Sikorsky X2 coaxial heli developments.

[IFMU]

The palm beach post published a correction:

Correction: A previous version of this story about Sikorsky Aircraft's X2 high-speed helicopter suggested the aircraft requires a runway to land. Sikorsky says the X2 is capable of vertical takeoffs and landings.

Can be seen at the original link: Palm Beach Post X2 article

-- IFMU

[Graviman]

CEFOSKEY, seeing these machines flying around in service will make for exciting times!

IFMU, that's feedback in action.

[riff_raff]

It's too bad they're going to cover the X2 rotor hub with a fairing. That rigid rotor hub structure is a real pretty piece of design work. I wonder if most people really appreciate how difficult it is to design a lightweight hub structure and pitch bearing mechanism capable of handling the huge moment forces produced by a rigid rotor. It was likely a difficult design problem at the small scale X2 demonstrator. One can only imagine how hard the problem will be to solve with the exponentially larger moments imposed by a larger diameter rigid rotor.

Don't know what's in store for the X2's future though. It was dropped from consideration in the JHL/JFTL program. Apparently it didn't fare too well against conventional tiltrotors in the JHL trade studies. To make up for it though, it looks like the program management threw Sikorsky a bone by suggesting they dust off their VDTR concept.

[ramen noodles]

riff raff,
Dont worry about JHL, its as dead as a doornail, because FCS is dead. Imagine using 250,000 lb rotorcraft to carry tanks, the enemy would surely be defeated - they'd laugh themselves to death!

[Dave_Jackson]

riff_raff,

I wonder if most people really appreciate how difficult it is to design a lightweight hub structure and pitch bearing mechanism capable of handling the huge moment forces produced by a rigid rotor. It was likely a difficult design problem at the small scale X2 demonstrator. One can only imagine how hard the problem will be to solve with the exponentially larger moments imposed by a larger diameter rigid rotor.

This is one of a number of advantages that the Interleaving-ABC configuration offers.

For the same disk-loading as the Coaxial-ABC the Interleaving-ABC, and the Intermeshing-ABC, will have physically smaller disks and therefor blades with shorter spans.


Dave

[Graviman]

Dave,

Does the circle density in your plots relate to induced velocity? I'm a bit bothered that your calc does not account for interaction between one rotor and another. There should be some azimuthal variation with each rotor.

Interesting point about interleaving reducing the blade loads for a given rolling moment (ie snapiness). The problem is this does not help you in level flight. The retreating side needs to be off loaded regardless of config.

CEFOSKEY, riff_raff don't like your fairings. Is you jus' gonna take that?

Riff_raff, agreed hub is a nice piece of design. Not sure about loads going up exponentially with radius, though. All components are scaled up equally, and stress reduces with depth^2. Since a machine twice the mass would require twice the rotor diam then hub bending moment also goes up 2x2=4, which keeps stresses in a hub twice the size in trim. Problem is generally MGB which now handles twice the power at half the output speed...

Commercial sector wouldn't mind a fast helicopter.

[Dave_Jackson]

It is with some trepidation that I comment on perceived improvements to rotorcraft, since Senior Pilot has made it apparent that it is not a subject for this forum.


Mart (Graviman)

Going off-topic related to the X2, but staying on-topic related to the ABC.

Does the circle density in your plots relate to induced velocity?

Yes. The above drawing shows the the rotor disks as having the same diameter. In reality, the disks of the Interleaving would be smaller for a comparable craft with the same gross weight. In the drawing, the density of the circular lines represents the induced velocity during hover.

I'm a bit bothered that your calc does not account for interaction between one rotor and another. There should be some azimuthal variation with each rotor. ........ The retreating side needs to be off loaded regardless of config.

The above drawing shows the craft with its rotors. There should not be any significant interaction between the two rotors.


Regarding aerodynamic interaction between the rotors and the fuselage;

During cruise, the retreating sides of the rotors are over the fuselage. The retreating blades will be producing minimal thrust due to the ABC.

During hover, the downwash on the fuselage would be reduced due to the inclusion of Active Blade Twist.

In both the above cases, the passage of 8 blades over the fuselage will increase the frequency of any downwash, and this is a benefit.


Dave
Just trying to find and disseminate improvements in VTOL craft.

[IFMU]

From cnet news:

Sikorsky 'progressing' with X2 helicopter effort



-- IFMU

[Graviman]

CEFOSKEY/IFMU, I gotta get me one of those!

Judging by trees/fence: Is that a full size mockup?

----

Dave, i've been quietly developing my own calc for rotor performance (based on Blade Element Momemtum Theory & Vortex Panel Method - but only capable of hover currently). I'm not convinced about the rotors not interfering in the intermeshing zone. For a given segment of air you are potentially doubling the lifting force, so that in practice induced velocity in this region will go up to reestablish stream function pressure equilibrium. Induced velocity will be far from uniform during hover, which will reduce benefit of greater rotor area.

