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.
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.
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.
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.
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.
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.
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.
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.
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.
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.)
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.
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.
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 editor [AT] piperflyer [DOT] com.
PowerFlow Systems, Inc.