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Sikorsky X2 coaxial heli developments.

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Sikorsky X2 coaxial heli developments.

Old 3rd Jul 2009, 14:10
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I see Sikorsky have got the propellor to go round but could someone please point out to them that the world speed record to beat for a Compound Helicopter is 311mph ,not 250mph .Still way to go !
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Old 5th Jul 2009, 01:00
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Is the X2 truly a compound helicopter? It has no wing.

Edit: found this on aero-news.net.



X2 flies with prop
-- IFMU

Last edited by IFMU; 5th Jul 2009 at 01:26. Reason: Found the latest X2 story
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Old 5th Jul 2009, 01:38
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Somebody from the IOC should check those vertical stabs for growth hormones...

I/C
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Old 5th Jul 2009, 16:58
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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.
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Old 5th Jul 2009, 17:14
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That 52 kts presumably marks the bottom end of where the prop starts to have advantages over the rotor for forward thrust. I'm keeping my fingers crossed that the envelope expands nicely all the way up to 265kts.

The three vertical fins had me confused for a while - just a matter of perspective...

Last edited by Graviman; 6th Jul 2009 at 11:28. Reason: Ooops!
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Old 6th Jul 2009, 00:37
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Thanks Nick, that clears up a fundamental misunderstanding I've been carrying along. Glad to see you back with us.

Heli1,
The boys at Sikorsky may have gotten their speed records mixed up, but they haven't gotten knots confused with MPH yet. They are talking 250 kts, which is 288 MPH or so. What is interesting is they talk of a record of 250 kts and a comfortable cruise at 250 kts. I would think the record and the comfortable cruise would be separated a bit.

-- IFMU
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Old 6th Jul 2009, 02:47
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Now, I'm no aeronautical engineer, but with regard to those fins/endplates...

When-oh-when will manufacturers figure out beforehand that any helicopter that has more surface area ahead of the rotor than behind is going to have stability problems in flight?? You'd think this was rocket science or something!

I looked at that photo of the X-2 on airliners.net and noted the vestigal fins back there. The very nice profile view shows a classic "teardrop" shape of the fuselage, which should be nice and aerodynamic, but...the mast is waaaaay aft of where it "should" be, and it doesn't take an engineer to see that. I'd bet real money that it wasn't tons of fun as they went faster and faster in forward flight - not to mention horrible weathercock stability. Ergo, they ended up with those barn doors back there.

Has anyone ever seen fins/endplates that shrank over time? Not! Dear Lord, look what happened in the transformation from 105 to 117. So methinks the X-2 is stuck with those endplates.
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Old 6th Jul 2009, 11:38
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FH1100 Pilot,

Agreed that X2 will not likely loose those vertical stabilisers, unless flight testing reveals a reason to. I was suprised by the area when seeing X2 from front (took a while to understand what i was seeing).

The problem (as you point out) is fuselage centre of pressure relative to centre of mass. I'm not really sure what could have been done different, since engines behind rotor reduces likelyhood of control damage in the unlikely event of catastrophic engine failure. The rotor is already as far forwards as possible, considering package for MGB, swash plate & actuators, vibration control system etc.

Fuselage has been kept as short as practical, so the penalty is the additional vertical stabilisation. I don't see it causing practical difficulties though.
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Old 6th Jul 2009, 13:28
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I'm no stability and control expert, but I'll bet the prop in the back contributes mightly to directional stability...
And the X-2 has a pretty awesome fly-by-wire system. Perhaps we're witnessing the dawn of the RW equivalent of the F-16 (which can't be controlled without the computers...)
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Old 7th Jul 2009, 01:53
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When-oh-when will manufacturers figure out beforehand that any helicopter that has more surface area ahead of the rotor than behind is going to have stability problems in flight?? You'd think this was rocket science or something!
Methinks it is harder than rocket science. Rockets have nice clean airflow running down their length, put some silly little fins on the bottom and you are good to go. But, like you imply, a lot of the manufacturers seem to end up redesigning the tail. The F28A I learned to fly in didn't have enough vertical area, or maybe it was enough but the air back there was not flowing backwards nice and orderly like it does on a rocket. I used to fly the enstrom with the ball a little out to keep it from wagging itself to death.

When I look at the airliners.net photo, it seems the distance from the mast to the tip of the nose is about the same as from the mast to the tip of the tail, just forward of the prop. However the tip of the nose is a skinny little thing, the tail offers a lot more surface area at the same displacement even in its original version. Also in the airliners photo it looks like he is talking on a hand mike. They should really have given him a boom mike.

