Sikorsky XH-59A

by Erik McIntyre

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This is a very unique and little known bit of info from the company that brought such infamous ships as the “BlackHawk” and the “Super Stallion”.  The ABC was a concept craft built to compete as the RAH-66 program, in part due to its unique rotor design. Its very hard to find actual photos of this ship. In later years, Sikorsky also built an S-76 ship (for those of you familiar) that was front/rear cockpit design. A drastic deviation from the side by side design of the original. The pilot sat in front and dropped down a bit from the rear allowing the “backseater’ to have the same field of view. This ship is all but impossible to find, since Sikorsky only showed this in a company calendar.

“Advancing Blade Concept (ABC) was a serious contender for the RAH-66.  However, the transmission weight penalties were too high and so the trade study decided that the advantages were outweighed (no pun intended although I will gladly take credit for it) by the weight disadvantage. In a zero sum game, weight for the ABC xmsn would come out of avionics or armor.  ABC is in mothballs as it outlived the design life of a number of critical components.  All who flew it were impressed with its capability.  We had also envisioned using it in conjunction with a pusher prop ala the Cheyene.  The Helix is a form of ABC being coaxial but you can see by the mast its not weight or design efficient.  As we put more effort into conventional rotors they get better and lighter.  Its going to take a lot of money to catch up the ABC given the lack of work in the last 20 years.”---unknown 

"The ABC had independent control for each disk, and very stiff blades (over 25% hinge offset) so tip clearance was not as big an issue as it is on other co-axials. This meant that the swashplates could be tilted to wash out lift on the retreating sides, but keeping it up on the advancing side. The mast was very strong, so the moment battle between the rotors (one "leaning" to the right, one to the left) was cancelled at the transmission. This let the rotors blast on out to high speed, and just as importantly, let the ABC develop very high load factor. It pulled 2.5 G's at 25,000 feet and cruised at over 250 mph, as the thrust engines pushed it along in autorotation (it was an autogyro at high speed!) Not too shabby. Of course the speed was driven by two jets that ate gas like there was no tomorrow, so the speed was really only good for short bursts. In a production configuration, the lift engines would drive the prop shafts, so the cruise efficiency would be much better. One really unique benefit of the ABC was the purity of the controls. The high offset made the aircraft snap to pitch and roll commands like a fighter, but with no cross coupling like the Boelkow family. The couplings were generated by each rotor, of course, but the rotors developed them in opposite directions so they cancelled at the head, and the pilot was left with a very crisp clean feel.

I flew it to about 250 MPH and was really impressed." ~ Nick Lappos, PPRuNe, August 27, 2001

There are certain circles that say the XH-59A competed against the XV-15. which went on to become the V-22. The design nature automatically suggests that this wasn’t true. That the XH-59A was more in line with a fast attack helicopter. The confusion comes into play when Sikorsky submitted it’s “X-wing” design that was supposed to compete with the now Osprey. This site, courtesy of Dryden/Nasa explains the program, http://www.dfrc.nasa.gov/Gallery/Photo/X-Wing/HTML/index.html.

Erik 

Photos and text © by Erik McIntyre