Sikorsky X2 coaxial heli developments.

[Dave_Jackson]

Originally Posted by Graviman
IFMU, the advantage here is that the pusher prop can be used to pull energy out of the airstream. The immediate advantage is a lack of need to drop collective and flare in autorotation from cruise. This also means that the main rotor does not have to be at flat pitch during autorotation, with forward cyclic, which allows good yaw authority. Coaxials can suffer from poor yaw control in autorotation, as the differential collective pitch does not transfer torque as when in level flight.

Mart, would you please provide your source(s) for these intriguing statements.

Dave

[Graviman]

Dave, this stuff was cited by Prouty about the AH-64 Cheyenne in Helicopter Aerodynamics. Cheyenne was a compound helicopter which settled on a seperate pusher prop after eliminating various alternatives. Having a pusher prop doesn't eliminate the need to do anything in autorotating, but it does give the pilot more choices. It makes sense to me that X2 would take advantage of these. I think you can also make out a rudder line on the X2 central vertical stabiliser.

http://en.wikipedia.org/wiki/AH-56_Cheyenne

Also i just noticed the use of NASC delta shaped intakes on the X2. I wonder if there is an advantage to be had here for spinning out debris from turbine ingestion. I've got X2 as my workstation backdrop, and it attracts a lot of interest. I think Sikorsky is onto a real winner with this project.

[JohnDixson]

AH-56 Engine Failure

I think I recall reading that the AH-56 had an automatic prop beta control, which would feather the pusher in the event of failure of the single T-64 engine.

John Dixson

[Dave_Jackson]

Mart,

Thanks for the reply.

IMHO, IMFU's concern about efficiency is valid. Why add the aerodynamic power loss of the propeller, plus the mechanical power loss of the powertrain to drive the main rotors when they can be driven directly from the airflow?

John mentions the feathering of the propeller. In addition, Sikorsky might have found it wise to insert an overrunning clutch in the propeller's powertrain for safety. This will preclude back-driving the rotors.


Prouty state that "One of the problems inherent in the compound helicopter configuration is that when entering autorotation at high speed the wings tend to produce so much lift that the rotor is starved for for the necessary thrust to keep it spinning.".....
.... the pilot could reduce the pitch of the propeller - thus converting it into a windmill that could extract energy out of the passing airstream. This could keep the entire drive system turning until the aircraft had slowed down to about 80 knots, at which speed it was put into conventional autorotation.

It would appear that this problem only exists because of the wings. I speculate, that this may only be applicable to the X2 if Sikorsky has a future intention of locating small wings under the large root cutout area of the new 'reverse taper' blades, to improve the craft's high speed performance.


Since we are limited to speculation;
I speculate that significant technical information on the X2 may be a long time in coming, if at all. Unlike the original coaxial-ABC, this project is not funded by the government and there may be little incentive on the part of Sikorsky to divulge much more than information for marketing purposes.
__________________

PS. You may wish to reassess you last statement regarding coaxials and yaw control.


Dave

[busdriver02]

While I agree that loads of info won't be forthcoming from Sikorsky any time soon, however if they do win a contract based on this tech, we might get some more good info then.

[Graviman]

Dave, John, agreed now that the pusher prop is probably of limited use in autorotation. Most likely it would be feathered to minimise drag, and may well have an overrun clutch. I'll go back and see if i can reword that post to avoid any misinformation.

Dave, i thought that coaxials could suffer reduced yaw control in autorotation for a given pedal movement? The little vector sketch in Prouty shows the rotors in windmill brake state generating some torque, so is probably not that valid. That central rudder will likely overcome any reduced effectiveness though.

CEFOSKEY, that's a shame that the X2 technology is being constrained in the marketplace before it has demonstrated it's potential. Hopefully that armed escort contract will allow this development to market. To my mind it is the right way to go for high speed helicopters.

[Dave_Jackson]

For those who might be interested.

Some calculations from CEFOSKEY's links:

[IFMU]

Some more media coverage

http://www.shephard.co.uk/Rotorhub/S...2-2077277627c1

http://www.aintv.com/ then look for "x2 revealed"

Dave,

That disk loading seems amazingly low compared to an S76 or other high performance single rotor helicopter.

-- IFMU

[Dave_Jackson]

IFMU.

You are correct. The weight was WAY out.

The page has been corrected and the disk loading is now 7.6 lb/sq-ft.

