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Bob Axsom

Well Known Member
I recently read a comment by an RVator that his old David Clark headset was no longer adequate and another that said straight (not turned down) exhaust tips caused such a loud drumming sound in the cockpit that he had to buy some turned down extensions. Because of these and other comments I feel I should make a recommendation based on my experience in our RV-6A which is both used for long distance travel and races with the turned down exhaust tips sawed off.

I bought the materials and the upholstery video from Becki Orndorff for insulating the cockpit and with the O-360 running anywhere up to 2730 rpm and I have never had the noise discomfort that has been described here. I have the aluminum foil black rubbery sheet material on the back of the firewall inside the cockpit. On the floor between the stringers I have the thick black sheet rubber covered with carpet. On the side walls I have thin white foam covered with my upholstery material. My sliding canopy seal is unique and I don't know of anyone that has duplicated it yet but it is sealed all around and the design includes a side skirt rib hack sawed from 3/16" aluminum bar stock with a sheet rubber seal glued to it to seal against the side of the roller tracks. Aft of the track I added an upturned 1/16"x3/4"x3/4" angle to provide a fuselage to skirt overlap and eliminated the aft skirt hang down skag. All of the canopy perimeter except the black seal to the roller track is the soft white "P" strip from Aircraft Spruce. The plane is no more noisy inside than my old Archer II was. As far as weight is concerned 100 lbs is equal to 1 knt in speed per the old rule of thumb.

I just felt I had to give this experience to provide food for thought in the finishing stages in building an RV.

Bob Axsom
 
Hi Bob,
Thanks for that. It is indeed food for thought. One thing that comes to mind, though, is that all of this is subjective. I wonder if anyone has access to a db meter and could actually measure the sound inside their cockpit. It would be very intesting to compile a list of a few different types of aircraft and compare that with an RV.
 
Not always a speed trade

Bob Axsom said:
As far as weight is concerned 100 lbs is equal to 1 knt in speed per the old rule of thumb.
Click to expand...

You may be right about the speed impact but:

100 lbs is equal to 100 lbs of bagage or fuel
100 lbs is equal to ~0.1g
 
Speed and Transport are Everything to Me

Speed and transport are all that matter to me and the insulation probably weighs less than 10 pounds rather than the rule of thumb 100 pounds. To each his own.

Bob Axsom
 
Hi Bob,
I was wondering if I could take a look at your plane sometine and maybe get your thoughts on a few things. I am right now in the stage of building to add insulation to the cockpit. I come to Springdale quite often to deliver to Trutrak and could drive over to Drake or wherever you are located.

Thanks
jim
 
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Bob Axsom said:
Speed and transport are all that matter to me and the insulation probably weighs less than 10 pounds rather than the rule of thumb 100 pounds. To each his own.
Click to expand...

My 6A was weighed with floor carpet and insulation but currently has none. I much prefer the quieter insulated 9A's that I've flown.........so it's getting insulated this winter. And BTW, I don't have down turned pipes, and it doesn't seem to drum. Or I just could even be deafer than I think I am. I definately need ANR's though!

L.Adamson
 
Sure Jim

rv9aviator said:
Hi Bob,
I was wondering if I could take a look at your plane sometine and maybe get your thoughts on a few things. I am right now in the stage of building to add insulation to the cockpit. I come to Springdale quite often to deliver to Trutrak and could drive over to Drake or wherever you are located.

Thanks
jim
Click to expand...

I am at Drake Field. Call 479-267-5206 and we can work it out.

Bob Axsom
 
Thanks Bob,

I'll call when I get a chance to come and if your busy we'll do it on another trip. Thanks again
 
Animation of vibratory modes for a rectangular panel:

http://www.kettering.edu/~drussell/Demos/MembraneSquare/Square.html

I'm building an -8, a model which has shown some tendency to crack belly skin in the first panel behind the gear structure. I've been thinking about exhaust system configuration and sound energy, and doing some reading, and I recently had a nice exchange with my engineering mentor. So a few thoughts. I can be right or wrong about any of them, but they should all make for interesting conversation.....

