Sikorsky S-80 / YCH-53E and IRB

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1971 factory model of 3-engine USN Sikorsky YCH-53E with its intended 7-blade IRB rotor (Improved Rotor Blades). I believe the IRB flight test vehicle was, in fact, a 2-engine CH-53D, possibly with a 6-blade rotor.

Note early tail surface arrangement of YCH-53E. Photos show YCH-53E trials in US Navy and US Marines markings.

Any photos or info of the Sikorsky IRB project will be appreciated.

 

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Both prototypes of Sikorsky YCH-53E, around 1973.