Colorado Rotor Heads

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Main Rotor Vibrations, Troubleshooting, and Tracking
Colorado Rotor Heads – April ’97 meeting

Author: Tom Toledo

Vibration is inherent in any type of machine. Unusual vibrations are generally produced by a malfunction. Worn bearings, unbalanced assemblies, or loose hardware can cause these malfunctions. Severe vibrations in a RC helicopter rotor system can cause serious damage to or destruction of the model in a short time. Because of the high stress load on a rotor system during operation, the slightest out-of-balance or out-of-track condition can create these damaging vibrations. For this reason, helicopter designers have been forced to use many different methods of damping and counter-acting vibrations to an acceptable level. Driving engine, driveshafts, rotors, and gearboxes at different speeds can help control sympathetic vibrations. A sympathetic vibration is created when a vibration from one source causes a vibration at the same frequency in a second part or component. You will see examples of this and other types of vibrations in the following paragraphs as we discuss possible causes in a rigid or semi-rigid model helicopter rotor system, along with main rotor and tail rotor tracking.

Note: Bear in mind that a certain amount of vibration exists in all helicopter rotor systems because of the rotating parts and the stresses created during flight. When the vibrations reach an abnormal level, the pilot should take responsibility for locating and correcting the cause of the vibration before attempting to fly the machine again.

There are basically 5 main types of vibrations found in a model (or full-scale) helicopter that can create damage or pre-mature wear and tear to the mechanical and radio components. They are lateral, vertical, low frequency, medium frequency, and high frequency vibration.

A lateral vibration (two beats per revolution) can be caused by an unbalanced rotor system; for example, one blade is heavier than the other is. Improper span-line alignment between the two blades can also cause an unbalanced condition. Loose, worn or cracked parts often contribute to an unbalanced main rotor and manifest themselves as a lateral vibration. There are three types of lateral unbalance: chord-wise, span-wise, and a combination of the two.

To isolate a chord-wise imbalance from a span-wise, apply a strip of masking tape around the tip of one blade and hover the helicopter. If the lateral vibration decreases and begins to increase again a short time later, it indicates that a span-wise balance is okay, but a chord-wise imbalance exists. The blade’s chord-wise Center of Gravity will have to be adjusted.
To isolate a span-wise imbalance, apply a strip of masking tape to the tip of one blade and hover the helicopter. If the vibration increases, remove the tape and install it on the opposite blade. Tape is then added one strip at a time until the vibration is eliminated. The tape is then replaced with equal weight, or weight approximating equal moments is secured at a specific location on the blade. For example, the tape weight may have to be doubled if the replacement weight is added at the midway point of the blade’s span (or 3-4X tape weight if added at the blade retention bolt).
Frequently, what I have found is a combined chord-wise and span-wise imbalance existing in most rotor systems. With knowledge and experience it is possible to correct both imbalances at one point on the rotor blade but it usually requires each imbalance to be corrected separately. If you are still unsuccessful in balancing the rotor system by accomplishing the previously discussed procedures, inspect the rotor system for loose, worn, bent or cracked parts. To name a few:

	Check the flybar for misalignment between the head, differing lengths/distance to paddles, and of course, look for a bent flybar.
	Check paddle alignment for out-of-track paddles.
	Check the spindle or cross shaft for bending.
	Ratchety or worn thrust or ball bearings in blade grips and mixing levers.
	Uneven distance between rotor hub and blade grips (shimming?)
	Uneven torque on blade retention bolts (uneven blade lead/lag) and mixing levers.
	Bent main shaft.

A vertical vibration is caused by a blade being out of track, which is identified by a helicopter bouncing up and down during hover or flight. A blade lifting the helicopter in one quadrant of rotation, and suddenly losing lift in the remaining quadrant during each rotation cycle causes the vibration. When this lifting force is experienced once during each revolution, it is referred to as a one-to-one vibration. To correct this vibration, first apply tape to one of the blades and have an observer watch the blade path during the hover and identify which blade or tape color is high or low. If there is no observer available then run the helicopter on the ground at about 1/3 throttle/collective and kneel down to observe the blade path. You will have to make the determination based on your rotor RPM if you want to raise the low blade or lower the high blade.

