Re: AH64 tail rotor

[CivetOne]

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.

[Tim]

"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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[sean trost]

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.

[mm]

The Apache tail rotor is, as you note, actually a pair of stacked tow-bladed
teetering rotors. There have been claims that the unusual blade spacing
you're asking about was intended to reduce noise. While the spacing may
give some noise benefit, the real reason for the orientation is simple: it
was how the blades would fit. If you look at an Apache tail closely you
will see that if you tried to orient the outboard rotor at 90 degs from the
inboard rotor, the pitch links for the outboard rotor would interfere with
the inboard blades. (The interference geometry is largely a result of the
skewed teeter hinge, which is needed to get the desired pitch-flap coupling.
This pitch-flap coupling reduces the flapping of the tail rotor in rotor
flight.)



"Tim" > wrote in message
...
> Hi,
>
> Please can anyone explain to me what the tail rotor on the AH64 Apache is
> all about? I see that the two pairs of blades are actually in different
> planes and only at about 30degrees spacing.
>
> What's the thinking behind this?
>
> (I'm into radio controlled helicopters and considering how/whether to
> replicate this on a model)
>
> Thanks,
>
> Tim
>
>
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