Vortex ring effect question.

[TheMoose]

I’m curious. How realistic the vortex ring effect is represented. In real life does a helicopter pilot deal with this every day? A frequent occurrence? Or just a freak of nature that they need to be aware of and deal with it when it happens…

PS. I don’t mind the effect; actually it’s a nice addition. But just curious on how frequently it happens in real life…

Cheers

[AlphaOneSix]

I happens very rarely in real life because pilots are trained to know what it is and how to avoid getting into it in the first place. Where I work, we train for it all the time...we just head up to about 2000 feet AGL, go to a hover, and put it into settling with power on purpose, just to see what it feels like and looks like in the cockpit, and at 2000', there is plenty of room to get out of it.

[starbird]

Its real enough that pilots are trained to avoid getting into them. I've read a number of stories of helis getting into and out of them. I can't say how realistic it is in black shark, but everything I've seen about them talks about violent shaking and a high descent rate. The only way out is to get some forward velocity and fly out of it.

 

The way to avoid them is by not descending too fast in a hover. I'm guessing this is what the descent autopilot was designed for, and why it descends at such a slow rate. The printed manual says to avoid descending in a hover greater than 5 m/s.

[ruprecht]

The conditions to avoid, and the warning signs are drummed into you very early in your training.

 

If you're lucky enough to experience it for real in a controlled environment, it is scary as hell. If you're unlucky enough to experience it accidentally, I imagine it's even scarier, but the training is designed to make the recovery actions automatic.

[ericinexile]

Helo pilots who've posted elsewhere have said that VRS is easier to encounter in DCS than in their experience. They've gone on to say that even when instructors are trying to demonstrate vrs, it is sometimes difficult to feel the effect enough to be meaningful to the student. But with the Ka50, who knows? It does have a smaller rotor diameter which could make it more prone to vrs. Rotor RPM is slightly lower than other designs, Does that make a difference? It might incorporate significant blade twist which could be a factor. In my reading, (Prouty's Helicopter Aerodynamics), the writer is more focused on tailrotor VRS which poses more of a problem with attack helicopters which must demonstrate hover capability over a wide range of wind speed and direction.

 

Personally I like the vrs model, even if it later proves to be too aggressive. It enforces a little discipline in the way I fly.

[ryuzu]

As others here have said, you fly to avoid getting VRS and train for the recovery - consider it like stalling a fixed wing.

 

However, it would be a misinterpretation to assume VRS only occurs in a hover - the only requirement is to have low air speed (usually trained as less than Effective Translational Lift - some use 17knots as the cutoff airspeed) but you may still have ground speed if you are downwind.

 

The most likely time to get into VRS is when flying slowly downwind, perhaps setting up a hover, landing or attack run. The loss of ETL during the process will lead to an unexpected descent rate which, unchecked (perhaps because you're looking at a target) can develop into VRS as the airspeed decays. In the downwind scenario the pilot still thinks they have some airspeed since they still have ground speed.

 

I've got it more often in the sim than in reality (in reality, like a fixed wing stall - I've only had it when I was looking to get it so far!) but in the sim wind awareness and kinesthetic senses (seat of your pants) are gone so you don't get that nagging feeling to look at the VSI... In reality, you feel a rumble as you pass through ETL when slowing down so that acts as a reminder to check your descent rate.

 

r.

Edited July 10, 2009 by ryuzu

[ruprecht]

Pinnacle landings are also a prime candidate. You get a little steep, few visual references, you slow a little and increase descent rate... pow. At least in that case you can nose over and fly down the side of the hill!

[JDski]

I get into them in the sim by doing a quick wingover sort of 180 without a descent. What's up with that?

[Griffin]

But with the Ka50, who knows? It does have a smaller rotor diameter which could make it more prone to vrs. Rotor RPM is slightly lower than other designs, Does that make a difference?

 

- Ka-50 nominal rotor RPM is 313

- Mi-28 is around 242 RPM

- AH-64A is around 289 RPM

(I'm not guaranteeing they are correct but close enough.)

 

With my logic, smaller rotor diameter equals higher RPM.

Can't answer your question though. :)

[Wags]

In my experience with the patch (yes, it really is coming), VRS onset is much more apparent now.

