One day at the Western Museum of Flight, I was touring a hangar of the museum housing a Harriet jump jet, gazing at memorabilia and desktop models of aircraft pertaining to the history of aerospace industry in southern California, when I happened to come upon a collection of artifacts and documents regarding vertical take-off aircraft built by the Doak Aircraft Company. One of those artifacts was a wind tunnel model marked with the label "Doak XF-1", and on the wall of the glass casing for Doak-related artifacts and memorabilia was an engineering drawing. I had remembered hearing the name Doak because it built one of several US Army VTOL research aircraft, but even though the Bell X-22 is the best-known tilt-duct fan aircraft, I didn't know until recently that the Doak company was based in Torrance, the very home of the Western Museum of Flight. Given that the location of the Western Museum of Flight was also the headquarters of Doak Aircraft, I thought it might now be a perfect idea to dedicate this post to telling the history of the Doak company and the VTOL aircraft designed and built by that manufacturer.
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| The Doak DRD-1 prototype primary trainer (civil registration NX25698) |
The Doak Aircraft Company was founded in Hermosa Beach, on October 7, 1940, by Edmund R. Doak, Jr., a self-taught engineer and vice president of Douglas Aircraft Company. The company later moved to a new factory based in Torrance in 1941, undertaking the construction of wooden parts and products for aircraft under a contract signed with the furniture firm Kroehler in 1942. Every major US aircraft manufacturer was subcontracted to build subassemblies for military aircraft, including molded plywood fuselages for the North American AT-6 Texan and Vultee BT-13 Valiant trainers, and doors, hatches, and gun turrets for a wide variety of combat aircraft. Throughout the course of US involvement in World War II Doak reached a peak employment of 4,000 workers (Lobb 2006). Doak also built a primary trainer aircraft of its own, the DRD-1, of which a single example (civil registration NX25698) was built and flown in 1941. The DRD-1 was powered by one Continental R-760 radial piston engine and featured laminated plywood construction, with a length of 26 feet (7.92 meters) and a wingspan of 35 feet (10.67 meters); despite being submitted for evaluation by the US Army Air Corps, the DRD-1 was not ordered into production or given a military designation.
Doak ducted-fan flight/wind tunnel test vehicles on display at the Western Museum of Flight in Torrance, California, from left to right: XF-1 wind tunnel model, Model 11 wind tunnel model, full-scale XF-1 tethered test vehicle. On the upper left is a three-view engineering drawing for what would later become the VZ-4.
Doak's real aviation interest, however, lay in vertical take-off and landing aircraft. Since 1935, he had been experimenting with ducted fans and similar air-moving concepts, and Cold War tensions in the late 1940s and early 1950s raised worries among the US military top brass that the Soviet Union and its Warsaw Pact allies in Central and Eastern Europe might carry out air attacks on unprepared airbases in West Germany, Italy, or France with combat aircraft or theater ballistic missiles. These concerns, along with recognition of the built-in speed limitations of helicopters thanks to the aerodynamics properties of the rotor blades, prompted Edmund Doak to submit a VTOL aircraft design to the US Army's Army Transportation and Research and Engineering Command at Fort Eustis in Newport News, Virginia, in 1950. He believed that his design could take off and land in a small area, hover and loiter over an enemy target, and fly backwards like a helicopter without the usual noise and vibrations of a helicopter, while incorporating the forward flight mode, high speed, weapons load, and combat mission flexibility of a fighter aircraft. Since conventional combat planes were susceptible to Soviet attacks on unprepared airbases with concrete runways, the US Army found Doak's idea attractive, and on April 10, 1956, Doak received an Army contract to build a VTOL research aircraft to test the ducted fan method of vertical take-off and landing. As a first step in testing his ideas for ducted-fan aircraft, Doak built a plethora of ducted-fan vehicles for flight and wind tunnel testing from 1950 to 1957, including the Model 11, Model 13, and XF-1.
The Doak VZ-4 (VZ=VTOL Research), known by the company designation Model 16, was an aircraft of all-metal construction, powered by a single Lycoming T53 turboprop engine buried in the fuselage that drove two wingtip-mounted tilting ducted fan propellers through a T-shaped box on the engine that transmitted power to the propellers via a 4 in (10.2 cm) aluminum tubular shaft and two smaller shafts. Each propeller had a diameter of 4 feet (1.22 meters) while the fan ducts each had a diameter of 5 feet (1.52 meters), swiveling upwards during vertical take-off and landing with a liftoff rotation speed of 4,800 revolutions per minute, and tilting forwards during forward flight. The wings and tail unit were of all-metal construction, and the cockpit had the pilot and observer seated in tandem, with the pilot using a standard stick and rudder to control the VZ-4 in flight. The VZ-4 was originally made of uncovered welded steel tubing, but later molded fiberglass was installed over the nose section and thin aluminum sheeting placed over the aft fuselage. To save time and money, some component were used during the construction of the VZ-4, including seats from a P-51 Mustang, landing gear from a Cessna 182, and the duct actuators from a Lockheed T-33 (trainer version of the P-80/F-80 Shooting Star jet fighter).
Top: The Doak VZ-4 performing one of its first hover flights, 1958. Note that the first two digits of the VZ-4's serial number are incorrectly reversed (the serial number on the vertical stabilizer should have read 56-6942, not 56-9642).
Bottom: Desktop models of the Doak Model 20 and Model 22 ducted-fan transport projects.
