Sikorsky X2 coaxial heli developments.

[Graviman]

Originally Posted by Ian Corrigible

Good to see Alastair Campbell has found a new job. The X2TD's problem has gone from being a "transmission failure" to the "detection of a gearbox manufacturing problem" to being "currently out of service for transmission upgrades."

Ian, this failure will have resulted in a huge investigation into root cause within Sikorsky (sometimes called a six sigma investigation). The engineers involved would have systematically checked out each possible reason in turn until establishing exactly what the trouble was. This is the only way to be sure that it does not reoccur. While doing this exercise nobody would have understood exactly what the failure meant.

I mentioned earlier about the fatigue analysis depending upon many coupon tests representing the part. These all assume that the part was manufactured to a fixed process. With prototypes the process is often still in development so can be subject to more variation than anticipated. On a highly stressed part this variation is tested even more so. This does not mean anybody made a mistake, just that the manufacture of the design is still settling down.

Clearly it is not an ideal situation, but that is why flight testing is so conservative. It was picked up in a ground test specifically designed to pick up this nature of problem. Nobody was put at risk and the problem has been resolved.


Dave, that intermeshing config would not be immune from this type of problem. Don't forget that these are very high hinge offset rotors designed for a machine with a snappy response. This application would make even the most hardened stress engineer squirm in his boots.


That is just the devlopment of any engineered product. You can take my word for it that the car you drive will have seen many many problems at prototype, which were all resolved for production. X2 is a high profile project so you are getting to see the development process up close and personal.

[JohnDixson]

X2 Airfoil

Dave, in my previous note I was referring only to the S-61F's prior experience with cruise main rotor autorotation, as applied to the background with which the ABC proceeded. Byron Graham was a pilot in each.

Your sentence with re to the X2 airfoil is to the point: the airfoil is the most complex I have seen on a flying prototype, and to date I believe that the X2 team is happy with it. The next testing phase is where we shall find out.

Thanks,
John Dixson

[riff_raff]

CEFOSKEY,

I find the explanation of "metal flaking" in the X2 MRGB quite reasonable. This "metal flaking" is most likely due to case spalls on the bearing races or gears. It is typically a fairly benign failure mode that is easily picked up by the lube system chip detectors. And if you're going to have a drivetrain failure, this is the type to have.

In the particular case of the X2 MRGB, I would speculate that the metal flaking is probably from the final stage gears. The reason I say this is that the X2 appears to have a fairly rigid rotor system with high mast moments. It is very difficult to isolate the MRGB output gear meshes (usually a planetary gearset) from these high mast moments, which tend to cause misalignment at the gear mesh points. The misaligned planet gear meshes tend to edge load and/or not load share properly, which in turn leads to excessive tooth contact stress levels and premature failures like case spalls or pitting.

Designing a lightweight transmission for use with a rigid rotor system is a very difficult engineering task. Sikorsky's drivetrain group definitely has their work cut out for them. I'm sure more drivetrain problems will continue to pop up as they push the performance of this aircraft. It's just the nature of the business.

riff_raff

[Dave_Jackson]

Food for thought

Perhaps Nick has the answer.

He was involved with the patent application 20070125907 ~ Variable speed gearbox with an independently variable speed tail rotor system for a rotary wing aircraft.
Item 18' is shown as a blank box. It transmits power for primary forward thrust during a high speed flight. However, the patent does not show what is inside this box. Could it be a box of corn flakes?


OK, so much for corny jokes.

_________________________

Speculation;

1. The center of lift, and perhaps the centers of drag, percussion and mass etc. etc., have been moved further out the blades then they were on the earlier ABC. This will have increased the length of the moment arms and thereby increased the loads on the transmission.
2. If the rotor speed IS variable then here is better speculation. "In conventional helicopter flight operations, the rotor speed is designed to operate at a fixed value. A constant rotor speed is chosen to avoid a host of dynamic conditions that would increase rotor vibration and blade loads. Rotors that operate over a wider range of angular velocities will surely encounter more diverse dynamic conditions as changes in centrifugal stiffening modify blade structural mode frequencies and increase (hingeless) bending moments. While the design of the rotor must account for the dynamic conditions, there may still exist in the flight envelope dynamic conditions that result in unacceptable loads. " From ~ OPTIMAL AEROELASTIC TRIM FOR ROTORCRAFT WITH CONSTRAINED, NON-UNIQUE TRIM SOLUTIONS

CEFOSKEY you don't have to say what the problem is. Just node to the left for 1. or node to the right for 2.


