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Sikorsky S-69 / XH-59

In February 1972, Sikorsky announced the development of an experimental helicopter designated S-69, which was designed to study the Advancing Blade Concept (ABC). This new system consisted of two rigid, contra-rotating rotors 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. This system did not even require a wing to be fitted for high speeds and to improve manoeuvrability, and also eliminated the need for an anti-torque rotor at the tail.

The aim of the project was to evaluate the ABC with this helicopter, first using scale models for wind tunnel tests at the Ames NASA research center, and then the real aircraft, which flew on 26 July 1973. Unfortunately, however, this prototype was lost in an accident a month later. Following an enquiry, design modifications were requested, plus improvements to the control system. Tests were resumed in July 1975 with a second aircraft. When test flights as a pure helicopter were completed, a new experimental phase began with the addition of an auxiliary turbojet. In 1983 Sikorsky proposed further modifying the aircraft as the XH-59B, with a shortened fuselage and ducted fan providing forward thrust.

G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984

Sikorsky S-69 / XH-59

In late 1971 the Army Air Mobility Research and Development Laboratory awarded Sikorsky a contract for the development of a single-engined research helicopter prototype designed specifically to flight test the company's Advancing Blade Concept (ABC) rotor system. The resultant Model S-69, which was allotted the military designation XH-59A and the serial number 71-1472, made its first flight in July 1973.

The XH-59A's ABC system consisted of two three-bladed, coaxial, contra-rotating rigid rotors, both of which were driven by the craft's single 1825shp PT6T-3 Turbo Twin Pac engine. During high-speed flight only the advancing blades of each rotor generated lift; this off-loaded the retreating blades and thereby eliminated the aerodynamic restrictions caused by blade-stall and the high mach number effect of the advancing blade tip. This, in turn, produced greater stability and manoeuvrability while eliminating the need for either a supplementary lift-generating wing or an anti-torque tail rotor. The XH-59A's streamlined fuselage more closely resembled that of a conventional airplane than a helicopter, having a cantilever tail unit with twin endplate rudders, side-by-side seating for the two crewmen, and fully retractable tricycle landing gear.

The crash of the first XH-59A early in the flight test programme led to the construction of a second prototype incorporating several significant control system modifications. This second machine (73-21941) flew for the first time in 1975, and in 1977 was converted into a compound rotorcraft through the installation of two 1350kg J60-P-3A turbojet engines. The modified machine was jointly evaluated by the Army, Navy, and NASA beginning in 1978, and was later able to reach and maintain speeds in excess of 515kph in level flight. The first prototype was ultimately rebuilt as a compound rotorcraft under a NASA contract and subsequently test flown (with the new serial 73-29142) by mixed Army, Navy, and NASA crews at NASA's Moffet Field, California, test facility. Both XH-59A aircraft were officially transferred to NASA following the 1981 end of joint Army/Navy participation in the tri-partite flight test programme.

S.Harding "U.S.Army Aircraft since 1947", 1990

In 1972 Sikorsky designed the S-69 for the US Army, gaining a contract for two XH-59A prototypes to evaluate an Advancing Blade Concept (ABC) rotor system comprising two counter-rotating three-bladed rigid main rotors, with a 1361kW Pratt & Whitney Canada PT6T-3 Turbo Twin Pac to power them; the S-69 requires no tail rotor and has a conventional horizontal tail surface with endplate fins and rudders. Additional power is provided by two pod-mounted 1361kg thrust Pratt & Whitney J60-P-3A turbojets, one on each side of the fuselage, and the S-69 has demonstrated a speed of 488km/h. In 1982 these aircraft were developed into a new XH-59B configuration with advanced rotors, new powerplant, and a ducted pusher propeller at the tail. This approach was seen as a possible solution to the Army's search for a new light attack helicopter (LHX), and further funding was recommended. The S-69/XH-59 programme was abandoned, however, and the need for LHX was only answered in the 1990s with the selection of the RAH-66 Commanche.

D.Donald "The Complete Encyclopedia of World Aircraft", 1997

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Photo Gallery 

A Sikorsky XH-59A ABC in a landing configuration with gear lowered. The ABC concept is that the lift load of the aircraft at high forward speeds is carried primarily by the advancing rotor blades.

Technical data for Sikorsky XH-59

Crew: 2, engine: 1 x Pratt Whitney of Canada PT6T-3 Turbo Twin Pac turboshaft, rated at 1360kW and 2 x Pratt & Whitney J60-P-3A turbojets, 1350kg of thrust, rotor diameter: 10.97m, fuselage length: 12.42m, height: 4.01m, take-off weight: 4960kg, max speed: 518km/h, cruising speed: 185km/h, ceiling: 4570m

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Fred Deuel, e-mail, 16.12.2009reply

I'm looking for a set of 3-views for this perticular helicopter to build a radio controled model. Any help would be greatly appreciated.

syahrul alam, e-mail, 02.10.2009reply


Charles Verax, e-mail, 27.04.2009reply

The advanced blade concept (ABC) Coaxial rotor system is a very innovative design. I am amazed the United States did not continue with the S-69 project from the 1970's. The s-69 /X-59 reached speeds of 322 mph!

The Russians have had their Kamov coaxial helicopters in use since the late 1950's, mainly as Navy shipboard helicopters. However, the Russians more recently developed the Ka-50 /52 gunship which has proven to be a very stable platform for an attack helicopter. It is fast (over 200 knots) because its contra rotating rotors reduces the effects of retreating blade stall and dissymmetry of lift.

