Sikorsky + Boeing pitch ‘X-2’-based design for US Army JMR TD effort

[IFMU]

Perhaps the army will plot the data for the 6000 or so LB Schweizer X2 against the twice as heavy Sikorsky raider and make up their own minds.
Bryan

[Ian Corrigible]

"We're gonna need a bigger engine..."

Sikorsky-Boeing select T55 to power SB-1 Defiant demonstrator

Interesting development, given that the parallel AATE/ITEP engine development program is currently focused on the 3,000 shp class GE3000 & HPW3000. It also reinforces the point made previously with regards to the FVL's sizing.

Originally Posted by riff_raff

Sikorsky looks to be making good progress with their S-97 demonstrators

...Though it's interesting that this article -- clearly written with the blessing of Sikorsky -- suggests that FF has slipped to 2015.

I/C

[IFMU]

I/C,
I had missed the date in the PM article, thanks. I had heard nothing but first flight this year. Usually the programs folks spin only in one direction. There was a classic AIN article that predicted an early X2 flight:
X2 could make first flight by month?s end | Aviation International News

The actual first flight took about another 9 months if memory serves. I followed X2 avidly back then.
Bryan

[SansAnhedral]

Perhaps the army will plot the data for the 6000 or so LB Schweizer X2 against the twice as heavy Sikorsky raider and make up their own minds.

That would be prudent, provided they understand the idea of diminishing returns and non-linear relationships.

2 data points does not make a curve.

[The Sultan]

With the need to use the T55 the $ per seat mile looks to be about double an equivalent tilt rotor. I guess the defiant is as draggy as it looks and one rotor always in the dirty air of the other brings into question hovering efficiency.

The Sultan

[riff_raff]

I would hope that the Army is astute enough not to drink the kool aid.

According to Defense News we should hear who the Army selects to build the JMR TDs by the end of next month.

The technical requirements for FVL would seem to favor a tilt rotor, but Sikorsky decided to go with a compound. Sikorsky is not stupid and they have spent quite a bit of time over the past 2 or 3 decades studying tilt rotors (anyone recall their VDTR concept?). If they felt a tilt rotor was a better technical approach they definitely have the expertise to design and build one. Maybe Sikorsky thinks the Army operating a large tilt rotor will create too much friction with the (fixed-wing) Air Force, so the only politically acceptable option for an Army FVL heavy configuration would be a rotary-wing aircraft.

It will fun to see what happens in a couple weeks when the TD selection is announced!

[horlick97]

Please pardon my ignorance. May I seek some help on the following:

What's the difference between X2's coaxial and AVX's coaxial?

I read that X2's rotors are rigid. Does that mean AVX's are not?

What is the implication of this difference? Is it solely limited to the separation between the two rotors?

I understand the larger the separation, the larger will be drag. But, it can't be worse off than Chinook (i.e. adjusted for similar size of the aircraft) which tendem rotors also need to be similarly separately vertically, can it?

In fact, due to the tandem configuration, the rotors in the Chinook have to be vertically separated more than if they were to be coxial (i.e. for similar lifeting capacity). This is due to the lateral geometry on how the rotors will flex. You know what I mean? Sorry I can't sketch it here.

Bottom line is, if the rigid rotors meant for the Defiant cause too much problem for upward scaling, can they be replaced by conventional coaxial rotors, or something in between, i.e. less rigidity and more vertical separation? Again, my premise is that the vertical separation can't be worse off than that in the Chinook.

Likewise, if the vertical separation of the AVX is causing to big a drag penalty, could it not add some rigidity in the rotors and reduce the vertical separation so as to reduce drag.

Isn't Defiant's and AVX's offering using the same technology along the same continuum?

Apologies if my trend of thought is confusing. Appreciate anyone's help.

[horlick97]

Another question I have is what's the difference between Defiant's forward thrust propellers and AVX's ducted fans?

Wouldn't AVX's split to two ducted fans require and additional differential/transmission which will impose weight penalty?

[riff_raff]

AVX's website gives their side of the story on why their coaxial compound is better. Your comment about the added complexity and weight penalty of two pusher props (or fans) versus one is correct, but the weight penalty is probably not significant. The benefit of using two smaller pusher fans is that they can be located well off center and up, which allows a rear fuselage ramp door.

One thing that I suspect all of the JMR competitors have considered, both for the tilt rotors and compounds, are variable speed drivetrains. In the case of AVX their design has small forward wings. So at high cruise speeds it appears they might be planning to slow the main rotors and use the wings for added lift, while diverting a greater percentage of power to the pusher fans. Sikorsky just stated that their SB-1 JMR design will use a T55 turboshaft engine that will be modified to allow a wider range of operating speeds (probably something like 85% to 105%). So it appears Sikorsky also plans to slow their main rotors in cruise.

[SansAnhedral]

If they felt a tilt rotor was a better technical approach they definitely have the expertise to design and build one.

That is a bit of a stretch. Concept development is one thing, but expertise in helicopter design does translate especially well to the world of tiltrotors, especially in the arena of rotor dynamics.

But I think your suggestion of the political implications of a single mast un-winged craft is more to the point.

And regarding scaling:

"There is a question on the scalability on the X-2 technology at the medium class," said Scott Starrett, Sikorsky's vice president for government business development. "When you get to the utility-medium or attack-medium class, it scales nicely." However, with size and weight increases "you starting getting up to that kind of payload and physical size and it gets to be a different challenge for the technology."

For the so-called "ultra-class," which would be a vertical lift machine the size of a C-130 tactical fixed-wing transport, Starrett said that tilt-rotor technology would be the technology of choice.

