The Future of Rotorcraft 4

[Intermesher]

Reprint of a posting on rec.aviation.rotorcraft.com



The bad news.

Most modes of transportation have changed relatively little during the past 70 years. For example, the automobile had, and it still has; four wheels, an enclosed heated passenger compartment, a reciprocating engine and a steering wheel. In addition, the speed limit has not changed, whereas the speed limit of the computer doubles every two years. In part, this slow development is because the transportation industry is a mature one.

This is inability to improve is particularly apparent in the field of rotorcraft. Boeing has stated "The Chinook was developed in the late 1950s, less than a decade after the B-52 bomber entered service. Since then, two follow-on bombers have been fielded, but no new heavy-lift helicopter."

This lack of significant advancement in rotorcraft has resulted in a diminishing number of people being involved in rotorcraft R & D. In addition, this small collection of people is thinned out even further by their diversity of interests. Jokingly, at one extreme are the few whose 'research consist of discovering whether a NC nut or a NF nut is used on a NF bolt. At the other extreme are the few whose research consists of modeling the blade vortex in 5D.

This reduced activity at the various levels of rotorcraft development manifests itself in many ways. One of these is in the peripheral support industries, such as publications. How many times can the same ideas be regurgitated?


The good news;

I am convinced that the field of rotorcraft has fallen behind that of other modes of transportation. There is an opportunity to catch-up, but, it will only come about when its leaders step out of the box. This box is the mindset that has embraced the abominable tail-rotor and excluded the more efficient latterly-located-twin-main-rotors.

Frustrated defense departments and some industry leaders are now starting to look in this direction. They are also looking at very-light rotorcraft, to be used as UAVs,

There is absolute no reason why the recreational/experimental side of rotorcraft cannot experience a developing and exciting future by moving in the same direction.


This posting may appear to be self-serving, but there is no commercial interest on my part.

Dave J.

http://www.UniCopter.com

 

[GraviMan]

Well, the beauty is in the detail (as Einstein once commented). I would like to see rotor rings, or shroud research. The field of rigid rotor control (without direct electronic control) is another area that got sidelined. Variable stators are worth pursuit, as well as twin rotors.

Envelope extension regards speed and efficiency are next...

Mart

 

[GraviMan]

Intermesher, I wasn't sure whether to start a new thread on this...

I seem to remember you talking about maximum velocity of the Unicopter being potentially 500kts. The RRPM was to be designed to reduce, in order for these speeds to be achieved without compressibility effects on the blade tips.

One thing has always bothered me, however. If the blade slows down, the region of zero airspeed on the retreating blade will move outboard. Eventually all the air will be flowing rearward, throughout the cycle. This means that when the blades are forward of the hub, the airflow will cause structural divergence. There is a serious risk that the blade could flex upwards sufficiently to snap off.

Is there any technology that you are considering that would avoid this problem? Wouldn't a rotor ring, or even static shroud, help both structurally and aerodynamically. NASA's X-15 had to use fly-by-wire to avoid structural divergence in the sweep forwards wings. I am curious, since the idea of a 500kt helicopter really appeals.

Mart

 

[Intermesher]

Graviman,

For rotorcraft to achieve higher forward speeds, plus an improved L/D ratio, a number of interrelated features must be unified in a single craft. Some of these features have been used in previous rotorcraft and some are new. A listing of them can be seen in the block at the top of web page.

http://www.synchrolite.com/UniCopter.html

The objective of the UniCopter's web site is to list the required features, explain their function, and then attempt to develop methods of successfully achieving them.

Listed features that have been previously applied in flying rotorcraft are;
~ The intermeshing and the interleaving configuration. [Flettner, Kaman, Kellett, and others]
~ An intermeshing configuration with 3-blade rotors [Kellet]
~ The Advancing Blade Concept [Sikorsky's XH-59A]
~ Two speed rotors [Sikorsky's XH-59A]
~ Separate horizontal thrust device [many]

Many of the listed new features have been proposed by others.
~ Low tip speed and wide chord [Wieslaw Stepmewski]
~ Reverse Velocity Rotorcraft [Harold Lemont] [Sikorsky]
~ Independent root and tip control [can't recall who]
~ Higher Harmonic Control etc. [Many are working on various methods]


The block in the above mentioned UniCopter web page has links to all these subjects. To satisfy your principle concern, you may wish to initially click on [Reverse Velocity Utilization].

If there still are questions, please ask.

Dave

 

[GraviMan]

I've had a brief (lunchtime) look at the reverse velocity utilisation, but it doesn't answer my main concern.