Also, a contra rotating rotor system uses 100% of its power to produce lift because each blade cancels the torque from the other. Whereas a conventional single rotor helicopter wastes about 20% of its lifting power to turn the anti torque tail rotor. Plus you never have to worry about a tail rotor failure.

Therefore, I believe the United States is behind the power curve because it has not developed a coaxial /contra rotating helicopter. At least Schweizer and Sikorsky in the past few years has been developing a coaxial helicopter design with the X2 technology demonstrator. The X2 made its first flight on August 27, 2008 from Schweizer Aircraft's facility at Horseheads, New York. The flight lasted 30 minutes and began the flight test program. I am eager and optimistic that the X2 will open the door to more advanced coaxial helicopter designs in the United States.

Of course, there have also been many advanced conventional designs that were cancelled in the 1970's and 80's and more recently the RAH-66 Commanche. In the 1970's the Sikorsky S-67 Blackhawk and the Lockheed AH-56 Cheyenne were two extremely advanced gunships, but were cancelled because they were ahead of their time and had cost overruns. The S-67 set a world speed record, could do aerobatics, had speed brakes on its wings, could carry 7 passengers (therefore it was an assault helicopter like the Mi-24 Hind).

The Cheyenne gunship was a compound helicopter because it had a pusher prop, in addition to a tail rotor (which spun off of the same drive shaft) which allowed it to reach speeds of 220 to 245 knots.

Currently Piasecki has modified a UH-60 Black Hawk into a Vectored Thrust Ducted Propeller VTDP, compound helicopter, that has a tail-mounted ducted propeller, called the ring-tail (auxiliary propulsion system) and wings.

The VTDP Black Hawk (compound helicopter) has fixed wings that work in concert with the tail to unload the rotor during lift. “By unloading the rotor from its lift and propulsion responsibilities, you can delay the onset of retreating blade stall, which limits the speed of helicopters”

The ability of the compound helicopter to fly long distances at speeds of 230 knots makes the VTDP technology a viable option for upgrading the Army’s Black Hawk fleet, Keep in mind most conventional helicopters fly no faster than 140 knots.

Compound helicopters have “potential advantages in speed and range over conventional helicopters, but have disadvantages in hover, cost and complexity. A suitable application would be one in which the speed and range advantage outweighed the disadvantages. Of course, the coaxial rotor design does not have the weight disadvantage of a compound helicopter and has excellent lifting capabilities.

It will be interesting to see if the new X2 Demonstrator and the VTDP Black Hawk (compound helicopter) will ever make it into production. For some reason, the United States likes to cancel innovative helicopter designs. So, keep an eye on these two projects. They could create some major advances in rotor wing technology in this country that have been long overdue. Otherwise, I guess the Russians will always be way ahead with their coaxial Kamov helicopters!

clare astley, e-mail, 12.03.2009reply

I have a comment about helicopters in general. I think it's about time the rotating disc was utilized, which would consist of a airodynamically shaped disc with blades extending beyond its diameter. Only a small part of the conventional rotor blade provides the required lift.In forward flight a rotating disc with blades would be better.

N/A, e-mail, 20.02.2009reply


Daniel Rusanowsky, e-mail, 01.01.2009reply

As explained above the rotor system was RIGID, no lead & lag or flapping hinges, only pitch change. since the two rotors were only two (2) feet apart the blade tips could only deflect a maximum of 9 inches from horizontal down and the same up, a total of 18 inches to prevent collision of the rotor blades during operation and flight. Any flapping and lead-lag motion was absorbed by the rotor blades.This aircraft in the slow speed mode was controlled and flew exactly like any single disc helicopter with one exception directioal control. This was accomplished by rudder pedal application in the cockpit which changed the pitch of the rotor blades on each head thus changing the torque on the fuselage turning it in the direction required, also at the same time the yaw control (pedals) are coupled with the rudders on the tail of the aircraft would be affective at speeds above 35-45 knots. Cyclic pitch controll affects the main rotors just like any helicopter tipping the rotor disc forward, aft, left or right and increasing or decreasing collective pitch would make the aircraft fly in the direction required and increase or decrease the speed. The ROTOR RPM is CONSTANT for all flight regimes, the disc size is 36 feet, with these two factors and 0 airspeed the blade tips travel at a constant miles per hour (MPH) speed, lets say for discussion purposes 400 MPH. This will keep the blade tip speed from exceeding MACH ONE the speed of sound, when the thrust jets are in operation and the airspeed is at say 300 MPH, the blade tip speed will not exceed MACH.

Danilo, 22.11.2008reply

Too Low, Too Bad.
It flew like an airplane probably.

Ben, e-mail, 09.11.2008reply

Did the fixed rotors provide a lift only primary function, or did the offloading of the retreating blade also imply forward trust? I'm unclear how the aircraft moved forward and maneuvered without the auxiliary turbines (and what was the speed deferential turbines vs. no turbines)?

Thomas Rønnow Larsen, e-mail, 30.10.2008reply

Very interesting helicopter. With a rotor diameter of only 11 meters, I wonder what the rotor RPMs were at speeds around the 500 km /h? Were they reduced at these high speeds?
Excellent site!

me, e-mail, 24.07.2008reply

u r gay

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