Is Your Engine Worn Out? How to Tell & What to Do About It

Is Your Engine Worn Out? How to Tell & What to Do About It

 

Smart owners who monitor key performance indicators can tell if an engine is still good or whether “it’s time.” If your engine is due for an overhaul or replacement, STEVE ELLS has a list of options which can save you time, money and maybe even both.

 

The day before the start of what I’m now calling the best EAA AirVenture Oshkosh ever (See page 60 for Steve’s AirVenture report. —Ed.), I stood before an enthusiastic group of Cessna Flyer Association members at the annual Gathering at Waupaca, Wisconsin. It was 7:30 a.m. Sunday morning. I made sure everyone was awake by asking a scary question.

I asked how many owners thought they had an engine overhaul looming on the horizon. Seven hands went up. Those owners reflected the concerns of many owners. Engine overhauls are expensive; not to mention they can be time-consuming and stressful.

It is difficult for owners who don’t deal with overhauls on a daily or weekly basis to be able to tell when “it’s time.” An engine can be worn out, but it will still start, develop power and appear to be operating normally. On the other side of the coin, it’s also certainly possible for an engine to be running well and in good condition far beyond the manufacturer’s recommended time between overhaul (TBO).

I’m going to provide a few guidelines for determining your engine’s health.

The engine’s bottom end (and why it matters)

The air-cooled direct-drive engines we fly behind are stout; especially the “bottom end” portions. The bottom end includes the case, crankshaft, connecting rods, camshaft, lifters and accessory gears and accessory housing.

Just because the compression is low in one, two or all cylinders does not mean the engine is ready for an overhaul. Cylinders can be removed and rebuilt, or replaced with new cylinders without disturbing or compromising the bottom end. But when an engine’s bottom end is worn out, nothing short of an overhaul will restore it to airworthy condition. 


Oil pressure

Idling oil pressure when the engine is hot is an excellent indicator of the health of the bottom end of an engine. The hot idling oil pressure of our engines should always stay above the lower red line on the oil pressure gauge. 

The oil pressure limits and acceptable range are in every owner’s manual and pilot operating handbook (POH). As a rule, Lycoming engines have a 25 psi low oil pressure limit and Continental engines have a 10 psi low oil pressure limit. 

One of the most important factors in maintaining oil pressure is the clearance between the crankshaft journals and main crankshaft bearings. The spinning crankshaft in an engine is supported by a cushion of lubricating oil under pressure. 

Since there is a gap between the outside diameter of the journals of the crankshaft and the inside diameter of the main bearings surrounding each journal, the oil that’s pumped in also flows out through the gap between the two. The size of the gap is a major determinant of idling oil pressure. When the gap grows due to wear, the leakage through the gap increases and idling oil pressure goes down. Low idling oil pressure almost always signals that the bottom end of your engine is worn out, or that there’s another problem with the bottom end.

Oil consumption limits

It’s rare for an air-cooled Avgas-burning engine to not use any oil. Manufacturers are tasked with producing engines that must perform in conditions ranging from below zero F outside air temperature (OAT) to 100 F plus OAT. The engines must produce rated power in missions where the aircraft may take off from extremely hot temperatures on the ground, only to climb rapidly to altitude where OATs are below freezing. Given all the metallurgical expansions and contractions that take place due to these extremes, air-cooled aircraft engines are intentionally built to larger tolerances than any automobile engine.

Oil usage is one of the trade-offs that result from building air-cooled engines that perform as well as ours do. 

If your aircraft’s engine uses oil, that’s normal. But how much is too much? Luckily, there’s a formula for that. Lycoming’s Service Instruction 1427C, “Lycoming Reciprocating Engine Break-In and Oil Consumption,” provides the following formula: 

0.006 x BHP x 4 ÷ 7.4 = quarts per hour.

Let’s find the allowable oil consumption for a 180 hp engine. BHP is an acronym for brake horsepower, so the formula works out like this. First, multiply: 0.006 x 180 x 4 = 3.6. Dividing that by 7.4 yields a maximum oil consumption for a 180 hp engine of 0.58 quarts per hour, or a quart every 1.7 hours. 

