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<< Lancair Builders' Mail List >>
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It is my opinion that laying up more than 3 layers of bid at one time, i.e.,
6, 8, or 10 bid in one sandwich, is false economy of one's time.
In multiple bid layers, the strength is only as good as the attachment of the
first layer.
If the multiple bid layer is so thick that it resists the conformation of the
parts being joined, the result may be that the most important lower layers
pull away from the surfaces and leave bubbles that can't be seen, and most
certainly can't be worked out if they were seen.
Bubbles of any size are death to a bond. They must be absent.
Even if bubbles don't appear under the first layer, there may be a tendency
for the heavy multiple layer to move away from the contours and wick in epoxy
under the first layer to allow the movement.
A thick layer of epoxy is almost as bad as bubbles. Epoxy by itself has no
strength. The strength comes in the bonded matrix of fiber glass with the
epoxy. Thus, when the first layer is separated from the contour of the parts
being bonded by a thick layer of epoxy which was not worked out, the result
is a very poor joint.
So, how do you avoid bubbles and thick epoxy layers in your multiple thick
bid buildups?
Simple--you build up the layers 2 or 3 bids at a time, working out the
bubbles and pushing the layers into the contours with a stiff brush.
Yes, it is two or three or four times as much work. But, when you are going
250+ kts and input some drastic control movement sometime, your 8 bid layup
has the strength of 8 bids, not the strength of 1 bid, and your plane is not
going to come apart.
The extra care and time is well worth the extra effort.
David Jones, Pecatonica, Illinois
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LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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