In the FW world the older glasairs and lancairs had small tails optimized for low drag. They were of course unfettered by regulations for certified aircraft. The later versions seemed to grow bigger tails for better longitudinal and yaw stability. It's a classic fight between performance and handling qualities.

At the speeds they have been to would the tail have even been much of a player yet?

-- IFMU
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Old 7th Jul 2009, 16:07
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X@ Tail Surfaces

FH1100,

Your quote: "You'd think this was rocket science or something!", struck a respondent chord.

I'd bet that the advanced design groups at Eurocopter, Bell, Boeing and Sikorsky, and MIL and everyone else for that matter, might argue that arrangement of all the pieces and parts of a new helo design so that the CG comes out in at least a controllable position and so designing the tail surfaces so that they provide the required static and dynamic stability, while not creating some of the known tail surface forced probem areas, is in fact a lot tougher than "rocket science"!

Thanks ,
John Dixson
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Old 7th Jul 2009, 16:23
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Shawn Coyle said:

I'm no stability and control expert, but I'll bet the prop in the back contributes mightly to directional stability...
And the X-2 has a pretty awesome fly-by-wire system. Perhaps we're witnessing the dawn of the RW equivalent of the F-16 (which can't be controlled without the computers...)
Shawn,

It seems to me, at least intuitively, that having the propeller thrust behind the CG would actually be a destabilizing factor in yaw (and perhaps pitch as well). During deceleration I imagine the reverse is true. However, as you go on to say, the FBW/AFCS probably makes it transparent to the pilot.

-Stan-
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Old 14th Jul 2009, 11:40
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John, agreed - rockets don't have to do anything to the air other than move it out of the way.


Shawn,

That's an interesting thought. I think it is just a question of the advantages of FBW becoming more significant as the envelope increases, especially for a coaxial machine. Pilot will not be able to trim cyclic laterally to compensate for any pitch-roll coupling (opposing rotor control), although differential cyclic is required to offload retreating rotor. Also the lead angle can be tuned to compensate for any altitude and coning angle (load factor). There is also some wisom for a performance machine if pilot reflexes can become tuned to exactly the same pitch & roll rates, for a given stick input, in any flight condition.

I wonder if the FBW goes as far as the system on Euro-fighter Typhoon where pitch and roll are referenced from flight velocity vector rather than pitch attitude vector (control advantages at high AOA)? Maybe there is also some collective control to avoid VRS and deal with engine failure? I need to read up some of the articles/papers i have accumulated...
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Old 14th Jul 2009, 20:46
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FBW for Rigid Coaxials

Graviman, you are on target w/re to your comments to Shawn. And the areas where FBW are of advantage only begin with the subjects that you noted. All of the controls in the XH-59 were mechanical, thus there was this very sturdy looking crank in front of the copilots shins that was a swashplate phase angle adjuster! Actually, it is a tribute to a very smart team that they accomplished what they did.

And then there is the very basic subject of control sensitivity vs needed control range. That always present compromise was as you might imagine exacerbated by the very high control power/sensitivity of the rigid rotored ABC. In fact, a control system change in the direction of lower sensitivity ( thus lower range ) was at the heart of the accident in the first prototype and one could easily argue that with a FBW control system, the "problem" would have been a non-issue.

So as the X2 proceeds, they have the tools within the FBW system to develop the solutions to the complex tasks and implement them behind the scenes, so to speak, such that after development is complete, a relatively simple, straightforward flying task should present itself to the operational pilots.

Thanks,
John Dixson
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Old 15th Jul 2009, 00:39
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Originally Posted by slgrossman
It seems to me, at least intuitively, that having the propeller thrust behind the CG would actually be a destabilizing factor in yaw (and perhaps pitch as well).
-Stan-
I thought in the fixed wing world that tractor propellers were considered destabilizing and pushers stabilizing. If I remember correctly there were issues with pitch stability on the old Northrup flying wings when they transitioned from pusher propellers to jets.

If you consider the thrust vector to be normal to the pusher propeller disk, I think it would be stabilizing. Consider a disturbance which pushes the nose up so the airplane is at a positive angle of attack. Then break the vector into a thrust which is orientated in the direction of travel, the left over vector (for a pusher) will try to push the tail up, reducing the angle of attack.

