Ejection systems in Helicopters.

Helicopters and tilt-rotor aircraft
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skrip00

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Unread post29 Sep 2006, 21:08

Also, take into consideration those two choppers fly higher and faster than most normal choppers.
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habu2

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Unread post29 Sep 2006, 23:09

Six seconds sounds like a lifetime... (no pun intended)
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kmceject

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Unread post03 Oct 2006, 06:33

Ejection seats in helos were rejected for several of the reasons given above in US service. The main one is that it takes time to eject that you don't have at low level. Systems tested included jettisoning the entire crew module, jettisoning the seat, or rocket extraction. The K-37-800 is an extraction system.

All three cases depended on jettisoning the rotors first. This leads to a VERY rapid sink rate on a helo and that makes it likely that the ejection sequence would be interrupted by ground interface at low levels. Not to mention as RoAF said it would be rather hazardous to fly in tight formation as most pilots are taught to do so for mutual aid. In the 80s in NYC where I lived a helo had a mishap atop what was the Pan Am building at the time (Met Life now) and it shed a blade that flew about eight blocks before intersecting a pedestrian, literally...

MKopack- great points, but extraction seats are significantly lighter than ejection seats, less than 100lbs vs 150-250lbs for seats (crew capsules start around 450, modules way up from there...). When you consider the weight of an armored crew seat on a attack helo it isn't that great an excess weight, and with modern engines helos typically have enough torque to spare a little if needed.

One other quick point is the K-37-800 pulls the crew out at an angle, leading to potential injuries. That was a problem when they tested the YANKEE a long time ago in AH-1s. I have reports on this stuff somewhere, but haven't had time to review them.

Kevin
The Ejection Site
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kmceject

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Unread post10 Oct 2006, 18:44

Did a little review of a 1973 vintage publication (for NATO countries) on the issue. The authors investigated several different concepts and had some interesting things to say about the issue. Manual bailout was listed as a decent concept for a low cost retrofit, but the min altitude issue was very problematic given the statistics. Sideward extraction was also a retrofit possibility, but with issues. Should a two-man tandem design like the Apache be fitted with extraction systems for example the fastest method would be simultaneous extraction in opposite directions (hoping the rotor disk wasn't too low on one side...) This would be problematic in the position of the collective interfering with one crewman's extraction.

For new design/built aircraft investigation was done into upward ejection or extraction, sideward ejection with L-shaped trajectory, or escape capsule. For upward ejection/extraction the rotor severance issues come into play. Concepts were pretty interesting including severing the rotor mast (which they suggested would need a method to raise and lock the collective inputs prior to that to allow for a predictable flight path of the disk.) Others included sequenced and simultaneous severing of rotor blades. Vibration was an issue, as well as risk of entanglement with the egressing crew and other vehicles in the area. Testing was done on sequenced ejection of blades in a set azimuth. Don't be in a formation in that direction and you probably would be safe! Simultaneous blade severance came in two flavors- all at once or pairs (in even number blade disks only). Technically all these concepts were reasonably valid, but most required a significant test progam to validate.

The L-shaped ejection path was one of the most interesting, but probably psycologically impossible one to accept. Rather than jettisoning the rotor, the seat would fire sideways (at up to 11G) and fly horizontally until it cleared the rotor disk, then rocket upwards for clearance. Technology for this existed in '73, but would have required a long, expensive test program to make it worthwhile. It also would require significant design changes to the cockpit to allow for the ejection path. (Stowing of cyclic/collective and side consoles, etc.) In the 1990s the CREST and 4th Generation Ejection Seat programs showed that complete trajectory control of a seat (with accelleration rate controls too) could be accomplished if desired so this concept would be easier to do now if the requirement came up.

Escape capsules as defined in the publication would be severance of non-essential portions of the vehicle and recovery of the crew areas by parachute cluster. This would have to be a blank blueprint incorporated design, and would have had many issues that would make it difficult. The diagrams in the report of a CH-47-class bird with six parachutes or the CH-53-class design with six parachutes and a huge group of airbags below is rather interesting.

Beyond this report I reviewed a video I have of a test using a CH-53-class bird on a rocket sled at Holloman AFB (846th Test Squadron) using a YANKEE or RANGER Extraction system. The sled seems to be moving at a low (100kt or less) rate and the rotors are spinning. They are severed on an azimuth of about 2 o'clock sequentially then the roof seems to explode in shards of plexiglass and the two rockets are simultaneously launched out by catapult. The ignite at the end of the 3 meter lanyards and extract the two 'pilots'. It is a very quick clip so I can't tell much more than that, but I'd say there were no rotor entanglement issues (although the trajectories were not stable), and the extractions appeared clean.

I know AH-1 Cobras were similarly tested and the system was found to work reasonably well. I am not in the 'know' on why they were not produced but seat manufacturers have told me that they could make a system IF a requirement ever came along from the 'customer'.

By the way, the report conclusion is very intense on its insistance that an escape capability should be provided as soon as possible. I do know that crew seat improvements have made it significantly more safe to crash land than the versions available when the report was written. Most ejection seatseat manufacturers have designed impact absorbant crew seats for helicoptors for example...

Kevin
The Ejection Site
ps as to weight increases they list some 60kg per crewman for ejection seats and less than that for extraction systems. These weights were for 1973 technology though. The McDonnell-Douglas Minipac and Martin-Baker Mk 15s were designed later and are significantly lighter/smaller ejection seats for trainers and for consideration for helicopter use.
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Roscoe

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Unread post11 Oct 2006, 16:04

That Holloman video you describe was one of several that were shown at the college lecture I mentioned...all the others were abysmal failures. As a tester and engineer I accept that failures often happen on the road to success, but those were brutal, the failure mechanisms were different in every case, and the final one that eventually worked was deemed to be "lucky" by the lecturer. An analysis was then done to see how many crews would actually be saved even if it did work as designed...it was never intended nor possible to be a zero-zero system so there were a lot of flight conditions where it simply would not help and the crews had a better chance of surviving if they would try to set it down instead. Turned out to not be a worthwile investment.
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kmceject

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Unread post11 Oct 2006, 17:45

Roscoe,

Just to be clear, your earlier post seemed to indicate the blade sep method was the problem. The YANKEE is a proven system, and I have several 0-0 videos of it so I know that works for fixed wing. It was even live tested as the 'away' system in the days before manikins got all the fun jobs. The issue to me is more that the rotors were always the main problem.

Kevin
The Ejection Site
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LWF

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Unread post11 Oct 2006, 23:07

Besides, most helicopters don't need ejection seats since they do better by auto-rotating down. Unless the tail is taken out, then they spin down.
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Unread post12 Oct 2006, 04:32

That's how it was presented. But that was also over 20 years ago so I trust your judgement on this one. Either way, it came down to a business case that it wasn't worth it. Any Helo so beat up you can't autorotate will probably not be stable enough to allow aircrew extraction.
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