Kellett XO-60

Display Status:

This object is on display in the Vertical Flight at the Steven F. Udvar-Hazy Center in Chantilly, VA.

Collection Item Summary:

The XO-60 represents the final evolution of the Autogiro before the type became obsolete. In 1943, Kellett produced one XO-60 and six YO-60s for Army Air Force evaluation. The slow loitering speeds and almost vertical takeoff and landing abilities of the XO-60 made it ideal for military observation duties. However, the arrival of the Sikorsky R-4 helicopter, an inability to hover, limited payload, and ground resonance problems kept it from production.

Kellett equipped the XO-60 with a jump takeoff clutch that allowed the engine to spin up the rotor to takeoff rpm. The pilot then set collective pitch and caused the aircraft to lift nearly vertically, while the clutch switched power to the tractor propeller to pull the aircraft forward in autorotation. However, jump takeoffs and near vertical landings were difficult maneuvers and resulted in a number of accidents.

Collection Item Long Description:

Kellett XO-60

The XO-60 represents the finale of the U.S. military's unsuccessful association with Cierva-licensed gyroplanes, known as Autogiros, during the 1930s and '40s. By the beginning of World War Two, the practical helicopter had begun to appear and the Autogiro, its rotary-wing antecedent, faded into oblivion. In the United States, the two manufacturers of Autogiros, Pitcairn and Kellett, both embattled survivors of the Great Depression, continued to seek government contracts to sustain their businesses. Both remained obstinate in their belief that the Autogiro, a proven technology, would be of far more use to the military in the forthcoming conflict than the largely experimental and delicate helicopters then under development. However, when the U.S. Army Air Force took delivery of the Kellett XO-60 in 1944, it was to be the last of the Autogiros produced in the United States.

The technology of the Autogiro had made significant advances during the late 1930s. In 1923, Juan de la Cierva developed and flew the first Autogiro. Aircraft of this class, now known as gyroplanes, utilize a rotor to provide the majority of lift. However, unlike a helicopter, an engine does not drive the gyroplane's rotor. Instead, the rotor acts like a windmill and the forward motion of the aircraft provides sufficient airflow to allow the blades to spin in autorotation and generate lift. Autorotation occurs when the center of pressure of the airfoil acts forward of its axis of rotation. A conventional tractor or pusher engine with propeller provides the necessary thrust to maintain the forward motion required for autorotation. The forward speed required by a gyroplane to maintain level flight is usually much lower than that required to keep an airplane from stalling, thus they can takeoff in very short distances and fly at very slow speeds. A trained pilot could make near vertical landings in an Autogiro, even in calm wind conditions.

The autogiros of the 1920s and early '30s relied on airplane control surfaces, and stubby wings to provide control and stability. The rotor was completely freewheeling and the blades could not change pitch. By the early 1930s, Pitcairn, Kellett, and Cierva began to develop a means of replacing the stubby airplane wings and control surfaces with a lighter, more efficient control system, using the rotor itself. This method, known as direct control, consisted of a mechanism that tilted the entire rotor head in the desired direction - an option that was impractical for helicopters. A further advancement was the development of the "jump" takeoff. A clutch mechanism allowed the engine driveshaft to bring the rotor to over 150% of cruise flight rpm while the aircraft sat on the ground. Then, simultaneously, the pilot used the clutch to switch power to the driving propeller while moving another lever to increase the pitch of the rotors collectively to about nine degrees. This caused the Autogiro to "jump" vertically up to heights of 4.6 meters (15 feet) by which time the propeller had begun to pull the aircraft forward with sufficient speed to guarantee autorotation. The pilot then altered the pitch of the rotor to an in-flight blade pitch of three degrees. Using this procedure, the Autogiro could clear a 15-meter (50-foot) obstacle from a dead stop in only 76 meters (250 feet) of horizontal travel - a feat unmatched by any airplane of the time.

The ability to now takeoff as well as land almost vertically seemed to make the concept of the vastly more complex helicopter redundant. As the rotor remained unpowered in flight, it did not require a tail rotor or other torque-compensation-mechanism like a helicopter. However, by 1940, the armed forces in Germany and the United States had begun to appreciate the potential value of a hovering capability in rescue, liaison, troop insertion, and anti-submarine warfare and hovering was something that an Autogiro could still not do.

In 1937, the Pitcairn Autogiro Company and the Kellett Autogiro Corporation, both based near Philadelphia, pushed their Congressman, Frank Dorsey into sponsoring a Bill that would provide Government funding to the struggling Autogiro industry "in the interest of National Defense." The resulting Dorsey-Logan Act passed with a $2,000,000 appropriation. However, the advisors to the committee reviewing the Bill, including noted engineer Alexander Klemin, recommended that the scope of authorization widen to include other rotary wing aircraft. Laurence LePage, a hopeful helicopter entrepreneur, had influenced Klemin. LePage had shown to the Army a film of the Focke-Wulf Fw 61 helicopter, then a Nazi propaganda star, hoping that fears over a German head start in helicopter development would result in a contract for which he could compete. LePage's efforts paid off and the committee expunged the word "Autogiro" from the bill and replaced it with "rotary wing and other aircraft." Nonetheless, both Pitcairn and Kellett pursued military Autogiro designs in an attempt to secure some, or all, of the Congressional appropriation.

