Coaxial v conventional MR + TR

[Head Turner]

Having read the previous items on Notars and Fenestron and conventional tail rotors I contemplate over the ratioale that all rotor systems havent become coaxial.
I understand that the notar/fenestron are most inefficient at hover, using more engine touqre than the conventional tail rotor blade system. But provide elements of safety for persons on the ground and quieter operation together with advantages in forward flight.
The coaxial appears to be the obvious answer to the power situation and be tailrotorless and thus as safe as you can get for the persons on the ground.
So why is the coaxial rotor system such a poor choice for the S92 and all modern helicopters?

[Dave_Jackson]

So why is the coaxial rotor system such a poor choice for the S92 and all modern helicopters?

Head Turner,

The first production helicopters started to appear just before and after the end of the Second World War. At that time, the German configurations were primarily side-by-side (including intermeshing), the Russian's were primarily coaxial and the American's were primarily main & tail.

After the war, Germany was not allowed to perform research and development on aircraft. Russian had lost 12-15 million people and it was economically destitute. Other countries, such as England and France had problems similar to those of Russia. America became the leader in aerospace; partially, thanks to the Germans.

Creative people, by nature, are a proud and 'bull-headed' lot. Therefore, why would the Bells (Youngs) and the Sikorskys want to change from that which they believed in and had spent much of their lives working on?

Subsequent research, lobbying and funding has been directed primarily toward the single rotor configuration. In my not so humble opinion, if the same effort had been directed toward twin rotor configurations, rotorcraft technology would be further advanced then it currently is.

To me, the V-22 is an example of a dichotomy in which there exists a frustrated client (the US military) wanting improved vertical aircraft, and an industry that is incapable of delivering.

[NickLappos]

Head Turner,

While some "natural" properties seem to be favored, the rule "if it is stupid, but it works, it is not stupid" must be applied. One of the lessons that Igor Sikorsky learned was that the apparent "natural" symmetry of the coax was one of the reasons it took decades to develop a successful helicopter. It took that long to abandon the coax, with its complex packaging, blade meshing problems, high drag and soft yaw control to make the first several generations of helos.

[Lu Zuckerman]

I may be wrong but I believe that the first USA attempt at development of a coaxial helicopter was the Hoppicopter developed by a friend of mine (Maurice Ramme) now deceased. It went through several stages of development starting with a strap-on version and then evolving into a single person helicopter with a rudimentary landing gear and a seat for the operator. Unable to get financing, Mr. Ramme sold his design to a British Company that was planning on further development and ultimate sale to individuals. That plan eventually went south and the program died.

Check this website:

http://avia.russian.ee/vertigo/hoppicopter.html

:E

[Head Turner]

Thank you for your replies.

Aerodynamically what are the advantages/ disadvantages of the coax v Main + Tail rotor designs.

Surely the coax idea is the answer to power loss at low speeds that the fenestron and notar suffer.

[Dave_Jackson]

Most coaxial rotors have three blades. The two 3-blade rotors are somewhat more aerodynamically efficient than a comparable single rotor with six blades. This greater efficiency is due to the counterrotating swirl of the two rotors and the vertical gap between them. The lack of a tail rotor also adds to the coaxial's efficiency.

Countering this is the fact that few, single rotor helicopters have six blades and a larger rotor disk with fewer blades is more efficient than a smaller rotor disk with many blades.

[Head Turner]

So what you are saying is that for vertical lift the coax is the best design, but for overall efficiency the three bladed main rotot coupled to a two bladed tail rotor benefits from the reduction in rotor drag and has more tail rotor authority/control.
I don't quite see how the rotor drag coming from the six blades being greater than in a conventional design can then be more efficient in lifting.
I can only guess that it's the tail rotor which is producing more drag than the extra three blades.
Anyboby have knowledge of this.

[zalt]

The most, perhaps ONLY, attractive co-axial rotor helicopter (IMHO) was the Sikorsky Advancing Blade Concept demonstrator from the mid-80s.

But this is more than just as aerodynamic issue. Quoting a certain Nick Lappos from Iteravia Jan 1986:

"Because of of its more complex control system and twin rigid rotors, the ABC is not as weight-efficient as a conventional helicopter. A penalty of about 10% would be likely in a production ABC."

