A little helicopter history [Archive]

Graviman

4th Apr 2007, 11:44

Hang on Dave, you saying all helicopters would be made out of folded card? Gonna meet son of single rotor pioneer in a couple of weeks, so i'll mention it. Exciting stuff BTW - you know what they say about meeting your heros. :}

Seriously, for this thread lets assume that the two technologies had been developed side by side ;). Clearly X2 is where the two meet, so you could say there is already a convergence. If intermeshing had been standardised helis may have achieved higher speed sooner, but may have suffered in sidewards flight. From the handling point of view symetry is good, but single rotor control systems have evolved to produce linear rate control (SAS, AFCS, FBW etc). High rotor hub separation gives a fast response, but then so does high effective hinge offset. Efficiency of two systems is in practice probably similar (although a paper only exercise could probably prove otherwise).

On balance, i think when an engineer is confronted by contraints of cost reliability and performance he/she will go for the simplest solution to deliver. To my mind MR/TR is that solution, but it is not without compromise. As market expectations shift so will the required compromise, and i will be interested to see how X2 affects this...

Mart

IFMU

4th Apr 2007, 14:10

It invokes speculation as to what rotorcraft would look like today; ~ if the war had gone differently.
I'd speculate that they would have taken all our best helicopter engineers and had a great lineup of single rotor helicopters!

-- IFMU

NickLappos

4th Apr 2007, 14:20

There would be hundreds of single rotor helos with Iron Crosses on them, and we would be speaking German.

My Dad was a B-17 crewman in the Eighth Air Force, maybe he helped change their minds....

Chukkablade

4th Apr 2007, 14:30

I think its fair to say he might have had something to do with it Nick:D

Joking aside, I read the book 'Combat Crew' by John Comer once. I really dont think I'd have had the nerve to do what those guys did week after week. Balls of pure steel.

Nothing but admiration for that generation of veterans, and although I'm not the type to be easily impressed, I feel genuinely humbled whenever I'm around any of those lot. I owe them more than a beer when I get the chance.

NickLappos

4th Apr 2007, 15:14

Chuk,
I just finished "Masters of the Air" by Miller, and found it simply great.

I was just placed on the Board of Trustees of the Mighty Eighth museum in Savannah GA and am quite honored. What is simply amazing is that only 9 months after Pearl harbor, the Eighth had several bomb groups dropping bombs from England - 9 months to assemble 3 bomb groups, each with 25 complex 4 engined bombers, 2,000 total crew, with support equipment, training, building the machines, etc etc. And in March of 1945, 3 1/2 years after Pearl, of the available 26 bombing days over Germany, they launched 20 1,000 plane raids! This took a war machine of over 330,000 airmen. As an index, today is 4 years from the start of the Iraq war!

Graviman

4th Apr 2007, 15:52

Serious respect to your old man, Nick, and all that flew in that conflict. Occasionally in UK you bump into ex-RAF/USAF guys of that era, and i never cease to be amazed how down to earth these guys are. Definately the real deal in the hero stakes.

Mart

Dave_Jackson

4th Apr 2007, 19:36

I'd speculate that they would have taken all our best helicopter engineers and had a great lineup of single rotor helicopters!

I'd speculate that the most famous single rotor helicopter (and perhaps the best ;) ) was the one with a single blade and a solidity ratio of 1.5.

Anyone want to speculate as to who the designer was?

Dave

IFMU

4th Apr 2007, 20:27

Dave,

That would be Leonardo. But is he really the best designer? He hasn't gotten any further than you have! :)

-- IFMU

To invent a flying machine is nothing. To build it little. To make it fly is everything.

Graviman

4th Apr 2007, 21:06

Hardest part these days, IFMU, would be to find the cash to do it. Not so much for the endeavor itself, but for the time it would take to to do it. You'd be suprised how much a small team of dedicated engineers can accomplish, but they have to eat, sleep, pay mortgages etc.

Also a decent CAD/FEA package, and machines to run it on, is going to set you back a bit more. Then the testing and developement takes more time and cash. Sikorsky was smart to recognise a new market while he was still building the Clipper flying boats.

Mart

B Sousa

4th Apr 2007, 22:56

As an index, today is 4 years from the start of the Iraq war!

Nick is there a point to be made about that statement. Something like in the same period of time we are still just plodding through wheras this should have been over by some means long ago??
Im begining to think this is worse than Vietnam as at least then they would send whole units out to take care of business.

IFMU

4th Apr 2007, 23:46

Hardest part these days, IFMU, would be to find the cash to do it.
Mart,

I'm building a homebuilt plank, I have to work for a living, and I'm fairly lazy. Somebody more ambitious could do it, that's why we have Enstroms and Robinsons. So, it could be done, I would propose.

