Rudder counterbalance ?

[George Kennie]

I have used a balance tab on the rudder of the last 3 of my designs and so far
have been happy with the results.
I happen to like the aesthetics of a forward sweeping rudder trailing
edge.However, this has the effect of unbalancing the rudder area above and below
the datum line, which accentuates proverse roll couple. So when you add the
balance tab, it increases the area at the top of the rudder, thus raising the
center of pressure and negating the proverse effect, in fact, if you're not
careful, you can pretty easily produce adverse couple, so it becomes a pretty
involved juggling act making sure that you have the dihedral at the correct
setting to compensate for all these discretionary forces that you build into the
model.
Georgie

John Ferrell wrote:

> I have thought a little about it but have not done anything towards proving
> my theories.
>
> First, by counterbalance I assume you mean there is an area ahead of the
> hinge line that acts to reduce the power required to operate the surface.
>
> That will impart a "twisting" load on the hinge line. It will induce more
> drag closer to the rudder centerline and higer up the rudder post. I would
> think this force would tend to pull to the canopy (may be good, may be bad).
> since the servo load is less, the damping action would be less. It would be
> more susceptable to flutter with a loose return side on the pull-pull
> rudder.
>
> I have a Goldberg Extra 300 and a Midwest Cap232 that both have this
> arrangement and no problems. I am not aware of either of the planes having
> the problem.
>
> Just my opinion, go ahead & shoot it down...
>
> John Ferrell
> http://DixieNC.US
>
> ----- Original Message -----
> From: "Peter Pennisi" <pentagon.systems at bigpond.com>
> To: <discussion at nsrca.org>
> Sent: Thursday, April 28, 2005 5:29 AM
> Subject: Rudder counterbalance ?
>
> >
> >
> > Hi all,
> >
> >
> > Most of you would be aware that a number of Composite ARF "IMPACT" have
> > failed from suspected rudder flutter.
> >
> >
> > I raise a couple of questions to this forum;
> >
> >
> > I know that the purposes of counterbalances are to reduce the loads on
> > servos and linkages in our application but what are the side effects.
> >
> > Can a rudder counterbalance create undesired torsional stresses on the
> > fuselage?
> >
> > Can a poorly configured and tensioned pull-pull linkage to the rudder be
> > more susceptible to flutter if the rudder has a counterbalance?
> >
> > What other types of forces are at play with counterbalances?
> >
> > I am just trying to find a reason for the relatively high failure rates
> > against this design.
> >
> > It is the same old thing- why are some people having problems and others
> > don't. (Similar story to 4-stroke exhaust headers)
> >
> > I will be test flying my model soon so I am obviously concerned.
> >
> > Regards,
> >
> > Peter
> >
> >
> >
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