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-   -   Sikorsky SB-1 flies for first time (https://www.pprune.org/rotorheads/619699-sikorsky-sb-1-flies-first-time.html)

The Sultan 18th Apr 2019 04:17

Only two flights in a month. Not an impressive demonstration pace.

This guy is struggling to make the program sound viable. Really not enthusiastic about what he is spinning.


IFMU 18th Apr 2019 14:35

Thanks for the link! I hadn't heard they had a second flight. Not sure I agree that he was struggling to make the program sound viable, but I don't think he is very knowledgeable about the technology.

Jack Carson 18th Apr 2019 20:32

The X-59 36 Years ago
 
I had an opportunity to fly the X-59 with the two J60 engines (3000 lbs. thrust each) installed in March of 1983. It was a short 45 minute flight that began with a short running takeoff followed by what would be categorized as a maximum performance climb at 45 degrees pitch up. Once leveled off we made a level speed run at just under 250 KIAS. As we rotated during the take off roll the torque from the PT-6 twinpac (1800 ESHP) reduced to 0%. The rotor system operated as an autogyro from takeoff rotation until we slowed for our descent to approach for landing. The PT-6s smoothly re-engaged providing power for hover and landing. The short end of the story is, the X-59 flew more than 100 hours and achieved a pretty significant flight envelope 36 years ago. It required a combined 6000 lbs. of thrust from the two J-60ís to push this little 11,000 lb. aircraft out to a maximum of 263KIAS. I have to believe that the SB-1 Defiant will require more than a T-55 engine to meet its goals. To say that the vibrations during my flight were significant would be an understatement.


The Sultan 19th Apr 2019 00:37

​​​​JC

From Bell site:



Bell Helicopter's compound research helicopter, a highly modified Bell UH-1, attains a speed of 316 mph in level flight. This is an unofficial speed record and has remained unchallenged.
That is 275 kts which indicates with enough thrust even a conventional helicopter can get to high speeds. Like the -59 this test bed had no range and shook like hell.

IFMU 19th Apr 2019 03:26


Originally Posted by Jack Carson (Post 10450765)
I had an opportunity to fly the X-59...

Jack,
When I was recruited to design and build the X2 FBW I spent a lot of time studying the ABC and talking to the old timers. Everything you say about the ABC is true. The X2 and its successors are different in two respects. The blade planform is a leap way beyond what you had on the ABC. The drag is way lower even without the nifty inter-rotor fairing. Also due to the driveline, speed of the main rotor is controllable. It doesn't autorotate away like the ABC, even at flat pitch. The ABC was a cool aircraft, and it advanced the state of the art of blade construction, but still seemed to be designed around hover. You probably remember the twist of the blades. What is good for low speed is bad for high speed, and visa versa. Time will tell for the Defiant, numbers don't lie.
Bryan Cotton

Commando Cody 19th Apr 2019 06:23


Originally Posted by The Sultan (Post 10450897)
​​​​JC

From Bell site:




That is 275 kts which indicates with enough thrust even a conventional helicopter can get to high speeds. Like the -59 this test bed had no range and shook like hell.

There used to be a saying that the F-4 proved that with enough thrust, even a barn door will fly.


Jack Carson 19th Apr 2019 15:49

IFMU
There are always compromises. Having enough blade twist to a achieve hot high hover capabilities while achieving reasonable cruise speeds and range is challenging.

SplineDrive 19th Apr 2019 21:40


Originally Posted by Commando Cody (Post 10434760)
What Rugen is describing is pretty much what was always planned for FVL-M. His statement about, " We know a lot about that lift-offset compound already..." doesn't make a lot of sense given X2's track record (or lack thereof) so far.

What the Army may well know is that a flatwise rigid rotor will have very significant levels of vibratory hub moments and vertical shears unless the number of blades +/- 1 is higher than any significant harmonic air loads in the blades. With 3 blades, the XH-59A had blade 2P, 3P, and 4P translating into vibe hub loads and blade 2P is always quite high leading to epic levels of vibration. A four bladed rotor avoids that 2P generating vibe hub loads, but 3P, 4P, 5P are still reasonably large and it's nearly impossible to not have a flatwise blade mode in that region, amplifying one or more of those air loads. The 4 bladed X-2 rotors will have less-than-epic, but still incredibly high levels of vibratory hub loads.

The aerodynamic benefits of the ABC rotor are significant but they come at considerable cost.

