Kellett XR-8
| Kellett XR-8 | |
|---|---|
 The second prototype of the XR-8 (XR-8A, USAAF serial number 44-21908) |
|
| Type: | Experimental helicopter |
| Design country: | |
| Manufacturer: | |
| First flight: |
August 7, 1944 |
| Number of pieces: |
2 |
The Kellett XR-8 Synchropter is a helicopter made by the US manufacturer Kellett Aircraft Corporation in the 1940s, of which only two machines were built. An occasionally claimed renaming to XH-8 according to the system valid from 1948 apparently no longer took place, since neither of the two units was in use at that time.
history
The XR-8 was Kellett's first attempt to work in the field of helicopter construction and, in June 1943, after the complete cessation of autogyro construction, also led to the name change from Kellett Autogiro Corporation to Kellett Aircraft Corporation . In the same year in which the USAAC ordered seven Kellett O-60 gyroscopes , on June 29, 1940, they also announced the prospect of purchasing an XR-8. A firm contract to build it wasn't until May 1943, with the USAAF then ordering two machines at a price of 92,864 US dollars each.
The first (USAAF serial number 43-44714) of two XR-8s had their maiden flight on August 7, 1944. Initially, the flight attempts were only carried out tied up. During the short hover flights with only one mounted rotor, the strength of various connections was tested by attaching different weights to the fuselage. With both rotors installed, static and then dynamic investigations were carried out to check the distances between the rotor blades. To do this, the rotors were first turned manually and the overlapping blades set in motion. In the subsequent motor-driven test, high-speed cameras were also used to determine the blade spacing. Only at a speed of 200 min −1 were the control devices unlocked and the influence on the rotors determined in all potential flight states.
The next stage of testing can be characterized as "half-tied up" because the cables were still attached to the ground, but allowed the machine to move sideways and upwards to a limited extent. This was followed by the first free hover flights. As a first result of these tests, the coupling device between the motor and the rotors was newly designed and built. Overall, the complex rotor system was seen as promising in the long term, but in the short term it turned out to be more difficult to bring to series production compared to the single-rotor design with a tail rotor, as used in the competitor's Sikorsky R-4 .
The main problem that emerged during the entire test was the danger that the two rotors could touch, especially if there were abrupt changes in direction. Backward and sideways flights at more than 8 km / h were strictly prohibited. When the additional restriction was added that an autorotation could not be initiated below 150 m ground clearance, it became clear that the XR-8 could only continue to have an authorization as an experimental aircraft.
As early as November 1943, Kellett suggested using the XR-3 gyroplane ( USAAC serial no. 37-380) still in the factory to support the development of the XR-8. After the problems that arose when testing the XR-8, the Air Materiel Command (AMC), as the client, changed the original contract to equip the XR-3 with the rotor head and control system of the XR-8. In addition, a camera was installed above the rotor, which showed that the blades were bent extremely severely in all directions. Allegedly, after looking at the pictures, the test pilots were about to refuse further flights.
The end of the project came when, in September and October 1944, three incidents with rotor blades touching each other occurred at short intervals. As a last measure, the three-blade rotors were exchanged for a version with two blades, which, however, did not help, but on the contrary caused additional vibrations in the system. On December 6, 1946, the project was discontinued because no further funding was expected from the US government.
A landing on the roof of the main post office in Philadelphia and an overland flight over a distance of 550 km ( Upper Darby - Schenectady and back) are to be mentioned as special events during the test . Although there is evidence that two prototypes were made, the history of the second machine is not available in the literature.
construction
The fuselage of the XR-8 had a squat, egg-shaped shape. The crew sat on two seats side by side in the aerodynamically shaped bow. A six-cylinder Franklin O- 405-9, which was installed in the fuselage behind the cockpit , provided the drive . The rotor system based on the Flettner system consisted of two adjacent and intermeshing rotors with opposite directions of rotation. A torque compensation via a tail rotor was therefore not necessary. The two rotor shafts were installed at an angle of 12.5 degrees to the right and left when viewed from the front and 12 degrees to the front from the side. The distance between the rotor heads was 1.21 m. The rotors could be tilted 8 degrees in all directions for control. One blade of the three-bladed rotors weighed 20 kg. The machine was nicknamed "eggbeater" (English for whisk, in the sense of a hand-operated mixer) because of the rotors that intermeshed, which were unusual at the time.
The fuselage consisted of an all-metal tubular space construction, the front half of which was metal-clad and otherwise covered with balloon fabric. To increase the directional stability, a two-part horizontal stabilizer with two rigid side windows was adjustable on the floor.
Technical specifications
| Parameter | Data |
|---|---|
| crew | 2 |
| Hull length | 6.89 m |
| Rotor diameter | 10.97 m |
| Overlapping rotor circle diameter | 12.19 m |
| Height above everything | 4.46 m |
| Empty mass | 1053 kg |
| Max. Takeoff mass | 1351 kg |
| Top speed | 167 km / h |
| Greatest hover altitude | 1000 m |
| Summit height in level flight | 3000 m |
| Range | 270 km |
| Engines | an air-cooled six-cylinder Franklin O-405-9 boxer engine with an output of 245 hp (180 kW) |
See also
literature
- Francis Allen: Ambitious 'Eggbeater' - Kellett's XR-8 far ahead of its time. In: AIR Enthusiast No. 111 May / June 2004, pp. 26-30.
- Howard Levy: Kellett Gyrations Part 2. In: Airplane Monthly December 1995, pp. 36-39.
- Airplanes from A – Z. In: AERO - The illustrated compilation of aviation. Book 100, p. 2796, Cavendish.
Web links
Individual evidence
- ^ John M. Andrade: US Military Aircraft Designations and Serials. Midland Counties Publications, 1979, pp. 118, 162.
- ↑ Flight recordings of the XR-8 on youtube.com