Future Rotorcraft Concept

[Intermesher]

The following link is to a picture of a proposed future rotorcraft.

http://rotored.arc.nasa.gov/activity/HiSpeedRotor.html

This craft utilizes the Reverse Velocity Rotor Concept. The link

http://www.brazd.ru/books/ahs58/book/58af089.pdf

provides an overview of its features.

Are then any participants on this forum that would enjoy using this proposal as the basis for discussing the pro and cons of its features?

 

[GraviMan]

Couldn't download pdf, but the concept of pusher propeller rotor craft is not that new.

McDonnell (Hughes) XV9: Pusher piston prop: >200mph
Lockheed XH51 (compound): Twin jet pushers: >300mph
Bell model 533 (based on XH40): Twin jet pushers: 250mph

Nasa concept: Twin turbine fan: 415mph

Well, why not...

Mart

 

[Intermesher]

Graviman,

The web site probably will download. It just takes one heck of a long time. There are only about 15 pages, but they are colored slides.

Sikorsky is promoting it as the 'Reverse Velocity Rotorcraft Concept'. My concern is with their main rotor concept. I don't think that the craft is capable of making efficient transitions between hover and fast forward flight.

 

[GraviMan]

Hi Intermesher,

It did take a while to download, but was worth reading. Seems like Sikorsky is doing it again - i.e. sticking to a main and a tail while everyone else looks at tandems (intermeshing or otherwise).

I am fascinated with the reverse flow aerofoil. Either way this will be needed in either tandems, advancing blades, or intermeshers. The tail rotor makes sense, since I imagine it vectors to port for hovering, acting as a conventional tail rotor. Wonder how far down the line this concept will go...

Mart

 

[GraviMan]

Just did a bit of digging, since I remembered an alternative to high speed VTOL rotorcraft that was also commisioned. Most of the links have the usual UFO drivel, but the attached PDF makes very interesting reading.

http://www.cufon.org/cufon/Silverbug.pdf

I have seen video footage of these things hovering in ground effect, but don't know how much further the project got. The main problem was, not suprisingly, weight. In a rotorcraft you just have the blades, this required all sorts of internal ducting and structure.

It wasn't quite there in the 60's, but maybe with modern composite materials it would stand a better chance. I think the idea of actively generating the wing/rotor tip vortex is very neat. Maybe it will resurface...

Mart

 

[Intermesher]

Graviman,

re: Sikorsky's Reverse Velocity Rotorcraft

The idea of using a Reverse Velocity Concept is good. One concern, to me, is that they are not proposing to develop and take advantage of Active Blade Twist.

Part way through the transition, the outer portion of the retreating blade will be in forward velocity whereas the inner portion will be in reverse velocity. This blade will be unable to provide any lift. To negate dissymitry of lift, all lift must be removed from the advancing blade, as well.

Probably the "2-P control for transition" means that the lift is provided solely by the forward and the rear quadrants during transition. The greatest thrust in these two quadrants will be when the blades are at 0º and 180º azimuths. This is because the thrust must diminish to zero as the blades rotate to 90º and 270º azimuths. The large long fuselage is located directly under this large downwash.

re: SilvarBug

A friend acquired a full size mockup of a very similar craft from the family of a friend of his who passed away. It's about 15 feet in diameter and doesn't look viable, for the same reasons you give. I passed your web link on to him.

Dave

 

[GraviMan]

Very interesting Intermesher, I hadn't thought it through like that.

I presume that the original variable gear ratio (hydraulic presumably) was designed to counter that problem. At max forward velocity the blades would be rotating only very slowly, if not there would be a risk of the tips going through mach 1. The two speed gearbox, changing ratio at ~300kts from memory, is a practical compromise.

Active blade twist is only a concept at the moment, and unproven at that - unless I'm mistaken. Sounds like a very complicated solution which could have very catastrophic failure modes. They would likely be better off considering a turbo generator/motor cvt system - these seem to be improving all the time, although very high electric rotor velocity is required to keep weight down.

There have been a few convertiplanes that have proposed stopping the rotor completely above a transition velocity. It may well be that the Sikorsky also does this, effectively giving it a 3-speed box. If it works it will definately give the V22 a run for it's money.

Personally I'll stick to gravitic methods, but then I've done the calcs (at long last) to prove it's viability. My chance of building and testing a prototype? About the same as developing my own convertiplane. <:-(

Mart

 

[Intermesher]

Graviman,

Good thoughts, but at some point in the transition they have to make the change to the rotor's speed. And, of course, change the pitch on the retreating blade from positive to negative at the same time.

Perhaps the craft might have so much power that it temporarily puts very high disk loading on the forward and aft quadrants and just 'forces' itself through the transition.
Alternatively, it might do it in a steep dive.

 

[GraviMan]

Hehehe :-D. Good point, well made!