The same formula applied to a 300 hp engine yields a maximum oil usage
of 0.97 quarts per hour. 

The only drawback with very high oil consumption is that it limits flight leg length. If your engine has a 4-quart sump, you’re not going very far if your engine is going through 2 quarts an hour.

Many owners are unaware that each engine and airframe combination has an oil level “sweet spot,” where consumption slows. 

Above this level, much of the oil is discharged out the crankcase breather tube. The oil is not being consumed; it’s simply being pumped out the breather tube. If you see a lot of oil on the belly of your airplane aft of the breather tube, you are probably over-oiling your engine.

The sweet spot in the 1966 Cessna 182J Skylane with a Continental O-470-R I used to own was 9 quarts, even though the oil capacity was 12 quarts. The consumption rate for my current Lycoming O-360 is 1 quart every five hours. My average cross-country leg is around four hours so I just carry some oil and add about a quart at every stop. 

The key is to first fill to the sweet spot for your airframe/engine and then use consumption from that level to determine your engine’s oil consumption. 

Oil leaks

Damaged engine cases can cause persistent, hard-to-find oil leaks. Cases can and do crack, leading to loss of oil. 

Lycoming narrow-deck engines—the standard configuration before the mid-1960s—can develop a difficult-to-find leak when the engine case through bolts are loosened and then retightened during a cylinder change or top overhaul. The sealing O-rings between the case halves often fail to reseal the through studs after the cylinder(s) are reinstalled and torqued down. The result is a persistent oil leak past one or more of the through studs. 

There’s no way to stop that leak, nor is there a way to fix a leaky crankcase crack short of engine disassembly. 

Section 6-4.12 of Continental Motors Publication M-0, “Standard Practice Maintenance Manual for Spark Ignited Engines,” covers crankcase inspections and allowable cracks. There is a provision for continued operation of certain engines with limited cracks in noncritical areas of the crankcase. However, the engine will continue to leak oil through the crack. 

I once found a leak in my engine by thoroughly cleaning the outside of the engine, then adding a small amount of fluorescent dye to the oil. I bought the dye and a black light at the local auto parts store. I waited for a dark night, then after a ground run, found the leak by shining a black light on the engine. I rebuilt the engine soon afterward. (Be aware of all regulations and the potential hazards before introducing a foreign substance into an aircraft’s engine or oil. —Ed.)

Oil screen and oil filter inspections

Always cut open the spin-on oil filter and inspect the filter media for contamination. I cut the paper media at the edges so I can unfold it for visual inspection. 

Engines that don’t have a spin-on filter will have a pressure screen. Remove it at every oil change and flush it.

If the filter media or screen reveals a quantity of metal that exceeds a quarter teaspoon, Lycoming mandates grounding the airplane until the cause can be found. Lycoming Service Bulletin 480F describes proper procedures for oil filter or screen inspections as well as corrective actions if the inspection shows contamination. 

Jacqueline Shipe’s article “I Found This in my Oil” (May 2017 issue of Cessna Flyer) provides a pictorial guide to oil filter inspection. —Ed.

 

Black oil

If the engine oil turns black in the first 10 hours after an oil change, yet the compression readings are good, combustion gas byproducts are blowing past the pistons and piston rings into the bottom end of the engine. The oil will continue to lubricate, protect and cool the engine, but due to the contamination from combustion byproducts, it’s a good idea to shorten the oil change interval. 

Compression tests and borescope valve inspections

Never pull a cylinder based on one compression reading. Compression test results can vary from flight to flight. Always fly the airplane to bring temperatures up into normal operating range. If you have a low reading, go fly a bit, and then perform a second, and possibly a third compression test. 

Lycoming’s guidelines specify that each cylinder’s compression reading should be above 70/80, and within 5 psi of the engine’s other cylinders. When compression readings fall below 70/80, Lycoming says that’s the result of wear and should be further evaluated. 

There are very detailed instructions in Continental Publication M-0, Chapter 6-4.11.1 through 11.3 describing procedures and guidelines for compression tests. For instance, tests are only valid if a calibrated compression testing tool is used. The calibration procedure provides a low limit compression reading number for that specific testing tool.

