the Intermeshing Configuration is Best

[Dave Jackson]

With the hope and pleasure of starting a controversy;

~ The intermeshing configuration is better then the single rotor configuration. ~


The following is offered in support of this position. http://www.unicopter.com/B280.html

[Fool on the Hill]

Intermeshing can cause problems. Loose one gearbox and the whole thing really intermeshes quite spectacularly. If you must have two rotors why not go upper lower contra-rotating like the Hormone and Helix. As for one Vs two. One rotor requires a larger disc area to produce the same lift as two. This leads to space and blade tip velocity problems. However intermeshing requires alonger airframe for the two rotor heads. Again space problems. As before the Hormone configuation releaves this And no tail rotor like a single rotor machine.

[ACORN]

Yeah, but one above the other must reduce the lift produced by the lower disk so therefore reducing the MAUM in comparison to Chinook style with clean air through each rotor. True or false?

[heedm]

Dave, that link has some good arguments regarding perceived problems to an intermeshing rotor, but it doesn't provide any argument as to why the intermeshing rotor would be better than a tail rotor.

I think the intermeshing rotor may prevail in certain roles. Heavy lift is an area where it may shine. Tandem rotor has proven itself there, and some of the same benefits come from intermeshing rotors, with different drawbacks.

What exactly are your reasons why you believe an intermeshing rotor is better than a tail rotor?

Matthew.

[Dave Jackson]

Fool on the Hill

>Loose one gearbox and the whole thing really intermeshes quite spectacularly.<

I think that this fear is more physiological than factual. To my knowledge, no intermeshing helicopter has had an accident due to the failure of rotor synchronization. The intermeshing helicopter must have the rotor-rotor portion of its gearbox as simple, as strong and as reliable as possible.

The final stage of a single rotor helicopter's gearing has a degree of criticalness as well. This is because its on the rotor side of the overrunning clutch, so it can drive the tail rotor.


> If you must have two rotors why not go upper lower contra-rotating like the Hormone and Helix. <

The intermeshing configuration has a number of advantages over the coaxial, such as a lower disk loading and true symmetry in forward flight.
________________

ACORN

True. The tandem configuration of the Chinook does result in more clean air through each rotor, particularly in forward flight.
________________

heedm

I agree that no one aircraft can serve all tasks. But; the intermeshing helicopter is perceived as only being good for lifting heavy loads because Kaman, the only manufacture of intermeshing helicopters, elected to build their craft that way.

The Germany patent on the intermeshing helicopter will be 100 years old, next year. Up until the mid-forties, the intermeshing configuration had as much chance of becoming the preeminent design as any other. In fact it was, questionably, the best helicopter when WW II ended. At the end of the war, the only surviving FL 282 was brought to the States, and evaluated by the Prewitt Aircraft Company. The report praised it and the craft was offered to American helicopter manufactures for testing, should they wish. I do not know whether others tested the craft or not, but it was eventually destroyed. I suspect that reduced government funding and, more importantly, the inherent pride of inventors in their own ideas resulted in the demise of the intermeshing helicopter.


> What exactly are your reasons why you believe an intermeshing rotor is better than a tail rotor? <

I believe that the only obstacle to the proliferation of the helicopter is the difficulty and cost in learning to fly it.

The intermeshing configuration offers symmetry of flight. The symmetrical FL282 even had a rotor (not engine) governor and thereby had automatic entry into autorotation. The cost of the helicopter is not a concern since the cost of supply will come down when the demand goes up. As per the Ford model-T.

[Cyclic Hotline]

Dave,
You might want to investigate that last statement a little more.

The worst civil helicopter accident that has occurred to date was a British International Chinook that desynchronized. 45 of the 47 souls died in this accident, after the fwd gearbox failed. That anyone survived was a true act of god!

Date: 11/06/1986
Location: Sumburgh Head, Shetlands, Scotland
Airline: British International Helicopters
Aircraft: Boeing-Vertol Chinook
Registration: G-BWFC
Fatalities/No. Aboard: 45:47
Details: Crashed due to rotor failure.

Columbia Helicopters have lost at least 2 BV 107 helicopters from shaft failures leading to desynch's.

I cannot vouch for all the military accidents, but I know of a number that have occurrred. These accidents tend to be of a catastrophic nature.

Link to the Mannheim CH-47 accident. http://www.aviationcrashes.com/photo...135CH-147C.jpg

All well and good until it comes unglued. There is no comparison between a tail rotor failure on a conventional helicopter and a tandem rotor system.

The main obstacle to the proliferation of helicopters is the ability (cost) to design and build quality machinery and the lack of control over the people operating them.

Edited to remove image and replace with link. (Apologies for posting that, it was innapropriate).

