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.

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]

Blades that extend

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-P...y=PN%2F4142697

http://patft.uspto.gov/netacgi/nph-P...y=PN%2F3768923

http://patft.uspto.gov/netacgi/nph-P...radenburgh&OS=

[henshaw]

Thank you

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.

[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]

Follow up

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. 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]

Good Info

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]

Variable Camber-Chord

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]

Time to CAD something up

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!

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 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]

Dancing Wu-Li Masters

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. 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.