Dave

[IFMU]

Dave,

What if it is neither 1 or 2? Seems like you gotta give CEFOSKEY a choice 3. It is entirely possible, actually probable that they designed the thing based upon real analysis and historical data.

It would not suprise me in the least if some of the media has gotten some of the details wrong. Often screwed up media makes for good threads here on pprune.

-- IFMU

[Dave_Jackson]

IFMU,

You are correct. The post is speculation. The 'delay' could be due to other reasons.



CEFOSKEY, Please fill in the blank,

3. ________________________________________

Dave

[NonSAC]

Cornflaker apparatus...

Dave,
You’ve asked an interesting question. Reviewing the relevant publication helps to an extent, but doesn’t definitively answer the question.

2007-0125907 is the publication number of US Patent Application No. 11/292,556, since issued and now US Patent No. 7,434,764 (the ‘764 patent).

To understand the ‘black box’ 18’ referred therein you can look to the patent application incorporated by reference. More specifically, the specification of US 7,434,764 describes 18’ as a translational thrust system and incorporates by reference US Patent Application No. 11/140,762. This application, published as US 2006-0266883 and now US Patent No. 7,413,142 (the ‘142 patent), describes a translational thrust system where a transmission shaft 46 drives a spur gear 84, preferably a sun gear, located between a multitude of planet idler gears 86, and spur gear 88 with inwardly [toward axis of rotation of tail rotor drive shaft] facing gears, thereby driving the translational thrust system 30. See the ‘764 patent at paragraph [0035], Fig. 2, and Fig. 3.

The transmission described in the ‘142 patent is fixed ratio; Fig. 3 describes an embodiment with engine, main rotor, and tail rotor rpm’s of 2100, 320, and 2140 respectively. The innovation here seems to be the counter- rotating main rotor blades and tail rotor system.

The ‘764 patent, in contrast, describes a rotary wing aircraft where the innovation includes counter-rotating main rotor blades having variable velocities with respect to the tail rotor speed, albeit through a very different transmission.

Comparing claim 1 of the '764 patent as filed to the allowed claim 1, and reading paragraph [0026] while looking at Fig.2, it appears that the variable speed functionality is effected in the main gear box rather than the black box out on the tail boom. Regrettably, there is very little additional disclosure about the specific means - the variable speed systems 30A and 30B – beyond their placement on the main rotor drive and being downstream from the tail drive.

I wonder whether the X2 transmission more closely resembles an embodiment disclosed in the ‘142 patent rather than one of the embodiments disclosed in ‘764 patent. As the ‘142 patent includes counter rotating main rotor blades and a pusher system, whereas the ‘764 includes counter rotating main rotor blades, a pusher, and a means to decouple their respective speeds, I would guess the X2 in its present form to more closely resemble one of the less ambitious embodiments of the ‘142 patent.

On an unrelated note, review of the prosecution of the ‘764 patent illustrates how claims can get narrowed during prosecution - a subject discussed previously in the context of the electric motor driven transmission Sikorsky patent application now before the PTO. Notable and applicable here is that the ultimately allowed claim 1 is much narrower, and I believe much clearer, than the originally filed claim.

Cheers!

As-filed claim 1:
A main gearbox of a rotary-wing aircraft comprising:
a gear box;
a tail rotor system driven by said gearbox; and
a variable speed system driven by said gearbox downstream of said tail rotor system.

