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Sikorsky S-97 Raider Ground Tests Today

Randy Daytona

Cold War Relic
pilot
Super Moderator

Brett327

Well-Known Member
None
Super Moderator
Contributor
Interesting. Does a coaxial configuration reduce the effects of retreating blade stall - either aerodynamically or by allowing a comparatively slower Nr?
 

Randy Daytona

Cold War Relic
pilot
Super Moderator
The S-97 is what is known as an ABC (Advancing Blade Concept) - basically it simply disregards the retreating blade while the advancing blades on each side balance each other with the pusher prop providing forward thrust. The rotor system is a variable speed - another limit on helicopter forward speed is when the blade tips start to go trans-sonic. If you slow the rotor system, you can increase forward speed. The problem you will have as you try and scale a coaxial to a bigger design is blade flapping - i.e., you don't want the blades to hit each other. You can increase the distance on the rotor hub between the blade systems but that increases drag and the rotor hub can be up to 40% of the total drag.
 

HokiePilot

Well-Known Member
pilot
Contributor
Interesting. Does a coaxial configuration reduce the effects of retreating blade stall - either aerodynamically or by allowing a comparatively slower Nr?
Yes

With an advancing blade on both sides, the retreating blade doesn't have to take the lift. The blade pitch on the retreading blade is reduced by control system. With a reduced pitch, it doesn't have to go near critical AOA.

Lower Nr would actually aggravate retreating blade stall.
 

Brett327

Well-Known Member
None
Super Moderator
Contributor
Yes

With an advancing blade on both sides, the retreating blade doesn't have to take the lift. The blade pitch on the retreading blade is reduced by control system. With a reduced pitch, it doesn't have to go near critical AOA.

Lower Nr would actually aggravate retreating blade stall.
Fascinating - many thanks! How does lower Nr aggravate RBS? That is counter-intuitive to my liberal arts brain. Lower Nr, retreating blade retreats less fast, no?
 

JTS11

Well-Known Member
pilot
Contributor
Retreating blade needs to be going faster to get Bernoullis going over wing
 

Brett327

Well-Known Member
None
Super Moderator
Contributor
That makes sense now, I was giving the airstream more credit than it was due. So, what they need is a blade that is mechanically retarded as it advances, then mechanically advanced as it retreats. Sikorsky engineers - get crackin'! :)
 

Hotdogs

Leeroy Jenkins
pilot
Lower Nr, results in higher AoA to produce same amount of lift, which starts to increase the size of transverse flow region, tip stall, and ultimately resulting in blade stalling.

Curious if this system is a bearingless, hingeless system or a fully articulated one like most Skorisky birds.
 

RobLyman

- hawk Pilot
pilot
None
I'm heading to Sikorsky next week for an aircraft ferry mission. I'm hoping to get a peek of it. I got to see the tech demonstrator X2 last time I was there. I'm sure no photos will be allowed up close.

BTW, the X2 had ridged rotors. No hinges.

Maybe I'll have something else to report the week after next when I get back.
 

Randy Daytona

Cold War Relic
pilot
Super Moderator
The S-97 is a rigid rotor system. The question is: how large can you make a rigid rotor? Until the new Bell Y/Z came out, the biggest rigid rotor I knew of was the Bell 412 (11,900 pounds, titanium hub). Anything larger than 20,000 pounds seems to require a traditional fully articulated system. The question of being able to scale up is important because the motherload of helicopter contracts - the eventual replacement of the H-60 for the Army. This competition appears to be between the Boeing / Sikorsky SB-1 Defiant (compound helicopter) and the Bell / Lockheed V-280 Valor (tiltrotor). Interesting that in the V-280, the engines do not rotate upwards like the V-22, only the rotors.

http://news.usni.org/2014/10/03/u-s-army-selects-bell-sikorskyboeing-build-prototypes-next-generation-helicopter-program

http://defense-update.com/20140812_valor-and-defiant-to-race-for-the-army-future-helicopters.html#.VNTMFS7Y-lc
 

Uncle Fester

Robot Pimp
None
Super Moderator
Contributor
Interesting concepts. One of the first things I thought was neither looks like it could be folded or broken down very easily for C-5 loading. I thought that was more or less a universal helo requirement?
 

SynixMan

Space Cadet
pilot
Contributor
Interesting concepts. One of the first things I thought was neither looks like it could be folded or broken down very easily for C-5 loading. I thought that was more or less a universal helo requirement?
C-17, but generally yes. H-60s all have hinges that the blades can be folded on. Not sure if the AH-1Z/UH-1Y has the same.
 

RobLyman

- hawk Pilot
pilot
None
There is very little new about the X2/X97. Most of the concepts have been tested before. What appeared to be new to me when I visited with some of the the X2 engineers was that the X2 was the first time many of the technologies were combined in a very synergistic manner. Rigid rotor, advancing blade concept, pusher prop (more versatile and effecient then a pair of fuselage mounted turbofans), fly by wire, active vibration control system, and a few other goodies combined into a very elegant X2 built with many off the shelf components. Randy Daytona has a point. This elegance may vanish as the airframe is scaled up. Lets hope not.

Edit: Blade flapping might be managed with fly by wire. Control inputs to the swashplate(s) can be varied with the flight regime to reduce flapping, vibrations and pilot work load.
 
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SynixMan

Space Cadet
pilot
Contributor
Looks pretty cool. Hopefully they're smart enough to consider marinizing it right away for the Navy.
 
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