ChopperAddict's Hints & Tips

CHOPPERADDICT SPECIALISES IN THE BUILDING, REPAIRING & 
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INFORMATION - Some common causes of Rotor head vibration

There 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.

1 - Main shaft not straight

Obvious 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.

If the top of the mainshaft can be seen to be wobbling, even only a little bit, REPLACE IT.

2 - Rotor blades out of balance

Follow the instructions given all over the place on how to balance your rotor blades correctly.

3 - Rotor blades not tracking correctly

Follow the instructions given all over the place on how to track your rotor blades correctly.

4 - Feathering shaft not straight

Again 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

5 - Flybar not straight

Sounds 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 REPLACE IT

6 - Flybar Paddles not balanced

Surprisingly 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.

If 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.

7 - Flybar paddles at uneven distance form holder

As described above, measure the distance form the inside edge of each paddle to the hub, and ensure that both are the same distance.

8 - Loose fitting screws

There 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.

Also 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

Regularly 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.

9 - Worn mainshaft bearings

Yes, 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. 

Then 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.

Wrap 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 - REPLACE IT

Repeat for all the bearings and then refit, using new bearings if necessary.

NB - 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 !

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