Assault on Bin Laden: mystery of the downed chopper

[TomS]

So, if you want to see what the tail rotor of a "normal" crashed Black Hawk looks like, here you go. From the raid on Syria this morning that led to the death of ISIS leader Abu Ibrahim al-Hashimi al-Qurashi.

https://twitter.com/x/status/1489236861046431747

View: https://twitter.com/kasimileri_/status/1489236861046431747

 

[RavenOne]

Could they not have had a stealth Huey that didn’t have a mile long tail? Kamovs would be my choice for a confined area…no way to quiet contra rotating props for a smaller landing footprint?

Not the range and the skids …unless its the old ISTAR EH-1H Quick Fix I or in this case JuH-1H with retractable skid.

But Huey would not be ideal in long range intense SF work

Cheers

 

[RavenOne]

Has anything surfaced lately? It is a wild wild shot but wondering with regard to the ongoing Russian invasion of Ukraine, have seen an increase in United States Army Aviation assets between Poland, Lithuania, Latvia on FR24 or adsb exchange such as:

- Sikorsky UH-60M Blackhawks (either locally 12th Combat Aviation Brigade based in Germany or the forward deployed likes of 82nd Abn Div 18th Aviation Brigade et al )

- Fixed wing RC-12 (ELINT / SIGINT) Guardrail or ISR Super King Air 200

But the piece de la resistance 3 weeks ago a local enthusiast and professional photographer Dalibor Ankovic (owner of DA Photography) outside Ramstein AFB snapped pair of Nightstalker MH-47G (without AAR) probe landing at the base.

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Cheers

 

[yasotay]

Has anything surfaced lately? It is a wild wild shot but wondering with regard to the ongoing Russian invasion of Ukraine, have seen an increase in United States Army Aviation assets between Poland, Lithuania, Latvia on FR24 or adsb exchange such as:

- Sikorsky UH-60M Blackhawks (either locally 12th Combat Aviation Brigade based in Germany or the forward deployed likes of 82nd Abn Div 18th Aviation Brigade et al )

- Fixed wing RC-12 (ELINT / SIGINT) Guardrail or ISR Super King Air 200

But the piece de la resistance 3 weeks ago a local enthusiast and professional photographer Dalibor Ankovic (owner of DA Photography) outside Ramstein AFB snapped pair of Nightstalker MH-47G (without AAR) probe landing at the base.

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Cheers

*Jedi hand wave* These aren't the helicopters your looking for."

 

[RavenOne]

Has anything surfaced lately? It is a wild wild shot but wondering with regard to the ongoing Russian invasion of Ukraine, have seen an increase in United States Army Aviation assets between Poland, Lithuania, Latvia on FR24 or adsb exchange such as:

- Sikorsky UH-60M Blackhawks (either locally 12th Combat Aviation Brigade based in Germany or the forward deployed likes of 82nd Abn Div 18th Aviation Brigade et al )

- Fixed wing RC-12 (ELINT / SIGINT) Guardrail or ISR Super King Air 200

But the piece de la resistance 3 weeks ago a local enthusiast and professional photographer Dalibor Ankovic (owner of DA Photography) outside Ramstein AFB snapped pair of Nightstalker MH-47G (without AAR) probe landing at the base.

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*Jedi hand wave* These aren't the helicopters your looking for."

i know lol was referring to the unit's presence in Europe at this particular point in time ...

Last time officially the Nightstalkers were in Europe was for the Csar exercise Jaded Thunder which laughingly was held for the first time outside CONUS here in UK, albeit oop north at RAF Leeming. Their presence was a pair of MH-6 Little Birds ...ferried in back of a C-17A to the base. From what my memory serves me.... the list of participants include.

-352nd SOW from here at Mildenhall with CV-22B and MC-130J
-AFSoC AC-130H or W gunship
-442nd Fighter Wing Oa-10A Warthogs
-resident 100 Sqn BAe Hawk T1
-160th SOAR MH-6E

And going back quarter of Century - elements of 160th deployed to Brindisi AB in Italy for ops over the Balkans. Led to believe they were MH-60A/L.

