Thanks Steve and Andy. It's interesting once you start looking, isn't it?
I've just done a bit of looking online, and there seems to be two answers, or a two part answer...
Quote:
The tail rotor has a "scissors" configuration, with the blades arranged at alternating 55 and 125 degree intervals, also to reduce rotor noise.
http://www.vectorsite.net/avah64.htmlSo far so good, and this 'noise' statement is repeated on most sources. But:
Quote:
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.
http://www.airtalk.org/ah64-tail-rotor-vt35959.htmlOK, so it was 'noise' advantage first, then a shipping advantage (which, interestingly, would match the Sea Otter's requirement.
The more things change, the more they stay the same?)!
But then, in the same discussion:
Quote:
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.)
So in this case, it's just so two sets of blades will simply fit together.
Anyone able to add to or correct any of the above, please?
Regarding the automotive car cooling fans, does anyone have any comment on those? Again, I'd be interested.
Cheers,
Login
Register
FAQ
Search
