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High-Speed Helicopter Revolution: Sikorsky's Dual-Rotor Design


• Dual Rotors: Single-rotor helicopters tend to roll at high speeds as their retreating blades lose lift. The X2’s opposed rotors create equal amounts of lift on both sides.

• Rear Propulsor: For maximum thrust, engine power is split between the main rotor and an aft propulsor.



check out Sikorskys newest concept bird :





Gee, sure reminds me of something..




I've always wanted a Lockheed AH-56 Cheyenne


Either way, can't wait to get there faster than the King Air.



Edited by Goldy
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Here's a little info on the Advancing Blade Concept (ABC) prototype from the mid-seventies...basically the predecessor of the X2 (Sorry in advance for the length).




"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


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"


...and a Nick Lappos email...


From: nlapposNOSPAM@miami.gdi.net (Nick Lappos)

Subject: Re: mu -a (Sikorsky ABC helicopter?)

Date: 11 Aug 1999

Newsgroups: rec.aviation.rotorcraft


In article , ""gridiron\"@antispam@bigfoot.com"n wrote:


>Doug Marker wrote:


>> We are seeking confirmation that any helicopter has ever broken the mu-1

>> barrier other than

>> in a wind tunnel test.


>> We have conflicting reports that the Sikorsky ABC may have done so but

>> the deeper we

>> research the more confused the Sikorsky story gets.


>> Firstly does anyone know for fact if it has been done ? (preferably with

>> a reference)


I flew the ABC for a bit, and have the facts. It went 300 miles an hour (259

knots) using the J-60's as auxiliary thrust. When blasting along, it was

actually in autorotation, with the rotor freewheeling as the jets pushed it


I have never heard of a supersonic rotor of any kind, wind tunnel or

otherwise. Sounds quite improbable to me, considering retreating blade stall.



>The S-69 ABC had two rotors turning in opposite directions so that at

>all times there was an "advancing blade". Of course Charlie Kaman, a

>whole lot of Russians and others had done this for years.


Doug implies that there was nothing really new about the ABC, since others had

flown co-axials before. Actually, the XH-59A ABC (Sikorsky designation

S-69) was very rigid, with extremely stiff blades, unlike the very low offset

Kamov designs (including the KA-50 Hokum/Werewolf). The extra rigidity is

quite unique, and allowed the ABC to operate beyond stall on the retreating



With a more conventional Coaxial, stall makes the blades flap a bunch, and tip

clearance between disks becomes an issue. Self mid-air collision can ruin

your whole day.


This ABC post stall operation relied on the upsweeping blade on each side of

the helo to keep roll and pitch control (in a single rotor at stall, you lose

control because the down sweeping blade gives up the ghost). ABC could pull

about 1/2 to 1.0 more G's than an equivilent helicopter, and could do so at

altitude. The ABC demonstrator pulled 2.5 g's at 25000 feet! As the waterboy

would say, "Not too shabby!"


>I don't know the speed they reached with the 69.


Cruise at 225 knots, dive at over 260 knots.


>into the S-72 which had a more conventional single rotor system but it

>also had the two big "pusher" jets mounted on the sides along with some

>pretty long wings for a helicopter. I think the acronym was Rotor

>System Research Aircraft (RSRA)? The idea was to have propulsion and

>lift that would support any type/size of rotor system, even one that

>would not lift the airframe. -vic

The RSRA was originally developed by Sikorsky for a NASA contract to build a

flying wind tunnel that would have the ability to adapt itself to several

rotor designs. It was based on the fuselage of the S-67 Blackhawk, close

observers will note the similar tail cone and vertical fin design.

The RSRA became the X-wing, which held the promise of stopping its very stiff

rotor and becoming a high speed airplane. The trick was to use the rotor

blades backwards on one side, achievable because the blades were oval in cross

section, and relied on air slots on the leading and trailing edges to control

the circulation around the airfoil. A massive compressor on the aircraft

produced the air to feed the rotor. The program was humming along but ran

into several snags that kept driving the cost up until it ran out of money.

Some of the difficulties were part of the more conservative attitude after the

Challanger shuttle disaster, with more concern about safety, even though the

S-72 had crew extraction seats. When I last saw the airframe, it was parked

at Edwards, in shrink wrap.


Nick Lappos


"Been there, done that, the T shirt wore out a long time ago."




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...And if you liked that, here's the modified H-60Sikorsky/Piaseki with VTDP (Vectored Thrust Ducted Propeller), aka the Ringtail. You guys can do the homework for the full story. Oh, that program was offed a while back, but who knows...





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