From: Fredmoreno@aol.com
Message-ID: <8259e2bd.36e80cdb@aol.com>
Date: Thu, 11 Mar 1999 13:35:07 EST
To: lancair.list@olsusa.com
Subject: Acetone, MC and bubbles and oil cooler doors
Mime-Version: 1.0

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Jerry Grimmonpre got it right about the bubbles coming up the following
morning.  Took me a while to figure it out as well.  I thought it was night
gremlins.  I put down a nice BID strip, no bubbles, clean my tools, go to bed,
check in the morning, and bubbles!  

When you clean with acetone or MC, two things happen.  First, as Jerry
indicates, some gets wicked into the pin holes or into the core via in holes.
It does not evaporate quickly.  Second, when the surface solvent does
evaporate, it cools the local area.  Then you cover it all with fiberglas, and
then the area warms a bit as the epoxy exotherms and the area returns to room
temperature, and voila! the solvent vapor pressure rises and pushes out a
bubble.  In the case of bonding on top of core, the air in the core is enough
to push out bubbles if there is much of a temperature change.  

Solution: as Jerry said: warm the area gently with your heat gun immediately
prior to applying the fiberglas (especially when working above core), apply
fiberglass and as the area cools it will suck a bit or resin in instead of
pushing air or solvent vapor out. 

For acetone users: be aware that the acetone vapor is heavier than air, and if
the air in your shop is still, you could be collecting a flammable mixture
around foot level.  If it finds an ignition source and cooks off, it will ruin
your whole day as well as your projec.t  USE LOTS OF VENTILATION WHEN USING
ACETONE.  What you smell is not necessarily what is going on around you feet.
Furthermore, you smeller stops working with continuous exposure, so you can be
fat dumb and happy until the mixture lights off.  Be careful. 

Jim- on the oil cooler doors: you are right: call them drag reduction doors
only.  On W.W.II installation, the doors were actually cowl flaps that were
closed to accelerate the flow when exiting to get some additional momentum and
decrease drag a bit more.  ON a Lancair, the ultimate would be to duct the oil
cooler air flow back out of the cowl through a nozzle with an adjustable
outlet to further reduce the drag.  You might actually get a knot or two more!

 The only exception I can think of for the oil cooler doors is operation in
very cold areas in which the oil can congeal in the oil cooler, dramatically
reduce oil flow, and when the vernatherm opens, there is insufficient flow in
the cooler to carry away the heat.  I expect multigrade oils have largely
eliminated this problem, but since you fly in Minnesota in the winter, it
could happen to you.  Close the door to keep the cooler warm and the oil
flowing.

Fred