Any cylinder with a compression reading above that limit is airworthy, provided a borescope internal inspection of the cylinder does not show cylinder wall scoring or extreme wear and the exhaust valve does not show any signs of burning. 

Many A&P technicians are not aware of the proper compression testing procedure for Continental engines. If your mechanic calls saying your compressions are too low, make sure he reads and understands the Continental procedures which are spelled out in detail in Chapter 6-4.11.2 of Continental Motors Publication M-0.

I strongly recommend that all airplane owners download this manual (it’s free) from the Continental website. There’s
a wealth of general information that, in my opinion, is useful to all air-cooled
airplane engine operators.

Now what?

Let’s assume that you’ve gotten some bad news from these tests. You’re facing an engine overhaul or replacement. What are your options?

There’s a choice of factory new, factory overhauled, factory rebuilt, repair station overhauled or field overhauled engines.

This is also an excellent time to research the STC data on the FAA website to find out if there are any engine upgrades such as installing a more powerful engine in place of the original engine. Some airframes may be eligible for engine upgrades via STC. An upgraded engine may be able to give you better performance and/or reliability. 

Finally, you may want to consider replacing your worn-out engine with a lower-time used engine. 

Cessna OEM engines

Obviously, buying a new “zero-time” engine from Lycoming or Continental will be the most expensive option. A factory rebuilt zero-time (exchange) engine is usually the next most expensive, followed by a factory “time since major” overhaul where the manufacturer overhauls your current engine. 

There are some very good reasons to deal directly with Lycoming and Continental. First, the price quoted is fixed, meaning there won’t be any unexpected price “modification” phone calls. 

Second, it’s broadly accepted that a factory zero-time engine will add value to any airplane. Remember that there are two flavors of factory zero-time engines. A brand-new factory engine is built from all new parts. A rebuilt engine is built with a combination of new parts and used parts which meet new limits. Both come with fresh, zero-time logbooks.

Third, and maybe the most important, is that you can continue to fly your airplane until the day your new engine is drop-shipped to your hangar or the nearest maintenance shop.

It’s a great advantage to have the removed engine and the new engine side-by-side during an engine change. This ensures that all the fittings are available and that routing questions can be answered without having to rely on memory or digital photos taken prior to engine removal. 

All Continental and Lycoming factory engines are sold with a core charge. The core charge for a Lycoming O-360-A1A engine is currently $16,400. If a buyer wants to keep the engine that’s been removed, or can sell it for a better price than the core charge, he/she is free to do that. However, the core charge must be paid if an engine is not returned to the factory.

The window to return the removed core engine is usually 90 days. 

Recent offerings

A relatively new option in new engines is Superior Air Parts’ Type Certificated fuel-injected 180 hp Vantage engine. It is approved for Cessna 172R and 172S models via an STC. 

Several companies have obtained or are in the process of obtaining approvals for installation of diesel engines in some Cessna models. 

 

Repair station or field overhaul

There are excellent non-manufacturer overhauls and not-so-good non-manufacturer overhauls. The excellent ones are built to new limits. The not-so-good are built to what’s called service limits. It’s legal for a shop to build an engine to the worn end of the manufacturer’s service limits guidelines. Of course, the engine won’t last as long as a “new limits” overhaul. When you’re gathering quotes from overhaul shops, make sure that you specify that you want your engine overhauled to new limits. 

Choosing a factory overhaul means your airplane will be down while your engine is removed, boxed for shipping, overhauled and shipped back. During a repair station overhaul or field overhaul of your engine your airplane will be down while the engine is disassembled, the parts inspected and certified, and the engine is reassembled and tested. Smaller repair stations and field overhaul shops typically must box and ship the ferrous parts and the engine case to a specialty shop for inspection and certification. 

There are 77 Type 1 (less than 400 hp) engine repair stations listed in the FAA’s repair station directory. Repair stations have submitted organizational plans and plans for parts accountability and quality assurance to the FAA.

What is included in an overhaul?

Factory engines typically come with a carburetor or fuel injection system, two magnetos and ignition harness, spark plugs, starter, oil cooler and engine-driven fuel pump. This is also the case with most non-factory overhaul options, but you’ll want to double-check to make sure these items are included.