Allowed claim 1:
A main gearbox system for a rotary-wing aircraft which drives a main rotor assembly and a tail rotor system comprising:
a main gear coaxial with a main rotor shaft of the main rotor assembly;
an input shaft driven at engine speed;
a tail-take-off shaft driven by said input shaft, said tail-take-off shaft driven about a tail-take-off shaft axis of rotation;
a reduction stage shaft driven through said tail-take-off shaft to drive said main gear, said reduction stage shaft driven about a reduction stage shaft axis of rotation which extends across at least a portion of said main gear;
a variable speed system upstream of said reduction stage shaft and downstream of said tail-take-off shaft, said variable speed system operable to drive said main gear speed at a variable speed relative to the engine speed; and
a tail rotor system driven through said tail take-off shaft, said tail rotor system driven at a constant speed relative to the engine speed.

[Dave_Jackson]

NonSAC

Unfortunately, my posting #642 was not very clear. In addition to mentioning the blank box [item 18'] to the propulsor it should have also mentioned the blank cylinder [item 110] to the rotors.

Both of these blank enclosures represent the undefined variable-speed units.

IMHO, this requirement to drive a pair of main rotors and a propulsor, while simultaneously providing variable speed to the rotors and propulsor, present an extremely difficult challenge for the power-transmission section of any engineering department.


However having said that; the title of application 20070125907 and subsequent patent 7,434,764 says;
"Variable speed gearbox with an independently variable speed tail rotor system for a rotary wing aircraft."

Please correct me if I am wrong, but it appears to me that Sikorsky can patent their 'gear-trains', as shown, but in no way can they consider this as patenting anything related to the design of variable speed mechanisms. This being the case, why does the phrase "variable speed" have such prominence in the title of their patent? Why does the phrase "variable speed" appear 25 times within the patent.

________________________

Thank you for the side note related to the narrowing of claims during prosecution.
Interestingly, the above patent 7,434,764 has a divisional application patent 7,651,050, dated January 26, 2010.


Thanks for your much-appreciated explanations.

Dave


Edit: 1/ Bold - by me. 2/ Clarified.

[Rigid Rotor]

What are the vibration levels?

What are the vibration levels on the X2? With all that hinge-offset, how have they managed it? I remember the early flights on a prototype hinge-less rotor with 14% offset (without vibration dampers then) - the vibrations did threaten to shake loose our teeth fillings!

[The Sultan]

CEFO

Unless he is phote'ed with missing teeth or blood coming out of one or more orifices a test pilot will say it is smooth, ready for production, what are we waiting for, etc...

I remember on the Commache first flight that this was their response, forgetting to mention all of the cockpit screens went blank.

The Sultan

[Ian Corrigible]

Not wanting to put words into his mouth, but I seem to recall Nick once saying that his main concern on the vibration front was the interaction between blade wake and the pusher prop at high speeds.

Interesting quote from Mark Miller in the article posted by CEFOSKEY ("open to teaming or investors"). Reminiscent of the lukewarm commitment to the project's future given by some of SAC's execs over the past year. Without wanting to provoke Mart (again ), I'm beginning to wonder whether we will ever see an application for the technology, or whether the beancounters are now questioning its cost/benefit ratio in the marketplace.

Regardless, Jeff Pino himself seems more optimistic about the project's progress once flights resume, saying last week that he expects to see the 250 kt milestone achieved by July.

I/C

[IFMU]

Originally Posted by CEFOSKEY

The TD incorporates a lot of AVD parts from the 92

In this old video, they mention the vibration control is there to smooth it out as they push up to the high speeds. I wonder if they have turned it on yet.

Originally Posted by The Sultan

Unless he is phote'ed with missing teeth or blood coming out of one or more orifices a test pilot will say it is smooth, ready for production, what are we waiting for, etc...

Standard work up in your parts? If they really have a problem I sure hope them boorish spares are up to the task.

Originally Posted by The Sultan

I remember on the Commache first flight that this was their response, forgetting to mention all of the cockpit screens went blank.

Too bad Nick isn't here anymore. Maybe John Dixson can confirm that Comanche shook like you say.

