The New Flight Manual--Coax Propaganda, Maybe?

[Smokin Hole]

OK, I admit there is a bit of a troll quality to the thread title. The flight manual isn't propaganda. In fact it adds 147 pages of (mostly) very informative material to the already good original manual. But I can't help but feel they may have taken some liberties with the Aerodynamic Features of Coaxial Configuration Helicopters section.

 

The section highlights the Coaxial advantage step by step. The most important advantage is gained through the lack of power loss and side slip that is an unavoidable feature of conventional designs with tail mounted anti-torque mechanisms. Well there's no propaganda there. But as you continue reading you are left with the impression that the Ka50 is, through its coaxial configuration, unique in its speed, maneuverability and stability. We learn why the Ka50's unrestricted yaw-rate makes that heavy old turret found in lesser machines completely unnecessary. We learn about the funnel. And here it is viewed as a real battlefield maneuver and not just a very cool airshow element. And we learn about the relative superior safety advantage of coaxial designs over conventional designs. If you didn't know better, you will finish this section of the manual thinking that the Ka50 is Russia's front line attack helicopter and that the Mi28 was dragged back to the hangar in humiliating defeat.

 

But is the Ka50 really that great? The initial yaw rate, in hover at least, isn't spectacular. And the speed isn't really all that fast. And, yes, it is true that there is zero risk of the rotor disk flapping back and shearing off your tail-boom (which I hear is bad). And that's why there are certain things you should not do in a conventional helicopter. But rotor disk intersection is every bit as career ending--and it is never mentioned.

 

The new manual is great in most respects. And, no, it doesn't lie. But...

 

In summary' date=' coaxial-rotor helicopters are generally safer to fly than the single-rotor helicopters.[/quote']

 

...it might be a little, er, enthusiastic about a design that certainly has its place but also has its limitations.

[Slazi]

Nice post. So I take it there's a new manual out for me to read? To be honest, this sounds a lot like the previous manual for BS I.

[Smokin Hole]

Oh really? My bad then. The version I used in the comparison was from 2008.

 

[EDIT: I don't know. I do think this session is new. The "old" BS1 manual I was comparing is in my 1.02 installation. That should be the latest.]

Edited November 14, 2011 by Smokin Hole
See post

[RvETito]

Well, that might be the simple statistics as well. I'm not certain about the correct numbers but I believe more than the half of the helicopter combat losses worldwide are due to tail rotor/transmission hits. While a main rotor (even two of them) is much harder to completely lose it's purpose.

 

The advantages and disadvantages of both design is well known. That's a subject to another discussion. And what you quote in the manual - "coaxial-rotor helicopters are generally safer to fly than the single-rotor helicopters" is something that I would agree. Rotors intersection is something that can occur after extreme violations of the helicopter's flight limitations and the history knows only two such cases, with Ka-50 prototypes for the record. But the coaxial design is really safer for pilot point of view and easier to master. Not sure about the Mi-28 but Ka-50 has definitely wider range of flight limitations than Mi-24.

[EtherealN]

If you didn't know better, you will finish this section of the manual thinking that the Ka50 is Russia's front line attack helicopter and that the Mi28 was dragged back to the hangar in humiliating defeat.

 

The Mi-28 won through a few specific things:

1) It's a Mil product. This might sound silly, but there's the usual politics - Mil always designed army choppers, Kamov navy choppers.

2) Ka-50 is single-seat. HUGE RED FLAG.

3) Ka-50 is obsolete. See points 1 and 2 and you'll get an explanation of why it was starved of development funds practically since '91.

4) The Ka-52, which did not have item #2 talking against it, is a modern bird and is entering production for front-line service with ~140 ordered so far. (If memory serves.)

 

But rotor disk intersection is every bit as career ending--and it is never mentioned.

 

Errr... YES it is. ;)

 

"Comparing the flight safety of coaxial-rotor to single-rotor helicopters, proponents of single-rotor systems often cite the issue of rotor-blade overlapping with coaxial-rotor helicopters."

