While speculating about why the engine might have been purposely shut down,.....or not, I re-read the report, looking for the calculated cockpit vibration levels, following the loss of the outer B blade at station 161. The report does not have any calculation of that estimated value. Was the ship 1P vibration ( both vertical and lateral ) such that the pilot inadvertently did that while intending something else? Was the environment such that the pilot was rendered virtually unable to do anything of consequence? I’d guess that the Robinson engineering folks did that calculation soon after notification.
The other aspect that occurred to me just after posting this a minute ago was that with large 1P levels, it is altogether possible for the pilot to aggravate the levels-a pilot induced ocscillation ( PIO ) on top of the unbalance caused disturbance. That sort of thing is dependent on a number of factors, but if the basic disturbance is below 6 hz, pilots can respond so as to increase the severity. Preventative measures to prevent PIO is a whole other subject and probably not relevant here.
After awhile, something eslse occurred to me regarding the pilots action in that vibration environment: was the pilot being bounced around so that he couldn’t read the Nr indictor if he wanted to.
I have a data point here. During the development of the UH-60 ( well before the fly-off part of the competition ) we were have vibration problems that were challenging our ability to get them ( there were variously caused issues ) resolved. In desperation ( true story ) we took set of S-61 blades, cut them down, modified the grips to fit the elastomeric head, balanced them on the whirl stand and flew them. Well, not so good: the speed maxed out at 137KIAS and the vibration levels were roughly the same. Then someone suggested, that to get a real baseline with the 61 blades, we should take all of the vibration fixes then on the prototype off the machine and fly it bare. I think at the time we had the stacked 3P and 5P bifilars, nose absorber, cabin absorber, and 50 lb fixed absorbers under each pilot seat, mounted on structure. All of that came off. How fast did we go? 20 kts paced forward speed, and the reason for stopping was that the vibration levels ( N/rev or 4P ) were such that the instrument panel disappeared in a blur-nothing was readable. Turns out we were done for quite a separate reason as well-we had blown by all of the GE T-700 engine vibration do not exceed limits.
Last edited by JohnDixson; 4th Apr 2021 at 16:28.
Reason: Another thought