Expert: Paul Soderman - 7/3/2008

Question
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

PS: My most recent flight in this machine:
http://www.onlinecontest.org/olc-2.0/gliding/flightinfo.html?flightId=-102864859


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.

Good luck and happy flying.
Paul



---------- FOLLOW-UP ----------

QUESTION: Paul, first thanks for your answer and your volunteer fire-fighting work !

Perhaps I should read the books you recommend prior further questions,!
However, given:
- Speed range of interest 50 - 115 knots
- nose opening for vent about 1.5" diameter
At low speeds, I'm not sure I'll get any airflow with a wool
noise filter. Can you give any more specific ideas about:
- what do I do where duct ends (dumps into cockpit) to
 keep it quiet ? Isn't the lip always > 7 degrees effective ?
- would a valve built from concentric tubes with a lot of
 holes (rotate to open/close) mitigate noise ?
- does lining the vent with felt have any positive effect,
 or does it just cause reduced flow (the Germans think
 fake rabbits fur helps but I'm not convinced it doesn't
 just block the flow as well as the noise)

Thanks again for your thoughts, I'll certainly avoid sharp
edges and corners in all ducting,
Best Regards, Dave


Answer
Dave
You do need to read some of those or other books about silencers.  The airflow does not go through the steel wool, it passes down an open duct with perforated walls that allow the sound to enter the wool and cavity surrounding the primary duct.  The primary duct is the same diameter as your original duct.  There is pressure drop because of roughness from the perforations, but that can be dealt with to some extent in a number of ways (smaller holes, larger duct area, screens, etc.).  The loss can be predicted.  

I don't understand what you mean by lip angle.  A diffuser is a conical duct section, the shape of the exhaust lip is not too important as long as it is smooth and streamwise.  Ideally you trap the sound before the flow exhausts.  

Your valve idea might be good.  

Lining a duct with absorbent material, even rabbit fur, would reduce noise with some flow loss due to roughness.  Depth, length, and type of material are important and depend on acoustic frequency and desired attenuation. There is no reason to block the flow unless you use internal splitters or center bodies.  If you add a lining, you will have to enlarge the duct diameter and insert a lining that gets the duct back to the original diameter, which is similar to my perforated duct idea.  You can either do some research on silencer design or cut and try some things... or you can pay someone to do an analysis and give you an engineering design.
Paul