-- IFMU

Graviman

5th Apr 2007, 11:48

IFMU, congrats on your homebuild. Not disagreeing with you, but your machine is based on a proven design, i assume. VS-300 had many technical difficulties to overcome, not all of which were foreseen. It underwent many major revisions, and suffered several crashes, as it was introducing new ground. I'll gain better insight in a couple of weeks. :ok:

Modern equivalent would be X2, which clearly is being taken at a sensible pace. Any future development of X2 capabilities would likewise take a similar effort - it is only prudent to plan accordingly.

Mart

Dave_Jackson

5th Apr 2007, 16:38

The VS 300-C had one attribute. All FOUR of its rotors faced upward.

However, three were used for control. Then someone told Igor that others were using main-rotor cyclic control. :eek:

________________________

"Not one successful single-main-rotor helicopter existed anywhere. It was just the type of challenge that Igor Sikorsky needed ...." ~ William E. Hunt, Sikorsky Advanced Design Staff.

Apparently, the challenge was to build something different, not necessarily build something better. :ugh:

Graviman

5th Apr 2007, 17:07

Dave, i realise last post was tongue in cheek, but i'm not sure i agree. Remember i've actually spent a while studying an actual FL282, and beautifully engineered it was too. Although there were control advantages at that stage, there were also vibrations introduced with the use of two blades (3 or 4 is possible of course). That site you link also shows an example of how tricky they could be to fly.

Is the MR/TR design a compromise? Yes, no doubt. At the time cyclic control had not been developed to the point where it was fit and forget, and there was no Prouty to refer to either! Sikorsky's concept was to figure a way to get a single rotor to fly in a convenient package. Since the tail is needed for a rudder device anyway, combining it with an anti-torque function is a logical step. It introduces control difficulties, true, but it works. Since at that time the objective was hover and slow forward speed, retreating stall was a necessary compromise.

Side by side designs preceeded MR/TR, including Berliner in the US, but were never really taken up by anybody. Tandem had even been flown by that time, but again was seen as too complicated a solution.
http://www.nasm.si.edu/research/aero/aircraft/berliner.htm

A good engineer understands the problems then figures out which ideas won't work. What is left, no matter how much development is needed, is the right answer.

Mart

Dave_Jackson

5th Apr 2007, 19:31

Mart,

If you want a better understanding of Igor and his technical ability, or lack thereof, you might appreciate the semi-technical book 'Heelicopter : pioneering with Igor Sikorsky' ~ based on a personal account by William E. Hunt.

I strongly suggest that European Rotorcraft technology faltered because North America won the Second World War, economically.

Nick gives a related reason for European failure when he mentions the bombing of German plants. As a matter of fact, Flettner was bombed on three separate occasions and had to relocate on two of them.

Please differentiate between American marketing and German engineering. ;)

For what it's worth, I have owned a Porsche 928 for 25, trouble free, years. When looking at total cost, it is probably the cheapest car that I have ever owned. IMHO, it is STILL technologically, and perhaps aerodynamically, superior to any car that North America has ever built.

http://www.austinev.org/evalbum/imgm.php?n=455a.jpg&w=300&h=225

Dave

IFMU

5th Apr 2007, 20:10

Apparently, the challenge was to build something different, not necessarily build something better.

Sounds more like somebody else here we know & love!

-- IFMU

Graviman

5th Apr 2007, 20:56

Dave, that 928 is a beautiful looking machine.

I have worked in Munich and can speak german (and french), and will say that i learned my outlook for engineering excellence working there. By the same token my Uncle saw the demise of the UK rocket industry to Europe, my father the demise of the UK aircraft industry to Europe, and me the demise of the UK car industry to Europe. Both my grandfathers were at D-Day, so it is a bitter pill to swallow.

I can believe that Anton Flettner spent agreat deal of time investigating various options before choosing intermeshing. I can also believe that the MR/TR concept was discarded because of the possible control problems. But that doesn't alter the fact that the MR/TR concept was developed somewhere else. Intermeshing is a good concept for particular applications, but it too is a compromise. The compromise here is limited yaw authority and sideways flight performance.

Having all the powertrain in one location is good for reliability, so it would not suprise me if we saw new intermeshing developments designed for high speed flight. However i suspect high speed helicopters will develope into coaxial for high performance military machines, intermeshing for general purpose, and tandem for transport. But for a cost effective solution MR/TR will remain, albeit limited by retreating blade stall. There will be other solutions, including the current craze for tiltrotors, but in the end the market will be drawn towards best performance for a given investment...

