Experimental, WWII

H-4 “Spruce Goose” – The Biggest Aircraft that Only Flew Once

This is the H-4 Hercules, a ginormous flying boat built from wood that stands as one of the largest aircraft ever built. With wings wider than a football field, it was powered by eight massive engines and could carry two Sherman tanks.

It was built by Howard Hughes, an eccentric billionaire who loved women, making movies, and flying aircraft. Built too late to see the war, Hughes was investigated for wasting government funds on the H-4.

Today, it stands as a remarkable, yet flawed aircraft, and arguably Hughes’ most infamous project.


The Need for a Flying Cargo Ship

Since 1939, the United States had been providing Britain and Europe with the critical supplies they needed to fight Germany in the Second World War. Millions of tons of cargo was transported over the Atlantic Ocean to the UK by ships, that brought ammunition, food, weapons, fuel and more.

Aware of how important this was to Britain and it Allies, Germany began attacking these vessels with unrestricted submarine warfare. These silent hunters could pop up, fire off a spread of torpedo’s and then submerge in an instant.

By the time the United States’ entered the Second World War in late 1942, German U-boats had already sunk hundreds of thousands of tons of Allied shipping. Some counters had been developed, like military escorts and anti-submarine aircraft, but the U-boats were still a major threat.

Liberty Ships en route to Europe.
Liberty Ships would cross in groups in the hopes of providing each other protection against submarines.

In that year, industrialist and shipbuilder Henry Kaiser proposed another approach: avoid the U-boats by avoiding the ocean entirely. What if aircraft could be used to transport supplies, rather than ships?

Of course, each aircraft would carry less, but this “flying cargo ship” could cross the Atlantic much faster and had quicker turn-around times. It would also eliminate the U-boat problem.

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Kaiser (ironically, the son of German parents) took his idea to eccentric billionaire Howard Hughes. The two of them planned for an aircraft capable of carrying up to 750 troops or 150,000 lbs (68,000 kg) of cargo.

Hercules Blueprints.
Drawings for the large flying boat.

It was also to be a flying boat, meaning it could land pretty much anywhere that had a coast. If it wasn’t, an aircraft of this size would be severely limited in the runways it could land on.

They created the joint Hughes Kaiser Corporation, and designated this monstrous aircraft the HK-1 (Howard Kaiser-1).

The project was not popular among military planners, who thought such an aircraft would be impractical and draw resources from more important projects. However, using their considerable government connections, Hughes and Kaiser managed to obtain an $18 million government contract for the construction of three aircraft.

H-4 wings under construction.
Wings for the aircraft under construction.

Aluminium, the standard aircraft construction material, was in short supply at the time, so the HK-1 was to be primarily built from wood.

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The project very quickly hit delays and cost overruns, mostly due to the unusual construction materials and Hughes’ famous need for perfection. This caused a frustrated Kaiser to leave the project in mid 1944. Hughes continued alone, renaming the aircraft the H-4 Hercules.

H-4 Hercules Design

The H-4 Hercules was the largest and heaviest aircraft ever built at the time. Its wooden construction led to it being nicknamed the “Spruce Goose”, however, the H-4 wasn’t actually built from spruce.

It was constructed from birch, using a method Hughes had acquired from the Fairchild Aircraft Company called Duramold. Duramold saw layers of thin birch veneer bonded together with glue, and then steamed.

The many layers with different grain directions provided a greater weight-to-strength ratio over typical spruce beams. Other types of wood were used in the aircraft too though, such as poplar and balsa.

H-4 Hercules in Los Angeles.
The H-4 was so big that you could stand up inside its wings.

Special joins were used to connect different parts of the airframe together, as the typically-used glue blocks were too weak due to irregular expansion. The skin of the H-4 was glued to the wooden frame. The skin was held in place with around eight tons of nails, which were all removed once the glue had cured.

Fourteen fuel tanks were installed in the aircraft.

Naturally, the aircraft was highly flammable, so Hughes implemented a large and complex fire suppression system inside the H-4.

H-4 Hercules inside the fuselage.
Inside the H-4’s fuselage, looking toward the rear. The red bottles near the bottom of the image are the fire extinguishers. The beach balls were a novel way of increasing buoyancy in the event the aircraft ditched at sea. Image by Burley Packwood CC BY-SA 4.0.

36 carbon dioxide bottles were fitted into the cargo bay, and were connected up to a series of valves and pipes. Through these pipes, the bottles could be emptied into the specific areas that were on fire. This system even allowed all 36 bottles to be dumped into a single area if necessary.

Howard, a big proponent of aircraft safety, ensured that the H-4 had extremely safe flight surface controls.

The pilots had the first “artificial feel” system, which multiplied the pilots’ force on the yokes, making the H-4 feel as light as a much smaller aircraft.

H-4 Cockpit.
The pilots’ seat inside the H-4 cockpit. Image by Valder137 CC BY 2.0.

The actual control surfaces were moved by a complex, electronically operated hydraulic system of valves and cylinders, that gently and precisely translated the pilots’ inputs.

