FRDS Gen 1; Vibration Isolation System

In 1991 the owner of the company I currently work for developed a fire retardant delivery system (FRDS) for a Texas agricultural aircraft company, Air Tractor for fighting forest fires. This system controls bomb bay doors on the bottom of a 800 gallon hopper that is normally full of fertilizer. The FRDS takes into account head pressure, g’s and a number of pilot inputs to achieve a linear flow rate by hydraulically servoing the doors, or firegate. Pilots use this control over flow rate to lay down even “lines” of water or retardant out in front of a forest fire.

One of the first projects I was assigned when I started work for the company I am currently with was a vibration isolation system for the FRDS control enclosure. At that point some systems had been in the field operating for over 10 years. 10 years of vibration in an aircraft adds up on all the wires and connectors inside electronics. As a result there were a lot of failures due to fatigued parts from year to year. It was obvious a better mounting scheme was needed over the old rubber mounts used.

The first step was to determine how bad the current mounting scheme was, which would help us determine how much improvement was needed. I designed and built a vibration table to shake the enclosure at various frequencies (1hz-30hz). I used accelerometers to measure the difference between the input and output forces. The differences in these values would let me measure how effectively mounts performed at isolating the system from shock.

After initial testing the old mounts proved horrible. They actually amplified the input vibrations at key frequencies up to 4x. It was clear something needed to be done to improve vibration and shock isolation. I then set out to find a mount that would help with the known natural frequencies in the aircraft as well as shock loads from landing and wave hits on the amphibian aircraft. I arrived at the wire rope isolator after testing a few other silocone rubber type components. The wire ropes performed much better than expected, we saw a reduction of the input forces by as much as 4x across the entire frequency range 1hz-30hz. With this solution the vibration isolation was improved 8x over the old mounts. These wire ropes in combination with sorbothane bumpers for over travel have now been working in the field for a number of years. As a result failure due to fatigue has been greatly reduced.

Below are a few pictures of the FRDS with isolators installed. The isolators were sandwiched between 2 L plates. This was done to ease retrofitting older systems with the isolators. The newer gen 2 FRDS system have the wire ropes attached directly to the enclosure.



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Monday, December 22nd, 2008 at 10:00 am in category blog

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