Comments--"Arm 36g": Implications for separation of VS
I appreciate the thanks for my study of the forces surrounding the failure of "arm 36g". Perhaps it is more a case of being glad that I was able to put together a written summary that was actually understandable. In any case the effort to do so really focused the mind.
At the time, I just concentrated on the one piece-- the aluminum arm and the immediate area around it. Its failure could be analysed without going into a lot of speculation. Well, the answer or endpoint of any analysis had to be that it broke at the bolt hole. And then a picture of the break turned up (meaning one clear enough to see almost the exact starting point of the crack). And after I'd helped the cause by having only one of the two bars break, someone commented that's exactly what had happened (while I was still checking the spelling).
By helping the cause, I mean finding the least amount of force that would explain why a part broke. Today I browsed around a bit in the qestion of how much force it would take to remove the VS. I looked at HN39's calculations and his sketch of the dimensions of the anchorage points. He is right-- the static pull to get it tumbling forward is 66 g's, perhaps even more.
That was a problem to him. It is a force that seems too high for the small damage seen to some debris. I have myself been reluctant to disagree with Bearfoil's instinct that there had to have been some damage at altitude, to the VS.
The only way that reasonably low forces (low to my mind, anyway) can remove the VS at water level is if there was prior damage to the aftmost anchorage area of the VS, making both clevises ineffective. Damage in this area is up in the carbon-composite. I think side loading from torque about the vertical axis of the fin would fit the bill here. Torque is caused by rudder action.
If that is the case, VS separation at water level could be accomplished by a forward deceleration of 16 g's, applied as an impact force to the forward four attachment points. That's kind of a nominal minimum; perhaps 16-20 is a better way to express that.
With this deceleration, the "arm 36" will have an upward lift of 10 g's required of it by the departing VS. Otherwise, if the VS attachments were undamaged at water contact, the force at "arm 36" would have been 40 g's upward. With this force, both of the pair of aluminum bars would have broken and we would be seeing a lot more damage in this area.
So the study of the aluminum bar fracture, after reflection on the total picture today, seems to compel the conclusion that the damage at the bottom rear of the VS, in the carbon-composite, had to have occurred at altitude, prior to water entry.
That is, in my previous sequence of events in the loss of the VS-- "a, b thru f"-- "a" occurred at altitude and due to side loading, likely torsional; and only "b thru F" occurred after water contact.
I appreciate the offer of help in placing some of my notes offline to save bandwidth. There is some background material on my work with fiber-composites that is more general than specific.
OE