[ericinexile]

- Ka-50 nominal rotor RPM is 313

- Mi-28 is around 242 RPM

- AH-64A is around 289 RPM

(I'm not guaranteeing they are correct but close enough.)

 

With my logic, smaller rotor diameter equals higher RPM.

Can't answer your question though. :)

 

I'm not sure where I read that but in hindsight it doesn't make much sense. Smaller rotor diameter usually does mean higher disc loading and higher RPM. It also means faster TAS before retreating blade stall and mach problems. Furthermore, high RPM would probably reduce flapping lowering the chance of an ugly blade intersection.

 

Thanks for the correction.

Edited July 10, 2009 by ericinexile

[th3flyboy]

In my experience with the patch (yes, it really is coming), VRS onset is much more apparent now.

 

Could you give us a little bit of info as to what you mean by much more apparent? It would be interesting to know a little more of what to expect with this patch.

 

As for what I've discovered about VRS...

Well, rule number one is never take off and just go straight up... doing that is asking for trouble. If you find yourself in VRS, the best bet is to lower your collective, and push forward, the more thrust you have pushing you up, the worse off you are. VRS happens when the air under your helo is too thin to be able to lift it up by your rotors beating the air into submission. You can say that the rotors are beating the air and ground into submission to keep you up, but if there is not enough air between you and the ground, you tend to fall. That is something that is very easy to happen if you stay still for too long. One of my tricks is I never enter a full hover, I always keep a little forward speed up, even if it is just 1kph. I don't take chances when I don't have to, and I've fell to VRS so many times it isn't funny.

Edited July 10, 2009 by th3flyboy

[ericinexile]

Could you give us a little bit of info as to what you mean by much more apparent? It would be interesting to know a little more of what to expect with this patch.

 

As for what I've discovered about VRS...

Well, rule number one is never take off and just go straight up... doing that is asking for trouble. If you find yourself in VRS, the best bet is to lower your collective, and push forward, the more thrust you have pushing you up, the worse off you are. VRS happens when the air under your helo is too thin to be able to lift it up by your rotors beating the air into submission. You can say that the rotors are beating the air and ground into submission to keep you up, but if there is not enough air between you and the ground, you tend to fall. That is something that is very easy to happen if you stay still for too long. One of my tricks is I never enter a full hover, I always keep a little forward speed up, even if it is just 1kph. I don't take chances when I don't have to, and I've fell to VRS so many times it isn't funny.

 

Well, I have to disagree a bit with your description. A vortex ring is just that--a ring of air-mass encompassing the outer circumference of the rotor disk that has been induced by the rotor to descend and rotate like a vortex. The inner part of that ring ("viewed" from above), the donut hole if you will, is downwash and is how a rotor generates lift. But the ring itself is just a mostly useless bit of turbulence and induced drag and is a constant fact of life for the rotor. This bit of physics becomes a problem when the rotor enters deeply into this ring, usually by descending into it, killing downwash production on a large portion of the rotor disk. Increasing collective only strengthens the ring, further killing lift. The takeoff scenario you mentioned is not hazardous because of VRS but rather because it places the helicopter outside a safe autorotation envelope. Hovering is safe. Hovering with a significant descent rate is not. Again, VRS didn't become a big threat until attack helicopter requirements pushed the hovering capability of the tail rotor. Then someone invented the tilt-rotor...

Edited July 10, 2009 by ericinexile

[wickedpenguin]

VRS didn't become a big threat until attack helicopter requirements pushed the hovering capability of the tail rotor. Then someone invented the tilt-rotor...

 

Not sure what you mean by that statement (maybe I'm reading it wrong), but the V-22 Osprey has had its share of VRS issues, including a crash that killed all 19 onboard. The Osprey also suffers from unequal ground effect issues. For instance, when they land aboard ship, one rotor typically extends out over the water while the other is inboard over the deck. With an identical power setting on both engines, the inboard engine will generate more lift since it is Hovering In Ground Effect (HIGE). The outboard is HOGE. The aircraft could feasibly snap roll off the side of the ship if not handled properly.

