My seat time in 206's showed me that the rotor did a whole lot of "dancing around" in flight, even with the cyclic perfectly still (I fly with a LOT of friction on the cyclic). The tip path plane would be jinking up and down and this way and that, evidently in response to gusts, or maybe in response to the weird airflow over the blades and through a rotor system in forward flight. Hey, do I look like Ray Prouty?

Thanks to the nature of the underslung rotor, most of these little "jinks" (as I call them) won't even cause the fuselage to respond. Still, unsophisticated pilots will try to damp out these little motions themselves with the cyclic, thinking that the control inputs are "necessary." They're not, and these pilots are only making things worse because if you move the cyclic even 1/16th of an inch, the swash plate is moving a proportionate amount. I've noticed that small excursions of the tip path plane *usually* correct themselves without my interference. The fuselage itself surely provides some additional aerodynamic stability to the rotor. However, if the fuselage is disrupted, small adjustments to the yaw trim usually brings it back, as long as the pitch or bank excursion is small. Unlike fixed-wings, which change course when you change heading, a helicopter does not.

The Bo-105 rotor system does the same kind of dancing around in high-speed cruise. It literally can make passengers sick (especially the ones riding in center passenger seats who can't look out a side window and can't see out the front). Trouble is, because the Bolkow has no mechanical flapping hinges, all of these little excursions are transmitted directly to the cabin. Additionally, they are not always self-correcting as they seemto be in an underslung system. Maybe this is because the fuselage does not help at all in keeping the rotor on an even keel, to borrow a nautical metaphor.

The strange thing is, pilots generally ascribe adjectives like "stable" to ships like the Astar, which has a fairly stiff hub. (Then again, I know guys who even term the Bolkow stable. Yeah, riiiiiiight!) Personally, I never found the Astar to be so, but I have very, very little time in it. But it makes me wonder if helicopter pilots really know what "stable" is.

My feeling is that for rigid rotor systems to attain high-speed flight, some sort of sophisticated artificial stability should be added. My feeling is that rotors need to be limber enough to waggle around as they do their cyclic thing. And all this waggling needs to be damped before it can influence the mast.

We pilots like the sporty maneuverability - and safety - that articulated (especially those with high hinge offset), rigid and near-rigid rotors provide. But sometimes that control response comes at the expense of passenger ride quality. But I'm not sure. Just seems to me that two-blade systems "ride" better in lively air.

S.C.A.S.