NSDQ

cheers

 

[Q-nimbus]

Has anything surfaced lately? It is a wild wild shot but wondering with regard to the ongoing Russian invasion of Ukraine, have seen an increase in United States Army Aviation assets between Poland, Lithuania, Latvia on FR24 or adsb exchange such as:

- Sikorsky UH-60M Blackhawks (either locally 12th Combat Aviation Brigade based in Germany or the forward deployed likes of 82nd Abn Div 18th Aviation Brigade et al )

- Fixed wing RC-12 (ELINT / SIGINT) Guardrail or ISR Super King Air 200

But the piece de la resistance 3 weeks ago a local enthusiast and professional photographer Dalibor Ankovic (owner of DA Photography) outside Ramstein AFB snapped pair of Nightstalker MH-47G (without AAR) probe landing at the base.

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Cheers

I would be suprised if the SSSSHHH-Hawks are in Europe as I’m pretty sure there are no American boots on the ground in Ukraine, apart from military advisors and intelligence liasons (who are doing an incredible job by the way).

It wouldn’t be impossible if some are kept on standby, but still unlikely I believe. You might as well keep ‘m ready for immediate loading stateside as it’s much, much more secure.

Also, unless they want to drop some guys into Moscow, there’s not really an operational need for the SSSSHH-Hawks as the ‘regular’ Black Hawks can probably do the same job with much lower risk. The fact that Ukraine can penetrate into Russia with their drones proves you don’t really need something very exotic.

The most important thing to consider are the political consequences; if US soldiers were caught operating against the Russians that would be seen as an act of war, and rightfully so. The risk/reward ratio probably isn’t worth it.

Regardless of the above: at this point I’m very skeptical whether they are still operational or not. Actually, they never really were operational. We know with relative certainty that the SSSSHHHH-Hawks were taken out of storage specifically for the Bin Laden operation, and while the mission was a succes, we all know what happened to one of the helicopters… so I wouldn’t be surprised if they put them back into storage or maybe even buried them around Groom Lake (apart from the surviving helo from Operation Geronimo to hopefully be put on display one day).

I’ve also said that I believe the SSHHHH-Hawks weren’t very stealthy, but mostly very quiet. I would be surprised if there aren’t some upgrades/conversions available today for normal Black Hawks which more or less have the same effect.

So, no Stealth Hawks in Europe I believe (but I hope I’m wrong!)

Now, if someone would photograph an RQ-170 or maybe even a RQ-180 flying around Europe that would be very, very interesting indeed…

 

[yasotay]

I also doubt that the "special" MH-60 is still in service. At least not the one that survived the OBL Raid. The technology associated with that platform is now very old. Some considerations though. I do not think that the aircraft in question were drastically different from a conventional Blackhawk. No doubt there was some "clean up" with major radar reflecting components, but I am not inclined to think there was an entirely new fuselage to go with the platform. Perhaps a new tail boom and some special adaptation bolted to the nose. I think that the tail rotor itself was more of a noise reduction effort than specifically designed to overcome radar returns. The tail rotor is the largest noise producing component on the H-60, which is otherwise fairly quiet in forward flight. One also should remember that MH-60 are significantly heavier than stock UH-60 with all of the extra equipment they carry. So a suboptimal tail rotor in association with high weight likely makes the platform somewhat unwieldy in higher altitudes. There was likely some unique electronics on the platform as well.

 

[RavenOne]

Has anything surfaced lately? It is a wild wild shot but wondering with regard to the ongoing Russian invasion of Ukraine, have seen an increase in United States Army Aviation assets between Poland, Lithuania, Latvia on FR24 or adsb exchange such as:

- Sikorsky UH-60M Blackhawks (either locally 12th Combat Aviation Brigade based in Germany or the forward deployed likes of 82nd Abn Div 18th Aviation Brigade et al )

- Fixed wing RC-12 (ELINT / SIGINT) Guardrail or ISR Super King Air 200

But the piece de la resistance 3 weeks ago a local enthusiast and professional photographer Dalibor Ankovic (owner of DA Photography) outside Ramstein AFB snapped pair of Nightstalker MH-47G (without AAR) probe landing at the base.

Log into Facebook

Log into Facebook to start sharing and connecting with your friends, family, and people you know.

www.facebook.com

Cheers

I would be suprised if the SSSSHHH-Hawks are in Europe as I’m pretty sure there are no American boots on the ground in Ukraine, apart from military advisors and intelligence liasons (who are doing an incredible job by the way).

It wouldn’t be impossible if some are kept on standby, but still unlikely I believe. You might as well keep ‘m ready for immediate loading stateside as it’s much, much more secure.

