About the coaxial rotor

[Piffer]

I read at Wikipedia some aerodynamic advantages of coaxial rotors:

• Settling with power - Reduced
  
• Retreating blade stall - Eliminated
  
• Ground resonance - Eliminated
  
• Low-frequency vibrations - None
  
• Medium frequency vibrations - None
  
• High frequency vibrations - None
  
• Transition from powered flight to autorotation - Eliminated
  
• Anti torque system failure in forward flight - Eliminated
  
• Anti torque system failure while hovering - Eliminated
  
• Height-Velocity Curve - Eliminated

Can you tell me if all this statements are true? My logic says the height-velocity curve (dead-man's curve) is the same than the "classic" helos.

[Force_Feedback]

Well, wikipedia isn't the most reliable source of information, I agree, the dead mans curve should also exist with coaxial rotors, but maybe it has to do with resistance generated by the main to tail rotor reductor.

No mention of blade clashing as a new side effect?

[britgliderpilot]

Well let's go through these one by one with a bit of discussion . . . .

 

 

[*]Settling with power - Reduced

 

Is this the same as Vortex Ring State?

 

You can still settle into a region of flight where the vertical speed of air through the rotors as the helicopter descends is faster than the rotors can push the air downwards. With two rotors I imagine it is slightly reduced over a single rotor, but it can still happen.

 

It's going to be something for rookie Black Shark pilots to watch out for, certainly.

 

 

[*]Retreating blade stall - Eliminated

 

Not eliminated so much as partially cancelled out. Both rotor disks will suffer retreating blade stall in opposite lateral directions, so no roll effects occur . . . . however, if you think about it logically there should still be a pitching moment due to the loss of lift towards the rear edge of both rotor disks.

 

 

[*]Ground resonance - Eliminated

 

I don't fully understand this one, so can't comment too much . . . . the sources I've read so far say it can't happen to two-BLADED helicopters, but don't mention twin-ROTOR helicopters.

 

Each of the Ka50's rotor disks are three-bladed, so I'd expect it to still be a problem.

 

 

[*]Low-frequency vibrations - None

[*]Medium frequency vibrations - None

[*]High frequency vibrations - None

 

. . . . not sure what it's talking about here.

 

 

[*]Transition from powered flight to autorotation - Eliminated

 

I don't understand exactly what this means. There are some interesting effects with autorotation in a coaxial helicopter, especially if you've suffered blade damage, but I doubt I'd be able to reveal exactly what they are here . . .

 

 

[*]Anti torque system failure in forward flight or hovering - Eliminated

 

Well there's no tail rotor to fail, but if you lose part of either rotor disk then your helicopter will still suffer yaw stability problems.

 

You won't suffer losses due to tail rotor failure, but then I've seen footage of survivable tail rotor failure losses. The Black Hawk crashes in Somalia were tail rotor failures, and crew members survived.

 

If you lose your rotor system in a Ka50, you're just going down. Puts all your eggs in one basket, as it were.

 

 

[*]Height-Velocity Curve - Eliminated

 

For those who haven't seen one, the height velocity curve illustrates regions of helicopter flight (in terms of height and speed) where the aircraft is unsafe in the event of an engine failure because:

 

1. There's not enough time to safely autorotate.

Primarily because by the time you realise you need to autorotate, the rotor speed (and therefore rotor energy) is decaying. In order to safely flare you need to build rotor energy through autorotation.

 

In this area of the Height/Velocity curve you're not travelling fast enough for the rotors to build rotational kinetic energy from the air passing over them, before draining that kinetic energy in the flare.

 

2. There's not enough time to cope with a mechanical failure because you're too low to allow reaction time.

 

I don't see why a coaxial system would eliminate either of these possibilities - you can still get decaying rotor RPM in the event of a double engine failure in the Ka50, and you still need time to deal with it.

 

Low altitude failures are always a problem. There is a spare engine on the Ka50 to pick up load, but IIRC you can't hover on it at high take-off weights.

[Yo-Yo]

I read at Wikipedia some aerodynamic advantages of coaxial rotors:

• Settling with power - Reduced
  
• Retreating blade stall - Eliminated
  
• Ground resonance - Eliminated
  
• Low-frequency vibrations - None
  
• Medium frequency vibrations - None
  
• High frequency vibrations - None
  
• Transition from powered flight to autorotation - Eliminated
  
• Anti torque system failure in forward flight - Eliminated
  
• Anti torque system failure while hovering - Eliminated
  
• Height-Velocity Curve - Eliminated

Can you tell me if all this statements are true? My logic says the height-velocity curve (dead-man's curve) is the same than the "classic" helos.

 

Very strange statements...

Ground resonance - there's no reasons for CR (coax rotor) to have not this effect because the origin of this effect is blades hunting and counter rotation can not eliminate the effect of rotor CG movement due to fuselage acceleration. The first Kamov's rotor suffered even from the AIR RESONANCE so the vertical hinge dampers are used.