Not sure how they get around the transition point on the retreating blade, but it is more likely to be a combination of science and brute force. How is this accomplished in the Unicopter? I presume the retreating blades generate no lift (+ve or -ve)?

I'm starting to see the entire point of the unicopter design - been a bit slow up till now ;-). Yeah symmetric outboard lift does makes sense. I'll definately keep my eye on that site...

Mart

 

[Intermesher]

One way to maintain lateral symmetry of lift is to have twin rotors, such as coaxial or intermeshing, and utilize the Advancing Blade Concept [ABC]

http://www.UniCopter.com/0890.html.

Another, which would be applicable to all rotor configurations, is Independent Root & Tip Control [IRTC]

http://www.UniCopter.com/1096.html.

Sikorsky's Reverse Velocity Rotorcraft Concept cannot incorporate ABC and does not consider [IRTC]. [IRTC] would allow the retreating blades to provide a percentage of the thrust at all times, from hover, through transition, and all the way up to fast forward flight. Having thrust on the retreating side will mean that this craft can also provide an equal amount of thrust on the advancing side.

As the forward velocity of a helicopter increases the root of the retreating blade becomes subjected to an ever-increasing reverse velocity. Meanwhile, its tip is subjected to a decreasing conventional velocity. Somewhere between the root and the tip of this blade is a zone of zero airflow, This zone moves out toward the tip as the advance ratio (forward velocity / tip speed ratio) [mu] increases.

The IRTC concept operates on the principal that there are two cyclic controllers, one for blade roots and the other for blade tips. At a mean advance ratio, the root pitch angle of the retreating blade is negative, and the tip pitch angle is positive. The location of zero pitch (actually zero angle of attack) will be at the zero airflow location on the blade's span. Changes in mu will cause a relocation of the zero airflow location along the span of the blade. Changes of the two cyclics will keep the zero pitch at the same location as that of the zero airflow. Of course, at mu >1 the whole blade will have negative pitch. At all times, the retreating side is providing lift.

 

[GraviMan]

Hmmm. Still thinking about this, while reading Unicopter site (good site BTW).

Initial reaction is that IRTC would be mechanically very complex (read expensive). ABC (intermeshing or concentric) seems to make more sense to me, since the retreating blades can be feathered to produce no lift. This would put a high bending load on the hub bearings, but as long as these were designed for this loading I can see no other problem. Obviously the hub assy would also need to be designed to handle the two cantilever loads.

In combination with a pusher prop, this would allow very high speeds without any real change in aircraft config from hover. Obviously there would need to be a mechanism that reduced advancing blades angle of attack as speed increased. Certainly rigid rotors are the way to go for affordable choppers.

Am I making sence, or have you alreaddy considered this route?

Mart

 

[GraviMan]

Forgot to add:

...Obviously there would need to be a mechanism that reduced advancing blades angle of attack as speed increased. Since there is a rotor govenor there anyway, one thought would be to have the rotor speed reduce with dynamic pressure (i.e. rpm proportional to 1/v^2)...

Mart

 

[GraviMan]

Just thinking (wish I could edit earlier posts):

If, in the Unicopter design, the retreating blade was feathered there would be less blade interaction - i.e. less rotor drag and vibration. Since the rotors are rigid, they could also be given a shallow V allowing the advancing blades to be in plane. If controllability warranted a small amount of dihedral would also be possible, although not suitable for inverted flight.

Another thought: since the blades are composite why not try to go for an elliptical plan? This may not be ideal for hover, but would allow a more efficient &quot;wing&quot; at higher speeds (i.e. more uniform airflow velocity). Admittedly a taper plan is easier to build, and probably fits the flight envelope better. Section will still need to be optimised, not forgeting the reverse flow in feathered/retreating position.

Just a thought...

Mart

 

[Intermesher]

Graviman

Your first post makes very much sense. There are many reports on the technical aspects of the Sikorsky ABC. They make for very interesting reading.

The Sikorsky ABC handled the changes in forward velocity primarily by changing the phase angle on the cyclic control.

You're correct about the reduced V-angle between the masts when the rotors are rigid. Previous intermeshing helicopters had angles around 25-degrees. The theoretically UniCopter has 18-degrees.

Another benefit from the Reverse Velocity Concept is that blade profiles can be selected based on improved divergence Mach numbers (at advancing tip) with no consideration given to high coefficients of maximum lift (at retreating tip). A symmetrical NACA 0006 airfoil may be close to the optimal profile and the additional drag from the reverse airflow over this profile will assist in rotating the rotor(s).

 

[GraviMan]

Intermesher,

Forgive me, but I am probably going to bounce all sorts of stupid questions off you for the next few posts. There is a lot to take in on the Unicopter site, but it's very rewarding.

RPM govenors:
Wouldn't the best strategy be to simply use the rotor govenor to vary collective pitch for fixed rpm. The &quot;collective&quot; lever could then directly control engine power (or torque if you prefer), letting the govenor sort out the actual collective. OK there would be a slight delay, depending on govenor control strategy, but absolutely no risk of rotor stall - ever.