Also, unless they want to drop some guys into Moscow, there’s not really an operational need for the SSSSHH-Hawks as the ‘regular’ Black Hawks can probably do the same job with much lower risk. The fact that Ukraine can penetrate into Russia with their drones proves you don’t really need something very exotic.

The most important thing to consider are the political consequences; if US soldiers were caught operating against the Russians that would be seen as an act of war, and rightfully so. The risk/reward ratio probably isn’t worth it.

Regardless of the above: at this point I’m very skeptical whether they are still operational or not. Actually, they never really were operational. We know with relative certainty that the SSSSHHHH-Hawks were taken out of storage specifically for the Bin Laden operation, and while the mission was a succes, we all know what happened to one of the helicopters… so I wouldn’t be surprised if they put them back into storage or maybe even buried them around Groom Lake (apart from the surviving helo from Operation Geronimo to hopefully be put on display one day).

I’ve also said that I believe the SSHHHH-Hawks weren’t very stealthy, but mostly very quiet. I would be surprised if there aren’t some upgrades/conversions available today for normal Black Hawks which more or less have the same effect.

So, no Stealth Hawks in Europe I believe (but I hope I’m wrong!)

Now, if someone would photograph an RQ-170 or maybe even a RQ-180 flying around Europe that would be very, very interesting indeed…

Interesting what do you mean by taken out of storage For the OBL mission?
was not aware they were.

cheers

 

[yasotay]

It is claimed (not officially confirmed) that the platforms were actually test vehicles, used at Groom Lake for analysis. Equally rumored is that they were not popular with aircrews who flew them. Popular account has it that they were maintained at a.certain level of airworthiness for missions such as OBL Raid, but not flown frequently.

 

[publiusr]

Can you fast rope while under autorotation?

 

[Manuducati]

Can you fast rope while under autorotation?

To fast rope you need a stationary bird.
Autorotation is basically using the helicopter movement to create lift, so no movement means no lift.

 

[rooster]

It is claimed (not officially confirmed) that the platforms were actually test vehicles, used at Groom Lake for analysis. Equally rumored is that they were not popular with aircrews who flew them. Popular account has it that they were maintained at a.certain level of airworthiness for missions such as OBL Raid, but not flown frequently.

All rumors

 

[publiusr]

Sometimes scuttlebutt is all you get. Even with above board civilian circles people fight like cats and dogs—a la real space.

 

[RavenOne]

It is claimed (not officially confirmed) that the platforms were actually test vehicles, used at Groom Lake for analysis. Equally rumored is that they were not popular with aircrews who flew them. Popular account has it that they were maintained at a.certain level of airworthiness for missions such as OBL Raid, but not flown frequently.

Not popular due to the handling ?? I guess the likes of the CofG would be affected.and wondered any excess loading on the airframe with the mods.

cheers

 

[yasotay]

@rooster - Correct

@RavenOne - The SOAR aircraft do have a lot more "equipment" at the front of the aircraft than standard versions of the aircraft. If there was "extra, extra" equipment on those particular aircraft, then the even more overweight special blackhawk would be a challenge in the hot high conditions that existed. The new tail rotor seems suboptimal for yaw control, perhaps on the very edge of controllability even in lower atmospheric conditions. Loss of tail rotor effectiveness is a very known phenomena with helicopters. This could explain the dislike of the aviators who flew them as they certainly prefer to have optimally maneuverable aircraft for missions (who wouldn't). As to the completely new stabilator on the rear of the aircraft, someone with more aero-engineering background will have to venture a guess.

 

[Q-nimbus]

I just came across an interesting bit in the book ‘American Cipher’ by Matt Farwell and Micheal Ames. It’s about Bowe Bergdahl’s ordeal in Afghanistan and Pakistan.

According to David Sedney, Deputy Assistent Secretary of Defense for Afghanistan at the time, there were two options drawn up to save Bergdahl: the ‘high’ option called for 8 MH47 Chinook’s carrying upward of 200 special operators and the ‘low’ option involved sending over a small team of Seal Team 6 into Pakistan in… stealth helicopters!

Not a single mission order was ever given though.

This was in 2009 and according to David Sedney, the plan was ‘updated, adapted and re-drafted’ in 2011 for Operation Neptune Spear.

While no definitive proof, it would seem the SSSSHH-Hawks were available in 2009 already.