[Dave_Jackson]

Because of of its more complex control system and twin rigid rotors, the ABC is not as weight-efficient as a conventional helicopter. A penalty of about 10% would be likely in a production ABC.

This is where the intermeshing configuration will be the winner.

Assuming that the rotors of a coaxial and an intermeshing helicopter are identical; the lateral offset of the intermeshing rotors will give a better figure of merit. Not only will this advantage overcome the mentioned 10% penalty of the coaxial, it will also be more efficient then MR + TR helicopters.

___________________


The greatest loading on a rotor in forward flight is at the tip end of the retreating blades. The following web page very clearly shows that this area, on an intermeshing configuration, is outside the downwash of the other rotor;
Rotor - Disk - Downwash - Intermeshing in Forward Flight (http://www.unicopter.com/0995.html)

[Head Turner]

The Russians obviously favour the coax system having built the 'Chernaya Akula' attack helicopter, Ka - 50 (NATO designation = Hocum).
With a 170 Knot top speed and a carrying capacity of a 3 ton truck this was an awsom machine.
QUESTION. Is this technology lost for ever?

[Dave_Jackson]

Head Turner

A response to the questions on your post of 7th, Oct; which I missed.

For vertical lift, the coaxial may be best for extremely low disk loading. A number of universities are using this configuration, in an attempt to achieve man-powered vertical flight. The rotors are turning so slowly that their aerodynamic interaction is just about irrelevant.

For powered vertical flight, the optimal lift-to-power ratio will be achieved by the side-by-side rotor configuration.


This may answer your question about the; Required Power Comparison for Various Rotor Configurations, in Hover (http://www.unicopter.com/B360.html#Required_Power_Comparison). Hope this helps.

[Dave_Jackson]

Head Turner,

Just in case your are seriously interested, the following 25-page report addresses your original question. It is excellent, and it was (and still may be) available on the net for free.
____________________________


A Survey of Theoretical and Experimental Coaxial Rotor Aerodynamic Research
Coleman, Colin P. (NASA Ames Research Center, Moffett Field, CA United States)
NASA Center for AeroSpace Information (CASI)
NASA-TP-3675 , 1997

The recent appearance of the Kamov Ka-50 helicopter and the application of coaxial rotors to unmanned aerial vehicles have renewed international interest in the coaxial rotor configuration. This report addresses the aerodynamic issues peculiar to coaxial rotors by surveying American, Russian, Japanese, British, and German research. (Herein, 'coaxial rotors' refers to helicopter, not propeller, rotors. The intermeshing rotor system was not investigated.) Issues addressed are separation distance, load sharing between rotors, wake structure, solidity effects, swirl recovery, and the effects of having no tail rotor. A general summary of the coaxial rotor configuration explores the configuration's advantages and applications.
No Digital Version Available - Order This Document

[greenthumb]

@ Head Turner

Is this technology lost for ever?

Russians and chinese have some days ago signed a contract to develop a new attack helicopter on base of the KA-50.

Due to the russian financial problems in russia only 4 KA-50 are built. The next stage is the KA-52. Now built two times.

Dave_Jackson is right. The intermeshing could be the winner.
For long liners the coaxial is the right deal. Lift to power ratio couldn't be reached by the others.

[Head Turner]

Thank you all for your knowledge and thoughts.
From what you are saying, the coax is ok but the intermeshing is better.
Next Question.
Why then are there so few of the 'intermeshing' helicopters?
Q2. Is it the expected look rather than the right look that drives the sales of helicopters and therefore manufacturing?

[Shawn Coyle]

The KA-50/52 may be capable of 170 Knots, but you'll hear it coming at least 5 counties away - noiser than any Huey or Cobra or Chinook.
The KA-32 is an awesome machine for lifting loads and logging, but not one you'd want to work underneath, due to the downwash- even at flat pitch on the ground, you don't so much walk away from it as get pushed. Downwash is an issue with coaxial designs, and complexity of the rotor controls is nearly beyond imagining.
The coaxial designs also normally have featured pretty boxy fuselages and very draggy back ends for directional stability.
Everything is a compromise.

