Rotor blade flexing video

It's true! It makes you wonder why we don't see more aluminum blade spars failing in fatigue. Aluminum has pretty poor fatigue resistance. Many extruded spars are just plain old commercial 6061-T6 -- not even the most fatigue-resistant member of the aluminum-alloy family.

But it's hard to argue with cold fact -- these things work and don't, in fact, break unless badly abused.
 
I question the short video as to whether this is normal gyroplane rotorblade, or even helicopter rotorblade movement. Being filmed in black & white indicated it was filmed many decades ago. Anything more recent would have been in color.

In the youtube comments below the video, someone identified this video of a BO105 helicopter. Made in Germany, info I found indicates the rotorblades as being fiberglass, and the four blades were hard-mounted to a hub bar that has no hinges.

I had suspected the blades in this video to be wooden, and were shown w/ such extreme movement out at the tips to be a result of their wood construction as well as being driven hard by powerful engines. I thought possibly this was a piston powered helicopter, and that the evident motion out at the tips to be engine torque pulses. But, they were powered by turbine engines.

Kind of like the end of a whip, or a human line of rollerbladers holding hands, and then "cracking the whip", resulting in forces that no one can maintain a grip out at the end, flinging the last one or two people off. No hinges means the blades themselves react to engine forces, collective commands instituted by the pilot, and maybe even wind shear encountered while flying.

We fly a teetering rotorblade on our gyros. It also isn't powered by the engine. I suspect our rotorblades do not exhibit such flex as this video shows. Also, every helicopter I have seen in person has hinges for each blade. Consequently, wouldn't video of a hinged blade show very little movement compared to this example?

Engine powered, (possibly old design) fiberglass rotorblades hard mounted to the hub bar mechanism that doesn't allow the rotorblades to flap?

Aluminum rotorblades, I'm guessing (since I'm no expert) would show a plank (the rotorblade being videoed) being steady, because the teetering hinge of a two-bladed gyroplane. I've also have no idea of how an aluminum helicopter rotorblade would look, but also suspect it would look like the gyroplane rotorblade in flight. Since it has hinges for up & down flapping, as well as lead-lag ones, those hinges would probably prevent such violent movements out at the tips.
 
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It is not from a BO-105. It is much older, and quite famous, done with a Sikorsky S-56 / CH-37 Mojave, and the link shows just a short clip from a much longer film..
The BO-105 has a large titanium rotor head and blade materials that absorb the flexing that would ordinarily go into hinges, but the S-56 is a huge twin radial piston engine helicopter with conventional metal blades and full articulation hinges.

Here's a better video that will confirm my assertion about the model

https://www.youtube.com/watch?v=srjbnvTWRJI
 
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P.S. Note that the flexing is cyclic, that is, distortion repeats with blade position in each rotation (look at the condition as it passes the tail rotor each time and you'll see what I mean). These look to me like air load responses, not reaction to engine torque pulses. Forces on the blade vary with each span-wise station and each cyclic position, and the blade is only supported at one end, so perfect rigidity should not be expected.
 
No argument there! By the way, the CH-37 had a 72 foot diameter rotor, so that's one really big blade you're seeing.

The pair of twin-row radials means 36 cylinders driving it, with over 4000 hp from 90 liters displacement. I'd bet the whole gas tank on my Bell wouldn't be enough to warm up those engines, much less fly..
 
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