Makes one wonder how long JSOC had the Stealth Hawks at their disposal prior to 2009…

 

[quellish]

@rooster - Correct

@RavenOne - The SOAR aircraft do have a lot more "equipment" at the front of the aircraft than standard versions of the aircraft. If there was "extra, extra" equipment on those particular aircraft, then the even more overweight special blackhawk would be a challenge in the hot high conditions that existed. The new tail rotor seems suboptimal for yaw control, perhaps on the very edge of controllability even in lower atmospheric conditions. Loss of tail rotor effectiveness is a very known phenomena with helicopters. This could explain the dislike of the aviators who flew them as they certainly prefer to have optimally maneuverable aircraft for missions (who wouldn't). As to the completely new stabilator on the rear of the aircraft, someone with more aero-engineering background will have to venture a guess.

The tail rotor appears to be optimized for both acoustic and radar signature reduction. Earlier programs to quiet (medium sized) helicopters had similar blade counts and a similar blade shape was also tested. The "cap" on the tail rotor is very similar to earlier efforts to reduce the RCS of helicopters. An ice-cream cone "cap" was part of the OH-58 RCS reduction kit. The tail is also shaped to reduce the RCS of the aircraft (much like the AGM-129)



That said, given this:



And what we know about past efforts to reduce the acoustic signature, I am very surprised that the wreckage of the Mystery Machine does not show a 5 blade rotor head. Not just that, but the 4 rotors appear to be standard Wide Chord blades. I would expect specialized blades, and more of them. The main rotor blades are a major contributor to both the RCS and the acoustic signature, but at least from what we can see are completely conventional - while the tail is very much not conventional. I do not think the tail would receive this level of treatment unless the main rotors had been addressed.

 

[yasotay]

Likely decision was to stay with standard rotor due to weight considerations. Also the dynamic components (rotors, rotor hub, drive shafts) are the hardest components to develop. A new rotor system would in essence have required a completely new aircraft. Compromises were likely made to have an aircraft in short order. It is possible that the aircraft, which likely resided at Groom Lake were originally developed for testing purposes with no real intent to use them.

 

[quellish]

Likely decision was to stay with standard rotor due to weight considerations. Also the dynamic components (rotors, rotor hub, drive shafts) are the hardest components to develop. A new rotor system would in essence have required a completely new aircraft. Compromises were likely made to have an aircraft in short order. It is possible that the aircraft, which likely resided at Groom Lake were originally developed for testing purposes with no real intent to use them.

They were based north of Groom Lake, the Army crews from Eustis and Campbell units commuted on the Beech Janets from Vegas for an extended period of time.

 

[TomcatViP]

With appropriate intelligence and signal analysis, they would probably have known what radar will be encountered and what lower grade rcs measures would have been sufficient (flight geometry, rotor rpm...).
It's possible that the aircraft comes in multiple configurations to reduce the risks of technology dissemination following a crash. For example, swapping a somewhat classical rotor hub to a more stealthy one would have adverted unnecessary risks with a sufficient level of penetrating capability.
???

 

[Hanz2k]

Likely decision was to stay with standard rotor due to weight considerations. Also the dynamic components (rotors, rotor hub, drive shafts) are the hardest components to develop. A new rotor system would in essence have required a completely new aircraft. Compromises were likely made to have an aircraft in short order. It is possible that the aircraft, which likely resided at Groom Lake were originally developed for testing purposes with no real intent to use them.

They were based north of Groom Lake, the Army crews from Eustis and Campbell units commuted on the Beech Janets from Vegas for an extended period of time.

Feel free to correct me, but the biggest source of noise is the interaction of acoustic waves between tail and main rotor - especially symmetrical ones. That’s why Fenestrons and NOTARs are so quiet. In Europe we have a lot of EC135 and many times I hear them 3-4 sec before they flight over me.

Looking into the picture - tail rotor looks more like non-symmetrical blade configuration from AH64 which should contribute to noise reduction while not presenting high risk from engineering standpoint.

 

[Nicknick]

Maybe the good old military glider could have been an alternative, it would be much easier to built with stealth properties than a helicopter and it would have been much quiter too. Of course, helicopter would still be needed afterwards to pic them all up.

 

[TomS]

Maybe the good old military glider could have been an alternative, it would be much easier to built with stealth properties than a helicopter and it would have been much quiter too. Of course, helicopter would still be needed afterwards to pic them all up.