The success of attempts to "insulate" against sound transmission will vary between RV models not sharing common structure because individual panels vary in dimension. Some panels will merely mimic (re-transmit, if you will) the sound energy applied to their outer surface. However, due to size and dimension, some panels may have a natural frequency matching the exhaust frequency(s). Those panels will resonate. Interior noise from those panels will be louder, and the panel will be subject to fatigue cracking too. I don't have the equation(s) for predicting the natural frequency of rectangular or square panels, but perhaps we have a reader who does, and would be willing to check some typical panel sizes?

Resonant or non-resonant, the approach Bob suggests (the rubber sheet stuff) should help. If the panel is resonant, changing its natural frequency by stiffening it with some additional stucture may be the best first move.

The 2nd and 4th order exhaust noise is a relatively low frequency. For example, 2nd order firing frequency at 2500 RPM is only 83 hz and the 4th order harmionic is 163hz. At these frequencies we should be able to check belly skin vibration by direct observation with a good variable rate strobe. I have one, and I have seen some absolutely amazing things with it, among them entire fabric skin panels pulsing in and out like the gills of a fish. No reason to believe the same isn't true for metal panels. (I've also seen control pushrods go resonant at some particular engine RPM, which will scare the snot out of you if you think about it.) Strobe observation is very cold work in the winter, but when the weather warms I'd be happy to do a survey.

At low frequencies, sound energy from a point source is not particularily directional. If I understand correctly, when frequency is less than the speed of sound divided by tailpipe diameter, you can expect sound energy to not be a lot less when measured in the quadrant behind the pipe outlet than when measured in the quandrant in front of the pipe outlet. There is a matter of "near field" vs "far field", near being a half wavelength of so. At these frequencies, the belly skin is very much in the near field. The rule is pretty absolute in the far field; the near field is more complex, and I'm too dumb to understand it all. I can say we're way down the frequency spectrum here (f = 0.1 c/d or less), so pointing the exhaust outlet away from the belly doesn't fully explain any perceived reduction in cockpit low frequency noise, or skin panel cracking for that matter. The tailpipe can supply higher frequencies; those can be directional, and so some preceived reduction may well be in those higher frequencies.

Distance from the point source is a big deal. Sound energy is subject to an inverse square rule. Again there are near and far field differences, but in general, if you double the distance available energy is 1/4. Moving the pipe outlet away from the belly is important to reduce energy at the belly skin.

A 90 degree turndown of the tailpipe has the potential to introduce a new vibration to the airframe, and it has nothing to do with sound. The exhaust gas exiting the pipe has mass and velocity, so the tailpipe reacts like a poor rocket nozzle. Because the exhaust gas comes in pulses, so does the thrust, and the pipe can vibrate up and down. This may shake certain airframe parts depending on pipe attachment, or work pipe expansion and ball joints pretty hard.

Note the distinct difference between sound energy (pressure waves in a media) and those exhaust gas pulses (moving mass with intertia). Throwing those pulses of hot expanding gas against the belly skin panels should beat the panels pretty throughly. I suspect moving them along parallel to the surface would also rattle belly belly skin; I imagine them expanding and mixing with the ambient air in some turbulent manner. Clearly pointing the pipe downward a little bit would have a large effect here.

I'm gonna go work on my airplane now <g>
 
slash-cut pipe tips?

Has anyone with the drumming floor problem tried slash-cut exhaust tips? I would have thought with the reported extra drag penalty of turned-down tips, a horizontal, but slash-cut tip would be worth a shot.

Just a thought. I'll probably try it on my RV-8, but it could be some time before I can publish the results... :D

A
 
Pulses from prop?

Just so you guys are aware, there's an RV guy who has a little experience with exhausts. Anyway, he tried running the pipes all the way back to the spar on a 7A. No difference in "belly pulses". His conclusion is that the pulses appear to be from the prop. Name is Vetterman.
 
Yes but ...

