The XH-51A Model (186) was an experimental helicopter made by Lockheed Aircraft. It first took flight in 1962.
The XH-51 was a specialised test-bed for technologies developed in joint research between the US Army and Navy.
It was a three-bladed, single-engine helicopter that made use of a rigid rotor. The skid landing gear was retractable.
The joint research program ordered two XH-51As. These had four seats and were powered by a 550 shaft horsepower Pratt & Whitney PT6B-9 turboshaft engine.
During initial flight testing it became apparent that the rigid bladed were not very good at higher speeds, becoming unstable. So the XH-51 was refitted with a four-bladed rotor.
One of the XH-51A helicopters was modified in a compound helicopter, adding an additional source of thrust. As such it was equipped with wings and a 2500 horsepower Pratt & Whitney J60-2 turbojet.
Using this setup the XH-51 compound version reached a speed of 413 kilometers per hour or about 257 miles per hour.
Two demonstrator craft were built, designated Model 286. They had five seats and carried over the four-bladed rotor from the Model 186.
Lockheed never sold any Model 286- or 186- derived units and only three X-51 craft were ever built.