Extendible Main Rotors

[henshaw]

Hello, I am new to this Forum. I am an experienced fixed wing pilot with about 25 years flying experience. I humbly know little about rotor craft. My day job is Founder and Chief Design of a company called MindMatter Innovates. I had a crazy idea and please bare with my fixed wingeness!

I've heard of failed attempts to design and implement extendible main rotors but when I try to research extendible rotors I come up empty handed. Tried patent searches, Google, NASA... I've heard of some unworkable ideas such as telescopic blades. Typically there are centrifugal forces to the order of 20 to 30,000 lbs on these blades, let alone asymmetric and variable centre of pressure & gravity. I can well imagine that a rotor blade which does not extend/retract has enough complexities both aerodynamically and structurally to be an engineers nightmare at best. I can almost bet if one adds another degree of freedom into the equation that certainly can sour the morning coffee.:ugh:

Q1: Has anyone heard of such a device and if so can they point me in the right direction please?

Q2: I have a preliminary idea design for an extendible main rotor that is simple, passive or active depending upon the requirement, and that would not implement single point failure at any point in the design or use. If I waved my "magic" design wand (wouldn't we all like to have one of those?) and put one of these little creatures into your main rotor would that be useful or useless?

I am not a helicopter pilot however If I may, I would think that if a pilot had use of extendible rotors then they should start out retracted at start up (higher rotation speed aka Ice Skater) and extend as the pilot required further lift. I would think that there might be a possibility of reducing the angle of attack of the main rotors if one extended the rotor disc therefore reducing the potential to stall the rotor and increasing torque output by running with a finer pitch on the ascent. Perhaps even having a quicker ascent. The downside I would think would be greater aero drag in forward flight and greater bending moments on the rotor. I guess the rotor would have to stiffer.

The design in mind would not deter torque transmital in any way. It would maintain the rotors perpendicular to the line of thrust so there would be no deterioration or losses there. The added weight would be close to the CG so further moments would not be a major draw back.

I do appreciate the complexe nature of rotary winged aircraft and in no way do I wish to make the reader feel that this addition is a simple one two three deal. It's just an idea right now, it works very nicely when I animate it parametrically on my SolidWorks station. The claims that I make about torque et al have already been tested and validated in a University Lab via another project that I'm currently working on. It was one of those wake out of bed oh my gosh let's try this out in a sketch kind of thing. Typically, or actually I've never posted a forum about ideas as I'm in the idea business. However, because this is complexe in nature and because I don't typically call on Helicopter companies I thought I'd give it a shot.

I would love some feedback at your convenience. I had first put this posting in another section and got feedback that I should ask a Mr. Nick Lappos as he should probably know something about this. Thanks.

Henshaw.

[slowrotor]

I have a book that has a photo of a Sikorsky model with blades that roll in and out.
That is, the thin blades roll onto a reel like film in a camera. They extend and stay tight from centrifugal or centripetal force.


The book is Vertical Flight and the caption calls it the "Roll Up Rotor."

Maybe Nick has some thoughts about the Sikorsky proposal.

[ron-powell]

Hello H:
I believe the idea is already out there, a Sikorsky design.

The blade is hollow with a threaded rod running from blade grip to tip. Only the outer section of the blade is attached to this rod via bearings– the outer section slides over the inner section when the blade is retracted.

The rod is attached to a bevel gear at the blade grip next to the mast. Another bevel gear located around the mast/shaft turns one way or the other, turning the blade gear and rod, thus extending or retracting the blade outer section.

Hope this helps.

[NickLappos]

There are several Sikorsky designs that made it to the ground test phase, with good to mixed results. The rotor was called the Trac rotor. Evan Fradenburgh was the chief aerodynamicist for Sikorsky, and the designer. He was one of the first helicopter engineers, joining Igor in the early 1940's. One design used a jack screw and another used a composite belt.

Here are a few patents, more can be found if you use "variable" and "diameter".

Good luck!!

http://patft.uspto.gov/netacgi/nph-Parser?Sect2=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN%2F4142697

http://patft.uspto.gov/netacgi/nph-Parser?Sect2=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN%2F3768923

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.htm&r=10&f=G&l=50&d=PTXT&p=1&p=1&S1=Fradenburgh&OS=

[henshaw]

Thank you all. Nick thanks for the direction. Was the Sikorsky invention fully commercialized? I don't recall ever seeing this type of innovation on helicraft.

Henshaw :)

[NickLappos]

henshaw asked, "Was the Sikorsky invention fully commercialized?"