Mart

Dave_Jackson

5th Apr 2007, 23:42

IFMU,

Who? Lu? :O

Mart,

You mention; "The compromise here is limited yaw authority and sideways flight performance."

1/ The yaw authority during hover may very likely be weaker than that of a tail-rotor craft. However, here are 2 potential solutions, should they be required; http://www.unicopter.com/1322.html and http://www.unicopter.com/1183.html

2/ The concern about sideways flight performance is more confusing.

Quadrirotor mentioned last week "As the Flettner was not allowed to taxi, I wounder if there is any problem when you try to take off with an heavy (or moderate) crosswind." This concern is similar to your's.

Quadrirotor was unable to recall where he had come upon this information. Can you recall your source so that I can dig into it further?

Thanks,

Dave

PS.

The Prewitt Aircraft Company evaluated the FL-282 for the USAF at the end of the war.
"The lateral static stability is substantially neutral. There is, however, considerable damping to lateral roll, and lateral control presents no special problem. Directional stability is influenced by collective blade pitch angles (good at high pitch, poor at low pitch)."

More of the summary from this 147 page evaluation. http://www.unicopter.com/0474.html#Evaluation

IFMU

6th Apr 2007, 02:01

For what it's worth, I have owned a Porsche 928 for 25, trouble free, years. When looking at total cost, it is probably the cheapest car that I have ever owned. IMHO, it is STILL technologically, and perhaps aerodynamically, superior to any car that North America has ever built.
Dave,

I would dare to comment that many would agree that Germany and engineering excellence go together. I don't know if it is genetic, cultural, or if it's in the beer. I have observed in my travels that many good engineers in the states are of German descent. Part of the diverse mix that makes the States a technological powerhouse.

One question that I would pose: If Flettner was so far ahead of his time, why hasn't Germany picked up the synchrocopter torch and marched on with it? They have an excellent helicopter company (MBB, even the birds are jealous). They have excellent aerospace in general - sailplanes, bizjets, commuters, unlimited acro monoplanes. Have the current generation of Germans been so cowed by their loss in WW2 that they abandoned all things German and took up the ways of the west, i.e. single rotor helicopters and weak beer? Or have their brightest minds in the helicopter field come to a different conclusion than you have?

-- IFMU

Dave_Jackson

6th Apr 2007, 03:41

IFMU,One question that I would pose: If Flettner was so far ahead of his time, why hasn't Germany [or anyone successfully] picked up the synchrocopter torch and marched on with it?

A very valid question. It has also perplexed me for many years. The following web page is an unbiased (I hope) search for that answer; or combination of answers. Intermeshing Configuration - Concerns (http://www.unicopter.com/B280.html)

At this point in time, I have not committed to any configuration. The only commitment is that of finding Nick's elusive 'Free Lunch'. However, at this point in time, the Intermeshing and the Interleaving appear to be the most promising for the future, IMO.
__________________

One might ask the same question of Sikorsky. They have known for 30-years what the pros and cons of their Coaxial ABC were. In addition, 30-years ago they stated the desired modifications to be made on a future craft.

Why, 30-years later, do they now think that it is a good idea? :confused:

Dave

NickLappos

6th Apr 2007, 09:27

The mystery is how one could believe all those foolish design engineers in all those factories in all those countries can't figure out what Dave knows and just hurry up and build one!

Seriously, the synchropter is LOADED with problems and inefficiencies. It is one of the forgotten configurations (like NOTAR) that awaits someone with both passion (like Dave's) and money to resurrect it. And like NOTAR, it will again prove how marginal it is.

IFMU

6th Apr 2007, 16:48

Why, 30-years later, do they now think that it is a good idea?

From the Sikorsky website:
This suite of technologies includes advanced blade technologies to significantly increase lift without increasing drag, fly by wire, active vibration control, advanced hub drag reduction, and an integrated propulsion system that intelligently controls the power shared by the aft propulsor and the main rotor.

And, I thought that their stated position was that technology has marched onward far enough to overcome the shortcomings of the original ABC.

-- IFMU

Dave_Jackson

6th Apr 2007, 18:11

Nick,

Yes, "the synchropter is LOADED with problems and inefficiencies." The 'synchropter', word and craft, are the creation of Charles Kaman.

No, the Intermeshing Configuration, as developed by Flettner, is quite different.

You appreciate facts over propaganda, therefore you will find the follow excerpt from the Prewett flight test and technical report interesting and informative.

http://www.unicopter.com/Prewitt-1.gif

If you desire additional factual information, the following excerpts from this report should be even more interesting.
You certainly must know of the test pilot Dave Driskill.

