aircraft power plant. Small trim balance weights are added to the propeller/crankshaft assembly to correct for errors in mass distribution and to reduce power plant vibration due to mass imbalance to the lowest level practical.

How is it done?

The engine/propeller combination is balanced right on the aircraft, in a flight ready state. A small vibration sensor (accelerometer) is attached to the engine in a location where vibration due to mass imbalance is maximum. A small tach pickup (photo-tach) is also mounted to the cowl or engine to produce a propeller tach signal. The engine is operated and the vibration and tach signals are processed by the electronic balancer.

The balancer produces a vibration level (magnitude) which corresponds to the amount of mass imbalance which exists. The balancer also provides a "phase angle" which corresponds to the location of the mass imbalance on the propeller disk. The vibration level and phase angle are used to compute a balance solution (weight amount and location). The balance solution is added to the propeller and the measurement is repeated until the vibration level is found to be acceptable.

My aircraft runs fine. Why should I have my prop dynamically balanced?

By all means, have your propeller balance checked. The average airplane which has not had a Dynamic Prop Balance has a vibration level due to propeller/crankshaft mass imbalance of about .450 inches per second (IN/S) velocity. This level is more than four times higher than what is considered to be an acceptable vibration level for propellers and represents a significantly higher level of wear and fatigue on engine components and accessories. This average level is usually very noticeable to the pilot and occupants. Yours may be higher or lower than the average but only a mechanic with a vibration analyzer can tell for sure. On the average, 19 out of 20 fixed-wing aircraft can benefit from Dynamic Prop Balancing yet many will never have it done.

My engine and prop were overhauled recently. Should I have my prop dynamically balanced?

Both new and used components need to be dynamically balanced. In fact, the best time to dynamically balance a propeller/engine combination is right after overhaul when components are fresh. In the DSS study, no appreciable difference was found between the vibration levels of recently overhauled and longer time propeller/engine combinations. Even brand-new aircraft with zero time engines and propellers need to be dynamically balanced.

What level of propeller vibration is acceptable?

In most cases, the vibration level due to mass imbalance can be brought down to under .100 IN/S very easily. In the DSS study, the average level seen post-propeller balance was .039 IN/S. When propeller vibration levels are this low, the operator will generally see a significant reduction in component wear and fatigue and will find that the aircraft "feels" like a completely different machine.

In addition to that the mechanic now knows what all of the other engine vibration levels are (Prop/crankshaft mass imbalance usually dominates them) and can utilize that information if additional work is needed.

But won't having my propeller dynamically balanced "mask" other engine problems?

No! An engine with an internal problem which results in unusual vibration will not respond to balancing in the same way that an engine which only suffers from mass imbalance will. A qualified mechanic will use ALL of the information available to make a judgment about your engine, including the vibration response.

THEORY: Reciprocating Engine Vibration

Piston Engine Vibration

An internal combustion engine produces power in the form of controlled explosions. These explosions produce powerful pulses of energy that cause the engine to vibrate in response. Engine designers do their best to make these forces cancel out to minimize vibrations. But, no matter how well the designer does his job, he cannot eliminate all inherent vibrations in an engine. Therefore we need to remember that it is perfectly normal for an IC (Internal combustion) engine to produce a characteristic vibration spectrum signature. Vibration analysis of IC engines then must focus on "variations" from the "normal" vibration signature.

Normal Vibration Signature

Each combustion pulse acts much like a hammer blow, hitting the engine block with a pulse of energy. The vibration spectrum of such a pulse is a series of vibration spectral lines. These spectral lines will be at integer multiples of the firing rate of each piston. In a four stroke engine the piston fires every other revolution, therefore the fundamental spectral line will be at 1/2 the engine RPM, often called the 1/2 order vibration. The result will be a vibration signature that has spectral lines at the 1/2 order, 1P, 1-1/2P, 2P, 2-1/2P, 3P ... etc. In our experience, we have found that most 4 and 6 cylinder horizontally opposed IC engine produce these spectral lines in varying patterns but the levels are usually in the range of .2-.5 IN/S. For unknown reasons we have found it is not uncommon for an engine to produce a 2P or 2-1/2P vibration in the range of .5-1.0 IN/S. These readings are taken on the ground at typically 2500 RPM.

