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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