Supersonic Rotorcraft ...

[Dave_Jackson]

... well maybe not quite supersonic, but very fast .

The forward velocities of today's helicopters are limited to approximately 200 knots. This is due to the combination of compression on the tip of the advancing blades and stall of the tip of the retreating blades. There have been various attempts to minimize this limitation, but the successes have been marginal.

A logical way to overcome this problem is to slow the rotors down. Unfortunately, the combination of fast forward velocity and slow rotor rpm means that the retreating blades are experiencing reverse airflow. Of course, air that is flowing over an airfoil in the reverse direction causes a lot of drag, plus other undesirable features.

A couple of years ago Sikorsky proposed a Reverse Velocity Rotorcraft and gave the blades an elliptical profile. Unfortunately, an elliptical airfoil may reduce the drag on the retreating side, but it will increase the drag on the advancing side. This concept was apparently withdrawn.
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Optimal Reverse Velocity Usage
http://www.unicopter.com/1369-2.gif

IMHO, for rotorcraft to achieve significantly faster forward speeds the rotor must have conventional airfoil profiles. In addition, the air must flow from the leading edge to the trailing edge at all times. A means of achieving this is to cause the blades to oscillate about their pitch axii as they pass in and out of the region of reverse velocity.

The Reverse Pitch Rotor Concept (http://www.unicopter.com/1357.html) is intended to achieve this. :8


An objective of this thread is to place the Reverse Pitch Rotor Concept into the public domain.

[Graviman]

My reason for studying rotorcraft has been primarily to understand the limitations of these machine. Compressibility is one limit i do not see as ever being overcome in a rotorcraft. Supersonic prop designs do exist, but the design would seriously hurt the machines ability to hover (mostly through disk loading). Better to find another technology...


Regarding your reverse pitch rotor concept, i gave a similar idea some thought when i was considering Unicopter. It introduces some serious materials difficulties, not least due to the extreme strain leading to fatigue problems well outside the limits of current composites. For a start when did you last see wing-warp ailerons, despite the serious advantages this would offer composite aircraft?

The "Piezoelectric Active Fiber Composite Actuators" are yet only a pipe dream in a laboratory, capable perhaps of small object manipulation. While i suspect the application to rotor blades would make a worthy PhD thesis, it is not available off the shelf nor will it be for quite some time. Good engineering effectively applies the available to the known to push the boundaries of the possible. Trust me, i spend a lot of "thought time" on the fringes, but willingly accept that advanced ideas have to be simplified until they are doable.

The performance advantages are also limited. Although you reduce aerofoil drag, you don't overcome the fact that no dynamic pressure (ie airflow velocity) around the zero velocity circle will result in no lift. This still allows a leak path from high pressure underside to low pressure topside. This is made worse by the fact that either side of this the aerofoil will be stalled. Interleaving will help, although above midspeed the zero vel circles coincide at two positions above the fuselage (ie not too serious). My aversion to interleaving is simply the complexity of drivetrain to keep rotors reliably intermeshed - similar to expensive V-22.

The above all leads me to the final conclusion that you are very hard pressed to beat the outboard advancing, feathered retreating, pusher prop (twinned for yaw control), intermesher - especially if gyro augmented for stability. Coaxial should just be seen as a stepping stone, although N.L. may strongly disagree.

I genuinely don't understand why you seem to have abandoned the (tandem seat) Unicopter concept... :ugh:

Mart

[Edit: On realisation that reverse pitch aerofoils will most likely be stalled around zero lift circle anyway (ie performance penalty)]

[Dave_Jackson]

This concept is a potential candidate for the future of rotorcraft. Its success is totally dependent upon highly advanced active blade twist (http://www.unicopter.com/B263.html#Active_Blade_Twist), which in turn is dependent upon the inevitable advances in materials and actuators.


I genuinely don't understand why you seem to have abandoned the (tandem seat) Unicopter I haven't. This concept could be applied to the UniCopter, in the distant future.

[Graviman]

"This concept is a potential candidate for the future of rotorcraft."

OK, but for reasons given i still prefer the intermeshing concept.


