Kellett XR-8

By the start of World War II, the Kellett Aircraft Corporation had established itself as a leading developer of rotorcraft for the U.S. military. The company's autogiros were the first accepted by the U. S. Army Air Forces (AAF) in 1943 for development service trials. W. Wallace Kellett, company president, hoped to develop the autogiro into an effective military observation platform but these efforts were scotched by Igor Sikorsky's VS-300 helicopter. Sikorsky used the VS-300 to demonstrate performance, safety, and stability characteristics well beyond those of the autogiro. At the end of December 1943, the AAF denounced the autogiro and officially backed the helicopter. Kellett foresaw the autogiro's fate and had already applied the rotorcraft engineering principles used to develop them to the design and construction of helicopters. Kellett was inspired by the work of the German engineer, Anton Flettner, who had designed several successful intermeshing, counter-rotating, two-blade, twin-rotor helicopters. Flettner's Fl 265 helicopter first flew in May 1939. An improved model, the Fl 282, saw limited production and operational service throughout the latter half of World War II with the German navy and air force. Someone at Kellett coined the snappy moniker 'synchropter' to describe this type of intermeshing rotor configuration.

Kellett submitted a proposal describing the characteristics of a successful synchropter-type helicopter on November 11, 1942. The Experimental Engineering Section at Wright Field, Dayton, Ohio, had originally discounted the synchropter design on a theoretical basis, but model testing showed it might be more efficient than Sikorsky's single rotor layout. The single tail rotor on a Sikorsky robbed the main rotor of horsepower but the synchropter needed no tail rotor. On January 7, 1943, the AAF approved Kellett's proposal and on September 11, the service issued a contract to Kellett worth nearly $1,000,000. The contract specified a synchropter twin-rotor design that would not suffer the vibration and weight penalties of similar helicopter with laterally spaced main rotors. This was a veiled reference to the first helicopter the AAF tested, the Platt-LePage XR-1. That machine was then mired in testing difficulties and later crashed. The contract specified that the Kellett Aircraft Corporation was to construct two demonstration aircraft in addition to developing a two-bladed rotor system as a possible alternative to the intended three-bladed design.

The helicopter that Kellett designed was designated the XR-8 by the AAF. It looked like a giant egg with two rotors perched on top. When the rotors turned, they 'meshed' with a whisking action that quickly produced the nickname 'egg-beater.' Soon the term applied to any helicopter. The steel-tube, frame fuselage on the XR-8 was skinned with sheet steel and fabric and a large, bubble-shaped, Plexiglas canopy covered the nose. Each rotor spanned 10.9 m (36 ft) and turned three fabric-covered blades made of metal and wood. The rotors were mounted side-by-side and 1.2 m (4 ft) apart so that both rotors swept an area 12.2 m (40 ft) wide. Both rotor shafts leaned away from each other at a 12.5-degree angle from straight up. This arrangement was a critical one for it allowed the rotors to spin without touching. The blades themselves consisted of plywood ribs fixed to a steel tube spar and covered with a thin-veneer plywood skin. A single engine powered both rotors through a transmission, using spiral-bevel gears. A crew of two sat side-by-side in the cockpit. Like most early helicopters, a single collective lever was mounted between the seats. The egg-shaped fuselage tapered aft to a fixed vertical stabilizer at the tail. The aircraft rested on a fixed, tricycle landing gear mounted on vertical oleo struts to soften landings and takeoffs.

On August 7, 1944, the XR-8 flew for the first time with Dave Driscoll, Kellett's chief pilot, at the controls. It became immediately apparent that directional control was lacking. Kellett fitted vertical stabilizers to each side of the tail and he redesigned the rotors to tilt fore and aft differentially for greater yaw control. These efforts greatly improved control but a much more serious problem soon emerged. On September 7, 1944, while maneuvering during a test flight, the collective control column nearly jerked out of the pilot' hand. Post-flight inspection showed that two of the intermeshing rotor blades had touched when one blade flapped against the other. Kellett designed the blades to flap, to accommodate the stresses of flight, but they were never designed to make contact. They whirled in opposing directions at a combined speed of several hundred miles per hour. This incident caused considerable concern at the Engineering Division of the Air Materiel Command and Kellett was ordered to design a rigid rotor system. Kellett received a contract for this work on May 10, 1945. The company continued to test the XR-8 and the helicopter was soon flying at speeds approaching 161 kph (100 mph). However, fearing catastrophic blade contact, maneuvering was severely limited. Kellett attempted another fix by installing a two-bladed, twin rotor system in one of the two XR-8s built. This version was designated the XR-8A and it flew in March of 1945 but tests uncovered severe vibration that would require a massive reengineering effort to solve. The two-blade rotor solution was abandoned.

