The Kellett XR-8 (later redesignated XH-8) was a helicopter built in the United States during World War II. It was a two-seat machine intended to demonstrate the feasibility of a twin-rotor system, and while it accomplished this, it also demonstrated a number of problems that prevented further development of this particular design.
Design and development
The successful demonstration of the Sikorsky VS-300 had the USAAF favoring the helicopter over the autogiro as an approach to rotary-winged flight. Realizing this, the Kellett Autogiro Corporation made a proposal to the USAAF on 11 November 1942 for the development of a twin-rotor helicopter that would eliminate the need for a tail rotor and its attendant loss of power. Initially discounted on theoretical grounds, the proposal was re-examined in the light of tests done with models by the Army's Experimental Engineering Section, and was accepted on 7 January the following year. This was followed on 11 September with a contract for nearly $1,000,000 to build two prototypes with the three-bladed rotors contained in Kellett's proposal, along with an alternative two-bladed system.

The resulting aircraft had a stubby, egg-shaped fuselage with a single tail-fin and tricycle undercarriage. Two seats were enclosed side-by-side behind an extensively-glazed nose and the two rotors intermeshed with one another, offset by 12??. The fuselage construction was of steel-tube, skinned in sheet metal and fabric, and the rotor blades were built of plywood ribs and skin attached to steel tubes. The intermeshing rotors quickly earned it the nickname "eggbeater".
Operational history
The first flight took place on 7 August 1944 with Kellett chief test pilot Dave Driscoll at the controls. A lack of directional stability was discovered, and was corrected by the addition of two extra tail fins. A far more serious problem was discovered on 7 September, when it was found that a blade from each rotor had collided while the aircraft was in flight. The Air Force therefore ordered Kellett to design a new, rigid rotor system for the XR-8.

In the meantime, the two-bladed rotor system was trialled on the second prototype, beginning in March 1945. This proved immediately unworkable, with severe vibration that was prohibitively difficult to resolve. Similarly, it became apparent that the rigid rotor system was going to require extensive redesign of the aircraft, and this effort was dropped as well.

On 23 January 1946, the XR-8 was accepted for service trials with its original, non-rigid rotors in place. However, the program was canceled almost immediately, and the prototype was eventually handed over to the National Air and Space Museum where it remains in 2008.
Variants
XR-8 - initial version (1 built)
XR-8A - version with two-blade rotor system (1 built)
XR-8B - version with rigid rotor system (not built)







Specifications (XR-8)
General characteristics
Crew: 1
Capacity: 1 pax
Length: 22 ft 7 in (6.88 m) fuselage
Height: 11 ft 0 in (3.35 m)
Empty weight: 2,320 lb (1,052 kg)
Gross weight: 2,975 lb (1,349 kg)
Fuel capacity: 34 US gal (28 imp gal; 130 l) normal; 81 US gal (67 imp gal; 310 l) maximum
Powerplant: 1 ? Franklin O-405-9 6-cyl. horizontally-opposed air-cooled piston engine, 245 hp (183 kW)
Main rotor diameter: 2? 36 ft 0 in (10.97 m)
Main rotor area: 1,017.5 sq ft (94.53 m2) each rotor
[b]Performance[b]
Maximum speed: 100 mph (160 km/h, 87 kn)
Service ceiling: 10,000 ft (3,000 m)
Hover ceiling: 3,000 ft (910 m)
Disk loading: 35.2 lb/sq ft (172 kg/m2)
Power/mass: 12.14 lb/hp (7.375 kg/kW)

Kellett XR-10
The Kellett XR-10 was a military transport helicopter developed in the United States in the 1940s that only flew in prototype form. It was designed in response to a USAAF Technical Instruction issued for the development of a helicopter to transport passengers, cargo, or wounded personnel within an enclosed fuselage. Kellett's proposal followed the general layout that the company was developing in the XR-8, with twin intermeshing rotors, and was accepted by the Air Force on 16 October over proposals by Sikorsky, Bell, and Platt-LePage.

The XR-10 resembled a scaled-up XR-8, although its twin engines were carried in nacelles at the fuselage sides, driving the rotors via long driveshafts, and the aircraft was skinned entirely in metal. The first of two prototypes flew on 24 April 1947, and at the time, was the largest rotorcraft to fly in the United States. During test-flights, however, the same problem that had been encountered with the XR-8's rotor system emerged when blades from the two rotors collided in flight. With fixes in place, flight testing continued, but on 3 October 1949, the first prototype crashed due to a control system failure and killed Kellett's chief test pilot, Dave Driskill. The project was abandoned shortly thereafter, and a 16-seat civil variant, the KH-2, never left the drawing board.



Specifications
General characteristics
Crew: Two pilots
Capacity: 10 troops or 6 stretchers or 3,550 lb (1,610 kg) of cargo
Length: 29 ft 2 in (8.89 m)
Empty weight: 9,550 lb (4,340 kg)
Gross weight: 15,380 lb (6,990 kg)
Powerplant: 2 ? Continental R-975-15 , 425 hp (317 kW) each
Main rotor diameter: 2? 71 ft 0 in (21.64 m)
Performance
Maximum speed: 100 mph (160 km/h, 87 kn)
Range: 350 mi (560 km, 300 nmi)
Service ceiling: 15,000 ft (4,600 m)