No, these concept were tested and then not exploited, perhaps the rotor is complex enough now, without the need to make it more so. The idea has merit where the disk loading is very high (and therefore the power needed is also high) especially in a tilt rotor, where the rotor efficiency could be increased by 20% in a hover if the blades were "grown" to cover the fuselage in a hover, then shrunk to prop-sized for the conversion to cruise.

[ShyTorque]

Please stop this talk right now. Any talk of things moving about on my rotor blades gives me the heeby jeebies. :eek:

;)

[Dave_Jackson]

Henshaw,

A US patent search using the following query will bring up an number of relatively modern patents.

"AN/Sikorsky AND ABST/variable"

Dave

[henshaw]

Dave thanks for the heads up on the Sikorsky patent search. It seems there is a parent patent from 5,620,303 1997 on up - VDR that covers a complicated mechanism to achieve the goal. I can't imagine the weight penalties, along with failure modes. No wonder why this stuff doesn't fly!

Shy Torque, thanks for the input. I guess helicopter pilots have enough to handle in their hands already both figuratively and literally:yuk:. I get it.

Nick your input has been invaluable. You saved me hours and hours of digging, which becomes even more dificult when you don't know where to look and how to call it. It seems that the best direction I have gathered so far is to direct this idea to VTOL and Windmills. I am getting more interested in this idea as I can now compare what is public info and what I'm thinking about. I'm getting more convinced that maybe I'm not entirely crazy! Imagine that. I'm going to take ths now to an alpha mock up and see what happens. Interesting aircraft the S-69, XV-3, V-22

Thanks again guys I wish you all well and safe journeys!

Henshaw

[WillDAQ]

mmm... Suggest you look up the function of 'lag dampers' on complex rotor systems. They're used to solve a specific problem (lack of lead/lag damping in rotor systems) that could well preclude the feasibility of using such a system in flight.

There are a large number of research efforts on going around the work relating to this sort of area (ie. variable blade properties) and it's a tough subject.

[Graviman]

Hmmm, agree with WillDAQ, tricky design problem this. I don't see system as responding fast enough to be useful in collective.

If you move blade out Nr droops as square of change of radius (since angular momentum = mwr^2). Disk area also goes up as square of movement, but lift for each aeroelement is 0.5 Cl Ro V^2 S. This means that for a short while the reduction in Nr overpowers the increase in disk area - ie a reduction in lift. The lift only increases once Nr has recovered.

Even worse if you try to force the system to keep a constant Nr you will introduce many horrible lead-lag oscillations. The best you can hope for is to design the system to trim the rotor with say a 10+ second time constant in order to keep the blade AOA optimal.

Perhaps you could vary it to keep blade AOA optimised at different payloads,
but little else. Even this worries me since it would change all the blade eigenmodes, and might cause a flexural mode to hit an excited rotor speed ratio (eg chordwise bending and 5P or 6P). I wouldn't consider this system until i had developed a production tip servo vibration damper.

Besides does the extra weight/cost justify the performance increase? Why not just spec as large a rotor diam as you can then reduce Nr for reduced payload. You still need the tip servos to avoid off optimum Nr rotor modes, but final system would be much more reliable.

If the justification is high speed flight, X2 still feels to me like the right solution. Not sure what the news on that front is...

[henshaw]

This is good intelligent information. Thank you very much. If I may...

I see the extendible blade in its full extension and most fine pitch (default) at liftoff. This could possibly allow a greater weight carrying capacity at slow speed or no speed liftoff portions of flight. There would be no concern of Nr/torque as we're still on the ground and we liftoff only when proper Nr/torque is available.

For forward higher speed flight one would retract the blades for transition. This would provide the result of faster Nr as you retract inward which I would think is desirable???

Further when one would want to reduce lift (descend) and reduce Nr they would extend the blades and go back to fine pitch most similar to constant speed props for landing fixed wing a/c. There would be that immediate reduction in lift as described above along with a settling of forces as time dragged on. Not unlike the effect of flaps retracting-extending on a fixed wing aircraft.

In thinking of similes here, I wonder how fixed wing pilots first felt about flaps? They slow you down but they alter lift and pitch therefore requiring correction in flight. The corollary holds true after takeoff.

Having said the above and after reading and understanding your comments (I sincerely hope) am I missing something in this description please? Does this not make sense in the sequence of events as described?

