Sikorsky SB > 1
 

A couple of agility data points I would be interested in hearing the results on would be hover points: 1) F/A pulse or step input and 2) Lateral pulse or step input at maximum control input rate. Both V-280 and Defiant.
Otterotor

Commando.....there is a built in fallacy to this "less is more" idea....because in reality "less is less"....you start losing aircraft by becoming predictable or for any other rreason such as putting more eggs in fewer baskets....losses can quickly be your un-doing.

Otherwise the logistics issue you describe works fine.

When is "fewer" in reality "too few"?

The reality is we're going to have less assets and resources available in the future. The threat environment is also going to be higher. Given that, the question becomes what's the best way to reasonably achieve our goal? You may indeed be able to have two UH-60s for each V-280, but that's just one component. What gets the mission done and what it costs to do so is really the question. Once in production we could probably turn out H-19s real cheap. Of course we'd have to have a lot more of them, the maintenance would be horrendous and we'd have to have lots more places for them to operate form. Similarly, We could buy a slug of P-51s or F-86s for the price of an F-15, but would that be a smart move?


Given today's threat and fiscal environment (we're going to have fewer aircraft and bases even if we built H-60s forever), the cruise and range desires for FLRAA make sense to me.

We've been discussing rotor diameter of SB>! and how many will fit in an LZ. Came across this picture of it in a hangar at some kind of recent presentation. Look at the height! How will we operate and maintain this in a forward area?
https://cimg7.ibsrv.net/gimg/pprune....001410e800.jpg

Looks to be (just from eyeball) about the same height as the aft rotor on a Chinook, which seems to manage just fine, albeit with a larger logistical footprint than a typical small rotorcraft.

Except that rotor height on a medium lift asset that does not have the range to self-deploy makes it a non starter to fit into any airlift platform, much less a deck elevator and into a ship hanger.

A picture is worth a thousand words. This is one tall MutherFr. Sikorsky- Boeing always talk about rotor diameter and overall length, but never height. Chinook is 18.3 feet maximum height. My eyeball says Defiant could be taller to top of the hub. Now lets talk about rotor hub drag.

I definitely agree on the maintenance aspect. Think of the crane that is necessary not only to remove the rotor but to lift the gearbox! Based on aircraft proportions, that coaxial gearbox probably weighs more than a CH 47 gearbox also.

The thing is, this aircraft is to be a Black Hawk, not Chinook replacement. It would have the penalties of the CH-47's height but not the lifit and volume benefits. Also, the Chinook has maintenance platforms built into the housing of its aft rotor. How do people get to that upper rotor/hub without some pretty tall scaffolding?

Whenever Sikorsky depicts a maritime SB>1, it's always shown based on an LHA/D or larger ship. DDGs and the like appear to be non-starters.

Some wag elsewhere (not me) said that in this case you remove the fuselage from the transmission.

Just Dropped
 

It's good to see they've got another flight under their belt. Note the operative words in the video about demonstrations are "going to" instead of "have".

Another statement concerns me, "We showed that 230 knots that's what the Army asked for..." . Last I heard, what the Army asked for was 250 knots as the threshold, which the SB>1 team originally claimed they'd be able to achieve. Hopefully the Army hasn't lowered the requirements to accommodate one of the contractors.

Also notice they never mention range. LR in FLRAA is for long range. Remember when the Sikorsky program manager said they were proposing both a high speed config or a "long range" config and pressing the Army to select which is more important. So it is clear they will not meet any of the original program goals while their competitor has.

The FLRAA program speed requirements have indeed been lowered in addition to lots of other requirements changes over the last year... this doesn't change the fact that SB>1 had a design Vh much higher than 230 knots and it's taken a long time to get to this reduced goal.

As of Dec. 2019 I believe they were still @ 250 knots and in their Dec. 2020. announcement I believe the Army said the reason they were limiting further competition to just Valor and Defiant was because they said they were the only two competitors they were confident could meet the requirement. You are correct in stating, though, that even if the requirement was lowered, Sikorsky Boeing said they could do 250.

