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# Sikorsky X2 coaxial heli developments.

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# Sikorsky X2 coaxial heli developments.

20th Oct 2009, 12:49

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regarding definitions

The post above quotes the FAI as follows;

HELICOPTER: A rotorcraft with a power driven rotor system whose axis (axes) is (are) fixed and substantially perpendicular to the longitudinal axis of the rotorcraft (my underlining)

by defintion this means the X2 isnt a helicopter but more correctly a

ROTORCRAFT - An aerodyne that derives the whole or a substantial part of its lift from a rotary wing system

or am I reading it wrong?

I don't doubt the X2 may prove to be a fast machine, whether this technology can be upscaled to a useful rotorcraft remains to be seen rather than a one man whizz about. Ultimately I believe that the G-LYNX record will never be beaten by a conventional (non augmented) helicopter.

DM
20th Oct 2009, 15:08

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Mart [graviman]

"while rotational component of tip speed at cruise is
4.02m x ( 2pi x 360rpm / 60 ) = 152m/s (45% sound)

So if we let tip get up to 85% sound then machine can travel at
85% - 45% = 40% sound"

Check your algorithms. Do you not mean 2 x 4.02m x (pi x 360rpm / 60 )
_______________

"The limiting speed for X2 is likely to be from compressibility of the advancing tips."

Lets hope you are correct.

It appears that the limiting speed on the XH-59A was caused by the drag of the retreating blades. This high drag wanted to drive the rotors at a rotation speed faster then the engine was providing.

This appears to be the reason for the change in the X2's blade profile. They must now put up with the drag of the long 'spar' at the root of the blades, but they have eliminated the negative lift (and its drag) that was created by the reverse velocity causing a negative lift at the root of the XH-59A's retreating blades.

Dave
20th Oct 2009, 17:55

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Dave,

Don't forget that X2 uses autorotation for cruise, so there will be a longitudinal cyclic trim position for minimum total drag (obviously with collective lowered). Actually, i'm not sure whether the FBW would need to keep an eye on longitudinal trim as well as collective. The idea of having to watch NR limits while cruising is an interesting thought - one for the TPs...

Ultimately, i have no doubt that X2 has been subject to the very latest CAE analytical techniques. So the question really remains how accurate are the predictions. If the project engineers believe the machine has the capability to achieve 240-250kias then i have no real reason to doubt them.

Chris, was all of the aero-mechanical-dynamics done in Ansys? Were there any other test beds to validate the CAE model?

----

Dangermouse,

G-LYNX will always remain the worlds fastest single rotor helicopter. Credit where credit is due.

If i read the FAI definition properly then X2td still falls into the category of pure helicopter. X2 is a rotorcraft with a power driven coaxial rotor system whose axes are fixed and substantially perpendicular to the longitudinal axis of the rotorcraft. I is confused?
20th Oct 2009, 19:19

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Mart

"Don't forget that X2 uses autorotation for cruise"

PS. I was wrong to correct your algorithm. Haste makes waste. Sorry

Dave
21st Oct 2009, 00:16

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HELICOPTER: A rotorcraft with a power driven rotor system whose axis (axes) is (are) fixed and substantially perpendicular to the longitudinal axis of the rotorcraft
I think this describes x2 exactly. However, a bit back in this thread we came up with:

Originally Posted by NickLappos
IFMU,
Here are the definitions:
A compound helo has auxiliary propulsion - a jet or prop. It is said that the world record Lynx used about 7% aux thrust from its engines (a small enough percentage to allow it to be classed as a true helicopter).
A winged helicopter has a wing that develops lift.
A winged compound has both.
So if we believe Nick, it is a compound helicopter. It seems clear to me that the X2 can never take away G-LYNX's record, as it is a different class of machine. None of that is necessarily bad.

-- IFMU
21st Oct 2009, 01:31

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More Boorish spares

Found this link on the previous PM page I referenced before:
X2 Sikorsky Helicopter Aviation Inspiration - Engineering Inspiration for the X2 Sikorsky Helicopter - Popular Mechanics

It has some interviews of some of the boorish spares. Funny they hid the young guy in the back. Must be one of those "old age and treachery overcoming youth and skill" sort of things.

