Model Forum / Radio Controlled / Helicopters / June 2008

I thought that was a tittybar?

I think the reason for that is because the flybar can be setup to augment or
damp control inputs to the main blades.  The problem is that there are so
many variables that effect how the flybar does what it does.  Flybar length,
weight of the paddles, size of the paddles, airfoil shape of the paddles,
percentage of mixing ratios between the flybar side and swashplate side of
the cyclic inputs, etc., etc.  There are some consistancies that can
generally be counted on.  "Everything else being equal", a heavier flybar
will be more stable than a light one.  Paddles with a rounded leading edge
will be more docile than ones with a sharp leading edge.  The rest of it is
pretty much an exercise in experientation to figure out what the individual
pilot prefers.  There just doesn't seem to be one easy formula to plug in
and be certain you're going to get the response you want.

Bell didn't have a "fly" bar on it's early machines.  It had a "stabilizer"
bar.  The bar was weighted but it was also aerodynamically blind.  It
received no cyclic inputs from the pilot.  Instead all cyclic commands to
the main blades were routed through the bars mix arms, just like our
bell/hiller mixers.  The difference being that, the bar simply acted as a
stabilizing gyro which made the rotor system easier for the human pilot to
control.  Hiller, on the other hand, used aerodynmic paddles on the end of
it's flybar, like our models do, to control the tilt of the bar itself.  All
cyclic commands from the pilot went to the flybar paddles only.  No cyclic
command from the pilot went straight to the main blades like our models do.
The pilot was flying the flybar and it, in turn, was controlling the main
blades.  It made for a stable control system but also one that was
relatively unresponsive.  That's one of the reasons it's not around any
more, IMO.  Actually, the earliest versions of Bells rotor system didn't
have the stabilizer bar and were known to be quite touchy on the controls.
The stabilizer bar was added to delay a percentage of the pilots control
inputs to the main blades.  This made the rotor much easier to control but,
unfortunately, it did it's job a little too well.  The system reportedly had
a lack of precision so Bell added a set of hydraulic dampers to the system.
If you look at one of the old Bell 47's (the MASH style heli's) you see a
structure mounted on the rotor mast, just below the stabilizer bar.  These
structures have a linkage between them and the stabilizer bar's see-saw
carrier.  These hydraulic dampers force the flybar back to a position 90
degrees to the rotor mast within around 3 to 5 seconds.  That damps the
pilots controls enough to keep them from being overly touchy but not
allowing them to be too dead when more agressive control responses are
needed.  What kind of testing they had to go through to figure out the right
balance in all this, I have no idea, but it had to be an interesting time in
their R&D department.

I think you're right, fixed wing and rotory wing stability are two
completely different topics.  I've also read that a rotor can be optimized
for hover, or for translational flight and what's ideal for one isn't
necessarily good for the other.  I really don't think it's all that critical
with our models mainly because, compared to the full size birds, we're so
grossly overpowered that we'd simply never feel the couple of percentage
points difference in one flight mode vs the other.  Generally speaking, I
believe that anything  you do to the models fly bar system that improves
stability in a hover, will also help in forward flight, at least that's been
my experience when adding or subtracting flybar weights.

FWIW!

Fly Safe,
Steve R.