Why so few coaxial helicopters?

[RagnarDa]

Why are almost only Kamov making coaxial-rotor'ed helicopters? As I understand it there are quite a lot of benefits compared to traditional single-rotor designs (stability, no torque, all energy is used to produce lift etc). From playing around in ToH:Hinds i started to appreciate how easy the Blackshark is to fly. There is ofcourse some added complexity when having two rotors in the same mast but aside from that I can't help to think that it shouldnt be that more difficult to manufacture nor expensive. After all, Kamov seems to be doing it quite good already. There are also other very succesful non-conventional rotor designs like the tandem-rotor Chinook. What are your thoughts?

[Weta43]

Sikorsky is developing a 287mph hybrid co-axial for the US millitary AAS project:

http://www.army-technology.com/projects/sikorsky-s-97-raider-light-tactical-helicopter/sikorsky-s-97-raider-light-tactical-helicopter4.html

[AlphaOneSix]

Negatives of a co-axial design: Weight, complexity, drag. For Kamov, their biggest advantage is that they are easy to fit on a ship...and they are great slingload platforms.

 

Proponents of tandem and co-axial designs like to point out the delicate nature of tail rotors, however, a helicopter with a tail rotor can still land more or less intact (sometimes good chance, sometimes slim chance) when the tail rotor fails. When one of the big rotors fails on a tandem or co-axial (not likely, I know), you don't get out of that alive.

 

The Sikorsky X2/S-97 programs are dealing with two of the negatives... Composite rotor blades to reduce weight and increase stiffness (allowing the rotors to be closer together, reducing drag and weight even more), and possibly a fairing around the mast to further reduce drag. There is still complexity, but two out of three isn't bad. The X2 itself is retired, I don't think they plan any more flights in it. It's top speed in level flight was 250 knots.

 

The S-97 Raider program is expected to have its first flyable prototype maybe by the end of 2013.

[Weta43]

their biggest advantage is that they are easy to fit on a ship

And are easier to fly ( are much easier to fly without a functioning tail rotor :)), have more lift for the same engine power, and so can operate at higher altitudes &/or carry heavier loads.

[Yurgon]

Proponents of tandem and co-axial designs like to point out the delicate nature of tail rotors, however, a helicopter with a tail rotor can still land more or less intact (sometimes good chance, sometimes slim chance) when the tail rotor fails. When one of the big rotors fails on a tandem or co-axial (not likely, I know), you don't get out of that alive.

 

And when one of one big rotors fails on a chopper with standard tail rotor, chances of getting out alive are equally bad. :lol:

 

But seriously: Would you say that tandem and co-axial helicopters are more prone to catastrophic rotor failure than single-main-rotor helicopters? Any stats on Chinook, Ka-28 and Co that would suggest that?

[aaron886]

Negatives of a co-axial design: Weight, complexity, drag. For Kamov, their biggest advantage is that they are easy to fit on a ship...and they are great slingload platforms.

 

I've heard (and forgive me for what is doubtless only a fixed-wing term) that directional maneuverability is also generally inferior.

[Pilotasso]

Another disavantage of coaxial helis, they are VERY noisy and dont cope well with noise restrictions.

[AlphaOneSix]

But seriously: Would you say that tandem and co-axial helicopters are more prone to catastrophic rotor failure than single-main-rotor helicopters?

 

No. That was not the point I was trying to make, however, so it seems I failed in delivering my message. My apologies.

[AlphaOneSix]

I've heard (and forgive me for what is doubtless only a fixed-wing term) that directional maneuverability is also generally inferior.

 

Well, co-axial helicopters use a mix of differential torque and actual movable rudder surfaces for directional control.

 

Differential torque is altering the collective pitch of both rotors so that the combined lift effect of both rotors is unchanged, while causing the torque generated by one rotor to increase and causing the torque generated by the opposite rotor to decrease. The torque differential creates a yawing moment opposite of the direction of rotation of the highest-torque rotor. This works great in a hover but it needs help at higher airspeeds, so a movable rudder is incorporated to add additional yaw authority during forward flight.

 

Further, co-axial helicopters (except Kamov*) are susceptible to "authority reversal" in an autorotation. The torque effect that must be overcome in a traditional helicopter is caused by the transmission of power from the gearbox to the rotor. In other words, if there were no power to the main gearbox, the rotors would tend to slow to a gradual stop. In other words, the torque that needs to be offset is caused by the "gearbox driving the rotor". For example, the nose want to go left, so you press the right pedal to make it go right. In autorotative flight, the torque effect reverses. The rotor drives the main gearbox, and there is still torque, but it is BACKWARDS, albeit at a much smaller amount than during powered flight. In a traditional helicopter, it's really not an issue, you press the pedal to make the nose go...that is, right pedal still makes the nose go right and left pedal still makes the nose go left, just as in powered flight. In a co-axial helicopter however, a lot of the yaw is produced using differential torque, which means that in an autorotation, the pedal effect reverses. Pressing left pedal actually makes the nose go right, and right pedal makes the nose go left. In addition, as already mentioned, the torque effect is not nearly as pronounced as during powered flight, so there is less overall torque to work with, resulting in reduced yaw authority. While the aircraft still has a decent amount of forward airspeed, this loss if offset for the most part by the rudders, but as the aircraft slows down, the rudders become less effective.

