Why do you need to balance a rudder to prevent flutter? This is a
very-often-heard question, even within an airplane design company, so don't feel
bashful about asking. Here's my version of the answer ...
The vertical fin deflects under load, just like a wing or horizontal
tail. We are interested in dynamic effects here -- not the steady state
loads that you'd get in a steady side slip. Something applies a side load
to the vertical fin (a gust or a rapid rudder input) and the fin bends towards
one side. If the center of mass of the rudder is behind the hinge line,
the rudder will lag behind the deflection of the fin. This means that the
rudder is deflecting in the direction to INCREASE the side load on the fin, thus
amplifying the deflection. The fin overshoots it's steady-state deflection
(any moving thing will do that) and starts to move back towards the steady-state
deflected position. As it changes it's direction the rudder once again
lags behind, thus providing a driving force to move the vertical fin farther
than it otherwise would have gone.
If the center of mass is ahead of the hinge line, the nose of the rudder
will lag behind the fin deflection, thus the rudder applies an aerodynamic force
which REDUCES the side load and therefore the tendency of the fin to
deflect.
The key here is not that we're trying to make the nose of the surface hang
downwards (well, we are, but gravity is not the issue), but rather than we're
trying to move it's inertia (it's center of mass) to be forward of the hinge
line, so deflections of the structure to which it's attached make the surface
move to generate aerodynamic forces which DECREASE the load which made the
structure deflect in the first place. It's the same for a wing/aileron, a
stabilizer/elevator, or a fin/rudder. We just use gravity to help us
measure where the center of mass is. It doesn't matter whether the surface
is used horizontally (ailerons and elevators), vertically (rudders), or
otherwise (ruddervators on a Bonanza, for example).
As an exercise to the curious, why don't we balance flaps?
- Rob Wolf
LNC2 80%
(see you at Oshkosh -- I'm taking the airlines)