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

[horlick97]

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?

[riff_raff]

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.

[riff_raff]

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

[horlick97]

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.

[riff_raff]

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.

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.

[KenV]

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.

[horlick97]

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?

[Hilife]

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.

[JohnDixson]

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'

[SansAnhedral]

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?

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

[KenV]

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.

[KenV]

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.

[JohnDixson]

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.

[horlick97]

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?

[busdriver02]

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.

[tottigol]

you mean this?

[riff_raff]

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.

[Shawn Coyle]

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

[SansAnhedral]

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.

[Ian Corrigible]

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