The beast has risen yet another time.
To: Dave Jackson
I found the information above very interesting. Bell single rotor pilots have for
a long time slowed the rotor by increasing collective pitch. However with
the new designs (multi blade rotors) they can't do it anymore for the same
reasons that pilots of articulated rotor system helicopters including the
Robinson's' are restricted from this activity.
On blades that can flap the pilots are restricted from pulling collective to
slow the rotor down because of blade stall. As the blades slow down they
lose the velocity necessary to generate the lift associated with increased
pitch. This will cause the blade(s) to stall out and drop down and hit the
fuselage. On most Sikorsky systems they have both droop stops and anti coning
locks. These devices are spring loaded and are weighted. When the
rotational velocity of the blades slows the springs overcome the centrifugal
loading and bring the stops into the static position. The droop stops come
in first and then the anti cone locks. This must happen in the proper
sequence or, the blade(s) can drop down far enough to contact the fuselage
or possibly ground personnel.
In the response by Robinson (which was technically correct) they in fact
supported many of my arguments on the PPRuNe forum. They stated that with
the mechanical advantage between the blade span and the tusk length (75:1)
you could exert a great deal of force on the contact point if you pulled a
blade tip downward. The force would be reacted by the tusk stop and would
then act on the teeter friction possibly damaging the stop. If you take
that point further imagine the force reacted by the stop when the rotor has
gyroscopic rigidity and the blades flap and the tusk hits the stop.
1) The reactive force necessary to move the disc out of its' rotational
plane and possibly cause mast bumping is sufficiently strong as to cause the
tusk to fail. This can result in blade incursion.
2) If the tusk does not fail the rotor can be made to teeter to a point
where you encounter mast bumping.
3) Consider the forces involved and the reactions with both blades flapping
to the point where the tusks are contacting the stops (see 1&2 above).
It all boils down to the blades flapping excessively. What causes this
flapping to the maximum of the blade movement? As the Robinson response
indicated pulling pitch to slow the rotor down can cause flapping (due
mainly to loss of lift) and according to the FAA statement in the Robinson
POHs excessive flapping excursions can be caused by:
1) Incorrect recovery from a Zero G encounter.
2) Side slipping.
3) Flying out of trim.
It was for this reason that Robinson pilots are cautioned from performing
any of the above maneuvers.
My total argument against the Robinson design was the design of the
rotorhead. It was my conclusion that all of the problems would go away if
they had a three-blade rotor system like the Schweizer 300 series
helicopters.