Vortex Ring State in gyros?

thomasant

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So, the question whether gyros can get into vortex ring state depends on the RRPM working like an engine drive for a short period.
I believe both renowned gyro experts Doug and Chuck have explained the subject well.
Of course, we can all agree to disagree as I don't believe that there will ever be any consensus.

Irrespective of what it's called, that's what happened to me. And I'm sticking to my story.
 

XXavier

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I see Vh = (Thrust/2·rho·Sr)^0,5

What's Sr?


If I get it right, in the real world there isn't a clear boundary between VRS and autorotation, but a fuzzy region in-between...
 

Jean Claude

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The border between autorotation and entry into the VRS is very clear (Red line)
It's just that once you enter VRS, the exit (green line) can be more or less close to autorotation.

Vh is the reference parameter. It is the theoretical induced speed produced by a rotor of area Sr, in hover flight.
For a 2600 N and a 7 m rotor, we find Vh = 5.26 m/s
So, with this helo, when it increases his vertical descente from 0 to 0.4* 5.26 = 2.1 m/s, then it certainly will enter in VRS
And it can exit of VRS only when his vertical descente will more 1.5*5.26 = 7.9 m/s or less with Vx

I can not imagine how a gyroplane powered by the inertia of its rotor could enter the VRS zone from below, with an disk AoA upper 45 degrees.
More generally, how a aircraft could enter the VRS zone from below (decreases the drop), if VRS increases drop?
 
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WaspAir

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I would add the additional question of how long it takes (in time) for a VRS condition to develop when compared to how long a rotor-overspeed "hover" lasts.
 

bryancobb

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Why does everyone have such difficulty with the phrase "Descending Into Your Own Rotor DownWash at Greater Than 300FPM Rate of Descent at a Zero or Near-Zero forward speed?"

The downward velocity of the column of air your rotor has blown downward must be greater than -300FPM or you cannot descend into it, if your descent rate is required to be greater than 300FPM.

In short, The Vortex Ring State CANNOT ever exist at all on a gyro with fixed pitch that always has airflow upward and never blows a column of air downward.

I would learn a lot from an airflow diagram showing what's happening in a perfectly vertical, stabilized, gyro descent, in calm air, at the fastest vertical descent rate that the gyro can muster.

I don't think there would be ANY recirculated vortices. I don't think there would be ANY downward-flowing air. I know there would not be ANY downward-flowing air in excess of minus 300FPM . I'm pretty sure the VSI would never scare you with your descent rate.
 

Jean Claude

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nomie

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In short, The Vortex Ring State CANNOT ever exist at all on a gyro with fixed pitch that always has airflow upward and never blows a column of air downward.
Hi. Newbie here trying to learn from the discussion.
Can someone explain the above bold statement please? I always assumed the above holds true for the driving region, but that in the driven region even though the inflow is up through the rotor, the outflow would be relatively downwards just like a normal fixed wing in order to generate lift? The Rotorcraft flying handbook omits showing the outflow though so the above was an assumption. Would be nice to have clarity :)
 

XXavier

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Hi. Newbie here trying to learn from the discussion.
Can someone explain the above bold statement please? I always assumed the above holds true for the driving region, but that in the driven region even though the inflow is up through the rotor, the outflow would be relatively downwards just like a normal fixed wing in order to generate lift? The Rotorcraft flying handbook omits showing the outflow though so the above was an assumption. Would be nice to have clarity :)

In the case of pure vertical autorotation, an observer moving with the gyro would detect no net downwash, because for that observer there's only a net upwards flow velocity WBl, due to the fact the induced flow WRi produced by the rotor has less velocity than the sinking speed Vsink, but the downwards-directed induced flow WRi has nevertheless a real existence, that could be appreciated by an external, static observer.

Of course, that induced flow is not constant across the radius of the disk, and the picture shows a simplified interpretation of the facts, but that flow does anyway exist...

WBl = Vsink – WRi



Captura de pantalla 2020-02-03 a las 7.41.17.png
 

thomasant

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Why does everyone have such difficulty with the phrase "Descending Into Your Own Rotor DownWash at Greater Than 300FPM Rate of Descent at a Zero or Near-Zero forward speed?"

