AboutPaul Soderman Expertise Aeronautics, Fluid Mechanics, Aeroacoustics, Noise Control, Muffler Design, Wind Tunnel Research.... I know nothing about India - do not ask about schools, jobs, application requirements, career choices, etc. for India. Please, no text message verbiage; I prefer full words in full sentences. Thanks.
Experience 37+ years with NASA as Aeronautical Engineer in research lab (retired). Specialty in aerodynamics, aeroacoustics and wind tunnel research. Publications 95+ technical papers and book chapters. Education/Credentials: BS/MS Aero Engineering - U. of Washington. Graduate studies - Stanford U. Awards and Honors AIAA Associate Fellow (American Institute of Aeronautics and Astronautics
Expert: Paul Soderman Date: 6/30/2008 Subject: Air-vent duct noise
Question Hi Paul - I fly an Antares 20E that I love. But, it has one annoyance that's driving me crazy - the ventilation is incredibly noisy. I'm hoping if you have a minute you might be able to provide me a few pointers.
I had an LS-6B with this problem. I sawed out all the factory ducting up into the tip of the nose, created new ducts with everything smooth with gradual expansion, ran a duct back to an eyeball output near me. Worked great: both silent and greatly improved airflow. Repeated it on my second LS-6B. The LS-6 problem was limited to the intake ducting (it had adequate exit air routing and volume). But my mods were strictly eye-ball engineering which worked out.
The Antares has lots of ventilation problems. Inadequate exit air from aft fuselage, inadequate exit path from cockpit, intake duct expands too rapidly and causes separation, intake duct blows directly on canopy support structure which creates turbulence and noise, etc. I've reviewed this with Axel Lange in Zweibruecken and he's pointed me at where I can cut holes for air ducting - but I still need to design the ducts so they are quiet.
OK, so now to my question: Can you recommend things I should read to educate me on how to keep from creating another screamer with the ducts and intake valve ? I understand 7 percent is a magic number but I'm not quite sure how to apply this. And creating a valve that remains quiet when partially open seems a challenge.
I need some basic guidelines like duct contraction and
expansion ratios to avoid noise (separation), how to make
a quiet valve (no roar on partial opening), over speed
ranges from 45 to 115 knots (I'll live with noise at
higher speeds).
Thanks in advance for any and all help,
Best Regards, Dave
Answer Hi Dave
Sorry for the tardy answer, I have been helping fight forest fires in Northern California. But I am off today, and I love your question because it relates to the work I did at NASA. Your questions, however, are geometry dependant so I can only give you general guidelines.
1. Some good references for flow noise control are Engineering Noise Control by D. Bies and C. Hansen and Noise and Vibration Control by Beranek (or newer edition with Beranek and Ver).
2. Good aerodynamics results in minimal noise.
3. Duct contractions can be quiet if sharp corners are avoided and the shapes is similar to what you would find in a wind tunnel (see Low-Speed Wind Tunnel Testing by Rae and Pope). The contraction area ratios are often large - say 8 to 1.
4. Diffusers are more sensitive because they can separate and cause turbulence noise if the wall angle exceeds 7 deg. The wall angle can be increased if aerodynamics splitters or center bodies are used. These can also be made sound absorbent as can the walls. Bies and Hansen show a perforated exit plate in a diffuser that traps sound.
5. Corners should be rounded. Avoid sharp edges that induce separation.
6. Valve noise comes from separation and turbulence from sharp edges. I don't know of any quiet valves, you might check manufacturer websites for noise data. You can incorporate a simple muffler between you and the valve. I have had luck with a perforated plastic duct surrounded by a larger outer solid duct and steel wool between. If high airspeeds are required, a protective cloth may be used around the inner duct to keep the fibers contained. See the above references for details.