* Pilot's Comments & General Comments http://www.unicopter.com/Prewitt-2.html
* General Comments (cont.) & Conclusionshttp://www.unicopter.com/Prewitt-3.html

IFMU,

All of the listed improvement, with the exception of fly-by-wire, were proposed 30 years ago.

* Improved blades of composite construction with reduced twist, etc,etc.
* The original craft had provisions for the inclusion of vibration dampers ~ which were never installed.
* Reduced drag.
* Reducing the engines from 4 to 2 and integrating the rotors with a single propeller.

The question remains unanswered;
"Why, 30-years later, do they now think that it is a good idea?" :confused: Or, is there more to the delay then we are told?

Dave

Graviman

6th Apr 2007, 22:20

Dave, sideways flight performance would be limited by one rotor effectively climbing while the other was effectively descending. This is just due to the angle of the rotors for intermeshing package and the components of crosswind relative to each rotor in turn. This means that if rotor blade twist & taper is optimised for say hover, then sidewards flight means both rotors are operating away from ideal conditions.

Even when pilot corrected for rotor induced yaw to crosswind, there would be an apparent drag operating above the mass centre (since figure of merit lower than ideal). This might have a destabilising effect, which coupled with flapback which is also present in a single rotor machine, meant the pilot needed additional correction to keep the machine on track. This would certainly explain why FL282 flight test indicates neutral lateral static stability, compared to posive static stability on a single rotor. I don't have any references for this i'm afraid but it seems logical enough.

Do you have any additional information on figure of merit for this intermesher? I am curious in general whether having rotor "curved" around machine improves FM. Normally the airflow starts off nominally static, is accelerted to a velocity throught the rotor, then ends up as downwash with twice this velocity. The stream tube thus contracts, with a resulting inward radial component. Having the rotors angled in that way may keep flow normal to rotor over some portion, avoiding radial flow component. Then again a single rotor could in theory be designed with a downward cone/bowl if it was stiff enough (ie very).

--

Nick, granted the commercial sector has sensibly developed concepts based on commercially proven designs. The only real advantage of intermeshing config is that each counterrotating rotor can have increased swash plate package. If Dave ever gets his Independant Root & Tip Control to work then this increased package space may prove advantageous. IRTC (or whatever the abv.) may then overcome the sidewards flight difficulties. It is a development risk, and certainly there is no immediate justification of this config over coaxial.

--

The question remains unanswered;
"Why, 30-years later, do they now think that it is a good idea?" Or, is there more to the delay then we are told?

Actually Dave that is a very easy one to answer. 30 years ago the market did not expect high speed helicopters, so did not demand it. With the new tiltrotor craze, ABC is the best way to answer future commercial requirements for high speed helis.

Mart

IFMU

7th Apr 2007, 01:07

All of the listed improvement, with the exception of fly-by-wire, were proposed 30 years ago.
Dave,
About 137 years ago, Jules Verne proposed the idea of a nuclear submarine. But, the technology wasn't ready. It did catch up to the idea, and the second Nautilus was named for the genius of the fictional one that preceeded it. It's not always enough to propose it, you have to execute it too.
Improved blades of composite construction with reduced twist, etc,etc.
Where would these improvements have come from? Wasn't the majority of blades in the era metal? Weren't the blades they made pushing the envelope of what they could do back then? Were they using CFD or sliderules?
The original craft had provisions for the inclusion of vibration dampers ~ which were never installed.
Which are passive devices. Would they have been good enough? Do you need multiple sets so you can detune multiple rotor speeds? How much would they have weighed? Did the old ABC have payload to throw away compared to an S61?
Reduced drag.
Clearly, we have made NO progress in reducing drag from 30 years ago. The sleekest German gliders of today barely get more L/D than the metal craft of 30 years ago. The very latest Sikorsky or Westland blades can't be any better than an S61 or a Wasp. The engineers at helicopter companies have put all their efforts into defeating the intermeshing configuration, rather than advancing the state of the art.
Reducing the engines from 4 to 2 and integrating the rotors with a single propeller.
This, they could have done 30 years ago. I give you that. But was it enough to proceed with the design?
Where are we in Nick's blog chart?
Wait, we are overdue for Lord Mount's step:
Haven't we hashed this out before:
http://www.pprune.org/forums/showthread.php?t=250086
-- IFMU

Graviman

8th Apr 2007, 10:33

IFMU, Dave has a point in that if intermeshers had been developed with the same vigour as MR/TR then high speed helis may have occured sooner. In truth with X2 pointing ABC technology towards the market place, i would say the two techniques are converging. It is almost accademic to worry about exact layout of counterrotating rotors, as long as they are there to counter retreating blade limitations. As for tail rotor, well turn it 90' to make a nice pusher prop.