Once a good ground or hover track is attained, the helicopter is then flown to determine whether blade crossover exists. Blade crossover is caused by one blade being more limber than the other. This condition is mostly evident in wood blades and can exist because of manufacturing tolerances (different wood density between blades), different flying time on blades or an unbalanced condition. Any one of these conditions allows a light or limber blade to climb with increased airspeed because of the various forces created during flight. Blade crossover is experienced when blades are almost perfect in track when hovering and during forward flight you will notice the blades are out-of-track (mostly when viewed from the side and during a low pass).

A low frequency vibration of one or two-per-revolution is caused by the rotor. One-per-revolution are of the two basic types, the lateral or vertical; both were discussed earlier in this paragraph. Associated with the one per revolution is an intermittent one-per-revolution vibration. This is a vibration started by a gust of wind causing a momentary increase of lift on one blade resulting in one-revolution vibration. The momentary vibration is normal, but it can be picked up by a loose collective/cyclic control system and fed back to the rotor system causing several cycles of vibration, which is undesirable.

Note: Two-per-revolution vibrations are inherent with a two-bladed rotor system, and a low level of vibration is always present. When the two-per-revolution vibration level rises to an unacceptable level, it is caused by faulty dampers (O-rings) or a loose and worn rotor system (control rod ball and clevis, washout unit, mixing levers, etc.) or even from excessive binding.

Loose skid mounts; struts, or canopies can be a source of vibration that feel as though they are coming from the rotor system.

A medium frequency vibration at four to six-per-revolution (6000-12000 beats per minute) is common in most rotor systems. An increase in the level of the vibration is caused by a change in the capability of the main frame/fuselage to absorb vibration from the main/tail rotor, driveshaft, and engine because of loose hardware, frame damage, or the load. Usually this vibration is caused by loose components that are either a regular part of the aircraft or external pieces. A few that come to mind are:

Loose or flexible tail fins, boom supports, engine/start shaft mounts, tail rotor gearbox mounts, canopy not mounted securely, skids not secured to landing gear struts, too tight of a gear mesh, bent or twisted main frames, loose radio components, fuel tank mounting, worn or damaged ball bearings, etc.

A high frequency vibration can be caused by anything in the helicopter that rotates or vibrates at a speed equal to or greater than the tail rotor. This narrows it down to mostly the tail drive wire/shaft, tail rotor gears and tail rotor and engine/fan/start shaft assembly. The first step in isolating this vibration is to check the tail rotor blades for proper track and balance. Then check for:

Bent tail rotor hub spindle, tail rotor output shaft, misaligned torque tube U-joints or bent/twisted drive wire, misaligned or off-center fan assembly, bent start shaft, misalignments between engine/clutch/start shaft, engine mixture settings, loose muffler/pipe or mount, worn or damaged engine bearings, etc.

A vibration coupled with a high pitched squealing noise could indicate a possible seized or spinning bearing race in the start shaft or clutch assembly area. You want to land immediately if this happens!

In summary, a two bladed rotor system usually has problems with a low, medium, or high frequency vibration. A recent test comes to mind where two aircraft mechanics, experienced in vibration analysis of full size aircraft, attached an AVA (Aircraft Vibration Analysis) unit to an XCell .60 helicopter. Sensors were attached to the helicopter and it was then hovered to record the vibration in stock form. They immediately recorded overall high levels of vibration and confirmed it through visual observation with the initial thought being the rotor system were to blame. Although, they eliminated at least half of the initial vibration by carefully balancing both main and tail rotor systems, the remaining vibration source was pointing to the engine.

As a side note, once the main/tail rotor systems were balanced, the visual observation noted that the helicopter was not vibrating and appeared to be smooth. The sensors told otherwise, as there was still at least another 30% reduction needed for vibration levels to be at an acceptable level. Another significant reduction in vibration was achieved by careful needle valve adjustments of engine mixture settings. They summarized, however, that these careful adjustments made to the mixture could not be set consistently and the vibration levels could fluctuate significantly from day to day. Because of the inherent design of our single cylinder two-stroke engines we will continue to have opportunities in reducing overall helicopter vibration levels for some time to come…or at least until we can all afford a smoother running turbine engine!

Tom Toledo
tom.toledo@peterson.af.mil
(303) 680-4972

The Colorado Rotor-Heads is a group of R/C helicopter pilots based in Denver, with members all along Colorado's Front Range. Our members belong to many of the Front Range clubs from Colorado Springs to Fort Collins. As a result, you may see our members at many of the local club fields and even the occasional cul-de-sac. If you would like to learn more about the Colorado Rotor-Heads, or R/C helicopters in general, please visit our web site at www.coloradorotorheads.com.

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