Piasecki PA-97

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PA-97 Helistat

Type	Experimental heavy-lifter
Manufacturer	Piasecki
Maiden flight	26 April 1986
Retired	destroyed in crash, 1 July 1986
Number built	1
Program cost	$40 million
Developed from	Navy ZPG-2W blimp, H-34J helicopter

The Piasecki PA-97 Helistat was an American experimental heavy-lift aircraft, built by Piasecki by fastening four obsolete helicopters to a framework beneath a helium blimp. ^[1] ^[2]

Design and development

Helistat design concept

The Helistat concept was to augment of 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. travelling 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. ^[2]

To maintain coincidence of the dynamic and static lifts (otherwise the envelope would pitch as helicopter power increased), it's impractical to use a single helicopter rotor and 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.

PA-97 prototype

The PA-97 was built under a 1980 U.S. Navy contract for the Forest Service to demonstrate a heavy vertical air lifter for harvesting timber from inaccessible terrain. The single demonstrator used a Navy ZPG-2W blimp and four H-34J helicopters. The combination of a large blimp with powered lift made the 343 feet (104.57 m) long helistat the largest dynamic lift aircraft in the world. ^[2]

The helicopters used were aged examples of a long-established design. Their tail rotors were removed, their fuselage shortened and they were attached to a crude tubular aluminium framework beneath the helium envelope. Although each of them retained its cockpit, they were all controlled by a single pilot flying the vehicle from the port rear helicopter. Four freely-castoring twin-wheel bogies beneath the framework provided the undercarriage. Criticism has been expressed of the structural qualities and stress analysis of this framework. ^[3]

Test flights were made from the Naval Air Engineering Station Lakehurst in Lakehurst, New Jersey, making use of is long-established hangars for handling large airships. First flight was on 26 April 1986. ^[4]

On 1 July 1986, the PA-97 crashed after completing a test flight, killing the pilot, 39 year old Gary Oleshfski. ^[5]^[3]

A 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 pilot correctly increased power and collective pitch to lift clear of the ground and reduce this vibration. However the vibration was sufficient to cause a structural failure as the starboard rear helicopter broke off its mounting, its rotors cutting into the envelope. The unbalanced lift then made the vibrations worse and all the helicopters broke free, including the pilot's.