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Comment by rkon | Hacker News Reader

From the Army Times article:

That crash landing might have been caused by a phenomenon known as “settling with power,” which occurs when a helicopter descends too quickly because its rotors cannot get the lift required from the turbulent air of their own downwash. “It’s hard to settle with power in a Black Hawk, but then again, if they were using one of these [low-observable helicopters], working at max gross weight, it’s certainly plausible that they could have because they would have been flying so heavy,” the retired special operations aviator said, noting that low-observable modifications added “several hundred pounds” to the weight of the MH-60, which already weighs about 500 to 1000 pounds more than a regular UH-60 Black Hawk.

http://www.armytimes.com/news/2011/05/army-mission-helocopte...

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A WSJ article stated that the high walls around the compound induced a vortex ring effect. http://online.wsj.com/article/SB1000142405274870456940457629... In a normal landing the rotor downwash would spread out across the ground but in this case the walls channeled the moving air back up and into the rotor again, causing it to lose lift. Of course I don't know whether that's true or not, but it seems plausible. https://secure.wikimedia.org/wikipedia/en/wiki/Vortex_ring#V...

There's also Loss of Tail Rotor Effectiveness[1] (LTE), caused by the tail rotor entering a vortex ring in much the same way the main rotor is affected in settling with power. LTE will cause a helicopter to yaw sharply in a direction opposite the main rotor's rotation.

The position of the wreck, with the tail broken over a wall, makes me think there might have been a yaw problem. Or they might have been in a high hover, had a problem, and hit the wall on the way down.

[1] http://en.wikipedia.org/wiki/Loss_of_tail-rotor_effectivenes...

The layout of the compound definitely had some design or goal to it. To my eye I couldn't tell what the effect was--religious was my first guess, but what you're saying makes sense. After all, he was 1800 yards from the Pakistan ~WestPoint. Coincidences of that magnitude don't occur flippantly.

I'd love to know why this was downvoted. Sometimes HN is a really hostile and narrow-minded place.

I can't say why it was downvoted, but my reaction to your comment was that it was completely off-topic to the post you replied to: the parent post was talking about vortex rings off the wall and your post speculated about architecture. Also, you made strong claims with no evidence (religious motivation for the walls); the stronger the claim, the more support it needs.

Please reread the comment "To my eye I couldn't tell what the effect was--religious was my first guess, but what you're saying makes sense." Which is pretty inconsistent from what you said "made strong claims with no evidence (religious motivation for the walls); the stronger the claim, the more support it needs."

I made no claim whatsoever, I divulged my first guess/naive speculation and said at first glance from my uninformed perspective I did not know exactly what the intentions were behind the architecture, but that I thought maybe it had to do with religion--him being a religious extremist.

I replied to the parent comment because I thought maybe the architecture was designed with the goal of producing vortex rings--making it harder for a helicopter to allow to allow people to repel.

You're also a hypocrite considering the comment you made previous to the one I'm replying "This is kind of off-topic, but I've always wondered...:http://news.ycombinator.com/item?id=2515028

How is that any different than what I did, if not worse?

Sometimes HN is just looking to be overly critical and point out flaws that only exist if you obsess over being pedantic.

Because you are making a couple leaps of logic. The compound definitely has a purpose to its shape? Maybe that is just the shape of the property they could buy. My property is kind of a trapezoid instead of a rectangle, thats just the way I bought it. And I highly doubt the walls were made to create vortex rings in helicopters. More plausible explanations are preventing people from seeing the inhabitants and slowing down ground based assault. I'm sure Osama knew that SOF can fast rope in on the inside of the walls, but the wall makes them either come up and breach (giving the defenders more time to react) or forces the SOF to drop in from a helicopter right over the property which gives the defenders an oppurtunity to shoot down the helicopter with small arms fire or an RPG. Not to mention the courtyard is overlooked by a 2 story building that makes it a shooting gallery. Those are far more likely reasons for the design than thinking Osama fired up some CFD program and designed his stone walls to mess with the rotors of a helicopter.

Reading comprehension.