Thanks

_________________

For comparison these are some of the specifications on the previous Sikorsky S-69 (XH-59A) ABC

Dave

[Graviman]

Hmmm, some interesting technical information there, Dave. Noticeable how the X2 disk loading is higher than S69. That is almost a statement of intent that X2 will be a fast machine.

Chris, if X2 looks good now then those fairings will just be the cream on the coffee!

[Graviman]

Dave, i'm about half way through that Leishman paper. It is interesting to note that a practical coaxial with vertical rotor seperation will only suffer about 22% increase in power over widely seperated rotors, and not the 41% often quoted. To my mind this provide further justification of the X2 design over the lateral seperation of say a V22.

For a given disk loading, hence power required for HOGE with a specific payload, Sikorsky's X2 concept will always be able to land in places too tight for any tilt rotor. Also the lower roll inertia means X2 based designs will be better suited to landing on the heaving deck of a ship. The numbers just add up.


Maxtork, this paper details how you estimate coaxial performance:

http://www.baldwintechnology.com/MTR_AHS06.pdf


Expect to see X2 spawn a new generation of transport concepts...

[Dave_Jackson]

Mart,

I totally agree that twin main rotors (or perhaps an even number of main rotors ) is the future.

The question of Rotorcraft vs PropRotor, and the sub-questions related to which configuration in each of the above two categories is optimal will probably depend on the specific applications for the craft, plus what future technological advancements come about.

IMHO, the coaxial-ABC is a step in the right direction but it is inferior to other twin-rotor alternatives. Maybe we will eventually find out why the coaxial-ABC was rejected from the VHL completion.

Dave

[Graviman]

Dave, get your ABC intermesher project off the ground and i might agree.

For big government contracts, politics plays as much a part as engineering.


I congratulate Sikorsky for having the gumption to do X2 on it's own to prove to the world that this design will work.

[chopper2004]

X2 from Heli Expo

heres a pics I took on Sunday in Houston

[IFMU]

Thanks for posting the pics chopper2004. Mart, looks like there is a name on the canopy on the second photo, maybe that answers your question about who is flying it. 007?

Maybe we will eventually find out why the coaxial-ABC was rejected from the VHL completion.

Mebbe it shows so much promise that somebody is concerned to see it compete against a tilt rotor.

-- IFMU

[Graviman]

Good luck with the first flight of X2, Chief Test Pilot K. Bredenbeck.

Thanks for the pictures Chopper2004, it looked like a great event.

IFMU, i think that was very much the concern for the JHL contract. V-22 represents many 1000s of man-hours investment. To simply pull the plug on tilt-rotor would leave a lot of folks very annoyed. Sikorsky have taken a very sensible approach and developed X2, in the same way that Bell originally developed XV-3 and XV-15, as a technology demonstator aircraft.

In the long run, i believe that X2 represents the next generation of helicopter development. The fact that the engineering numbers describe a machine with good payload-range performance means that there is great commercial potential. I wish Sikorsky luck because i would like to see the potential realised.

[Dave_Jackson]

Mart,

The fact that the engineering numbers describe a machine with good payload-range performance ....

Would you please provide the source of this fact.

Thanks
Dave

PS.
Here is another interesting article on coaxials that you might like.
A Survey of Theoretical and Experimental Coaxial Rotor Aerodynamic Research

[Graviman]

Dave, compared to V-22, X2 will have a lower disk loading so will require less power for a similar MAUW. My source:

http://en.wikipedia.org/wiki/V-22_Osprey
Disc loading: 20.9 lb/ft² @ 47,500 lb GW (102.23 kg/m²)

http://en.wikipedia.org/wiki/Sikorsky_X2
Disc loading: ? lb/ft² (kg/m²)

http://www.unicopter.com/1465.html#Calculation
Disk loading at GW: 6.2 lb/sq-ft - S69 spec quoted

Even accounting for errors and design difference in X2, the difference is disk loading is such that coaxial has to have better hover performance.
I'm confused, i thought you had a guestimate disk loading for X2 at some point?

CEFOSKEY, it a bold experiment indeed. Good luck to all involved!

[Dave_Jackson]

Mart,

"... compared to V-22 ...

Thank God. For a moment, I thought that you might have been comparing it to the Interleaving-ABC.

I'm confused, i thought you had a guestimate disk loading for X2 at some point?

It was removed because of the problem in determining 'Effective' Disk Area.

Dave

[IFMU]

Top secret video of the first X2 flight!

http://www.youtube.com/watch?v=G9ZTfbcao1M

Quick! Before they pull it!

-- IFMU