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common causes of Rotor head vibration
are of course a great many possible causes of vibration on the head
assembly of any helicopter, but some are maybe a little less well known
than others. This page is going to try to cover ALL of the
possible reasons for vibration. They are in no particular order.
- Main shaft not straight
really, but still needs stating. To check it quickly, the easiest way is
to remove the "Jesus Bolt" that holds the main rotor hub to
the mainshaft, disconnect the rods to the swash plate, and lift the head
off in one piece. The you can spin the mainshaft up and if
necessary, line the top of the mainshaft up with something else behind
it, and then slowly spool the helicopter up.
the top of the mainshaft can be seen to be wobbling, even only a little
bit, REPLACE IT.
- Rotor blades out of balance
the instructions given all over the place on how to balance your rotor
- Rotor blades not tracking correctly
the instructions given all over the place on how to track your rotor
- Feathering shaft not straight
obvious, but still needs stating. To check this quickly, the easiest way
is to remove the rotor blades, and then use a box spanner or hex driver
or whatever your particular heli uses to hold the feathering shaft in
place and twist it at one end only. Watch the other end very
carefully, and if it does not rotate in the same plane all the time (in
other words you can see it wobbling as well), REPLACE IT
- Flybar not straight
obvious, but it is surprising that fly bars can be out of true, but
still appear to be straight. To check this out quickly, the easiest way
is to loosen any grub screws or other fixings that hold the control arms
to the flybar, then do the same for any fixings that hold the flybar in
the flybar holder. This should let you rotate the flybar freely in
its holder, and see if it really is straight. If not, either
remove it and straighten it if you feel you can do so, successfully, or
- Flybar Paddles not balanced
this is something that a lot of pilots forget about checking, but it is
equally important to ensure that each paddle is at least exactly the
same weight. To check, remove flybar entirely (after marking the
exact position of the inside of each paddle when fitted securely) then
refit paddles to those positions and place on a knife edge or similar to
find the centre point of balance. Mark that and then measure the
distance from there to the inside of each paddle. It should of
course be the same distance.
not, apply tape to the lighter paddle in the normal manner until the
balance point is dead centre. Then reassemble, ensuring that the
paddles are refitted to the original marks. Double check that the
distance of each paddle away from the flybar holder is the same.
- Flybar paddles at uneven distance form holder
described above, measure the distance form the inside edge of each
paddle to the hub, and ensure that both are the same distance.
- Loose fitting screws
are of course a lot of little screws in most rotor heads today, and some
of them may seem to be fairly inconsequential, but in fact they can
easily be crucial. One of the main culprits of this syndrome are
those little tiny screws that lock the flybar holder in place.
This is especially true if you have a CNC head, and several people have
even reported the need to grind a little off the end of these screws to
stop them bottoming out and causing binding when fully tightened
down. With all CNC parts, including heads, it goes without saying
that you should use thread lock on ALL SCREWS.
check the washout levers to ensure that they are correctly tightened so
that there is no rock on the mounting bearing. If there is, this is
magnified by the washout levers and causes a really nasty vibration as
well as poor cyclic control responses
check the balls on the swash plate for any sign of looseness. It
is amazing how often you do find that over time one or more does manage
to get itself loose, even if it is thread locked into a CNC swash
plate. Oh, by the way, do NOT be tempted to use thread lock in
plastic threads, it will not work and will cause damage to the plastic.
- Worn mainshaft bearings
pretty obvious, but it is fairly hard to identify a bearing that is
getting loose, especially one such as the centre bearing used in the
Belt CP, as this has three main bearings, unlike many helicopters that
only use two main bearings. It is a very good idea to check these
every 100 flights at most. To do so, remove the bottom "Jesus
Bolt", disconnect the rods from the servo horns and pull the
mainshaft and head out in one piece.
loosen the fixing bolts for both sides of the bearing holders and slip
the bearings out, or if necessary remove them from the aircraft
completely in the bearing holders.
each bearing in some soft cloth or wooden jaws or similar and lock them
into a vice tightly enough to hold them in position, but don't
distort the outer case. Now get an old main shaft if you have one,
slip it into the bearing, and try to rock it LEFT & RIGHT (not up
and down) as you will be able to see how much movement there is on the
inner bearing casing. If it is noticeable to any real extent -
for all the bearings and then refit, using new bearings if necessary.
- when ordering bearings the way the measurements are written are as
follows : 4 * 8 * 2.5 means that the outer diameter is 8mm, the
inner diameter is 4mm, and the thickness of the bearing from side to
side is 2.5mm. Don't get them the wrong way around !