Crane drops wind turbine rotor - GIF

[schuylergrace]

You ever watch the wings on the airplane you are flying on as you taxi along? Or as you fly through turbulence?

I understand, from having built a number of large flying scale model aircraft and taken engineering classes covering such designs, the necessity of both light weight and flexibility in wing structures (which wind turbine blades essentially are). But I was surprised how flimsy those blades seemed to be. A long while ago, I watched a 777 (if I remember correctly) static wing deflection test, and they bent that thing nearly 30 feet before it failed! I know these were different sorts of loads from that, but the blade on the ship side appeared to go limp like a noodle for its entire length when it struck.

[hemmjo]

That 777 wing test is indeed impressive. If you analyze that test further and apply it to the blade dropping situation it makes perfect sense. We must remember the force used in the wing test was provided by hydraulics. The force was increased slowly, stopping to remain steady at a few points. Then slowly increased until the failure point was reached. When the wing failed, the stress which caused the failure dropped to zero almost instantly. Any further damage that would have occurred after the initial failure was prevented. This allowed engineers to further evaluate points of failure to gain even more understanding of the failure. If you watch closely both wings fail at the same instant. The wings do not seem to fail at one specific weak spot, instead they break apart along the entire length at the same instant. (The test makes me wonder, what would have happened if they had stopped and held the force at 150% of the designed load. If they left that load applied for an extended time, hours, days, weeks, would the wings have eventually failed?)

In a "real life" failure, as in a crash, or dropping those blades, the forces are applied instantly, and overwhelmingly. The forces continue to act on the failing system. The system is greatly weakened after the initial failure, this allows subsequent damage to become excessive, making the damage more spectacular.

[schuylergrace]

I think that was the test I was at, hemmjo! I remember I was surprised we were down on the floor because I figured we'd be up in a control room or some more protected area. If you look at how that wing failed, the main spar snapped/exploded and although the shock wave from that event transmitted throughout the entire wing, the structures outside the primary failure area still seemed to be reasonably structurally sound. I may have been interpreting what I saw with the turbine blade incorrectly, but it looked like the entire blade failed at once, as if it were being held together by components in tension, like cables inside the blade. Or the skin could have been the component in tension, and when it ripped away, there was no longer any structure. And of course, the shock wave traveling outward from the point of impact probably destroyed other components away from the original damage, so that could be the cause, too.

Thanks for that literal blast from the past!