Dave, i can understand from the comments i made several posts ago why you might be drawn towards interleaving as a solution for lateral stability. My concern with this is that you are now introducing distributed drivetrain for flight critical rotors, which pushes up component cost & mass if nothing else. Also the increased frontal area will reduce highspeed performance - in a fixed wing most of the profile drag comes from the wings, not the fuselage! Going to ABC tandem would improve drag, but not driveshaft reliability.

This is why i am keen for X2 to be a success. It's only real detriment at this stage is the loss of payload for additional rotor mass, which reduces return rate for more expensive config. In practice each rotor need not be as strong as single rotor it replaced, for any given manouvre, sincle load will be shared between two rotors. Additionally twist & taper will be optimised for highspeed not hover, which is why IRTC intermeshing still has a look in. The symetrical control you seek Dave is already present in coaxial.

Now if, after all this, X2 did not see the light of day? Well http://www.unicopter.com/RollLaugh_2.gif

Mart

Dave_Jackson

8th Apr 2007, 20:01

Mart,

Three quick points about the Intermeshing configuration, based upon actual hands-on knowledge;

1/ The Prewitt report speaks well of the Flettner Intermeshing handling characteristic. Particularly, when it is compared to the Sikorsky craft of the same era.

2/ The Kaman Intermeshing (Synchropter) was used at one point in time for the initial training of Armed service pilots. It is said that this was discontinued because the training did not prepare the pilots well enough for the more difficult single rotor configuration.

3/ I spoke to Dick DeGraw specifically about his Intermeshing 'Hummingbird' on two occasions. He never mentioned control problems, other than the transition into autorotation, which is a slightly greater concern on the Coaxial because the Coaxial is totally 'differential collective'.

Obviously, the Intermeshing configuration is not perfect. The challenge and the pleasure is in the belief that it can be brought closer to perfection than can the other current configurations.
____________

I am also leaning toward the Interleaving configuration as being the optimal one for future rotorcraft.

The "distributed drivetrain", which concerns you, will likely be much less complex than that on Tilt-rotor craft.

For now, simply consider the application of the Advancing Blade Concept to the Interleaving configuration. ~ for fast forward flight (mu = 1.0) and for hover.
Consider the ABC-Interleaving in respect to the bottom sketch on this Web page http://www.unicopter.com/1506.html.
Then consider the ABC-Interleaving in respect to the sketch on Web page http://www.unicopter.com/1505.html

Forget for now, the fact that the coming future developments of Active Blade Twist and of Individual Blade Control will do MORE for the Interleaving configuration than for any other configuration.
_________

If and when there is a desire for a technical discussion on the Interleaving Configuration, I would suggest that it be started in a thread of its own.

Dave

Graviman

9th Apr 2007, 11:37

Dave,

OK, maybe intermesher is less of a compromise than i thought for sideward flight. Certainly it has the same, and possibly greater, advantage of coaxial for compact powertrain. I understand that if you chose a config already championed you will lose the initiative, but it sounds like a good horse to me! ;)

Appreciate what you are saying about interleaver not tilting, but powertrain will not be that much simpler. The driveshaft has to be able to take the full engine torque from one engine to the other rotor for OEI, potentially during high roll manouvres, and this has to be seen as a full reverse torque fatigue loadcase. This means that driveshaft will need to be large diam and will need many intermediate bearings, with surrounding structure, to avoid shaft resonance. Chinook, or tandem in general, powertrain might not be a bad place to look. This means that outlay and running cost pushes machine into transport applications to get the return, so we are not talking about a privately owned machine with this config.

Appreciate what you are saying, on site, about same diam (hence mass) rotor reducing disk loading on interleaving over coaxial. I think you need to do a separate study to understand how much of a compromise the increased width of the machine will be. Dedicated transport destinations will be able to cater for a wider config, but there is a definate market advantage to being able to squeeze in to a landing site. The hypothetical risk is you then lose out on sales for machines which can get into tighter spots, since future coax manufacturers (like 'sky) would simply increase the rotor diam to stay competetive. In weight terms my guess is that increasing rotor diam by 30-40% is lighter than the extra pylons and driveshaft mass, and this still lets coaxial be 20-30% narrower. Just something that needs consideration.

Mart

Dave_Jackson

9th Apr 2007, 18:06

Opps! Revised this post before seeing your following post.
Considering the amount of participation, this subject could be handled by e-mail.

