The Lockheed XH-51 (Model 186) was a three-bladed, single-engine experimental helicopter designed by Lockheed-California, a division of Lockheed Aircraft, utilizing a rigid rotor and retractable skid landing gear . The XH-51 was selected as the test vehicle for a joint research program conducted by the United States Army and United States Navy to explore rigid rotor technology.
Read more about Lockheed XH-51: History, Variants, Survivors, Specifications (XH-51A)