The downward velocity of the column of air your rotor has blown downward must be greater than -300FPM or you cannot descend into it, if your descent rate is required to be greater than 300FPM.

In short, The Vortex Ring State CANNOT ever exist at all on a gyro with fixed pitch that always has airflow upward and never blows a column of air downward.

I would learn a lot from an airflow diagram showing what's happening in a perfectly vertical, stabilized, gyro descent, in calm air, at the fastest vertical descent rate that the gyro can muster.

I don't think there would be ANY recirculated vortices. I don't think there would be ANY downward-flowing air. I know there would not be ANY downward-flowing air in excess of minus 300FPM . I'm pretty sure the VSI would never scare you with your descent rate.
Using terms like "everyone", "never", "cannot ever", "ANY" seem to depict a sense of superior knowledge on the part of one who I believe does not even fly gyroplanes. So that means that folks like Doug Riley and Chuck Beaty are talking through their hat??

"Seek first to understand, then to be understood" is a good quote by Stephen Covey.
 

WaspAir

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Hi. Newbie here trying to learn from the discussion.
Can someone explain the above bold statement please? I always assumed the above holds true for the driving region, but that in the driven region even though the inflow is up through the rotor, the outflow would be relatively downwards just like a normal fixed wing in order to generate lift? The Rotorcraft flying handbook omits showing the outflow though so the above was an assumption. Would be nice to have clarity :)
The driving and driven regions refer only to the angle at which the aerodynamic force vector points with respect to the rotational axis, not the direction of flow through that section of the disc, or whether that section is generating lift. It's an idea that is pertinent to rotor rpm but not lift distribution or flow patterns.

If the the vector is angled sufficiently far forward to be ahead of the axis of rotation, it will provide an accelerating force to keep the rotor spinning. If the vector angles back a bit, behind the axis of rotation, it will be trying to retard rotation. Stable rpm comes from balancing of these contrary effects. But that doesn't address the net flow, the strength of downwash, or where lift is generated. If a region is not stalled, it's making lift, and "driving/driven" is irrelevant to that issue.
 

farmboy

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The driving and driven regions refer only to the angle at which the aerodynamic force vector points with respect to the rotational axis, not the direction of flow through that section of the disc, or whether that section is generating lift. It's an idea that is pertinent to rotor rpm but not lift distribution or flow patterns.

If the the vector is angled sufficiently far forward to be ahead of the axis of rotation, it will provide an accelerating force to keep the rotor spinning. If the vector angles back a bit, behind the axis of rotation, it will be trying to retard rotation. Stable rpm comes from balancing of these contrary effects. But that doesn't address the net flow, the strength of downwash, or where lift is generated. If a region is not stalled, it's making lift, and "driving/driven" is irrelevant to that issue.
Thanks, WaspAir. That helps me understand what has been a puzzle to me for as long as I have been reading the forum. I appreciate that!
 

bryancobb

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Using terms like "everyone", "never", "cannot ever", "ANY" seem to depict a sense of superior knowledge on the part of one who I believe does not even fly gyroplanes. So that means that folks like Doug Riley and Chuck Beatty are talking through their hat??

"Seek first to understand, then to be understood" is a good quote by Stephen Covey.
OK. Help me understand then. I am not superior to anyone. I feel I have a very thorough understanding of a rotor system on any flying machine and I have it, not because I feel I am all that and a bag-o-chips. My basis for feeling I am an expert is that I have been a continuous student of literally dozens of great teachers/instructors since 1980 +/- when I first touched the flight controls of a rotorcraft.

Chuck Beatty, Prouty, and other well respected rotorcraft patriarchs came up in an age that aligned with what was known at the time. I have a 1st edition Fundamentals In Aerodynamics textbook from Georgia Tech (circa 1925+/-). In the chapter titled "Unconventional Aircraft," it compares gyros to helicopters and says "should a helicopter's engine fail, it would most certainly tumble from the sky."

