RogerBW's Blog

Piasecki PA-97 Helistat 19 March 2014

The Helistat was a hybrid airship/helicopter combination, designed for heavy vertical lift.

In principle it's not a bad idea. Compared with a conventional large helicopter, the vehicles it was bidding to replace, it uses less fuel because much of the lift comes from the gas envelope. Of course there's more aerodynamic drag, but at helicopter speeds this needn't be hugely significant.

Because putting a single huge rotor on an airship would be impractical, the Helistat prototype (built under a US Navy contract for the Forest Service, who wanted it for logging in remote places) consisted of a single obsolete ZPG-2W (N-class) envelope, coupled with four obsolete Sikorsky H-34 helicopters (with the tail rotors and booms removed to save weight). The controls were linked together so that the assembly could be handled by a single pilot in one of the helicopter cabins. Everything was linked together by a structural framework.

Test flights were conducted at Lakehurst throughout May and June of 1986, apparently with some success. However, on 1 July, a gust of wind caught the aircraft while it was on the ground, and the landing gear started to shimmy. Some of the helicopters entered a ground resonance state, a state in which the rotors vibrate excessively and generate off-axis lift and turning forces. The pilot correctly increased power to lift off, but one of the helicopter bodies broke free, slicing into the gas envelope. This made the vibration worse, and the framework disintegrated, leading to all the helicopter bodies breaking away; this killed the pilot.

The basic problem seems to have been a woefully optimistic calculation of the stress dynamics of the connecting framework. Given that making it stronger would inevitably make it heavier, and the whole point of the thing was to be light to carry heavy loads, further development was not undertaken.

The SkyHook JHL-40 could be considered a lineal descendant (though the designers wouldn't thank you for doing so), but whether it'll ever actually get built is anyone's guess.

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