-- IFMU
21st Oct 2009, 11:33

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IFMU,

Agreed. G-LYNX was the climax of the single rotor helicopter, while X2 opens a thrilling new chapter on compound helicopters.

Having read the article i must admit to finding this project inspiring. I started this thread what feels like years ago to capture as much technical detail as practical about what i recognised to be a revolution in helicopter design. It is the closest i can currently get to being involved without actually being there.
21st Oct 2009, 12:51

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being really anal here!

my view (for what it's worth, not much probably) is that the X2 shouldnt be referred to as a helicopter by just the use of that one word.

IFMU, the X2 rear rotor AXIS is not perpendicular to the longitudinal one, it's along it thereby eliminating it from the pure helicopter definition.

so, it is a compound helicopter/rotorcraft.

I know its just pedantic but coming from that part of the world G-LYNX originated from it's a matter of some pride here!!

effectively for rotorcraft we have 5 types of vehicle that could hold seperate speed records.

Helicopter; rotor lift, no augmentation - Lynx, UH60 etc
Winged Helo; Rotor and wing lift no augmentation - Mil24, Mil6
Compound helo; rotor lift+ thrust augmentation - X2
Compound winged helo; rotor and wings lift + thrust augmentation - AH56
Tilt rotor - V22, BA609

until of couse someone invents something new.

hypothetical question; how fast would the cuurent fastest coaxial helo go with a pusher prop (say a KA50 with a prop on the back)

DM
22nd Oct 2009, 00:45

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Originally Posted by dangermouse
IFMU, the X2 rear rotor AXIS is not perpendicular to the longitudinal one, it's along it thereby eliminating it from the pure helicopter definition.
dangermouse,

I'm good with that, for what it is worth. I would argue the thing on the back is a propeller and not a rotor, but I agree X2 is not a pure helicopter. I do think it has some pure helicopter attributes that come along with low disc loading, such as hover efficiency and autorotation capability, but it would seem to me that it can't steal G-LYNX's record based upon the definitions that we all seem to agree on. If X2 accomplishes what they say it will, then its accomplishments will stand on their own merit without having to compete against G-LYNX.

Furthermore, you should be pround of G-LYNX. Heck, I'm proud of G-LYNX and I have no ties to Westland. No real ties to England either, as we are two countries divided by a common language and that little spat back in 1776 or so. But, the helicopter tribe is small enough, that when somebody in it does something exceptional, how can you not sit back and say "hey, that was cool." I think that people who cannot appreciate other's achievements have perhaps never achieved anything themselves, and are therefore ignorant of what it really takes.

-- IFMU
22nd Oct 2009, 11:51

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Lynx – The World’s Fastest Helicopter 20 Years On

I'd like to make sure this thread captures the technical excellence that is G-LYNX

Lynx – The World’s Fastest Helicopter 20 Years On

The 11th August marks the 20th anniversary of the Westland Lynx helicopter setting a new world helicopter speed record of 249.1 mph (400.87 kph). Although other attempts have been made, 20 years on Lynx retains the title as the world’s fastest helicopter.

The Lynx helicopter, registered G-LYNX, flew a 15km course across the Somerset Levels at 500 ft on a calm hazy evening back in 1986 with Westland Chief Test Pilot Trevor Eggington at the controls and Derek Clews, Flight Test Engineer alongside. The average speed achieved over two runs was 249.1 mph (400.87 kph), beating the record held by a modified Mil “Hind” helicopter by over 20 mph (32 kph).

The world record flight was made possible by the introduction of the advanced technology composite blades developed as part of the British Experimental Rotor Programme (BERP) project. The BERP programme was carried out in co-operation with the UK Ministry of Defence to produce an advanced technology rotor blade, which would exploit the advantages to be gained by aerodynamic tailoring through the use of modern composite materials and construction methods. The first BERP blades were flown on 9th August 1985 and such was the progress achieved G-LYNX was able to claim the World Speed Record just one year later. The BERP blades are now fitted to all Lynx helicopters in service and the same technology was used for the EH101 main rotor blades.

