Item 0896

OTHER: Helicopter - Outside - Intermeshing - Kellett XR-8 & XR-10

Picture:

Information re Rotor Development:

"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. All rotor blades are designed to flap to accommodate the stresses of flight but the rotors 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."

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In Vertiflite, Fall 2005, page 54, Prouty presents a slightly different version of Kellett's demise.

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It should also be mentioned that M. Kellett died a year later (1951) and this would not have been a 'positive' for the company.

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"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."

"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."

My comment;~ This last sentence could be considered as the starting point for the development of the Dragonfly and the UniCopter.

 

Exerts from Article [Ambitious 'Eggbeater' ~ Kellett's XR-8 ~ Far Ahead of Its Time]; Air Enthusiast - May/June 2004:

Have hard copy.

Doomed to be Experimental:

"Unlike the smaller and less complex rival [Sikorsky's XR-4], the XR-8 was saddled with the twin rotor concept which, though promising more in the long term, was more difficult to perfect. The problem with colliding blades [three blades per rotor] soon became chronic, particularly in sudden maneuvers. .............."

Rotor Brake:

"The brake, which was operated by the same control as were the clutches, could only be applied when the control was in neutral or from the friction clutched position."

Rotor Torque:

"The horizontal stabilizer was retained by two bearings - on each side of the fuselage - and had an arm that was incorporated into the spar, which permitted adjustments to be made in order to counteract torque." My thought ~ This seems to imply that the craft must have forward velocity. Torque about Y-axis less in UniCopter

Yaw:

"The motion of the rudder pedals caused differential longitudinal tilt plus differential collective pitch change of the rotors to occur."

Directional Stability:

".... directional unstable in forward flight, due to the outboard rudder fins being buffeted by the rotor slipstream." "This resulted in an increase of the vertical fin area by 28%, but in turn caused excessive directional stability at high speed,..." My thought ~ Locate the VS aft of the rotor disks and OTHER: Flight Dynamics - Control - Twin Vertical Stabilizers

Blade - Blade Contact:

".... rotor blades' tendency to make contact during certain manouvers" My thought ~ Absolutely Rigid Rotor Concept (ARR)

Vibration:

".., a two-blade system was installed, replacing the original three-blade layout, but this caused excessive vibration." My thought ~ OK

Autorotation:

".., although control was poor in power-off flight, making the aircraft definitely unsafe in auto-rotation." My thought ~ Both Flettner and Kaman never mentioned problems with autorotation. Could the third blade make any difference???

 

Location of Specific Information:

This Web Site:

Use search engine, located on initial page, to find all locations in this web site.

Other Web Sites:

http://www.nasm.edu/nasm/aero/aircraft/kellett_xr8.htm

http://avia.russian.ee/vertigo/kellett_xr-8-r.html

http://avia.russian.ee/vertigo/kellett_xr-10-r.html

http://www.hiller.org/exhibits/copter-news-v3n8/images/kellett-xr-10.jpg

http://www.fly2mqi.com/xr8.asp The XR-8 Kellett Helicopter. John David Driskill, the test pilot who died, was the first person to be issued a civilian helicopter license by the CAA. The crash was caused by a broken connecting rod.

 Miscellaneous Information:

Type: XR-8

Helicopter, two built. It had a small egg-shaped fuselage, which tapered into a very deep tail structure. It had two-bladed intermeshing rotors.

Function: experimental helicopter
Year: 1944 Crew: 2 Engines: 1 * 245hp Franklin O-405-9
Rotor Span: 40ft Length: Height: Disc Area:
Empty Weight: 2320lb Max.Weight: 2975lb
~ 6.07 lbs / hp ?
Speed: Ceiling: Range:
Armament:

Both rotor shafts leaned away from each other at a 12.5-degree angle from straight up.

They redesigned the rotors to tilt fore and aft differentially for greater yaw control. These efforts greatly improved control.

Line drawing of rotor head; [Source ~ MDD p.180]

Type: XR-10

Utility helicopter broadly based on the R-8. The two engines were installed in nacelles on the sides of a compact, box-like fuselage. It retained the intermeshing rotors and intermeshing two-bladed rotors. It was at the time the largest helicopter under development for the USAF. One built.

Function: utility
Year: 1947 Crew: Engines: 2 * 525hp Continental R-975-15
Rotor Span: 65ft Length: Height: Disc Area:
Empty Weight: Max.Weight: 15500lb
~ 14.75 lbs / hp ?
Speed: Ceiling: Range:
Load: 10 seats

Line drawing and description of rotor head; [Source ~ MDD p.181]

Posting:

Can anyone suggest where it might be possible to get information, preferably of a technical nature, on the Kellett XR-8 & XR-10 intermeshing helicopters?

Replies:
"Try Fort Rucker, the Helicopter Collection. They got a superb library."
It appears that they do not have a web site.

"There are file cabinets full of Kellett E-size blueprints at the AHS Museum outside Philadelphia."

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Last Revised: June 9, 2008