Besides, my point still stands: During cruise the retreating side will still become unloaded - this is a limitation of stall BVI from vortex shedding. So intermeshing still needs a high hinge offset to work.

As long as ideas are kept practical, i have no problem with technical challenges. A good idea should be able to survive on its merits - X2 is a good idea...

[maxtork]

Yeah it's full size alright. I got to look around it a bit yesterday. Pretty slick looking machine thats for sure!



Max

[Dave_Jackson]

Mart,

My previous posting obviously needs some clarification, or elaboration, regarding hover.

• The Advancing Blade Concept (ABC) allows for dissimilar thrust between the 'advancing' and the 'retreating' sides of the rotors. During hover, each of the intermeshing rotors will be asked to produce 2/3 of their lift on their outside (advancing side during cruise) and 1/3 on their inside (retreating side during cruise). This will result in a relatively uniform induced velocity over the total effective disk area.
• As previously mentioned; during hover, the induced velocity directly over the fuselage, will be further reduced due to the inclusion of Active Blade Twist.

These are two very significant advantages that the Coaxial-ABC cannot exploit.

In addition, the intermeshing configuration is proposed for transportation requirements. In this capacity a significantly greater portion of the craft's time will be spent in transit v.s. hover. In cruise the advantages of an Intermeshing-ABC configuration will again be greater then those of the Coaxial-ABC, AND those of the Intermeshing-ABC.

_________________

As long as ideas are kept practical, i have no problem with technical challenges. A good idea should be able to survive on its merits - X2 is a good idea...

In addition. I have three practical concerns about the merits of a small or medium Coaxial-ABC, which are also of some concern to the Intermeshing-ABC.

One concern:- Consider the high disk-loading of the coaxial rotors, as shown in the above pictures. Then consider that the pusher prop is at the rear of the combined disks. It will be 'pulling the rug out from under this disk'. In other words, the propeller will be stripping a significant portion of the 'lift' out from under the two rotors.


Dave

[Graviman]

CEFOSKEY,

One or two questions (you might know the answers):
How does that collective work? Rearwards for increased thrust?
Does sidestick include yaw control?
Is the terrain display something Sikorsky is actively working on or is it just on the wish list?

----

Dave,

That prop position is just a compromise required by the drivetrain. Don't forget that tail rotor drivelines will be designed to all sorts of inhouse analysis procedures. Besides prop is only really active in cruise where mass flow is high enough to reduce required thrust. The other way to see it is that any streamline passing through rotor disk and prop disk is effectively just being revectored by prop so that rotor does all the hard work of increasing total pressure (treating aerofoils as a planar pumping machine). I've read up a bit since that stimulating discussion about ground effect pressure under the rotor.

So your intermesher is a sub 200kts machine then? In that case it is not ABC and cannot be compared to X2 (threadjack caution light ). If it was ABC then my point about retreating blade unloading stands, by definition of ABC. In hover, if you fatigue the rotors hard enough, you could achieve any downwash pattern you wanted. Best to keep mechanism simple = durable...

[Dave_Jackson]

Mart,

...... in cruise where mass flow is high enough to reduce required thrust.

Are you saying that the thrust [lift] from the rotors is less in forward flight than it is in hover????

So your intermesher is a sub 200kts machine then?

Who said that????
Please, look at the [Line of zero airflow. Mu = 1.0], in the drawing on post #465.


P.S. You have a couple of PPRuNe private messages and emails, dated mid-March and a week ago.


Dave

[Graviman]

CEFOSKEY,

Me thinks heli training just got a whole lot less expensive!
It does look like a seriously capable system though.

----

Dave,

X2 papers are much appreciated!
I've been doing a lot of hours recently so didn't check my email computer - apologies...

If your intermesher can fly above 200kts then it must unload the retreating blade, unless you have some super new way of pushing up stall rel-AOA. But let me back peddle a little: a low hinge offset rotor could rely on rotor coning to generate the advancing side lift. As long as rotorheads are not teetering advancing side does not need to balance out lift with retreating side. So with lateral symmetry there will be no net rolling moment in flight.

OK, i agree with you: You can use laterally seperated low hinge offset rotors with retreating side unloaded. The lateral seperation then allows a fast roll response time.

For you question about prop position, what i mean is that for any streamline:
thrust = (mass_per_sec x velocity_induced) + (press_local - press_farfield)
At cruise mass_per_sec is higher so you need less induced velocity.
Also pusher prop is effectively just revectoring streamtubes, but main rotor is generating most of the total pressure increase required for lift and thrust. Pusher prop effectively has it's power reduced since it is drawing air from a region of higher total pressure under the main rotor system.

[Dave_Jackson]

Mart,

But let me back peddle >-------> less induced velocity.

