






I. Stealth Choppers -- Key to the Strike?




II. History of the Stealth MH-60






III. Launching the Mission







According to a Pakistani news report one document reads:






IV. The Strike -- A Crash, but Mission Success






V. More Questions Raised







Comments     Threshold


Black hawk down
By Phoque on 5/6/2011 5:59:19 PM , Rating: 2
I was surprised that the chopper crashed because of "settling with power". I would have believed such a sophisticated chopper to have electronic aids that would have prevented this from occurring without pilot inputs.

RE: Black hawk down
By rbuszka on 5/9/2011 12:55:13 PM , Rating: 2
Vortex Ring State (VRS), or "settling with power", occurs when the helicopter is within ground effect, and descends too quickly. It is caused by the rotor's downwash being sucked back into the top of the rotor and being recirculated instead of being directed straight down and generating lift. The result is a dramatic reduction in lift, and this can cause the helicopter to drop to the ground rapidly. Once the helicopter enters VRS, it cannot be stopped by any mechanical or aerodynamic means a helicopter typically possesses, much less an 'electronic aid'.

From the photos I've seen, if the main rotor head was also covered with a similar dishpan-style fairing, it would have reduced the area of the rotor head capable of generating lift, leaving only the outer portion of the blades acting upon the surrounding air. Because the outer portion of the rotor blades travel the fastest, they generate the most lift, so occluding the inner portion of the rotor disc with fairings wouldn't cause that great of a decrease in available lift, but it does make the rotor much more susceptible to VRS. This has to do with the thrust disc no longer being a 'disc' but a 'ring', which makes it much easier to set up a toroidal airflow pattern in which the downdraft is being recirculated by the vacuum created by the rotor blades at a high angle of attack (which is used when helicopters are trying to land very rapidly).

The article is incorrect - all H-60 helicopter variants are manufactured by Sikorsky or international licensees; Boeing (Vertol division) and Lockheed-Martin are Sikorsky's competition, so it is unlikely that the design would have been licensed to them. Instead, the Black Hawks would have been modified by the Boeing/Lockheed-Martin joint venture. The RAH-66 Commanche helicopter was a previous 'stealth' helicopter with low-observable features such as a fairing over the main rotor head, an exterior shape with angular facets, and an internal ducted fan-tail design instead of a conventional tail rotor. However, that project was canceled, and it would not have been able to carry out a mission like this because it has substantially no 'cabin', just a cockpit. As we've seen here, adding low-observable features to an aircraft where such features were not initially considered in the aircraft's design can lead to reduced handling qualities and even dangerous performance characteristics such as the rotor's susceptibility to VRS. The tail rotor would also be somewhat more susceptible to VRS in hover, potentially leading to a loss of control if the tail rotor's blade pitch were suddenly increased.

