Sikorsky S-69/XH-59A with auxiliary turbojets
Role Experimental compound helicopter
National origin United States
Manufacturer Sikorsky Aircraft
First flight July 26, 1973
Retired 1981
Primary users NASA
United States Army
Number built 2

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.


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.[citation needed] The S-69 was the demonstrator for the Advancing Blade Concept (ABC).[1]

The first S-69 built (73-21941) first flew on July 26, 1973.[2] However, it was badly damaged in a low-speed crash on August 24, 1973 due to unexpected rotor forces and insufficient control systems.[3] 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.[4] 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 156 knots (289 km/h; 180 mph), but with the auxiliary turbojets, it demonstrated a maximum level speed of 238 knots (441 km/h; 274 mph) and eventually a speed of 263 knots (487 km/h; 303 mph) in a shallow dive. At 180 knots (333 km/h; 207 mph) level flight, it could enter a 1.4 g bank turn with the rotor in autorotation, increasing rotor rpm.[5] Airframe stress prevented rotor speed reduction and thus full flight envelope expansion.[5] The XH-59A had high levels of vibration and fuel consumption.[6][3]

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.[5][7][8] 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.



The Advancing Blade Concept system consisted of two rigid, contra-rotating rotors (30 inches apart)[9] 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.[10][11] This system did not require a wing to be fitted for high speeds and to improve maneuverability,[5] and also eliminated the need for an anti-torque rotor at the tail.[12] 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.[5]

Surviving aircraft

Airframe 73-21941 is in storage at the NASA Ames Research Center[13] and 73-21942 is on display at the Army Aviation Museum, Fort Rucker, Alabama.[14]

Specifications (S-69)

Data from U.S. Army Aircraft Since 1947,[15] Illustrated Encyclopedia,[12] US Army Research Laboratory[5]

General characteristics

9,000 lb (4,082 kg) without turbojets
or 1,500 hp (1,119 kW)[5]
2x three-bladed co-axial rotors, with 30 in (762 mm) separation, turning at 345 rpm


156 kn (180 mph; 289 km/h) without turbojets
15,000 ft (4,572 m) without turbojets

See also

Related development

Aircraft of comparable role, configuration, and era

Related lists


  1. ^ Michael J Taylor: The Illustrated Encyclopedia of Helicopters, page 20. Exeter Books, New York, 1984. ISBN 0-671-07149-1
  2. ^ Taylor 1976, p. 390
  3. ^ a b Lawrence, Thomas; David Jenney (31 Aug 2010). "The Fastest Helicopter on Earth". IEEE Spectrum. Archived from the original on 30 January 2017. Retrieved 1 August 2017. [first airframe] the very stiff coaxial rotors produced greater-than-expected nose-up forces when flying forward. The aircraft's control system also turned out to be inadequate. [second airframe] the helicopter vibrated so much at these higher speeds that its pilots struggled to control it ... The heavy and fuel-hungry jets pushed the aircraft to high speed
  4. ^ 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
  5. ^ a b c d e f g h 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.
  6. ^ Robb, Raymond L. Hybrid Helicopters: Compounding the Quest for Speed[dead link] p. 48, Vertiflite, Summer 2006.
  7. ^ Goodier, Rob (September 20, 2010). "Inside Sikorsky's Speed-Record-Breaking Helicopter Technology". Popular Mechanics. Retrieved 22 September 2010.
  8. ^ Croft, John. "Hyper Helos: Prototypes coming off the drawing board and into the race", Flightglobal.com, 3 July 2008. Accessed: 9 March 2012.
  9. ^ Kocivar, Ben. "Turbofan-powered flying carpet" page 68, Popular Science, September 1982. Accessed: September 2014.
  10. ^ Chandler, Jay. "Advanced rotor designs break conventional helicopter speed restrictions (page 1) Archived July 18, 2013, at the Wayback Machine " Page 2 Archived July 18, 2013, at the Wayback Machine Page 3 Archived July 18, 2013, at the Wayback Machine. ProPilotMag, September 2012. Accessed: 10 May 2014. Archive 1 Archive 2
  11. ^ Jackson, Dave. "Coaxial - Sikorsky ~ S-69 (XH-59) ABC Archived November 6, 2014, at the Wayback Machine" Unicopter, 9 March 2012. Retrieved: 22 May 2015. Archived on 6 November 2014.
  12. ^ a b Apostolo, G. "Sikorsky S-69". The Illustrated Encyclopedia of Helicopters. Bonanza Books, 1984. ISBN 0-517-43935-2.
  13. ^ Bagai, Ashish. "Sikorsky XH-59A ABC (S-69)." airliners.net, March 29, 2011. Retrieved: June 8, 2011.
  14. ^ Baugher, Joe. "1972 USAF Serial Numbers." Retrieved: June 8, 2011.
  15. ^ Harding, Stephen (1997). U.S. Army Aircraft Since 1947. Atglen, PA, USA: Schiffer Publishing Ltd. p. 251. LCCN 96-69996.
  16. ^ Lednicer, David. "The Incomplete Guide to Airfoil Usage". m-selig.ae.illinois.edu. Retrieved 16 April 2019.