[ 26 August 2001: Message edited by: Cyclic Hotline ]

[collective bias]

A K-MAX in europe recently (1-2yrs ago?)had a catastrophic failure in the lift generating dept.
Those are really yuccky pics CH. Keep that up and I'll have to stop reading prune before beddybyes...

[Fool on the Hill]

One incident that I do know of is the loss of two RAF chinooks in one day. Both were on first test flights after maintenance, one in England and one in the Falklands. As the aircraft lifted it to the hover there was a bang and the front rotors slowed down and mingled with the rear rotors. It turned out that the techs had put the drive shaft to the front gerabox in backwards on both machines. both were written off but no lives were lost

[sling load]

Dave Jackson,

You obviously don't read much about helicopters, if you did, you would know about the problems encountered by the Marines when the CH-46 was introduced. They experienced synchronised shaft failures that caused many accidents and many fatalities. Before you make such claims, you want to get your head out of your computer and do some research.

Your opening of your post "with the hope and pleasure of starting a controversy" is not in the spirit of pprune. All helicopters perform specific functions relative to their design, each has its advantages and disadvantages, each has its unique role in aviation.

[Thud_and_Blunder]

F-o-t-Hill,

Just to return some of the loyalty our techs in the RAF show us, I should point out that at least one of the transmissions involved in the 2 1989 incidents had been incorrectly assembled by contractors (at Perth?), not RAF groundcrew. It may well be that both came from the same source, but time and distance prevent me from verifying...

I couldn't do the techies job, I'm just glad they're willing to.

[Nick Lappos]

Guys,
I fully support Dave Jackson's wish to start a controversy. If it degrades into an on-line slasher flick, it is our fault for failure to exercise decorum, not Dave's fault for stating a premise that might prove interesting! "It's Johnnie's fault I hit him! He said something I disagreed with!"

Regarding syncropters, tandems and such:

The standard single rotor helo has stood the test of time, but as technology moves forward, small changes in metalurgy, computers and other esoteric fields can change the balance and make old ideas appear new. The helicopter itself was held from development by the need for an engine light enough to allow the relative inefficiency of the rotor to be over come with brut force. Similarly, the tilt rotor and vectored thrust had to wait until engines got light enough to allow gobs of power to be pissed away while they hover.

I'll bet an electrically motored dual rotor system helicopter, with no interconnect shaft, but with computer controlled digital synchronization could trigger new development. The ways that technology reviels new doors to walk through is almost completely unpredictable. Who would have inagined that lasers (I remember back in 1958 when the first ruby lasers were developed) would revolutionize communications via fiber optics? That they would be used as supermarket scanners, eye surgery tools and fancy pointers for professors, enormous money wasters for US politicians trying to shoot down missiles while terrorists truck the bombs in via Budget rent-a-truck?

I think the complexity, weight, drag and control issues with today tandems and synchropters relegate them to the role of interesting sidelights to the helicopter world. Over 95% of all helos having Igor's trusty tail rotor, the best single test of what works. After all, like Beta vs VHS, it isn't what is best that always counts.

BTW, I understand that two Kamov KA-50 coaxial types (the awesome Russian gunship that eats Tigres for breakfast, and Long Bows for lunch) have had catastrophic accidents due to self midair.

Give me the old fashioned kind of midair, thanks, and make mine a single rotor helo, to go.

[Gash Handlin]

This is probably a stupid question and please go easy on me as I have almost no knowledge of rotary types other than being able to point and say helicopter

As the failure of the gearbox on an intermeshing design causes such "spectacular" results is there any particular reason why the cabin isn't stretched the extra 10 feet or whatever to provide physical seperation between the blade tips?

I realise there are probably aerodynamic reasons why the rotors have to be synchronised but at least the physical separation would prevent contact between the blades if the synchronisation is lost, wouldn't it?

or should I get me coat?

[Nick Lappos]

Gash Handlin asked why we don't just prevent intermeshing by lengthening the fuselage the extra 10 feet or so.

(I edited this post, as I looked up the Chinook and found that it has 20 feet of overlap, the below corrects the original post that was up for an hour or so)

Really, the problem is the integrity of the system, since a connecting shaft break, even without intermeshing problems, will lead to loss of the machine anyway, since either each rotor must have its power supply, and perfect speed control, or the game is over anyway.

The amount of overlap is a ticklish thing, since the weight of the fuselage goes up a lot as the body is lengthened (maybe 150 pounds per foot, plus the loss of lift due to downwash on the greater fuselage area (maybe another 125 pounds per foot). The cost of the extra 20 feet of length might be 3000 pounds weight plus 2500 pounds lost hover performance, total about 5,500 pounds, maybe 11% of the total gross weight, 22% of the payload.

The lift of tandem rotors actually increases as they are separated, so the two play off against each other. If we completely separate the rotors of a Chinook we can drop the power required to hover by about 10% (which would improve the hover performance by 10% or so), but we'd have to lengthen the fuselage by the 20 feet. So we'd gain 10% and lose 12% for a minus 2% net and a much larger, more expensive to build machine (you pay for empty weight).

