To: Dave Jackson
“>" If that is what you mean the point of reference can be several feet outboard of the hinge and still not make any pitch change if the two points are coincident with each other."<
I don't fully understand the above sentence. Perhaps you could elaborate.
Response: I meant that if you were to scribe a virtual line from the centerline of the cone hinge for two feet and at that point attach the pitch link to the pitch horn. As long as the attach point remains coincident with the centerline of the cone hinge and the blade flaps there will be no pitch change. However if the connection point is raised and is no longer coincident with the cone hinge there will be a pitch change and in this case pitch will be extracted. If the blade flaps downward the pitch will be increased. On the actual head the two points are several inches apart.
“Using the Robinson R-22 as an example; only because it has two types of hinges. If the blade 'teeters up' then pitch will be removed. If the blade 'flaps or cones up' there will be no change to the pitch”.
If this were true then it would blow the hell out of Nick Lappos's and Frank Robinson's story about pitch flap coupling causing the blade to respond to cyclic input and flying down over the nose in 72-degrees. With teetering alone the pitch flap coupling is just like a Bell. If the two points are not coincident there is pitch change. With flapping on the cone hinge it is just like a Sikorsky or any other articulated head and there will be a pitch change. The greater the difference in the two points the greater the pitch change.
There is pitch flap coupling when the blades flap individually. This may be part of the problem in rotor incursion or mast bumping. I don’t know what 100% Nr is on the Robinson but let’s say 400 RPM. It is specifically stated that certain maneuvers and an improper recovery from zero G can cause severe flapping loads. At that rotational velocity with the blades flapping up and down at a high rate and extreme excursions there will be high frequency pitch changes from positive to negative (not negative pitch) and imagine the aerodynamic changes that occur during this time and the vibratory forces that would be generated.