I read everything Chuck and others write, with the idea in the back of my mind, that their concrete foundations were poured in the 1920's and 30's when that GA Tech textbook was still in use. I do know that Chuck takes major issue with Instructional texts that are used and have been used to train helicopter and gyro pilots for decades. These books are the "source-control-documents" for information that the FAA and most other aviation agencies the world over use to teach, write test questions, and evaluate the knowledge of licensed pilots.

The FAA Rotorcraft Flying Handbook
The FAA Pilots Handbook of Aeronautical Knowledge
The Robinson Safety Course Book
King Schools Videos and Test Prep
US ARMY Programmed Texts for IERW
US ARMY Fundamentals Of Flight

On principle of rotor system behavior about which Chuck has not been able to convince me "he is the way, the truth, and the light..." is why he feels a gyro flies better with blades that have twist that makes tips have more pitch at the tips than at the root. I know with certainty, a blade that is twisted the other way more evenly distributes the aircraft weight along the span instead of making the tips lift all the weight. When the tips lift all the weight, the blades bend upward more and since gyro RPM may get low sometimes without any way for the pilot to quickly get RPM back, the tips may bend upward beyond the safe flying line.

In his early days, he used to fly with Hughes 269 blades mounted upside down and spun backwards to get the "wash-in" I mention above. I'll bet WaspAir's FAA Certified gyros didn't have wash-in. They most likely have washout so the weight supported doesn't bend the blades. I'll also bet he can autorotate vertically with those positively twisted blades.
 

WaspAir

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I've never actually set them next to each other for a true side-by-side comparison, but my belief has always been that a genuine factory supplied J-2 blade differs from the early 269 blades (dimensions and construction being essentially the same) by having no twist at all (neither wash-in nor wash-out). Unfortunately, I don't have any handy documents to cite to confirm or refute this, and my former J-2 is in New Zealand, making direct measurements impractical.
 

thomasant

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OK. Help me understand then. I am not superior to anyone. I feel I have a very thorough understanding of a rotor system on any flying machine and I have it, not because I feel I am all that and a bag-o-chips. My basis for feeling I am an expert is that I have been a continuous student of literally dozens of great teachers/instructors since 1980 +/- when I first touched the flight controls of a rotorcraft.

Chuck Beatty, Prouty, and other well respected rotorcraft patriarchs came up in an age that aligned with what was known at the time. I have a 1st edition Fundamentals In Aerodynamics textbook from Georgia Tech (circa 1925+/-). In the chapter titled "Unconventional Aircraft," it compares gyros to helicopters and says "should a helicopter's engine fail, it would most certainly tumble from the sky."

I read everything Chuck and others write, with the idea in the back of my mind, that their concrete foundations were poured in the 1920's and 30's when that GA Tech textbook was still in use. I do know that Chuck takes major issue with Instructional texts that are used and have been used to train helicopter and gyro pilots for decades. These books are the "source-control-documents" for information that the FAA and most other aviation agencies the world over use to teach, write test questions, and evaluate the knowledge of licensed pilots.

The FAA Rotorcraft Flying Handbook
The FAA Pilots Handbook of Aeronautical Knowledge
The Robinson Safety Course Book
King Schools Videos and Test Prep
US ARMY Programmed Texts for IERW
US ARMY Fundamentals Of Flight

On principle of rotor system behavior about which Chuck has not been able to convince me "he is the way, the truth, and the light..." is why he feels a gyro flies better with blades that have twist that makes tips have more pitch at the tips than at the root. I know with certainty, a blade that is twisted the other way more evenly distributes the aircraft weight along the span instead of making the tips lift all the weight. When the tips lift all the weight, the blades bend upward more and since gyro RPM may get low sometimes without any way for the pilot to quickly get RPM back, the tips may bend upward beyond the safe flying line.

