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This is the thread that never ends, Yes it goes on and on my friend..."
It seems that the same group of people join in this subject whenever it starts. We mostly get closer to agreement each time, and the understanding of the underlying physics increases.
Nick's right when he talks about summing the spins. The energy argument is a little mixed up, and it actually applies to special cases only. One important point, the angular momentum is not constant, it is conserved. If you apply a moment to a spinning object you
change the angular momentum in magnitude, direction, or both.
In the past I've found that many don't accept some of the jumps in a rotational dynamics based description of gyroscopic precession, or they don't have the background to understand it. If you want a fairly complete attempt at explaining gyroscopic precession, I put one on page two of the
Helicopter Dynamics: Gyroscopic Precession thread. I warn you, it's long.
Don't believe what Dave says about a gyroscope requiring high mass and high rpm. Gyroscopic effects happen on all rotating systems. Read my previous message in this thread.
The biggest problem with gyroscopic precession is that the term is used completely inaccurately when describing helicopter flight. It is not a force. It is not an
explanation of why a force applied to a spinning object appears to manifest itself 90 degrees later. It is an
example of that apparent 90 degree lag.
When you spin a top, if the surface it's contacting offers little friction, then the top spins about one axis until that axis leans from the vertical. Once it leans from the vertical, gravity tries to pull it over. Instead of falling over, the top continues to spin about it's axis, but the axis rotates in space, or
precesses.
That's what gyroscopic precession is.
The top precesses rather than falling over because the force of gravity causes a moment about the contacting point of the top. Since the top is spinning, that moment adds more spin to it, but about a different axis. When these spins are added, the result is the top wants to spin about it's axis while leaning 90 degrees away from the direction that the force was applied. Gravity continues to act on the top, so the top continues to "fall" 90 degrees away from the direction the force was applied. The result is the precession.
Lu. I think you're right about the 18 degree left roll in a Robinson. I was watching one the other day, and he couldn't fly straight, always banked left. He probably understands physics, unlike the few other pilots who have flown Robinson helicopters.
Matthew.