1/2 Order Vibrations

It turns out that if all of the pistons produce nearly identical combustion pulses, the 1/2 order vibration will be very small, .1-.3 IN/S. When any one cylinder produces less power than the rest, the 1/2 order vibration will increase from .3 IN/S on up to over 1 IN/S for a misfire. Mechanics know very well what can cause one cylinder to be weak. Plug misfire, bad plug, plugged injector, broken ring, leaky valve, low compression, bad magneto, worn cam, collapsed lifter, etc. Any of these things can cause a higher than normal 1/2 order vibration.

1/2 order vibration are especially troublesome as they can be felt in the cabin by the pilot. Low frequency vibrations are not well isolated by most engine mounts, and the vibration will shake the entire aircraft. If not taken care of, a 1/2 order vibration can loosen rivets, hinges, and pivots all over the airframe, as well as causing premature pilot fatigue. This is a serious safety issue.

1P Vibrations

1P vibrations are usually dominated by propeller imbalance. Happily we can correct this with propeller dynamic balance. 1P vibrations can also be caused by unequal piston mass. Pistons and cylinders are often replaced 1 jug at a time, and once in a great while the wrong weight piston is used due to error. In horizontally opposed engines, this will produce a 1P vibration in the horizontal plane, but little vibration in the vertical plane. An out of balance prop will produce nearly equal vertical and horizontal vibrations. If the rear of the engine has a high 1P vibration that is not corrected by balancing the prop, piston mass imbalance is the most likely cause.

What are the causes of unwanted (excess) cabin vibration?

An unwanted cabin vibration can be generated by a variety of problems:
- Propeller mass imbalance (mass distribution problem)
- Propeller aerodynamic imbalance (blades not pitched equally or airfoils not matched)
- Abnormal engine vibration (combustion problems or unmatched engine components)
- Normal engine vibration transmitted abnormally into the airframe (poor engine to airframe isolation)
- Aerodynamic excitation of airframe by prop wash.

Why go to the trouble of balancing my propeller first?

In each of the situations above, the very first step in troubleshooting an aircraft vibration problem is to dynamically balance the propeller. Because propeller imbalance is by far the most common cause of excess cabin vibration, we eliminate the majority of cockpit vibration problems with this first step.

Okay, so now my propeller is balanced, what comes next?

After the propeller imbalance is eliminated, other engine vibrations can be analyzed and traced to their source. A vibration analysis signature is taken on the engine and then compared to signatures taken from similar engine models. With some engines, a vibration signature check will point to a combustion problem, perhaps caused by a dirty injector, a dirty spark plug or an ignition problem.

Although rare, sometimes a vibration signature check will point to an internal engine problem. When an engine problem is found this way, the engine can usually be repaired before the problem becomes unmanageable.

My engine vibration is normal, what is causing my unwanted cabin vibration?

 If the engine vibration is reasonable, then vibration readings are collected in the cabin so that the "felt" vibration can be evaluated. While the engine may be producing "normal" vibration levels, these normal vibrations may be transmitted abnormally to the airframe due to lack of isolation between the engine and firewall. A vibration check can point the way to detection of isolation problems such as bad engine isolators, hoses which do not have adequate service play, exhaust system to airframe contact, etc.

What about aerodynamic excitation of the airframe?

Even with good mechanical engine to airframe isolation, vibration analysis can be used to detect and identify other unwanted cabin vibration. The air mass which envelops the aircraft can provide a transmission path for unwanted vibration. Any time the propeller is producing thrust, propeller induced air pulses can excite parts of the airframe such as thin or poorly attached windshields. With some aircraft, replacement of the windshield with a thicker aftermarket unit will reduce cabin vibration considerably.

Please contact us for more info.


 




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