A rigid rotor system proposed for installation on the XR-8B proved untenable because it required significant reengineering so the aircraft continued flying with the flapping rotor system. The AAF accepted the aircraft for service trials on January 23, 1946. However, insoluble engineering difficulties and Kellett's ongoing financial situation forced the government to terminate contract at the end of 1946. Helicopter technology was progressing rapidly and Kellett realized that an entirely new, more practical aircraft was needed if the company hoped to compete in the burgeoning helicopter market. The successful introduction of Sikorsky helicopters into military service generated official interest in a new, large utility helicopter. An AAF Technical Instruction issued August 31, 1944, called for the development of "a large helicopter capable of carrying wounded personnel, passengers, and cargo within the fuselage." Kellett won this contract on October 16, 1944, beating out Sikorsky, Bell, and Platt-LePage. Clearly, the AAF did not see the XR-8's teething problems as insurmountable at that time. Kellet's large new helicopter was designated the XR-10. The XR-8 continued to fly to aid in developing the XR-10.

The XR-10 strongly resembled the XR-8 in layout but Kellett hoped to incorporate all the lessons learned from the first synchropter. When it flew on April 24, 1947, the XR-10 was the largest rotorcraft in the United States. It could haul six stretchers, ten troops laden with combat equipment, or 1,612 kg (3,550 lbs) of cargo. Kellett foresaw a civilian version that he designated the KH-2. The XR-10 could fly to almost 4,560 m (15,000 ft), more than twice as high as XR-8. The new helicopter also cruised 24 kph (15 mph) faster. Kellett seemed poised to win a hefty production contract when tragedy struck. A blade on each rotor touched during a test autorotation. Fixes were hastily applied but during a subsequent test on October 3, 1949, the XR-10 crashed when the control system failed, killing Dave Driskill.

Kellett never recovered from the dire financial straits caused by the crash and subsequent loss of business. Both of Kellett's synchropters ultimately failed but they did prove the viability of the synchropter configuration pioneered by Anton Flettner. Charles Kaman successfully exploited synchropter-engineering concepts pioneered by Kellett and Flettner and developed his own unique series of helicopters. Today, Kaman is still manufacturing rotorcraft of the syncropter configuration.

The AAF donated the XR-8 that is now preserved in the collection of the National Air Space Museum, along with other early helicopters such as the XR-4. This XR-8, AAF serial number 43-44714, was the first of two constructed and flown. Although a technical failure, it showed military rotorcraft planners a direction not to follow in the development of bigger and better helicopters. In the museum setting, the XR-8 provides a compelling contrast to more successful designs.

Rotor Diameters: 10.9 m (36 ft)
Length: 6.9 m (22 ft 7 in)
Height: 3.4 m (11 ft)
Weights: Empty, 1,052 kg (2,320 lb)
Gross, 1,349 kg (2,975 lb)
Engine: Franklin O-405-9 air-cooled in-line, 245 horsepower

References and Further Reading:
Air Material Command Rotary Wing Branch. "Summary of Rotary Wing Projects of Aircraft Projects Section." 1947.
Labermont, Paul. "Helicopters and Autogyros of the World." New York: Philisophical Library, Inc., 1959.
XR-8 curatorial file, Aeronautics Division, National Air and Space Museum.

Roger Connor, Russell Lee, 7-21-00

Transferred from the United States Air Force.

