Jim – Here are a few things I would suggest to isolate the source of vibration. Keep going down the list and you should find the problem.
1. Remove the main rotor blades and start the engine. Since this is an experiment, take the helicopter well away from the pit area and run up the engine to about hover rpm. Look at the normal places you would see vibration, such as the fuel tank, canopy, tail fin, ect. Obviously if the vibration stops then the main blades are out of balance – at least for this scale application. If the vibration is still there, move down the following list.
2. Now remove the tail rotor blades (leaving off the mains) and do the engine run up again.
3. Disconnect the tail rotor drive from the main gear – so nothing is turning inside the tail boom – and again perform the engine run up.
4. Notice we are taking off pieces of the helicopter one at a time to try to find the source of vibration. I would hope by now one of the above steps have found the offending piece. However, if not remove the head and do the engine run up again.
5. If the vibration is still present, there are only two more possibilities – the engine/clutch combination or the main drive system/main shaft. This should be rather easy to check out for loose bolts, worn bearings, misaligned engine installation, etc.
As anyone who flies helicopters knows, vibration can come from many sources and is extremely tricky to isolate and cure. However, it is the silent killer to many of our helicopters, and why we have problems in flight, so curing the problem as much as possible is definitely worth the time and effort. And no matter how frustrated I may get in the process of finding and eliminating vibration, I always remember there is an engineering cause, and all I have to do is play detective to find the problem and apply the correct cure. I know we can’t remove all vibration, but whatever portion we can find and remove will make our flying that much more enjoyable, and reduce the wear and potential causes of future mishaps.