Written by Ashley Davis Sunday, 16 April 2006 00:00
The following modification can be performed on any Swift suffering from vibration during spool up or spool down. This vibration has also been called the "funky chicken".
This particular phenomenon is caused by the feathering spindle being too tight inside the rotor head block. It can also be exacerbated by the undercarriage. Century have produced new head block's and a new undercarriage in response to this problem. The new head block's are supposed to have more room for the feathering spindle. The new undercarriage has webbing all the way around to strengthen it and stop vibration.
The first task in this modification is to sand down the ball in the centre of the feathering spindle. This is extremely easy to do. I used some medium wet and dry sandpaper in order to sand down the central ball. I achieved this by putting the feathering spindle in a drill and then just holding the ball in the sand paper. A few seconds spinning the drill at high speed is enough to remove the required material to make the feathering spindle nice and loose within the head block. The aim is to make it such that the feathering spindle can move freely back and forth inside the head block. Below are some before and after pictures showing the central ball.
No modification is required apart from fitting the new fully webbed undercarriage. Below are some pictures of the new undercarriage showing the webbing.
Greasing the spindle and dampers
When refitting the feathering spindle make sure that plenty of Teflon based grease is put around the central ball. Additionally the dampers should have a liberal coating of Teflon grease so that they can move freely on the feathering spindle shaft.
The last part of this modification is to move the mounting points for the tail boom supports. By default the tail boom supports are connected to the undercarriage on two tabs about one quarter of the way down the legs of the rear undercarriage struts. In practice I have found that these mount points to vibrate and can cause some vertical vibration on the tail boom.
The fix to this is to find a new mount point for the tail boom supports that will provide a much stiffer and vibration free mount point. The obvious solution to this problem was to mount the tail boom supports onto the bolts holding the undercarriage onto the main body of the helicopter.It was here that I met a little problem. The tail boom supports need to be spaced away from the carbon battery tray as otherwise the undercarriage bolts cannot be secured properly. I used extra long 3 mm bolts in order to bolt the undercarriage and the boom supports into place. The spacer I used was the small brass spacer found in some T-Rex kits as inserts in the blade root hole.The carbon rod for the boom support also needs to be modified in order that the plastic boom support end can be rotated through 90° to be bolted on to the underside of the undercarriage. This is a simple modification requiring the plastic end to be removed. Then a hole needs to be drilled at 90° to the factory drilled hole. I used a 1.5 mm drill bit in order to make the new hole. The plastic boom support end cap can then be replaced and secured with the small Phillips head screw. Below are some pictures of the tail boom support modification.
Having made these modifications I now have a very smooth running helicopter with no vibration at spool up or spool down. Since moving the boom supports to their new location I have also eliminated some vertical vibration on the tail boom. The whole helicopter is now running much more smoothly.
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