You can bet that the designers placed the rotors right where they wanted them!

[ 26 August 2001: Message edited by: Nick Lappos ]

[ 26 August 2001: Message edited by: Nick Lappos ]

[Fool on the Hill]

Thud and Blunder,
Apploigies to the techs. I realise that they do a very complicated and often unrewarded job. My source was the RAF flight saftey mag Air Clues so I wrongly assumed the crews were RAF. Just to assure you, if I didnt have complete faith in the techs working on my aircraft I wouldnt fly. However I believe they do a great job, but it was interesting that two machines were lost on the same day.

[Cyclic Hotline]

There is no such thing as a stupid question - stupid answer maybe, but not the query!

The main reasons for the development of the tandem rotor system, was the ability to counteract torque whilst providing all the power solely to provide lift. A tail rotor uses a considerable amount of power, detracting from the available horsepower for main rotor lift performance.

The designs that evolved coupled a compact airframe with an excellent lifting capability. The drawback, was that in order to achieve both goals, the rotor systems would have to intermesh. In the instance of the Chinook, the rotor system is 60 feet in diameter (no metric crap here!), but the distance between rotor centres is 39.17 feet. The Chinook is hardly a compact helicopter to start with, but if it were to be built with the rotor systems not intermeshing, the distance between rotor centres would have to be at least 60.1 feet. If you now combine all the additional airframe thay you would require (say, 50% stretch) you are starting to look at a massive increase in empty weight, hence payload reduction and useability.

Like most things in life, the result is a compromise between risk, performance and design.

The K-Max is actually quite an efficient design, in so far that it is a compact, efficient little single seat airframe. Generates a lot of lift for that T-53!

There were many other experiments with dual rotor system, the Bristol Belvedere was an interesting machine.

The Russians also built a large number of intermeshing and non-intermeshing tandem designs. The ultimate was the the Mil V-12. If you look at these photo's you will see the physical scale of separating the rotor systems. It takes a lot of horsepower to run one of these! http://www.duffeyk.freeserve.co.uk/mi-v12.htm
http://www.royfc.com/mi12001107-3.html

Of course, like so many great innovations in the helicopter world, Igor Sikorsky was the leader. Here is Igor's early attempts at solving the problems of helicopter design, forerunner of everything in the rotary field! Don't forget the ABC, Sikorsky's more recent venture into this realm. http://www.russian.ee/~star/vertigo/sikorsky.html

The Yak-24 kept up the development work; http://www.russian.ee/~star/vertigo/jak-24-r.html

All the Kamov products are here: http://www.russian.ee/~star/vertigo/kamov.html

All the Mil's: http://www.russian.ee/~star/vertigo/mil.html

Not to be outdone, that famous manufacturer of chainsaws McCulloch got in on the act with a couple of small tandems! http://www.russian.ee/~star/vertigo/jov.html
http://www.pimaair.org/mcc_mc4c.htm

There are many more examples, including a number on the website below. Just another interesting topic relative to overcoming the practicalities of building helicopters.

Lots of credit to whoever put this website together. It is really quite excellent: http://www.russian.ee/~star/vertigo/helicopters.html

[inthegreen]

Not to speak for Dave Jackson or anything, but I think there's been some confusion in terminology. He spoke of an intermeshing rotor, which is usually understood to be a synchropter design such as the H-43 Kaman Huskie. Although a Ch-47's rotors do intermesh, they are usually referred to as a Tandem rotor and not an intermeshing rotor. And yes there have been many spectacular crashes in the CH-46 (bv107), CH-47 (bv234) family.
Like Dave, I believe the intermeshing design was a great idea because of it's overall efficiency. It has two advancing blades and the weight of only one gearbox, without the power loss of a separate antitorque system.


1) Vs. Tail rotor:
A tail rotor equipped helicopter uses 15% of total power in a hover to counteract torque, which is power unavailable for lift. Plus the obvious advantage of not having a tail rotor to lose.

2) Vs. Tandem:
A tandem rotor helicopter requires a second gearbox and synch shaft which adds several thousand pounds to the Ch-47. Plus the aforementioned problem of desynch.

3) Vs. Coaxial:
The lower rotor has a higher induced velocity and therefore higher induced drag and lower lift. Coaxial helicopters generally have much higher downwash velocities, making them difficult to use in all situations.

4) Vs. Notar:
The Notar spends just as much power on antitorque as a tail rotor aircraft, maybe more. It has the added disadvantage of not gaining any of that back in forward flight the way a tail rotor unloads at cruise speed.

I agree with the bit about rotor danger when approaching from the side. That is a serious drawback. There are also cyclic and pedal combinations that can cause unusual effects, although training and time can overcome that.
It may not be the best aircraft design for the jobs that we do, but from a pure engineering standpoint, it is the most efficient.

