The Helistat concept was to augment the helicopters' dynamic lift with the static lift of an air buoyancy envelope. This would give greater maximum lift capability for heavy-lift work. At low weights (i.e. traveling to site without a payload) it would also free up the helicopters' rotor thrust for forward thrust, requiring less dynamic lift and lower fuel burn.
To maintain coincidence of the dynamic and static lifts (otherwise the envelope would pitch as helicopter power increased), it is impractical to use a single helicopter rotor, so multiple rotors are arranged around the center of buoyancy of the envelope.
Differential changes to the collective pitch (i.e. thrust) of the rotors gives powerful control forces. Propulsion and retardation are obtained from the cyclic tilt of the rotors, as for a normal helicopter. Yaw moments are produced by the differential cyclic tilting of the rotors (i.e. one side forward, the other back). In forward flight, the ruddervators at the tail of the blimp also add their pitch and yaw control moments.
https://en.m.wikipedia.org/wiki/Piasecki_PA-97A gust of wind from the rear of the aircraft induced some movement across the tarmac. The undercarriage responded badly to this, the bogies shimmying uncontrollably. Vibration in the framework then coupled with a helicopter phenomenon known as ground resonance. The vibration was sufficient to cause a structural failure as the starboard rear helicopter broke off its mounting, its rotors cutting into the gasbag.The unbalanced lift then made the vibrations worse and the remaining three helicopters broke free.
What were they thinking in this Heath Robinson or rather Piasecki design?
From NTSB report.THE HELISTAT, A HYBRID A/C WITH 4 H-34 MAIN FUSELAGES ATTACHED TO A FRAME ALONG WITH A ZPG-2 HELIUM FILLED ENVELOPE HAD JUST COMPLETED IT FIRST HOVER TEST FLT SUCCESSFULLY AND LANDED. A PWR LOSS WAS NOTED ON THE NO. 3 HELICOPTER AND THE TEST WAS TERMINATED AND THE MOORING MAST CALLED FOR. PRIOR TO RE-MOORING A WIND SHIFT CAUSED AN UNCOMMANDED LEFT TURN WHICH THE PILOT COULD NOT CONTROL WITH THE FLT CONTROLS. WITH A TAILWIND, NO WHEEL BRAKES OR GND STEERING A TAKEOFF WAS ATTEMPTED. THE 4 MAIN LANDING GEAR WHICH HAD NO SHIMMY DAMPNERS STARTED TO SHIMMY. THE FOUR HELICOPTERS STARTED TO REACT TO THE SHIMMY WITH GROUND RESONANCE. AS THE HELISTAT FINALLY LIFTED OFF, THE FOUR INDIVIDUAL HELICOPTERS BROKE OFF AND FELL TO THE GROUND. ONE PILOT RECEIVED FATAL INJURIES, 3 RECEIVED SERIOUS INJURIES AND ONE MINOR INJURIES. THE HELISTAT WAS DESTROYED. THE PRW LOSS ON THE NO. 3 HELICOPTER WAS TRACED TO A MISSING THROTTLE LINKAGE CORRELATION PIN. WHY THE PIN WAS MISSING WAS NOT DETERMINED.
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