The point being made by Graham and myself was that new builders should avoid
the problem by leaving a clearance.
It has been illustrated that the problem is temperature variable because the
application of heat reverses it.
It is a fact that the loosely fitting control stick sleeve makes it vulnerable
to out of square fitting, with resultant binding of the extremities at either
side of the vertical stick position.
It is writ that the coefficient of expansion of aluminiun is about 23,5 times
ten to the minus 6 per DEGREE CENTIGRADE.
It is up to the factory to specify what that clearance could be, but if the
worst case pinch is half a thou, your man with the plastic bag spacing washer
looks to be a good bet.
The stiffness is caused by the aluminium rubbing on the tufnol. Assuming the
pressure is constant across the surface, the stiffness is going to be
proportional to the radius at which the friction is acting. Now, if the large
section of the bracket is leaning towards the tufnol, you have the worst
pressure and friction at by far the biggest radius. Half a thou does not sound
very much, but if you think how much weight you would have to hang onto the
bush assembly to make it strech that far, you can see that the consequenses
would be noticable.
Imagine a man standing on that flange whilst you were trying to rotate it.
Then try to imagine the flange changing in diameter from 1 inch to 1 foot
(internal dia changing at same rate). The sitffness would increase from
"noticeable" to "impossible".
My solution of creating a clearance on the largest radiuses between the
control stick yoke and the tufnol bearing worked, and it does make sense.
OK. So the stiffness may be transitional as we go through the phases of,
1.chilled alum bits with warm tufnol to, 2.chilled alum,bits with chilled
tufnol, thro, 3. everything warmed up again by the cockpit heating. The point
is that the skeleton is out of the cupboard, and new builders can avoid it.
Meanwhile, those of us who have made really nicely fitting assemblies which
stiffen can look for ways of rectifying the problem, or living with it. Grind
it, saw it, rebuild it, worry about it, or forget it. If youve got it, you
will probably want to talk about it!
It is good to see the subject getting such a good airing, but there must be
other problems. ---------Now about those hinges!--!!!
Regards Bryan Allsop
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