Vibration - yes because there is no way to compensate too.

Transition to autorotation is the same as for the classic but there's no need to reduce rudder input. The yawing control for the CR is even harder than for classic.

 

The main advantages of CR are: higher efficience because of bi-plane effect,

higher hovering/flight power because no power to tail rotor is required;

almost symmetric roll/yaw controls with no cross-links (pitch-rudder-cyclic);

no limits for backward and side fligh becuse of tail rotor vortex ring state.

[RvETito]

I read at Wikipedia some aerodynamic advantages of coaxial rotors:

• Settling with power - Reduced
  
• Retreating blade stall - Eliminated
  
• Ground resonance - Eliminated
  
• Low-frequency vibrations - None
  
• Medium frequency vibrations - None
  
• High frequency vibrations - None
  
• Transition from powered flight to autorotation - Eliminated
  
• Anti torque system failure in forward flight - Eliminated
  
• Anti torque system failure while hovering - Eliminated
  
• Height-Velocity Curve - Eliminated

Can you tell me if all this statements are true? My logic says the height-velocity curve (dead-man's curve) is the same than the "classic" helos.

 

Few things to add. I can't see one correct point in this list.

 

Retreating blade stall is even more important in CR scheme, because it's the reason for the rotor cone inclination. The lower rotor inclines to the left(counter clockwise rotation seen from above) while the upper rotor inclines to the right(clockwise rotation) and as a result the gap between both rotors decreases on the 2-4 o'clock sector in forward flight. That's why CR helicopter have severe flight limitations of maximum airspeed and right pedal input(LR pitch increase, UR pitch decrease).

 

Ground resonance- all helicopters, no matter the design and rotors layout suffer from this phenomena. Kamov helicopters in particular have ground resonance valves built-in the shock absorbers of the landing gear.

 

Vibrations- of course there are. It's inavoidable when you have so many rotating components. Low frequency vibrations occur in case of rotors dynamic unbalance, or if the rotor system is balanced then they appear when the rotor makes a transition from axial flow(in hover) to slanting flow(forward flight). The nature of those low frequency vibrations is aerodynamical- interference between rotor downwind and fuselage and they appear at about 50km/h when the helicopter is slowing down and also when it accelerates up to this speed. But this also common for all helicopters.

 

In general the main advantages of the CR are:

- better efficiency coefficient(EC) compared to single rotor with the same diameter and fill ratio(all blades area to rotor disk area)- the UR is narrowing the flow and it's external boundary intersects the LR disk at about 75-80% of the diamater. Thus the LR interacts with additional flow mass which gives 7-8% bigger EC. If we add the 10-12% output power for the tail rotor that must be spent in the classic design that gives about 20% more power efficiency- to create on and the same lift the CR needs 20% less power. I'll illustrate this right away- the Mi-8MTV and the Ka-323 have the same engines and similar TOW but the Kamov lifts 5 tons load in sling operations while the Mil can handle up to 3.5-4 tons.

- much easier to fly- CR features indipendant controls like Yo-Yo said. As a result this makes it's flying very easy and natural, close to airplane. I can tell ya- once a pilot sits in a Kamov helicopter he will never want to fly a classic helo again.

- better maneuverability- turn rate, yaw control precision at any speed, climb rate you name it. The combat debut of the Ka-50 in Chechnya has shown it's advantages in complex mountain terrain compared to the Mi-24 for example. Kamov helicopters are still holding the world record for rate of climb.

- compact construction- mandatory for carrier operations. You won't see anything but Kamov helicopters on russian ships.

- combat reliabality- all records since Vietnam indicate that about 75% of all accidents are caused by tail rotor/transmission damage which is long and badly protected.

 

Drawbacks:

- risk of blades collision- especially in high speed maneuvers combined with right pedal input.

- shorter service life of gearbox and rotormast assembly because of the bigger loads;

[Piffer]

Thank you, Tito, Yo-Yo and Britglider!

[Alfa_Kilo]

Are there any major advantages of conventional tail rotor helicopters over co-axial rotors, other than simpler gearbox? Kamov JSC said that cabin height requirements led them to employ a tail rotor (compact GB advantage) in the Ka-60/62, but their smaller machines, the Ka-126/226 & Ka-115 are coaxial, with fairly voluminous cabins for their respective classes. Was the overall height an important criterion (maybe imposed by the size of transport a/c cargo holds) in the Ka-60 design?

[Sealpup]

Are there any major advantages of conventional tail rotor helicopters over co-axial rotors, other than simpler gearbox? Kamov JSC said that cabin height requirements led them to employ a tail rotor (compact GB advantage) in the Ka-60/62, but their smaller machines, the Ka-126/226 & Ka-115 are coaxial, with fairly voluminous cabins for their respective classes. Was the overall height an important criterion (maybe imposed by the size of transport a/c cargo holds) in the Ka-60 design?

