Also known as: HK-1, HK-4 Hercules, Hughes H-4 Hercules, the Hughes Flying Boat, the “Flying Lumberyard”
Date: Design work began in 1942; a prototype was completed and one test flight was accomplished in 1947
Definition: In terms of wingspan, the largest aircraft ever built; because of its wooden construction, also one of the most controversial airplanes ever built.
Significance: The Spruce Goose flying boat was designed during World War II to transport cargo or troops over long distances; its sole flight, over a distance of one mile in 1947, is a landmark in aviation history; its massive size and distinctive wooden construction have made it a true American icon.
Development
With a wingspan of 320 feet—longer than a football field—the Spruce Goose has the distinction of being the largest aircraft ever built. Planned and designed during World War II, when materials such as aluminum were in short supply and were reserved for the most urgent military projects, the Spruce Goose earned its name from its nearly all-wood construction. Only the flaps, or control surfaces, were made from fabric; the remainder of the plane was fashioned from layers of plywood especially constructed at the Hughes Aircraft Company plant in Culver City, California. Despite its nickname, the “Spruce Goose,” only about 5 to 10 percent of the craft is constructed of spruce; the remainder is birch plywood. The name stuck, however, because, in the words of one worker, “nobody could think of a word that rhymed with birch.”
The idea for such a gigantic seaplane originated with F. H. Hoge, Jr., a member of the Planning Committee of the War Production Board. After German submarines sank some 300,000 tons of British and American shipping during May, 1942, Hoge proposed to solve the submarine problem by building flying boats to transport cargo and troops across the Atlantic Ocean. Unlike conventional aircraft, flying boats could land or take off on bays or harbors and did not need long, land-based runways.
The idea intrigued the industrialist and shipbuilder Henry Kaiser, famous for building the Liberty Ships during World War II. In July, 1942, he suggested that the United States build an “aerial freighter” of at least seventy tons, a “gigantic flying ship” beyond anything imagined by the nineteenth-century science fiction writer Jules Verne. Kaiser asked for help from the billionaire Howard Hughes, a crack designer and pilot who had broken several airspeed records during the 1930′s.
The project was approved in October, 1942. A team from Hughes Aircraft Company would design the craft and build one prototype and two additional planes. Once tests were completed, Kaiser’s companies would begin regular production. The project was initially designated the HK-1 (HK for Hughes/Kaiser). Once design work had begun, Hughes employees voted to name it the H-4 Hercules. Hughes himself disliked the popular name of “Spruce Goose” and preferred to call the aircraft “the Flying Boat.”
The project fell well behind schedule very early, mainly due to a multitude of design and construction problems. Kaiser dropped out of the project, and Hughes was forced by various government bodies to defend the project. Only continued support from the War Production Board and the personal intervention of President Franklin D. Roosevelt kept the project going.
Design
The problems involved in designing and building such an airplane were massive. Kaiser had suggested that the overall size of the first prototype be seventy tons, but Hughes made the work more challenging by changing the size to some two hundred tons. The goal was an aircraft that could carry 130,000 pounds of cargo or 750 troops (twice the passenger load of a modern Boeing 747).
Working at the Hughes Aircraft Company plant in Culver City, California, and at other sites, the Hughes team tested a variety of shapes for air and water efficiency. The final design model, based on decisions largely made by Hughes himself, recorded the lowest air drag of any seaplane ever tested at the National Advisory Committee for Aeronautics’ Research Center at Langley Field, Virginia. Instead of a double-hulled plane, Hughes choose a single-hulled design which would require a wingspan 50 percent larger than the next largest plane of the time, the Martin JRM Mars. It was also decided that the aircraft would have a sizeable single vertical tail.
The final design divided the interior of the fuselage into two decks connected by a spiral staircase: a flight control deck for the operating crew and a cargo deck. Two railroad cars could fit in the interior cargo space, on a floor that was designed to carry 125 pounds per square foot. If planks were provided for its tracks, a 60-ton army tank could drive inside, under its own power, without the need to dismantle any part of the tank. The hull also contains eighteen watertight compartments, twelve of which might flood without sinking the craft.