The parallel Marine Corp effort for a new medium lift which is paralleling the FLRAA has the speed requirement broken down into a required cruise speed of 280kts and a desired of 320kts, both of which are well within reach of existing V-280 technology. Does the dumbed down FLRAA requirements have similar required/desired metrics? if so, can anyone share the wording. Can't see the Army wanting to, once again, take an aircraft inferior to the Marines (e.g. V-22, CH-53K....).

This is quoted from a Breaking Defense article published April 05, 2019

Speed: The Army’s minimum acceptable cruise speed, the threshold requirement, is 250 knots (288 mph); its preference, the objective requirement, is 280 (322 mph, incidentally the intended cruise speed of Bell’s V-280). The Marines’ threshold is 275 knots (316 mph), almost as high as the Army’s objective; their objective is 295 (339). And that actually understates the difference, because the Army only asks for this performance at maximum continuous power — the highest the engine can sustain over a long flight — while the Marines want it at 90 percent of maximum continuous power.

The Marines have even higher speed requirements for brief sprints, something the Army doesn’t address".

Fastest level flight speed so far for SB>1 is 230 knots (264.7mph); for V-280 it's 305 knots (351mph).

Bell Dismantles SB Offering
 

Bell has announced the retirement of the V-280 after it has demonstrated all original FLRAA requirements in a three year/214 flight hour campaign. During this effort the aircraft demonstrated speeds 25+ knots above Bell's goal and 55+ knots above the original FLRAA goal. This compares to the SB-1 barely reaching a speed 20 knots below the original speed goal. As to other program goals the V-280 demonstrated those in flight testing on the actual aircraft while SB continues to promise future attempts to try to achieve in flight or pushing simulations as an acceptable alternative. Bell noted this in their end of testing press release with:

This level of trolling will make it difficult for the Army to give SB any credit for not demonstrating something Bell was able to. Now that the SB-1 performance shortcomings have been bared for all to see, it time for asking why SB could not achieve the requirements even when they were given an extra 1 1/2 years to try to catch up. In this time SB was only able to accumulate around 15% of the V-280 flight time. One needs to ask: What are the concept or reliability issues with the ABC that make it unsafe to even test, let alone risk our warfighters in?

Initiating high g turns, rolls and or dives, causing failure to maintain rotor separation. Safe separation is something never demonstrated by the X2, Raider or Defiant, under conditions of a hard maneuver. These hard maneuvers should be demonstrated by a real aircraft, including when there are strong crosswinds.

The Raider that crashed is proof the rotors are too close to each other. The Raider unexpectedly rolled to one side, then as the pilot corrected the roll, the rotors hit each other. The close spacing of the rotors decreases drag substantially. I have to believe Sikorsky has already told the Army that their demonstrators can't pull high g maneuvers. But that they will increase rotor spacing for production. Which will make a slow, tall Defiant even slower and taller.

Noneo wrote:

You are correct in identifying one of the few performance points the 97/SB-1 programs demonstrated which was the ability of the rotors to collide in moderate roll reversals. You are, however, naive in thinking the SB team will admit this concept flaw. They will instead hide behind a claim that a simple tweak to the fly-by-wire control laws has eliminated the chance of this flaw reoccurring. Only after the loss of a couple more aircraft will they propose changing rotor separation.

It's going to suck to be one of their test pilots.

Lets not omit the old XH-59. One wonders what the public perception of the X2 concept would be if the videos of both the XH-59 and S-97 prangs had made their way online like some of the old V-22 mishaps managed to.

https://i.imgur.com/s5fWKok.png

https://i.imgur.com/RwDNhDZ.png

https://i.imgur.com/y2zOH6W.png

https://i.imgur.com/150dfsr.png

Both programs were lucky the rotor collision events occurred on, or near, the ground or this "feature" of the ABC would not be so amusing.

FLRAA RFP
 

Article in Defense News:

https://www.defensenews.com/land/202...t-competition/

Finally new stuff to talk about.

Recently read an article on the Bradley replacement where they had to cancel the original program because only one vendor responded. They then dumbed down the requirements and went out for new responses. Apparently it is unfair to have requirements that only the best can meet. This explains the FLRAA changes.

