Lu asked:
1) As a Snake driver did you ever ask yourself or anybody else what causes the 2-per rev bounce and why the frequency of the bounce was two time the rotor speed and not three or four times?
Nick answered:
That is because in the case we discuss, the 2/rev is the mode that tickles the transmission mounts. On some machines, it could be 1/rev or another that drives the limit cycle instability. On some Sikorsky's, it is about 2/3 per rev that makes things hum, and we work to damp this until it is gone. I guess you would believe that a 2/3 per rev mode is excited when the blade looses 2/3 of its lift, huh?
Lu asked:
2) As an engineer can you explain in layman’s terms how the energy is developed that would lift and drop a 1000 pound dynamics system by 2+/- inches at a frequency of about 500 times per minute.
Nick answered:
Lu, you should be able to explain this, as it is fundamental to understanding the nature of dynamic oscillations, a field of study for all engineers. Simply put, the vibration needs almost NO ENERGY, Lu. When a system is in resonance, the natural vibration is at that frequency, and very little energy is needed to make the oscillation keep on going. The trick is to understand that when the transmission is dancing up and down, it gets a down push just as it needs it, so only a little push is needed to keep the dance up.
Remember the analogy of the small boy on a swing. That boy could not lift himself up to head height through normal strength, yet he can do it on a swing by simply adding a little energy when the swing needs it, so his dynamic interaction with the swing is perfectly timed. For a boy to make a swing go from head height in one direction to head height in the other takes almost no energy, as the swing is simply converting the kinetic energy of speed at the bottom into potential energy of lift at the top.
Somehow, you confuse this vast impressive motion with a great need for energy, and you are wrong.
The energy needed to bounce the transmission up and down is small, I estimate that a 1,000 pound force might stretch the mounts an inch or so, so the potential energy would be about 1,000 pounds times 0.083 feet, or 83 foot pounds. This could be applied to the system each second by a small 1/6 horsepower electric motor!
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