Sikorsky S-69

The Sikorsky S-69 (military designation XH-59) is an American experimental compound helicopter developed by Sikorsky Aircraft as the demonstrator of the co-axial Advancing Blade Concept (ABC) with United States Army and NASA funding.

Development In late 1971, the Army Air Mobility Research and Development Laboratory, which later became a part of the Army Research Laboratory, awarded Sikorsky a contract for the development of the first prototype. The S-69 was the demonstrator for the Advancing Blade Concept (ABC).Michael J Taylor: The Illustrated Encyclopedia of Helicopters, page 20. Exeter Books, New York, 1984.

The first S-69 built (73-21941) first flew on July 26, 1973. However, it was badly damaged in a low-speed crash on August 24, 1973 due to unexpected rotor forces and insufficient control systems. The airframe was then converted into a wind tunnel testbed, which was tested in the NASA Ames Research Center 40x80 feet full-scale wind tunnel in 1979.Felker, Fort III. NASA NASA-TM-81329, USAAVRADCOM-TR-81-A-27 Performance and loads data from a wind tunnel test of a full-scale, coaxial, hingeless rotor helicopter. http://hdl.handle.net/2060/19820004167 A second airframe was completed (73-21942) which first flew on July 21, 1975. After initial testing as a pure helicopter, two auxiliary turbojets were added in March 1977. As a helicopter, the XH-59A demonstrated a maximum level speed of , but with the auxiliary turbojets, it demonstrated a maximum level speed of and eventually a speed of in a shallow dive. At level flight, it could enter a 1.4 g bank turn with the rotor in autorotation, increasing rotor rpm. Airframe stress prevented rotor speed reduction and thus full flight envelope expansion. The XH-59A had high levels of vibration and fuel consumption.Robb, Raymond L. Hybrid Helicopters: Compounding the Quest for Speed p. 48, Vertiflite, Summer 2006.

The 106-hour test program for the XH-59A ended in 1981. In 1982 it was proposed that the XH-59A be converted to the XH-59B configuration with advanced rotors, new powerplants (two GE T700s), and a ducted pusher propeller at the tail. This proposed program did not proceed as Sikorsky refused to pay a share of the costs.Croft, John. "Hyper Helos: Prototypes coming off the drawing board and into the race", Flightglobal.com, 3 July 2008. Accessed: 9 March 2012. Sikorsky and its partners funded the development of the next helicopters using the Advancing Blade Concept, the Sikorsky X2 and Sikorsky S-97 Raider, from 2007.

Design

The Advancing Blade Concept system consisted of two rigid, contra-rotating rotors (30 inches apart)Kocivar, Ben. "Turbofan-powered flying carpet" page 68, Popular Science, September 1982. Accessed: September 2014. which made use of the aerodynamic lift of the advancing blades. At high speeds, the retreating blades were offloaded, as most of the load was supported by the advancing blades of both rotors and the penalty due to stall of the retreating blade was thus eliminated.Chandler, Jay. "Advanced rotor designs break conventional helicopter speed restrictions (page 1) " Page 2 Page 3 . ProPilotMag, September 2012. Accessed: 10 May 2014. Archive 1 Archive 2Jackson, Dave. "Coaxial - Sikorsky ~ S-69 (XH-59) ABC " Unicopter, 9 March 2012. Retrieved: 22 May 2015. Archived on 6 November 2014. This system did not require a wing to be fitted for high speeds and to improve maneuverability,J. Rudell et al. Advancing Blade Concept (ABC) Technology Demonstrator report: USAAVRADCOM-tr-81-D-5, United States Army Research Laboratory, April 1981. Size: 11 MB. Accessed: 10 March 2012. and also eliminated the need for an anti-torque rotor at the tail. Forward thrust was provided by two turbojets, which allowed the main rotor to only be required to provide lift. It was found to have good hover stability against crosswind and tailwind. With jets installed, it lacked power to hover out of ground effect and used short take-off and landing for safety reasons.

Surviving aircraft Airframe 73-21941 is in storage at the NASA Ames Research CenterBagai, Ashish. "Sikorsky XH-59A ABC (S-69)." airliners.net, March 29, 2011. Retrieved: June 8, 2011. and 73-21942 is on display at the Army Aviation Museum, Fort Rucker, Alabama.Baugher, Joe. "1972 USAF Serial Numbers." Retrieved: June 8, 2011.

Specifications (S-69)

Sikorsky S-69

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