Cone hinges Vs Flapping hinges
To: 3top
Quote:
"By the way Lu, I believe the Robinson cone hinges are the best invention since the underslung head, they actually take any bending moment caused by coning completely out".
"Arguments that the coning hinges do flap work are not valid. If you look up the maintenance manual, you will find that there are precise procedures how to adjust the frictions on the head system. if it is set up right it will work fine. If for any reason the friction values change in one hinge it becomes apparent immediately as a mainrotor vibration. At this time it has to go into the shop to adjust this. No big deal".
If you look at the patent application for the three hinge rotor system referenced above Frank Robinson makes the same point about reduction of the bending stresses when compared to a Bell system which has very high bending stresses at the root. Whether the Robinson’s’ cone hinges are the best invention since the underslung rotor is a bit of a stretch. If the cone hinges were not incorporated the rotor system would be so heavy as to minimize any payload for the helicopter.
Adjustment of the friction on the cone hinges preloads the system to minimize any relative movement on the stack-up of the parts in the assembly. The point you made about the change in friction level on one cone hinge causing a vibration is correct however it is also true when there is an imbalance between elastomeric elements in a multi-blade system. For instance if the elastomeric dampers on the Apache differ by more than 5% the aircraft can enter into air resonance and self-destruct. The same is true for hydraulic or friction dampers on other helicopters. The Robinson head is in effect a semi rigid semi articulated rotor system and being semi articulated (free to flap) it is subject to the same forces that generate leading and lagging.