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Complaints: Major Vibrations!
Symptoms:
Tail fin shakes and vibrates rapidly, like a hummingbird’s wings. Tail bounces up and down, or left to right. It seems that the gyro has to work extra hard to keep the tail straight (pilot is forced to make many corrections, even while in a hover). The entire heli does a very defined wobble. The fuel in the fuel tank ripples and shakes. There is a constant buzz noise that seems unordinary. Tail belt clangs and shakes in the boom (make sure it is tight before attributing this to vibration). Fasteners seem to come loose far too often. There seems to be a “radio glitch” that you can’t track down, no matter what channels you try. Ay of your electronics fail far too often or prematurely.
Source #1: The Engine
A two-stroke engine is a very powerful and light workhorse. With this power comes a whole lot of vibration. The very design of the model two-stroke engine contributes to the amount of shake it causes. You have a crankshaft with a large piston attached at one side and a relatively small counterweight at the other. The job of that counterweight is to attempt to create equilibrium at the shaft, meaning that it offsets the centrifugal forces the piston assembly creates. Unfortunately, this counterweight cannot offset all the force differences caused by the piston and the combustion chamber when the engine is running at up to 16,000 or more RPM. The vibration caused by the engine will be of a very high frequency—it manifests itself as a buzz or very fast shake. This high-pitched vibration is also the kind that can wreak havoc on electronics and fasteners when it is amplified enough. The first thing to do with an engine is make sure it is tuned correctly. Set the needles properly and get the mixture right. A poorly tuned engine can induce vibration problems on its own, so get that cause singled out right off the bat.
Source #2: The Main Drive Train
The main drive train (which consists of the flywheel, clutch, bell, start shaft, and pinion) creates roughly the same vibration symptoms as the engine. This is due to the fact that these parts generally spin at the same speed as the engine. If you have a high frequency shake that won’t go away and an inspection of the engine yields no answer, this is where to check next. It is important to have these parts as balanced as possible, as an out of balance main drive is the cause of many pilots’ vibe problems.
Inspecting these parts is very simple, provided you use the right tools. On many helis, the start shaft is integrated with the clutch and flywheel assembly. Once bolted to the motor, this becomes a fairly long rotating mass. Add into the equation of the high RPM that these parts reach and you can see how even a small imbalance can be amplified to a large problem. It is recommended that you balance and dial indicate the flywheel and cluth assemblies during the build. If you can get these components spinning straight and balanced, they will not add any new vibration to the heli. This will also give the ball bearings that support the main drive a much longer life. By the way, be sure to check out all the ball bearings in the heli during this process, as a bad bearing can cause vibrations and glitches as well.
Source #3: Main Rotor System
The main rotor system consists of the main gear, main shaft, and everything attached to the main shaft. The vibrations caused by problems in this system need not be confused with those in the main drive or engine. This assembly runs at a much lower RPM usually (between 1500 and 2000). Because of this, the symptoms you may see when ther is an imbalance in this system will usually manifest themselves as a bounce or wobble. Unlike high frequency vibrations, this is basically a shake that is slow enough for your eyes to follow. So if your heli’s tail rotor bouncing up and down at a fairly defined rate, it would be a good bet to look here.
Checking the main rotor assembly is roughly the same as the main drive. First check the main shaft and spindle for any bends. Rolling them on a piece of flat glass is a great way to do this. Make sure that the head block seats completely and square with the main shaft, as the head needs to spin true with the main shaft to be vibration free. Lastly, be sure to balance the main blades. An out of balance set of blades will cause a wobble in short order.
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