Section 5-12, under "Flight Safety".

[Yo-Yo]

I do not know if it's mentioned but coax rotor helicopter always has better energy performance in comparison with the tail-rotor design IF THEY HAVE ROTORS OF EQUAL DIAMETRE. Or vice-versa: if they have the same energy performance the coax design can have the rotor of less diametre.

 

I mean they have the same engines of course.

 

One can compare rotor diametre of Mi-8, Mi-24, Ka-32 and Ka-50 and their energy performance. In the simpliest way it can be shown as vertical speed * mass* g, i.e. excessive power.

Edited November 14, 2011 by Yo-Yo

[EtherealN]

That was one of the reasons why the soviet navy went for Kamov designs as well, if I remember right. Smaller diamater makes it easier to operate from ships.

[joey45]

^^ No tail rotor

[Smokin Hole]

No question coax is more stable. If you ever get into RC helicopters there is a very good chance that your first will be a little micro coax. You can chase your cat around the house with it, let it coast to a hover, put the radio down to grab a beer from the fridge and come back to find your cat staring at it within a few feet from where you left it. The US designer Kaman also capitalized on the stability of the (albeit different) twin-rotor design. Back in the 50's he had a little media display where he soloed a simple "housewife" after just one lesson. The Navy found them to be so easy to fly that they refused to use them as trainers. I don't disagree with the Coax Advantages section of the manual. I just think it is a bit skewed. From reading it one wonders why every helicopter isn't configured this way. Regarding tail-boom strikes. These also require violent violations of the flight envelope. They are also usually preceded by mast bumping or other indications that you are headed for trouble.

[213]

But is the Ka50 really that great?

 

yeah

[hassata]

Back in the 50's he had a little media display where he soloed a simple "housewife" after just one lesson.

 

I could use his help :D. Just kidding, I'm no longer married. Err, wait . . .

[EtherealN]

From reading it one wonders why every helicopter isn't configured this way.

 

In the words of the manual:

"Between the late 1970s and early 1980s, all necessary prerequisites were developed for the development of a combat coaxial-rotor helicopter"

 

So, to start off - one reason would be that for combat helicopters, not even Kamov considered it a good idea until very recently because the technology just wasn't there yet. It's a bit like "if an unstable design makes a plane so much more maneuverable, why aren't all of them made to be unstable?" Because until very recently, it just wasn't an option as far as technology goes.

 

And of course, in many cases there are other considerations that dictate design - big deal in them being expertise. If you never made a coaxial, you are less likely to bet the health of a huge project on trying to make a functional coaxial. Say the US were to decide it needs a replacement for he Apache and task Sikorsky with making it happen - even if Sikorsky were to agree with Kamov that coaxials are better, I'd personally be suprised if they actually decided to go coaxial: it would delay the project and make it a lot more expensive and risky simply because they'd find themselves having to develop new tech for themselves. (Though they did take initial steps in the coaxial direction with that one-off speed-demon of theirs.)

 

Think of it like this: why did we not see properller planes with double, contra-rotating, airscrews in world war 2? This would have almost eliminated the considerable problem of torque, but it wasn't until the end of the war that prototypes of that design started showing up - because it's darn complex to design it properly in a robust way.

[TAIPAN_]

I remember reading that section when BS1 was released and I was thinking this chopper is freakin badass.

 

But then I try to spot Sam launches and fly at te same time, or respond to close range small AAA at the same time as trying to avoid. I want a partner in my shark.. Also the only warning system being for getting lased is quite limited in usefulness.

 

I think the missions we are put in are the problem though - if I was in command I wouldn't be sending a Ka-50 to escort mi-8s through a AAA infested zone that also has Manpads and Strelas liberally roaming the countryside.

 

In a reconnaissance mission though, or a tactical assassination style strike its very nice. If I was a member of a terrorist camp I'd also be quite freaked having this thing popup 4km away as well - if I saw it..

