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Co-axial rotor systems... a couple of Qs

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Co-axial rotor systems... a couple of Qs

Old 21st May 2008, 18:22
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Co-axial rotor systems... a couple of Qs

To anyone who has flown, or has professional knowledge of, the co-axial rotor design found on Kamov helos, perhaps you can answer a couple of Qs coming from a PPL(H) with time only in the R-22, Bell-206 and Hughes 500?

1) What are the ball park rotor stall speeds (percent RRPM) for co-axial helos? Perhaps a very generic question, but feel free to answer with specifics for the types you have flown

2) If a co-axial rotor system fully stalls, can it be recovered by flaring and inducing airflow through the disk, or do the blades simply fold up (as I was taught is the case with the R-22 and Bell 206 teetering head systems)

3) Autos: I understand that yaw control becomes an issue during the flare, and am also informed that standard auto for something like the Ka-50 is a 120 knot descent with a late flare and a 3g bounce/rolling touchdown. Does this sound about correct? Is there any way to auto a co-axial to the ground with zero forward velocity following the flare?

4) Flight envelope-wise, what conditions must exist for the two separate rotor disks to be at risk of collision? Low RRPM and high forward velocity, for example? Similarly, are the typical danger areas for tail boom strike (low-RRPM and low forward velocity) to other types also applicable to co-axial types?

5) Does the co-axial design lend itself particularly to performance in terms of manouverability?

6) Are there any additional complications from entering VRS in a co-axial type, or are entry criteria and recovery procedures the same as normal?

Any help much appreciated!
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Old 23rd May 2008, 01:29
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Paging Mr Dave Jackson.....

I'd be amazed if anybody managed to consistently achieve an intended 3g bouncing arrival. Arrival would be the operative word I suspect......

Despite the fact that Kamovs are generally built like brick outhouses, I'm inspired to ask;

Can anybody say spinal compression?
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Old 23rd May 2008, 12:55
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An educated guess...

1. similar to single rotor, as lift will be shared on cioaxial.

2. Unlikely. Structural integrity will be as single rotor.

3. Yaw effectiveness will be reduced in autorotations. Flare will not suffer.

4. In theory non, but in practice only if aircraft is ubjected to reduced g - so same conditions to cause rotor strike.

5. Not especially.

6. Similar to single rotor.
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Old 23rd May 2008, 15:33
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Let's see:
Rotor stall - I'm not aware of any helicopters that have a significant rotor stall issue in the normal operating range of airspeed and Rotor RPM. Why is there such an interest for a co-axial?
Who teaches rotor stall on Bell 206? I've never heard of it being an issue, except perhaps for very high weight/ high density altitude, low rotor RPM autorotations...
AUtorotations for co-axial rotors with mechanical flight control systems are a problem because of the way yaw control is mechanized. What works in the proper direction in powered flight for yaw is reversed in autorotation. So all the current co-axial machines use large vertical stabilizers and rudders and need to have forward airspeed to keep yaw working the 'normal' way. The KA-32 Flight Manual has a minimum airspeed in autorotation of about 60 KIAS if memory serves me well. Below this airspeed 'Special Techniques' are necessary. Flare and landing shouldn't have much yaw corrections needed, and the landing certainly would not be 3G.
Risk of coliision of the two rotor systems is minimized by the large vertical separation - the worst case is high speed and I believe pulling G. Happened to one of the KA-50 prototypes, I understand. But not a problem in the KA-32.
As far as maneuverability, I don't believe co-axials have any particular advantage or disadvantage - Nick Lappos would elaborate.
Vortex Ring State symptoms and recovery should be the same as single rotor.
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Old 23rd May 2008, 15:44
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Thank you for the responses.

I am researching co-axial systems within the context of military operations - where the normal operating range may regularly be exceeded, and where high weight, high density operations at the limits of the RRPM envelope may well be exceeded. My current research suggests that low-RRPM conditions well below the placarded stall speed of about 80 per cent RRPM in the Ka-50 are recoverable. I have yet to find a pilot who can actually confirm that, though!

My specific interest is in the Ka-50, but I kept my questions generic since it occurred to me that few (if any) here are likely to have flown it, and that co-axial rotor systems may have specific peculiarities regardless of which type they are installed on.

I understand that the two Ka-50 self-midairs were the result of excessive negative g at high airspeed. I was therefore wondering of there were any other areas of the envelope that would lead to self-midair.
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Old 25th May 2008, 16:47
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Not sure about the KA-50 midairs being caused by negative G. Could be positive G is an issue.
One of the things that most people don't realize is that the KA-50 is perhaps the loudest helicopter in existance. I was in St. Petersburg many years ago for the European Rotorcraft Forum, and they brought a KA-50 to the Peter and Paul Fortress - you could hear it coming 10 miles away, and it wasn't a cold day.
How you could ever expect to sneak up on someone with the machine is beyond me, but then again, maybe the noise was intended to scare the folks it was being flown against...
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Old 25th May 2008, 17:57
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The typical articulated coax has rotor rpm limits very much like a single rotor helo, and has no exceptional advantage in low low rpm operations.