[Quote now edited out, but i felt the response still valid]
"Why put two people in an easy to fly craft that is intended for in-flight activities, when one person will do?"

For an advanced high speed rotorcraft, i would say that instruction would be essential. I suggest tandem since it offers little aerodynamic penalty (same frontal area), while allowing much increase in versatility.

"This concept could be applied to the UniCopter, in the distant future."

In an outboard advancing intermesher, if the retreating blade produces lift you get vortex slap against the advancing blade. Vortex slap is the principle noise source in a Chinook - and it is protty noisy. It is also the indicator of an uncordinated turn in a teetering head, rotor basically hitting its own vortex. Potentially fatigue issues could arise, if it was not considered in the design. Finally, canopy drumming also becomes a problem.

Mart

[Dave_Jackson]

Graviman,

Re: Vortex induced noise and vibration from retreating blades (intermeshing & interleaving):[list=1] The teetering Bell 206L-3 in Prouty's book has a tip speed of 763 fps from rotor rotation. Assuming a forward speed of 100 knots, the advancing tip will be experiencing a tip speed of (763 + 169) = 932 fps.
The tip speed of the retreating blade on the Reverse Pitch Rotor (http://www.unicopter.com/1357.html), when the craft is traveling at 500 mph, is only 587 fps.

The size of the vortices is also dependent upon the angle of attack. The usage of the Advancing Blade Concept (http://www.unicopter.com/B263.html#ABC) will reduce the angle of attack of the retreating blade.

At 500 mph the craft will probably be out in front of any potential problem.
[/list=1] Dave

[Mac the Knife]

Could the Magnus wing effect ever be applied to rotorcraft (i.e. a rotorblade that spins as well as rotating) ?

[Graviman]

Dave, the problem with votices is only when they slap against something like another rotor or a wing. At the 100kts speed chosen there will be considerable noise/vibration as intermeshing retreating blade sets up vortex for advancing blade to hit. Hence my thoughts on feathered retreating intermeshers.

I haven't looked into it in great detail, but Chinook sounds to me like rear/higher rotor wake impinges front/lower rotor. Has anyone found that tandems get quieter as they go faster?


BTW another concern with interleaving over intermeshing (or even conventional) is the required heli rotor span for same disk loading - the V22 problem...

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"Could the Magnus wing effect ever be applied to rotorcraft"

www.fanwing.com

I imagine it only works well at a narrow speed range. Too slow and high disk (line) loading is a problem. Too fast and the effective aerofoil shape becomes inefficient.

Besides, i prefer a Flymo to a Briggs&Stratton... :D

Mart

[quadrirotor]

The best concept for a supersonic hello is this one seen in the film "THE 6Th DAY" with Schwarzenegger:

http://ww2.planepictures.net/29/87/1106201162.jpg

[tecpilot]

Hi Dave,

The noise will be increased to a airflow velocity inbound Mach 1,1 in 6 to the power of the airflow velocity. Must be shocking!!!

I have some material from shukowsky (russia) about the supersonic rotor, but unfortunately is my academic english not good enough to translate it comprehensible :(

They have it fully metric and with other terms.

I'm 2 weeks out, but i will see...

[Dave_Jackson]

Mac the Knife

The Magnus effect is an interesting one but it may not have enough 'effect' for helicopter rotors. The related Coanda effect has been used on helicopters, but its 'effect' is also somewhat marginal.

Some trivia; The world most innovative helicopter builder, Anton Flettner, built a ship that utilized the Magnus effect (http://www.efluids.com/efluids/gallery/gallery_pages/flettner_rotorship_page.htm).


Graviman,

There will be aerodynamic rotor-to-rotor interaction on all craft with twin rotors (inc. main+tail), with the exception of the side-by-side configuration. The web page UniCopter ~ Vibration (http://www.unicopter.com/UniCopter_Vibration.html) considers vibration and 'wasted' thrust on the Intermeshing configuration.

On the Interleaving configuration, the greatest vortex coming from the retreating blade will be at the tip. You will notice on sketch 1441 (http://www.unicopter.com/1441.html#Drawing) that the retreating tip is basically outside the fuselage, plus it is below the other rotor.