[Dave_Jackson]

Head Turner,

Why then are there so few of the 'intermeshing' helicopters?

Five years ago, I started researching VTOL concepts; everything from weird dreams to functional craft. This ongoing research and development has averaged about 3,000 hours per year.

There has been a concerted effort to try to keep an open mind. In addition, no metal has yet been cut and therefore the commitment to a specific configuration is not as strong as that held by current helicopter manufactures.

Nick Lappos has said that "there is no free lunch" and he is absolutely correct. No one configuration can satisfy all VTOL demands; now and on into the distant future. However, in the second year I came to the realization that the underdeveloped intermeshing configuration offered the most. In the ensuing three years of research and development, nothing has come up to change this belief.


This web page may provide the answers to your question; ~ Intermeshing Information (http://www.unicopter.com/A085.html)

Dave J.

[Head Turner]

DJ Your attachments have made VERY interesting reading and extremely informative.

[zalt]

Dave

One 'objection' I'd heard in the past that I didn't see addressed by the web page that meshed rotorcraft are not capabble of cruise speeds typical of current helicopters.

[Dave_Jackson]

zalt,

Your concern about "meshed rotorcraft are not capable of cruise speeds typical of current helicopters" is a popular belief.

This misconception has come about because Charles Kaman decided to utilize the intermeshing configuration's superior Figure of Merit. He left United Technologies (Sikorsky) when he was told that they "already had one chief engineer and did not need another". It appears, his $5,000.00 bankroll was insufficient to take on the established United Technologies head-to-head, so he went for a niche market.

IMHO, everything points to the intermeshing configuration as actually being the BEST one for high-speed rotorcraft.

Without getting into the boring detail stuff, it might be noted that;
~ In the beginning, the intermeshing Flettner FL-282B had a top speed of 93 mph where as its single rotor contemporary, the Sikorsky R-4B, had a top speed of only 75 mph.
~ Since then, the Russians have produced fast coaxials, plus a side-by-side helicopter that held the rotorcraft speed record. The intermeshing configuration fits between these two.
~ For the future, the aerodynamicist Stepmewski's preference is an ABC Synchropter, with a cruise speed of up to 260 knots.

[zalt]

Dave

I ignored the Flettner FL-282B / Sikorsky R-4B comparison purely because 93mph is rather pedestrian!

[NickLappos]

zalt,

I agree. I have flown faster backwards.

[Nigel Osborn]

Nick
But weren't you trying to go forwards at the time? :ok:

[NickLappos]

Nigel,

Yea, I was!

As Daniel Boone said, "In all my trekking I was never lost, but there was a 3 or 4 day period when I was a mite disoriented."

[Head Turner]

I have found that there is in Japan a project to build a small co-ax twin engined single seater. Contact is www.ezycopter.com. It seems to be in the very early stages of development

[Dave_Jackson]

Head Turner.

Interesting. Thanks.

It appears that the Ezycopter rotor heads do not have provision for teetering or flapping. With only two blades per rotor, there will be problems in forward flight, unless their "innovative and patented collective and cyclic controls" are doing something really neat.

[Head Turner]

DJ
From what I can discover there are 4 blades to each rotor.
Does this provide any control benefit over a 2 bladed design? Mechanically I don't see how, as control is control.
This would be fascinating to see as a working prototype.

[Dave_Jackson]

Head Turner

The pictures on their web site appear to show 2-blade rotors, for a total of 4 blades.

This page should explain the problem that they may be faced with
DESIGN: Dragonfly ~ Rotor - Disk - Lift Distribution re: Vibration (http://www.unicopter.com/1142.html). The page is for an intermeshing configuration but it will also apply to a coaxial. It is based on the blades crossing at 45º +/++/+++ 90º azimuths, but the problem will apply to any crossing azimuth.

[tecpilot]

Is this technology lost for ever?

Found the following message from monday:

Russia’s Air Force Will Fly on Newest “Alligator” Combat Helicopters

In the near future, up to 12 multirole Ka-52 “Alligator” helicopters will reach the inventory of the Russian air force, air force commander-in-chief, General-Colonel Vladimir Mikhaylov, announced on Monday.

...

