Beginning in rotary wing

[david perry]

Thanks to all who offered help and advice. I had my first lesson today with
ATS. Superb fun on the Raptor. I think I'm hooked.

One question though...from a scientific point of view - how do the paddles
work? I gather they add stability to the rotor disk, i.e. they damp the
movement of the disc..but HOW? And why do I suddenly not need paddles if I
go for three or more rotor blades? And why does a jet ranger, or a Robinson
for that matter, which does have two blades, not need paddles? Yes I know
early Bells had balls on a fly bar...not paddles? How so?

Anyone know?

David

[Tim]

"david perry" <david_perry@btconnect.com> wrote in message
news:b9rh4p$g35$1@sparta.btinternet.com...
> Thanks to all who offered help and advice. I had my first lesson today
with
> ATS. Superb fun on the Raptor. I think I'm hooked.
>
> One question though...from a scientific point of view - how do the paddles
> work? I gather they add stability to the rotor disk, i.e. they damp the
> movement of the disc..but HOW? And why do I suddenly not need paddles if
I
> go for three or more rotor blades? And why does a jet ranger, or a
Robinson
> for that matter, which does have two blades, not need paddles? Yes I know
> early Bells had balls on a fly bar...not paddles? How so?
>
> Anyone know?
>
> David
>
>
The paddles and flybar form a "Hiller servo rotor" which provides damping to
the rotor and also provides the forces for cyclic pitch changes. By altering
the pitch of the flybar paddles, the aerodynamic forces acting on the
paddles cause the flybar to tilt, which via pushrod connections to the main
rotor blades, causes a change in pitch for the main blades. Have a look at
the rotor head when the model is shut down and play around with the cyclic
inputs; you will see that they only act on the paddles, but then if you tilt
the flybar, to simulate the aerodynamic forces at work on the paddles, you
will see the corresponding change in pitch of the main blades. (Noe that the
collective pitch control however acts directly on the main blades). The
damping aspect of the Hiller system comes about as the paddles also act as
weights on the end of the flybar - read on.

The flybars with weights only, are referred to as "Bell stabilising bars",
and act as a gyroscopic stabiliser. These were originally incorporated, as I
understand it, to assist pilots overcome the instability due to rotor tip
and root vortices, and to make the helicopter easier to hover. I'm guessing
that where you have two bladed heads without a flybar, then there is some
arrangement of dampers to deal with the instability issue.

There is a further benefit of the Hiller rotor for modellers, in that it
takes less servo effort to change the pitch of the flybar paddles (less
aerodynamic resistance) than it would to change the pitch of the main blades
directly. The magnitude of any aerodynamic forces acting on the main blades
are greatly reduced before they reach the servo, so sudden gusts etc are
accomodated within the head itself, rather than putting undue loads on the
servos.

I'm sure there's a lot more to it than this; I've only really scratched the
surface (and I'm sure others will correct my errors!). As to why mutli blade
heads don't have flybars, perhaps it's because they're inherently more
stable, or maybe it's simply because, if you've got lots of blades, there's
nowhere to put the flybar paddles?

Speaking of multi blades heads, is there a commercially available seven
blade head? A scale MH-63 Pave Low (Super Sea Stallion) would be a nice
project...you'ld also need a five balded tail rotor, if I remember rightly.

Tim


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