Chopper blades normally have the airflow at the tips flowing from the leading edge. If the chopper speed goes up and RRPM slows down, there will come a point in the rotation when flow is from tip to root (ie along blade). This is only really a problem if reverse flow occurs over retreating blade tip. In this instance the effect will be worst as rotor just passes the forward most position in cycle (over canopy).

If this does occur, even with rigid rotors, there is a real risk that any flexing will increase blade lift. The practical result would be structural divergence, resulting (sooner or later) in blade failure.

I take it that by having a broad/flat tip the concept is to push up rotor RRPM, so that tip remains in leading to trailing edge flow. I just can't visualise any other way the system could work, unless you have active tips - like had to be fitted to the NASA aircraft (i'm not sure what the designation was, but it had swept forward wings).

Mart

 

[GraviMan]

BTW, how do you find the time to put so much work into Unicopter & Synchrolite?!? Is it a project you are doing on your own or is there a collective?

I have recently placed an order for the various recommended reading on single rotor, counter-rotating, and convertiplane helicopters. I intend to read these, after going through friend's notes on aero-engineering. Actually the second degree (physics) seems to complimemt the first degree (mechanical), since i usually seem to end up explaining the notes to her!

My main interest at the moment is to understand whether intermeshers or interleavers offer the most advantages. I still like the idea of swirl removing stators, and some kind of tip shroud to remove losses. Who knows, i may be able to help in a limited way. Although, i've still got two more courses left...

Mart

 

[Intermesher]

Graviman,

The ultimate objective for rotorcraft, excluding the tiltrotor, is to exceed mu = 1. At mu > 1, and at 270-degree azimuth, the complete blade is in reverse airflow.

The CarterCopter is a compound craft that hopes to achieve this high speed by unloading its rotors. Theoretically the wings will provide most or all of the lift and the centrifugal force of the high inertial rotor will restrain the blades from doing anything fatal. Perhaps the Tennis Racket Effect [

http://www.synchrolite.com/B329.html#Tennis_Racket_Effect

] plays a role. IMHO, the compound configuration is a looser. The additional weight and the aerodynamic disadvantages of this configuration would be better spent by providing additional strength in the rotor system [

http://www.synchrolite.com/0815.html

].

Sikorsky's Reverse Velocity Rotorcraft Proposal [

http://www.synchrolite.com/1281.html

] eliminates the wings. IMHO, this concept also has problems, as mention on this page and in a previous thread on this forum, but it is a step in the right direction. A better understanding of this concept, can be seen at the slow downloading slide presentation [

http://www.brazd.ru/books/b0005/book/58af089.pdf

].


Questions and criticism appreciated.

Dave

 

[GraviMan]

Trouble is even these still don't answer my main concern. Perhaps i should explain it another way...

If RRPM was extremely low, and chopper speed was very high, at a given instant in time the rotor assy could almost be considered stationary. If a blade is stationary above the canopy, then the main airflow is from tip to root. At the tip, there will naturally be some vector component flowing from leading to trailing edge. If mu>>1 then this component is so small as can (for this "demo") be neglected.

There is no such thing as an absolutely absolutely rigid rotor, since it would need to be machined from unobtanium - which would be a science in itself ;-) . Any flexure upwards in a practical assy, due to a thermal gust say, would cause flexure so that lift increased. The net result is structural divergence, with potentially catastrophic results.

The only solution i can see harks back to the discussion about using coupled pitch-coning. Since you are already considering independant root and tip control, then this does offer a solution. What you will need is to a cyclically variable tip-pitch coning couple. The couple will try to centre "rigid" blade flap-flexure above canopy, but have limited or no effect above tail.

I can see the hub mechanism becoming extremely complex - read heavy and expensive. It may actually be time to consider (partially at least) the same thing the fixed wing guys have grudgingly accepted - fly-by-wire. Since there are already moves to introduce cyclic higher harmonic control, it may be an idea to think about electro-mechanical control by default for the blade tips.

In the same way that an earlier thread discussed tip jets, and another servoflaps, you could have servo motor controlled blade tip aero servos. I think it may be necessary to accept a certain paradigm shift in control strategy for the machine you are visualising. Specially designed/tested electric motors and control systems are now FAA CAA rated, so can be used. A company in England (AMTL - Andover (UK)) are actually considering steer-by-wire for cars...

Mart

 

[Intermesher]

Graviman,

"There is no such thing as an absolutely absolutely rigid rotor, since it would need to be machined from unobtanium"

True; there is no such thing as 'absolutely rigid' but some guy used the expression 'rigid rotor' for a rotor that was anything but rigid. It doesn't leave much wiggle room It was originally called it a 'Really Rigid Rotor' (RRR). Perhaps it should be renamed back to it's first name.


