UH-1H missing from Coffs Harbour 6th Sept 2019
"...Separation..."
Why would that have been an end to an intelligent discussion. BlakMax is a world renowned expert in an interesting field that gains relevance by the day. His treatise on bond physics is fascinating, and when considered in light of various kludge repairs that get done on composites or bonded metal structures is of more than passing interest. His Boron patches were pretty darn good... Following the physics of a bonded edge with "additional structural" rivets clears the mind on the state of the designs we fly.
For the subject UH1H event, the wreckage mapping will tell a story, as will detail analysis, but the conditions make rotor separation a possibility, to be disproved by the debris field analysis. The instrumentation is interesting, lots of possibilities there, however driving around dependent on geriatric gyros without comparators etc is not much worse off than an iphone or ipad taped to the panel with their own mems sensors or using a modern external AHRS sensor, all of which work well, until the batteries fail, you get an inbound call, the ipad times out.. etc. My little 2 seat jet has the same gyro that came out of the USSR in the middle of the Viet Nam war, and the ipad is a desirable safety enhancement to relying on a T&S to keep the blue side upper most.
My recollection of the UH1 separation sequence is that it would end up with a strike through the cabin quite often. which is similar to the RHC mast bump cases. MR-tail boom impact other than blade sailing, seemed to be more prevalent with heavier single rotor designs where the TPP can alter rapidly and the fuselage pendulous response is slower, thinking of CH-53's and similar (not a mast bump case of course, they could get to stop pounding but not a characteristic mast bump as such).
Wasn't a nice day to go fly by the sounds of it.
Why would that have been an end to an intelligent discussion. BlakMax is a world renowned expert in an interesting field that gains relevance by the day. His treatise on bond physics is fascinating, and when considered in light of various kludge repairs that get done on composites or bonded metal structures is of more than passing interest. His Boron patches were pretty darn good... Following the physics of a bonded edge with "additional structural" rivets clears the mind on the state of the designs we fly.
For the subject UH1H event, the wreckage mapping will tell a story, as will detail analysis, but the conditions make rotor separation a possibility, to be disproved by the debris field analysis. The instrumentation is interesting, lots of possibilities there, however driving around dependent on geriatric gyros without comparators etc is not much worse off than an iphone or ipad taped to the panel with their own mems sensors or using a modern external AHRS sensor, all of which work well, until the batteries fail, you get an inbound call, the ipad times out.. etc. My little 2 seat jet has the same gyro that came out of the USSR in the middle of the Viet Nam war, and the ipad is a desirable safety enhancement to relying on a T&S to keep the blue side upper most.
My recollection of the UH1 separation sequence is that it would end up with a strike through the cabin quite often. which is similar to the RHC mast bump cases. MR-tail boom impact other than blade sailing, seemed to be more prevalent with heavier single rotor designs where the TPP can alter rapidly and the fuselage pendulous response is slower, thinking of CH-53's and similar (not a mast bump case of course, they could get to stop pounding but not a characteristic mast bump as such).
Wasn't a nice day to go fly by the sounds of it.
For the subject UH1H event, the wreckage mapping will tell a story
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For six days after the crash the seas in the area were wild, at this point the wreckage could be anywhere and post crash damage done in those wild seas would be extensive.
A very interesting contribution, thanks blakmax ! I would imagine that bonding composites to non-composites has had a lot of research put into in the development of the latest Airbus and Boeing products. Although in a fixed wing aircraft I can't imagine that there are stresses remotely similar to those encountered in a rotor system, other than the vibration aspects (which EK learned the hard way with flap fairings near the engines that needed replaced at well below their design limits due to cracking of the attachment points).
Without intending to be morbid, having watched many RNZAF UH-1Hs as a lad, and twice riding in one as a pax in the outward facing transmission tunnel seat - am curious.
How does a main rotor separation sequence on a UH-1 end up with blade going through cabin?
Each blade is ~25 feet long - and the mast anchors to the transmission at the rear of the cabin.
I would have thought on separation the rotor's inertia would have meant it continued `flying' and the fuse just dropped.
Does this just happen if you are in forward flight at the time - mast snaps - airload on disc causes it to rapidly pitch extremely forward, at the same time as the fuselage momentum continues through the blade arc?
Forgive the naive question - I am just a humble plank driver...
How does a main rotor separation sequence on a UH-1 end up with blade going through cabin?
Each blade is ~25 feet long - and the mast anchors to the transmission at the rear of the cabin.
I would have thought on separation the rotor's inertia would have meant it continued `flying' and the fuse just dropped.
Does this just happen if you are in forward flight at the time - mast snaps - airload on disc causes it to rapidly pitch extremely forward, at the same time as the fuselage momentum continues through the blade arc?
Forgive the naive question - I am just a humble plank driver...
The mast might break, but the blade is still attached to pitch change links from the swash plate, which could cause some ridiculous pitch inputs as the blade leaves its normal orbit. Thus the path thru the cabin / tail boom.