Beware, Russians generally set 100% as the autorotation upper limit, so that low cruise rpm can be 85%, low emergency limit much lower. These numbers look low relative to a helo that has 100% as its normal rpm, but the tip speed is not lower.

Stall affects the control system vastly by overloading the pitch links and servos, coax or not. This means the controls would have to be very heavy to support the post stall needs of your low rpm mission segment, a price no coax helo manufacturer has yet decided to pay.

Also, a coax would seem to have an advantage in stall because a fully lifting blade is available on both sides to keep cyclic control, but the stalled blade will wander and flap, and self-midair is a real problem. Word is the loss of the KA50 was due to maneuvering in cyclic, not zero

This is an example of how the ability to mix myths helps establish pop-aerodynamics - a teetering rotor flaps and removes itself at zero g, thus an articulated coax must have seen zero g to have a rotor problem. Monty Python had similar logic, if a witch floats, and wood floats, then witches must be made of wood......

The payload advantage that some subscribe to coax configs is not real, because the disk loading is the hover performance driver, and the two coax rotors weigh more than a single rotor, more than a tail rotor system weighs. Also, the wide separation between the two disks (to prevent noise complaints when the blades collide with each other) makes the cruise drag very high, usually about 10% higher than a single rotor. This makes the payload less at any significant range, since the gas burned robs payload. Coaxs are great at shipboard ops, where the wind can come from any direction without any practical limit, and short, stubby tail cones make deck spotting easy, and where mission ranges are miles, not hundreds of miles, so the fuel burn at cruise is a smaller segment of the mission.

The ABC coax concept that X2 uses resolves the post-stall flapping/blade contact issue to some extent by having very rigid rotors so that almost no flapping can be observed. Also, the rigid blades can be brought closer together, so that the drag is less than a conventional coax, but still higher than a single rotor. The KS-50 has two articulated systems, with one at 2 1/2% offset, the other around 7%. An ABC rotor has about 25% offset on both rotors.
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Old 25th May 2008, 18:01
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I see somebody is getting prepared for DCS: Black Shark

BTW that autorotation presentation video. 120 kph IAS, was not Knots, but Kilometers per hour - as it is Russian design, and they use the metric system for everything. And with this "3g touchdown" performed, I would expect ground resonance very shortly.

For those that have no idea what video I'm talking about, here it is:

I understand that the two Ka-50 self-midairs were the result of excessive negative g at high airspeed. I was therefore wondering of there were any other areas of the envelope that would lead to self-midair.
Low G maneuvers and sharp, or prolonged yaw to the right may be also dangerous as anything making the lower rotor to cone. The Ka-50 maneuvering envelope might be similar to that of a Cobra. Not really a dogfighter Which is kinda funny to read in late 80's books about it, where they say it was designed to fight other helicopters... on the other hand the same authors claimed that F-117 was supersonic

Last edited by Lt.Fubar; 25th May 2008 at 18:16.
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Old 25th May 2008, 19:03
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The ability for helicopters to fight each other is more a function of weapons system design than maneuvering ability. At standoff ranges, high speed precision weapons are key and if you ever get into the merge a stabilized turreted gun with a lead computing and motion compensating fire control system would easily overcome higher turn rates or extension speeds.

MD did some interesting air-to-air work with the Apache 30MM system which never got into service. That gun is horrible for traditional air-to-air work because of the low speed of the round and rate of fire of the gun, but with a proper fire control system it was pretty effective against drone targets.

The Marine Corps proved the air-to-air capability of the Hellfire missile when they hit an F-18 with one during a gooned up non-autonomous [email protected] designation shoot out in Yuma a few years back. The F-18 was lazing and the shooter geometry was backwards (shooting at the F-18). The missile saw the designating pod as the spot and took off one of the F-18's vertical fins.
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Old 25th May 2008, 21:53
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I asked the designers of the Mi-28 why the KA-50 was designed the way it was - they said it was not designed for NOE or anti-tank warfare as we knew it. It was designed to do a single high speed pass through the front lines on pre-determined tracks with pre-assigned targets, literally spitting death as it came, and then once past the front lines, turn around and head back home.
It had a very limited traverse on the single gun, a cockpit you could barely see out of, and a single pilot.
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Old 26th May 2008, 05:51
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A few KA-50 maneuverability and weapons action (last shows a loop)





