Originally Posted by
TTSN
This is from a post about another Robinson accident being discussed on this forum in 2020.
Quote:
Originally Posted by
Arnie Madsen View Post
I am not anti-Robinson but we must acknowledge that very light blades are more sensitive to turbulence & mast bumping.
Heavy blades retain their plane of rotation long enough to handle sudden strong gusts.
But they require extra strength (heavy) laminations & spar at the root end to handle bending loads at the root (blade coning)
Frank came up with a good solution to save ounces and pounds in the rotor system
By incorporating coning hinges at the root which relieved any coning stress
Thus he could build very light blades .... which had the additional benefits of lower centrifugal force on the hub and feather bearings which meant he could build a lighter head.
Early blade de-laminations were the first red flag , but were attributed to outsourced blade manufacturing errors
Then came the numerous unexplained rotor incursions into the cabin or boom. A big red flag ... not to mention further de-laminations through the years
Easy for me to say ... but if he (stubborn frank) had halted production , built a blade similar to the H269 , strengthened the hub to take the extra loads , the problem would have been solved.
Of course those several pounds of weight would have reduced payload .... but extra pitch along with extra power from the de-rated engine should solve that ..... working the Lycoming a bit harder would likely reduce TBO from 2200 hrs to 1800 hrs which is still excellent for piston helicopters.
I am also suspicious of having 3 hinge points on a 2 blade rotor .... if a split second disturbance caused
one blade to pivot on a coning hinge it would immediately put everything out of phase and would self-destruct.
i replied to this post
TTSN
There ... you just hit the nail firmly on the head. In 25 years plus of following Robinson rotor issues this seems to be the view of a number of the worlds experts. The numerous unexplained Robinson rotor divergence issues are unique but so is the rotor head design with a teeter and 2 coning hinges. I’m sure this particular accident was due to being out in the wrong kind of weather and the result would have been the same regardless of what type of light helicopter was being flown. More frequent checks on the condition of the coning bolts and closely monitoring preload (resistance to movement) should be introduced. A Robinson operator told a story on here (PPrune) a couple of years ago of finding the broken off end of his coning bolt during a pre flight check in the hangar (a little alarming I know). I’m not a Robinson hater in any shape or form and would be willing to assist in any way I could reducing or solving this issue.
I am also suspicious of having 3 hinge points on a 2 blade rotor .... if a split second disturbance caused one blade to pivot on a coning hinge it would immediately put everything out of phase and would self-destruct.
Yes/No/maybe...
Coning hinge... : If the tensions get messed up, it gets ugly very quickly, but not like these cases. Having had a coming hinge fail in flight, it was a rapid progressive ugly, but not instantaneous. I detected the anomaly climbing out over the C-17 building, and did an abbreviated circuit to the grass. Landed. Instructor with me picked up to the hover to get his thoughts and he was visibly being shaken by the cyclic, not something that normally happens. After a discussion he hover taxied back to the lines about 2 minutes and by landing the head was toast. It was progressive, but not instantaneous. The R22 that had one blade shear from the TE to the D cell still took time and that was with a failed coning hinge. Apparently did fly funny. Spectacular photo..
Effect of gusts doesn't have much relationship with blade mass, lock number is affected of course.
Just wasted 7 hrs on a flight re reading Johnson, and can't see that there is a torsion harmonic issue, that would be over 109% NR for the....??? 7th mode??? (It put me to sleep)
Concorde's noted wind noise brings up the thought that a yaw event occurred, but most causes would leave evidence in the blades, hub, control runs.
No question that teeter heads are not happy with low g, but Concorde and Queenstown do not suggest low g from environmental conditions. The comment of disorientation, the bay is often like a milk bottle interior with the haze taking out a distinct horizon, the NTSB not making a comment on that doesn't preclude it as an issue.
The RHC deserves a good evaluation of the rotor head to resolve the matter, it has a place in the industry as a valuable tool