From: JMcKibbin@aol.com
Message-ID: <9923e6a4.24cceced@aol.com>
Date: Sun, 25 Jul 1999 18:42:53 EDT
Subject: Elevator and Rudder Balance
To: lancair.list@olsusa.com
Mime-Version: 1.0

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Doug W wrote

>We made plaster of paris molds and heated some lead on a hot plate to make 
the >counterweights on my hangarmates plane.

I also tried using a plaster-of-paris mold and, based on my experience, 
cannot recommend it.  As the molten lead was being poured into the mold, 
substantial quantities of steam were released which caused the lead to 
spatter to the extent that I gave up after only pouring about one-third of 
the counterweight.  Not only was this unacceptably dangerous (IMHO) and 
messy, it resulted in a very porous counterweight.  Subsequently, I found out 
that the active ingredient in plaster-of-paris is hydrated calcium sulfate 
(gypsum) with the chemical formula CaSO4.2H20.  Reaching back into the dim 
memories of my freshman chemistry course, I believe that this formula implies 
that the two molecules of water are bound mechanically rather than ionically 
with the gypsum molecule.  Regardless, when the plaster-of-paris is heated to 
around 325 F (lead melts at around 620 F) the water is released and is 
instantly turned to steam.  This is presumably what caused the fireworks that 
I witnessed.  Perhaps there is some technique which would have prevented this 
which Doug W or others could share with us.  It may be that the shape of the 
mold contributes to the problem.

I also considered the lead shot/epoxy mix method which has been advocated by 
several on this site.  While I consider this an acceptable method and would 
have used it if I hadn't found an alternative, I have the following 
misgivings:

1 The density of this material is less than pure lead.  According to my 
material science textbook (also only a dim memory), the maximum theoretical 
packing factor for spheres of the same size is 74%.  Epoxy will fill the 
voids but is only about one-tenth as dense as lead.  Unfortunately, the epoxy 
will probably not allow the lead shot to lie contact-to-contact thus reducing 
the packing factor still further below the maximum theoretical.  This can be 
improved if spheres of two drastically different sizes are used and I would 
suggest that anyone using this method try mixing a couple of different sizes 
of shot.

2 It will be difficult to properly prep the inside surface of the elevator 
just prior to bonding.  Prepping the inside of the elevator just prior to 
closeout is probably sufficient but just not the best.  Considering that the 
elevator may sit around for several years between closeout and addition of 
the counterweight, daily temperature cycling will likely condense some 
unwanted material on the bonding surface even though it is closed up.  Of 
course, any energizing of the surface by sanding will have long since 
disappeared.

3 There is no way to secure the counterweight from sliding backwards as with 
the bid tapes shown in the instruction manual.  Some would argue that the 
bond of the shot/epoxy mix to the skin is sufficient but this is a very 
concentrated weight in a very light structure.

4 It would be just my luck to have one piece of shot which did not bond or 
was otherwise jarred lose and would forever be rolling around inside the 
elevator.

While none of the above are show-stoppers, I felt that the cast weight method 
would result in a better job if a convenient way to do the casting could be 
found.  In discussing my problems with some boat-builder friends, they noted 
that they routinely pour molten lead into wooden forms when making keels.  
Although it chars the surface of the wood and makes a little smoke it is 
otherwise not very exciting.

With this in mind, I constructed a wooden mold.  This is illustrated in a 
JPEG file which I have forwarded to Marv for inclusion in the attachments 
section.  It consists, from bottom to top, of a 1x4 base which is a little 
over 6 inches long, a layer of sheet metal and the mold itself which is a 
piece of wood about 3 inches thick.  The whole thing is held together by 4 
deck screws which come up from the bottom.  The mold was hogged out of a 
piece of redwood using a Forstner bit and cleaned up with a wood chisel.  The 
bottom of the mold (i.e., the sheet metal) forms the forward face of the 
counterweight.  The rounded short side is the outboard end and the flat side 
is the inboard end.  The free surface of the casting is, by deduction, the 
aft face.  One mold works for both left and right sides.

I don't know if the sheetmetal is required but I didn't want any moisture 
which was trapped in the 1x4 to be released under the casting and cause 
porosity as was the case with the plaster-of-paris mold.  Originally, I used 
aluminum sheet which, while it retained its integrity, buckled somewhat so I 
changed it for a scrap of sheet steel which seems to work well.  The metal 
sheet gets very hot during the casting process.  It would probably be safer 
if it didn't extend all the way to the edge (ouch).

I used redwood for the mold because it was a readily available scrap, was 
thick enough and, most important, I knew this piece was very dry.  I suspect 
that a denser wood might be better.  The mold is deeper than it needs to be 
but a 2x4 (i.e., 1-1/2 inch thick) is too thin, especially for the left 
counterweight, as you will want some freeboard.  The redwood charred on the 
first pour but seems to have suffered little after that.  Once the lead is 
poured and completely cooled, the mold can be disassembled and the casting 
carefully driven out with a small wooden block and a hammer.  The outboard 
side of the mold is tapered to fit the elevator and this facilitates removal 
of the casting.  If I were doing it again I would also taper the top and 
bottom surfaces slightly.  With the casting removed from the mold, it can be 
trimmed to a nice "honeymoon fit" with a wood rasp or coarse metal file and 
then floxed in place.  With a little care in the design of the mold and 
little bit of post-casting rasp/file work, it is surprising how close a fit 
can be achieved without starting with a male mold plug.  I plan to adjust the 
weight later after finish painting by drilling out the excess as is SOP.

In short, I think this method avoids some of the safety concerns of other 
methods (i.e., anything involving water), produces a higher density weight 
than is possible with the lead shot method, is very easy to construct and can 
be made from materials readily found in the shop.

Jim McKibbin


[Thanks for the detailed explanation, Jim.  Your photo is already online.

      <Marv>     ]
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