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S97 Raider

Old 25th Apr 2017, 21:20
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See no reason to dismiss option A
The pool of aerodynamics expertise is pretty shallow and the number of people really conversant with coaxial rotors is single digit at best. Factor in a huge scale up such as this program postulates, it is outside the envelope.
My guess is powerpoint engineering met reality. This seems to happen a lot lately, wonder why.
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Old 25th Apr 2017, 21:29
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RJ, Your article also mentions the H-60 that went down during the bin Laden raid and as others have mentioned there was the 60 on the golf course a few weeks ago. Using your logic is the H-60 also an 'self crashing' aircraft that can't make a 'round trip'? Or are you applying a double standard?
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Old 26th Apr 2017, 18:05
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ET, understanding of coaxial rotors isn't as thin on the ground as you suggest... it's not a new configuration, there's wind tunnel data, aeroelastic modeling, subscale rotor testing, and of course flight data for several generations of ABC style aircraft. In addition to that, there is the experience of the Kamov, Hiller, and other designs that are to varying degrees, relavant... certainly more than single digit numbers of people are really conversant.

The design will manageably scale. It may or may not be the best fit to customer requirements, but that's partly what the JMR effort is for :-)
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Old 26th Apr 2017, 19:45
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Aerodynamics are not really the issue when it comes to the scalability of the ABC configuration - its the feasibility of manufacturing a physical rotor head, mast, and blade set that can withstand the exponentially greater loads driven by increases in gross weight, while maintaining overall weight and rotor spacing that keeps drag at a manageable level.

Consider also it is no real mystery why we have not seen any high gross weight conventional rigid rotor helicopters, in spite of the control power and savings in maintenance and component cost (no more hinges, elastomers, bearings, etc) they could provide.

Last edited by SansAnhedral; 26th Apr 2017 at 21:08.
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Old 26th Apr 2017, 21:58
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Originally Posted by SplineDrive
ET, understanding of coaxial rotors isn't as thin on the ground as you suggest... it's not a new configuration, there's wind tunnel data, aeroelastic modeling, subscale rotor testing, and of course flight data for several generations of ABC style aircraft. In addition to that, there is the experience of the Kamov, Hiller, and other designs that are to varying degrees, relavant... certainly more than single digit numbers of people are really conversant.

The design will manageably scale. It may or may not be the best fit to customer requirements, but that's partly what the JMR effort is for :-)
Honestly, I have no confidence that the designs will scale.
We don't really have a firm grasp even on conventional rotor systems engineering, else we have long since corrected the design flaws that led to the EC225 getting its certificate pulled for North Sea operations.

AFAIK, there are no 30,000 lbs GW ABC vehicles flying anywhere. What there is is much smaller and the development performance to date suggests strongly that the designs don't scale.
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Old 26th Apr 2017, 23:33
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What are the issues that exist for the 97 but did not seem to limit the 53,64. and 67 or the Cheyenne?

What is unique to the ABC concept that other Rotor Systems do not share when it comes to size increases?

Did the Russians not have some success with Co-axial Rotor systems?

Then there is the Piasecki X-49 pusher that uses a Blackhawk airframe and Rotor System....how does that compare to the 97?

https://tacairnet.com/2015/08/05/pia...-in-existence/

https://ntrs.nasa.gov/archive/nasa/c...9970015550.pdf
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Old 27th Apr 2017, 04:05
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SAS,

To help you catch up:

- the 53 and 64 are tradional helicopters with 60's performance and abysmal safety records

- the 67 ended up scattered across a field in England due to poor maneuverability and/or pilot error

- the Cheyenne was an expensive failure with design flaws many related to the rotor

As for the Piasecki, this is a joke going nowhere.
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Old 27th Apr 2017, 15:48
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Originally Posted by SASless
What are the issues that exist for the 97 but did not seem to limit the 53,64. and 67 or the Cheyenne?

What is unique to the ABC concept that other Rotor Systems do not share when it comes to size increases?
ABC craft, by definition, utilize a closely spaced high hinge offset rigid rotor system. This means no flapping or lead/lag hinges, and therefore colossal moments through the rotor head and into the mast, even for small diameter rotors and smaller gross weights.

As I alluded to before, why is it that you've never seen a rigid rotor from a Bo105 installed on a helicopter much larger than a light twin (topping out at ~12k lbs), much less in a closely spaced stacked arrangement? Because the loads into the structure increase exponentially, to a point where the required construction to carry them becomes prohibitively sized. The amount of blade flapping also will increase, as there are no materials which maintain the stiffness to weight ratios required at larger radii. This drives greater rotor spacing, which is a poison pill to the ABC concept meeting its stated performance. Then there is also the complex aeroelastic tuning aspect.

The comparison to Kamov is getting tiresome; the only commonality between these designs is the usage of a coaxial drivetrain. The Russian designs are all articulated, hinged rotor systems (hinges and flapping alleviate loads). There is no doubt you can scale something like that up to CH53 size, though to clear flapping it would on the order of 50 feet tall!