Five hydraulic systems were installed for additional safety. These included two main systems and three backups.

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On either side of the 220 ft (67 meter) long boat-shaped fuselage was a pair of giant wings, measuring 320 ft (98 meters) from tip-to-tip. This, combined with its seven-story high tail and weight of over 120 tons meant the H-4 was the largest and heaviest aircraft ever built at the time.

Pratt & Whitney R-4360 Wasp Major nacelle on the Hercules.
One of the eight huge engine nacelles, each containing a 3,000 hp Pratt & Whitney R-4360 Wasp Major.

To get such a big machine of the ground, the aircraft was fitted with eight, yes, eight, Pratt and Whitney Wasp Major R-4360 air-cooled radial engines.

Each engine had 28 cylinders, displaced 4,362.5 cu in (71.5 litres) and produced 3,000 hp. The R-4360s were the largest and most powerful piston aircraft engines built by the US, and were also used in the B-36 Peacemaker bomber.

A Waste of Government Funds

The massive aircraft’s development was riddled with setbacks as technological problems were overcome and hampered by huge cost overruns.

It only became ready for testing in 1946 – long after the war had ended. Even if it had arrived before 1945 it wouldn’t have had much use, considering the submarine threat in the Atlantic was significantly reduced after 1943.

H-4 Hercules wings being built.
The giant wings of the H-4 Hercules under construction.

As the H-4 had no landing gear, it had to be transported to the ocean for testing. This involved moving the aircraft, which had been broken down into its major, enormous parts, 28 miles from Culver City to Long Beach, California.

Cables, power lines, signs and other objects were removed in preparation for the journey. Thousands came out to watch the enormous wings and fuselage of the H-4 roll by at 2 mph.

The Hercules' fuselage during transportation.
The aircraft’s fuselage during its move to Los Angeles.

However, with the war now over and this absurd project still ongoing, many in the government began to question Hughes’ handling of government funding during the conflict.

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He had spent $18 million of government funding, and a further $7 million of his own money on the H-4 project. All to produce a single aircraft out of the originally contracted three.

By 1947 this had culminated into a scandal, and Hughes was called to a Senate hearing to investigate the matter.

H-4 Hercules wing being transported by trailer.
One of the wings is pulled through a town, witnessed by large crowds.

He fought back against his detractors, arguing that the H-4 had pushed the technological limits of the day, and thus his work was slowed as he pushed these boundaries.

He also argued that his own perfectionism had hampered the project. During the hearing, Hughes famously said “I put the sweat of my life into this thing.

H-4 Hercules fuselage during its transportation to Los Angeles.
This image shows just how large this aircraft was. It dwarfs entire buildings.

I have my reputation rolled up in it, and I have stated that if it was a failure I probably will leave this country and never come back, and I mean it!”

After the hearing Hughes dumped resources into preparing the aircraft for a test flight, to prove it technically worked, and therefore wasn’t a waste of time and money.

H-4 being put together in Los Angeles.
H-4 under final assembly in Los Angeles, California.

The First and Only Flight

On November 2, 1947, Hughes performed taxi tests of the H-4. On board were 36 people, comprised of press, industry representatives and crew. With Hughes at the controls, these were only meant to be taxi runs – he himself had said so.

However on the third and final taxi run, Hughes requested that the flight engineer set the flaps to 15 degrees – the take-off setting.

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Rumbling along the water, Hughes opened the throttles up until around 75 mph, when the 120-ton behemoth gracefully left the water – it was flying!

Hercules first flight.
The Hercules during its only flight.

The aircraft remained airborne for 26 seconds, covering around a mile at a speed of 135 mph. Hughes then gently brought it back down onto the water to an applauding crowd.

What happened? Was this flight an accident? Well, no one knows. Hughes never confirmed whether he meant to get the H-4 into the air, but it has spawned decades of debate, with it looking most likely that he knew exactly what he was doing.

The test proved the H-4 could fly, but for many this wasn’t a true “flightworthy” test, as the aircraft only reached an altitude of 70 ft – therefore not leaving the ground effect. The test didn’t prove its manoeuvrability, its high-speed characteristics or whether it could reach higher altitudes.

H-4 Hercules in the Evergreen Aviation Museum‎.
The H-4 Hercules at Evergreen Aviation Museum‎. Image by Valder137 CC BY 2.0.

One of the biggest questions was of cargo load; could it have ever gotten off the water with its planned maximum take-off weight of 400,000 lbs (180,000 kg)? It seems not.

Hughes planned to fly the aircraft again, but due to his deteriorating mental health and other commitments this never happened. Still, he hired a full-time crew of 300 people to maintain the aircraft in flightworthy condition in case he ever decided to fly it.

The staff reduced in number over the years, but remained at work on the aircraft until Hughes died in 1976.

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In 1993, nearly 50 years after its legendary flight, the H-4 was moved from Long Beach to the Evergreen Aviation & Space Museum in Oregon, where it remains today.