Sans,
There is surely more drag due to the extra rotor head, but you went on about lost cabin area due to volume, so I answered that misimpression.
Your worldview of helicopter drag seems drawn from airplanes, where "presented area" and cross section drag are dominant. Rotorcraft are far different, and need a fresh way of examining their efficiencies. Mostly, you must look at the entire package, especially the power consumed by the rotors to produce the lift and thrust, and the hover performance lost when large wings lay within the rotor downwash field.
A twin rotor head design carries an inherent drag penalty, for sure, mostly driven by the extra head but also vastly compounded by the tall mast and space between the rotors. Mikheyev of Kamov published several papers that described his estimate at 10% extra for his designs, I have no reason to doubt that.
The fairing we were talking about on the X2 designs, and the much reduced X2 rotor separation (which drops the upper rotor height substantially) help limit the frontal area drag to very reasonable numbers relative to older coaxials. The head separation on a Kamov is about twice as much, because the low hinge offset rotors have to be kept apart (to prevent self-midair collisions). The very rigid ABC/X2 rotors have almost no flapping, so they can be kept closer together, and with the ensuing lesser drag.
The most important power loss/drag increaser at speed is not the frontal area, but the large power penalty of the retreating blade stall. The rotor alone consumes a vast amount of the power at higher speeds, You could reduce the presented area to a point in space and the rotor blades will consume 3/4 of the power anyway. Helicopters must have balanced lift on both sides of the disk, so the retreating blades, operating at very high angle of attack, and in some reversed flow, are famously inefficient. This is while the advancing blades are approaching Mach 1 and consuming power due to drag divergence. Can't slow them down, or the retreating side gets worse!
The X2/ABC basically tilts its swash plates to dump the angle of the retreating blades, and making sure the advancing blades tote the lift needed while the retreating blades reduce their angles and loaf along. As Mach 1 is being reached, the rotors are slowed down as well, which keeps the high speed drag under control.
What this means, SansAnhedral, is that the X2/ABC design has much lower drag at high speed than helicopters. Its L/D is very attractive, especially when you compare its hover efficiency. Added benefits over other high speed configurations include: efficient helicopter rotors for good payload and good altitude hover performance, fantastic handling with low inertias and nimble pitch, roll and yaw handling, compact design so it fits into small areas.
This was all examined when the FVL was being considered by the Army, and speed, range and altitude performance were all part of the new way of doing business. Need independent proof of these comparisons? Boeing took months to examine tilt rotors, compounds and X2s, and came to Sikorsky to join the team, even though they have a great deal invested in tilt rotors, and know them very well. Defiant is very much a 50-50 partnership with a company that makes 50% of the V22, and chose to spend their effort and money on the X2/ABC configuration.
BTW, another misimpression is the mistaken belief that Sikorsky tried to hold back the speed goals of JMR/FVL. I have no idea who told you that, my suggestion is that if they tell you it is 2 o'clock and your watch agrees, throw your watch away! Sikorsky and Boeing are spending a ton building a machine (Defiant) that will be far, far faster!
Last edited by NickLappos; 28th May 2015 at 18:17.