Pitcairn was the first of the two American autogiro manufacturers to go after the Dorsey-Logan Act appropriation under a type-specification issued on August 25, 1939, for a rotary-wing military aircraft capable of clearing vertically a 15-meter (50-foot) obstacle. In May 1940, Pitcairn's entrant, the PA-38 two-man observation Autogiro, lost out to Laurence LePage's XR-1 helicopter (see NASM collection), designed with Havilland Platt. Although the XR-1 proved to be an unsuccessful aircraft, it was nevertheless the start of the Army's close association with the helicopter.

An enthusiastic young Major H. Franklin Gregory ran the rotary wing program of the Army Air Corps. He had become involved with the Autogiro during the Army's first association with the type. In early 1936, the National Advisory Committee for Aeronautics (NACA) requested Army pilots to fly two Autogiros (a Kellett YG-1 and YG-2) for a research program. The Air Corps tapped Gregory, then a fixed-wing observation pilot, for the duty. While flying for the NACA, he realized that the slow speeds of the aircraft and their short takeoff and landing capability (STOL) made the Autogiro ideally suited to artillery spotting. Gregory quickly talked his superiors into sponsoring field tests with Autogiros in the observation role. Assigned to lead the project, Gregory and his pilots achieved mixed results. One problem was that, like many inexperienced Autogiro pilots, he attempted to land as he would in an airplane - a technique that could (and did) lead to a destructive "ground resonance" oscillation in the rotor. Gregory concluded that a successful observation aircraft had to carry two people and a "tactical" load and have the capability to make vertical takeoffs. In the meantime, the Army proceeded with the organization of a rotary-wing school to train Autogiro pilots.

Once it received word of the Fw 61's successful helicopter flights in Germany, the Army Air Corps decided to retain its Autogiro school in anticipation that rotorcraft, either Autogiros or helicopters, would play a role in the conflict that seemed increasingly likely. Nonetheless, as head of the Army's fledgling rotary-wing program, Gregory became convinced that only the helicopter could satisfy the needs of the service on the battlefield. The Army's involvement with the Autogiro would probably have ended there had it not been for a demonstration of the Pitcairn PA-36's jump-takeoff capability, witnessed by members of the Air-Ground Procedures Board on April 18, 1941. On August 2, 1941, the Board secured proposals from Pitcairn and Kellett to provide direct-control jump-takeoff Autogiros for evaluation in the liaison and observation roles. The contract stipulated that Kellett would deliver one XO-60 and six YO-60 prototypes and that Pitcairn would deliver one XO-61 and five YO-61s.

In spite of Kellett's previous experience producing direct-control Autogiros, the O-60 program proceeded slowly under the direction of Richard Prewitt. The design was a direct development of Kellett's KD 1-A, which Gregory had tested extensively under the YG-1B designation. The primary differences were the addition of a jump-takeoff system and a new blade dampening system. The fuselage consisted of chromoly steel tubing with a fabric covering, while the rotor blades consisted of chromoly steel spars with a laminated plywood airfoil. The much larger tandem, fully enclosed cockpit featured a rear swivel-seat for the observer so he could turn rearward and chart positions on a large map shelf. The O-60's bubble canopy offered excellent downward visibility, as did Plexiglas panels in the floor. One advantage of the O-60 over other observation aircraft was that the pilot could easily fold the rotors back upon landing and taxi the aircraft along roads to a dispersal site - a concept that field-testing validated. Kellett's chief test pilot, J. David Driskill, put each YO-60 and the one XO-60 through their paces before delivery to the Army Air Force.

The only material difference between the "X" and "Y" models was the landing gear. The simpler XO-60 relied on a rigid gear arrangement, while the YO-60s relied on a high-travel strut arrangement. Normally the "X" series experimental model precedes the "Y" series service test model, but in the case of the O-60 program, the reverse was true. The USAAF accepted the first YO-60s in August 1943 and the XO-60 in February 1944.

The Army Air Force selected the 445th Test Squadron of the 50th Fighter Group, based in Orlando, Florida, to evaluate three of the YO-60s. Testing began in earnest, but the YO-60s proved to be dangerously difficult for fighter pilots to handle without prior Autogiro experience. The jump takeoff procedure was at the margins of the aircraft's performance envelope, and loss of control during these phases of flight caused several accidents. The control stick oscillated heavily if rotor rpm dropped below 200 rpm, so a lock held it in place until the clutch spun the rotor up past that point. However, even then, the pilot had to hold the stick firmly forward and to the right to counteract engine torque and the gyroscopic forces of the rotor. The additional cockpit controls included a clutch handle and a rotor pitch lever that substantially increased workload during the demanding jump takeoff procedure. Because of the conventional landing gear configuration, the rotor had a substantial backward tilt during takeoff. In a rolling takeoff, this was an asset because it aided autorotation. However, in a jump takeoff, the rotor actually lifted the O-60 slightly backwards, which meant that the propeller had to work that much harder to pull the aircraft forward in autorotation and increasing the difficulty of the maneuver even further.