Loads on a rotor have highest peaks at any contact with ground from skid or wheels, and at entry into autorotation. The root strain there is much higher than any other time. Pitch link loads get high with the RTT type manoeuvre which is comparable to the conditions where jack stall can occur in one or two well known types. The blade loads otherwise are alleviated by hinges in the nain. The UH 1 rotor head is a robust structure although some in plane loads are absorbed by the structure and the coning loads out of plane are taken by the structure as well. TPP is damped by the counter weight input.. By comparison, those driving the elegant RHC 3 hinge heads in their ride should take care that the coning hinges have the right tension relative to the teeter hinge, the blade structure needs that relief, as does proper TPP control. If that head starts teetering from a coning hinge bad stuff can happen.
Just musin'
tartare, the usual cause of rotor separation due to mast bumping is a result of low "G'. In this case the rotor hits the mast at the nine o'çlock position and separates. Because the blade on the right is travelling forward and that on the left is travelling rearwards there is a dissymmetry of lift between the two blades, the right having greater and the left less, the disc then rolls left with respect to the fuselage and impacts the tail boom 90° following separation ie .046 of a second after separation, rotor RPM being 324.
A video on the subject.
Had the unpleasant experience of being on the side lines of a fatal mast bump in Vietnam. We had a formation of five aircraft for a troop lift, but on arrival only four were needed, me being #5 was delegated to suntan duty. I was on my second day of flying duty having just returned from ten days R&R in Hong Kong with my wife and anxious to get back into it, so tried to swap with #4 and let him suntan. Unable to persuade him he went off and some short time later he had a mast bump. One take off technique used was to accelerate to Vne on the tree tops and then cyclic (zoom to FW folk) climb to altitude. The push over at the top of climb was where you had to be careful not to get into a low "G" situation. Often used when you were heavy and/or likely to encounter gun fire. The wreckage was winched into the back of a Chinook and delivered home
The accident in question.
Information on helicopter 68-15568
A video on the subject.
Had the unpleasant experience of being on the side lines of a fatal mast bump in Vietnam. We had a formation of five aircraft for a troop lift, but on arrival only four were needed, me being #5 was delegated to suntan duty. I was on my second day of flying duty having just returned from ten days R&R in Hong Kong with my wife and anxious to get back into it, so tried to swap with #4 and let him suntan. Unable to persuade him he went off and some short time later he had a mast bump. One take off technique used was to accelerate to Vne on the tree tops and then cyclic (zoom to FW folk) climb to altitude. The push over at the top of climb was where you had to be careful not to get into a low "G" situation. Often used when you were heavy and/or likely to encounter gun fire. The wreckage was winched into the back of a Chinook and delivered home
The accident in question.
Information on helicopter 68-15568
Great video - it highlights the reason for the roll in low G conditions which is the TR thrust producing a right roll around the C of G.
the usual cause of rotor separation due to mast bumping is a result of low "G'. In this case the rotor hits the mast at the nine o'çlock position and separates. Because the blade on the right is travelling forward and that on the left is travelling rearwards there is a dissymmetry of lift between the two blades, the right having greater and the left less, the disc then rolls left with respect to the fuselage and impacts the tail boom 90° following separation ie .046 of a second after separation, rotor RPM being 324.
The disc is liberated from the mast usually at the 2nd strike of the shaft by the inner edge of the head, that is characteristic. At the instant the rotor mast fails, the disc is free to go it's own way. The wreckage I've reviewed has normally an intrusion of a blade into the cockpit, but not strikes on the boom. boom strikes and cutting of the boom have been seen with a gross cg shift from structural failure of the TR/TRGB. I've never seen any video of the instance of loss, if anyone has such video, it's probably not much fun for public presentation but would be worth looking at. I would suspect that the blade intrusion would occur from a pitch up of the fuselage due to the horizontal stab down force acting on the aircraft without the opposing nose down moment from the rotor.
Low g mast bump results from induced roll from the asymmetric thrust line of the TR which is above the CGz of the system. The low thrust results in increased flapping if opposing cyclic is applied to negate the roll, vs the correct recovery of loading the disk up with aft cyclic.
Not being difficult, just would be interested in any details that would otherwise describe why in mast bump cases blade impact into the cockpit is usually seen. Prouty, Johnson, Stepniewski & Keys, or Leishman give the dynamics for when there is a shaft connected, but nothing covers the disconnected case. Your comment on dissymmetry is definitely the case up to the point of failure, thereafter... best seen from a distance. If considered as a gyroscope, then the disc forward velocity would continue to give a different velocity to the advancing and retreating blades, which would suggest a precession outcome which would be a flap back of the disc. at the point of failure, as has also been suggested, the shift fails but the pitch links for a moment will remain attached until overloaded, and the input to the cyclic blade angle would be dependent on whether the control horns lead or lag the blade. The image below shows a blade and links, as does the photo of the head. if the head is released from the shaft, then then both pitch links are going to be reducing blade pitch by almost the same amount... The advancing blade will have higher resultant aerodynamic forces and going to a negative pitch is going to reverse the direction of flapping momentarily. musing out loud.. If the blade chops the tail off, then the flap back is the mechanism. If the blade goes through the overhead, then the pitchlink reversing flapping may be the process.
