The S-56 (also known as the Sikorsky CH-37 Mojave), was born in the 1950's and was one of the large piston engines ever made. It was retired about 10 years later and gave way to helicopters such as the CH-53 Sea Stallion.

In the video you can see the rotor blade as it cuts through the air. One can imagine the immense forces at which the rotor blades are subject to.

Thanks to Nick Bates for pointing it out.

S-56 blade bending in flight