From this news article which mentions the helicopter crash in Abbottabad :
That doesn’t make sense to me… I know about the concept of ground effect with planes, which increases lift, but why would the height and thickness of the walls of a structure decrease the lift of the helicopter?
Likely the walls interfered with the flow of downwash from the rotors, causing some kind of recirculation that resulted in a settling-with-power situation.
Not sure of how accurate any of the reports are, but if there were no mechanical failure at all, it wouldn’t seem likely that they would destroy the chopper and leave it there. If it were only a downwash situation, the craft could be unloaded and the pilot would have attempted to take it out of the compound. OTOH, if it landed during a lack of control situation, maybe it was just too close to the structure to take off again safely.
The crash may not have been initiated by mechanical failure, but the crash itself may have resulted in subsequent damage that rendered the helicopter unflyable.
That’s a reasonable explanation. It doesn’t sound like a national security issue, so we may get the whole story before long.
I’m a little surprised that this issue wasn’t identified previously in the prepatory drills they ran within the ‘identical’ compound mock-up.
It fell onto its side when it came down. So destruction was much easier than recovery.
Bear, do you have a cite for this? I’m not doubting you, I’d just like to see the actual reports.
One of the photos of the site, taken after everything was done and all parties gone, shows the tail of the chopper hanging over the wall. So perhaps it struck the wall and broke off the tail rotor when it came down, rendering it non-airworthy.
It could have been windy, or they could have been trying to evade fire from the ground, I suppose.
I read it today. I think it was in Stars and Stripes. That issue is at work right now. I will check it tomorrow morning.
Also would be interested where in the States the mock up was, in terms of comparative altitude. Abbottabad is 4,000 feet above sea level, which could have an impact on the lift generated by the rotor blades, which wasn’t evident if the practice runs were at a lower altitiude
Paging JohnnyLA!!
I figured they had to land right friggin’ now and get on with the mission. Maybe there was a chance to maneuver, etc, but I would think that once you are there and the others are good to go, you have to go NOW. Five seconds to back off, fight it, etc could mean utter failure.
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Here is a picture-ish of the crash.
http://www.boston.com/bigpicture/2011/05/osama_bin_laden_killed.html#photo6
It was in “the segregated area” at Bagram in Afghanistan. How high is Bagram?
Ah. initial reports I saw said the mock up was in the States. My suggestion is way off then because Bagram is ~5,000 feet above sea level.
Looking at the picture, I suspect the helo was doing a quick stop and the tail simply hit the wall.
Elevation above sea level combined with heat is a factor called “density altitude”, and it has a big effect on the performance of fixed and rotary wing aircraft. Hot air is less dense, which means the aerodynamic surfaces have less to work with in producing lift. It also has an effect on engine performance. Hot air combined with a high elevation in the first place means a very high density altitude.
Taking this into account, it then becomes a matter of how much weight you’re trying to lift with a given amount of engine power. In this case, it may have been too much for the helo during landing.
It now appears that this was a special modified ‘stealth’ helicopter. The modifications included rotors that turn slower and provide less lift, but are quieter.