In his early days, he used to fly with Hughes 269 blades mounted upside down and spun backwards to get the "wash-in" I mention above. I'll bet WaspAir's FAA Certified gyros didn't have wash-in. They most likely have washout so the weight supported doesn't bend the blades. I'll also bet he can autorotate vertically with those positively twisted blades.
The discussion is about VRS in gyros.
Something happened to me during my crash on 15 Dec 2018.
I would like to draw your attention again to the description that Doug Riley explains in his post which I have quoted in post #17 of this thread. He describes the condition quite well. Compare that description with the condition experienced in a helicopter during VRS. It sure makes me wonder if we all understand everything about gyros. Based on the inputs from other posters, we can now understand that the airflow pattern in the gyroplane rotors is not always from below. Different flight conditions and rotors will determine that.
Of course, the vsi is not going to register a sudden drop from below about 20 ft above the ground. You will experience the "splat", which is what I did.
If anyone experiences a "splat" on landing due to rotors that have been over sped in a tight G turn while coming in to land, and is still alive to wonder about what happened, then I do believe it warrants further examination and not just accept a simplistic conclusion like yours that "The Vortex Ring State CANNOT ever exist at all on a gyro with fixed pitch that always has airflow upward and never blows a column of air downward."

Powering out in a gyro is not the same as applying the collective to cushion the landing in a helicopter.
 

bryancobb

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thomasant,

Thanks for your 1st hand account. Just keep one thing in mind. The aircraft must be descending into the column of air at a velocity that exceeds the speed of the column of air. If your event created some descending column of air like you suspect, then you must have flown into that descending column of air with a pretty high rate of descent already.
 

thomasant

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Bryan, you're welcome.

Again I feel you're making some assumptions. But have you read the post by Doug Riley? It is in post #17. What are your comments on his explanation? There is a mention of the aircraft being able to hover for a short period of time and then descend in it's downwash. Here's a link to the whole thread:


I believe that being the expert you claim to be, you will be able to shed some light on those. Also, what do you think could happen to a rotor system when the blades suddenly lose their lift due to some reason, do you think that the machine would drop very fast without prop thrust? Consider laminar airflow, turbulent airflow, etc on airfoils. I remember what happened to Steve Weir when his rotors detached completely. I'm just trying to find some answers.
 

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I have not read this entire thread, but the heading got my attention.
I do not claim to be an expert, I do hold a valid Helicopter licence and I do hold a gyro licence with some time in both types.

Its not possible IMO to suffer vortex ring with a gyro, this is a condition associated to powered rotors.
We have had this discussion in the past in a class and on forums.
What did come out of this discussion is the very difference between VR and settling with power as many times people claim this is the same thing.
just my 2 cents....
 

thomasant

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My takeaway from this thread:

1. I agree that VRS does not exist in gyroplanes due to the rotors being in autorotation, and have not advocated that it exists.
2. There can be a period of reversal of airflow if the rotors speed up over normal flight autorotative RPM.
3. The gyroplane can be made to hover for a short period depending on the inertia of the blades during this airflow reversal.
3. A condition of "settling without power" can then result, and can be dangerous close to the ground at low airspeed if recovery action is not taken.

For those that are flying gyroplanes regularly, (like Birdy who has done this regularly in his over 6000 hours of gyro time) all of the above can be experienced by following the procedure given by SandL in the thread "What is Drag Over"; and I quote his method below from post # 12:

helicoptering
helicoptering
I'm getting there but not brave enough to give it a go for the time being
however later in the summer (if we have one this year) I may be tempted to give it a go.
so to perform this manouver (without moos)
I go up to say 1500 ft then ease back on the throttle and gently dive to 1000ft at say 70kts ease the stick back to a hover thus increase G loading (gyro has more weight to lift) so blades increase speed and airflow goes downwards through the disk (would hear the blads slapping the air ?)

I then enter a vertical decent and in effect fall through the dirty air for a few seconds recovering at say 7-800ft lower the nose and gradually open the throttle. returning to straight and Level (S and L as in sandl)

does that sound ok ?

things to watch out for don't pitch up too sharply when entering the hover, keep a little throttle on during the hover to retain rudder authotity. make sure I have plenty of air under me for recovery.

is that it ? I have a single seat Bensen with Pod, CLT and light 22ft draggon wings.
Thanks
Peter


Personally, I do not advocate anyone trying this out. I have nothing more to add other than re-direct those that do not agree with me to the above thread. Best to direct specific questions to the experts with actual experience.
Fly safe.
 
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