Last edited by SansAnhedral; 27th Apr 2017 at 17:00.
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Old 27th Apr 2017, 16:07
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Thank you for the explanation.

That does explain the large separation of the two rotor systems on the Kamov as compared to the much smaller vertical separation of the ABC system.

Knowing the Rigid Rotor system is only on the BO and BK did not strike me as notable but in light of what you describe it does make sense. The BK is a derivative of the BO and not much increase in size. I have never heard a conversation about why those are the only two "Rigid Rotor" aircraft.

Sizing of Rotor Systems....diameter, blade chord, and numbers of blades has always seemed a Black Art of sorts....then compound that by adding a second set of Blades and I can see how complex it all becomes.

The Sikorsky ABC competed with the Bell Tilt Rotor back in the early 80's in an Army Test Program....with the ABC concept not getting the Nod.... which resulted with the advent of the V-22 Program.

Did the possible shortcomings of the ABC Concept, as you are describing now, become factors that led to the Tilt Rotor concept gaining acceptance by the Military and the ABC not?
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Old 27th Apr 2017, 16:29
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The ABC versus tiltrotor competition of the 70s used craft (XH-59 and XV-15) sized below what is being compared today (SB>1 and V-280). At the time, Sikorsky used to illustrate larger versions of the rotor system scaled to fit medium lift class aircraft, but as the technology proved toublesome from a vibration and flight control standpoint through testing, they were never put to task proving it was feasible.

Now that FBW and better AVC have addressed some of the shortcomings revealed 40 years ago, the elephant in the room remains the sizing.

Fundamentally, gimballed rotors used on tiltrotors don't have the same concerns with scaling, and whether or not that factored heavily into the JVX decision making process is debatable. That's not to say there aren't other tiltrotor designs that do not make dubious scalability claims - Karem's OSTR to wit. Yet more issues with a large rigid rotor concept, this time with the added challenge of tiltrotor dynamics. Karem seems to believe he can simply just tune high with some rather mythical ultralight and ultra stiff construction material.
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Old 27th Apr 2017, 16:52
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The Tilt Rotor has some scaleability issues too doesn't it?

OGE Hover performance for the V-22 remains a limiting factor in helicopter mode....and is quite modest considering the amount of horsepower the engines are capable for producing.

That comes from the compromise that must be made when combining helicopter and airplane performance using Prop-Rotors.

That being the Power Demand that is generated by the Rotor Design itself....hover performance demands huge amounts of power well in excess of that needed for cruise performance and that has been the problem for the Tilt Rotor in the past. The Bell Aircraft was eighteen months behind due to reworking the aircraft to accept 1800 horsepower engines as I seem to remember when preparing for the Demonstration Flights at Fort Rucker back in the Eighties.

The two aircraft used for that "competition" were not production aircraft but were instead technology demonstrators. The Army saw the ABC as having more potential as shown by their decision not to get involved in the Tilt Rotor Contract as did the Navy/USMC and USAF.

Which seems to be the bigger problem for the 97....stresses and weight issues in the Rotor Head/Mast or thrust being produced by the Pusher Prop?

The goal is 265 Knots Dash Speed is it not?
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Old 27th Apr 2017, 17:51
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Well for JHL, the program office actually had settled on a (quad) tiltrotor solution only - it was determined it was basically the only technology that could meet the requirements. It will be interesting to see how that develops when the program is restarted.

Specific V-22 HOGE performance are not an indictment of tiltrotors in general but more of the specific sizing requirements placed on the Osprey driven by shipboard operations. For example, V-280 is purported to have much lower disc loading and improved hover performance due to a suite of improvements in sizing and rotor control. It also comes down to doctrine, does a high speed vertical lift transport asset need to spend long periods of time loitering in a hover?

If the army saw promise in the ABC, then its surprising they chose to put zero funding towards Sikorsky's proposed XH-59B which added a ducted pusher. That snuff was essentially what shelved the concept for 30 years.

All in all, I think that the S97 is at the upper end of the feasible size of an ABC craft, and likely can dash around 230kt, but I am not holding my breath for it to demonstrate some of the severe high-G maneuvers SAC advertises. The X2 which was about half the gross weight seemingly never pulled them off. S-97 also will not achieve anywhere close to the range and cost numbers they have floated.

My theory regarding the slow progression of Raider testing is that they have possibly had some rotor component failures.
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Old 27th Apr 2017, 17:56
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Sans, running through the scaling math for beams, it looks like that for a given number of blades, solidity, and load factor, the spacing between the rotors can indeed be a constant proportion as gross weight is sized up. Loads, radius, airfoil thickness, blade stiffness, and tip deflection all change so that the blades are strained the same. I don't think that new materials are required as gross weights increase, though I can certainly imagine that specific pitch bearing joint technologies work well at certain size ranges and may practically constrain the hub design at some point.