[Graviman]

henshaw,

The analogy is more like extendible wings, than variable camber flaps. Variable camber aerofoils would benefit helicopter performance, if they were practical. I don't see how a variable radius rotor helps, when you could more easilly design a variable Nr system.

Swing wings occasionally come up for fixed wing commercial transports, but the mass of the bearing outweighs any fuel saving. Flaps allow a practical solution to allow a high speed cruise achieve a low speed landing and take-off. I think the same difficulties would be applicable to extendible rotors.

[henshaw]

Thanks Graviman. Yes, I see the analogy. Given the choice I would rather opt for flaps or variable camber because the variable/constant is less wild than articulating joints in motion. That makes perfect sense. The simplest things are most often the hardest to come by. Who hasn't felt this in their career.

So if I follow the logic that variance in camber or in chord is desirable then why not nest two airfoils into one another and hinge a swing of the second airfoil at the root (main rotor bearing) ? The first airfoil remains fixed so as to not disturb structure/forces/moments/lag/lead....

There would be no further bearing requirement as the main rotor bushings would already do the job of fixing and swing on the second airfoil. The leading edge of the second airfoil would remain always nested in the first airfoil. That would allow for internal support and effectively throw the turbulent transition point aftward, although not entirely perpendicular to the line of thrust. There would be a centre of pressure movement probably running inboard to the disk? Birds effectively do this when they fan their feathers in flight. But I can also see how tricky comparing fixed and rotary wings can be.

Regards,
Henshaw

[Graviman]

Now that is lateral thinking! This is a nice simple solution which allows the same rotorblade to produce a large range of Cl. If it behaves itself flexurally you could use it to improve retreating blade stall.

I imagine the root would effectively mimic the structure of the blade, so you could vary assemby pitch as well as camber. The problem is potentially always the cyclic control forces, since camber introduces a negative pitch. Hydraulics are a good start, but system should remain controllable if for example there was contamination in the hydraulic circuit.

There is likely an optimum combination of pitch and flap, so control system may actually be suprisingly simple. I would consider the design as if trying to retrofit it to the current fleet of helicopters. This keeps development cost down and helps highlight unknowns - safety comes first...

[henshaw]

Hi Graviman,

I guess this is lateral both laterally and literally! Your Scottish and I'm Canadian so that's pretty lateral wouldn't you say? I wouldn't want to walk into a pub in your country and start advertising that mind you! :D

If you could get me a profile or profiles of some popular airfoils used for rotor blades I could then model it up and see what happens. I run on SolidWorks.

Cheers

Henshaw

[Graviman]

henshaw, i'm cheating though since i was born in Hertfordshire!
Not far from where Harrier was developed. ;)
(between Cambridge and London)

Modern high performance machines are moving away from symmetrical aerofoils like NACA 0012 to aerofoils with camber. Listing given here:
http://www.ae.uiuc.edu/m-selig/ads/aircraft.html

You could do far worse than Theory of Wing Sections (http://books.google.com/books?id=DPZYUGNyuboC) by Abbott & Doenhoff. It has a nice section at the back detailing 60's aerofoils. I would google for more up to date designs like vertol VR-7.

Solidworks eh? I'm an NX and Ideas user myself, both CAD and FEA. At some point i'm intending to get up to speed on CFD too. PM me your details, so i can help out more. Time will be limited for next two months though (Quantum phys exam - seriously!).

[henshaw]

Thanks for the links Graviman,

I'll pull an airfoil section from one of these choices, model it up and PM to you. I don't know if the parametric properties will remain but I'll try to save as an IDEAS format or worst case a .dxf (why do they still make AutoCad? No idea).

As for Quantum Physics exam, sounds like fun I'm sure you'll do well, and good luck. Don't forget to read The Dancing Wu-Li Masters if you're in to that sort of stuff.

Please remember that I'm not a Helicopter pilot. I crash it every time in flight sim too.:ouch: I'm a fixed wing pilot but I do have a great respect and curiousity for these fascinating machines. So whatever I input comes from ignorance if that's the right word. Good evening.

[riff_raff]

Both the US government (taxpayers) and Sikorsky have pumped a lot of engineering hours and dollars into evaluating the variable diameter rotor concept, for both rotary wing and tilt rotor applications, over several decades. Take a look in the patent files and you'll see lots of Sikorsky patents covering the topic. The conclusion of all the studies they did was the same: it's a non-starter. It looks great in theory, but in practice it's a nightmare. The mechanical system design, reliability and dynamic issues presented by the variable diameter rotor concept would make it unsuitable for use on any type of aircraft.