The following may answer your concern.

"I can see the hub mechanism becoming extremely complex - read heavy"

This is the hub (or is it 'nub') of the very concept.

Basically, the UniCopter has two EXTREMELY rigid rotors which ubstitute for both the wings and the rotor(s) that are found on a compound helicopter.
For instance;
~ The spar of the blades is made from pultruded carbon. [

http://www.synchrolite.com/1054.html

]
~ The rotors have 'static masts' [

http://www.synchrolite.com/0844.html

]
~ The mast bearing is one large Crossed Cylindrical Roller Bearing [

http://www.synchrolite.com/1171.html

]


The UniCopter is intended to be a new generation of rotorcraft. There is no theoretical reason why this helicopter could not be taken up to a high altitude and then the engine and rotors stopped - dead. The craft put into a dive, the engine restarted, and the rotors restarted. Then the pilot pulls the craft out of the dive, just like a stalled airplane. Theoretically, the craft could even fly inverted, given sufficient negative pitch.

In other words, these two strong intermeshing rotors serve as rotors and wings. The lateral symmetry, significantly reduced cross coupling, pusher propeller, and extremely rigidity rotors should make the UniCopter behave very similar to that of a fixed wing craft in forward flight.


As always, constructive or destructive criticism solicited.

Dave

 

[GraviMan]

"It was originally called it a 'Really Rigid Rotor' (RRR). Perhaps it should be renamed back to it's first name."

Howabout "Really absolutely rigid rotor, no really it absolutely is..."


"Basically, the UniCopter has two EXTREMELY rigid rotors which ubstitute for both the wings and the rotor(s) that are found on a compound helicopter."

Yes but from a structural divergence point of view, stiffer and lighter just means higher eigenfrequencies. The behaviour of an aerodynamic axis ahead of the main structure path will still result in flutter. Check out the following sites to see NASAs soln to the problem.

http://www.dfrc.nasa.gov/Newsroom/FactSheets/FS-008-DFRC.html

http://www.dfrc.nasa.gov/Gallery/Photo/X-29/

This is why i suggest either active tips, or tip pitch-cone coupling in the earlier post.

"The UniCopter is intended to be a new generation of rotorcraft."

Agreed, and i really like the lateral thought going into it. I originally got on the right path with rotor craft design thanks to yourself, and this site. This doesn't mean that i completely agree with all of the unicopter design. That is partly why i am going to do some homework...

"The craft put into a dive, the engine restarted, and the rotors restarted."

Agreed, but there will likely be a Vne speed beyond which there will be rotor assy structural divergence. IMHO, only a control system (mechanical or otherwise) can extend this to 500kts.

"As always, constructive or destructive criticism solicited."

I am actually very impressed with the way you are going about the unicopter design. To many ideas get tied up in patent b*ll, before they have a chance to succeed. I have never seen your approach, and i hope it pays off.

Mart

 

[Intermesher]

Graviman,

Your concern about 'structural divergence' may be a valid one, particularly at extremely fast forward speeds.

My perception of future 'rotorcraft' is that of vehicles which would be faster than current helicopters but not necessarily faster than a tiltrotor. At this moderately fast forward speed, the low Mach number, the small but existing centrifugal force, the tennis racket effect resulting from the CF, the low angle of attack at the tip, the short moment arm about the feathering axis and the possible requirement for power pitch assist, will probably (or hopefully) negate this concern.

If 'structural divergence' is a problem then the simplest and perhaps most effective solution will be to lay up the plies in the blade so that there is a flap (out-of-plane bending within the blade) - pitch coupling.

Just one person's opinion however.

Dave

 

[GraviMan]

As I was saying, before my toes were trodden on. Since i understand from your site that you are considering higher harmonic control, it may be worth investigating electrical servomotor control for the tips. This would allow IRTC and high speed flutter control. Mechanical is equally possible, but i would keep an open mind. AMTL in the UK specialised in motors for aero surface control, and are extending the designs to steer by wire in the automotive field (read VERY reliable).

I'm not convinced about relying only on the tennis racket effect, since (as you point out with rotor assy precession) the aerodynamic loads will overpower the inertial/centrifugal loads. You could (by lay-up, cables etc) design the blade tips to flex down as blade cones/flexes up. This would likely be a combination of flex and twist, to have max effect on airflow velocity vector relative to blade tip. You may well be able to design the system in as part of IRTC, since it too will need to be cyclic.

