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

"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.

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

I/C

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

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

2 data points does not make a curve.

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

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!

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.

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?

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.

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?

Well this certainly is interesting

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

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.

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.

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?

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.

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?

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.

http://i.imgur.com/e10GOXe.png

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

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.

riff-raff

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.

Walnut shells.

Does anyone know the status of the Groen Brothers Gyrolifter concept? Would this not be a promising concept also? In fact, would the Fairley Rotordyne not have proven the concept?

We should be hearing from the Army in the next 2 weeks who they have selected for JMR TD contracts. Too bad the forum moderator does not permit gambling, or we could start a betting pool on who the contract winner(s) will be. :ok:

Looks like the Army made a decision on JMR, but apparently we'll have to wait until all the contract details are hammered out before they announce who got the funding.

Army Narrows Playing Field for Joint-Multi Role Helicopter, But Few Public Details Announced - Blog

It will one of each system.

Coaxial: Either Boeing-Sikorsky or AVX.

Tiltrotor: Either Bell or Karem.

It will be good if award would allow the combination of the features to be employed, eg, Bell non-tilting engine with Karem's variable speed, or AVX's configuration with Boeing-Sikorsky's rigid rotors. But, from the article, it seems this approach is not on the cards.

Based on what the Army has stated publicly I'd guess that all four competitors will get some amount of funding. But I'm pretty sure only Sikorsky and Bell will get contracts for flight demonstrators. This is a very high-profile development program for the Army, and I can't imagine them being willing to risk one of the flight demonstrator contracts on small companies like Karem and AVX that have limited resources and no track record managing projects of this scale.

I agree with you that the rotor systems proposed by AVX and Karem appear to provide some worthwhile performance benefits. But these rotor systems will also likely add cost. One thing I liked about the Bell concept was that they made an effort to reduce cost (ie. the single piece straight wing, non-tilting engines, etc). The Army does not want a repeat of the massive budget overruns and schedule delays that become common with all aircraft programs.The US defense budget is rapidly shrinking and funding for development programs is one of the first things to get cut.

A few corrections are in order.

1. The S-97 Raider will NOT be the demonstrator for JMR-TD. The SB-1 Defiant demonstrator will be a full-scale 30,000 lb class machine. Sikorsky/Boeing is very confident the configuration will scale up to this size. But that is near the size limit for this configuration. Significantly larger rotorcraft will require a different configuration, either single rotor, tandem rotor or tilt rotor.

2. Sikorsky/Boeing and their partners have ALREADY spent $250M of their own funds, and plan to spend roughly 4x what the government will provide to develop and build the SB-1 demonstrator.

3. CG envelope for just about any aircraft is quite limited at max payload. However, CG envelope for lighter payloads is much larger on an inline tandem rotor than either a single rotor or side by side tandem like a tilt rotor. Since 90% or more of missions are performed at well below max payload, that is a significant advantage operationally.

4. Sikorsky/Boeing perceives they have an advantage over Bell/Lockheed's tilt rotor based simply on the fact that this is an Army program. The US Army is not keen on tilt rotors and has bought not a single V-22. Bell has some serious selling to do to overcome this Army bias. However, similar conventional wisdom in the 90s was that USAF would never buy a turboprop to fill their JPATS requirement. Beechcraft proved them wrong. We'll have to wait and see if the alleged Army bias against tilt rotors is true.

Thanks KenV for the insightful discussion.

May I enquire:
What is the factor that limits the X2 configuration to only upto FVL-M? Is it due to the difficulty in having rigid rotors beyond this size?

Now that SB Defiant has been shortlisted for the JMR-TD, but it will have no way to go beyond FVL-M, this will mean the services will have to rely on another configuration for the bigger machines. That being the case, we'll see:

Upto the size of blackhawk - rigid conaxial rotors, ABC, X2 configuration.

Beyond that: a combination of tiltrotors if speed is needed, or tandem if speed is non critical.

Is the going to be the case? Will this approach lead to sub-optimality?

There are bound to be compromises, but I don’t think so.

JMR is the precursor to FVL and it has always been my understanding that the end solution is likely to be 3 airframes of varying sizes in order to cover the Apache/Black Hawk/Chinook weight range, so you never know, we might even see a high speed 60,000lb tandem coaxial, contra-rotating rotor Chinook replacement.

Ken V
 

You posted:

"3. CG envelope for just about any aircraft is quite limited at max payload. However, CG envelope for lighter payloads is much larger on an inline tandem rotor than either a single rotor or side by side tandem like a tilt rotor. Since 90% or more of missions are performed at well below max payload, that is a significant advantage operationally."