 

It could be that the gearbox associated with the coax design takes up a larger vertical footprint than a conventional gearbox. So if they wanted more head-room in the rear cabin of the Kasatka, that could be a reason to go with a more conventional layout. Of course, I've never been able even lay eyes on a Kamov product, let alone poke around in the gearbox area, so I'm just going off the top of my head.

 

Also, am I the only one that thinks the Ka-60/62 is an AS-365 clone? Sleek, twin engined, fenestron tail, cetacea name (Dauphin 'dolphin', Kasatka 'Orca')

[Alfa_Kilo]

Also, am I the only one that thinks the Ka-60/62 is an AS-365 clone? Sleek, twin engined, fenestron tail, cetacea name (Dauphin 'dolphin', Kasatka 'Orca')

 

You are not. ;)

 

The Kamov has slightly more power though, and I suppose its of more modern construction.

[Blaze1]

Few things to add. I can't see one correct point in this list.

 

Retreating blade stall is even more important in CR scheme, because it's the reason for the rotor cone inclination. The lower rotor inclines to the left(counter clockwise rotation seen from above) while the upper rotor inclines to the right(clockwise rotation) and as a result the gap between both rotors decreases on the 2-4 o'clock sector in forward flight. That's why CR helicopter have severe flight limitations of maximum airspeed and right pedal input(LR pitch increase, UR pitch decrease).

 

Ground resonance- all helicopters, no matter the design and rotors layout suffer from this phenomena. Kamov helicopters in particular have ground resonance valves built-in the shock absorbers of the landing gear.

 

Vibrations- of course there are. It's inavoidable when you have so many rotating components. Low frequency vibrations occur in case of rotors dynamic unbalance, or if the rotor system is balanced then they appear when the rotor makes a transition from axial flow(in hover) to slanting flow(forward flight). The nature of those low frequency vibrations is aerodynamical- interference between rotor downwind and fuselage and they appear at about 50km/h when the helicopter is slowing down and also when it accelerates up to this speed. But this also common for all helicopters.

 

In general the main advantages of the CR are:

- better efficiency coefficient(EC) compared to single rotor with the same diameter and fill ratio(all blades area to rotor disk area)- the UR is narrowing the flow and it's external boundary intersects the LR disk at about 75-80% of the diamater. Thus the LR interacts with additional flow mass which gives 7-8% bigger EC. If we add the 10-12% output power for the tail rotor that must be spent in the classic design that gives about 20% more power efficiency- to create on and the same lift the CR needs 20% less power. I'll illustrate this right away- the Mi-8MTV and the Ka-323 have the same engines and similar TOW but the Kamov lifts 5 tons load in sling operations while the Mil can handle up to 3.5-4 tons.

- much easier to fly- CR features indipendant controls like Yo-Yo said. As a result this makes it's flying very easy and natural, close to airplane. I can tell ya- once a pilot sits in a Kamov helicopter he will never want to fly a classic helo again.

- better maneuverability- turn rate, yaw control precision at any speed, climb rate you name it. The combat debut of the Ka-50 in Chechnya has shown it's advantages in complex mountain terrain compared to the Mi-24 for example. Kamov helicopters are still holding the world record for rate of climb.

- compact construction- mandatory for carrier operations. You won't see anything but Kamov helicopters on russian ships.

- combat reliabality- all records since Vietnam indicate that about 75% of all accidents are caused by tail rotor/transmission damage which is long and badly protected.

 

Drawbacks:

- risk of blades collision- especially in high speed maneuvers combined with right pedal input.

- shorter service life of gearbox and rotormast assembly because of the bigger loads;

 

Hi AirTito

 

In order to achieve higher speeds would it be possible to use a stiffer rotor design such as that used in the Eurocopter Tiger. I'm thinking this may help prevent rotor collision in a coaxial design?

 

Blaze1

[GGTharos]

You can never prevent a collision. It has less to do with stiff blades and more with rotor disk tilt. You'd have to further separate the rotors, but then I think you start losing some of that counter-rotating rotor efficiency.

[Blaze1]

You can never prevent a collision. It has less to do with stiff blades and more with rotor disk tilt. You'd have to further separate the rotors, but then I think you start losing some of that counter-rotating rotor efficiency.

 

Thanks GGTharos

 

I did some digging around and found this: http://aeronode.com/aero/18/sikorsky-twin-rotor-x2-ground-run-in-october

Also this: http://www.aviastar.org/helicopters_eng/sik_s-69.php

 

Blaze1 :thumbup:

Edited July 9, 2008 by Blaze1

[GGTharos]

There you go. :)

[Vekkinho]

Thanks GGTharos

 

I did some digging around and found this: http://aeronode.com/aero/18/sikorsky-twin-rotor-x2-ground-run-in-october

Also this: http://www.aviastar.org/helicopters_eng/sik_s-69.php

 

Blaze1 :thumbup:

 

Nice find!

[Udat]

Here is an old NASA article on Coaxial rotor systems: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970015550.pdf

[ricktoberfest]

Almost as old as this thread!

 

 

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