[PeterP]

One addition to EtherealN's posting:

 

Almost every rotary-wing UAV that is in development shows a coaxial-rotor design - and it's very likely that this technology will find its use also in every manned vehicle once it is established.

Just give the things time to envelop.

Edited November 15, 2011 by PeterP

[Yo-Yo]

No question coax is more stable. If you ever get into RC helicopters there is a very good chance that your first will be a little micro coax. You can chase your cat around the house with it, let it coast to a hover, put the radio down to grab a beer from the fridge and come back to find your cat staring at it within a few feet from where you left it. The US designer Kaman also capitalized on the stability of the (albeit different) twin-rotor design. Back in the 50's he had a little media display where he soloed a simple "housewife" after just one lesson. The Navy found them to be so easy to fly that they refused to use them as trainers. I don't disagree with the Coax Advantages section of the manual. I just think it is a bit skewed. From reading it one wonders why every helicopter isn't configured this way. Regarding tail-boom strikes. These also require violent violations of the flight envelope. They are also usually preceded by mast bumping or other indications that you are headed for trouble.

 

Coax system has more collective authority for airframe rotation and less MOIs in pitch. So the lag between rotor cone pitch and airframe pitch is much more for classic scheme.. Thus the probability of blade - tail boom intersection is higher for it.

[Haukka81]

I remember reading that section when BS1 was released and I was thinking this chopper is freakin badass.

 

But then I try to spot Sam launches and fly at te same time, or respond to close range small AAA at the same time as trying to avoid. I want a partner in my shark.. Also the only warning system being for getting lased is quite limited in usefulness.

 

I think the missions we are put in are the problem though - if I was in command I wouldn't be sending a Ka-50 to escort mi-8s through a AAA infested zone that also has Manpads and Strelas liberally roaming the countryside.

 

In a reconnaissance mission though, or a tactical assassination style strike its very nice. If I was a member of a terrorist camp I'd also be quite freaked having this thing popup 4km away as well - if I saw it..

 

 

Yeah, i miss RWR too. Many missions have radar sam's and shilka's so it would be good to have one.. :D

[Bushmanni]

The game that hooked me to helicopters was Jane's Longbow 2. While it's flight model was pretty basic it seemed to have performance characteristics correct. My impression of the Apache was that it can barely fly with combat load. You could hover only in ground effect and climbing over a small mountain was a struggle. Then I get to fly the Shark and it feels like a moon rocket compared to the Apache. Ka-50 has huge advantage in cruise and climbing speed and it's very easy to learn to fly. Pretty much the only thing I don't like in Ka-50 flight characteristics is it's slow yaw acceleration that makes using rockets a bit more harder than I would prefer. But with my impressions based on these two games I'd take coaxial rotor system any day over conventional one if only flight characteristics matter. The ultimate combat chopper would be Apache with coaxial rotor system and with a electrical ducted fan in tail helping with the initial yaw acceleration.

[EtherealN]

Pretty much the only thing I don't like in Ka-50 flight characteristics is it's slow yaw acceleration that makes using rockets a bit more harder than I would prefer.

 

Pardon my curiosity, but I don't see how those two relate.

[Mikoyan89]

Coax system has more collective authority for airframe rotation and less MOIs in pitch...

Perhaps i could understand for the more collective authority,but why it has less MOI in pitch(at least i read this as "moment of inertia")?if i may ask....

[rud]

I apologize if someone covered the basic theory already, but...

 

For the same mass, moment of inertia is higher if that mass is further from the center of rotation. For the simplest case of a point mass the moment of inertia is just mass times the squared distance from the axis of rotation (m*r^2). A 1 kg mass 10 meters from the center of rotation has 100 times the moment of inertia of a 1 kg mass at 1 meter.