IFMU 19th Apr 2019 22:27


Originally Posted by Jack Carson (Post 10451354)
IFMU
There are always compromises. Having enough blade twist to a achieve hot high hover capabilities while achieving reasonable cruise speeds and range is challenging.

If you look at the installed power required for high speed flight, you are more than covered. Also with 8 blades there is enough solidity to absorb the power required for high/hot even without all the twist.

IFMU 19th Apr 2019 22:33


Originally Posted by SplineDrive (Post 10451558)
The 4 bladed X-2 rotors will have less-than-epic, but still incredibly high levels of vibratory hub loads.

They certainly can, but we were balancing the X2 using conventional techniques. Then vibes were cleaned up with AVC. The predominant harmonics are not exactly as you say but not sure if that info was published.

SplineDrive 19th Apr 2019 22:56


Originally Posted by IFMU (Post 10451586)
They certainly can, but we were balancing the X2 using conventional techniques. Then vibes were cleaned up with AVC. The predominant harmonics are not exactly as you say but not sure if that info was published.

Right, I'm speaking from a "first principles" textbook point of view, the full problem is certainly more complex, but unlikely in a positive way. Question is, can that AVC clean up the vibes to a production level across an entire flight envelope? I'm sure the Army will be interested in that answer.

IFMU 19th Apr 2019 23:08


Originally Posted by SplineDrive (Post 10451597)
Right, I'm speaking from a "first principles" textbook point of view, the full problem is certainly more complex, but unlikely in a positive way. Question is, can that AVC clean up the vibes to a production level across an entire flight envelope? I'm sure the Army will be interested in that answer.

We did it on the X2. Not just AVC, you need to balance too. Most likely the Raider and Defiant guys have gotten further as management during X2 didn't allow as much dedicated vibe work as we would have liked.

CTR 20th Apr 2019 14:49

AVC a Flight Critical System on Defiant and Raider?
 
I have been wondering for a while. Is the AVC system on the Defiant and Raider considered a flight critical system? By that I mean, is it like a flyby wire flight control system where redundancy is required and complete system failure can result in loss of the aircraft?


IFMU 20th Apr 2019 18:11

Definitely not for the X2. I can't imagine it is flight critical for its progeny.

CTR 20th Apr 2019 19:12


Originally Posted by IFMU (Post 10452092)
Definitely not for the X2. I can't imagine it is flight critical for its progeny.

Has enough flight envelope on any X-2 been expanded enough to be remotely sure?

IFMU 20th Apr 2019 20:10


Originally Posted by CTR (Post 10452136)


Has enough flight envelope on any X-2 been expanded enough to be remotely sure?

Yep. 252 kts level flight for the X2 and something like 262 in a shallow dive.

SplineDrive 21st Apr 2019 23:36


Originally Posted by IFMU (Post 10452166)
Yep. 252 kts level flight for the X2 and something like 262 in a shallow dive.

There is far more to a full flight spectrum than straight and level flight, even high speed flight, nor is it a forgone conclusion that high speed flight is the worst regime from a vibration control standpoint. From released videos and statements, it appears the S-97 has performed a wider variety of maneuvers, Nz levels, etc. than the X-2 demonstrator did, even without yet hitting its top line speed goal. Sikorsky has been publicly pretty mum on what is has learned about vibration control on that aircraft, so it's hard to project much to SB>1.

However, that said, if vibrations on a rigid rotor can be significant, and I believe math and available evidence says they can be, then an AVC system has to be suitably powerful. An active system capable of (nearly) canceling large vibrations is technically capable of nearly doubling them with incorrect system operation. It might not need to be strictly flight critical, but it could well be mission critical.

CTR 22nd Apr 2019 02:11


Originally Posted by SplineDrive (Post 10452830)
It might not need to be strictly flight critical, but it could well be mission critical.

From what I have been told, PIO from severe vibration can be a concern. At least with FBW with notch filters this can be addressed. This may allow AVC loss to be only mission critical.

SplineDrive 22nd Apr 2019 21:15

Yup. I’m sure cribbing S-97 line drawings generated some laughter at Bell.

PAXboy 6th May 2019 22:21

As a rank outsider who knows nothing about Helis and had never heard of this machine until deciding to browse this forum:

Looking at the machine, I have to ask, why would anyone do that? As far as I am aware, the Osprey is not a runaway success and this machine looks like it's trying to tackle the same problem from a slightly different angle.


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