In the summer of 1986 the largest aircraft in the world sat on a ramp at Lakehurst, New Jersey, three quarters of a mile from where the Hindenburg had burned. It was 343 feet long. It was a Navy blimp with four helicopters bolted underneath it.

On 1 July it lifted a few feet off the ground, started to shake, and came apart.

The problem it was built to solve

The United States Forest Service had 20 million acres of commercial forest classed as marginal — too steep, too remote, no roads. Getting timber out meant either building a road, at around a million dollars for six miles, or flying it out with a heavy-lift helicopter, which was expensive and limited.

Frank Piasecki had an answer, and he had been carrying it around for a very long time. He filed a patent for a combined balloon-and-helicopter lifting unit in March 1958. It was granted in November 1961. Then, as one recent account puts it, absolutely nothing happened with it for over a decade.

The idea is elegant on paper. A blimp big enough to lift the aircraft’s own weight, but not the cargo. Helicopters to lift the cargo and to provide the control a blimp does not have. Static lift and dynamic lift acting through the same point. In theory, you get a machine that can hover with 25 tons on the hook without needing an engine large enough to hold all of it up.

Built from what was lying around

Piasecki won the contract in January 1980 partly on price and partly on speed: the company said it could deliver in two years where others wanted five, and it proposed making extensive use of surplus government equipment to keep the cost down.

So the envelope was a ZPG-2W airborne-early-warning blimp gas bag that had been crated at Lakehurst since the Navy shut down its lighter-than-air operations around 1961. The helicopters were four surplus Navy H-34Js, piston-engined machines already long out of service, with their tail sections removed. They were mounted two per side on a tubular aluminium frame around the centre of buoyancy.

One pilot, in the left-rear cockpit, flew the entire thing. Flight engineers occupied the other three. Pitch and roll came from differential collective pitch across the four rotors; yaw from differential cyclic tilt; thrust from tilting the discs. All of it mechanically interconnected. No fly-by-wire.

The undercarriage was four freely castering twin-wheel bogies. They had no shimmy dampers, no brakes and no steering. Remember that.

The warnings

They started early. A General Accounting Office report of 2 June 1981 is one of the more quotable documents in the history of American procurement.

The GAO had asked four forest industry associations and thirty-two potential private users what they thought. Their overall assessment of the concept was recorded as poor. Eleven said weather and wind would ground it much of the time. Thirty-three preferred cable systems or conventional heavy-lift helicopters. Exactly one was willing to contribute money.

The GAO also noted that the Navy contract administrators said they were hamstrung in getting the data they needed to judge the engineering adequacy and safety of the vehicle being built, and that the Forest Service had let the contractor begin fabrication concurrently with design.

Navy, NASA and FAA officials criticised the workmanship on the interconnecting structure during construction. One Navy engineer was quoted in blunter terms.

1 July 1986

It flew free for the first time on 26 April 1986. The vectored-thrust airship patent Piasecki had filed in 1979 was granted on 27 May. Five weeks after that, on the evening of 1 July, the Helistat completed a hover test successfully and landed.

A power loss was noted on the number three helicopter, so the test was terminated and the mooring mast called for. Then the wind shifted.

The NTSB’s narrative of what followed is worth reading exactly as written:

“Prior to re-mooring a wind shift caused an uncommanded left turn which the pilot could not control with the flight controls. With a tailwind, no wheel brakes or ground steering a takeoff was attempted. The four main landing gear which had no shimmy dampeners 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.”

Footage of the Helistat’s last minute at Lakehurst.

One rotor cut into the gasbag on the way down. One pilot, aged 58, was killed. Three other crew were seriously injured, one slightly, and a base firefighter was also hurt. Frank Piasecki, then 66, watched it burn.

The power loss on number three was eventually traced to a missing throttle linkage correlation pin. Why the pin was missing was never determined.

Ground resonance, and why four rotors made it worse

Ground resonance is a helicopter’s own private nightmare. If the rotor blades bunch unevenly around the disc while the aircraft is in contact with the ground, the resulting out-of-balance force can couple with the natural rocking frequency of the aircraft on its landing gear. Each cycle feeds the next. A machine can tear itself apart in seconds.

The defences are well understood: properly damped blade lag hinges, and properly damped landing gear. The Helistat had four rotor systems, a large flexible frame connecting them, and undercarriage bogies with no shimmy dampers at all.

What ground resonance is, and why it destroys helicopters so quickly.

Afterwards

The Forest Service did not try again. A spokeswoman told UPI the day after that testing had already dragged on for years beyond what it was supposed to and that, with the aircraft destroyed, there were no plans to resurrect it.

Piasecki’s own response, delivered through a company spokeswoman, was almost defiant.

There is an odd footnote. That same year, 1986, President Reagan awarded Frank Piasecki the National Medal of Technology.

The patent expired in 2003. SkyHook International and Boeing then proposed the JHL-40, a heavy-lift hybrid built on much the same logic, and were ready to start construction in 2008 when the credit crisis killed it. Lockheed Martin’s LMH-1 and Hybrid Air Vehicles’ Airlander have kept the hybrid-lift idea alive since, without anyone yet cracking the heavy-lift job the Helistat was built for.

The modern version of the same idea — hybrid airships for cargo into places with no runways.

The Helistat remains the only American helistat ever to fly. It managed five hours in total. Whether the concept was sound and the execution was not, or whether it was always going to end this way, is a question nobody has been willing to spend the money to answer since.

Sources: NTSB Aviation Investigation Final Report NYC86FHD01; U.S. General Accounting Office report MASAD-81-31 (2 June 1981); U.S. Forest Service Heli-Stat fact sheet; Piasecki Aircraft Corporation; Peter Lobner, Modern Airships (Lyncean Group); Forest History Society; UPI, 2 July 1986; TIME, 14 July 1986; Wikipedia; Wikimedia Commons.