I guess beauty is in the eye of the beholder... took another look through patents and found what looks like the application for the SB>1 rotor hub. Compare the patent art with the first flight image without rotor fairings. Figure 3 looks like the bottom main rotor head.

US Patent Application showing what looks like SB>1 main rotor system

High resolution first flight image showing bare rotor system

Since you have experience in the Huey, you’ll appreciate the evolution of rotor head technology from greased steel bearings to... steel bearings and TT straps. All metal construction, highly loaded oscillating steel bearings, and even the blade lugs look like they have doublers bolted on. Speaks to the load magnitudes involved on a rigid rotor head of this size. Doesn’t look light or prone to long bearing lives.

Thanks for the drawings and picture.

Yes, beauty is in the eye of the beholder! LOL

That MR head is mighty complicated. It all undoubtedly works, but what is the real % gain over a conventional helicopter in terms of cost/maintenance/operational benefit? It seems a lot like an idea looking for a requirement at the moment, I was at West Palm Beach when the Comanche was canned...

Until they achieve and demonstrate a single promised performance metric, or even allow a non-company pilot in the thing to objectively assess the vibration, I would say the jury is still firmly out on that one.

Still no customer pilot has flown the Defiant?

Unbelievable that the US Army can be so patient.

Maybe the U.S. Army doesn’t have a good dental plan for their pilots.

I fly out of Fort Pierce and managed a glimpse of the Comanche many years ago, a cool machine.

As for the coaxial design, the obvious gain is no tail rotor and assorted systems and penalties. Whether that outweighs the complexity of 2 rotor heads I have no idea, but the Russians have been using them for years.

The advantage is not having to rely on getting any lift from the retreating side of the disks.

Sure, in a one-disk system, the advancing blade can generate a lot of lift, but it has to throw most of it away, because it can only provide the dismal amount of lift that the struggling, half-stalled retreating blade provides. Otherwise, the sides are not balanced. The advancing side, at top speed, could provide around 7 times more lift than it is allowed to generate. But use a coaxial system, and there is an advancing blade on both sides, forget the retreating blade, and now because all that lift can be used, the diameter can be smaller. Simples. Tccchhhk.

To get most of that advancing blade benefit, though, simply being a coaxial rotor system isn’t enough. You have to be able to carry large moments across the hub and hold what would normally be an unusual rotor trim on each rotor so the center of lift on each rotor is offset from the rotor shaft. The Kamov style coaxial rotor systems don’t do this, so they don’t get an advancing blade benefit. Only relatively rigid rotor systems can. Unfortunately, the ability to carry large 1P moments in a rotor also means you carry large 2P, 3P, 4P, 5P, etc. moments as well. This is where the real problems begin. While 1P moments are very large (and generally commanded), 2P moments are also quite large (and unhelpful). 3/4/5P harmonic blade loads are smaller (still unhelpful), but also significant. In a flapping rotor hub, you have some design freedom to select the number of blades and blade tuning to choose how to minimize the impact of these harmonic loads, and with enough blades, rotors can fly quite smoothly. In a rigid coaxial rotor, the same problem exists, but there are more design constraints. If the rotors have more than 3 blades, the large 2P loads will generally cancel out at the hub, hence the 4 bladed designs on the current ABC aircraft. A 4 bladed rotor will interact with 3/4/5P harmonic blade loads. Those harmonic blade loads WILL end up as vibratory hub loads that must be mitigated somehow. Studying a helicopter dynamics textbook will show that. It’s not just a matter of “poor blade tuning” these aircraft struggle with, it’s the intersection of concept and physics.

Maybe Sikorsky finally gets a really excellent anti-vibration control system working for all flight conditions. Maybe they lose both FVL contracts. We’ll see.

I used to be a Sikorsky/X2 guy. I'm probably not at liberty to say the particulars but I'll say you have your harmonics wrong with respect to what was creating vibes and what wasn't.


I have no knowledge of Raider or Defiant, but they got it working for X2. I personally don't see any reason why it wouldn't work for the new aircraft. They have excellent dynamics people, not just in theory but supporting the aircraft in the field and in the air.

