Table of Contents
Heli vibration can be broadly categorized into two types:
Main rotor vibration
The main symptoms of this are landing gear vibrating, and the vertical tail fin vibrating left/right.
Tail rotor vibration
The main symptom of this is the tail boom vibrating up/down at high speed.
If you follow these directions step-by-step, you should be able to solve most vibration problems.
The basic methodology is to keep removing parts from the helicopter until the vibration stops, and when the vibration stops the last piece you removed was causing the vibration problem.
Note that some helicopter models are known to have problems. For example:
Many Hummingbird Elite CPs appear to have been shipped with warped plastic head blocks (CNE205).
Early models of the Robbe Eolo were shipped with warped tail parts which caused tail vibration.
Checklist for main rotor vibration
Check the obvious
Check if main gear mesh is too tight.
Check if the main gear is missing teeth.
Check if the tail belt is too loose or too tight.
Check main blade tracking. Put a piece of marking tape on one blade and run up the motor. Check if the blades are tracking properly. If the blades do not track properly, then see the section on blade tracking.
Check if the blade grip bolts are too loose or too tight. Hold the helicopter so the right side or left side is facing down, and see if the main rotor blades will stay horizontal to the ground. If they just barely stay horizontal then this is just right.
Check main rotor blade CG and balance. Balance the main blades on a blade balancer and ensure the CGs match and the blades balance. If the CG does not match or the blades do not balance, then see the section on blade balancing.
Check if the feathering shaft is bent. Disassemble the main rotor hub and remove the feathering shaft. Check the feathering shaft using the directions given for the main shaft below. The Corona doesn't have a feathering shaft, and instead has an aluminum rod which holds the subrotor on the rotor hub. This rod is very thin and has a tendency to bend after a few crashes. You will notice this because the subrotor paddles will "droop down" and will not be parallel to each other. This problem can be easily fixed by carefully bending the subrotor paddles up to straighten the aluminum rod or bye replacing the aluminum rod.
Check if the rotor head is balanced. Reinstall the head and swashplate assembly but remove the main rotor blades. Connect the battery and slowly apply throttle. If the heli is vibrating then you should check if the flybar is straight and centered and balance the main rotor head using a Tru-spin prop balancer or equivalent.
Check if the main shaft is bent. A quick test for this is to remove the main rotor head, swashplate and all the related linkages. Run up the motor without the head and hold a screwdriver against the top of the shaft as the shaft turns. If you feel vibration while holding the screwdriver against the turning shaft, then the shaft is obviously bent.
If you do not feel any obvious vibration, then the shaft may still be bent. To perform a more precise test for bent shaft checking, first you need a perfectly flat surface, preferably a sheet of glass. It isn't necessary to remove the glass cover from a frame; just use the entire photo or award.
Second, take a new shaft and place it on the glass and tilt the surface slightly. Watch it roll back and forth as you tilt the surface. The first thing you should notice is a straight shaft will easily start rolling from a stopped position. You do not need to tilt the surface very much to start it rolling. The second thing to notice is a straight shaft can be rolled at all speeds. If you tilt it only slightly, it rolls very slowly. If you tilt it a lot, it rolls very quickly.
Now remove the new shaft and place the suspect shaft on the glass. Roll the suspect shaft back and forth as you did the new shaft. If the suspect shaft requires more tilt to start rolling than the new shaft, then it is bent. If the suspect shaft cannot be rolled as slowly as the new shaft, then it is bent.
If you hold the surface to your eyes and roll the suspect shaft, then you may be able to see gaps between the shaft and glass as the shaft rolls. If you can see these gaps, then the shaft is obviously bent.
Check the main rotor bearings. Remove the main shaft bearings from the helicopter. Put the main shaft through each bearing and spin it by hand. If the bearing does not rotate smoothly or feels gritty, then replace it.
Checklist for tail rotor vibration
Check tail rotor blade balance
Reinstall the main rotor shaft and main gear, but do not reinstall the head and swashplate assembly. Remove the tail rotor blades and run up the helicopter. If the vibrations are gone, then the tail blades need to be balanced or replaced.
Check the tail blade grip spacing
If the tail blade grip screw is not screwed in equally on both tail blade grips, then the tail blade grips will not be the same distance from the center of rotation which will cause vibration.
Check the tail rotor shaft
Remove the tail rotor shaft and follow the directions in the Bent Shaft Checking section.
Check the tail rotor bearings
Remove the tail shaft bearings from the helicopter. Put the tail shaft through each bearing and spin it by hand. If the bearing does not rotate smoothly or feels gritty, then replace it.
Tail rotor hub balancing
If the vibrations remain, then the tail rotor shaft and hub need to be balanced. Put the entire assembly on a Tru-spin prop balancer or equivalent for balancing.
If the tail is vibrating left/right (usually noticeable by looking at the vertical tail fin) then this is usually caused by problems in the main rotor system, such as a bent main rotor shaft or unbalanced main rotor blades.
If the tail is vibrating up/down, then this is usually caused by problems in the tail rotor system, such as a bent tail rotor shaft or damaged tail rotor blades.
On a Corona, the gear on the hollow TR shaft (56633) can become chipped, and it's very hard to notice because the gear is inside the tail case. Try cleaning the old lithium grease out of the tail case with a Q-tip and check for orange pieces of plastic in the old grease. If you do find these plastic bits, then the TR shaft (56633) should be replaced.
Tail vibration can be caused by tail blade holders being too tight. Some helicopter upgrade tail hubs promise "dual or triple ball bearings which remove all slop..." This is a bad idea. Most variable-pitch tail rotors are designed as a flapping hinge rotor which require slop in the tail blade holder.
If this slop is removed, then this destroys the ability of the flapping hinge to equalize the thrust from both tail rotor blades. This causes high-speed vibrations which may weaken the tail blade holder screw and eventually shear it.
For more information on flapping hinges, search the web for the phrase "delta three angle" or go to the following URL: www.scotiabladerunners.ca/delta.htm