×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Tail rotor design for noise

Tail rotor design for noise

Tail rotor design for noise

(OP)
Wondering what design considerations there are for noise when making a tail rotor.

I see that speed is a big factor in this article on tail rotor design, but isn't there something with the airfoil or tip shape?

RE: Tail rotor design for noise

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.

RE: Tail rotor design for noise

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...

RE: Tail rotor design for noise

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

RE: Tail rotor design for noise

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


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

RE: Tail rotor design for noise

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.

RE: Tail rotor design for noise

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

RE: Tail rotor design for noise

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?

RE: Tail rotor design for noise

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]

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close