[Graviman]

Riff Raff,

Welcome to the Forum. The more i think about high speed heli design, the more convinced i am that the solution is to stick a wing on the retreating side. Obviously hingeless or bearingless main rotor is required here. It may not look pretty/symetrical, but totally avoids the retreating blade stall. Being a single wing the mass is less than a convertiplane, but ideally needs a root bearing to minimise hover download - especially as force would only be on one side. I'm guessing the mass might be competative to a coaxial system for similar performance potential. Interested in other thoughts on this.

Henshaw's idea has good potential though, since it may allow unflapped blades to operate nearer idea Ct/Sigma=0.125. I'll be interested in his developments on this. Just need to study now!

BTW, i notice you are another powertrains man. I'm hoping to learn all sorts of tricks of heli g/box design from you. ;)

[ShyTorque]

A single wing on one side of the helicopter would produce another (roll) problem in the hover from downwash.

I think we have a good compromise with the present designs, which have evolved over a number of years into a good old engineering and aerodynamic compromise. I remain still to be convinced by the tilt-rotor concept, which is too specialised and so inherently relatively inflexible. I think a better way forward would be to resurrect the Gyrodyne/Rotordyne concept.

[Graviman]

I've seen parts of the Rotordyne at heli museum, ShyTorque, it an interesting concept. The problem was noise from the tip jets, and the poor efficiency would have limited hover time. Rotor gearbox weights are not such a problem for a heli this size now.

The asym-wing concept would only work with a high effective hinge offset and advanced SAS or FBW. These are both concepts which are fundamental to the coaxial high speed heli, so are feasible. In essence i am suggesting, without backing from calculation, that the empty weight of the asym-wing may be lower than a coaxial with strong blades. This also removes the complexity of the second rotor.

[henshaw]

Hello Riff Raff, Graviman:

There are two concepts floating around here.

1. Extendible/Rectractible Rotor Blades.
2. Extendible/Retractible aerofoil Chord

After reading the threads of comments it seemed that an extendible/retractible chord would be "theoretically" more useful than an extendible/retractable rotor. Boiling down the idea "why do this" is simple "in theory land":

To offer further control of ascent/decent characteristics. How much it offloads or loads up torque, Nr, ... we don't know. Yes, you're right Riff Raff, in theory it all looks pretty, and yes in reality the complex mechanisms to get to the "pretty" end up making it "ugly". As Joe Sutter said in his book-when he was finished with the entire 747 design he stood back, looked at the damn thing like a piece of art and said yes that looks right. I know you're probably shaking your heads on that but I do believe in simple statements and simple solutions like that. Example: does the A-380 look right to you?

Graviman I'll send you over the first ideation sketch of the extendible rotor in good old .dxf From there you should be able to see what I mean by simple solutions. Same goes for extendible chords. I beleive there's always an answer. Every time I board a 747 I do so still not believing that this thing can fly. Yet she flies gracefully and safely. Our brains are more than capable of figuring this stuff out. ;)

[NickLappos]

Those who think that extending the blades somehow just increase the torque don't understand rotors very well. It goes without saying that the increase in rotor diameter also incurs a change in the rotor rpm to adjust tip speed appropriately. Usually, that means holding tip speed constant and reducing rpm. Torque goes up linearly with reduced rpm at fixed power, but lift goes up by the square root of the disk area. Therefore, lift increases markedly against power at fixed torque. Big rotors always lift more than little ones. Otherwise, helos would have rotors that fit into vest pockets.

If not for the mechanical clap-trap needed, the idea is a sound one. The first thing a designer does is size the rotor diameter for the engines and payload needed, and the ability to do so "on the fly" is (except for the mechanism) virtually all gravy.

[Graviman]

Nick, i was refering to the system ability to respond to transients. Angular momentum is mwr^2 or mvr, so that increasing radius momentarily drops the aerofoil speed. It requires a torque increase just until tip speed is recovered. My concern is that this introduces lead/lag modes, if the rotor radius is being continously adjusted. For steady state, or long time constant inputs, your analysis is spot on.

I'm actually slightly confused here. Isn't the rotor normally sized as large as practical for a given machine, to keep disk loading down? Why would a designer want to reduce the rotor diam for a given machine on the fly? The only reason i can think of is for reduced landing area, say for ship ops.

