To: Flight Safety
You have raised some valid points relative to the Robinson rotorhead design. I had posted on several occasions my thoughts on this subject and was ambushed by Robinson supporters and individuals that did not agree with my technical conclusions. You are the first to recognize that the flapping of the blade introduces leading and lagging forces (Coriolis).
It is true, when the blades flap they lead and lag. In the case of the Robinson rotorhead design there is no provision for leading and lagging and these forces are reacted by the coning bushings (hinges) and are transmitted directly to the teeter bushing (hinge) and are in turn reacted by the main rotor shaft and the transmission. The forces then pass to ground to the fuselage via the transmission attach points.
The magnitude of the leading and lagging is dependent upon the coning angle (flapping angle).
The frequency of the blade movement (tendency to move) is at a rate four times the rotational speed of the main rotor.
In reacting the lead / lag forces the cone bushings wear in an elliptical shape. In the wearing of the bushes the blades are actually allowed to move which exacerbates the condition. The teeter bush (hinge) will also wear but not at the rate of the cone bushes. It is my contention that in reacting the forces of leading and lagging the main rotor shaft can be fatigued as well as other structural parts of the blade.
Regarding other semi rigid rotor heads some of them use drag links which transfer the loads directly into the rotorhead. On the Bell 206 series the blades are restrained from movement by an adjustable clamping arrangement and those loads are reacted by the rotorhead. Some of the in plane vibratory forces may be reflected on the opposite blade but they are minimal. Bell in trying to minimize the Coriolis forces underslung the rotorhead.
It well within the realm of possibility that the vibratory forces that are manifested in the leading and lagging can be reflected on the blade and cause an aerodynamic / aeromechanical instability that would result in divergence.
If you had read my report, I raised a question as to the level of developmental testing performed on the rotorhead. According to FAA certification regs, the rotorhead on the Robinson helicopter constituted a new and unusual design and it required a higher degree of testing to prove its’ efficacy as opposed to a conventional semi rigid rotor head such as that used on the Bell.
On several other posts I alluded to a report performed by the Georgia Tech Aeronautics Department at the behest of the FAA. This report which was never finished was adapted by the FAA and Robinson and as a result the R22 and R44 were restricted from flying out of trim (balance) and from being sideslipped because the rotorhead design would lead to high flapping loads and result in mast bumping. This should have been discovered in the certification tests and not 10 years after the design was certified.
Another point that should be considered is the design of the internal workings of the rotorhead. The Robinson rotorblade has an internal lug (stop) which establishes the static droop of the blades. When the blades cone, this lug moves in opposition to the blade coning and the blade is free to flap (cone) without restriction. However when severe flapping loads are encountered the blade can drop down to its normal static position and the lug will engage the stop. During the high flapping excursions the blades have a high degree of kinetic energy and in making contact with the stop the forces involved will / can force the rotorhead down (teeter) and the blade can contact the fuselage or, it can result in mast bumping.
There is one other point that bears strong investigation and that is the rigging procedure for both the R22 and all of its’ variants and the R44 and all of its’ variants. The procedure is vague and ambiguous and places the mechanic in the position of making a decision in the procedure with out adequate instruction in the Maintenance manual. There are also very confusing instructions which can lead to the binding of the uniball on the swashplate and possible contact between the control system and fuselage structure. The control limits for different variants of each helicopter are set by a rigging stop plate. When the cyclic is rigged it is placed in the mechanical neutral of the rigging plate which means that the cyclic can only be moved a given amount in any direction. When forward cyclic setting adjustments are made on the blades there is a fixed angular setting. That means, that when the cyclic is moved aft for the setting of the cyclic pitch settings the settings for aft should be the same as for forward. They are not, which requires that the pitch setting on the blade pitch link must be changed. In doing so, the mechanic is told to check the forward setting to see if it has been changed. It will have changed but the mechanic is not told what to do. This type of instruction or the lack of instruction is typical of the entire procedure.
All of this is contained in my report.
------------------
The Cat
[This message has been edited by Lu Zuckerman (edited 01 March 2001).]