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"Bouncy, Bouncy"


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Oil leaks have been an irritation in the side of many pilots - especially when there is no real need for them due to poor maintenance or maintenance skills.  However, a fact of life is that they do occur and they make a mess, usually in the wrong place at the wrong time and can eventually cause airframe or component damage by causing resonance.

When designing a helicopter - the engineers have many things to consider.  One of these is the absorption of unwanted vibrations which could cause harmful resonances to occur.  To help determine and eliminate vibration absorption, they will spend a lot of time designing and testing engine and transmission mounts until the right type and combination of material is found.

For example - the bottom sprag mount rubbers on a Bell 47G are designed to absorb frequencies of around 650 cycles per minute while the Bell 47J has a rubber mount which looks identical but has a different frequency rubber (about 750 cycles per minute). Mixing them up can give your helicopter some weird and wonderful vibrations and bounces.

The Bell 206 rear transmission isolation mount is fine until it gets soaked in leaking oil.  Then all sorts of strange vibrations can happen.  I've flown a 206B and seen sliding windows turn to a blur at certain speeds as everything resonated!!

When isolated and away from base and confronted with an oil leak, many pilots (and maintenance technicians) wash their machines down with the only available cleaning fluid - Avgas or Jet fuel.  Besides the potential for fire, either through the aircraft's electrics or ungrounded static discharge, Avgas gradually attacks rubber engine and transmission mounts, causing them to soften or harden and then resonate at different frequencies - guess what happens next?

With the price of fuel so high and the potential for a rough and bouncy flying machine almost 100% - why do pilot continue with this practice?  I guess they never were told about the reason for the rubber mounts!  Think about it!

Donít put up with oil leaks Ė the designer didnít deliberately include them in the system! Get leaks fixed ASAP otherwise the ride could be rough and consequently another part of the machine may fail through metal fatigue.



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