(Sun, Tan, Wang, 2013)
Tao Sun, Jianfeng Tan, Haowen Wang, Investigation of rotor control system loads, Chinese Journal of Aeronautics, Volume 26, Issue 5, 2013, Pages 1114-1124, ISSN 1000-9361, https://doi.org/10.1016/j.cja.2013.07.029.
fdr, go back and read my post #76.
That separation was from firstly a huge yaw/roll input from the tail rotor, then a large cg change with nose down pitch as the whole TR and gearbox separated. The advancing blade came around the nose and sliced off the left side and tail boom.
That separation was from firstly a huge yaw/roll input from the tail rotor, then a large cg change with nose down pitch as the whole TR and gearbox separated. The advancing blade came around the nose and sliced off the left side and tail boom.
The wreckage I've reviewed has normally an intrusion of a blade into the cockpit, but not strikes on the boom. boom strikes and cutting of the boom have been seen with a gross cg shift from structural failure of the TR/TRGB.
Of course you're correct in that it may come through the cabin, to quote an extract from one report, "As the fuselage continued into the right roll the main rotor blades sliced through the cabin 3 times; first severing the roof of the canopy, then the control panel, and then the cabin floor through the center. The accident wreckage revealed that the cyclic pushover must have been so abrupt that once the low-G situation was reached it would have been impossible to recover from".
Be interesting if anyone has written anything on rotor dynamics post separation, perhaps the combinations and permutations are so complex as to prevent any understanding.
Based on the partial information and rumors available.
It’s likely, that if mast bumping and subsequent rotor separation occurred, it was the final result of the above, and had they been on board a fully articulated or rigid rotor system equipped AC, the outcome would have been similar. Just more likely the blades would have been attached as they hit the ocean.
Best we can do is learn from this seemingly avoidable tragedy.
In hindsight, a night in Coffs could have been a great night out.
As mentioned, a night in the “Bell Hotel” would have been unremarkable.
fdr, go back and read my post #76.
That separation was from firstly a huge yaw/roll input from the tail rotor, then a large cg change with nose down pitch as the whole TR and gearbox separated. The advancing blade came around the nose and sliced off the left side and tail boom.
That separation was from firstly a huge yaw/roll input from the tail rotor, then a large cg change with nose down pitch as the whole TR and gearbox separated. The advancing blade came around the nose and sliced off the left side and tail boom.
The B yaw control pickup by the TR drive shaft was one of the inflight tail boom chops that came to mind. I had missed your #76 post. You were most fortunate not to be on that flight. The photos that DAFS had at that time were sobering.
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Human remains found from helicopter that crashed off Port Stephens three weeks ago
Police have located debris and human remains from a helicopter believed to have ditched into the water off Port Stephens almost three weeks ago.
Late yesterday specialist divers located and retrieved debris from the aircraft, sitting approximately 30 metres under the surface off nearby Birubi Point.
The aircraft disappeared off radar about 6.30pm on Friday September 6, crashing into the water and killing five people onboard.
It is still unknown what led to the helicopter's crash.
Among the five killed were Sydney resident Jocelyn Villanueva and her fiancé Greg Miller, as well as Jamie Ogden and his best mates Grant Kuhnemann and pilot David Kerr.
The 53-year-old Bell UH1 helicopter they were travelling in was nicknamed "Huey", and had been restored back to operational capability.
Following its disappearance from radar, a massive multi-agency search was conducted with support provided from a Royal Australian Navy Coastal Minehunter vessel, the HMAS Huon.
No emergency beacons were activated and there were no mayday calls detected from the crew onboard.
HMAS Huon provided underwater search and reconnaissance assistance to locate and identify the debris.
Detectives from Marine Area Command are continuing investigations working closely with the Australian Transport Safety Bureau.
Police will prepare a report for the information of the Coroner.
Late yesterday specialist divers located and retrieved debris from the aircraft, sitting approximately 30 metres under the surface off nearby Birubi Point.
The aircraft disappeared off radar about 6.30pm on Friday September 6, crashing into the water and killing five people onboard.
It is still unknown what led to the helicopter's crash.
Among the five killed were Sydney resident Jocelyn Villanueva and her fiancé Greg Miller, as well as Jamie Ogden and his best mates Grant Kuhnemann and pilot David Kerr.
The 53-year-old Bell UH1 helicopter they were travelling in was nicknamed "Huey", and had been restored back to operational capability.
Following its disappearance from radar, a massive multi-agency search was conducted with support provided from a Royal Australian Navy Coastal Minehunter vessel, the HMAS Huon.
No emergency beacons were activated and there were no mayday calls detected from the crew onboard.
HMAS Huon provided underwater search and reconnaissance assistance to locate and identify the debris.
Detectives from Marine Area Command are continuing investigations working closely with the Australian Transport Safety Bureau.
Police will prepare a report for the information of the Coroner.
Last edited by Senior Pilot; 26th Sep 2019 at 07:12. Reason: Add quote: this helps Rotorheads know what you're posting about!