“In the near future, the combat composition of military aviation will be supplemented with 12 new generation Ka-52 'Alligator’ helicopters. This Kamov firm developed combat aircraft has kept the best qualities of the predecessor - the Ka-50 ‘Black Shark’ helicopter,” Mikhaylov said.

More capabilities have appeared with the Ka-52 for use in night combat and poor weather conditions, Interfax reports.

“It now is able to operate in a group, in permanent contact with ground controllers. This helicopter can go into combat against an aerial target for the first time in worldwide practice,” the CinC said.

The Ka-52 “Alligator” – Construction History

The prototype Ka-52 was 85 percent based on the Ka-50 “Black Shark.” The designers had endeavored to prevent the helicopter, which was lightened by a tonne, from losing the unique combat qualities of its predecessor.

Foreign companies took part in the development of the “Alligator.” The French Thomson, for example, assembled the avionics and monitors.

The Ka-52 is built in series at the Arsen'ev Progress aviation complex.

According one version, the “Alligator” was made as an export variant, while the Ka-50 "Black Shark" was oriented more at the domestic market.

“The appearance of the 'Alligator’ just at the right time suits the Ka-52, an aircraft which operates in the expectant, aggressive manner of a predator,” the firm’s general designer, Sergey Mikheev, explained.

The cost of the Ka-52 varies in the limits of 20 million dollars. Considering the helicopter’s capabilities, experts think the cost will be attractive for potential buyers. The aircraft is so new that right now it is difficult to compare with helicopters of the Western competitors: the “Apache” (U.S.), "Tiger” (Germany and France), and "Roywalk” ((sic)) (South African Republic.) The K-52 leaves its competitors far behind in the criterion of "cost-effectiveness.”

Best combat helicopter

The Ka-52 helicopter can be used for the resolution of a whole sphere of complex tasks: for the destruction of an enemy’s tanks, for combat with slow-flying aerial targets, for reconnaissance and support of ground forces. The Ka-52 can carry out combat actions both by day and by night.

The Ka-52 has maintained practically all the combat capabilities of the K—50, and it is on the level of the best combat helicopters of the world, although in some flight performance it nevertheless yields to its single-seat prototype. The Ka-52 has identical control from two operational seats. Both pilots can take control of the helicopter and weapons themselves if needed.

An emergency escape system is installed on the helicopter. It provides rescue of the two crew members practically in all possible situations in combat. Two K-37 ejection seats and a system for jettisoning the rotor blades are installed on the Ka-52. On may note that ejection from the helicopter is used for the first time in the world. The Ka-52 has greater combat capabilities in operation by night and in poor weather conditions than the single-seat Ka-50.

The side-by-side positioning of the airmen in the cockpit eases the interaction of the crew members and lessens the time for making a decision about the physical condition or the ejection of the second pilot. The canopy of the twin-place cockpit has been executed as it is on the Su-24 and F-111 airplanes, which provides the crew a good view and comfort.

The Ka-52 has not analogues in the world for level of the cockpits armor plating. The Ka-52 combat helicopter can be viewed as a combat training aircraft since it resolves the problem of crew training in the air "for itself" and for the single-seat Ka-50.

The Ka-52 helicopter possesses a low radar cross section ((MALAYA RADIOLOKATSIONNAYA ZAMETNOST’)). It can execute maneuvers which in principle are not possible for helicopters of the classical design. It can hover for a long time. The Mi-8 helicopter, for example, cannot hover for more than 5 minutes.

The Ka-52 “Alligator” is a fine anti-tank helicopter, which is equal to 10 - 12 tanks in effectiveness. Corresponding exercises took place in the Federal Republic of Germany, for its shakedown, and the helicopter destroyed in simulation 10 - 12 tanks," an expert in the area of aviation equipment, air force Lt. Colonel Mikhail Kryukov, told NEWSru.com.

Basic tactical and technical characteristics of the Ka-52 “Alligator"

Crew is two men.

Diameter of main rotor is 14.50 meters.

Helicopter length with turning rotors is 15.90 meters.

Maximum weight is 10,400 kilograms.

Maximum empty weight of the helicopter is 7,800 kilograms.

Static ceiling is 3,600 meters.

Practical ceiling is 5,500 meters.

Type of engine is 2 gas turbine Klimov TV3-117VMA.

