Needs more rotors! Also, did you use blender for this?

These co-axial rotors would need plenty more separation if Mu>0.25 due to aerodynamic flexing of blades.

The ABC concept uses exceptionally stiff blades. The X-2 has them just two feet apart, which is much closer than any of the conventional Kamov designs. I've halved that here to just 30cm, which I'm justifying via advances in material science. ^.^

If they're 30 cm apart, this means there's less... oh damn, I just noticed these are not contra-rotating. Never mind.

What do you mean they aren't...? Oh damn. Well gosh darn it that was a silly mistake. I'm going to have to fix that.

And yes I know that every rotation each blade will want to flex as the lift it generates changes relative to the air sped it's moving through (going backlwards and forwards). Due to the double five-bladed ABC though, each blade never supports more than 15% of the helicopters weight at high speed, and between 5-10% at low speed. This is significantly less range that a regular helicopter has to deal with.

Sorry about that.

Still, you have to (at least) consider how far the tips will reach at maximum angle of attack and your blades are pretty wide. Between cyclic, collective, G loading, forward/reverse airspeeds (at least I suppose those huge high-bypass engines are there to get the whole thing above Mu 1), even super-stiff designs can have a surprising amount of flex.

Ah, but Mu is also dependent on blade rotation. As forward speed increases, high speed rotation is no longer needed to put high speed airflow over the advancing blades. By slowing the rotors down, you can ensure you never reach Mu=1, and avoid the troublesome Mu=0.75 region.

The problem is not on the advancing blade, but on the retracting one. That's where the instability and oscillation begins, especially when parts of the blade are still seeing the "forward" airflow and others "reverse".

Actually, going over Mu=1 is quite possible and beneficial; check out what these guys are doing: http://www.cartercopters.com/

Not quite. From the mouth of someone who actually flew a helicopter like this:

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 blades. 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.
http://yarchive.net/air/sikorsky_ad.....e_concept.html

Visually you can also see the difference in blade 'flaping' moments between a single blade and compound helicopter in this document, along with a lot of fun CFD pictures:
https://dspace.ist.utl.pt/bitstream.....Sousa_2010.pdf - page 6

Alternatively there#'s a sample diagram on the bottom right of page 10 that illustrates that flapping is a much smaller problem for coaxils than for regular single blade helicopters.
www.humanpoweredhelicopters.org/art.....icles/nasa-tp-3675.pdf

Mmm, thank you for those papers. Very interesting.