Tail fan pitch control for forward flight:
Isn't this best done from what would have been the &quot;collective&quot; twist grip. Make it a slider if you think this would be more controllable. This would allow the cyclic to be used purely for attitude control - important for an aerobatic machine.

Cyclic control:
Assuming you go for a feathered retreating blade (neat idea about using the drag for autorotation BTW). Roll could be accomplished using differential collective, on top of the already slanted swash plate (to feather blades on retreat). There are two options for pitch control:
(1) Conventional cyclic operating fore/aft on both swash plates.
(2) Tilting the entire assy fore/aft, although this may give clearance problems. Unlike a conventional helicopter you will have no net gyroscopic forces to contend with, so the control forces will be low.
Both systems would allow a very fast response.

&quot;Rudder&quot; control:
The simplest system would be differential pitch control of the main rotors (conventional pedal control). Again this could be accomplished by direct pitch, assuming there was clearance to do so. There would be some gyroscopic torque, which would require some collective roll input. Then again this is no worse than having to coordinate stick and rudder, and more importantly would be symmetrical. Any additional forces over the pitch control will be easier to deal with using one's feet.

High forward speed flight rotor RPM:
Couldn't think of a satisfactory way of doing this without using an epicyclic gearbox. Maybe CVT units are reliable enough for the sort of power you are directing. A one way clutch after the gearbox/cvt should allow the rotor rpm govenor to do it's job, even if the whole powertrain falls to bits...

Lateral dihedral:
Hmmm, tricky one. Having tall tail fins will help. It may just be a case of optimising tail fin height/area to balance dutch roll against spiral divergence. Sorry, fixed wing terminology, but I don't know rotorcraft speak!

The whole point is to have reasonably straightforward (almost fixed wing) control mechanisms. Lets face it it is verging towards being a convertiplane, albeit without fixed wings.

Are we reaching, or do I have a long way to go?

Mart

 

[Intermesher]

Graviman,

We may be moving off topic, and are definitely covering a lot of features, but here goes.

RPM governors;
You are correct, except that during an autorotative landing the pilot will want to 'consume' the inertia of the rotor. In other words, he will give up RRPM for thrust at touchdown.

Tail fan pitch control for forward flight:
Using the pedals for yaw retains a similarity with the airplane. Using a laterally symmetrical configuration eliminates the tail rotor all together.

Cyclic control & Rudder Control:
With the Absolutely Rigid Rotor Doncept [

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

] nothing tilts or flaps. The phase angle is 0-degrees. In other words the response to pilot inputs is fast, strong and producing minimal cross-coupling.

High forward speed flight rotor RPM:
A slow RPM rotor is one solution to this. Ref;

http://www.unicopter.com/1093.html

When comparing the helicopter to the airplane, I believe that the UniCopter with is laterally symmetrical configuration and absolutely rigid rotors comes the closes to the airplane. The rotors on UniCopter could lose significant RPM, the craft be put into a steep dive, the RPM reacquired, and then the helicopter pulled out of the dive. In all respects, it should perform like an airplane, except that it has the ability to takeoff and land with zero velocity. Theoretically speaking.

 

[GraviMan]

&quot;We may be moving off topic, and are definitely covering a lot of features, but here goes.&quot;

Yeah, sorry 'bout that. Must admit I was planning to take a back seat on this thread, and let the knowledge roll in. There were no answers - guess you're the theory guy here.;-)

I could start another post, or is there already one I should be reading?

RPM governors;
Good point well made - Pilot needs ultimate control. I imagine most accidents then are a result of the rotorcraft equivalent of trying to &quot;stretch the glide&quot; in fixed wing.

Tail fan pitch control for forward flight:
Quite right. I was really refering to the generation of forward thrust. More like the Sikorsky concept of this thread and the Unicopter pusher prop concept.

Cyclic control & Rudder Control:
Again, good point well made.

High forward speed flight rotor RPM:
Interesting stuff...

&quot;When comparing the helicopter to the airplane...
...it (Unicopter) should perform like an airplane, except that it has the ability to takeoff and land with zero velocity.&quot;

This must be the ultimate (subsonic) aircraft.

Mart

 

[FranAle]

Hi guys:

Interesting, what about drag or OEI(one engine inpoterative) operation? The pilot´s forces for stability in emergency during autorotation or slide landing. Did you discuss about that?

just more for the same idea

 

[Intermesher]

FranAle;

Are you thinking about Sikorsky's single rotor concept, the UniCopter's intermeshing concept, or both? Could you expand on your question a little?

Thanks.

 

[FranAle]

Hi Intermesher

I'm sorry. I saw the Sikorsky concept, and 80 paxs suggest at least two powerplant and of course as you explain quite well many devices for control. I have no more data to support the materials involved into transmisión design, swashplates and things like that, but is a concern

Thanks

Francis