 

*Kamov has some nifty hardware that apparently takes care of yaw reversal in an autorotation automatically, so the pilot doesn't have to worry about it. I don't know how it works.

[RagnarDa]

The S-97 seems like the ultimate chopper except it lacks armour and stealth. BTW why hasn't anyone started making a DCS:Kiowa Warrior?

 

Another disavantage of coaxial helis, they are VERY noisy and dont cope well with noise restrictions.

 

How is this?

[Pilotasso]

The the counter rotating rotors augment noise levels. Its like slaping soundwaves on each other.

[BBQ]

I've heard (and forgive me for what is doubtless only a fixed-wing term) that directional maneuverability is also generally inferior.

 

Not sure what you mean by "directional maneuverability" -- but it was my understanding that the co-axial rotor configuration allowed greater maneuverability that the standard single rotor configuration. Not sure really, if it's true or not -- please chime in if anyone knows --

[NRG-Vampire]

Sikorsky is developing a 287mph hybrid co-axial for the US millitary AAS project:

http://www.army-technology.com/projects/sikorsky-s-97-raider-light-tactical-helicopter/sikorsky-s-97-raider-light-tactical-helicopter4.html

 

and the fastest: 300mph

[Nealius]

And when one of one big rotors fails on a chopper with standard tail rotor, chances of getting out alive are equally bad. :lol:

 

I guess you haven't heard of autorotation....

[Yurgon]

I guess you haven't heard of autorotation....

 

And I guess you haven't read correctly. Or do you suggest autorotation by tail rotor? :D

[aaron886]

Not sure what you mean by "directional maneuverability" -- but it was my understanding that the co-axial rotor configuration allowed greater maneuverability that the standard single rotor configuration. Not sure really, if it's true or not -- please chime in if anyone knows --

 

Pedal turns.

[Nealius]

And I guess you haven't read correctly. Or do you suggest autorotation by tail rotor? :D

 

I read perfectly fine. You didn't say anything about tail rotor failure, only main rotor failure. ;)

[NoJoe]

I guess you haven't heard of autorotation....

 

Sure, if the engine failed you could autorotate. But if the main rotor failed, there's nothing to autorotate with. :D

 

--NoJoe

[Nealius]

Is it even possible for the main rotor to fail without the engine failing? The only thing I can think of is the rotor getting damaged, which is different than a "failure."

[Jona33]

Is it even possible for the main rotor to fail without the engine failing? The only thing I can think of is the rotor getting damaged, which is different than a "failure."

 

If the rotor falls apart, that's a rotor failure.

[seikdel]

The S-97 seems like the ultimate chopper except it lacks armour and stealth.

 

Okay Folks, I've been wondering about this for awhile. How do you deal with doppler return off of rotors when you want a "stealthy" helo? The RAH-66 was supposed to be "stealthy" (conformal weapon bays and all that), but won't you always still have that massive return from the eggbeater on top? Is RAM really good enough to hide your rotors from a fast mover's radar?

[leafer]

I don't think anyone in the know will raise his hand to commit treason. :D

[Alfa]

One reason as far as attack helicopters is concerned could be the danger of blade collission during extreme evasive manouveres - at least some Ka-50s were lost during training due to this phenonomen.

 

The fix was to increase the distance between lower and upper rotor, but then there is the question of battlefield profile - i.e. harder to use terrain features to hide behind between pop-up attacks.

 

For an ASW platform like Ka-27PL(or its civilian Ka-32 version) neither of these issues are a problem, while the compact nature, hover stability, high payload capacity and lack of a tailrotor makes it a perfect design for operating from the confined space onboard ships.

[Alfa]

Okay Folks, I've been wondering about this for awhile. How do you deal with doppler return off of rotors when you want a "stealthy" helo? The RAH-66 was supposed to be "stealthy" (conformal weapon bays and all that), but won't you always still have that massive return from the eggbeater on top? Is RAM really good enough to hide your rotors from a fast mover's radar?

 

Maybe that was among the reasons why it was cancelled? :)

[GGTharos]

It was cancelled because planning and oversight were botched. The thing's price and weight continued to increase, and to the point where no one was certain what would be onboard and if the heli could take off with what ... may or may not have onboard.

 

In the end, it turned out that it wasn't needed anyway. Technology developed or envisioned for that helicopter has been implemented in the AH-64 and probably AH-1 and the Kiowa, as well as select versions of the UH-60.