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Newsgroups: rec.aviation.rotorcraft
Subject: Re: 2 vs 3 Blade
Date: Sun, 08 Oct 2000 17:40:06 GMT

In article <>, wrote:
>    I can see hangering and transporting advantages of 2 blade
>helicopeters and wonder what the advantages are for 3 blade.....
>    Think I'm down to Bell, Hiller, or Rotorway as a project choice. As
>well any pros and cons here?


Don't let the type of rotorhead get confused with the number of blades.  A
teetering rotor (all blades attached to a gimbal that is hinged at the mast,
like a typical Bell) can have more than two blades, for instance the V-22
Osprey has teetering rotors with three blades.

Generally, the more the number of blades, the less you'll see of  higher
frequency vibration.  Each blade carries its fraction of the total weight of
the helo, more blades mean a proportionately smaller share of the weight is
carried by each, so the differences between blades can be proportionately
smaller.  Generally, the designers know that the magnitude of the  N per
revolution vibration (where N is the number of blades) is much smaller as you
increase the number of blades.  In other words, a one-bladed helicopter has a
bunch of N per rev vibration!

Almost always, three blades make a smoother ride than two.

As for rotorheads, a teetering rotor generally has higher N per revolution
vibration, because the blades are tied together, and are not free to
independantly flap up to relieve differences in lift.  Articulated heads have
somewhat lower vibrations, but are more complex and more expensive to build.

Nick Lappos

Newsgroups: rec.aviation.rotorcraft
Subject: Re: R22 Teeter?
Date: Mon, 09 Oct 2000 15:09:18 GMT

In article <>, (Gary) wrote:
>The blades do NOT flap independently, they do it as a unit.  I'm not
>sure about the "tri hindge".
>I'm more curious about that 3rd short rod coming off the rotating
>swash plate (second rod from left, leading directly to the rotor).
>What is tha for?  Is that cyclic movement?


You are correct, the head is a classic teetering "semi-rigid" rotor head,
similar to the typical Bell 204/206/212 family.

Tri-hinge probably refers to the three total bearing/hinges in the head, the
two feathering and one teetering (flapping) hinge.

The third rod is the rotating scissors, the mechanism that spins the rotating
swashplate.  This keeps the pitch change links at the same vertical angle, and
is sort of a "timing belt" for the swashplate.  The drag of the swash plate at
high speed can be appreciable, so the scissors must be pretty strong.  The
round lump on the far side of the scissors is a counterweight to prevent a
1/rev from the mass of the link.

I don't see the stationary scissors, which prevents the stationary swashplate
from rotating.  It could be that the swashplate is keyed into the top of the
gearbox to prevent rotation, or maybe the scissors is at the other side of the

Nick Lappos

Newsgroups: rec.aviation.rotorcraft
Subject: Re: R22 Teeter?
Date: Mon, 09 Oct 2000 21:15:01 GMT

In article <8rst6k$3pgq$>, "Daniel Seehof" <> wrote:
>When they talk about tri-hinge, they mean the teeter-hinge and the two
>coning hinges which take stress (bending) out of the blade. Compared to a
>206 which doesn't have those coning hinges, the blade life-time is improved.
>The lower swash-plate is not rotating, because it's not connected to any
>rotating object. The only thing rotating is the mast and the scissors attach
>to the mast and the upper swashplate, thereby turning the upper one.

You said, ">The lower swash-plate is not rotating, because it's not connected
to any rotating object."  This isn't really correct, because the swashplate
bearing induces some substantial torque on the stationary swashplate,
especially in near stall maneuvers where the pushrods are really pushing down
on the whole assembly.  I would be surprised if this torque were not relieved
somehow.  It is usually with stationary scissors, but perhaps with a keyway or
some such anti-rotation device at the inner support.



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