The bulk of the noise is from the periodic impingement of the downwash (sidewash?) of the induced flow against the fin and tail boom. It's nice if the tip is not in the high subsonic to supersonic region as well. Eliminating the tail rotor is the best way to eliminate tail rotor noise, such as with the fan and slotted boom arrangement or dual counteracting main rotors.

I can't say as to how I have ever noticed that a majority of sound emitted from the tail rotor - it is usually the powerplant and the main rotor downwash impingement on the fuselage, at least to my ear.

The biggest consideration, past plain aerodynamic efficiency and effectiveness, is that tail rotors offer an excellent thing to strike trees, power lines, careless people walking around them, and that this often leads to an immediate disassembly of much of the remainder of the helicopter, either from uncontrolled spinning, flipping over, or by smashing into the ground.

The tail rotor on the Hughes 500P (AKA the quite one) and maybe the Apache (seem to recall there were may have been a wake issue as well) had unequal spacing (4 blades) to reduce noise.

A bit on the Hughes 500P
https://medium.com/war-is-boring/the-cia-built-a-s...

Unequal spacing changes the beat frequency of the sidewash and reduces the magnitude.

More accurately it splits the harmonic series from strong multiples of BPF to a more sort of broadband noise. I doubt the actual unweighted sound power changes much or in any general trend. The subjective annoyance will be reduced.


Cheers

Greg Locock


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I left off "increasing the number of blades decreases the per-blade sidewash magnitude."

I bet there is someone out there now considering creating a variable phase adjustment to create a spread-spectrum system.

I'd have thought for stealthing, the main rotors would be orders of magnitude more important, and more difficult.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

The cooling fans on the old mercedes i've had have an uneven spacing to the blades. Probably still well balanced, because mercedes.

Oh yes. An unbalanced cooling fan will do all sorts of exciting things. That's one of the considerations when designing the blade spacing, years back people would settle for an X shape, but its an easy vector sum.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

Wirlem… this is a question that likely has very general answers... while in-depth specific design practices will be secretive and proprietary.

A LOT of research and development goes into each decibel reduction coming from any machine... and machines with military implications expend even more capitol for the audible-stealth mode than for civilian operations/applications.

During the Vietnam war, the 'quietest' fixed and rotary wing aircraft ever built were used in 'stealth mode' for ultra-secretive [and still mostly classified] CIA missions. The missions flown have only partially been revealed after 4 decades… BUT the embedded technology/innovations have NEVER been declassified.

Having said all of this I am sure there are a few unclassified NASA and DOD reports for helicopter design for reduction of noise-exposure for the crew/pax and for exterior noise to come into 'environmental' compliance. There are likely other design guides for rotor-specific noise reductions...

Suggest starting with the basics for YOUR DESIGN... ANALYZE the noise signature of YOUR Tail Rotor Drive System [TRDS] mechanical components and then do the same for Your [now isolated] Tail Rotor System [TRS]driven by the TRDS... for starters... then test in-flight for the aft-fuselage and stabilizers contributions due to overall noise.

MIL-STD-1294 ACOUSTICAL NOISE LIMITS IN HELICOPTERS
MIL-STD-1474 DESIGN CRITERIA STANDARD NOISE LIMITS

It is then a matter of experimentation which leads to an analytical-modeling/understanding of YOUR TRS/TRDS/Structure... which can be driven mathematically to various conditions for noise analysis. Then TEST to verify the analytics of the combination of TRS and TRDS. Then refine Your design and thinking.

"Experience is a cruel teacher... First you take the test then learn the lesson." -Vern Law
"Nothing is a waste of time if you use the experience wisely." --Auguste Rodin
"Mistakes are the usual bridge between inexperience and wisdom."-- Phyllis Theroux
"Experience is one thing you can't get for nothing." -- Oscar Wilde
"Experience is a brutal teacher, but you learn. My God, do you learn." --C.S. Lewis

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]