Does anyone know why Cessna put tape on the backside of the belly skin? Maybe every foot or so they stuck a square piece of thick back tape. I also notice it on side panels occasionally. Bugs me every time I see it. Why would they do this? Makes a perfect place for corrosion to hide underneath the tape.
I'm not doubting you, well maybe I am, a piece of rubber is enough to stop the metal from rippling? The vibration thing is weak sauce too.. thanks Cessna
Big expanses of unsupported sheet metal tend to vibrate. An actual ME can no doubt explain it better, but the short answer is that little strategically placed extra mass can change the natural frequency of that vibration and greatly reduce its tendency to resonate with things like prop wash and engine vibration. The trick is to alter the natural resonant frequency with as little added mass (it being an airplane and all) and expense as possible. Hence little squares stuck in specific spots. In a car you just layer the whole thing in Dynamat... but if you care about the money and weight, you figure out how to do it with less material.
It’s the vibration deal. Well documented. I do seem to remember something about vibration damping techniques such as this from back in A&P school. Dang that was a long time ago! Too many years flying and working on larger planes.
"noise control patches" .... is what they are called in industry. Also called "sound deadner" Any thin metal container will be a megaphone of sound so they use several methods to dampen it .... patches , sprayed on rubber coating etc etc. Try it on an empty tin can , tap it with a spoon and it will ring like a bell .... stick a piece of bubble gum on it and you only get a dull thud.
A friend of mine who is a professional car audio guru showed me how completely covering the inside of a car with Dynamat is a waste and he could do the same with just a few of those thick black mats. Knowing where the harmonic nodes are on a hundred or so cars is pretty valuable info. Cessna knew where they were on their four or five mass produced airplanes.
So if over the years these pieces of tape come off or are removed, is this going to become a problem? Should they be replaced if missing?
The inside of the fuselage tube and in the center of panels is really not the site where you see much structural corrosion.
Also works on drums: https://rtom.com/moongel-damper-pad I don't know, though, why everyone seems to think moistening their drums rather than damping them is a good idea. Tim
Actually some of the worst I’ve seen as an A&P have been in those places. Mostly due to being kept outside and poor drainage from the belly area. I’m in the process of removing this tape and cleaning up fuselage corrosion right now.
Negative requirement to replace according to Cessna. The Service document that Captain provided address this and clears up some of the speculation as to it’s intended purpose. Per pg 3 of the Service Newsletter: (Note: The sound deadening materials purpose is strictly for dampening sound in skin panels and provides no structural advantage. Therefore it is not required to be replaced.) I’m assuming this is speculation, right? I’m in the process of removing the Cessna vibe tape to clean the corrosion it’s caused. If you have references to back your theory I would truly appreciate seeing it. It’s very important to me to get it corrected properly.
FWIW - my Musketeer has honeycomb bonded to the fuselage panels and halfway down the empennage. So not an uncommon thing in light planes. I suspect they did it to keep the sound down but also to stiffen up the panels as it's 1/2' thick.
"The sound deadening materials purpose is strictly for dampening sound in skin panels and provides no structural advantage. Therefore it is not required to be replaced." Per the service newsletter previously linked.
Yes , absolutely , especially on aircraft. The study of harmonics and vibrations is a fascinating subject. In one case an experimental aircraft used an automotive engine with a jack-shaft reduction drive on which the prop was mounted . Everything was "balanced" but there was a harmonic resonance (high frequency) that would crack the 4130 engine mounts and loosen all the rivets in the surrounding aluminum. The rivets would first exhibit a black powdery residue around the heads , then after a few more hours the rivets would begin to spin in the holes and the whole panel was loose. .
It also includes a very detailed and convoluted process to remove the corrosion found under the sound tape. I’ve never removed corrosion in such a detailed fashion and don’t even recognize half of the chemicals mentioned. Compliance to this method would also be extremely time consuming and difficult in the limited space available inside the floor panels. Is there any reason I couldn’t just use CorrosionX and maybe a scuff pad attachment instead?
I always figured it was padding for when I’m replacing the garbage disposal and hit my head. Minimizes the dents in the sink.
Thanks, I was talking more to the oil canning thing. The vibration damping makes sense, stills seems like a "low rent" solution to an outsider. Is this unique to Cessna, or do we see the same on Beech, Piper, Cirrus, Grumman, etc.?
Not sure, what have others done? Is that what they all do? Outside of the bottom of a kitchen sink (learned something new) I hadn't seen this trick anywhere else.. and the OP's point about moisture, etc., seemed valid Maybe it "is what it is" ..
Moisture is an issue. I had a couple of spots on my 182RG that required a lot of work to get the old pads and adhesive off. But if I remember correctly, the corrosion was more a result of the adhesive than of the pads.
Oil canning can happen with panels that have substantial areas of thin sheet metal without rigid structure behind it. It’s especially present in areas where there’s some stress applied, like the rear part of a fuselage. Why do that, then? Why have large unsupported areas of thin sheet metal? Weight. Don’t use .032 where .020 will do. Don’t put stiffeners or other structure where it’s not required. Adding a little mass like your sound deadening tape can often control it. I’ve seen pictures of cases where someone added a stiffening rib to stop oil canning... and ended up with stress cracks instead. I’m really not all that familiar with the innards of factory built aircraft, but it wouldn’t surprise me to see this kind of solution applied by other manufacturers. I have also seen it done inside of car door panels and some other places (in addition to the outsides of stainless sinks, as has been mentioned).