Revised AGAIN to add the following info about taxing:

Dave;
see:
http://avia.russian.ee/helicopters_eng/flettner_kolibri.php (http://avia.russian.ee/helicopters_eng/flettner_kolibri.php)
at the bottom of the text you have a column of photos and if you go to the Flettner fl282 v12, you have the notice:
"taxing on its own power was stricly forbidden..." it's not the only place where i saw this remark...I go on searching for explaination

As before :O Over and out.

http://www.unicopter.com/Closed.gif

:)

Graviman

9th Apr 2007, 20:32

Original response revised for clarity:

There may well have been a difference of opinion between Kaman and Sikorsky at UTC, but there are many technical reasons for the choice of coaxial over intermesher. For a start the rotor dynamics of coaxial will be a small step from single rotor, whereas intermeshing introduces sideslip yaw coupling and possible roll yaw coupling. The coaxial aerodynamics are also likely to be more like single rotor. The other factor is that a coaxial rotor shaft assy was likely seen as being an easier development than seperate shafts.

at the bottom of the text you have a column of photos and if you go to the Flettner fl282 v12, you have the notice:
"taxing on its own power was stricly forbidden..." it's not the only place where i saw this remark...I go on searching for explaination

Dave, this will be to avoid rotor strike, since fuselage is constrained to follow ground in taxi. It was more likely done as a precation, since the earlier FL265 actually crashed after a rotor strike. This config is only really suited to high effective hinge offset, and i would be interested to know how frequent rotor strikes are with two blade intermeshing designs.

Mart

Dave_Jackson

10th Apr 2007, 02:14

Mart, i would be interested to know how frequent rotor strikes are with two blade intermeshing designs.

Objection:

The risk of blade to blade contact.

Information:

re: Flettner

50 pilots were being trained to fly the Flettner FL-282. This included blind and bad weather flying, to which some accidents were attributed. One fatal accident on Nov 10, 1944 occurred when blind flying in a cloud and it is assumed that the pilot dived his machine then pulled back so violently on the controls that the rotor blades were forced into each other or the tail. Subsequently the diving speed was limited to 109 m.p.h.

"All attempts to get blade interference in flight have been unsuccessful." from US evaluation of Flettner FL-282. [Source ~ ETF p.25] (http://www.unicopter.com/B281.html)

re: Kellett (http://www.unicopter.com/0896.html)

The initial flights of the XR-8 showed that there was insufficient directional control. To solve this, the rotors were redesigned to give opposed longitudinal cyclic. The two rotors have three blades each and this additional flapping caused the blades of opposing rotors to occasional make contact with each other. They then produced and tested 2-blade rotors, like the Flettner, but this resulted in severe vibration. A rigid rotor system was then proposed but proved untenable because of the significant reengineering required and the unavailability of composite construction in those days.

The XR-10 was then build incorporating the lessons learned from the XR-8 but there was blade contact during an autorotation. On a subsequent test, the control system failed, killing the test pilot.

Graviman

10th Apr 2007, 10:35

Interesting facts, Dave. The one i had picked up on was on this
Flettner 265 web page (http://avia.russian.ee/helicopters_eng/flettner-265.php):

The Fl 265 first flight took place in May 1939. During this flight the blades struck each other and the helicopter was destroyed. A similar fate overtook the second one, because the pilot had for-gotten to fill his fuel tanks. The remaining four Fl 265s in the original contract were extensively tested on the deck of a cruiser with such encouraging results that work was speeded up on the Fl 282, a second intermeshing rotor helicopter to embody the experience acquired during the tests of the Fl 265.

This does pose an interesting problem with intermeshing, that risk of blade strike increases with increased number of blades per rotor. In a coaxial the risk remains constant, as the seperation is constant. You may recall, that trying to understand rotor dynamics, i did a rough hand calc to understand why Comanche had five blades on the rotor on this PPRuNe X2 thread (http://www.pprune.org/forums/showthread.php?t=245168&page=2&highlight=comanche):

Using flexural waves (frequency dispersive bending wave) for blades in combination with rotating frequency gives:

Rotating_Blade_Freq = SQRT( (N^2 x Static_Nat_Freq )^2 + ( N x Rotor_Freq)^2 ) ; where N = blade N/4 wavelength mode.

For Comanche example this would gives "actual" flexural modes within ranges:
1/4 mode: 1.12P - this mode causes the swashplate lead angle
3/4 mode: 3.36P to 5.44P (1.12 to 1.81 x 3P) - say 4.28P
5/4 mode: 5.60P to 13.56P (1.12 to 2.71 x 5P) - say 8.71P
7/4 mode: 7.84P to 25.69P (1.12 to 3.67 x 7P) - say 14.19P

The range is because the calc does not consider construction, so that lower range is for stressed skin only and upper range is for solid section.

