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Posted for Tom Nalevanko <tom@mstay.com>:

I noticed that Doug Pohl has what seems to be a common problem with his
rudder balance; he did not initially provide enough weight to compensate
for the primer and paint.

After having struggled with this problem on my elevators, I developed the
following technique that seems to work well (well, at least so far).

It seems that a common problem in counterweight balancing a control surface
is correctly estimating how much weight it will take to balance out the
primer and paint. Current suggestion to simply overbalance and drill out
any excess is a bit simplistic, particularly when the control surface ends
up underbalanced. A more accurate approach follows.

(Ideally, you might do the following before you even start building the
piece, so you don't have trim tab holes, etc to worry about.)

Before you close off a control surface, say a rudder for this example, take
the skin only for one side and balance it on fulcrum in two orthogonal
directions. For example, along its vertical and horizontal axes. This can
be as simple as sitting a long piece of angle iron on a table, angled-side
up, and sliding the skin to where it just balances, see-saw fashion. Draw a
line across the piece corresponding to where it balances on the angle iron.
Do the same in the other direction and where the lines "cross" is the
centroid of the weight of the primer/paint. That is, placing all the weight
of the paint and primer here is the same as spreading/spraying it out
normally. Now when the rudder is built and almost ready for closing, put
its hinge pins on some knife edges, place the counter weight in the piece,
put the top skin on it and then put a large cup of water on the "cross" to
simulate the paint. Add/subtract counterbalance weight until the rudder
balances.

You can balance the rudder before adding the simulated paint, then add the
cup of "paint", and see how much more weight balancing it out takes.
Suprisingly, it seems to take somewhat more counterweight then that which
is simply intuitive!

I hope this helps and speeds one more project into the air.

Tom Nalevanko
Ugly Duckling aka Stallion Builder

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