The three month programme to modify a standard Lynx to world speed record configuration was launched on 22nd May 1986 and included installing more powerful Rolls-Royce Gem 60 engines. These engines were cleared to operate at their maximum contingency rating, thereby boosting power by 45%. Engine power was further boosted by utilising a water-methanol injection system. To improve yaw control a new low set tail plane with vertical fins was fitted which also offload the tail rotor.

A drag reduction exercise was undertaken involving the fairing of the main rotor head, removal of external items such as steps, aerials and windscreen wipers, while joints were sealed and cooling ducts blanked where possible.
Dangermouse, nobody is trying to take away the Westland record for world's fastest pure helicopter.

Along with Concorde this has inspired a generation. But, technology inevitably marches on and the coaxial compound helicopter looks to be the way forwards. I would see X2 compound helicopter a spiritual successor, using the latest available techniques. One day X2 must also gracefully look on as it's spiritual successor goes from VTOL to even higher speeds. This is is just the way of things.

A conventional coaxial would have no chance at high speeds. The secret to X2TD is the advancing blade technology developed for XH59/S-69 helicopter (not sure if this was defined as compound).

Mart
22nd Oct 2009, 12:24

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didnt mean that to happen!

let's get back to the X2 and how its going...

so how's it going?

DM
14th Nov 2009, 01:52

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A little more popular media coverage:

http://www.popsci.com/bown/2009/prod...-x2-helicopter

But not much meat there. Who knows what they are up to. No sign of the boorish spares.

-- IFMU
14th Nov 2009, 22:41

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This is the briefing given by Carey Bond at Heli Expo 2009:

And by Kevin Bredenbeck at HeliExpo 2009:

Also a general video of the potential of ABC in LTH applications:

The point i found most interesting was the explanation of the twitchy control of the first flight. At that time the FBW was direct stick to head control with no feedforward/feedback stability augmentation. The pitch and roll control was actually slightly higher than had been anticipated by the predictive processes used to develop X2 control systems. The flight has since been analysed to understand the actual pitch & roll moments for a given swashplate movement which will then be incorporated into the control algoritms.

An interesting insight into the world of flight test there. Even more so as the rotor dynamics head into ABC regime. This is likely the main reason the flight test is being done in three distinct speed phases.

What i will be interested in understanding is how the control for the pusher prop developes to make it intuitive to use. Especially once the main rotor has gone into autorotation. I would imagine that collective would be automatically trimmed to keep Nr constant while TP varies thrust. I would imagine that prop pitch will also be automatically trimmed so that TP is only controlling engine power with twist grip.

Hopefully future briefs on this amazing machine will be just as informative...

Last edited by Graviman; 14th Nov 2009 at 23:01.
6th Dec 2009, 05:56

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Well, just to toss in a (probably rehashed somewhere) idea, why not have some wings? Short wings like the X-49A, but I imagine it'd make it more stable, make high speed maneuvering better, and it gives a CAS/attack helo version using this tech a nice place to mount weapons on. If it improves range, makes it easier to land with dead engines, makes it more maneuverable or stable, and all that jazz, then why not? It seems like they have added stub wings to it at some point, but I'm interested in knowing why.

Feel free to flame me and pick it apart. Also, yes, I do not know the math involved.
6th Dec 2009, 11:38

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Anything sticking out from the fuselage like a pair of stub wings would probably increase the power needed to hover. It may affect the ability to autorotate also. I'm no expert but that would be my slightly educated guess. They're probably looking for all the efficiency they can get right now.

If a military model were to be produced I'd imagine you'd see guns or pods hanging out the side, like the last video 2 posts up.
6th Dec 2009, 18:07

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Well, if they find that hover performance is decreased significantly and it does more bad then good, yes it's a bad idea and it's fine not to use it.