It’s important to take notice of and budget for what’s not included. Time and money must be devoted to inspecting, purchasing, repairing and in some cases overhauling the turbocharger and wastegate (if installed), the exhaust system, the engine mount, the cooling baffles, the generator or alternator, hoses, engine mount and vibration isolators, propeller, prop governor, vacuum pump and fuel boost pump. 

Though you don’t necessarily have to replace or rebuild all of these items at the same time as the engine overhaul, it’s certainly more cost-effective to address them when the engine is already off the airplane. Access is easier, and you can minimize installation and removal hours. 

Most of the larger overhaul shops have worked out favorable pricing with over-the-road shipping companies but shipping costs must also be included during overhaul budget planning. 

It’s also critical to compare the warranties offered by each vendor as there is no industry standard for coverage. 

Can I overhaul my engine myself?

The FAA considers the overhaul of all except a very few engines to be minor repairs, not major repairs. This assumes that the person doing the work adheres to the procedures in the manufacturer’s engine overhaul and inspection manuals. 

You as the aircraft owner (or anyone else) may overhaul your engines, as long as a certificated A&P mechanic oversees the work and he/she is willing to sign off the overhaul. 

If you or your mechanic aren’t ready to do it yourself, there’s no reason a local machinist with years of engine building experience can’t build your engine. Again, this assumes the factory overhaul procedures are adhered to and an A&P is willing to supervise and sign off.

There are some caveats to this approach. 

• Your A&P must agree to this option, and must oversee it to the extent that he/she will sign it off.

• You (or the builder) must use aircraft quality parts.

• You (or the builder) must comply with applicable engine manufacturer Service Bulletins.

• You (or the builder) must comply with all applicable Airworthiness Directives (ADs).

• You (or the builder) must follow the machining processes outlined by the engine manufacturer. 

• You (or the builder) must follow the engine manufacturer’s break-in procedures.

If you’re not sure about the details involved in a light aircraft engine overhaul, there’s a 36-minute video on rebuilding a Lycoming engine on YouTube. (See Resources for the link.
—Ed.) Once you watch the video, it’s easy to see that these engines aren’t complex, nor are they difficult to overhaul. 

Aircraft owners have another option to enhance their knowledge prior to attempting an overhaul. Lycoming offers engine teardown and assembly classes throughout the year in Pennsylvania.

Used guaranteed engines

Another option to get your aircraft back in the air is to buy a used, serviceable engine from an aircraft salvage yard. This is not as radical an option as it may sound. All reputable salvage yards guarantee (warranty) their engines.

Ideally, you’re looking for a first run or first overhaul engine which is mid-time or less. For instance, as of the writing of this article, Wentworth Aircraft had an O-360-A3A with 217 hours since major overhaul for sale for $15,500. Though this engine wouldn’t do me any good (I have an -A1A, not an -A3A, and the two aren’t interchangeable), it does illustrate that there are cost-effective used engines available. The used route is dependent on finding the correct engine. 

An advantage of installing a used engine is the lack of core charge. The $15,500 cost mentioned above could be reduced by a few thousand dollars if you’re able to find a buyer for your core. Your worn-out engine may be just what another owner or kitplane builder is looking for.

Another source for used serviceable engines are engine upgrade specialists. Check the Cessna Yellow Pages online or call CFA for more information about Cessna Flyer supporters. 

You can also often find good engines on the buy-and-sell pages of various online forums, via the For Sale/Wanted thread on the CessnaFlyer.org forums, or through the advertisers in this magazine. 

 

Takeaways

Not every engine showing trouble signs needs an immediate overhaul. However, if you and your mechanic have determined an overhaul or replacement is needed, there are several options. Take your time, do your research and you’ll be back up in the air soon. 