If you look at the Bin Laden compound, there was absolutely no where you could land a glider inside. There wasn't even room for a safe helicopter landing, one reason for the fast-rope insertion.

At best, a glider puts you outside the walls in some fields across the street. One helo landed there anyway, after the crash, but without a team inside, scaling the wall for the assault would have been very dangerous.

 

[Nicknick]

True, but one additional glider (could be remote controled) could have been built with an amoured nose to break through the outer wall.

 

[riggerrob]

True, but one additional glider (could be remote controled) could have been built with an amoured nose to break through the outer wall.

Explosives are cheaper and lighter.

 

[TomS]

True, but one additional glider (could be remote controled) could have been built with an amoured nose to break through the outer wall.

I think that's asking an awful lot from an aircraft running into a concrete wall. Besides, debris from the glider would likely just block any hole you made.

In the event, the second team just climbed the wall. Which they could do because the primary team was already inside and pulling focus from the defenders.

 

[Fluff]

True, but one additional glider (could be remote controled) could have been built with an amoured nose to break through the outer wall.

I think that's asking an awful lot from an aircraft running into a concrete wall. Besides, debris from the glider would likely just block any hole you made.

In the event, the second team just climbed the wall. Which they could do because the primary team was already inside and pulling focus from the defenders.

And some places make a wall by burying rebar vertically and slotting the blocks over the rebar.

 

[CJGibson]

True, but one additional glider (could be remote controled) could have been built with an amoured nose to break through the outer wall.

... And stick a pair of MK108s on it. Or have I been looking at German aircraft for too long?

Chris

 

[Nicknick]

I don’t want to go deeply into the “what if” category, for this mission, there haven’t been any military gliders around, so it wasnt an alternative. Despite that, breaking a simple wall with a special purpose plane, isn’t a no go, you can easily break through most walls with a car. Keep in mind, that the Messerschmitt gigant could even transport a small tank.

Military gliders have been replaced with helicopters, which offer a lot of advantages, but if stealth becomes most important, they might be useful in future missions. Building a stealth aircraft for a single mission might seem be too expansive, but it looks that operating stealth aircraft is especially expansive because of high maintenance cost:

https://www.forbes.com/sites/niallm...-combat-aircraft-infographic/?sh=5c2d9c207da7

For a one-way plane, the maintaining cost might be very low as long as good storage can be provided.

One drawback of military gliders is surly the high risk during the landing, this would require some development work.
I

 

[yasotay]

Interesting...

Stealth Chopper: The Secret Technology Behind the Bin Laden Raid Is Back

While the Black Hawks—stealthy or otherwise—remain elusive, the announcement of agreements such as this could still be taken as hints on whether research and development on the technology is ongoing.

nationalinterest.org

 

[TomS]

Interesting...

Stealth Chopper: The Secret Technology Behind the Bin Laden Raid Is Back

While the Black Hawks—stealthy or otherwise—remain elusive, the announcement of agreements such as this could still be taken as hints on whether research and development on the technology is ongoing.

nationalinterest.org

Lots of questionable stuff in this -- it rehashes a bunch of info from the Drive article about that one "stealth" EH-60 photo with very basic info about the raid.

The GKN announcement is very routine. They make a bunch of composite structures (tail cones, doors, bits of empennage) for Black Hawks at a plant in Tennessee. They also opened a plant in Mexico a few years ago to make Black Hawk parts -- I don't think that means Mexico has Stealth Hawks.

 

[yasotay]

They also opened a plant in Mexico a few years ago to make Black Hawk parts -- I don't think that means Mexico has Stealth Hawks.

Well now, that would be something!

 

[RavenOne]

Well let’s see what’s next gen for Nighstalkers as this article indicates that half of their A/MH-6 and Mh-60 be replaced by high speed platforms come 2030s…

Half Of Night Stalker Black Hawks, Little Birds Replaced With High-Speed Types By 2030s

The Army's elite 160th SOAR is set for a major transition, but it will retain some MH-60s and AH/MH-6s for certain missions.

www.thedrive.com

cheers

 

[TomcatViP]