This argument always seemed to show up in CH-47 vs CH-53 foreign sales competitions. The points we always responded with from the SA viewpoint is that one does not spend the millions to fly a big aircraft light, and when the 53 and 47 were loaded up, guess what, the CG ranges were about the same. In fact there were some comparisons where we had a slight ( very ) edge.

Just sayin'

Rigid rotor tip path plane divergence at larger rotor diameters.

The rotor scales essentially infinitely aerodynamically on paper. Building sufficiently light and rigid blades is the issue.

You can increase spacing to allow for blade flapping (particularly in maneuvers), but then drag increases exponentially as ~40% of helicopter drag is from the hub.

Note the published tip clearance from the 6000lb, 26 foot rotor diameter X2 was only ever measured in level flight, and was reduced by nearly 50% between 120 and 240 kt

http://i.imgur.com/F0vzuNO.jpg

It MAY be true that a user would want to fly a big expensive aircraft at or near its payload capacity, but the reality is that that is seldom the case in the real world for either fixed wing or rotary wing aircraft, especially military cargo aircraft. From the C-5, to the C-17, to the C-141, to the C-130, all carried average payloads far below their max payload, and that is equally true of the CH-53, CH-47, CH-46, CH-3, V-22 and others. Military missions are far too diverse to enable the users to fill the aircraft to anything near their capacity. Further, many (most) military missions "cube out" before they "gross out", meaning the cargo hold volume is filled before it reaches its payload weight capacity.

As for wide CG envelope, when you're under fire or about to be under fire, it's very nice to be able to load up quickly and take off before taking the time to calculate the CG. This is done routinely all over every theater of operations in which the US and its allies are engaged. A wide CG envelope is also very helpful when doing sling loads. The CH-47's three sling load hooks and wide CG envelope provide a very flexible external load capability.

Coaxial rotor systems are limited by the rigidity of their rotor systems. The bigger the aircraft, the bigger the rotors, and the bigger the rotors, the more the rotor blades bend up and down at the tips. With contra rotating rotors tip divergence will cause the rotor disks to strike each other, especially under maneuver loads. That would certainly be a very bad day for the crew. The only solution is to make the rotors more rigid, which dramatically adds weight and causes other problems, or space the rotors farther apart, which dramatically increases drag. So the advantages of the coaxial configuration are cancelled out by the increasing size.

To give some perspective of how much the blade tips bend under maneuver loads, consider both the MH-47 and MH-53 which are inflight refuelable. The pilots are warned and trained to NEVER "chase the basket". Chasing the refuel basket can result in maneuver loads that cause the rotor tips to impact the helo's refuel probe, which when extended extends beyond the rotor disk. A very bad situation for the crew.

Here's a video of an H-53 crew that survived a rotor/probe impact during aerial refueling:



BTW, this was a VERY experienced HAC at the controls. It can happen to the best.

Also
 

They are also taught that, once engaged in the drogue, pick it up vertically, such that an inadvertent release will result in the drogue going down, not up into the rotor.

Thanks for all the input regarding the limitations of the coaxial configuration when you scale it up.

The being the case, the only way to still employ the counter rotating rotors (so as to negate the need of a counter torque tail rotor) is to use the tandem configuration with the rotors interleave as in the Chinook.

I therefore thought an upsized Chinook will be a good idea for the heavylift solution. A further improvement could be to add a pair of wings and even hang a pair of forward thrust props for speed enhancements. With the wings, the main rotors may be slowed in forward flight, and part of the engine power shifted to power the forward thrust props.

Why the above was not considered?

KenV, not chasing the drogue has nothing to do with rotor blade flex. It's an issue of PIO, you're trying to chase the end of a whip instead of watching the hand that controls the whip.

http://www.diseno-art.com/news_conte...boeing-347.jpg

you mean this?

horlick97- There has been a substantial amount of effort put into performance simulations over the past 7-8 years in support of what is now the JMR program. Simulations were performed of every rotorcraft configuration you can imagine. Some of the simulation work was performed by companies participating in the program on their own proprietary designs, while other simulation work was performed by independent groups like NASA based on generic designs they created.

Here is a 2007 performance simulation study conducted by a group from NASA & AMRDEC, which includes a 250kt, 150klb GTOW coaxial rotor concept and a 300kt, 139klb GTOW compound tandem rotor concept. The individuals responsible for the work (like Wayne Johnson) are well respected in the industry, so the results of this study do have some credibility.

What about what Piasecki has been up to recently??
Winged, augmented thrust showed pretty good promised, from what I saw.

The X-49 fell well short of its speed goals, and the tech was not chosen to be pursued in JMR mainly due to those concerns.

The X-49A suffered from serious rearward CG, which resulted in a high AOA in the hover (and approach). This was also a concern with the AVX proposal.

I/C