ABC concept
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Joined: Apr 2001
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From: Hartford, Connecticut, USA
ABC concept
After reading about the S-69 ABC concept in the intermeshing thread, I looked at a couple websites and they said that the abc was able to fly faster because the dual rotors offloaded the retreating blades. My assumption is this, i believe that the angle of attack would have to be reduced in order to prevent RBS and the flap that would result from that. How did they reduce the angle of attack on the retreating blade? was it through some sort of mixing unit or something else? 
OR am i on the completely wrong train of thought here? I can't imagine not having to change the angle of attack on the retreating blade because doesnt RBS result in flapback of the blades? as well as a very large increase of drag? Ok i have said too much. I await your answers. thanks 
OR am i on the completely wrong train of thought here? I can't imagine not having to change the angle of attack on the retreating blade because doesnt RBS result in flapback of the blades? as well as a very large increase of drag? Ok i have said too much. I await your answers. thanks
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baranfin:
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.
[ 27 August 2001: Message edited by: Nick Lappos ]
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.
[ 27 August 2001: Message edited by: Nick Lappos ]
