My understanding of ABC is that the retreating blade is at a very low or zero angle of attack regardless of the angle of attack of the advancing blade. I guess the reason is to eliminate retreating blade stall. If this is correct then how can ABC work with any design other than a coaxial? With a coaxial as the blade is advancing on the left side, the other rotor has a blade advancing exactly opposite on the right side. This seems logical so the lift stays on balance left to right. If you do this with a two bladed syncropter such as the K-max you would have a blade on one side advancing while the other rotor has its blades aligned longitudinally with the fuselage. If the lift alternates left to right with each passing blade that would cause a huge vibration. My question is am I understanding this correctly, that a sychropter can not be an ABC design? I would appreciate it if someone corrects me and can lead me to a detailed explanation of the ABC designs.
Thanks,
Rene
Dave Jackson
27th Aug 2002, 07:54
Rene,
You are correct. The application of ABC reduces the opportunity for the retreating tip to stall. In addition, by also reducing the rotor's speed, compression on the advancing tip can be reduced, and this allows for faster forward flight.
The ABC operation of the coaxial and that of the intermeshing (synchropter) are very similar. If the rotors in either configuration have only two blades each, there will be a dissymitry of lift and an excessively large vibration. This is true of both the coaxial and the intermeshing configurations during forward flight. In your coaxial example, you have the two blades on each rotor advancing together. As you say, this will give lateral symmetry of lift, but as these advancing blades approach the nose they will create a longitudinal dissymitry of lift. Changing the azimuth of blade crossing will change the moments (more or less pitch or roll) of the vibration but it will have fairly little effect on reducing it.
The actual coaxial ABC and the two conceptual intermeshing ABC helicopters have three blades per rotor and this reduces the dissymitry of lift considerably. It appears that there is still a rotor-induced vibration, at a level slightly higher than that of a single rotor helicopter, but future higher harmonic control and/or individual blade control should make it insignificant.
If you wish more detail, you may find the following pages of interest;
Sikorsky ~ XH-59A ABC (http://www.unicopter.com/0891.html)
Advancing Blade Concept (ABC) (http://www.unicopter.com/0890.html)
Lateral Dissymmetry of Lift (http://www.unicopter.com/0871.html)
Dave J.