The Piasecki PA-97 Helistat was an American experimental heavy-lift aircraft, built by Piasecki by fastening four H-34J helicopters to a framework beneath a

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

-inflated

blimp A blimp, or non-rigid airship, is an airship (dirigible) without an internal structural framework or a keel. Unlike semi-rigid and rigid airships (e.g. Zeppelins), blimps rely on the pressure of the lifting gas (usually helium, rather than hy ...

envelope. The sole prototype was lost during a test flight, killing a test pilot and injuring another four in the course of a single incident.

Design and development

Helistat design concept

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 A helicopter pilot manipulates the helicopter flight controls to achieve and maintain controlled aerodynamic flight. Changes to the aircraft flight control system transmit mechanically to the rotor, producing aerodynamic effects on the rotor bla ...

(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 airlifter for harvesting timber from inaccessible terrain. The single demonstrator used a retired Navy ZPG-2W blimp envelope and four Sikorsky H-34J helicopters. The combination of a large blimp with powered lift made the 343 foot (104.57 m) long helistat the largest dynamic lift aircraft in the world. The helicopters used were aged examples of a long-established design. Their tail rotors were removed, their fuselages shortened and they were attached to a crude tubular aluminum framework beneath the helium-filled envelope. Four freely-castering twin-wheel bogies beneath the framework provided the undercarriage. Criticism has been expressed of the structural qualities and stress analysis of this framework. Test flights were made from the

Naval Air Engineering Station Lakehurst Lakehurst Maxfield Field, formerly known as Naval Air Engineering Station Lakehurst (NAES Lakehurst), is the naval component of Joint Base McGuire–Dix–Lakehurst (JB MDL), a United States Air Force-managed joint base headquartered approximately ...

in New Jersey, making use of the long-established hangars for handling large airships. First flight was on 26 April 1986. On 1 July 1986, the PA-97 crashed immediately after liftoff on a test flight, killing one of the pilots. A gust of wind from the rear of the aircraft induced some movement across the ramp. The undercarriage responded badly to this, the bogies shimmying uncontrollably. Vibration in the framework then coupled with a helicopter phenomenon known as

ground resonance Ground resonance is an imbalance in the rotation of a helicopter rotor when the blades become bunched up on one side of their rotational plane and cause an oscillation in phase with the frequency of the rocking of the helicopter on its landing gear. ...

. 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.

Specifications

References

External links

National Transport Safety Board Brief on Accident to Piasecki Helistat 97-34J registered N1897Z

Photograph

"From Aerologger to 'Balloondoggle'"
''Peeling Back the Bark'' blog, the

Forest History Society The Forest History Society is an American non-profit organization dedicated to the preservation of forest and conservation history."Forest History Society." Echo Project. Center for History and New Media, George Mason University. http://echo.gmu. ...