Henshaw, i though the variable camber design was for fixed chord? This way you allow the "clean rotor" to run near Ct/sigma of 0.125, but with the option of deploying flaps quickly for increasing lift. Rotor figure of merit efficiency would be reduced in the manouvre, but this is a temporary state. I was trying to think of a relatively simple mechanism which would improve the rotor efficiency without sacrificing the manouvre performance. Increasing chord to maintain constant Ct/sigma is a good future development, but basics first...

[Dave_Jackson]

Today's helicopters require the rotor to product the lift and the forward propulsion.

It would appear that the inclusion of a propulsive device, such as a propeller, will reduce the thrust that is demanded from the rotor, for a given forward velocity. This lower thrust from the rotor will mean a reduction of the high pitch angle on the retreating blades. In turn, this will result in a retreating tip that is further from stall and a retreating root that is producing less negative lift.

All of the above should result in a slightly fast forward speed for a given power. Perhaps, the Lynx speed record could be considered as an example of this.

In addition, since less of the 'given power' is being consumed by the main rotor it means that less power is consumed by the tail rotor or the drag of the rudder to counter the rotor's torque. This was previously look at by Sikorsky when they placed a swingable tail rotor on an existing craft.

Makes one think that a union of the helicopter and the gyrocopter may be in the future. http://www.unicopter.com/Think.gif

Makes one think that a union of a symmetrical twin rotor helicopter and the gyrocopter (http://www.unicopter.com/UniCopter.html) may be in the future. :)

[Graviman]

Dave, i've always supported your high speed intermeshing concept. Kaman (http://www.kamanaero.com/helicopters.html) apparently offered an intermeshing solution at the same time as Seasprite, but the Navy chose conventional. This may have held back counterrotating helicopter development. I suspect the reason was more to do with head clearance on deck than performance. Unfortunately development of X2 seems to have ground to a halt, which is a great shame.

The modified S-61 was part of the competition that lead to the AAFSS, for which Cheyenne was designed. The competitors were all compounded machines with Bell getting 274kts and Lockheed getting 263kts. This gives an idea of maximum achievable speed with a pusher prop compounded helicopter. I am suggesting an adaptation to compounding which would alleviate the retreating blade stall, while minimising hover download and parasitic mass.

The asym-wing is very much a compromise, with the objective being the minimum solution for the specific problem of retreating blade stall. A movable tail rotor may well be part of that solution. The single wing would produce anti-torque at speed, so tail rotor could become a pusher. Part of me recoils from the concept, but i'm sure it would be viable. Basically it has the lowest parts count. :uhoh:

[Dave_Jackson]

Mart; you mention; "Unfortunately development of X2 seems to have ground to a halt, which is a great shame."Perhaps there are problems developing appropriate airfoils for high-speed flight. In addition, the two rotors and the propeller are all in very close aerodynamic proximity to each other.

Recall that the nighttime promotional picture of the prototype showed a fuselage etcetera but no rotors or propeller. One would think that Engineering would develop and test the complexities first.

Perhaps this quote from the AHS Rotary Wing History thread (http://www.pprune.org/forums/showthread.php?t=292194#1) says much. "Perhaps his [I] greatest accomplishment was in his skills as a showman in selling the helicopter to the military, ...."

[Graviman]

Dave, unfortunately i suspect there may be some internal politics going on. I have quietly wondered whether there has been an incident which damaged the machine - i sincerely hope not. A project of this nature is not going to be cheap, and will tie up many engineers during it's development. I imagine much of the groundwork has already been done on the ABC.

Getting everything stiff enough, while keeping mass low, will be a headache. The one problem i am aware of is flexure in the extended driveshaft to the top rotor. Sikorsky may have been overly ambitious on rotor clearance/flexure.

This is what got me thinking about the asym-wing concept as a quick and dirty method to achieve the X2 intended performance. The Comanche parts bin could also be further used, particularly that 15% hinge offset rotor. The final design may not be a pure helicopter, but so what as long as the machine delivers 250+kts...

[Graviman]

Just a quick check on Sky website reveals nothing new on X2. There is one sound heavy lifter concept though:
http://www.sikorsky.com/file/popup/0,9604,2190,00.pdf

Designing 2 4-blade rotors to do the work of one 8-blade makes sense from the weight point of view. I guess it is smaller machines which would suffer cost and weight penalties. Of course tip servo vibration damping might allow the return to 3-blade designs, then again i imagine the each rotor 3P vibration was out of phase due to 60 degree azimuth seperation. I've never read about S-69 suffereing vibration issues.

