Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Tue, 24 Sep 2002 10:38:47 -0400
Message-ID: <redirect-1770236@logan.com>
From: "Gary Casey" <glcasey@adelphia.net>
Subject: mass balance
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<<Getting back to the condition where the elevators are not balanced
individually but balanced as an assembly, the result of this is that the
balancing torque must be transmitted through the hinge / control rod
brackets. This is fine if the forces are small. As the forces increase, or
as counterweights are added at this point, the stresses through these parts
increases and the potential for a failure from a crack or loose fastener
increases. If all the elevator counterweight  was hung from a bolted
assembly then flutter possibly could be induced if the bolts were simply
loose and not even failed. I cant say if the elevator center hinge weldment
has been reinforced since I built my airplane but I can say that I would not
add a counterweight arm to mine. But that is just my gut opinion.

We trust our lives to bolted assemblies every day but a wise man does this
as infrequently as necessary. IMHO individually balanced elevators are the
way to go. Nothing to break, nothing to fall off and nothing to forget
(counterweight wise).

Regards
Brent Regan>>

As usual, a very excellent post by Brent.  As an aside, the Cessna Cardinal
has a central balance weight attached by a bracket to the spar - there have
been numerous cracks in these brackets - your point exactly.  On the other
hand, the (Lancair) central arm is designed for considerable loads and you
are almost certain to never have a flutter-induced load at the same time as
a maximum pilot-induced load.  So adding some weight to the central bracket
probably doesn't increase the stress on the brackets (note I said probably).
A central weight might even be a good thing as the elevator itself has some
flexibility and most of the unbalanced weight is toward the center and the
balance weights are way out at the tips.  Putting a little weight at the
center will distribute the weight and reduce the torsional moment in the
elevators themselves (a good thing).  In fact, I looked at my integral
weights and thought that the elevator itself needed stiffening.  I am still
tempted to put some unidirectional carbon diagonally across the tips to
stiffen the joint between the weights and the elevator.

Gary Casey