-- IFMU
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Old 15th Jul 2009, 22:23
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IFMU said:

If you consider the thrust vector to be normal to the pusher propeller disk, I think it would be stabilizing. Consider a disturbance which pushes the nose up so the airplane is at a positive angle of attack. Then break the vector into a thrust which is orientated in the direction of travel, the left over vector (for a pusher) will try to push the tail up, reducing the angle of attack.
Now that I see your explanation I realize you're right. What I was thinking of was the aircraft's response to power changes, but that's not really a matter of stability.

Thanks very much,

-Stan-
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Old 16th Jul 2009, 11:32
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You guys are a bit off base. The rigid pusher does not simply deliver a thrust, it responds to flow changes with surprising changes in its thrust and moment. If the nose goes up, the prop will see a strong "P factor" that causes a yaw (remember the right rudder on climbout in an airplane?) This means the prop is always destabilizing, and often cross coupling as well (where yaw makes it cause a pitch probelm and vice versa).
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Old 27th Jul 2009, 11:40
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John,

Interesting feedback about FBW. Of course the other realisation is that castings and machined parts (like the shin height phase adjuster ) would require time consuming redesign to adjust flight characteristics. Software potentially requires recertification, but i imagine as many variables as can be thought are programmed in from the start. So much of the risk of envelope expansion by flight test is reduced to being control optimisation.

Was much of XH-59 flight testing taken up by stability optimisation?
I imagine that a with a snappy response the there can be a tendancy for the aircraft to have poor gust rejection, although faster response will produce less tendancy for PIO.


Ramen,

I imagine that a prop acts as a very high effective hinge offset rotor. So there will be a degree of flapback for any sideslip/yaw/pitch. I've heard Spitfire pilots talk about cross coupling controls (rudder for pitch, and stick for yaw). Of couse as an engineer i can't help but think about designing a teetering mech with delta3 for self-alignment, but i still admire the level of control required.
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Old 27th Jul 2009, 20:49
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XH-59 Stability Optimization?

Graviman, an anecdote to answer your question:

Nick Lappos and I were invited to help fly the ship back from a static demo at FT Rucker Alabama, where there was a US Army Aviation Commanders Conference. I got to fly from OZR to JAX and Nick flew down to West Palm.

Going cross country was a kick,, well, sort of. Get out on the runway via the rotor and the PT-6 twin pack, then start the J-60's and firewall those things, climb like a fighter to 10K level off and cruise for a few minutes, then shut down the J-60's because the fuel guage is going down as fast as the sweep second hand on your watch. Do a cruise/decerating descent to 1000 ft and flutter along at 80-100 kts. NO SAS.

Tail surfaces were sized for the high speed flight part of the envelope and stability up there was generally ok, but at 80-100 in the typical south Alabama summer afternoon, one kind of herded it. If you've flown an A Model CH-47 or CH-53A with SAS /AFCS turned completely off, you get the picture.

The flight test program had prioritized the flight envelope and performance assessment, and the basic controllability of the vehicle. Short and long term stability, required augmentation, vibration control and attenuation and the like, were yet to be addressed.

Thanks,
John
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Old 9th Aug 2009, 18:41
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John,

Genuine thanks for reply - i've had internet provider problems for the past week or so and couldn't thank you earlier.

I'd really have relished the opportunity to fly machines like the Chinook and Stallion. An ear infection as a kid meant i didn't get into flying until too old for any of the services. Now i have to get whatever experience i can between UK housing boom/bust gobbling up my hard earned cash.

Interesting about yaw stability of the coaxial XH-59 though. There must be some yaw stability from torque generated by rotor whirling in same sense as fuselage, but i can see that most would come from tail surface. I imagine the X2 will benefit from various rate gyros to give the machine a much crisper pedal response. Actually i wonder if this means rotors end up reducing response rate at 250+ kias.

By "short term stability" i imagine you mean response time to a given input (ie dynamic stability), and by "long term stability" you mean the need to retrim conrols (ie static stability)? I'm starting to realise that vibration attenuation at rotor blade source is very much in its infancy, with the Moog vibration absorber inherited from S-92 an interim step.

Something i have often wondered is how was the "fixed-wing" J60 control integrated with the PT-6 control? Was there just a seperate throttle? This is something i am curious about for X2, as i imagine the collective has sprouted some additional features. Would a mech to allow fore-aft movement of the twistgrip allow adequate control of longitudinal thrust (be it prop pitch or J60 thrust)?

Mart
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