Date: 1943-1947

Country of Origin: United States of America

Dimensions:
Rotor Diameters: 10.9 m (36 ft)
Length: 6.9 m (22 ft 7 in)
Height: 3.4 m (11 ft)
Weights: Empty, 1,052 kg (2,320 lb)
Gross, 1,349 kg (2,975 lb)

By the start of World War II, the Kellett Aircraft Corporation had established itself as a leading developer of rotorcraft for the U.S. military. The company's autogiros were the first accepted by the U. S. Army Air Forces (AAF) in 1943 for development service trials. W. Wallace Kellett, company president, hoped to develop the autogiro into an effective military observation platform but these efforts were scotched by Igor Sikorsky's VS-300 helicopter. Sikorsky used the VS-300 to demonstrate performance, safety, and stability characteristics well beyond those of the autogiro. At the end of December 1943, the AAF denounced the autogiro and officially backed the helicopter. Kellett foresaw the autogiro's fate and had already applied the rotorcraft engineering principles used to develop them to the design and construction of helicopters. Kellett was inspired by the work of the German engineer, Anton Flettner, who had designed several successful intermeshing, counter-rotating, two-blade, twin-rotor helicopters. Flettner's Fl 265 helicopter first flew in May 1939. An improved model, the Fl 282, saw limited production and operational service throughout the latter half of World War II with the German navy and air force. Someone at Kellett coined the snappy moniker 'synchropter' to describe this type of intermeshing rotor configuration.

Kellett submitted a proposal describing the characteristics of a successful synchropter-type helicopter on November 11, 1942. The Experimental Engineering Section at Wright Field, Dayton, Ohio, had originally discounted the synchropter design on a theoretical basis, but model testing showed it might be more efficient than Sikorsky's single rotor layout. The single tail rotor on a Sikorsky robbed the main rotor of horsepower but the synchropter needed no tail rotor. On January 7, 1943, the AAF approved Kellett's proposal and on September 11, the service issued a contract to Kellett worth nearly $1,000,000. The contract specified a synchropter twin-rotor design that would not suffer the vibration and weight penalties of similar helicopter with laterally spaced main rotors. This was a veiled reference to the first helicopter the AAF tested, the Platt-LePage XR-1. That machine was then mired in testing difficulties and later crashed. The contract specified that the Kellett Aircraft Corporation was to construct two demonstration aircraft in addition to developing a two-bladed rotor system as a possible alternative to the intended three-bladed design.

The helicopter that Kellett designed was designated the XR-8 by the AAF. It looked like a giant egg with two rotors perched on top. When the rotors turned, they 'meshed' with a whisking action that quickly produced the nickname 'egg-beater.' Soon the term applied to any helicopter. The steel-tube, frame fuselage on the XR-8 was skinned with sheet steel and fabric and a large, bubble-shaped, Plexiglas canopy covered the nose. Each rotor spanned 10.9 m (36 ft) and turned three fabric-covered blades made of metal and wood. The rotors were mounted side-by-side and 1.2 m (4 ft) apart so that both rotors swept an area 12.2 m (40 ft) wide. Both rotor shafts leaned away from each other at a 12.5-degree angle from straight up. This arrangement was a critical one for it allowed the rotors to spin without touching. The blades themselves consisted of plywood ribs fixed to a steel tube spar and covered with a thin-veneer plywood skin. A single engine powered both rotors through a transmission, using spiral-bevel gears. A crew of two sat side-by-side in the cockpit. Like most early helicopters, a single collective lever was mounted between the seats. The egg-shaped fuselage tapered aft to a fixed vertical stabilizer at the tail. The aircraft rested on a fixed, tricycle landing gear mounted on vertical oleo struts to soften landings and takeoffs.