 

 

Since coax heli's do away with the whole tail rotor assembly there is a lot less weight at the far back end of the aircraft. That mass is far from the axis of rotation for both pitch and yaw so getting rid of it helps lower those moments of inertia a lot. A lower moment of inertia means less resistance to angular acceleration, so the Ka-50 can whip itself around faster, so we love it that much more

[aaron886]

To be fair, the tail rotor assembly on most attack helicopters is not that heavy. More importantly, it can impart a very powerful rotational moment. I can't say for certain, but I wouldn't be surprised if the moment produced by the tail rotor offsets its additional weight. I highly doubt, if the coaxial rotor can turn more quickly, that it is by any great margin.

 

The benefit of the coaxial design is damage resistance, some additional amount of efficiency, and decreased complexity.

[Bushmanni]

Pardon my curiosity, but I don't see how those two relate.

 

With better yaw acceleration you get less laggy response from rudders which means it's faster and easier to point your nose to the target and make minor corrections. While the theoretical accuracy of rocket delivery itself isn't affected by yaw acceleration as with enough time you can align ourself just fine regardless of how fast your rudder responds but in combat time is of essence and you would always want to have the shortest possible run in to the target and least amount of time switching to new target.

 

To be fair, the tail rotor assembly on most attack helicopters is not that heavy. More importantly, it can impart a very powerful rotational moment. I can't say for certain, but I wouldn't be surprised if the moment produced by the tail rotor offsets its additional weight. I highly doubt, if the coaxial rotor can turn more quickly, that it is by any great margin.

 

The benefit of the coaxial design is damage resistance, some additional amount of efficiency, and decreased complexity.

 

The tail rotor can easily offset it's inertia in yaw axis as single rotor helicopters have better yaw acceleration initially than coaxial and in short yaw turns they turn faster but in longer yaw turns the coaxial will be way more faster as it's not limited by some aerodynamics issues with tail rotor.

 

I did some rough calculations according the graph in BS2 manual at p.5-8 and it would seem that co-axial and single rotor helicopters are matched at about 180 degree turn that is completed when angular motion stops. Single rotor system can perform any turn less than that slightly faster than co-axial. In addition to that single rotor system has no issues in initial acceleration which means better fine control in yaw axis. I have to say I can't understand Kamovs stance on no need of a turreted gun based on co-axial systems edge in yaw speed as that advantage is evident only in artificial situations.

 

The tail boom is going to hurt pitch axis nevertheless as it's not just the mass but it's location far away from center of gravity. And it could just as well be the tail rotor system that is simpler as the Ka-50 rotor head looks quite complex to me. But anyway I believe the future will be co-axial. Just look at the Sikorsky X2 that has pretty much solved all the problems the Kamov design has and also achieved some new performance benefits.

[EtherealN]

With better yaw acceleration you get less laggy response from rudders which means it's faster and easier to point your nose to the target and make minor corrections. While the theoretical accuracy of rocket delivery itself isn't affected by yaw acceleration as with enough time you can align ourself just fine regardless of how fast your rudder responds but in combat time is of essence and you would always want to have the shortest possible run in to the target and least amount of time switching to new target.

 

Sorry, but my reaction to this is that it sounds like you are trying to get your helicopter to correct for an improperly planned rocket attack.

 

Rockets are NOT precision weapons. They are airborne artillery. You set up your attack for a given axis, run in, stabilize, release your weapons in a configuration where you minimize CEP.

 

Introducing yaw moments in an attempt to create "corrections" defeats the attack run and you should instead abort and set up again.

[Shein]

I'm friends with the test engineer for the canceled comanche program, I asked him why america never designed a coaxial bladed helicopter. His answer was simply "because the mission requirements never needed it." I went on to state why not, when a coaxial system seems to be so much better than a conventional, and he explained to me the aerodynamics of it. Basically, a coaxial system, while better performing, has less of a maximum payload than a conventional system. The rotordisk of the conventional and the *combined* rotordisks of the coaxial produce the *same* amount of lift, 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. Those are heavy, and the added weight is why the coax can't carry as much as a conventional. Least this is all if I'm remembering it correctly... If you guys want, I can try and dig up the conversation and post what he said...

[aaron886]

Think about it... you can have a geared tail-rotor assembly, or you can have a whole second rotor/swashplate. That makes sense.