A little helicopter history

[Dave_Jackson]

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

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?

Dave

[NickLappos]

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]

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]

Yes, No, Maybe so

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.



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?" Or, is there more to the delay then we are told?


Dave

[Graviman]

Lots of answers...

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.

--

Originally Posted by Dave Jackson

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]

proposed vs. feasible

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]

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

Mart

[Dave_Jackson]

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]

Not really enough here for a thread sep...

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]

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
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 Over and out.

[Graviman]

Roger.

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.

Originally Posted by Dave Jackson

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]

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]


re: Kellett

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]

Interesting facts, Dave. The one i had picked up on was on this
Flettner 265 web page:

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:

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 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.

Mart

[Dave_Jackson]

Heck ~ Go for 12 blades per rotor.

[IFMU]

Outstanding idea. Then your rotors could be your gearbox.

[Graviman]

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

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]

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.

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]

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]

Mart, you said "the seperation is constant"

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

5. As the phase angle 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]

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