In its final design, the Spruce Goose has an overall length of 218 feet. Its 320-foot wingspan exceeds even that of the U.S. Air Force’s modern transport, the Lockheed C-5A Galaxy. The tail alone, at 113 feet, is more than eight stories high. The hull is 265 feet wide and the wings, at their thickest, are more than 11 feet thick. The craft has a gross weight of 400,000 pounds and a range of 3,500 miles. It cruises at 175 miles per hour and has a landing speed of 78 miles per hour.
Hughes chose to power the plane with Pratt & Whitney R-4360 engines. Eight of these twenty-eight-cylinder, 3,000-horsepower engines were mounted in the wings. The engines, radial in shape, sport four-blade Hamilton
Events in Spruce Goose History
May, 1942: Federal government official F. H. Hoge, Jr., proposes that the German submarine threat be countered by the construction of massive “aerial freighters” to fly cargo and troops across the Atlantic Ocean.
July, 1942: Industrialist Henry Kaiser proposes that the United States build “aerial freighters” of from 70 to 500 tons.
August, 1942: Kaiser suggests that billionaire and aviator Howard Hughes meet with him concerning the “aerial freighter” project.
November, 1942: A new Kaiser-Hughes corporation signs a federal government contract to design and manufacture Spruce Goose aircraft.
June, 1946: Sections of the Spruce Goose are moved from the Culver City, California, plant of the Hughes Aircraft Company to a final assembly site near Long Beach, California.
August, 1947: Hughes begins testimony before a congressional committee investigating the Spruce Goose project.
November, 1947: Hughes flies the Spruce Goose for the distance of a mile in Long Beach Bay. February, 1982: The Spruce Goose is moved to become a public exhibit alongside the ocean liner Queen Mary in Long Beach.
October, 1992: The Spruce Goose is shipped via ocean barge to Portland, Oregon, to become a public exhibit in McMinnville, Oregon.
Standard propellers more than 17 feet in diameter. There are a total of 448 spark plugs to service and maintain. Although the total engine horsepower of 24,000 is impressive, the engines’ ability to lift a craft of more than 400,000 pounds is a real achievement for both their makers and the Hughes team’s overall design efforts.
Flight controls that would respond reliably and quickly were a special problem for such a gigantic aircraft. The layout of the flight controls on the flight deck is conventional—a dual column and wheel to turn the elevator and ailerons, and pedals for the rudder. Less conventional is the way that the craft was designed to respond to these controls. Instead of a mechanical system, Hughes and his team chose a hydraulic system in which pressurized oil moves the control surfaces. Purely mechanical links between the flight deck and the rest of the plane would have required the strength of 150 to 200 men just to turn the controls. Mechanical links also are unreliable in such a massive aircraft. The Spruce Goose is so large that changes in temperature could cause metal parts to expand and contract, possibly jamming in the process.
In addition to wing fuel tanks, there is a central fuel system in the hull. Fuel lines in the wings, however, have slip joints to allow for wing deflections of as much as 13 feet during flight.
The Spruce Goose pioneered the use of a 120-volt DC electrical system in airplanes. This relatively high voltage leaves a safety margin in case of electrical leakage in any of the 32 miles of wire inside the Spruce Goose. It also allows manageable wire sizes to be used. (A 24-volt system, the engineers calculated, would have required solid aluminum rods 2 inches in diameter in order to carry the current.) Electrical relays are specially designed to work at high altitudes.
Construction
A special building at Culver City, claimed to be the world’s largest wooden building at the time, was used to build the subassemblies of the Spruce Goose. The most challenging construction problems involved the extensive use of wood. The Hughes team spent a great deal of time fashioning a wood construction process that would hold together in the stresses of flight. Although metal would be available in sufficient quantities during the last two years of World War II, that was not true when the Spruce Goose project began. At that time, aluminum and other key materials were reserved for higher priority war projects.