I'd say the rotor is less complicated - it lacks a lead/lag hinge. It does have twice as many rotors, if that is what you mean as complicated. And, I'd also give you the fact the upper pushrods go up through the main rotor shaft, like on an Enstrom. Is an Enstrom complicated?

For a 4 bladed rotor system, 4P blade loads create 4P vertical shears at the hub. 3P and 5P blade loads get converted to 4P vibratory moments in the fixed system. Depending on where particular blade natural frequencies lay, you can somewhat minimize some of these responses at the expense of increasing others. I, personally, would rather deal with a pure 4P vertical shear from the hub than 4P pitch or roll moments in the fixed system. 2P vibes should be low but can exist if there are tracking or stiffness differences between the blades.

Pretty sure I have the above general statements correct since they would apply to any 4 bladed rotor (and ignoring higher harmonics). The X-2D, S-97 Raider, and SB>1 all have different hub and blade configurations. It’s not a given that the tuning and harmonic airloads across all three ships are identical, despite being 4 blades each. In fact, looking at the respective hub designs in patent art, I’m pretty sure they’re not identical. So the X-2D experience might not entirely translate to the larger ships.

Ifmu Wrote:

The X-2 was a tiny concept demonstrator with an occupied area of around 5 feet on the center line. It is simplistic to get that small of an area smooth in only one direction and at one frequency such as N/rev. The problem becomes significantly harder trying to suppress vibration over a large three dimensional space, which the last S-97 pilot report I saw says they have failed at. In fact the pilot quotes indicated the vibration (and loads?) were so bad that the crew were reluctant (note I didn’t say scared) to go above 180 knots once it became obvious the latest cure for vibration being tested did not meet expectations. As the 97 still appears to be restricted to 180 knots for even high profile demos they apparently failed to overcome the issue.

https://www.verticalmag.com/news/arm...utm_content=V1

53 knots below Raider’s target speed w/o pylons/weapons installed, but an important milestone for Sikorsky. Thanks for the link.

But when will an Army pilot fly the Defiant
 

When I first saw the article, I was expecting that a customer pilot was finally flying the SB>1 Defiant. I can’t quite understand all the hype for an Army customer Pilot flying the S 97. I would’ve expected this to have already occurred years ago.

The article did note that an Army customer Pilot flew the V-280 back in February 2018. Would have enjoyed reading an article comparing the handling qualities of the two aircraft by the same pilot.

Taking Lowering the Bar to Extremes
 

From Sans earlier:

[QUOTESeems Sikorsky's own demonstration flights have implied that. After reaching barely over 200 kt in Sept 2018, they subsequently have flown progressively slower in their big-to-do demo flights - only reaching 190 kt in July 2019 and then 180 kt in Feb 2020. And of course, nothing since then at all.
][/QUOTE]

From above article:
Unless I missed a revision, the FARA min max speed was 180 knots. In what fuzzy world is 187 knots (+4%) considered well in excess of that requirement? In my world that difference is considered barely outside allowed measurement error. I know the commercial requirement is to demonstrate 10% margin to an never exceed airspeed limit, so they are (in their world) grossly short of the 198 knots required to get credit for being able to do 180 knots.

Sikorsky really needs better PR people. The speed thing is bad enough, the I not letting the Army sit at the real controls (paraphrasing) is worse. Implies that the 97 is not the easiest thing to fly. Wonder if their scared the Army pilot be slightly aggressive on the controls ending up in another collision between the rotors.

Firstly, that was not a flight of (an actual FVL aircraft) Defiant. Secondly, I must have missed the part in there where Charlie was permitted to make a single comment about the Raider flight (specifically the vibration). The fact it was explicitly mentioned that he was muzzled about the experience is telling.

It's definitely eyebrow raising that it took over 5 years for a pilot to set foot into the S-97. The V-280 had the Army XP flight 2 months after first flight!

The V-280 has had multiple guest pilots including, I believe, four Army. It's like they did decades ago with the XV-15, including at least one Senator.

Bell Tiltrotor Guest Pilots
 

Before it was sent to the Smithsonian Air and Space Museum, the actor Harrison Ford flew the XV-15. Maybe he will be the next V-280 guest pilot ;-)