Regarding Tilt-Rotor (TR) vs. ABC, there are a few things that need to be taken into consideration.
A TR is always going to have the penalty of the weight and area of the wing. Although you can have much more twist in the blades of a TR (because there is no need to design for the case where the blades are flying through the air sideways at higher speeds), the smaller the size of the air vehicle, the greater proportion taken up by the wing. On the other hand, once you stop hovering, you have the advantage of having the wing. Your propulsion source doesn't have to be your lift force as well, which gives you greater efficiency and performance. The ratio of hover to forward flight determines which is the most efficient. It's worth remembering that there were likely going to be two TR proposal for LHX until the Army reversed course and changed the specs so as to preclude a TR bid. From the information that leaked, it looked like the TR proposals would far outperform helos, even allowing for the hovering penalty.
A TR does offer lower risk. The XH-59 was pretty much a flop, and the X2, unquestionably an impressive achievement, was years late and has only demonstrated a few minutes at its design speed, whereas the XV-15 was routinely cruising at those speeds and above over 30 years ago.
A TR of course offers higher speeds. In the case of the V-280, the power driver was the 6K 95F requirement. With the power to do that, the 280 knot speed came as a byproduct, not a driver. It also offers longer range, because of that wing, and lower fuel burn, since it can run at proportionately lower power settings while wingborne. Self deployment is relatively easy for a TR, as was demonstrated when V-22s were suddenly deployed from Afghanistan to the Libyan theater. Self- deployment is especially important for them, since it looks like it'll be a easier (if the height permits it) to stuff an ABC vehicle in a C-17 than a TR.
The control system on a TR can be argued to be simpler since there is no need for a separate system to provide thrust. An ABC can fit in a narrower LZ, but it looks like a TR can fit in a shorter one, and there's no need to worry about what the aft prop is doing relative to obstructions and personnel. I can't tell if the arc of the proprotors on the V-280 extend beyond the nose. If you watch Sikorsky's videos on ABC operations, they show the propeller not being brought on line until airborne. This can have an affect on operations and will have a big impact on civil use of the concept for EMS, since one of the big concerns in that arena is the tailrotor and people near it. One other thing: With a TR, at least in the attack version, conventional ejection seats could be provided for the flight crew, not practical with ABC.
Final cost will depend a lot on how the gov't specs out the final requirement, and how much they add on to the desired capabilities and what they stuff into the contract. I simply do not believe that Sikorsky (or anyone) could deliver the Raider for $15 million. Like a TR, the flyaway costs of an ABC are going to be more than that of a conventional rotorcraft. Like a TR, you mke it up in the back end and needing fewer vehicles o accomplish your missions. If all they want is a conventional helo can do (watch what they do with the speed and range requirements), then neither advanced concept a chance. If they back off to 150-175 knots and lower range requirements, both of these technologies will go away.
As to why Bell left the 609, I suspect part of that was that they needed to concentrate all their resources on the V-22 program and that they hired certain people at senoir level (and they definitely needed new blood at that level) who were known not to be fans of TR.
One other thing: door gunners. While in an attack bird, remotely operated turrets are the way to go, in the assault mission they can not react fast enough, don't have the versatility and are much more expensive and trouble prone than guys in the doors. Consider a situation where the vehicle has landed to infil/exfil. The turrets are pretty much out of the picture. Also, when low and slow for whatever reason, you can do a lot more with door gunners than with a turret, especially if there are bad guys on both sides. It's no accident that USMC as decided to give priority to getting UH-1Ys over AH-1Zs.
Last edited by Commando Cody; 16th Apr 2013 at 02:45.