Alex as you stated that is his personal conclusion and it is significant. He is still experimenting. I have given some thought to moving the exhaust pipes and cooling air outlet ports outboard and baffling out the entire lower cowl center section avoiding all of the NLG and FAB clutter. I have had some communication with Mr. Vetterman and he told me he has already worked on this. I saw his plane at Mitchell, SD before the 2008 AirVenture Cup and it appears to be a four pipe system stacked two to a side exiting two fixed conic fairings. I do not know what his results are but it is flying so at least casual results are known to him. The outboard placement of the pipes would change the placement to a tighter portion of the lower fuselage skin and I would expect a higher frequency response.

Bob Axsom
 
I absolutely believe pressure variations from the prop can make the skin drum; note my previous comment about observing skin with a strobe.

However, I'm not so sure running the pipes "all the way back to the spar" means much with low frequency sound energy.

The reality of "quieting" is probably a lot like modifying for speed....there is no one thing that makes it go a lot faster. You improve many things in many areas and they add up.
 
The folks that talk about the "drum" experience

The folks that talk about the "drum" experience describe it as if it was a relatively low frequency throbbing sensed through the feet on the floor of the cockpit. I have no test observations or data but prop pulses at cruise speed seems unlikely especially 2 pulses per rpm isolated to the bottom of the fuselage in a dynamic flight situation (not static prop not allowed to move forward through the air ground operation). These kinds of pulses would also be experienced on the flat fuselage sides. Just rolling this over in my head, the prop pulse concept seems wrong.

What about piston power pulses, and prop orientation when they occur? There is something that stresses the bottom cowl to fuselage interface more than any other. I had to replace my bottom hinges with flanges, platenuts and screws early in my RV-6As operational life while the rest of the original hinge mounts have been fine for ~ 500 hours.

Bob Axsom
 
Lower cowl stress and noise

Bob Axsom said:
There is something that stresses the bottom cowl to fuselage interface more than any other. I had to replace my bottom hinges with flanges, platenuts and screws early in my RV-6As operational life while the rest of the original hinge mounts have been fine for ~ 500 hours.
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Bob,

I had the same problem as you after about 350 hours or so. I noticed that in addition to the inner hinge eyes breaking off, there were paint cracks at both corners of the adjacent cowl cooling exit. My interpretation was that both problems were caused by flexing of the bottom edge of the cowl exit due to turbulence of the exiting air.

I sometimes get the "drumming" noise others have mentioned. In my case it tends to happen when I'm in a steep climb and then go away as I regain speed. Could be that prop tip vortices are coming into contact with the belly at high angle of attack?
 
Bob Axsom said:
The folks that talk about the "drum" experience describe it as if it was a relatively low frequency throbbing sensed through the feet on the floor of the cockpit. I have no test observations or data but prop pulses at cruise speed seems unlikely especially 2 pulses per rpm isolated to the bottom of the fuselage in a dynamic flight situation (not static prop not allowed to move forward through the air ground operation). These kinds of pulses would also be experienced on the flat fuselage sides. Just rolling this over in my head, the prop pulse concept seems wrong.

What about piston power pulses, and prop orientation when they occur? There is something that stresses the bottom cowl to fuselage interface more than any other. I had to replace my bottom hinges with flanges, platenuts and screws early in my RV-6As operational life while the rest of the original hinge mounts have been fine for ~ 500 hours.

Bob Axsom
Click to expand...

Bob, you have a great point about the bottom being different. So, is it the exhausts? I don't know. But, the bottom is also different in how the air has to travel as it goes under. The sides and top of the cowl have much less divergence than the bottom. Perhaps that explains why the bottom is "different"?

I wouldn't think it is the position of the prop during the power pulses, as I don't think the angular velocity of the crank varies that much between power pulses.

Another difference is that the air coming from the cooling exit is messed up, and has to mix with the air to either side.

The mystery remains...
 
I had a 2002 Camaro w/ t-tops and dual exhaust. when I drove it at highway speeds with the t-tops removed and the windows closed, I'd get the drumming effect. Since the camaro does not have a prop, and the exhaust exits the rear of the car, I suspect it is caused by air pressure oscillations over the airframe.

Why it seems to be coming from the floor might be the large floor is enough to resonate with the small pressure differences.

(Just stiring the pot:D)
 
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