I don't understand your disk loading concern,


quadrirotor,

Heck.. It looks like it's built for 'show' not 'go'. :oh:


tecpilot,

I agree, there would be a lot of shocking things in trying to take a rotorcraft through the sound barrier. Think of the blades having to go into and out of supersonic three or four times a second.
http://www.unicopter.com/WrongForum.gif
Perhaps my reference to supersonic should have gone on the 'Hallucinations' forum. :D


Dave

[Mac the Knife]

Thanks - interesting link to Flettner too :ok:

[Graviman]

"There will be aerodynamic rotor-to-rotor interaction on all craft with twin rotors (inc. main+tail)"

Feathered retreating, outboard advancing, (FROA) intermesher will be no worse than a conventional.


"On the Interleaving configuration, the greatest vortex coming from the retreating blade will be at the tip."

Depends on twist & taper. My real objection is the drivetrain complexity. On paper 4x4 cars gives the best performance, but few are sold due to the cost.


"I don't understand your disk loading concern."

A single rotor the same diameter as the interleaver will require less power/weight (approx 1.4 times if tip passes close to hub). F.R.O.A. intermesher gets close, by having a compact rotor system.

I see the F.R.O.A. intermesher as a bit like a fixed wing, except the wing moves forward to meet the air at the optimum velocity. The retreating bit is just a convenient means of returning the blade. When i say feathered retreating i mean twisted and pitched so that blade is at zero AOA to inflow (at 270'), hence no vortex to spill off.

The trouble really is Dave that we are both disputing respective speculation. The only way to resolve the optimum solution is going to be a test rig, some smoke, and a strobe...

Mart

[NickLappos]

Best way to get supersonic in a rotorcraft is to go to 15,000 feet, pull both engines to idle, put the rotor brake on and wait. Just before impact, you will be going alot faster than any of the dreams you guys are spinning in this thread!

[Dave_Jackson]

Graviman,

"Interleaving configuration .... My real objection is the drivetrain complexity"
If the Interleaving drivetrain gets you excited, think about the Tiltrotor's drivetrain.

"I" STILL "don't understand your disk loading concern.". Let's not worry about it .

To me, it is a shame to not utilize the retreating blades, but as you say "... we are both disputing respective speculation."

Over and out.


Nick,

That's a good idea, but Roton beat us to it. :ok:
http://www.aero-space.nasa.gov/library/event_archives/showcase/rotary.jpg

Dave

[Graviman]

"Best way to get supersonic in a rotorcraft is to go to 15,000 feet, pull both engines to idle, put the rotor brake on and wait..."

Hehehehe!! This was part of the test program on ABC then? ;)

Just been reading the Fatigue Time Life proving requirements for helis: http://avstop.com/Helicopters/20/

You TPs really earn your way! :ugh:

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"...think about the Tiltrotor's drivetrain"

My point exactly. Operating costs will win friends with no one, and just wait until we get threads along the lines of OEI with driveline failure (even Chinook is not immune to sprocket tooth failure).

"I STILL don't understand your disk loading concern"

Draw your interleaver in plan, then see how big a single rotor would fit into the same "parking" space. Intermesher planform gets much closer to circle (optimum for hover). No need to use retreating blades, so why not just feather 'em?

Since v-RRPM is essential for high speed, blades need to be designed to run in plane anyway. Root/hub will need to take same bending moment as fixed wing, but that just lends itself to aircraft manouvreability. The trick is going to be careful rotor/blade design, to allow the right modes of stiffness and compliance - you're already thinking along these lines. The whole thing just strikes me a very good high performance rotorcraft package, with fewest technical risks.

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"...but Roton beat us to it."

Being able to deep cone the rotor was a good way to achieve supersonic auto. In theory you could use coning to produce a supersonic prop (king of a rotary swing-wing) - converts some disk flow velocity to axial flow. Trouble is during engine failure you have to allow for fast coning the other way. The original plan was rotor assist launch, but clearly they were eventually forced into the "keep it simple" philosophy...

Mart