Item 0890

DESIGN: UniCopter ~ Rotor - Disk - Advancing Blade Concept (ABC)

Sikorsky's XH-59A ABC

Abstract of 'An ABC Status Report': I have hard copy
The Advancing Blade Concept (ABC) uses two rigid counterrotating rotors in a coaxial arrangement to provide advancing blades on both sides of the aircraft. This makes use of the high dynamic pressure on the advancing side of the rotors at high forward speed, virtually ignoring the low dynamic pressure on the retreating side, while still keeping the rotor system in roll trim. Theoretically, such a rotor system will maintain its lift potential as speed increases. The XH-59A was designed to investigate this theory. A description is provided of the flight test program from May, 1980 to January, 1981. A summary is presented of the knowledge gained throughout the entire program, and current pitfalls are reviewed. It is concluded that the ABC has been verified, with the XH-59A envelope of blade lift coefficient as a function of advance ratio greatly exceeding that of conventional helicopter rotor systems.

Retreating Blade Consideration:

The Sikorsky ABC's rotor diameter is 36 ft., it has a -10 nonlinear twist, the rotor speed in auxiliary propulsion is 450 ft/sec. (240 rpm). At 217 knots (250 mph) the location of zero free air velocity on the retreating blade is at 14.5 radius (0.81 R). This is 819 fps (0.75 M up to 0.85 M at altitude) at the advancing tip.

Figures 13 in the report 'Advancing Blade Concept (ABC) High Speed Development' shows;

This is the penalty for the -10 of twist. They said that any future blades would have less twist.

The desired top speed is 300 knots (345 mph). This will be 959-fps (0.88 M up to 0.92 M at altitude) at the advancing tip. The total span of the retreating blade at 270 azimuth will be in reverse velocity.

More information on twist: DESIGN: UniCopter ~ Rotor - Blade - General - Twist

More information on the: Sikorsky's XH-59A ABC


Sikorsky's X2 ABC

Currently under development and testing. .


Stepniewski's Synchropter ABC

This is a 119-page evaluation of three different rotorcraft configurations as contenders for an optimal high-speed rotorcraft. The Synchropter (Intermeshing) ABC is his preference. .



The intent is for the advancing blades to provided a sufficiently higher lift then the retreating blades so as to give the combined rotor disk some dihedral without giving it so much that the vertical and lateral vibrations become too severe.

The application of ABC will also allow for faster forward velocity without retreating blade stall. The airfoil should therefor selected for its high-speed performance and not its stall resistant characteristics.

Note that for the Interleaving configuration the intent is to modify the ABC by removing some thrust from the inboard sides of the disks and adding this thrust to the outboard sides of the disks during hover. This means that outboard side of the disks will produce more thrust during both realms of flight.

Note: The pitch and loading on the advancing blade(s) will be higher then normal since the thrust is not evenly distributed between the advancing and retreating sides. This might be only a difference of + & - 10%.

Amount of ABC:

Initially consider the advancing blade supporting 120% of the load and the retreating side supporting 80% of the load.

This means that the advancing side is supporting 116.67 / 83.33 = 50% more than the retreating side.


The intent is for the thrust from the advancing blades on the outside of the two disks to be reasonably matched by the thrust from twice as many retreating blades on the inside interleaving area.


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Last Revised: July 15, 2008