Hughes HK-1 Hercules “Spruce Goose”


Howard Hughes’s famous HK-1 Hercules (#NX37602) is a cargo-type flying boat designed to transport men and materials over long distances. Originally conceived by Henry J. Kaiser, a steelmaker and builder of Liberty ships, the aircraft was designed and constructed by Hughes and his staff. The original proposal for the enormous, 400,000-pound wooden flying boat, with its spectacular 320-foot wingspan, came from the U.S. government in 1942. The goal was to build a cargo and troop carrier that did not require critical wartime materials; in other words, that substituted wood for metal. Throughout its construction, considerable controversy surrounded its funding. After a disgruntled U.S. Senator dubbed the HK-1 a “flying lumberyard,” the “Spruce Goose” nickname was coined Hughes despised the name!

The huge flying boat consists of a single hull, eight radial engines, a single vertical tail, fixed wingtip floats, and full cantilever wing and tail surfaces. The entire airframe and surface structures are composed of laminated wood (primarily birch, not spruce). All primary control surfaces, except the flaps, are fabric covered. The aircraft’s hull is divided into two areas: a flight deck for the operating crew and a large cargo hold. Access between the two compartments is provided by a circular stairway. Below the cargo hold are fuel bays divided by watertight bulkheads.

The Hughes/Kaiser flying boat was to be the largest airplane ever built (in fact, after its completion, it was three times larger than the largest aircraft built before it) and probably the most prodigious aviation project of all time. Only the courage and solitary dedication of Howard Hughes and his small development group caused this project to advance resulting in its historic first, and only flight. As Hughes kept meddling in the design, making things more complicated and causing lost time, Kaiser backed out. An urgent government project in 1942, the wooden flying boat had lost all priority by 1944. Hughes’s motives for making things so complicated are not clear. They seem to have resolved around his idea for an “aerial freighter beyond anything Jules Verne could have imagined.”

Although he thought big, Hughes paid careful attention to the smallest details. A true eccentric, he would sit for hours in the cockpit of his great aircraft deliberating the design of the control and instrument panel. Unfortunately, as a perfectionist, he could not make up his mind, and his many delays finally caused a Senate committee to look into the project. By the time the aircraft was finally completed in 1947, the U.S. government had spent a whopping $22 million — Hughes spent $18 million of his own money!

On 2 November 1947, Howard Hughes and a small engineering crew fired up the eight radial engines for taxi tests and thrilled thousands of on-lookers with an unannounced flight. With Hughes at the controls,¬†the flying boat lifted¬†33 feet off the surface of Los Angeles Harbor and flew one mile in less than a minute at a top speed of 80 miles per hour before making a perfect landing. This trial run was simple vindication from the detractors of the program and it is now looked upon as a great moment in aviation history. Was the flight an accident? When the chief designer asked Hughes whether he meant to lift it off the water, Hughes replied, “You’ll never know.” It is possible he flew it that one time just to prove that something that big could actually fly.

After the historic event, the “Spruce Goose” was returned to its large, specially designed hangar, never to fly again. As ordered by Hughes, it was constantly maintained and kept in flight-ready condition, including monthly runups of its engines, until he died in 1976. Over the past 50 years, the aircraft has endured to become a popular cultural artifact telling a remarkable story of sacrifice, determination, and technological development. The HK-1 was decades ahead of its time in the 1940s. It revolutionized jumbo flying bodies and large lift capability, shaping modern flight. The popular wooden flying boat is now appropriately regarded as a true American icon.

After a long period of storage, the HK-1 was presented to the Aero Club of Southern California and was for many years the star attraction while preserved in its own circular building, next to the former ocean liner Queen Mary, at Long Beach, California. The aircraft was moved in 1992 and now resides in a newly constructed facility at the Evergreen Aviation Educational Center in McMinnville, Oregon. To this day, it is still the largest aircraft, in terms of wingspan, ever to fly.

Note: The HK-1 designation was derived from the names Hughes and Kaiser. When Kaiser backed out of the project in 1944, Hughes redesignated the aircraft H-4 and, after the first flight, changed the tail number from NX37602 to N37602.


Official DesignationHK-1 (H-4) Hercules Flying Boat
Unofficial NicknamesThe Spruce Goose, Flying Lumberyard
Primary RoleMilitary transport
National OriginUSA
Original ContractorHughes Aircraft Company
Wingspan320 feet
Wing Area11,430 square feet
Length218 feet, 6 inches
Height at Tail80 feet
Tailspan113 feet, 6 inches
Height of Fuselage30 feet, 6 inches
Width of Fuselage24 feet, 5 inches
Volume of Cargo Hold165,000 cubic feet
EnginesEight Pratt & Whitney R-4360 Wasp Major radials (largest radial reciprocating engines ever built)
Horsepower3,000 shp each
PropellersEight four-bladed Hamilton Standards with a diameter of 17 feet, 2 inches (the four inboard propellers have reverse pitch capability)
Cruise Speed (est.)175 mph
Max Speed (est.)218 mph
Range (est.)3,000 miles (flew one mile during first and only flight)
Service Ceiling (est.)20,900 feet
Gross Weight400,000 pounds
Max Payload130,000 pounds
Total Cost$40 million
First Flight2 November 1947
Total ProducedOne aircraft