ChopperAddict's Hints & Tips



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INFORMATION - Some common causes of Tail/Boom vibration

There are of course quite a few possible causes of vibration on the tail assembly and boom 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 in these areas. They are in no particular order.

1 - Belt crossed twice in the boom

Sounds simple, but this can cause vibration, as well as binding on the tail.

2 - Tail blades not balanced

Although only small, these can very often be different weights, and therefore need to be balanced before fitting.

3 - Bent tail drive shaft

These can still get bent very easily, especially in what seems to be only a gentle crash.  In fact even grinding the tail blades on something hard can bend them. Easy enough to check, rotate the main head by hand and watch the outer end of the tail drive shaft to ensure it does not rotate in an ellipse, but runs true al the way along.

4 - Tail blades tightened up too much

The tail rotor blades should be nipped up a little looser than the main blades, so that they can drop with their own weight.

5 - Bad bearings in tail drive

It never fails to amaze me how rapidly these bearings seem to fail.  They really need swapping out every 5- - 75 flights to be reasonably sure of them.  To check them you usually need to strip the tail box down, and then rock the tail drive shaft in each one individually to see if there is excessive rock in the bearing itself.  If there is, REPLACE IT.

6 - Rock on the central blade hub

Check by gripping each blade and pulling/pushing them from the tips in and out towards/away from the tail box to check that the bearings in the grips do not have excessive wear/rock.  If they do - REPLACE THEM

7 - Boom support rods not tightened, or broken

These are typically aluminium or carbon fibre, but they fail quite regularly, often with the rods coming out or coming loose in the plastic ends.  If this happens, the rods fail to perform their main task, which is actually to dampen harmonics, and not necessarily to support the boom itself, so make sure they are tightly secured to the aircraft AT BOTH ENDS

8 - End float in tail drive shaft

Try pulling the tail rotor shaft sideways in and out of the tail gearbox.  If it moves any more than a tiny bit you need to find out why as it should not do so.

9 - Belt too tight

Obvious, but always ensure that your belt is not too tight.  An over tight belt causes a lot of strain on the tail drive shaft as well as the tail driven shaft, and will cause vibrations.

10 - Teeth missing on the tail drive gear in the tail gearbox

Always ensure that the teeth are in good condition. Missing teeth can cause hard to identify vibrations due to the belt jumping occasionally.

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