The New Flight Manual--Coax Propaganda, Maybe?

[Mikoyan89]

... If you guys want, I can try and dig up the conversation and post what he said...

Yes,i want :thumbup: thanks!

[sobek]

Those are heavy, and the added weight is why the coax can't carry as much as a conventional.

 

I seriously doubt that, there is a number of dual rotor heavy lift platforms, even the yanks have the chinook, the additional weight of the coaxial design versus none coaxial dual rotor is most likely miniscule.

[EtherealN]

Basically, a coaxial system, while better performing, has less of a maximum payload than a conventional system.

 

...because spending 30% of your engine output on something that does not produce lift gives you more payload... Errr. ;)

 

The rotordisk of the conventional and the *combined* rotordisks of the coaxial produce the *same* amount of lift

 

Incorrect.

The only way to make this correct is to make the coaxial's rotor diameter substantially smaller than that of the "conventional".

 

and since the upper disk has a longer moment arm much stronger and more robust couplings and mountings are required so it doesnt rip itself apart.

 

Kamov doesn't seem to have a problem with this.

[EtherealN]

I seriously doubt that, there is a number of dual rotor heavy lift platforms, even the yanks have the chinook, the additional weight of the coaxial design versus none coaxial dual rotor is most likely miniscule.

 

Kaman K-max: 1800 sHP, empty weight 2334kg, MTOW 5443 - lifting 3 kilogrammes per sHP. AH-64 lifts 2,6 kilogrammes per sHP.

 

This is rediculously simplified of course and things change a lot with different atmospheric conditions. But it should be noted that Kaman made a chopper that lifts more than it itself weighs, on a single and comparatively weak engine, thanks to going double-rotor. (Though they're not coaxial, their meshing, so they don't even get the same lift you'd get with a "proper" coaxial.)

[sobek]

(Though they're not coaxial, their meshing, so they don't even get the same lift you'd get with a "proper" coaxial.)

 

I thought that intermeshing is even more efficient because it doesn't force one rotor to be in continuous climbing state. :huh:

[EtherealN]

I'm not 100% sure on this, but my understanding is that that advantage is negated by both rotors being off-axis.

[Shein]

Okay, Call me wrong if you like but this is from an actual helicopter engineer... and the one who was the lead test engineer for the comanche to boot!

 

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Hey, why didn't we ever make a coaxial helicopter?

 

 

 

 

• 
  
  • Him: Simple - the design never met anyone's mission requirements. Watch this space, though - Sikorsky's X-2 is going to try it again.June 2 at 5:59pm ·
     
     
    
  • Me: I can understand that, but requirements aside it seems to me like its just a better design than a main/anti torque design...June 2 at 6:43pm ·
     
     
    
  • Him: Define "better." Give examples. Yes, this is a test.June 2 at 8:17pm ·
     
     
    
  • Me: Well I'm not helicopter engineer, but with a conventional system you have power from the engine that goes to the anti torque rotor, one way or another. That power is used ONLY to maintain heading and combat the torque effect of the main rotor. With a coaxial design, 2 main rotors moving in opposite directions, torque is canceled out as you have two rotors moving at the same speed in opposite directions. But since they both produce lift, and work together, no engine power is "wasted" on maintaining heading. You also dont have issues with things like cross coupling and whatnot. So where am I wrong? why aren't all helicopters coaxial?June 2 at 8:35pm ·
     
     
    
  • Him: Generally speaking, what would the difference in rotor system weight be for the coaxial vs main rotor/tail rotor, given identical cruise speed and payload requirements?June 2 at 10:12pm ·
     
     
    
  • Me: ...ah. So the coaxial system would weigh a crapton more, because you now have two main rotor disks instead of one, thus reducing the usable payload and giving it a slower speed due to the increased overall weight. so by "requirements" you mean more payload is needed more than the efficiency of a coaxial system, and a traditional main/anti torque system fits the bill better... am I warm?June 2 at 10:17pm ·
     
     
    
  • Him:You're getting there. Why would the coax weigh more? I'll give you one fact to play with - Lift is proportional to total disk area. With two disks, the total area of both disks for a coax will equal the area of the single disk of a conventional for a given amount of lift.June 2 at 10:21pm ·
     
     
    
  • Me: Because you have twice as many rotors? usually? I mean two rotor disks, even if they're smaller, is going to weigh more than a traditional one. But if lift is proportional to disk area, shouldn't the coax have twice as much lift? making the increased weight worth it? I can't think of another reason why the coax would weigh more... I mean traditional and coax both use two engines...June 2 at 10:24pm ·
     
     
    
  • Him: One step further. For a given lift (=aircraft weight), the area of the two rotors (coax) = the area of the single rotor (conventional), meaning smaller blades for the coax. Totatl rotor blade weight is thus roughly comparable. What's left to be different between the two?June 2 at 10:27pm ·
     
     
    
  • Me: Jeeze. the collective system? I really can't think of anything else!June 2 at 10:39pm ·
     
     
    
  • Him: That's part of it - controls. The other part is driveshafts and mounting structure. The lower rotor of the coax will be roughly the height of the single. The upper will be much further up, to keep the rotors from "shaking hands" as the disks tilt (forward blade up, retreating blade down). That means a long drive shaft. Then you've got the double swashplate and pitch rods, plus gearing to drive two rotors in opposite directions. Finally, that upper rotor is way up there, increasing moments and thus loads at the point the transmission mounts to the airframe, meaning much heavier mounts.
     