As to the issue discussed most: \ the speed requirement is as follows:

So the min acceptable speed was reduced from 250 kts to 230 kts to keep SB in the competition to avoid the Bradley replacement problem. Much to SB's despair the objective speed was defined as 280 kts which the V-280 easily meets and the ABC concept can never meet. Additionally, the 280 kt objective matches nicely with the Marine new medium lift minimum speed requirement.

I think everyone knows how this plays out. No vendor team will get both the FLRAA and FARA. Even if Bell's Invictus equals the Sikorsky Raider in every way, Sikorsky will be given the program. FARA will only go to Bell if the ABC's concept deficiencies are overwhelming so Bell gets the FLRAA without years of protest from the losers.

Once the two programs are awarded, the emphasis on Pacific operations will keep FLRAA safe while the FARA can be terminated for cause when Sikorsky stumbles (face it what program have they recently executed flawlessly?) with minimum impact to the US's warfighting capabilities.

What maneuverability?
 

From "Spyclip":

https://www.secretprojects.co.uk/thr....13812/page-46

"I can confidently say that there have been a total of zero published flight demonstrations where level 1 handling qualities were demonstrated with requisite rates in pitch, roll, or (particularly) yaw on any ABC aircraft historically.

If any of these had been accomplished over the Jupiter swamps with Raider I would eat my shoe if Sikorsky would decline to publicize such footage.

The fact that the V-280 was able to perform these maneuvers at double the gross weight of S-97 makes the ongoing ad nauseum Sikorsky marketing of mythical extreme rigid rotor agility that much more egregious.

All of this is to say absolutely nothing of SB1's lack of demonstrating anything of note whatsoever."
____________________________________________________________ _________

A "feature" of the Kamov is that the rotors occasionally hit each other:
https://www.bbc.com/russian/russia/2...licopter_crash

The Raider and Defiant are more complex than the Kamov Alligator. They have a very large pusher prop, some say the largest prop ever produced in the US for Defiant. In a fast maneuver, there will be large 1-P moments input to the airframe. The control system must account for this, which may be ?impossible?. Note the AVX proposal had two ducted fans, so would not have this problem.

The massive diameter prop on Defiant will have a massive roll torque input to the airframe. When a conventional helicopter loses tail rotor authority, the helicopter spins about its axis. Large gear reduction, large torque, transmitted through the ring gear to the airframe. Normally the only roll torque in a conventional helicopter is from the much lower torque from the engines. The Defiant has to have a substantial gear reduction from the engine to the prop. Large gear reduction, large torque. The AVX design was better.

Now your rambling is just getting silly. Why would you think the prop torque would be a problem? Do you think there is not enough cyclic trim to balance it?

Your last sentence is a straw man argument.

Did I say Raider/Defiant couldn't fly because of it? No. Is it better not to have a large reaction torque? Yes.

To create a roll moment with the cyclics, the rotors are tilted with respect to each other, reducing clearance between the rotors, compared to the AVX design. There is a structural weight penalty. Flying with the rotors tilted with respect to each other reduces efficiency.

Edit -If cyclic control is lost, the helicopter could flip. AVX is just more proof, the better design does not always win.


Care to discuss Raider/Defiant lack of demonstrated maneuverability?

The comment made in that post about yaw is interesting.

I think a historical knock on tiltrotor maneuverability was probably based on yaw rate, which is primarily affected by rotor flapping limits. The V-22 has around 10 degrees of flapping cooked into the hub and I do not believe meets level 1 HQ yaw rate (probably exacerbated in crosswinds). The V-280 on the other hand showed some very impressive ADS-33 level 1 yaw maneuvers for its gross weight, thanks wholly to the massive increase to 15 degrees of flapping. When ground taxi turning in their demo footage, the flapping can look pretty wild!

Inherent to tiltrotors, the control power from flapping is applied at the wingtips, creating a solidly effective yaw moment arm.