So back to the entire premise of Joint Multirole - a single design paradigm spread out among different size classes. If FVL-H is out of the box for an ABC coax, how is it the technology relevant to the fundamental pursuit of the program?

[SansAnhedral]

Originally Posted by Sansanhedral

Curious to see if this is a serious endeavor, or just an obvious effort to dog-and-pony-show FVL and instead sell more UH60X and AH64X ad infinitum. Boeing and Sikorsky already have a revenue stream with those models.

Well this certainly is interesting

Boeing proposes high-speed Apache, heavier Chinook - 6/26/2014 - Flight Global

[henra]

Originally Posted by horlick97

I understand the larger the separation, the larger will be drag. But, it can't be worse off than Chinook (i.e. adjusted for similar size of the aircraft) which tendem rotors also need to be similarly separately vertically, can it?

Tandems don't have to be vertically separated. In certain (most) flight regimes the Chinook blades will be intermeshing. Due to axis Offset the tips will be moving in the same direction where they intermesh. In a Co-ax they will only intermesh once and that only very briefly...
Plus the Chinook wasn't designed primarily for all out speed.
So no comparison here.

[riff_raff]

Sans- Thanks for the article link. As the article stated the Army would like to see some of the technologies being studied under JHL/JFTL/JMR make their way onto existing platforms since these aircraft will remain in service for many more years.

During the JHL program Boeing was given a $3.4M study contract for their Advanced Tandem Rotor Helicopter concept, which was basically an improved CH-47. There were also spin-off efforts from JHL such as FARDS and ARCD. Sikorsky got one of the ARCD contracts for a vibration reduction system and just announced some results of testing on a Black Hawk. Bell signed a $30M contract for a FARDS program that will test multiple new technologies on a 407 transmission. The Sikorsky HMVS definitely seems promising, and some of the drivetrain technologies being tested at Bell look very worthwhile. These types of technologies can be incorporated into existing rotorcraft without too much trouble, so we may see them put into service in the near future.

[horlick97]

Thanks Henra.

It is interesting to learn that the tandem's rotors need not be vertically separated due to the axis offset. Though I note this configuration would only be applicable within a very narrow range of situations, eg, limited to 2 or at most 3 bladed rotors, requiring a relatively larger longitudinal separation in order to minimise the intermesh, etc.

Having said that, I am still excited about this. Without the need for vertical separation of the two rotors in tandem, would this not make the tandem configuration viable for optimisation for speed? So, instead of the AVX's Defiant's coax with 2 fans for forward thrust, would a tandem (with both rotors intermeshing in the same plane) fitted with 2 forward thrust fans perform better for speed?

[riff_raff]

horlick97-

The CH-47 is actually a fairly fast helo for its size, and a bit faster than a UH-60. The CH-47's rotors are interleaving in planform, but they have a fair amount of vertical offset with the rear rotor plane being higher. Boeing recently proposed a rather strange compound version of the CH-47 with mast mounted wings above each rotor called ULOR. Boeing seemd to think they could achieve a >250kt cruise speed with this concept. And then there was the BV-347 from around 1970, which Boeing actually built and flew. Lastly, fore/aft interleaving rotors is not the only configuration possible. You can also locate the interleaving rotors side by side as shown here.

[horlick97]

Thanks riff_raff. I learnt something new.

The ULOR is interesting, though I must acknowledge I do not understand fully by the 'offset'. Anyway, I wonder why boeing did not offer this or the JMR. Could it be because the concept is still too new and there were not enough validation yet?

It is also enlightening for me to learn that the CH-47's vertical separation is strictly not needed but for noise minimisation. The interleaving would have ensured the rotors do not clash.

Back to the ULOR. Can the present CH-47 be fitted with forward thrust fans and allow the rotors to be unloaded during cruise? Isn't the X2 using this approach?

[SansAnhedral]

The CH-47's rotors are interleaving in planform, but they have a fair amount of vertical offset with the rear rotor plane being higher.

The rotor center of rotation is at a higher waterline, but the disc declination puts them at interference. In the -47 the rotors intermesh at any Nr > 0 where CF stiffening and flapping/coning is in effect. So basically, all the time.



A functioning synchronization shaft between the 2 rotors is absolutely essential, or the aircraft will instantly self destruct, (see: Mannheim 1982)

[riff_raff]

Sans- Thanks for clarifying my crappy description of the CH-47 rotor arrangement. I'm not an aero guy, but I believe the reason for locating the rear rotor on a pylon at a higher WL was to reduce the impact of downwash from the forward rotor on the rear rotor during forward flight or autorotation.

And an interconnection between the CH-47's two rotor drives to maintain phasing is indeed critical. The 1982 Mannheim crash you referenced is well known to rotorcraft drivetrain engineers. The root cause of the accident was loss of oil flow to the rear mast bearings, causing them to overheat and seize. After losing phasing the front/rear rotor blades impacted each other, and soon after the rear pylon/rotor separated from the airframe. The loss of oil flow to the rear rotor mast bearings was determined to be caused by debris clogging the bearing oil jet orifices. The debris was blast media (walnut shell) used to clean the gearbox housings during overhaul that was not completely removed from the oil galleries. As a result of this particular accident all rotorcraft transmission lube oil jets now typically incorporate a "last chance" debris filter.

[Hilife]

riff-raff

The root cause of the accident was loss of oil flow to the rear mast bearings, causing them to overheat and seize.

Blocked oil jets yes, but not the rear mast bearings, but the forward transmission pinion assembly.

This was followed by failure of the forward synch shaft, the result of which was that the forward blades took out the aft rotor.

A terribly sad day.

[Lonewolf_50]

Walnut shells.