Power is 2 x 2,260 horsepower (2 x 1,660 kilowatts.)

Practical flight range is 1,160 kilometers.

Operational range is 520 kilometers.

Maximum operational G-loading is 3.5 G.

Vertical rate of climb is 10 meters per second.

Maximum speed in a gentle dive is 350 kilometers per hour.

Maximum speed in level flight is 310 kilometers per hour.

Maximum flight speed sideways is 80 kilometers per hour.

Maximum flight speed backwards is 90 kilometers per hour.

The Ka-52 helicopter can carry the following armament: 12 supersonic “Vikhr’” anti-tank guided missiles, which are guided automatically to the target by a laser beam, standard pods with machine guns and cannon, 80 unguided 80-milimeter aerial missiles, and air-to-air missiles, and the helicopter is equipped with a 30-milimeter 2A42 cannon with 500 rounds. The combat load is 2,000 kilograms on four hard points.

[Dave_Jackson]

from the report; THE ABC HELICOPTER by M.C. Cheney, Jr., United Aircraft Corporation

"It is evident that an infinitely rigid rotor not burdened with roll trim requirement [i.e. twin rotors] could maintain constant dynamic pressure on the advancing blade by reducing rpm and thus avoiding exceeding the drag divergence Mack number until the rotor was stopped. This, of course, implies potential helicopter rotor forward speed capabilities exceeding 500 knots."

[RDRickster]

I wonder what the folks at Carter Copters think of their approach to the Mu Barrier. Last month, they got a DARPA grant to research this further...

http://www.cartercopters.com

[2rotors]

Dave,

Can you explain your comment:
"The greatest loading on a rotor in forward flight is at the tip end of the retreating blades. The following web page very clearly shows that this area, on an intermeshing configuration, is outside the downwash of the other rotor"

I checked your website and the diagram shows the curves below the blades to be at about the same vertical position. But since the advancing blade is far above the retreating blade there is much more area under the advancing blade curve. Does this mean the advancing blade is doing more lifting than the retreating blade? As you can see I am confused by what I interpret in the diagram vs. what you said regarding whether the advancing or retreating blade does more lifting.

Please clarify for us less technical types.

[Dave_Jackson]

2rotors,

Fig. 5 on Dissymmetry of Lift (http://www.unicopter.com/0994.html) shows the thrust distribution on an advancing blade and Fig. 6 shows the thrust distribution on a retreating blade. Both blades are producing the same thrust (actually ~ same moment), but it can be seen that the retreating blade's thrust is concentrated closer to the tip.

The diagram on Downwash - Intermeshing Disks in Forward Flight (http://www.unicopter.com/0995.html) has been modified to show this more clearly. The thrust curves are measured down from the horizontal reference line. The rotation of the rotors is inside forward (breaststroke), therefor the highest thrust is outside the thrust of the other rotor. This results in a more even distribution of the thrust about the disk, as can be seen from the curve representing the sum of the two thrusts.

[2rotors]

I see now that the inner half of the retreating blade is doing very little lifting, therefore the outer half must lift more to compensate.

Can you show (or email me) a similar diagram with the rotor rotating so the advancing blades are outboard? I am interested in how the total downwash distribution changes.

Thanks.

[Dave_Jackson]

2rotors;

To my knowledge, no intermeshing helicopter has rotors that rotate outside forward. There are reasons for this. Flettner or Kaman tried it and then went back to the breaststroke. Interestingly, Stepniewski's intermeshing concept (http://www.unicopter.com/1093.html) in 1997 proposed outside forward.

The diagrams are only rough approximations. You could sketch one up by just transposing Fig. 5 and 6.

I believe that any non-recreational intermeshing helicopter that was built today would incorporate the Advancing Blade Concept (http://www.synchrolite.com/0890.html). This will result in shifting much of the thrust from the retreating blades to the advancing blades, during forward flight.

[Head Turner]

I would like to thank everyone, and especially Dave Jackson for the knowledge and scientific data which has emerged in this topic.

From this information I would hazard at a guess that this method of rotary flight is just waiting for the technical breakthrough that someome will come up with.

Until then it remains that the main rotor and tail rotor design will continue to rule the rotary skys.