Skin vibration damping

[lewbloms]

The belly of our agricultural airplane is CRES, for corrosion resistance, thin (.018) for lightness. It is attached to the steel truss frame along the edges and to cross tubes with rivets to brackets. There is 1" to 2" of space between the truss and the skin, and cracking from vibration has been a perenial problem. Right now we attach cast urethane blocks to the truss tubes that press on the skin to reduce vibration. They work OK, but are rather heavy, the material is expensive, and casting them is labor intensive.

I wanted to replace these dampers with blocks of closed cell foam, but have been unable to find a suitable foam. Any suggestions? It has to be closed cell to exclude the various chemicals used in agriculture.

Another Idea I'm playing with is buying polyurethane tubes and cutting them into rings. These rings would be ty-raped to the truss tubes in two places so they line up with the tube. The skins resting on them would deform them to an oval shape (pre-load)so they would act as springs and dampen the skin motion. I lean toward polyurethane because it resists taking a permanent set. For lightness sake I want to use fairly hard polyurethane. I will need a couple of sizes so I'm looking at 2" and 3" tubes with 1/4" to 1/2" wall thickness. I can vary the springiness by changing the width of the ring, but don't want to go much less than 1/2". I'm not analytically gifted, especially when it comes to vibration, so I'm going to experiment with wall thicknesses and widths, but am not sure where to start with hardness. I'm thinking Shore A 70 would be a good starting place. Any comments on this approach?

 

[tomirvine]

First, you need to take some vibration measurements to determine both the forcing frequencies from the engine and propeller and to determine the natural frequencies of the skin panels.

The forcing frequencies are most likely due to the propeller's blade passing frequency with integer harmonics.

The forcing frequencies may be exciting the natural frequencies of the panels, resulting in fatigue.

This vibration data is needed to determine the proper damping treatment or other design modification.

There is a company called EAR Specialty Composites that makes a variety of effective damping materials (Disclaimer: I am not their salesman).

Tom Irvine

http://www.vibrationdata.com