Vibration is the enemy. It foams fuel, confuses gyros, cracks solder joints, cracks metal and plastic frame components, and it looks bad too. Every drivetrain component, from the piston to the rotor blades, is a potential source of vibration, so figuring out what's shaking your helicopter can be a monumental pain in the ass undertaking.

It can help to consider the frequency of the vibration, and then consider what parts of the helicopter are spinning at speeds that would produce vibrations of that frequency...

Frequency	Source	Destination
High 200-300 Hz (12,000-18,000 RPM at the crankshaft)	High frequency vibrations usually emanate from the beginning of the drivetrain, where things are spinning fastest. This means everything from the piston to the pinion, and the cooling fan is a common culprit.	High frequency vibrations often cause a "blurring" of the horizontal and vertical tail feathers, and foaming of the fuel.
Medium 6000-9000Hz	The tail rotor spins at about half the speed of the engine. The drivetrain is probably pretty well balanced, but a bent tail rotor output shaft or tail rotor hub can cause plenty of trouble.	Watch the end of the tail boom. If it's shaking differently from the rest of the helicopter, something is probably amiss back there.
Low 20-35Hz (Rotor head RPM ÷ 60)	The main rotor is the slowest-turning thing on the helicopter. It's also the heaviest spinning part, and thus it's the one with the most obvious symptoms when it isn't perfectly balanced. The blades must be matched, and must track perfectly; the flybar must balance, and the paddles must be parallel not only to each other but to the swashplate as well.	Major imbalances will not go unnoticed. I've seen a helicopter that couldn't even get the main rotor up to speed before putting Middle Eastern bellydancers to shame with its gyrations. Minor imbalances will be evidenced by shaking skids and canopy ears.
Ultra-Low 2-5 Hz	Kyosho Concepts are especially prone to this, but it's not unheard-of in other helis. I don't fully understand this myself, but it's believed to be a resonance of the feathering spindle dampers. A higher head speed (1700 RPM or so) will usually cure it.	You'll see a very distinctive slow wobble of the entire helicopter, best described as "the hula dance."

Here's a checklist, sorted by vibration frequency:

High

cooling fan runout
cooling fan balance
engine bearings
engine mixture

Medium

tail rotor blades
tail rotor output shaft
tail rotor hub
tail rotor drive shaft

Low

main blades
main shaft
flybar weights/paddles
blade axle

More food for thought

Any time a bearing goes bad, it becomes a potential source of vibration. The speed at which the bearing turns will not necessarily determine the frequency of the vibration, whichs makes tracking down a bad bearing a real pain in the ass time-consuming process.

Curtis Youngblood suggests placing tail drive shaft supports unevenly along the shaft. This way each section of the shaft oscillates at a different frequency, which in turn keeps a vibration in one section from inducing vibrations in adjacent sections, which in turn keeps overall vibration levels down.

Boom supports are susceptible to vibration, and two or three different manufacturers will be happy to sell you a boom support brace that brings them some additional rigidity.

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