-- IFMU

[JohnDixson]

Comanche Vibs/Screens

IFMU:

One of the things re the RAH-66 was the vibration story. I refer to 1/rev and n/rev. Just to be sure I sent a note to the project pilot Rus Stiles and he confirmed that from start to finish, they never made a 1/rev adjustment on the original blades and never had to do anything about the 5/rev. I only flew that machine once with Rus and from hover to Vh ( ~165 KIAS or so ) it was remarkably smooth. It was also amazingly easy to fly. It was.....but I'd better not get started down that road.

I had to send another note back re the screen issue. I recall that they did lose some screens, but I think the instrumentation screens were working fine so they continued. I'll let you know further when Rus gets back.

Rev 1: From Rus Stiles:

All the screens went blank – but, yes we had critical parameters on both the instrumentation and a display driven directly by the flight control computers.

This back up display shared media with the far right screen – when it detected loss of communication from the avionics, it automatically switched over to the flight control computer. It had critical warning/caution messages, air data and performance information.


Rus refers to the two large MFD's in each cockpit. That glitch was fixed and did not recur.



Thanks,
John Dixson

[IFMU]

video on UTC.com

John,

Thanks for the update. When I re-read The Sultan's post, he does not state that vibrations made the displays fail. I must have jumped to a conclusion, I am sure he was going somewhere else with that thought.

In surfing around today, I found the UTC annual report has an X2 video clip. One quote from the video: "Vibration levels, I tell you what, very nice." Maybe that is code for screens going blank, they don't say though. In the video clip I could only see the left screen and it seemed to be working.

Video at:
UTC Annual Report
You can skip the intro, though it has one X2 picture on it. When you get past that, click on the technology picture with the turbofan engine on it. Then there is a link to the video on the left.

-- IFMU

[JohnDixson]

X2 Vibs

The X2 team is happy with the vibration levels at the modest speeds achieved to date, but of course the next testing phase will be challenging in that respect as well as several others. As opposed to the XH-59 design, however, they have S-92 style active vibration absorbers installed and available for use if need be. I'm just a rabid fan on the outside now, but I have high hopes for that team.

Thanks,
John Dixson

[Dave_Jackson]

The XH-59 was was built to accept vibration dampers. Apparently they were never installed because Sikorsky stop work on the craft after it lost the competition.

In retrospect, it is unfortunate that they did not slowly continue with R & D, since testing the craft with dampers, etc. might have been beneficial to the development of the X2.

[NickLappos]

Dave said, ".......Sikorsky stop work on the craft after it lost the competition."
There was no competition, Dave. The XH-59 fulfilled its contract, a follow-on was discussed but not funded, and later (much later!) the X2 was built. A sister contract at the same time was the XV-15, both were administered by the same Army engineering group.

Ian, I believe what I once posted was concerns that the small prop on the X2 was in some pretty poor air behind the fuselage, so that it wouldn't be as efficient as one would hope.

The RAH-66 Comanche was pretty smooth, vibration was never an issue in any maneuver or speed I had flown, and there was no vibration absorber or suppression system ever installed. This is typical of stiff, thin gunships, where the structure is very rigid. Big open cabins with door cutouts and flat spans breed vibration problems.
Pilot lore that "bad" rotor heads produce vibration is just that, mythical. All rotors produce vibrations, what we feel is how the fuselage reacts to that vibration.

[IFMU]

Too bad Nick isn't here anymore. -- IFMU

I stand corrected! Welcome back Nick.

-- IFMU

[Dave_Jackson]

Hi Nick,


Me said;

Apparently ...... Sikorsky stop work on the craft after it lost the competition.

Nick said;

There was no competition,

David Lednicer said;

"I worked on the ABC late in its life .... The US Army decided they didn't want it. The tilt rotor helped bring about its demise."

Me say; I stand corrected; also.


CEFOSKEY get ready to apologize to Nick; also.
Years ago he corrected me to the fact that the XH-59 didn't crash. It had "a hard landing".
_____________

It's good that your back and fighting. Perhaps stimulating technical discussions can return to PPRuNe.


Dave

Editrd to add remark by David Lednicer.

[Dave_Jackson]

CEFOSKEY

This is all I know.

Dave