If i had to guess: Sky is waiting for tiltrotor verdict before making a pitch... ;)

[Dave_Jackson]

If i had to guess: Sky is waiting for tiltrotor verdict before making a pitch... ;) If i had to guess: Blue Sky (http://www.alphadictionary.com/goodword/word/blue-sky) is waiting for goverment money before making anything.... ;)

[NickLappos]

Gents,

As far as I know, the X2 is humming along, preparing for its next step. Funding has been slow, I hear, because the funds are purely internal R&D funds, what is left over after profit, and therefore subject to the natural market forces.

Several good friends, experts in their field are working daily on the aircraft.

[Graviman]

That is good to hear, Nick. So no news really is good news then in this case. My concern was that the project may have lost momentum.

Good luck to the X2 team. Take your time, and get it right. When it comes many jaws will drop...

[Dave_Jackson]

As Nick has mentioned, the objective of extendable blades is to improve the hover/cruse efficiency of the tilt-rotor (or tilt-wing) craft by having the disk area greater during hover than during cruse.

Perhaps the proposed V-44 quad-rotor (http://www.popularmechanics.com/science/air_space/1280611.html) , or a modified version of it, could provide this. During hover this craft would be supported by four rotor disks. During cruise each front and rear rotor would rotate so that their propellers shared a common axis of rotation. If the gap between the front and rear props was minimal they would act as coaxial propellers, as used by high speed aircraft.

In other words the total effective disk area in cruise is half of the total effective disk area during hover.

Dave

[TheMonk]

Extending a blade is probably not as hard as trying to retrieve or shorten it under power. lol
Monk

[Graviman]

Dave, that looks like a bandaid on a bandaid. I think i'll patiently wait for X2, if for no other reason than parts count!

The Monk, your observation is spot on and is my main objection to the extendible rotor concept. As you let the blades out you are extracting energy from the rotor system (ie it is pulling you). This means the kinetic energy and thus speed of the aerofoil must go down. As you pull the blades back in you are inserting energy into the rotor system (ie you pull it). This means the kinetic energy and thus speed of the aerofoil must go up.

This is why i say that for each adjustment the lift initially goes the wrong way. Any attempt to force the lift to remain constant for Nick's induced power saving forces lots of lead-lag energy into the system. The best you can hope for is a trim system with a 10+ second response time.

Much better to do something with the blade camber...

[henshaw]

Graviman,

That is correct. The concept I'm working on is for constant chord that extends (like flaps but not flaps) thus limiting the variables to offer as much control as needed. Again, I see variable length and variable chord as two different animals with different reasons for being.

Henshaw

[Dave_Jackson]

Here is another related idea, which might be of interest.

It is by Anton Flettner in 1955.

Air vehicle with Rotary Wing ~ US Patent 2,881,989 (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=2,881,989.PN.&OS=PN/2,881,989&RS=PN/2,881,989).


Dave

[Graviman]

This one actually flew, Dave:
http://airandspace.si.edu/research/aero/aircraft/images/herrick_hv2a.t.jpg
http://airandspace.si.edu/research/aero/aircraft/herrick.htm

The problem with a modern stop rotor is the need to stop the rotor without the tips breaking off at high KIAS. Active vibration supressing tips at the very least are required. These also need to handle potentially divergent forces when the rotor is at 180' azimuth...

[Dave_Jackson]

Mart,

The 'Stopped Rotor' is an interesting but very difficult concept. I do not believe that Flettner is considering this concept. His objective is to reduce the downwash of the rotor that is 'wasted' on the wings during hover and slow speed flight.

Last year's patent by Northrop Grumman ~ US 7,137,589 (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=7,137,589.PN.&OS=PN/7,137,589&RS=PN/7,137,589) is a variant of Flettner's.

Dave

[Graviman]

Right, i'm with you, Dave. Flettner tried many interesting things, including replacing the sail with a magnus effect rotor.
http://en.wikipedia.org/wiki/Flettner_ship
http://upload.wikimedia.org/wikipedia/commons/thumb/3/3f/Buckau_Flettner_Rotor_Ship_LOC_37764u.jpg/180px-Buckau_Flettner_Rotor_Ship_LOC_37764u.jpg

Ultimately I imagine X2 derived designs will consign compounds designs to history, if only because all the complexity/cost is in the rotor system. Many stop rotor concepts are still valid though, as the ideal is to be able to slow down rotors in flight. Once dynamic vibration suppression is developed for blade tips (reverse flow region), i would be curious what the top speed limit actually is. Maybe compressibilty effects...