Anyone at the tail stage should carefully note the instruction to scuff
and groove the bushes within. I have just towed up the country an early
builder's tailplanes and found both bushes on one side were no longer
bonded. Whether this was due to previous mountings, or vibration when
perched on the trailer posts is impossible to say.
The early instructions I understand didn't mention grooving, and maybe
the epoxy needs to be critically right to bond to metal -any chemists in
the audience?. The root end bush can experience some nasty twisting
forces as it's difficult to visually align the tail plane with the tube
in two dimensions, with no parallel edges to sight with. When you get it
in, the bush guidance is not enough to take it cleanly into the inner
bush, and a few clonks on the bush end may be enough to disturb a poor
bond. Insertion of a tufnol guidance cone in the tube-end might be a good
idea, and reinforcing the pip-pin recesses so that if there is a failure,
the pin head has a chance of preventing the axial departure of the
tailplane (12mm is enough to separate the drive pins).
Of course any misalignment of the inner bush will mean you won't get the
pin in anyway, but in the quoted case the bush could be rotated by a rod
through the hole when not on the tube.
Graham Clarke (G-EMIN 83)
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