Lu, it's funny that this question should raise itself again. I actualy forgot that I posted my comment on it ages ago.
Where the alpha angle thingy came from is when I was doing my BK117 ground school the engineer was teaching us about the Mast Moment Indicator. This instrument, unique to this breed of helicopter, literally lets you know how much bending of the mast you have, especially on the ground if you try to lead with the cyclic (If you fly any Bells you know what I mean). Sounds scary but it's true.
Anyway the engineer was talking about the alpha angle with respect to why we have small masts on more rigid type systems compared to long masts on teetering type systems. Very much a design discussion because supposedly you can make a much thinner and lighter mast with a small 'alpha angle', whilst you need a short (and therefore strong) mast for larger alpha angles.
Having said all that - I have no idea what it actually is.
The engineer sounded very convincing at the time and I do remember understanding it, but hey you can teach monkeys to fly these things but they may not remember it later.
P.S - Someone was trying to explain or ask about the 45 degree etc phase angle in Helimutt's thread (pg 5 I think). It sounded very close to this alpha angle thing.