>"Overbalanced is OK but underbalanced is not"? Please explain. I don't
>understand.
Jeff,

Having understood this at one time in my past I thought I'd take a crack
at it.  In articulating my thougths I'm gaining a better understanding
but have also come to the realization that I no longer have this topic
nailed.  Stop reading now if you need all the details.  Or stop reading
now and "Nike" so you'll be flying sooner.  (Nike = Just Do It!) As you
will see gray matter runs dry in about the fifth paragraph.  Perhaps one
of our JPL friends can clarify where my blather comes up short.

My laymans understanding has to due with dynamic instability of the
control surface, ie flutter.  We are mass balancing to negate the
possibility of flutter.

My recollection all revolves around a momentary upset of the airframe
such as a schwack from turbulence. The control surface reaction compared
to the airframe action is where the potential for problems developes.

The ideal case is when the aileron is 100% mass balanced for and aft of
the hinge line.  In this example the momentary upset will have a
"balanced" reaction from the aileron.  That is there is an equal and
opposite moment on both sides of the hinge line.  These moments cancel
each other.  The resultant reaction from the control surface is to
remain neutral.

The opposite and negative affect is when the aft portion of the aileron
is heavier than the forward portion (less than 100%
balanced-underbalanced).  In this situation consider the airframe
recieving a momentary upset, Down for example.  The opposite upward
reactive forces for and aft of the hinge line are unequal.  The heavier
aft portion having more momentum will tend to rise relative to the
descending airframe.  The airstream will then try to neutralize the
control surface.  As the heavier side moves back towards neutral it's
greater moment may take it through the neutral position swinging into
the airstream on the other side.  This asymetric balance on the aileron
can then possibly cause an excitation and subsequent harmonic vibration.
That is, it can increase  in vibration amplitude until it shakes itself
and your plane apart.

Finally, the third situation is greater than 100% mass balanced
(overbalanced).  This case is where excess lead is added to the leading
edge.  Taking the same example of a momentary downward force on the
airframe causes the leading edge of the aileron to rise slightly.  Again
the aileron is forced towards neutral from the airstream.  However, in
this example the greater moment in front of the hinge line has a
dampening affect.  So, even if the aileron goes slightly through neutral
as in the above example, it will tend to decrease in amplitude.  Perhaps
there will be a few dampening oscillations and then back to neutral.
Therefore,  overbalanced is safer than underbalanced because it will not
lead to flutter.

In the words of Forest Gump, "That's all I have to say about that".

Larry Henney
N360LH LNC2 IO-360 B1
16XS  Ft Worth, TX

PS:  Now you aero guys help me.  Am I correct in talking moments
compared to weight?  Is the blend of static stability correctly stated
compared to dynamic stability?  Have I lead this gentleman astray?  

How would one assess both ailerons and elevators mathmatically?  My
understanding is that 100% was always the known gaurantee of no flutter.
This however may also be true at some smaller mass balance.  Lance even
made mention of this when they were flutter testing the L360 with German
firm but made no comment after the testing.  Could we mathematically
consider an 80% mass balance?  Or perhaps 70%? On elevators?  On
ailerons?

Would empirical data be useful to provide a mathematical model to reduce
the weight?  Is Finite element analysis small enough to run on the home
computer.  When I studied it we had to use the CRAY which was big
(interplanetary like big!) at the time.  

All this to save an ounce and gain perhaps a quarter knot.  Hmmmmm?  You
bet it's worth it.