[Cyclic Hotline]

Well I guess you might have a point there! However, the initial statement was that The intermeshing configuration is better then the single rotor configuration
(No stupid questions, only answers! So I think that the response to the initial question posed was pretty reasonable.

I did a quick search for intermeshing rotors and just got a lot of information about food-mixers (honestly)!

However, there was one reasonable hit, and an excellent site for information about much of what has been discussed in this thread.
http://www.aerospaceweb.org/aircraft/question/helico pter.shtml

However;

A subgroup of twin non-coaxial rotors is the twin intermeshing rotor system, seen only on a few helicopters made by the Kaman company, including the H-43 Husky and K-MAX. The principles by which intermeshing rotors operate are the same as those previously discussed for other twin rotor helicopters. The difference is that the rotors are mounted very close together and actually intermesh, i.e. the blades on one rotor travel through the rotor disk of the other rotor. Of course, the intermeshing of the rotors is carefully timed to keep the rotors from chopping each other to pieces .

Whilst the Kaman products have seen some success, it needs to be noted that the most succesful helicopter produced by Kaman was the Seasprite - a conventionally configured helicopter with a tail rotor.

Not a flame. Interesting topic and an interesting response (as always).

[ 26 August 2001: Message edited by: Cyclic Hotline ]

[Dave Jackson]

Sling Load

There appears to have been a slight misinterpretation of the opening statement in this thread. It was a specific reference to the intermeshing configuration.

There was absolutely no intent to say that the other multiple rotor configurations, which includes the tandem, coaxial, interleaving, side-by-side, and quad, are any good, at all.
__________________

collective bias

I believe that the accident with the K-Max in Germany was due to the failure of an oil pump.
______________

Nick Lappos is by far the best person to comment on the following, but in my limited way, I do find the potential marriage of the intermeshing configuration and Sikorsky's Advancing Blade Concept very intriguing.

[Nick Lappos]

Let me answer inthegreen's points in turn: I used carrots to mark his points.

>>>1) Vs. Tail rotor:
A tail rotor equipped helicopter uses 15% of total power in a hover to counteract torque, which is power unavailable for lift. Plus the obvious advantage of not having a tail rotor to lose.

NL - I think you overstate. A typical tail rotor eats about 3 to 6% in steady hover, where it counts. (More in maneuvers, quite up to the 15%) The loss of efficiency for overlapped rotors is at least 10% in steady hover, so they are worse where it counts.
Regarding mechanical failure, the loss of synchronization is utter disaster when compared to the loss of a tail rotor (although a tail rotor loss is perhaps slightly more probable.)

>>2) Vs. Tandem:
A tandem rotor helicopter requires a second gearbox and synch shaft which adds several thousand pounds to the Ch-47. Plus the aforementioned problem of desynch.

NL - The transmission for a co-axial or synchropter is more complex, and actually approaches the complexity of that dreaded second transmission. Desynch is also a disaster. No free lunches!

3) Vs. Coaxial:
The lower rotor has a higher induced velocity and therefore higher induced drag and lower lift. Coaxial helicopters generally have much higher downwash velocities, making them difficult to use in all situations.

NL - The overlap area of the synchropter is nearly 100 %, so the effects mentioned on induced power are shared by it. The twin heads of the synchropter have all the forward flight drag of the second rotor of the co-axial. No free lunches!

>>4) Vs. Notar:
The Notar spends just as much power on antitorque as a tail rotor aircraft, maybe more. It has the added disadvantage of not gaining any of that back in forward flight the way a tail rotor unloads at cruise speed.

NL - No argument there! The Notar eats lots of power, and has only tail protection as a virtue, which the synchropter has as well.

What didn't get discussed:
Problems in autorotation with yaw control, and the need for more rudder area and controls with extra weight and complexity. Need for twin rotor controls, with two of everything - swashplates, servo sets, control mixers, etc. Lots of maintenance weight and cost.

>> (stuff snipped)It may not be the best aircraft design for the jobs that we do, but from a pure engineering standpoint, it is the most efficient.

NL - I don't think so, I think it is a wash with a single rotor. Of course, a wash means either will do, so let the buyer chose!

[H-43]

Nick you state that the transmission on an intermeshing helicopter is much more complex. I disagree after having worked on and worked with both the Kaman H-43 and several models of Bell products. The transmission in the H-43 is basically a large truck rear end where output is driven up instead of out. These transmissions are extremely strong and quite simple (if you can consider any helicopter component simple). Also the extra gearbox(es) going to the tail rotor are eliminated thus removing complexity so I feel that the argument of more complex is not really a factor.

Also having logged under both the Huskie and the UH-1H with identical powerplants I can say from experience that the Huskie is more efficient. Especially at high temperatures and altitudes.

Just my $.02

Brian