    So, for a given amount of lift, and an equal airspeed requirement, the coax has a heavier rotor system weight, therefore a lower payload.June 2 at 10:46pm ·
     
     
    
  • Me: And all of that really weighs that much? even with the newer alloys and whatnot we use these days?June 2 at 10:48pm ·
     
     
    
  • Him: Both systems get exactly the same benefit from advanced materials in terms of percentage weight reduction, so that's a wash.
     
    Even if you're just an airplane driver, you should learn to think like an engineer - there WILL come a time when the airplane isn't doing what you expect it to, and that ability to figure out why things are happening will save your sorry ass.June 2 at 10:50pm ·
     
     
    
  • Me: Believe me I know it. They drill it into us at school, and i've had it happen to me in EFMS class. I've also taken systems and components, turbines, and jet transport systems but I haven't studied helicopters at all. anything I know about them is due to my own interest, not due to my schoolJune 2 at 10:57pm ·
     
     
    
  • Him:Sounds like you got it.
     
    

[EtherealN]

What you are missing is this:

 

Same rotor diameter means coax gets more lift. Coax also does not spend wads of horsepower turning a tailrotor, so it has that power available to turn those larger rotors.

 

That argument only holds when comparing helicopters with the same rotor area - in which case other considerations may be relevant. In the case of naval operations (which has always been Kamov's forte) the reduced diameter you "spend" to get that lift makes naval operations a lot easier. But for a ground-based aircraft, you don't have that requirement - you can make your helicopter have the same rotor diameter as the conventional and still have it be smaller - due to not needing tailrotor separation - and thus you get more lift.

 

For example, the Mi-24 has a larger rotor diameter than the Ka-50, but the Ka-50 has a larger rotor area. And the Ka-50 has superior high-altitude performance. If we compare Ka-50 with AH-64, they have roughly the same rotor diameter - but the Ka-50 has twice the disc area.

 

That's the point. With a coaxial, you can do one of two things: get the same area with a smaller diameter and pay a minor weight penalty but get easier shipborne handling, or have the same diameter and get a LOT more rotor area. Of course, you are also a lot less susceptible to being irretrieavaly taken out by a shot to the tail, but that's a safety issue not a performance issue.

 

Another point that he does not mention, is that you also add weight through aircraft structure having to extend behind the rotor to house the tailrotor assembly. Rotor mast is not the only thing that comes in kilogrammes. ;)

Edited November 19, 2011 by EtherealN
typo

[Shein]

If you say so, this is really beyond me now; I just posted what he said...

I don't mean this insultingly, but are you an engineer?

[EtherealN]

Yes, but not on aircraft.

[EtherealN]

I'll say one thing though - the guy may well be right, but the questions may have been placed such that undue weight is placed on the primary part of a coaxial system - that is, the coaxial itself - and sort of "phasing out" the secondary features. I do not know the typical weight of a tailrotor assembly, but even then the typical weight isn't strictly that useful either. It is a matter of fact that having that tailrotor is a serious vulnerability in combat. For example, consider the below:

 

 

Had that been a conventional helicopter, that whirlybird would NOT have flown home. (It basically took a full salvo of 30mm HE in the tail.) So while there might be a general payload cost involved of unknown size, we also gain a lot in survivability. The point here is that the entire flight controls can be protected in armor without paying a lot in kilos for it, whereas conventional helicopters quite simply have to forget about armoring a critical component of it's controls. Armoring the entire tailboom and tailrotor assembly would basically mean the helicopter can't even get airborne. It is really hard to actually confirm whether there is such a general cost though, since just comparing weights on helicopters of the two designs is not enough - we'd also have to take into consideration differing materials, differing armour etcetera. So unfortunately there is very little useable data available freely to settle this.

 

But again, the important point is that he was talking about payload for a given lift, not payload for a given helicopter. And it is worth noticing that the Ka-50 does have very good high-altitude performance for a combat helicopter, it has no problem carrying it's weapons (and plenty of them), and at the same time being very heavily armored. So basically, I'm inclined to suspect that whatever inherent difficulties might exist as far as weight goes - Kamov appears to have solved them. :)

 

A further note one speed though - coaxials have greater speed potential generally since they can achieve a given lift with smaller rotor diameter, which means they are less likely to become limited by advancing blade tips going transonic or retreating blades stalling - and of course reduced issues from assymetric lift in high-speed conditions. This does obviously depend a lot on how the hub and blades are designed though, since disc intersections may become an issue before you reap the benefits of this.