So you have never speculated on something which there is no definitive answer for?

I said yes there was some purpose behind the shape, look at it, and like you said, it could be just the plot that was available, but... like I said the very first comment it was a "guess" so it's really not a leap of logic and my point was he had a hard to breach compound located conveniently near a military base--a highly suspicious coincidence."Those are far more likely reasons for the design than thinking Osama fired up some CFD program and designed his stone walls to mess with the rotors of a helicopter" Strawman much? Did Osama fly the planes into the World Trade Center? No. My point was the Pakistan military could have provided the safe-house. I never said I knew whether the walls were designed to make vortex rings, or conceal OBL, or make it harder to breach by a commando team, or if it was religious--but clearly it's not a randomly designed shape.

My point is people, like yourself and prewett, get off on down-voting people, arguing over some minutia in an attempt to achieve a superiority high, and wasting everyone's time arguing about some inconsequential aspect of their comment. I never said all along I was certain I said it was a "guess" so please spare me your smug high and mightiness about "making a couple leaps of logic."

Why don't you try reading a comment in its entirety next time?

Suggestions from one of your downvoters:

1) Keep it civil.

2) If you are going to make large jumps of logic like the one you've made, then that's fine. Just spell out what you are saying and why you are saying it because as it stands, I agree that your original comment seems very offtopic.

1. Did you even read my previous comment?

2. I made no "large jumps of logic" like I said, it was open speculation. Now please get off your high horse and quit repeating things that have already been said--everything you assert in your second point is proving wrong in my previous comment.

Do you not realize you're just discouraging people asking questions? That's essentially what you're advocating.

1) Yes I read your previous comment, that's why I'm telling you that you should be more civil.

2) You asked for an explanation, so I gave you an honest one. If you don't want to hear it, don't ask next time.

In the end, isn't the compound just oddly-shaped because whoever built it bought an oddly-shaped piece of land to put it on?

Given the level of skill, training and experience a pilot would need to be allowed to fly that thing, and after rehearsing the raid in a life size replica of the compound - I think it's unlikely.

Even doing a PPL you learn about the effects of vortexes, wind shear, turbine wash etc. And these guys have many, many, many orders of magnitude more training than that.

More likely some combination of weight, altitude, excess speed... but I guess we'll never know.

Apart from the obvious, such as landing under fire, dealing with the effects of the other helicopter in a confined area, or simply pilot fatigue after the fast, low flight to get to the compound, it could be some freak effect like that.

Flying deep into Pakistan undetected and landing in Bin Ladin's backyard is a hell of a trick, no ordinary pilot could do that.

"working at max gross weight" unlikely since the other heli would've been over weight (carrying both teams) and surely suffered the same fate.

I heard it was hot and high conditions that forced it to go down http://en.wikipedia.org/wiki/Hot_and_high

I did read at first that one second heli evacuated both teams after the first one crashed. But I also later read somewhere else that they dispatched a third heli after the crash?

The articles I saw indicated there were four choppers total. I'm not sure whether they were right there or just nearby, but the article gave the impression that they were there in case they had to engage the Pakistani Air Force.

Surely if they did need to engage the Pakistani Air Force (and it's claimed that they had Pakistani permission for the raid) they'd want fighter jets, not extra helicopters?

Presumably they had those too, we don't know. It does make quite a bit of sense to have a backup in case something goes wrong, even under the high-stealth circumstances.

Remember that we have no idea what was in those helos, they could've held more SEALs, or sophisticated medical equipment, or even heavy weapons (although if they were also blackhawks that's less likely.)

Oh, certainly. I agree, the extra helicopters would have been kept at a safe distance just in case one or more of the others had a problem. They've obviously learned a lot from Operation Eagle Claw (this is required reading for understanding this raid):

http://en.wikipedia.org/wiki/Operation_Eagle_Claw

There were four total, but only two were sent at first with the other two as backup. When one of those went down, one of the backup helicopters was sent in.

This is the importance of backups.

Remember that they would have burned a lot of fuel by that point, so they would no longer be a max gross weight.

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