Steve Ells has been an A&P/IA for 44 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and lives in Templeton, California with his wife Audrey. Send questions and comments to

Resources
Further reading and research
Lycoming Service Instruction 1427C,
Lycoming Service Bulletin 480F, and
Contintental Motors’ publication M-0
CessnaFlyer.org/forum under “Magazine Extras”

FAA STC data
rgl.faa.gov/Regulatory_and_Guidance_Library/rgSTC.nsf/MainFrame?OpenFrameSet

FAA Repair Station directory
av-info.faa.gov/repairstation.asp

Factory engines/rebuilds/factory overhauls – CFA supporters
Continental Motors Group
continentalmotors.aero/engines/gasoline-engines.aspx

Lycoming Engines
lycoming.com/services

Superior Air Parts
superiorairparts.com

FAA Repair Stations – CFA supporters
Airmark Overhaul, Inc.

airmarkoverhaul.com

Granite Air Center
graniteair.com

Poplar Grove Airmotive
poplargroveairmotive.com

Other rebuild and overhaul resource – CFA supporter

Progressive Air Services

progressiveair.com

Salvage yards – CFA supporters
Dodson International Parts, Inc.
dodson.com

Preferred Airparts, LLC
preferredairparts.com

Wentworth Aircraft
wentworthaircraft.com

Engine rebuild video

SkywardTech Inc.

youtube.com/watch?v=n-26eqLc4pQ

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Propeller Vibration & Dynamic Balancing

Propeller Vibration & Dynamic Balancing

Feeling a bit shaky when you’re flying (and tired when you land)? It might not be your nerves; but rather the side effects of excessive vibration. Vibration can originate from an aircraft’s engine, propeller or spinner, and if unchecked, can lead to further mechanical problems. Vibration can also cause pilot and passenger fatigue. Luckily, computerized dynamic propeller balancing is a cost-effective route to help dampen those pesky vibrations.

Aircraft propellers are manufactured to be durable, strong and able to absorb the stresses of flight. Depending on the model, propeller blades weigh anywhere from a few ounces to several pounds. The rotational speeds and aerodynamic loads imposed in flight on a propeller make it susceptible to vibration if it has even the slightest imbalance.  

Excessive vibration can come from an aircraft’s engine, propeller, spinner or a combination of all three.


Why vibration matters

No matter what its source, excessive vibration can lead to a number of problems. The engine’s vibration isolators are designed to filter out most of the vibration so that it is not transmitted to the airframe, but they don’t eliminate all of it. 

An out-of-balance propeller that causes the engine to vibrate in its mount will wear out the vibration isolators. Cracks in the airframe can form as a result of excessive shaking. Cracks can also form on the cowling itself, and on the spinner or spinner bulkhead. 

Vibration can cause cracked or loose exhaust connections. As an illustration of the importance of balance when it comes to exhaust systems, the makers of PowerFlow exhausts actually require a dynamic propeller balance for their exhaust systems to qualify for an extended warranty. 

That’s not all. Vibration is hard on engine components and can cause premature engine wear. On top of creating mechanical issues, vibration is also source of pilot stress and fatigue.
Where’s it coming from?

The frequency at which a vibration is occurring gives a clue as to whether the vibration is being caused by the propeller itself or the engine. 

Vibrations are counted by the number of vibrations per revolution (of the propeller). They are referred to as “two-per,” “half-per” and so on.  A one-per vibration occurs on every revolution of the propeller and is indicative of the propeller itself, or in rare cases the engine crankshaft. A half-per vibration occurs on every other revolution of the propeller and is usually caused by a cylinder malfunction. A small vibration occurring at a frequency greater than once per revolution (two-per or more) is usually indicative of bearing wear or a malfunction in an accessory such as the alternator. 

Vertically-mounted static propeller balancer
Causes of vibration: engine and mounts

As noted above, vibration can originate from sources other than the propeller. The engine and engine mounts can be a culprit.

Compression imbalances or a cylinder with excessively low compression can cause vibration. Extreme wear in crankshaft counterweights can also cause vibration. 

Worn engine vibration isolators permit excessive vibration, and can allow the front of the engine to sag downward.

A cracked engine mount can cause a great amount of vibration. 

Horizontally-mounted static propeller balancer
Causes of vibration: spinner

Spinners that have a heavy spot due to a manufacturing defect or repair can cause a slight imbalance which produces vibration. Laying the spinner on a flat table and slightly rolling it can sometimes detect a heavy spot on the spinner. If the spinner comes to rest with the same spot on the bottom each time it probably has a heavy spot.  A spinner with a heavy spot can make dynamically balancing a propeller difficult.

