Nick, to be fair I never said the transmission itself alone was large, or of large diameter. Note I mentioned QCA, which as a Sikorksy alum I am sure you know the acronym.
You have deftly danced around the issue I was trying to get at; a not-insignificant increase in frontal area. As has been published through many whitepapers during G-LYNX days, XH-59 days and so forth, its well understood that a major contributor to profile drag has always been frontal area, specifically the rotor head. I have read it mentioned that approximately 40% of drag on an edgewise flight helicopter is driven by the head/mast.
So lets take the X2, XH59 and S97, all of which were designed to have negligible payload within the cabin. This allows for a very clean installation using the area below the top deck to be used to house the QCA attachment along with the single engine power pack. Now, if you must reclaim this area due to the payload and egress requirements of FVL-M, you have 2 options:
- Mount the QCA and twin engines above top deck in pylon
- Mount the QCA and twin engines fully aft of cabin below the top deck a la X2/XH59/S97
The latter of which would almost certainly cause extreme CG issues, as I mentioned before.
It would stand to reason that this was realized by the Sik-Boeing design team as it would appear from they have chosen the former (over time the main gear has migrated in concept art as well, likely due to cabin rearrangement for CG):
So, in addition to all of this new area in the freestream due to dynamic systems, we have a larger diameter rotor, which increases rotor spacing. From layouts of the SB>1, the overall height looks something near 20 feet.
For small GW tatical aircraft like the S97 where you can use the fuselage itself to fair the dynamic system, and keep the rotor spacing low, the ABC concept works well.
I have a hard time fathoming how installed power is going to overcome the apparent large increase in drag from both pylon mounted engines+QCA and increased rotor spacing in edgewise flight on a larger version. I think this is even well-understood by SAC, considering the amount of lobbying done on the FVL IDRR in an attempt to keep the speed requirement closer to 170kt than what was settled on for MPS @ 230kt.