I would expect that when actual photos of the SB>1 are released, the inter-rotor spacing is essentially the same as the X2 and S-97 as a percentage of the rotor diameter.
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Old 27th Apr 2017, 17:58
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OGE Hover performance for the V-22 remains a limiting factor in helicopter mode.
The V-22 met the spec requirements for the program. The performance of the V-22 was primarily controlled by design constraints for shipboard stowage. That constraint is not an issue with the smaller V-280 which is also designed to meet the 6K/95 requirements.

As to the requirements for the V-22 can the 1/2 capable SB1 even fit in the hangar of an LH? It is extremely tall and you cannot fold that second rotor away.
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Old 27th Apr 2017, 18:56
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Originally Posted by SplineDrive
Sans, running through the scaling math for beams, it looks like that for a given number of blades, solidity, and load factor, the spacing between the rotors can indeed be a constant proportion as gross weight is sized up. Loads, radius, airfoil thickness, blade stiffness, and tip deflection all change so that the blades are strained the same. I don't think that new materials are required as gross weights increase, though I can certainly imagine that specific pitch bearing joint technologies work well at certain size ranges and may practically constrain the hub design at some point.

I would expect that when actual photos of the SB>1 are released, the inter-rotor spacing is essentially the same as the X2 and S-97 as a percentage of the rotor diameter.
Calculations for static beams don't account for aeroelastic tuning concerns, effective airfoil shapes that must fully contain the structure, nor mast/hub sizing for the fixed end of your cantilever.
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Old 27th Apr 2017, 19:02
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Originally Posted by SansAnhedral
Calculations for static beams don't account for aeroelastic tuning concerns, effective airfoil shapes that must fully contain the structure, nor mast/hub sizing for the fixed end of your cantilever.
I'm with spline on this one. He got it right in one.
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Old 27th Apr 2017, 19:04
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I implicitly addressed airfoil cross sections by holding them constant, blade tuning to similar fan plots is acheiveable across wide ranges of rotor diameters, and I explicitly addressed what is likely the biggest challenge in hub design. I do believe the design scales. Practical limits of ABC aircraft will be driven by other factors (like Sultan's shipboard observation above).
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Old 27th Apr 2017, 20:08
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If the army saw promise in the ABC, then its surprising they chose to put zero funding towards Sikorsky's proposed XH-59B which added a ducted pusher.
Remember what was going on in those years with the Economy?

Military Budgets were affected by those same factors and the draw down from Vietnam Years.

The Army also did not get into the Tilt Rotor business either.

Their Budget was being spent on things like the Blackhawk, Apache, Kiowa, Chinook, Cobra, and Commanche.

The Strategic Focus was Tank Killing in Europe as the Soviet's superiority in numbers of Tanks was seen as the major threat.

The Army spent a lot of money on everything from Lasers, Sand Enhancer, NVG development, and a host of other critical (or thought critical) items and concepts.....besides helicopter research and development.



http://www.history.army.mil/books/DAHSUM/1980/ch11.htm

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Old 27th Apr 2017, 20:41
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Spline claims that:

Loads, radius, airfoil thickness, blade stiffness, and tip deflection all change so that the blades are strained the same.
Exactly how do you propose you achieve stiffness for the same strain within a linearly scaled aero envelope without increasing structure and in turn weight?

An increase in size by a factor of 2 would require a quadratic increase in stiffness (EI), as your material modulus is invariant in our example case. Greater stiffness with the same materials requires more material, and results in larger weight and therefore larger loads. This is a snowball effect. Mach scaling a rotor radius 200% is a cubic function for mass, square function for loads.

A blanket statement like:

blade tuning to similar fan plots is acheiveable across wide ranges of rotor diameters
in light of the immense structural changes required by scaling up is dubious. I've seen it time and time again on trade studies of rigid rotors - from coax to tiltrotor.

In any case, the added mass has a cascading effect on mast moments and rotor hub structure also increasing the sizing in the non-blade rotating system as well.


If rigid rotors in general scaled as easily as you suggest here, then existing conventional medium and heavy helicopters would already implement that rotor system as it offers significantly better control power and a far less complex and costly rotor head assembly. Sikorsky also would not have candidly admitted that X2 technology does not scale beyond the medium class in 2011

Edit: The link is now dead, I wonder why? Alt reference

Last edited by SansAnhedral; 27th Apr 2017 at 21:20.
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Old 27th Apr 2017, 20:50
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Originally Posted by SASless
Remember what was going on in those years with the Economy?

Military Budgets were affected by those same factors and the draw down from Vietnam Years.
True, but the army was also reeling from the dumpster fire that was the AH-56 program. So I would argue their taste for rigid rotors was not all that well developed.

I've not seen anything published or documented that would illustrate a positive opinion expressed by the Army regarding XH-59A, so I am curious why you say:
The Army saw the ABC as having more potential as shown by their decision not to get involved in the Tilt Rotor Contract
The logical conclusion they did not get involved in either project goes back to your point regarding the budget situation.

Last edited by SansAnhedral; 27th Apr 2017 at 21:01.
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