On August 7, 1944, the XR-8 flew for the first time with Dave Driscoll, Kellett's chief pilot, at the controls. It became immediately apparent that directional control was lacking. Kellett fitted vertical stabilizers to each side of the tail and he redesigned the rotors to tilt fore and aft differentially for greater yaw control. These efforts greatly improved control but a much more serious problem soon emerged. On September 7, 1944, while maneuvering during a test flight, the collective control column nearly jerked out of the pilot' hand. Post-flight inspection showed that two of the intermeshing rotor blades had touched when one blade flapped against the other. Kellett designed the blades to flap, to accommodate the stresses of flight, but they were never designed to make contact. They whirled in opposing directions at a combined speed of several hundred miles per hour. This incident caused considerable concern at the Engineering Division of the Air Materiel Command and Kellett was ordered to design a rigid rotor system. Kellett received a contract for this work on May 10, 1945. The company continued to test the XR-8 and the helicopter was soon flying at speeds approaching 161 kph (100 mph). However, fearing catastrophic blade contact, maneuvering was severely limited. Kellett attempted another fix by installing a two-bladed, twin rotor system in one of the two XR-8s built. This version was designated the XR-8A and it flew in March of 1945 but tests uncovered severe vibration that would require a massive reengineering effort to solve. The two-blade rotor solution was abandoned.

A rigid rotor system proposed for installation on the XR-8B proved untenable because it required significant reengineering so the aircraft continued flying with the flapping rotor system. The AAF accepted the aircraft for service trials on January 23, 1946. However, insoluble engineering difficulties and Kellett's ongoing financial situation forced the government to terminate contract at the end of 1946. Helicopter technology was progressing rapidly and Kellett realized that an entirely new, more practical aircraft was needed if the company hoped to compete in the burgeoning helicopter market. The successful introduction of Sikorsky helicopters into military service generated official interest in a new, large utility helicopter. An AAF Technical Instruction issued August 31, 1944, called for the development of "a large helicopter capable of carrying wounded personnel, passengers, and cargo within the fuselage." Kellett won this contract on October 16, 1944, beating out Sikorsky, Bell, and Platt-LePage. Clearly, the AAF did not see the XR-8's teething problems as insurmountable at that time. Kellet's large new helicopter was designated the XR-10. The XR-8 continued to fly to aid in developing the XR-10.

The XR-10 strongly resembled the XR-8 in layout but Kellett hoped to incorporate all the lessons learned from the first synchropter. When it flew on April 24, 1947, the XR-10 was the largest rotorcraft in the United States. It could haul six stretchers, ten troops laden with combat equipment, or 1,612 kg (3,550 lbs) of cargo. Kellett foresaw a civilian version that he designated the KH-2. The XR-10 could fly to almost 4,560 m (15,000 ft), more than twice as high as XR-8. The new helicopter also cruised 24 kph (15 mph) faster. Kellett seemed poised to win a hefty production contract when tragedy struck. A blade on each rotor touched during a test autorotation. Fixes were hastily applied but during a subsequent test on October 3, 1949, the XR-10 crashed when the control system failed, killing Dave Driskill.

Kellett never recovered from the dire financial straits caused by the crash and subsequent loss of business. Both of Kellett's synchropters ultimately failed but they did prove the viability of the synchropter configuration pioneered by Anton Flettner. Charles Kaman successfully exploited synchropter-engineering concepts pioneered by Kellett and Flettner and developed his own unique series of helicopters. Today, Kaman is still manufacturing rotorcraft of the syncropter configuration.

The AAF donated the XR-8 that is now preserved in the collection of the National Air Space Museum, along with other early helicopters such as the XR-4. This XR-8, AAF serial number 43-44714, was the first of two constructed and flown. Although a technical failure, it showed military rotorcraft planners a direction not to follow in the development of bigger and better helicopters. In the museum setting, the XR-8 provides a compelling contrast to more successful designs.

Rotor Diameters: 10.9 m (36 ft)
Length: 6.9 m (22 ft 7 in)
Height: 3.4 m (11 ft)
Weights: Empty, 1,052 kg (2,320 lb)
Gross, 1,349 kg (2,975 lb)
Engine: Franklin O-405-9 air-cooled in-line, 245 horsepower

References and Further Reading:
Air Material Command Rotary Wing Branch. "Summary of Rotary Wing Projects of Aircraft Projects Section." 1947.
Labermont, Paul. "Helicopters and Autogyros of the World." New York: Philisophical Library, Inc., 1959.
XR-8 curatorial file, Aeronautics Division, National Air and Space Museum.

Roger Connor, Russell Lee, 7-21-00