The forward tip of the spinner should be aligned with the center of rotation of the propeller. If the nose of the spinner appears to wobble when observed by an onlooker outside of the plane as the engine is run, the spinner should be realigned by loosening the mounting screws and retightening them as the spinner is held firmly in place. 

Cracked or broken spinner bulkheads can also cause the spinner to wobble. It is a good idea to inspect them closely if any defect is noted.

Static balance weights for use on a controllable-pitch propeller
Causes of vibration: propeller

Vibration originating from the propeller is usually caused by a mass imbalance. A mass imbalance is when the center of gravity of the propeller is not in the same location as the center of rotation of the propeller. It is usually caused by the removal of material on blades to repair nicks or from differing degrees of blade erosion. Luckily, this can often be remedied by balancing the propeller and checking for correct blade track and indexing.

Sensor mounted on top of cowling with a clear view of the rear of the propeller
Static propeller balancing

Aircraft propellers are statically balanced at the time of manufacture and at propeller shops. Static balancing is the process of checking the weight of the hub and blades for even distribution. This ensures that the propeller is not subjected to any turning or bending force due to a heavy area on one of the blades or hub. 

During a static balance, the propeller is mounted on a mandrel resting on low-friction bearings so that the propeller is free to spin, with a minimal amount of force needed to move it. (See photo 01, page 26.) 

When the propeller is turned slightly, it should remain in the new position it is placed in without backing up or continuing to turn. The process is similar to balancing a wheel assembly. 

If an imbalance exists in the propeller or hub, the heavy area will cause the propeller to rotate so that the heavy spot ends up on the bottom. 

Some shops mount the propeller in a horizontal plane on the top of a shaft that has an indicator rigidly suspended from the bottom of the mounting shaft. (See photo 02, page 26.) 

As the propeller is turned, if the indicator on the bottom of the shaft leans to one side rather than maintaining a vertical position, the propeller has a heavy spot. 

Weights can be added or subtracted to the hub to statically balance most controllable-pitch propellers. (See photo 03, page 26.)

Fixed-pitch propellers are statically balanced by removing an allowable amount of material from the heavy blade. 

Static balance is initially adjusted at propeller assembly and fine-tuned after the propeller is completely assembled and painted. Propellers with de-icing (“hot props”) are adjusted after all anti-ice boots are installed. 

Reflective tape placed on a propeller blade
Propeller blade track

Once a propeller has been statically balanced and installed on the aircraft, the track of each blade should be checked. The blade track refers to the path that each blade tip travels. On a perfect propeller, the tracks will be identical. 

The track is checked by placing a solid object next to a propeller blade near the end so that the propeller blade is free to rotate past it, and marking exactly where each blade tip passes the object. There shouldn’t be more than 1/16 inch in difference between the tracks. 

The airplane needs to be chocked so that it can’t move and the propeller should be pushed in slightly against the engine as each blade is checked to remove the endplay from the thrust bearing in the engine. A blade that is out of track will cause an aerodynamic imbalance because its angle of attack will differ from the other blade or blades. Also, differing blade tracks can indicate that the propeller has been damaged in some way

Analyzer (above), sensors and cables (left). Ready for a test run.
Propeller indexing

The propeller index refers to the location on the crankshaft flange where the propeller is installed. Engine and airframe manufacturers designate where the propeller should be installed on the crankshaft flange with the No. 1 cylinder’s piston on top dead center of the compression stroke. Typically, on most small, two-blade, fixed-pitch propeller aircraft, the propeller is installed with the top blade aligned with the bolt-hole preceding the vertical position as viewed facing the propeller. This corresponds roughly with the 2 o’clock and 8 o’clock position.

There’s no reason to guess as to what indexing is correct. The maintenance manual for each aircraft model gives the specifications on where to install the propeller on the flange. Propellers installed in the incorrect location on the flange can cause vibration.

Dynamic propeller balancing

Dynamic propeller balancing is the process of checking for vibration while the propeller is in motion. The propeller is installed on the engine and the engine is run through its complete rpm range. 

