CH 37 Mojave - fly with a blade missing???

[Agaricus bisporus]

The single main rotor was five-bladed, and designed to function with one blade shot away in combat.

This quote from Wikipedia - sounds too far-fetched to be true, surely nothing could survive that sort of vibration? Or is wiki printing crap?

Does anyone know more?

CH-37 Mojave - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/CH-37_Mojave)

[SASless]

Perhaps John Dixson might care to weigh in on this one.

[JohnDixson]

Your second sentence re the Wikipedia article is accurate. I do recall hearing the same BS when I was in the Army.

Thanks,
John Dixson

Perhaps some additional history would help. In early 1966, two months before I got back from Pleiku and joined Sikorsky, the structural test prototype CH-53A was doing the buildup to the formal structural demonstration to be done at NAS Patuxent River. The crew was having difficulty getting the -0.5 "G" pushover done. On that machine the yaw boom out front had the yaw ( sideslip ) vane and an instrumentation pitot tube. The boom was mounted from the very center of the nose and secured/stabilized by four cables attached to a solid aluminum block thru which the boom protruded. This block was roughly 1/2 way to the tip of the boom. The yaw vane was attached to the top of the forward end of the boom, and in their so far unsuccessful attempts to achieve the required negative G, the main blade (s) had contacted the yaw vane sitting on top of the end of the boom. Since they were pretty close to the -0.5 G, the flight test team decided to take the yaw vane from the top of the boom end and put it on the bottom of the boom end. I should add here that were perhaps apocryphal reports that in their quest to get this manuever out of the way, the crew had even set the collective trim at low collective, pulled against it to set the ship up at the correct speed, and then let go of the collective, letting the trim actuator spring force snap the collective to full down.

Anyway, on the next flight the crew did the pushover and a main blade contacted the aluminum block, resulting in that blade failing at that point and setting up a horrendous 1P balance/vibration which in turn resulted in the tail boom failing. And that is a six bladed rotor.

Just to be clear, the problem is the asymmetry/unbalance. In the 80's we did a three bladed program on the CH-53D, looking at very high blade loading effects. Those three blades were mounted on the standard six bladed head, but symmetrically, thus avoiding balance/vibration issues.

[SASless]

BS in the US Army......tell me it ain't so!:ok:

[Brian Abraham]

John, while you are here talking about the 53 can you fill us in what happened in the following video. I recall seeing a similar video where the tail boom resonance was much more pronounced (caveat on memory) and think the aircraft was in a high hover rather than sling loading. Many thanks.

lWh8QXHQEUg

[Ned-Air2Air]

Poor Bastard - survived the crash and died from a broken neck after being dumped off the stretcher :eek:

[JohnDixson]

An IAF ( Israeli ) 53C as I recall. Prior to the tail boom failure, one can see the horizontal tail oscillating vertically at a large amplitude, and most likely at its first vertical bending mode frequency, somewhere in the 3-6 hz range. I cannot recall if this was a PIO caused situation, but will check. The 53 A,B,D and C all had the same built in collective friction set at about 3.5 lbs thru a phenolic block. That is, with the collective trim release trigger depressed, there is still a running friction of 3.5 lbs as the collective is moved. Having zero friction on the collective was a setup for a PIO ( typical to many machines ). PIO is: Pilot Induced Oscillation.

Thanks,
John Dixson

[Winnie]

With regards to the earlier question, I have a picture in a book of a RN Sea King that had to ditch due to severe vibrations, and after setting down on the water and shutting down, another machine took a picture as it rolls over (crew already egressed), with about a foot missing of the blade.

Also, in Norway in the 70'ies, there was an S-61N that shed a blade from the root, and there were no survivors. Had a relative as a pilot on that flight...

[SASless]

The Chinook also used a mag brake on the Thrust (Collective) Lever. Over time the brakes could lose effectiveness or a Pilot might hold the release button for too long a period....and a very small vertical bounce of any kind could turn into a Bronco Busting experience where you prayed for the Eight Second Bell!

The most effective cure was to lock the Lever in place by wedging a knee under it and firmly holding it down against the knee.


Winie,

As I recall....Helikopter Services lost two 61's due to that problem. I remember the first one clearly as I was flying that day. Sorry to hear you lost someone. We all experience a sense of loss when those kinds of tragedies occur.

[topendtorque]

No surprises there with the BS in wikipedia either.

It used to be a free to air sight, meaning that any goose could go on there, and publish non peer reviewed papers or anything pretty much and have it posted. Maybe that is what happened here.