The VZ-4 (serial number 56-6942) was completed by the end of 1957. It carried out its first hover flight on February 25, 1958, and the transition from vertical to forward flight was conducted on May 5 of that year. Although initial flight tests were successful, the overall short take-off and landing performance was less satisfactory than was hoped for, as the VZ-4 itself tended to make a nose-up while transitioned to forward flight mode. After 18 hours of tethered hovering and 32 flight testing hours, in October 1958 the VZ-4 was transferred to Edwards Air Force Base for a new round of flight tests, and it underwent another 50 hours of test flights, attaining a maximum speed of 230 mph (370 km/h), a cruise speed of 175 mph (282 km/h), a range of 250 miles (403 km), a flight endurance of one hour, and a service ceiling of 12,000 feet (3,658 meters) with the turbines (Stevenson 2014, pp. 14-15). The US Army formally accepted the VZ-4 in September 1959, leading to it being called VZ-4DA (the code suffix DA denoting the Doak Aircraft Company), and the aircraft was subsequently passed on to the NASA Langley Research Center in Hampton, Virginia, on May 5, 1960 for additional testing. Even before the Army acquired the VZ-4, Doak proposed two ducted-fan transport designs in 1959, the Model 20 with two ducted-fan engines and a butterfly-shaped tail, and the larger Model 22 with four ducted fans, of which two were at the tips of the low-mounted wing near the crew cabin and the other two were situated at the tips of a high-mounted wing just ahead of the vertical stabilizer. By this time, however, pressure from Congress meant that the Army could only field helicopters and that only the Air Force could operate airplanes, and even though the VZ-4 was classified as a plane, the USAF showed little interest in developing an operation ducted-fan VTOL aircraft, so in late 1960, Doak Aircraft laid off 90 percent of its employees and went out of business in 1961. The VZ-4, meanwhile continued to serve with NASA until 1973, when it was retired and eventually donated to the US Army Transportation Museum in Fort Eustis, Virginia, where it remains on display today.
Top: Designs by Douglas for transport derivatives of the VZ-4 (1-ton payload design on left, 4-ton payload design on right)
Bottom left: Douglas D-828 four-engine tilt-duct transport project
Bottom right: Douglas D-850 four-engine tilt-duct research aircraft (which lost out to the Bell X-22)
After Doak Aircraft vanished into history's dustbin, Douglas acquired the patent rights and engineering files for Doak's VTOL designs, while hiring four former Doak engineers to work at its naval aircraft division based in El Segundo. In 1960 Douglas had proposed two transport derivatives of the Doak VZ-4 for submission to the US Army, one with a 2,000 lb (907 kg) payload and another with a payload of 8,000 lb (3,628 kg), but these designs were rejected by the Army. On January 27, 1961, the Department of Defense announced the Tri-Service Assault Transport Program for a large VTOL transport to be jointly operated by the Army, Air Force, and Navy, and Douglas responded in late 1961 with the D-828 ducted-fan transport, which was similar to the Doak Model 22 but had shoulder-mounted wings, with tilt-duct fans at the housing 8 feet (2.44 meter) diameter six-blade propellers, each powered by four General Electric T64 turboshafts. The wings spanned 48 feet 5 in (14.77 meters) when unfolded or 30 feet (9.15 meters) when folded, and the fuselage was 50 feet (15.25 meters) long. However, the D-828 was not proceeded with because US armed services disagreed on the best method of vertical for a large VTOL transport, with the Air Force and Army favoring the tiltwing and tiltrotor arrangement and the Navy preferring the tilt-duct system. (The D-829 tiltrotor proposal envisaged in parallel with the D-828 initially seemed acceptable to the Air Force and Navy, but was rejected by the Air Force along with a North American tiltwing proposal in favor of the Ling-Temco-Vought VHR-477 tiltwing design [which became the XC-142] and thus is not considered here as it was not a tilt-duct project.) After withdrawing from the Tri-Service VTOL transport program, in the spring of 1962 the Navy announced a competition for a tilt-duct technology demonstrator. In response, Douglas proposed the D-850, which featured the same tilt-duct engine layout and placement as the D-828 but had a cockpit similar to that of the Bell X-15 experimental tiltrotor and a shorter wingspan, while eliminating the mechanism for folding wings and tails. However, Bell's rival design to the D-850, known as the D-2127, won the Navy competition for the tilt-duct research aircraft context in 1964, eventually receiving the designation X-22 and making its first flight on March 17, 1966.
The Doak Aircraft Company remains an unsung aircraft manufacturer from southern California when it comes to the development of vertical take-off machines, being largely overshadowed by Hiller Aircraft of central California in terms of name recognition. However, Doak helped to put to the test the concept of a VTOL aircraft with tilting ducted fans by building the VZ-4 tilt-duct technology demonstrator, and even though the VZ-4 never led to an operational production machine of its design, not to mention that VTOL designs envisaged by the El Segundo division of Douglas derived from Doak's unbuilt tilt-duct transport projects either did not make the cut due to wrangling among the US armed forces over what form of VTOL was suitable for large VTOL transport or lost out to design proposals put out by other manufacturers, the volumes of aerodynamic data gathered by the VZ-4 helped influence development and testing of the Bell X-22 (as a matter of fact Bell has historically led the pack of American aircraft manufacturers in building and testing various types of VTOL aircraft besides helicopters, from vectored thrust jet aircraft to convertiplanes).
References:
Cox, G., and Kaston, C., 2020. American Secret Projects 3: U.S. Airlifters Since 1962. Manchester, UK: Crécy Publishing.
Harding, S., 1997. U.S. Army Aircraft Since 1947. Atglen, PA: Schiffer Publishing.
Lobb, C., 2006. Torrance Airport. Charleston, SC: Arcadia Publishing. ISBN 978-0-7385-4662-9.
Stevenson, R., 2014. "Doak's One-Off." Aviation History (July 2014): 14–15.
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