Sorry, but I do not understand.

Also pusher prop is effectively just revectoring streamtubes, ....... .

I respectfully disagree. This subject is considered here; DESIGN: UniCopter ~ Pusher Prop - General - Tractor vs. Pusher
__________________


As an aside; Re your mentioning of retreating blade (reverse velocity airflow).

There is an ongoing PPRuNe thread on the V-22 Osprey. The discussion basically relates to the effectiveness of the Tiltrotor configuration, and its Achilles’ heel, which is the difficulty of its blades to serve the requirements of both a propeller and a rotor.

The ABC configuration has it's own Achilles's heel, that being the extremely high drag/lift ratio from its retreating blades.

Eventually, there may be an interesting 'showdown' between the Tiltrotor and the ABC for 'Best in Breed', where the breed is 'VTOL+speed'.


Dave

[IFMU]

It will be 'pulling the rug out from under this disk'. In other words, the propeller will be stripping a significant portion of the 'lift' out from under the two rotors.

Why then does a long-ez or other canard fixed wing work? Or is it because the propeller is aligned with the wing, and therefore strips an equal amount of sink above the wing as the amount of lift it is stripping out below the wing?

As a conventional helicopter moves forward, does the relative wind push the lift out from under the blades?

-- IFMU

[Dave_Jackson]

IMHO

Hi IFMU,

Why then does a long-ez or other canard fixed wing work? Or is it because the propeller is aligned with the wing, and therefore strips an equal amount of sink above the wing as the amount of lift it is stripping out below the wing?

Both the canard and the HS provide an aerodynamic upward pitch on the craft, which offsets the moment from the CG that is located ahead of the main wing's center of lift.

A canard creates a lift at the front of an airplane, whereas a horizontal stabilizer creates a downward force at the rear of an airplane. In other words, the canard is a contributor to the overall lift on the craft.


It appears that the centerline of the Long-ez's propeller is above the chord of the wing. I would speculate that this 'elevated' propeller would tend to increase the velocity of the air that is passing over the top of the wing in relationship to the air that is passing under the wing. This should also enhance the lift.

As a conventional helicopter moves forward, does the relative wind push the lift out from under the blades?

Using 'ground cushion' as an example, I would say yes.

During the transition from hover to forward flight, the helicopter will at some velocity "run off of the ground cushion". Logic suggests that if a tractor propeller was operational at this time it would contribute to the stripping of this 'ground cushion' from under the rotors.

Dave

[rjsquirrel]

"push the lift" is not physically correct in any way, IFMU. I truly hope you didn't find such logic in any aero discussion!

As Dave says, a canard is the same thing as a horizontal tail - it balances and controls the main wing. The big difference and advantage of a canard is that it can do this balancing act with an upforce - lift - instead of the downforce that a tail must use. Thus, a canard shares the lift, while a tail pushes down, making the main wing lift more - and use more power. The Wright Brothers used a canard for that reason.


No the prop cannot blow the lift anywhere, lift can't be blown away.

The collective on the X2 is a fly by wire stick that raises two swashplates, one below the transmission that pushes control rods inside the beefy mast to control the upper rotor, and one above the transmission that is rather conventional and controls the lower rotor.

[Graviman]

Dave,

I would be more convinced if the maths was there to back up your pictogram logic.

In the code i mention, actually designed for cooling fans, i have assumed that each streamline passes through the fan then through a cooling pack before accelerating away as thrust. Along each streamline total pressure (static+dynamic) must sum to zero, although will increase through the fan and decrease through the cooler. What is really happening is that a different streamline leaves the same position in a different direction as the aerofoil passes, but that would require a full 3D vortex panel model (ie much bigger matrix). It is early days but the results tie up well with testing. After i finish my OU physics degree i intend to adapt it for high speed helicopter design studies like yours.

The typical assumption in textbooks of total pressure being uniform under the rotor disk is just that: an assumption. It allows momentum methods to calculate a figure of merit, but beyond that does not tell you what the aerodynamics look like. The 1g lift must remain constant by definition. What changes is the power required to achieve that lift. A pusher prop will require less power than a puller prop, but will increase power required by the rotor. Total power requirement all balance out so that Lift=Weight and Thrust=Drag...

With equal respect, i would say that a refresher on first principles of aerodynamic theory would allow discussions of performance quantities rather than qualities for your project. The discussion about pressure increase below rotor in ground effect made me go away and do some reading to understand which of my assumptions were wrong.

----

RJS, interesting technical detail about X2 swashplate layout.

[Dave_Jackson]

With equal respect, i would say that you need to read some textbooks on basic aerodynamic theory. This will allow you to discuss performance numbers rather than make qualitative statements. Your project will then be taken more seriously by other engineers.

Mart,
Your above paragraph shows absolutely no respect.
In fact, it says little about me and perhaps much about you.