Anyhow along came the great global warming debate, sorry world government power grab and get rich quick some Indian jerk along with his mate Big Al debate, NTM the totally non peer reviewed IPCC rubbish.

Propagandists from both sides of the argument jumped on the 'Wiki' free propaganda gravy train.
But no, no more, as a couple of warmests got themselves on as moderators and blocked all the "skeptics", so's all you will see is one side of the story.

Don't believe me? Write some story up there about the polar bears multiplying and see what happens.

And - just the other day the jerk that claims ownership of the site had his hand out for some free loot, ho ho, like 'ell.

[JohnDixson]

SAS: Had a Colonel out for a fam ride at Rucker early in 1965, who started one of those vertical bounce/PIO events, when he touched the forward gear down with more than necessary vertical speed. Had to reach over and yank his hand off the collective, then explain why a 2nd Lt had treated an O-6 thusly.

Did get a response from a still active pilot friend, who reminded me that the USAF PAVE LOW machines received a longitudinal cyclic damper later on. He didn't think that the IAF had shared their accident investigation results with SA.

I failed to mention that with regard to external loads, there is always another possible source for a vertical dynamics issue, and that is the spring rate of the sling, and whether that spring rate results in a sling/load natural frequency that matches any of the fuselage first bending modes. Those bending modes ( speaking of the single rotor models with which I am familiar ) are usually in the 4-7 hz range, and this frequency range is, as I alluded to earlier, a range where we pilots can get into the act quite easily.
As one can imagine, this subject gets a bunch of attention during early development. Another note here is that as I was reminded, the possibility of a longitudinal cyclic PIO has to be kept in mind as well. An example of this possibility occurred during the early external load testing of the 53E, where evn with collective friction and damping, a closed loop longitudinal AFCS response ( in response to a control bump to try and excite things ) resulted in an oscillation between AFCS and the fuselage tail that was getting rather too exciting until the load was touched down. Solution was a filter in the longitudinal AFCS.

Thanks,
John Dixson

[THM]

Can't remember where I found this but it's from a magazine showing testing of a three bladed CH-53.

Not sure how the crew would remove the damaged blade and two others while in a hostile area considering it would take at least six guys to lift one blade.

http://norrpress.co.uk/3bladech-53.jpg

[Jack Carson]

In this instance the AFSC servo frictions and the resulting light control forces at the cyclic control became very low over time. The low control frictions combined with the very high gain task of placing a heavy boundary marker in a precise position placed the pilot in a condition where PIO (Pilot Induced Oscillations) started the tail wagging. I believe it was the lateral control axis that was suspect on this aircraft. Once the oscillations at approximately at 3 Hz became extreme the both pilots were being thrown about the cockpit rather violently, thus allowing PAO (Pilot Assisted Oscillations) to become the driving factor until the tail failed. Also in this mishap the tail may have been damaged on the previous attempt to place the marker in position. The first wave off was successful. The tail failed on the second attempt.

The solution to this event was to install mechanical flight control dampers in the pitch, roll and collective axis similar to those used on the 53E series aircraft. I don’t believe that there have been any similar events since the installation of the dampers.

[JohnDixson]

As I mentioned in a previous note, that 3-bladed test had the purpose of investigating very high blade loadings. There was no intention of providing guidance for a fly-home envelope with three blades, although it made for interesting bar-talk.

Thanks,
John Dixson

[HMH361GRUNT]

"Shot away" and "shot off" might be the different in lingo here. Yes the birds were built to sustain heavy damage to them. In school for ch-53's we were taught blade tracking and balance. In maintenance you always get the reports of how badly the blades can be damaged and the result. it seems as long as 75% of the main spar is left or 75% of the entire blade weight you can do an emergency landing under 500 ft. While cruising that # is significantly lower apparently. This isn't in any reports' or 'studies' but just good old maintenance knowledge. fun forum.

[HMH361GRUNT]

The Ch-53Es had damper attached to the flight controls right beneath the pilot and co pilot seats . You accessed them through the electronics bay doors on the right or left side of the front lower nose. I remembered an issue with our bird porpoising up and down and we replaced the dampeners and then the servo servo itself .... in this accident I see it is a A or D model we transferred to Israel in the 90 s .... I have seen these throw blade weights off and the nuts that hold on the tail rotor blades but I am sure on this instance John has it right. i

[jim63]

http://www.popasmoke.com/visions/images/image_4225.jpg