Basically, what i suspect Bill Welsh uncovered during his dynamics work on that project is that having only 4 blades for a rotor with such high effective hinge offset means that blade 3/4 mode comes a little to close to 4P for comfort. If you remember on this PPRune blade construction thread (http://www.pprune.org/forums/showthread.php?t=250317thread) i concluded that there was no reason to consider any number of blades in a rotor other than for dynamics or cost. I suspect that the decision was taken to go with 5 blades, as both 3/4 and 5/4 modes sit comfortably either side of 5P.

Interesting to consider what difficulties might occur in a 5 blade intermesher. Again this explains why 'Sky chose to go for the lower risk strategy of extending the work of single rotor knowledge to a coaxial. It could be considered that high effective hinge offset blades on a high speed intermeshing design pose additional difficulties. However, on any good project the difficulties are worth solving if the performance improvements merit. :ok:

Mart

Dave_Jackson

10th Apr 2007, 18:04

Heck ~ Go for 12 blades per rotor. (http://www.unicopter.com/0997.html)

IFMU

11th Apr 2007, 02:36

Outstanding idea. Then your rotors could be your gearbox.

Graviman

11th Apr 2007, 20:00

Bit worried here, Dave - i realised reading the page that you might not be joking? :eek:

Your 4-blade designs are fine. The reason to avoid 3 blade is that the blade 3/4 wave resonance would match the rotor 3P frequency. This means that if there was an upthrust at 90 azimuth say, all the blades would resonate in synch hence transfering 3P vertical mode to hub and thus fuselage. Comanche went for 5 blades, i believe, since it was found that the 3/4 wave resonance might match rotor 4P frequency (blade stiffness pushes up resonant freqs).

The reason blade 3/4 wavelength mode is a problem is that blade can be seen as a flexural structure, fully constrained at one end and free at the other. This is an over simplification but means that blade rotational frequency, for effective hinge offset calcs, will be 1/4 wave mode. It also means there will be odd order modes above this, 3/4 5/4 7/4 etc. That is how i did the dynamic guestimates above. Naturally having internal damping, or active tip damping also helps to minimise vibration at source.

Coaxial package protects for any number of blades required, so is a good choice. I would imagine 5 blades and above is getting risky on an
intermesher...

Mart

Dave_Jackson

11th Apr 2007, 22:01

IFMU,

Are you 'giving me the gears'? ;)

Mart,Bit worried here, Dave - i realised reading the page that you might not be joking? Joking??? The patent application has been applied for. The helicopter is going to be called 'the Barber'; because it has self-stropping blades. :rolleyes:

A test.
You say in post #33; In a coaxial the risk remains constant, as the seperation is constant. Now you've got me a "bit worried here". Think about the Advancing Blade Concept in conjunction with the Coaxial configuration.

Dave

Graviman

11th Apr 2007, 22:47

Dave, there will be similar blade flexure in both configs. In coaxial you can design the machine with sufficient clearance to start with. This does not alter with blade number, so going up to five presents no additional risks.

Interesting point though. In an intermesher advancing blades will be far apart from retreating blades. I think you would still want a good clearance though for rearward flight, although speed would be much slower. Also fast roll would pull advancing side towards unflexed retreating side, on one side or other (depending on rotation direction chosen).

I'm probably still safe in thinking coaxial better suited to high manouvre rate military apps, and intermeshing high efficiency civil apps. Good discussion though.

Mart

Dave_Jackson

12th Apr 2007, 03:39

Mart, you said "the seperation is constant"

Re Coaxial - Sikorsky ~ S-69 (XH-59) ABC

5. As the phase angle (http://www.unicopter.com/0998.html) is decreased, the clearance between the blade tips of the two rotors decreases. Minimum clearance occurs at the crossover of a retreating upper blade and the advancing lower blade on the left-hand side of the rotor disk. (Ref. Rotors articulated at the center of the mast have a phase angle of roughly 90º.)

1. The swashplate has an adjustable phase angle of 0º to 70º.

Opposed Lateral Cyclic:

1. Lift Offset: Differential displacement of the rotor thrust vectors can be controlled in two ways on the demonstrator aircraft: by introducing differential lateral cyclic (B1') through an independent control or by the variable phasing of the pilot inputs to the rotor (Г). Testing to date has concentrated on the use of variable phase angle.

Graviman

12th Apr 2007, 16:08

Dave, S69 would have varied the swashplate lead by altering both swash plate control mechs. Crossover point would be defined by main g/box synchronisation of rotors, so would not alter. Crossover points would be every 30 degrees, so if there was one on the left there would be one on the right. Statement about clearance with blade flexure still stands.

