yup what he said.
Good info.
all the best Sean "Apache driver in my dreams" Trost

CivetOne wrote:
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.