Landings also proved problematic in the O-60s. Pilots had to exercise extreme caution when performing near vertical descents, lest they allow the rotor rpm to slow and cause a rotor stall. The landing flare out was excessively severe and the aircraft landed tail first. The sympathetic vibrations caused by the aircraft rolling out over uneven surfaces could cause a rapidly divergent oscillation in the rotor blades known as ground resonance. This condition could (and did) result in the rapid disintegration of the rotor system. Accidents quickly claimed several of the YO-60s. The XO-60 went to Wright Field for testing and subsequently to Bowman Field in Louisville, Kentucky, where it suffered significant damage during a landing accident of its own on October 13, 1944.

The Army's Artillery Board evaluated the YO-60 at Camp Bragg, North Carolina, and the Infantry Board did the same at Camp Rucker, Alabama. The subsequent lead O-60 pilot, E. Stuart Gregg, then underwent helicopter training on the Sikorsky R-4 (see NASM collection). Based on his observations during the evaluations, he noted that the O-60 had some slight performance advantages over the Army's other light planes, but that its mechanical complexity, high cost, and difficult handling characteristics erased any positive aspects. He also noted that the R-4 helicopter, by then already obsolete, offered significant advantages over any Autogiro.

The pusher-engine Pitcairn XO-61 fared even worse than the O-60 series. Beset by cooling and mechanical problems, as well as stability issues, the first aircraft did not fly until April 7, 1943. The onset of ground resonance during a test several weeks later destroyed the airframe and by the end of the year, the USAAF had cancelled the contract.

By the time the USAAF accepted the YO-60s, both the military and Kellett realized that the Autogiro's prospects as a military aircraft were finished. Not only were the Sikorsky helicopters more capable, but the company had produced and delivered the aircraft in a remarkably short time, given the difficulties in manufacturing an entirely new class of aircraft. Within months after the YO-60s went to the USAAF, a Sikorsky YR-4B was performing the first medical evacuation flights from the frontlines by helicopter in the steaming jungles of Burma. Kellett was aware of the trend and began work on a twin intermeshing rotor "synchropter" helicopter, even before the delivery of the first YO-60. This XR-8 (see NASM collection) and its larger descendent, the XR-10, seemed to guarantee the company's survival, but technical problems with these aircraft proved insurmountable and the company teetered on bankruptcy and the end of the decade.

The YO-60s suffered an ignominious end, with most of them meeting their demise during accidents. One replaced an earlier Kellett model used for border patrol in New Mexico. The pilot quickly destroyed the aircraft, as he had no experience in jump takeoffs and other high-performance Autogiro maneuvers. The Army Air Force rebuilt the XO-60 after its crash, and delivered it to Freeman Field, Indiana, though by that time the Army Air Force had already decided it was a museum piece and it did not fly again. General H.H. Arnold set it aside, with only 18.6 hours of logged flight time, for the National Air Museum collection, along with numerous other experimental aircraft. The National Air and Space Museum fully restored the XO-60 in 1975. The XO-60 was the last of the Autogiros built in the United States. At the Smithsonian Institution, the XO-60 has joined the first Autogiro to fly in the United States, the Cierva C.8W, and the oldest surviving American-built Autogiro, the Pitcairn PCA-1A.

The gyroplane began to reappear in the 1950s as a sport aircraft for enthusiasts who are looking for small, lightweight aircraft. In the late 1990s, several companies developed cabin-class gyroplanes with potential military application as low-cost surveillance aircraft for internal security duties similar to the duties intended for the O-60. Although, the O-60 and the other Autogiros of the 1920s and '30s failed to have a substantial impact in the aviation industry, these aircraft played a pivotal role in the development of other rotary wing aircraft.

Ironically, the practical helicopter would have appeared much later without the Autogiro. The flapping hinge, invented by Cierva to overcome the lift differential between the advancing and retreating blades on his Autogiros, was equally essential for the helicopter. Additionally, rotor blade design advanced considerably with experience gained on the XO-60 and its predecessors.

Rotor Diameter:12.80 m (42 ft)

Length:6.61 m (21 ft 8 in)

Height:3.38 m (11 ft 1 in)

Weight:Empty, 889 kg (1,960 lb)

Gross, 1,198 kg (2,640 lb)

Engine:Jacobs R-915-3 radial, 330 horsepower

References and Further Reading:

Brooks, Peter W. Cierva Autogiros: The Development of Rotary-Wing Flight.

Washington: Smithsonian Institution Press, 1988.

Smith, Frank Kingston. Legacy of Wings: The Harold F. Pitcairn Story. Lafayette Hill,

Pa: T.D. Associates, 1981.

Townson, George. Autogiro: The Story of "the Windmill Plane." Destin, Fl: Aviation

Heritage, Inc., 1985.

Kellett XO-60 curatorial file, Aeronautics Division, National Air and Space Museum

R. D. Connor