We don't usually consider the power to weight ratio of a helicopter, mostly just its useful load.
But without diving into and calculating the power to weight ratios of various helicopters I'd bet money on either the S-64 or the K-MAX.
As you can see the Sikorsky S-64 is a very large helicopter with very little to it. It's got a cockpit, landing gear, fuel tank, rotors, two turboshaft engines and gearbox, and some fuselage to hold it all together. This helicopter was built in the 1960s to haul the heaviest loads possible. Especially useful in the vietnam war for hauling equipment around by air rather than cutting an impossible road through the jungles. It has an empty weight of about 19,000 pounds and a maximum takeoff weight of 42,000 pounds, which gives a useful load of 23,000 pounds. Its two engines combined give a shaft horsepower of 9,000. So 9,000shp/19,000lbs = 0.47.
Now, the kamen K-MAX
Again there's not much to this helicopter which cuts down on the weight. Its a lot smaller than the S-64 but still has a great useful load. It is a single seat, single engine helicopter with an intermeshing rotor design. The empty weight is 5,145 pounds and the maximum takeoff weight is 12,000 pounds, giving it a useful load of 6,855 pounds. The single turboshaft engine is capable of producing 1,500 shaft horsepower. So 1,500/5,145 = 0.29.
This actually surprised me because I had assumed that the K-MAX would have a comparable power to weight ratio given that its useful load is comparable as a percentage. So out of curiosity I checked the worlds largest helicopter, the Mi-26, if you can spot it behind that Jet ranger.
With an empty weight of 62,170 pounds and two turboshaft engines giving a combined 22,800 shaft horsepower - 22,800/62,170 = 0.36. Still less than the S-64!
I think you'd be hardpressed to find a helicopter lugging around that much horsepower with so little airframe to carry with it that gives a better power to weight ratio.
So I unofficially crown the Sikorsky S-64.
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