Animation of vibratory modes for a rectangular panel:
http://www.kettering.edu/~drussell/D...re/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>
"Quiet" Cockpit
Linear Mode