Last edited by Rich Lee; 26th May 2008 at 06:05.
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Old 26th May 2008, 13:28
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Old 26th May 2008, 14:56
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It's pretty difficult to tell at what airspeed the 'sideward' flight is done (supposedly to show the ability to point the gun at nearly anywhere). I've see it done at about 60KIAS, but never faster.
Nick could elaborate on this from Commanche experience, but my two cents worth-
The fire control system would have a very difficult job sorting out the windage corrections to get rounds even close to the target initially. It would take several seconds for it to sort out where the relative wind was really coming from, at a minimum.
The second problem is that during the sideways flying, there appears to be a reasonable amount of pitch movement - again a real problem for the fire control system to sort out. Even a small change in pitch is going to make a big difference in the trajectory of the bullets. Roll corrections are a bit less of a problem. Yaw errors also obviously a big problem, but it's not clear if the machine is yawing very much.
Interesting that we never see any footage of the KA-50 actually firing it's gun during this sideways flying, let alone getting rounds on a target.
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Old 26th May 2008, 15:33
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Sorting out a gun pointed 90 degrees to the free stream velocity is what turrets do every day they are used. The Comanche had a good airspeed system in the blades that read speed and azimuth, and so did the Mi28. I would imagine the KA-50 could/should also have that feature, since they didn't even bother to turret the big gun.

The KA-50 is really not used as a helicopter in the doctrine that employs it, it is more like a stol A-10, conducting swooping run-in attacks from altitude, and not NOE. The single pilot would get lost alot if the thing were brought down to NOE tactics.

The Mi-28 was the KA-50's competitor, and it is more classic in design, similar to the Apache (but without its excellent pilotage and targeting systems).
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Old 29th May 2008, 18:28
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Thanks very much for the additional responses, especially to NL. This is a pretty good precis of coaxial pros and cons.
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Old 30th May 2008, 23:03
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Some notes that might be of value.

2) If a co-axial rotor system fully stalls, can it be recovered by flaring and inducing airflow through the disk, or do the blades simply fold up (as I was taught is the case with the R-22 and Bell 206 teetering head systems)
I suspect that on a craft with two main rotors, which are vertically or laterally separated from each other, and which are extremely rigid, it may be possibly to utilize the same method of stall recovery as is used on a fixed-wing aircraft, i.e. drop the nose.

3) Autos: I understand that yaw control becomes an issue during the flare, ..................
You are obviously aware of how yaw control operates on a Coaxial during autorotation, But for others who are interested, here is a 'description of operation' presented on PPRuNe by HDW a couple of years ago. As you mention, the flare is the interesting part. HDW describes how the flare is handled on an Intermeshing. Unfortunately, this ability to used opposed longitudinal cyclic is not available on the Coaxial.

4) Flight envelope-wise, what conditions must exist for the two separate rotor disks to be at risk of collision?
The rigidity of the rotors will obviously play a major roll in determining the gap (vertical spacing) between the two rotors. The following rough gap-to-radius ratio may be of some interest. Then again it may not.

Ka-50 rotor radius 23.8 ft Gap between rotors looks like being somewhat over 4 ft. Gap/radius ratio = 0.168
Sikorsky S-69 (XH-59A) ABC Rotor radius 18 ft. Gap 2.5 ft Gap/radius ratio = 0.139
On this page and its linked pages is information related to the minimum tip-tip gap experienced, etc. etc. I think that it was 11 inches.


I would, perhaps presumptuously, recommend the Intermeshing configuration for consideration, in conjunction with the Coaxial during your preliminary design process. In the minds of most people the Intermeshing configuration is perceived a craft with a 'soft and mushy' control, but with a good lift. This is due to the direction that Kaman took the configuration. It is not due to Flettner, nor to Kellett's aspirations.

As you probably know, Prouty mentions the "Synchropter" in his book 'Military Helicopter Design Technology'. His short review of the configuration is interesting, however it does not assess the configuration in the light of today's requirements and technological advancements. I would not fault Prouty, since knowledge of the intermeshing configuration has been very limited, however his last paragraph needs some expansion.

In addition to the yaw control situation in autorotation, which is shared with the coaxial, he says; "Another problem for the synchropter is the difficulty in using more than two blades on each rotor because of the blade strike problems. This is one of the reasons that the three-bladed Kellett synchropters were not successful.".

~~ The Kellett 3-bladed XR-10 crashed when a pitch link failed, killing the pilot, Dave Driskill. The mechanic who parachuted out of the craft and survived reported the situation.

~~ In addition, from The Luftwaffe Profiles Series No.6 on the Flettner Fl-282; "two three-blade rotors - was tried out on the test bed. It proved to run extraordinarily smoothly, however this was not a consideration for military use."

~~ My opinion; Today's ability to have extremely rigid rotors makes it possible to produce 3-bladed Intermeshing rotors; and within certain spatial constraints it should be possible to produce 4-bladed Intermeshing rotors.

Just some thoughts for consideration when looking for the very best.

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