Now with the compound coaxial pusher prop design, you have the large and heavy static pusher prop assembly, gearbox/bearings, massive stabilizer, and tailwheel assembly - a huge amount of dead weight at the end of a very long moment arm from the mast. The only way to achieve yaw control is by differential torque between the rotors, which have been designed to be as stiff and lightweight as possible. This is a massively different situation from a conventional helicopter where the yaw moment is applied by the tail rotor at that same location perpendicular to the longitudinal axis.

I wonder, at the same medium to large gross weights, is a high flapping tiltrotor going to have substantially better yaw control than a rigid rotor coaxial design with a pusher assembly? I would say almost certainly. It's a real shame that Defiant hasn't even attempted (that we know of) high yaw rate maneuvers that could be compared to the myriad V-280 demos. Perhaps they have and the results were disappointing.

I am going to Ramble on. Even with a clutch, the prop placement of Raider/Defiant is an accident waiting to happen. Unlike a Black Hawk, with a high mounted tail rotor.

Igor Sikorsky, truly an aviation genius, used two jet engines on opposite sides of the helicopter, to provide forward thrust on the ABC demonstrator. Then someone at Sikorsky had a wet dream that they would use a single large pusher prop instead.

A cyclic input will tilt both rotors in the same direction. Therefore the clearance will stay approximately the same.

That being said, I think the coaxial rotor idea is not very good. A lot more complexity. a large minimum level of rigidity in order to avoid collision between the rotors (which will lead to a rought ride, even worse than the BO105).
The rotor head on conventional helicopters creates about 30% of the parasitic drag. Parasitic drag is the main reason for power consumption at high speed. The SB-1 has two rotor heads. These may be encased in fairings, but many protoypes with rotor head fairings in the past showed only a small reduction of parasitic drag.

The effect of "advancing blade theory" could have been created just as well with a comparatively small asymmetric wing. This means a conventional configuration with an asymmetric wing and a thrust vectored tail rotor, or tail rotor at an fixed intermediate angle and compensating fin would have much less drag, less complexity, less risk of rotor blade collision, and easier to inspect and mantain main rotor head.

The XH-59B was the intended follow on to the XH-59A, and it had a ducted pusher prop design going all the way back to ca 1980.

https://www.sikorskyarchives.com/ima...A)/S-69-11.jpg


Interestingly, Igor died a year before the XH-59A first flew. He was quoted posthumously by Ben Kocivar in popular science's Sept 1982 piece on the "ABC copter" as having once said:


Genius indeed ;)

There are two cyclics. Tilting both rotors in the same direction creates a sideways force, the same as a conventional helicopter, but with no tail rotor. So we end up with a sideways push as well as roll. We need to counteract the roll torque from the prop. Tilting the rotors in opposite directions creates a pure couple, force times the distance separating the rotors.

You make very good points. Interesting, thank-you. Wouldn't the design you propose still have retreating blade stall at high forward speed?

Thanks for posting, very interesting! I was not aware of the XH-59B. From the picture, the pusher is counter-rotating and shrouded. So better than Raider/Defiant!
I can't blame Igor for that, or for being frustrated with coaxial designs.

A coaxial design will be limited in yaw when operating at maximum lift. Reducing collective on one of the rotors to change the reaction torque, reduces lift. If Sikorsky could have demonstrated a high yaw rate, they would have.

A good summary of the Raider crash.
https://verticalmag.com/news/ntsb-re...ider-accident/

Totally wrong. The coax limitation for yaw is in autorotation. They leave room for differential collective on top of collective.

A conventional helicopter has to react the thrust from the tail rotor with lateral cyclic. It's no big deal.

An X2 rotor system would use lateral cyclic, same direction for both rotors, to react any torque from the prop. Also no big deal.

In the absence of education or experience in these things, you might find it helpful to learn to fly helicopters, or anything.

There is a major difference between a prop and a tail rotor. Answer how the side force is counter-acted when both rotors are tilted in the same direction. Don't just give an arm wave. The reaction torque from a pusher prop is a pure torque. The torque from a tail rotor provides side thrust and moments (above the cg). Conventional helicopters have a much smaller roll torque from the engines. Did you ever take a physics class?

When you get an answer from your bosses at Sikorsky, I am willing to learn and be educated.