Not sure if S-69 was FBW or not - did it preceed Shadow version of S-76? Also i'm not sure how much control pilot (ie Nick) had over swash plate lead angle, but my guess would be this was an automated function of the control system (Nr and IAS).

BTW did some back of envelope calcs today, in the best engineering tradition. I was mistaken about roll torque variation being reduced with blade number - once you go to 3 or more blades per rotor roll torque remains smooth. So only reason for RAH-66 to have had 5 blades over 4 is for a flexural mode near 4P, unless there is a survivability benefit. More likely machine started with 4 blades, then increased MAUM.

Unless you have internal damping, stick with 4 blades per rotor for an effective hinge offset less than 15%. But if blade 3/4 wave mode is well damped, with a fail safe system, the 3 blade rotor will be more cost effective.

Mart

Dave_Jackson

12th Apr 2007, 21:55

Mart,

The remarks in my previous post are not speculation! The number in front of each remark is the Sikorsky document that it came from.

You obviously have an interest in Sikorsky's ABC craft. Perhaps your technical speculation, and the rebuttals, will be reduced, plus your knowledge be enhanced, if you first read this, and the linked, web pages (http://www.unicopter.com/0891.html); or buy the Sikorsky documents.

IFMU

13th Apr 2007, 01:46

Not sure if S-69 was FBW or not
Look at the era they flew the S-69 during. Did your car back then have a carb or electronic fuel injection? I think that may lead you to an answer.

did it preceed Shadow version of S-76?.
It seems like in the past several decades the designation number has been close to the year of conception. I think the S-92 was announced in 1992, though it was longer before it was certified. In either case, the model number is a sequence designator, so the S69 would have preceeded the S70, 71, 72, 73, 74, 75, and 76. The S70 was the blackhawk, some of the ones in between were probably research aircraft, or the designation never went anywhere. Maybe they were top secret intermeshers.

-- IFMU

NickLappos

13th Apr 2007, 01:51

IFMU,
The S-1 was Igor's first machine, and it didn't make it over the trees. The S-2 did, and so on!

The S numbers were used for big proposal efforts as well, so not every S number flew.

The only Sikorsky machine without an S number was Comanche, which was a Boeing collaboration.

I flew the ABC, it had regular controls, lots of them! 225 knots was a scream, frankly!

Graviman

13th Apr 2007, 10:28

Dave, old chap, not rebuttals just observations. I just misread your last post as being a comparison between lateral machines and coaxial. Thanks for additional info though.

From your site i would say altering the phase angle just redirected the forward cyclic required to compensate flapback to load up advancing sides, otherwise compensating for flapback would transfer load back to retreating sides nullifying ABC advantage. I am suprised that opposed lateral cyclic wasn't preferred over lead angle trimming though. This may have just been easier to control in flight. For automated system opposed lateral cyclic would make more sense to my mind. :confused:

IFMU, thanks for clarification. I wasn't sure whether numbers were in order. Then again i'm here to learn this sort of stuff!

Nick, that project sound's like my definition of fun! How did you find lateral control at speed? I'm curious in context of altering phase angle to load up advancing blades.

Mart

Dave_Jackson

6th May 2007, 00:02

Mart,

In post #38 you mentionedIn coaxial you can design the machine with sufficient clearance ......... Premonition? This latest Sikorsky patent may bring a smile to your face. :)

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=7,210,651.PN.&OS=PN/7,210,651&RS=PN/7,210,651

or if the above does not work try;
http://www.uspto.gov/patft/index.html,
go to 'Patent Number Search' and enter the patent number 7,210,651

Dave

Edited to add the following United States Patent Applications by Sikorsky for coaxial transmissions;

20060269414, Variable speed transmission for a rotary wing aircraft.
20060269413, Rotor drive and control system for a high speed rotary wing aircraft.
20060266883, Split torque gearbox for rotary wing aircraft with translational thrust system.

Graviman

14th May 2007, 20:29

Skimmed patent, Dave (pics don't always work for this site). Intermeshing may well offer a simpler solution for forward flight (equal flapback both rotors). For sideward flight the risky area is the crossover azimuths fore and aft of the intermeshing hubs. So active servo technology would benefit any counterrotating machine.

The main arguement to my mind has to be the compactness of the main gearbox with intermeshing. Being able to keep the rotorshafts short is a benefit for resonance control. The increased package space also allows independant root & tip control. In practice active tip servo helps on both counts - assuming multiple redundancy...

Mart