A dynamic balance is performed using a vibration-detecting sensor mounted to the top of the engine, and a photo sensor mounted so that it has a clear view of the rear of the propeller blades. The sensor detects a reflective piece of tape placed on the rear of one of the blades each time it passes through the sensor’s beam. (See photos 04 and 05, page 28.)

The vibration sensor is an accelerometer containing a crystal which detects the direction and amount of force of each vibration. The sensor is calibrated and reads the force in inches per second (IPS). This information, along with the location of the reflective tape provided by the photo sensor, is transmitted to the analyzer. 

The analyzer attached to both sensors gathers information about the amount and frequency of any vibration, accurately records rpm and calculates the amount and location of weight to be added to correct an imbalance. (See photo 06, page 30.)

After the engine and propeller run, the specified amount of weight is placed in the location given by the analyzer. The weight is added according to the propeller balance equipment manufacturer’s instructions. Usually, the propeller is rotated by hand so that the reflective tape is lined up with the photo sensor. The number of degrees shown on the screen of the analyzer marks the spot needing the weight. A measurement is made from the accelerometer in the direction of propeller rotation, and the spot is marked.

On airplanes with Lycoming engines, the weight is usually added to one or more of the holes on the outer section of the starter ring. On airplanes with Continental engines, the weight is usually added by drilling a hole in the spinner backing plate. An AN3 bolt or #10 structural screw with a locknut and stacks of large-area washers are used to add weight. A maximum of six washers is allowed per screw. 

It may take several runs and additions and subtractions of weight to eliminate an imbalance, or at least bring it into a reasonable range. Sometimes, especially with Lycoming engines, the weight can’t be placed at the location pinpointed by the analyzer because a hole isn’t available at that exact spot. In that case, the weight should be halved and installed at two different holes on each side of the target location. 

The installed weight(s) should be checked for adequate clearance from the starter and other components by pulling the propeller around by hand and making sure the weight doesn’t come close to contacting anything as it rotates with the propeller. 

Vibration levels are labeled according to a standard scale. Vibration levels of 0 to 0.07 IPS are considered good. 0.07 to 0.15 IPS is considered fair. 0.15 to 0.25 is considered slightly rough. 0.25 to 0.5 is moderately rough. 0.5 to 1.0 is very rough, and 1.0 to 1.25 is considered dangerous. 1.20 is the maximum allowable FAA limit for a dynamic imbalance. 

Dynamic balancing errors

The person performing a dynamic balance should use high-quality, calibrated equipment. Erroneous sensor readings will cause weight to be added in incorrect locations. 

The propeller and spinner should be clean before the balance procedure is started. Spinners should be removed and cleaned on the inside as well, especially on propellers that require grease. 

Controllable-pitch propellers should be greased and serviced properly with nitrogen as required before the balance procedure is started. 

Finally, the weather conditions should be favorable. Accurate readings are best obtained when the engine and propeller is run in calm air. The plane should be pointed into whatever wind is blowing, not only to aid in engine cooling, but also because a tailwind or crosswind can affect the readings.

Troubleshooting other problems during a dynamic balance 

The propeller balancing equipment can also be used as a troubleshooting aid when an engine does not seem to be developing its normal power.

The photo tachometer gives an accurate rpm indication at full-throttle static rpm.  If the engine is not making its full rated power, further investigation is warranted. A lower-than-normal output is indicative of an engine defect or excessive wear. 

Most airplanes use mechanical tachometers. These tachometers are seldom accurate and can read high or low when compared to actual engine rpm. The photo tachometer is more accurate for troubleshooting use. It will also help determine whether a mechanical tach is reading incorrectly; and if so, by how much.

Conclusion

Most airplanes have at least a slight propeller imbalance, even if it hasn’t become bad enough to be noticed by the pilot. It is always best to correct vibration problems early because they tend to grow in magnitude as wear occurs. 

The benefits of propeller balancing greatly offset the cost. Reducing vibration helps reduce wear and fatigue, extending the service life of many components not only on the engine, but on the airframe itself. 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

Resources

PowerFlow exhausts
PowerFlow Systems, Inc.
powerflowsystems.com

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