H-16 Transporter

The H-16 Transporter Helicopter was an early tandem-rotor design being developed for Transport or Rescue by Piasecki. The prototypes were being evaluated by the United States Air Force and Army, but a crash of the second test aircraft in 1956 led to cancelling the project and it never went into production.
It was to be the world's largest transport helicopter. The first prototype rolled into public view in September of 1953. The Transporter as it is called was originally conceived as a long-range Rescue helicopter but its potential load carrying capacities led both the Army and Air Force to explore its possibilities for military usage.
The H-16 weighed more than twice as much as the largest helicopter in service at the time and could carry 40 troops, 32 litter patients, or 3 Jeeps. With a fuselage that was nearly 78 feet long and the combined rotor span of a 134 feet, the Transporter had a faster cruising speed than any other helicopter in service. It had a gross weight in excess of 15 tons.
On January 5th 1956, the second YH-16 test aircraft crashed while returning to Philadelphia from a test flight over New Jersey. The cause of the crash was later determined to be the aft slip ring bearings which seized. Because of this, the rotor shaft failed in flight, which in turn led to the aft and forward blades desynchronizing and colliding.
The aircraft was a total loss and the two test pilots were killed. This led to the cancellation not only of the H-16, but also the planned 69 passenger H-16B version.