The structure of the aircraft was created from laminated layers of wood. The process chosen for laminating the wood, called Duramold, had first been used by Hughes in 1934 to construct parts of his record-breaking H-1 racer airplane. Birch was selected because it created a stronger plywood than spruce. To secure the most suitable birch wood, a team of specialists was sent to inspect and purchase trees in Wisconsin.
Layers of wood were bonded together with three different types of epoxy glues, and heat and steam were applied to “cure” the glues. The process required special jigs and construction techniques, some of which have remained secret. Workers had to wear gloves, since the oil from fingerprints might weaken a glue joint. For the wings, some 8,000 nails were used to hold the wood layers together until the glues had cured. All had to be removed later with special nail pullers.
Considerable sanding of the plywood exterior was necessary. A coat of wood filler was applied to all exterior surfaces, followed by a layer of sealer, a layer of rice paper, and two coats of spar varnish. The final step was a layer of aluminized spar varnish, which gave the Spruce Goose its silver color. These steps produced a smooth, glossy finish that was said to be more air efficient than aluminum skins, which require large numbers of rivets, which cause drag.
Assembly and Flight
In the summer of 1946, one year after World War II had ended, the subassemblies of the Spruce Goose, including the hull, tails, and wing sections, were transported to a dry dock and assembly site near Terminal Island, in the vicinity of the Long Beach naval base.
In 1947, the project came under attack from Republicans in the House of Representatives, who insisted that the Spruce Goose was a fitting symbol of the wastefulness of the Democratic administration of President Roosevelt. One congressman termed the flying boat “the flying lumberyard.” Although Hughes defended the project passionately in congressional hearings, privately he told company workers to accelerate the project or “the next time you’ll see me in jail.” Hughes, who had spent much of his time during the war working on another aircraft, the XF-11, now worked at the assembly site for the Spruce Goose full time, although, characteristically, he did his work at night.
OnNovember2,1947, a test of the plane was scheduled which involved taxiing the craft across Long Beach Bay. Some members of the press were invited to ride aboard the plane, and Hughes took the controls. After two successful trips across the bay, Hughes increased the plane’s speed during the third attempt. He delighted a sizeable crowd of onlookers by lifting the plane off the water. After traveling for a mile at a height of about 70 feet, the craft landed smoothly. Although it was the only flight ever made in the Spruce Goose, it became a memorable moment in aviation history.
Although Hughes described the test flight as “just great,” he never flew the craft again. There are varying opinions as to why he made no further attempts. Some argue that the plywood construction was not totally satisfactory (some workers claimed that Hughes attempted to address this by adding a corrugated aluminum skin and metal stiffeners into the gigantic wing). Others believe that Hughes saw congressional criticism as a challenge and lost interest after the successful flight.
Yet Hughes continued to spend money on the aircraft. Although the original plans had called for three Spruce Gooses, no other versions of the planes were ever produced. The prototype remained in Hughes’s control for the remainder of his life, sitting in a hangar that was air-conditioned to provide the proper humidity to preserve the wood.
The craft was kept airworthy and the engines were fired up every month. The Spruce Goose was painted white. Hughes continued to make improvements, such as installing more powerful engines. When flooding damaged the Spruce Goose, Hughes built a larger hangar. While the
United States government spent some $22 million on the project, Hughes spent an estimated $7 to $18 million dollars of his own money to complete and maintain the Spruce Goose.
The Spruce Goose as an Exhibit
Four years after Hughes’ death in 1976, rumors circulated that the airplane was going to be disassembled so that pieces could be given to museums around the country. There were public protests, and the United States House of Representatives voted to declare the Spruce Goose a national treasure. Finally, the airplane was moved to another section of Long Beach, where it was put on display next to the ocean liner Queen Mary. Its new home was the world’s largest geodesic dome, some 400 feet in diameter.
In 1988, the owner of the Spruce Goose, the Aero Club of Southern California/Aero Exhibits, sold the aircraft to the Evergreen Aviation Museum of the Evergreen Aviation Company in McMinnville, Oregon. A large section of the geodesic dome was removed to allow the Spruce Goose to be disassembled and placed on an ocean barge for its long journey.