I imagine that this will have to be new ground for the Unicopter. The V22 has had to break new ground, and lets face it, all engineering projects do - unless they are told not to investigate other fields...

Mart

 

[GraviMan]

Hi Dave,

Looking at Stepniewski&Keys, i can understand why you recommended it. I've got some other stuff to study first, but i'm gonna enjoy reading it. Hopefully i can then offer more help get your project off the ground. In the mean time i'll keep asking questions.

Mart

 

[Intermesher]

Graviman.

I question that HHC offers much improvement to the rotor performance. Active independent root and tip control will offer far more, if and when it can be achieved.

Electrical servomotor control is a consideration. My hope is to provide power assist to the root control and none to the tip control.

Flap-pitch coupling can be achieved now with composite construction. It is achieved by the use of different thread materials (carbon has relatively little stretch whereas fiberglass does) and/or by laying down the threads with a specific bias.

Dave.

 

[GraviMan]

"I question that HHC offers much improvement to the rotor performance..."

I've not done a lit search, but HHC was originally done by Sikorsky (i think) to reduce vibration. What they found was that by allowing blades to follow local streamlines more accurately, the rotor torque went down. Figure of merit went up sufficiently to justify additional hydraulic power requirement. Cost and reliability have so far prevented the concept getting to market, but i would say it complimented IRTC.

"Electrical servomotor control ... power assist to the root control ..."

I just suggest it in the event that you find yourself running out of package space. I can see two sets of swashplates, along with a mechanism to provide the required adjustment with forward speed as becoming complex.

Having thought/learnt a bit since posting the thread

http://www.eng-tips.com/viewthread.cfm?qid=97039

i would say that this is the single most daunting challenge in the entire project. It really is how the pilot percieves the aircraft. Have you got any preliminary control system layouts? Are you doing it on CAD or just mockups?

"Flap-pitch coupling can be achieved now with composite construction..."

Yeah, i thought of several mechanisms to accomplish this. Basically a 4-bar linkage, where tip bends down more than root bends up (or vice-versa). Regardless of decisions on control systems research into this would benefit both helicopter and fixed wing design.

Being honest i still see the ultimate aeronautical transport as being a combination of both technologies - i'll read up to get started. Basically i just see flight as the most efficient method to redirect reasonably still air towards the ground...

Mart

 

[bobg3723]

Intermesher,

Would you keep us up to speed as to what extent you've modeled the aeroelastic properties of your rotor? It seems everything hinges upon that mechanism (pun intended).


To not get too far off topic, I've been trying to locate the CASA link for ordering this document:

Symbolic Generation of Elastic
Rotor Blade Equations Using a
FORTRAN Processor and
Numerical Study on Dynamic
Inflow Effects on the Stability of
Helicopter Rotors
T. S. R. Reddy, Ames Research Center, Moffett Field, California
June1986

This Acrobat document took awhile to d/l, but it had an interesting title. However, a few pages were deliberately left blank.

This link will start the download process:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870015022_1987015022.pdf

The only thing I'm aware of it how difficult it is to be granted a license in general.

BobG

 

[Lovelock]

In answer to Dave J. origin comments, I have developed a unique (as confirmed by international patent search) rotor system that appears to have substantial advantages over traditional design.
No tail rotor, no counter rotating blade system, predominantly self stable, lifting capacity not constrained by rotor diameter, lower noise level, and several other advantages.
Experimental captive frame models and some basic wind tunnel tests look promising. A computer model of theoretical lift capacity is most encouraging.
As a retired design engineer (structural), can anyone out there give me some advise on what to do next!
Is the idustry really interested in fundamentally different thinking, or is it so locked into the tried and proven that inertia to change now predominates future development?
P.S. I live in N.Z. I am not a US citizen, have no contact with the US DOD, Airforce, or civil aviation!!
Where to from here?
Jack.

 

[Intermesher]

Lovelock;

The US army, out of frustration, IS interested in fundamentally different thinking. In addition, Sikorsky is looking at a significant step forward.

http://www.unicopter.com/1465.html

Just a couple of thoughts.

Every idea has a risk of failure or a lack of practicality. Sometimes the 'inventor' is too close to see a potential, and perhaps fatal, problem. A lot of time might be saved by submitting the idea to scrutiny by others. In addition, I believe that no party can patent your publicly disclosed idea, and you have one year from this disclosure in which you can patent it.

The above should probably be confirmed by a patent lawyer.

The further you advance the concept before seeking private financing, the less you will have to give up to get the necessary money.

Dave