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Sikorsky X2 coaxial heli developments.

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Sikorsky X2 coaxial heli developments.

Old 8th Dec 2009, 21:41
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Mart,

The primary advantage of the Intermeshing over that of Coaxial is the reduced disk-loading for physical identically rotor-disks. In hover, the Intermeshing's larger 'effective disk area' provides a greater efficiency (Figure of Merit). In forward flight, the Intermeshing provides a more equitable disk loading because the rotor-rotor overlap of both rotors is in the 'reverse velocity' region.

I have no ties to any specific configuration, and change concepts of interest faster than changing the bed sheets. Your proposed research into the ABC concept would be challenging and hopefully productive. I would constructively suggest that your focus should be directed toward the segments of the blade that are required to work in the reverse velocity region during fast forward flight. The profile drag and negatively induced lift are the biggest hurdle for the ABC concept; be it Coaxial or Intermeshing.

IMO,the current blades on the X2 are an attempt to 'sweep the problem under the rug' and not an attempt to confront the problem head-on. In all fairness, perhaps the X2 is intended as a working platform upon which ideas to improve the blade can be tested ~ with your help.


Dave
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Old 8th Dec 2009, 23:38
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Well, it wouldn't make much sense in a normal config (transport etc) but in the attack role it'd make sense to have some stub wings, since you can mount weapons on it.

Six pylons, titanium bathtub, ABC tech, good hot/high perf, and good avionics/sensors would be a godsend for attack helicopters I think.
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Old 9th Dec 2009, 02:24
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In the fall 2009 edition of vertiflite, they have a 5 page X2 article written by Frank Colucci. One quote from page 28 of the article:

Originally Posted by Frank Colucci
For all the challenges, the ABC demonstrator was exceptionally responsive. The XH-59A flew snap turns, high-rate sideslips, and level transitions from hover to forward flight. It pulled 2g turns at 240 kt, limited by design considerations. One Sikorsky pilot estimated the ABC was 50% more maneuverable than a single-rotor helicopter of the same size, and the Army concluded that the XH-59A, with no anti-torque rotor to waste power, was more efficient in a hover than conventional helicopters. Plans for a fixed auxilliary wing on the XH-59A proved unnecessary. According to Art Linden, "the one thing we proved beyond a shadow of a doubt is we did not need the wing... [the rotor system] developed all the lift you could ever use."
So I would think you would not need a wing. The ability to generate lots of lift also can translate in the ability to pull G's, or to manuver. The LTH mockup shown in the same article shows some rocket-pod looking things. No sense putting wings on, that is only more drag. If adding more wings always made things better, we would all be flying around in triplanes.

I wonder how well the servo flap idea would work on the x2.

-- IFMU
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Old 9th Dec 2009, 02:47
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Servo Flaps

Thanks for the expanded answer, Dave.

I doubt if the aerodynamic impact of higher speed helicopter flight was on anyone's mind at the time Mr. Kaman pulled out of Sikorsky to start his own company, but the blade pitching moments created at free stream mach numbers above 0.92 have proven to create some problems, and one wonders whether the tie bar design present in the SH-2 design, for instance, is strong enough to prevent destructive behavior. I'm told that when Kaman did a higher-harmonic control system design that went into the wind tunnel, it had a pushrod as well as a servo flap, but that might have been for other reasons.

Thanks,
John
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Old 9th Dec 2009, 04:12
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John, this may, or may not, answer your last sentence.
Rick Andrew thinks that Kaman has a patent on a rotor blades control where a swashplate sets the root pitch and his servo-flap sets the 75% of span pitch. He is going to look up his copy of this and send it to me. I could not find anything relevant by doing a patent search on 'Kaman Aerospace' and by reviewing the Kaman folder. September 13, 2002
He came across the article. It was 'CTR Control System', on the Seasprite, in Aviation Week & Space Technology ~ Aug 5, 1968 ~ March 31, 2004
The only relevant meaning for the acronym CTR that I can find is Civil Tilt Rotor.
Very mysterious.


Dave

Last edited by Dave_Jackson; 9th Dec 2009 at 04:22.
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Old 9th Dec 2009, 12:41
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Hunt3r, have you heard something about a DoD contract then?


Originally Posted by Dave Jackson
The primary advantage of the Intermeshing over that of Coaxial is the reduced disk-loading for physical identically rotor-disks. In hover, the Intermeshing's larger 'effective disk area' provides a greater efficiency (Figure of Merit). In forward flight, the Intermeshing provides a more equitable disk loading because the rotor-rotor overlap of both rotors is in the 'reverse velocity' region.
Dave, i think the fairest comparison (when i get around to it) would be to compare machines capable of landing in the same ground box area. This would allow the benefits of better FM through such things as reverse velocity utilisation to be evaluated in a holistic way.