I guess I'm just thinking about the helo as the modern AH-56
7th Dec 2009, 16:54

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From the Journal of the American Helicopter Society, Oct 2009 issue.

Article ~ 'High-Resolution Computational Investigation of Trimmed Coaxial Rotor Aerodynamics in Hover'.

Excerpts from ~ 'Conclusion':

"4) The overall performance is well predicted for the coaxial system. The bottom rotor showed a degradation of performance due to the influence of the top rotor wake (~40% increase in power at same thrust at higher thrust levels). Interestingly, even the top rotor showed slight degradation in performance....."

"6) The interactions [of the blades on the two rotors] resulted in a fluctuation of 5%-10% in the integrated Quantities. Such a fluctuation could be significant for vibration and acoustic characteristics. ...."

______________________________

If United Technologies had kept Charles Kaman, logical evolution (such as today's movement toward highly rigid rotors) suggests that the advantages of the Intermeshing configuration would have established it as the predominant one.

Dave
7th Dec 2009, 18:42

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Kaman

Dave, I had always heard that Mr. Kaman left Sikorsky over his pushing the servo-flap design approach, not intermeshing or coaxial main rotors. That not the case?

Thanks,
John Dixson
7th Dec 2009, 22:03

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John,
Dave, I had always heard that Mr. Kaman left Sikorsky over his pushing the servo-flap design approach, not intermeshing or coaxial main rotors. That not the case?
You are correct in that Charles Kaman's proposal to the Engineering Manager at United Aircraft was the servo flap. But the servo-flap may have been only part of his reason for leaving.

Perhaps the following two excerpts from his book 'KAMAN ~ Our Early Years' clarifies his reasons for leaving.

At first I was mainly concerned with stability and control forces, and I applied my aerodynamics training to a new concept for rotor control.

I felt that somehow we had to find a means of aerodynamically stabilizing the rotor, while at the same time making it easier to fly. Since that was not part of my daily job at Hamilton, I continuously worked on the problem on my own time.

Primary among those potential improvements was my idea for aerodynamic servo flaps on rotor blades. My initial thought was that such a flap would be mounted on the trailing edge of the main rotor blade and would be connected by mechanical linkages working through a swash plate to the pilot's controls. It would be a good while before I would understand that this system would not achieve the desired result, and that it would have to take the form of a true servo system to see success. But this is getting ahead of the story.

The use of intermeshing rotors-a feature I would later envision for the Kaman K-125 helicopter-promised a significant power saving and greater efficiency over tail rotor designs. A tail rotor, used to counter the torque of the main rotor, used precious power without providing lift. But proposing an alternative to the tail rotor would have been premature - and out of the question for United Aircraft - as Sikorsky was completely committed to the tail rotor configuration.

___________________________

"Erle, I'd like to develop this system for United . I'll continue to function with my own basic job, and I'll do this work on overtime or extra time, or however. All I ask is that you pick up the costs and provide me with the place and a technician to the basic spade work."

His answer was swift, decisive and clear. "Charlie, we have our inventor at United Aircraft. His name is Igor Sikorsky. We don't need another one."

I think that his first patent was US 2,455,866, and it shows the servo flap on an Intermeshing helicopter.

Dave
8th Dec 2009, 11:35

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Hunt3r,

The advantage of ABC is that you get the benefit across the full speed range. Sikorsky/Schweizer spent a lot of time optimising the blade section/twist/span for the performance saught. Stick on wings just were not required to get the cruise/dash performance.

Dave,

I know that the original X2 design studies used the then available non-coaxial codes, and that wake contraction will affect the results for the lower rotor. However, in forward flight i don't imagine there to be much difference between coaxial or intermeshing. In hover there probably are some advantages to the lateral symmetry of intermeshing. The down side of intermeshing is tip clearance, which might have affected operational considerations.

When i (finally) get my physics degree out of the way next october, i'll pick up my rotor simulation tool again. I'd like to adapt it to handle ABC type of configs. It would be interesting to do a design study comparison of optimised coaxial vs intermeshing vs interleaving. Finally take some of that speculation out of our discussions.