Edited November 19, 2011 by EtherealN

[Shein]

Oh no, don't get me wrong I completely agree with you; Coax's are way better if you ask me, I was just trying to present arguments to the contrary because thats what I was told... gotta be fair ;)

 

...well, try to be fair... I think we all know the KA-50 is the most badass helicopter around...

Edited November 19, 2011 by Shein

[EtherealN]

All is fair in love and war, and for me the Ka-50 slots in on both. :D

[Shein]

All is fair in love and war, and for me the Ka-50 slots in on both. :D

 

Very well said :smartass:

[Tango]

"Single rotor designs are best" because that is what "we have experience with" is a pretty poor argument. Yes, experience is good, and initially may be a benefit over a new design, but if you don't try another design, you will never advance.

 

Imagine if they argued that the cycle is better than the car as we have more experience with them!!!

 

Best regards,

Tango.

[sobek]

Imagine if they argued that the cycle is better than the car as we have more experience with them!!!

 

The cycle is better than the car in a lot of ways. The cycle is the most energy efficient means of transportation available to man. :)

[tietze]

"Single rotor designs are best" because that is what "we have experience with" is a pretty poor argument

 

Design is a very practical thing that also depends on experience. So this argument is only poor if you disregard that a reliable design needs "experience", by being confronted with a lot of different empirical situations, be it through testing, experiments or having used it in a lot of different conditions...

 

Imagine if they argued that the cycle is better than the car as we have more experience with them!!!

 

But since that has never been the case (the bicycle is a rather new invention, compared to a four wheeler - not limiting a car to an automobile). It further obscures the argument by making it an universal question, rendering particular circumstances under which a car or bicycle have value irrelevant. Being better or best is a (normative) question that (at least) depends on what your aim, means and circumstances are. You always have to specify being better with regard to, under what circumstances and so on...

 

While not having read the manual (:poster_oops: flying what shark? - here comes a universal presupposition about how is often argued for new designs), I suspect that things that could make it look like propaganda is to be a bit too bold with the universal potential of this particular design. A modern sports car might get you easier than legs from A-B when roads are present it might not have been so easy without roads, gas stations, oil rigs and so on... Circumstances change and that particular design might become more viable than others. Paving the road, so to speak, for the coaxial rotor is probably going to get it going in some direction, for some time, for some people in some places... If it is done right ;)

 

If you already have a conventional helicopter and the whole sociotechnical infrastructure for this particular design, you might be hesitant about putting another rotor disc on top of that ;)

Edited November 19, 2011 by tietze
forgot an "also"

[Smokin Hole]

First, I saw 30% figured in this thread as the power lost to the rotor disk due to gearing the tail-rotor. That's high. It might be true on some helicopters at a narrow data point along their speed/altitude/mass range but it is not typical. Second, where is the fabulous yaw rate? The initial hovering yaw rate of the Ka50 (as modeled by ED) is pathetic. That low power in yaw makes the "unlimited sideslip" velocity sort of irrelevant because above 100 kph you run out of yaw authority anyway. Plus, the videos of the Commanche demonstrated what appeared to me to be much better sideslip performance and initial yaw-rate than I have ever managed with the 'Shark. If they want to sell the Ka50 based on the small rotor diameter for ship borne use then I certainly see the value. Same is true for high altitude OGE hover performance. Other than that, I think the coaxial advantages way oversold. None of this has stopped me from enjoying the hell out of BS since its release.

[Bushmanni]

Ka-50 or even Ka-52 isn't the epitome of co-axial helicopter, but it does demonstrate many of the benefits of the design. New materials and technology allow to solve the problems of the current Kamov design so when talking about co-axial helicopters in general this should be remembered. X2 is a good example of what can be achieved with co-axial system and I believe there isn't any change to achieve similar performance with single rotor.

[andysim]

The cycle is better than the car in a lot of ways. The cycle is the most energy efficient means of transportation available to man. :)

 

 

Its 99% energy transfer from the rider to the wheels, a car is hopeless at around 20%. Nothing else we have that even comes close execpt maybe on Ice

[galagamo]

Find me a safe chopper, and I'll find you a margin of error :P

[sobek]

Its 99% energy transfer from the rider to the wheels, a car is hopeless at around 20%. Nothing else we have that even comes close execpt maybe on Ice

 

To make that comparison fair, you'd have to take your muscles efficiency into account. The most inefficient part is the converion of chemical into kinetic energy.