Last edited by Graviman; 9th Dec 2009 at 19:18. Reason: Typo...
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Old 9th Dec 2009, 19:16
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Chris, those pylon mounted rocket pods don't look very "low observability". I'll bet that if the project goes that way then Comanche style features begin to reappear...
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Old 10th Dec 2009, 02:19
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Well, it'd be nice if this X2 attack version were really meant for CAS. A-10 tough. Big bullets, lots of rockets, etc.

The Apache/A-10 combo leaves a pretty big gap in terms of CAS. Not enough rockets/missiles.

I guess I'm thinking of the X2 as a fast, survivable, and heavily armed attack helo. Something that can really lay down the lead.

Last edited by Hunt3r; 10th Dec 2009 at 03:10.
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Old 10th Dec 2009, 02:49
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Like the Beech Starship pushers do not do well with FOD and such. Looks like a Hellfire will blow the pusher off.

Another thing, how fast has the X2 gone, seems really quiet so there most be little to report.

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Old 10th Dec 2009, 03:35
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Still stuck at 106 kts according to today's rehash of last month's Popular Science award.

Still, there's still another 3 weeks to hit that 250 kt goal...

I/C
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Old 10th Dec 2009, 04:48
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Cmon... 250 knots... 106 knots is pitiful for a design that could reach above that with some turbojets.
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Old 10th Dec 2009, 12:34
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Don't forget this is very much an experimental flight test program.

This means that for every little stretch of the envelope:
The stress engineers will be carefully pouring over the strain gauge data to see that the stress / fatigue calcs are correct.
The dynamics engineers will be carefully pouring over accelerometer data to see that the rotors flex in the way that was predicted.
The control engineers will be pouring over the FBW logs to see that the actuators are responding as anticipated.

Frankly, i would be astonished if this was the only program to achieve its stated aims without one group finding something that they need to understand better. Look at how many redesigns occured between first flight of VS-300 and R4. That's just engineering.

The most important aim for every flight is to walk back into the office...
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Old 11th Dec 2009, 02:36
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Taking it slow with the X2 flight testing

Sikorsky is probably taking it slow with the X2 flight test program due to still being a little gun-shy with control of a rigid rotor aircraft. Recall that they had a crash very early in the flight testing of the XH-59 (ABC) program, that set them back about 2 years.

One thing I find interesting about the X2 is the unique sound that its rigid rotors produce. It sounds more like a turboprop than a traditional helo's rotor or even the V-22's prop-rotors.

As for the max speed with the X2, I've seen lots of recent patent applications from Sikorsky covering two-speed drivetrain arrangements. Does anyone know if the rotor system on the X2 is intended to run at variable speeds?
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Old 14th Dec 2009, 12:30
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Riff_raff,

The main rotor reduces RPM with speed. I seem to recall that the design is two speed with autorotation for cruise. Not sure what the scheduling is, but the idea is to hand over power for forward thrust to pusher prop with speed. This keeps the fuselage (and hubs) level to minimise drag. The pusher prop also becomes more efficient as speed increases air mass going through it.

The tip noise on X2 should be far less pronounced than other helos. Normally a square tip produces very concentrated tip vortices, but the profile used on X2 will allow a much larger tip vortex with reduced vorticity. If you look at the just flown A400M you can see a similar idea in its turboprops - the other benefit being improved flow at high tip mach numbers. In a helo much of the noise (particularly during manouvres) comes from Blade Vortex Interaction (BVI) so less localised vorticity helps to reduce "blade slap".
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Old 14th Dec 2009, 14:48
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I find it pretty interesting that there has been so much hype about the X2, especially as the Russians have been operating the KA-50 for a number of years. It is a contra-rotating co-axial rotor system thought it does lack a pusher prop. Is the hype because of the design or because of the technology being applied to the design or because a western corporation is developing the product?
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Old 15th Dec 2009, 14:21
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Thank you for pointing me in the right direction...

TGZ
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Old 17th Dec 2009, 06:30
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TBH, I don't think that the X2 would really quite fit in the recon role like the Commanche. It should be able to do recon, but should also be able to fulfill CAS and anti-armor roles effectively.
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Old 18th Dec 2009, 02:30
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Hunt3r,

Is the X2 technology really only applicable to attack? I bet it would be just as applicable to other missions, like utility, offshore oil, executive transport, SAR, etc. It is still just a helicopter. If they succeed it will be a fast helicopter, with longer legs to boot.

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Old 18th Dec 2009, 14:07
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...or this one circa 1973...



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Old 19th Dec 2009, 04:41
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Well a faster helicopter would be better for everything. As transport in general, attack, recon, etc.

Also, the AH-59 would be a nice thing to think about...
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