coanda said:
I find it hard to get on board with some of this. Mainly because they crashed one, it seems, because they didn't simulate the compound properly. The people involved would be the best of the best - an elite. They must have known they'd run in to problems landing in a walled courtyard, and that they'd be operating a helicopter very near to its limits in an area which could create re-circulating air. I'm willing to suppose Murphy might just enjoy causing trouble...
The aircraft reported came down during a landing or fast rope insertion due to vortex ring state caused by the density altitude at the target being far less than predicted.
"I know what I've been told, which was that the temperature was 17 degrees higher than anticipated, and based on the temperature, and the load in the helicopter, the helicopter began to descend, and so it was a kind of controlled but hard landing."
Feinstein, 5/3/11 (http://www.reuters.com/article/2011/05/03/us-binladen-qanda-idUSTRE7426Z720110503)
The lower than predicted density altitude at the target can make it easier to enter the vortex ring state, and harder to get out of it (http://books.google.com/books?id=jFQ5NulQWysC&pg=SA11-PA5&lpg=SA11-PA5). A hard landing may have been the only option available to the crew due to the proximity of buildings and the other assault helicopter.
Blade vortexes are also the source of most of a helicopter's noise signature. It's likely that a VLO helicopter would include blades and blade tips designed to lower the noise signature by changing the blade vortex interaction, which may also make a vortex ring state easier to enter. The tail of the SHHH-60 that was left in Pakistan showed 5 or 6 short, wide chord blades, which appear to be intended to reduce vortex interaction between the tail rotor blades. The main rotor produces a larger percentage of the aircraft's noise, so it is reasonable to assume that similar quieting methods may have been used there.
coanda said:
Against:
I've never heard of a design like the suggested stealth hawk. No one has said anything about it before now as far as I can tell, there've been no rumours of a specific design for a stealth transport helicopter, no half caught glimpses at sunrise in far away places. I find this significant because of the amount of rumour regarding fixed wing types, and that rumour is generally linked to something in existence. Are helicopters missing?
The February 6 1995 issue of Aviation Week described a VLO helicopter observed at Groom Lake. In the early 1990s there was a program to develop a signature reduction kit for a particular UH-60 model that was flight tested and shown to work. Some OH-58Ds have been modified with a set of signature reduction enhancements, and it is safe to say that there may be signature reduction methods in use by 160th SOAR on their aircraft. The RAH-66 and UCAR programs both designed LO/VLO rotorcraft, and there is a long history of VLO rotorcraft patents in the public eye. Sikorsky, Bell, and Boeing all have dedicated signature/LO groups in house. Sikorsky and Boeing both have outdoor signature test ranges (I don't know about Bell). As OBB pointed out, LO Blackhawk modifications were studied at least as far back as the 70s:
http://up-ship.com/blog/?p=9714
Helicopters are somewhat easy to hide. They do not require as much space to test as high performance fighter aircraft, and many of them fit easily into transport aircraft like the C-5 or C-17. At a distance, a VLO helicopter may not look all that different from a conventional counterpart. There are many, many places in the US where a helicopter could be tested and operated covertly.
coanda said:
I keep coming back to what wreckage we've seen and I wonder if it's more the case that the tail boom has been re-engineered for low observability and is a fieldable kit to be used as and when necessary. Then it can be removed and the helicopter put back into normal use - hidden in fairly plain sight.
Field modification kits work if they don't drastically alter the flying qualities of the aircraft. From what we have seen of the tail, this modification would significantly alter how the aircraft flies.