"CivetOne" wrote in message
...
Hi Tim,

When Hughes Tool Co., Aircraft Division experimented with the OH-6A to
make it
very quiet, the engineers knew that a big source of noise was the tail
rotor.
To make the tail rotor quieter, they decided to increase the blade area
and
decrease the rpm. The most convenient (i.e., least costly) method was to
simply add another set of teetering blades to the tail rotor.

In addition to the addition of more blades, Hughes engineers experimented
with
blade phasing to decrease the noise level as much as possible. As I
remember,
the phasing they decided upon was about 45 deg.

When the AH-64 Apache came along, the Hughes engineers incorporated what
they
had learned from the OH-6A development work. The same basic design for
the
tail rotor of the quiet OH-6A was incorporated into the Apache design; two
pairs of teetering rotor blades. The same concept was used on the
4-bladed
tail rotor option on the MD 500D, E, and FF series of helicopters.

The one additional requirement for the Apache was to fit into a C-130
transport
without disassembling the rotors. In order to meet this requirement, the
tail
rotor blade spacing was changed to the 30 deg spacing you found. The
noise
signature suffered a litttle bit, but the transportaion requirement was
met.

Thanks for that. Just got to work out how to incorporate that into an R/C
replica...


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