"So, what we're looking to do is we're looking to evolve our existing aircraft and go hybrid electric. We're starting a program with DARPA in 24 [Fiscal Year 2024]. We're going to go out with a limited competition to take the 530 - we've actually acquired the aircraft - and what can be done to reconfigure the aircraft with hybrid electric [power],” he said. “The studies that we've done show that you can get anywhere from 25 to 100 percent increase in speed. So, if I'm flying in at 90 knots now, I can get 170-180 knots. That's huge."
The "530" Downer referred to is the Hughes 530 helicopter family, now produced by MD Helicopters. The 160th's current generation Little Birds are related to the 530 design, but Boeing is the prime contractor for those helicopters.
“It gives us an ability to have a platform that can keep pace with the other aircraft that we're going to be receiving from the Army with the FARA and FLRAA,” the head of SOCOM’s PEO-RW continued. “Or we can get anywhere between 25 to 75 percent increase in range.”
These claims are stunning to hear. Whether they will materialize is another story. If they come anywhere near those figures, that would be a massive enhancement.

Massive improvement indeed. I can't stop myself thinking that who would achieve that with SOCOM money, will have a very profitable days ahead on the mobility market.

 

[yasotay]

It will likely have to be a significantly redesigned 530 to get 170 knots

 

[quellish]

It will likely have to be a significantly redesigned 530 to get 170 knots

Redesigned to be a Prius with rotors!

 

[yasotay]

It will likely have to be a significantly redesigned 530 to get 170 knots

Redesigned to be a Prius with rotors!

...driving off a cliff

 

[Scott Kenny]

One thing that occurs is that there may be additional/other benefits from 'tricking out' a stealth helo.

A few years ago I spoke to the UK EH101 project manager and he pointed out that although the BERP rotors were designed for greater efficiency, and produced lower noise too, an unexpected advantage was they hugely reduced the likelihood of 'brownout' in hover/landing. He mentioned this included the insertion of troops - makes hovering safer so you can do it to reduced limits (e.g. at night), and inside the rotor disc (IIRC) the air is clearer, for the troops to see the ground etc.

So, although stealth may be a factor, maybe the 'tricked out' MH-60s (if so they be) were used because they offered additional attributes than low noise. Could explain why other, noisy, helos were used too, if they were.

I could definitely see the SOCOM types liking that!

Good point about slowing the rotor and VRS. Also, if the main rotor is slowed for acoustics then the tail rotor would be slowed in the same proportion. The tail rotor on the vehicle looks smaller and would have a higher disk loading and only a modest increase in solidity. The tail rotor would probably need to be sped up. This would point away from a simple bolt on kit unless an entire new tail pylon is bolted on at the tail fold hinge. A change in gear ratio might explain the seemingly oversize gearbox fairing compared to standard.

Combined responses:

If this is a UH-60 derivative, the changes are more than skin deep.

Compare the rotor head pitch link attachment of the wreakage (top image) with one from the UH-60 (detail from a photo taken here http://www.b-domke.de/AviationImages/Rotorhead.html#Sikorsky.)

This is definitely different. It IS a Sikorsky helicopter, but it is quite different than the UH-60. Maybe it has one or two additional blades and the rotor hub has been entirely redesigned.

Combined responses:

Likely decision was to stay with standard rotor due to weight considerations. Also the dynamic components (rotors, rotor hub, drive shafts) are the hardest components to develop. A new rotor system would in essence have required a completely new aircraft. Compromises were likely made to have an aircraft in short order. It is possible that the aircraft, which likely resided at Groom Lake were originally developed for testing purposes with no real intent to use them.

Back when this first wreckage showed up, I had an idea: H-3 transmission and rotor hub (though it looks like the wrecked hub was a 4-blade not 5) with H60 rotor blades. The H3s have a 62ft rotor diameter, H60s have a 52'8" rotor diameter. If I've done my math right, that's a 17% slower blade tip speed. Though I suspect that the better transmission would have been from an H53, as the H3 transmission is limited to 2500hp. H53 transmission would give even slower rotor tip speed, too, 27.8% slower than that of an H60.

 

[publiusr]

Now Bin Laden's compound had a tall section and a solid wall perimeter...as opposed to low-buildings and chain-link fencing where you might imagine testing done.

Might the design of the compound have impeded flow?

 

[Scott Kenny]

Now Bin Laden's compound had a tall section and a solid wall perimeter...as opposed to low-buildings and chain-link fencing where you might imagine testing done.

Might the design of the compound have impeded flow?

That's what I remember reading on another forum at the time of the raid. The chain link fences let the rotor downwash flow away from the area, while the solid walls forced the downwash upwards where it could get grabbed by the rotors again in a vortex ring state.