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Old 01-20-2013, 11:09 AM   #4 (permalink)
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Join Date: Jan 2010

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There's a bit more discussion on this topic there

Originally Posted by snemi00 View Post
In order to have a properly balanced tail or main rotor assembly, the distance from the centre of the tailshaft to the mounting hole for the blades must be identical. This is almost always accomplished on a heli by using the end of the spindle or the spindle face as the reference plane.
So, when the tail is assembled correctly, the outer thrust washer face is to be perfectly flush with the end of the tail hub. The screw head, or as included in the new tail assemblies, the washer that comes with the screw, are only there to hold the components in from extending out past the end of the hub. This is why the washer that comes with the screw does not fit over the spindle of the hub. It should bottom out and create a nice square stop for the thrust bearing to bear on.

If you add a washer that fits over the hub, then you will be pushing the thrust bearing in past the end. You would have to do the exact same thing on the other side to make sure it is balanced. But, it is impossible, since you will end up removing all the slop on one side differently than on the other.

The slop, in and out axially along the spindle direction, does not matter because as the rotor spins the amount of centripetal force is HUGE. This forces the grip assembly out and both of them should sit perfectly balanced at the end of the hub against the underside of the screw head or washer if you have put it in. It really doesn't matter if you can push the grips in 1mm or 3mm or whatever, all that matters is how far out they go.

A washer under the screw head is now used in the assembly as it is creates a very nice square face as a datum plane for the thrust bearing to be pressing on. Although the screw head alone could work, the underside of some screw heads are not as square and consistent as others and also have less surface area than the beefy washer.

As a side note, if you have a chance to look at the new Compass Warp machine, there is identical slop in the main rotor grips in and out. This is for the same reason as the SAB tail grips. A single reference plane to keep tolerance on that increases accuracy while making manufacturing simpler and more consistent.
Unfortunately, it creates user confusion as it is unconventional.
SAB Heli Division Rotorquest
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