[TheMoose]

This makes for interesting reading. I give thanks:D

[Rangoon]

Keep in mind also that recovery from Vortex Ring State, or Settling With Power (term mainly used in the U.S.) generally is getting the disc into clean air, which can be airspeed in any direction - forward, sideward, backward. If your best option is NOT forward, any direction will do. And if you have enough altitude, you can technically lower collective enough that you effectively enter autorotation. This will also solve the problem because vortex rings will not form with air moving *up* through the system. But 99% of the time airspeed any direction is the recovery (and lower collective a bit if altitude permits).

 

I wonder if the Black Shark is more susceptible only in the sense that it's heavier than a typical civilian helicopter. For that reason I can see things happening more quickly if the pilot is distracted.

[GGTharos]

The Shark seems more susceptible probably due to people not being able to feel the helicopter's motion.

 

Normally you'd correct for VRS before it ever happened because you'd feel the heli starting to descend.

[Rangoon]

Well that's true, we don't have the feeling in the seat of the pants. At the same time, most times it happens for real, it happens so quickly that all the pilot feels is the floor drop out beneath. That's just based on the stories I've heard from actual accounts where the pilot survived the ensuing crash.

 

If you're in a profile where you expect it *could* occur (such as OGE hovers, steep approaches, and downwind approaches), you're much more aware, sensitive to it, and ready to react. If you're in combat, you could well be thinking about other things.

 

In training, we enter it intentionally, slowly, feel every little (then large) vibration and then the rapid increase in descent rate followed by the recovery. But the stories I hear all involve the unexpected entry and it comes as a surprise until the helicopter is either wrecked or saved through luck or altitude, then the pilot goes "oh yeah, I shouldn't have done that." I can imagine in combat, with a heavy aircraft that it could happen more easily due to distraction and the fact that descent rates will build more readily. Although much of the effect of weight is relative since it has more to do with % of max gross weight since higher gross weight requires more power, more pitch in the blades, which leads to more significant vortex rings.

[Wichow]

The Shark seems more susceptible probably due to people not being able to feel the helicopter's motion.

 

Normally you'd correct for VRS before it ever happened because you'd feel the heli starting to descend.

 

When you put a real pilot (tested with Real F-18 pilots) in from of the good PC simulators (falcon4, DCS, etc.) all agree that it is more easy fly the real planes than PC simulator because they miss peripheral vision, G feeling, hotas etc.

 

Here is where the programmers must improve the Virtual pilot helps suppliant the lack of peripheral vision, G feeling etc . How; Sound, visuals and Force feedback.

 

To the DCS programmers, One suggestion/example. When the vortex is going to begin just move and zoom quickly the pilot head focus to VVI for one second and then restore the pilot focus at the original position apart more visible vibrations at the cockpit also you need hear more volume at the cockipt vibrations, we need to hear the metal rivets squirm of the Bird body, soft or hard depending on severity of the situation . Some technical, head focus in, will be applied for events into the theorical human field of view to supply the lack of peripheral vision, whether or not the event occurs into the PC monitor for example incoming sams, treats, etc.

Edited July 11, 2009 by Wichow

[wickedpenguin]

When the vortex is going to begin just move and zoom quickly the pilot head focus to VVI for one second and then restore the pilot focus at the original position apart more visible vibrations at the cockpit also you need hear more volume at the cockipt vibrations, we need to hear the metal rivets squirm of the Bird body, soft or hard depending on severity of the situation.

 

I disagree with that method you describe there. That would be distracting and disorienting, and would likely contribute to the problem. When I'm in a low-level, edge of the envelope situation, the last thing I want is the sim jerking my view around.

 

I don't really think the sim needs anything. You just need to have situational awareness.

 

The exception may be some auditory clues. For instance, some airliner sims have a "clacking" sound that represents the stall warning stick shaker. That keeps those of us without a force feedback controller informed of what we would be physically experiencing in the cockpit. Another thing I recall from my RL flight training is the amount of wind noise when entering a power-off stall. With the engine idle, it becomes far more pronounced.

 

For the real helicopter pilots, what does it feel like to enter VRS? What does it sound like? Are there any significant vibrations?

Edited July 11, 2009 by wickedpenguin

[ryuzu]

For the real helicopter pilots, what does it feel like to enter VRS? What does it sound like? Are there any significant vibrations?

 

Well each helicopter will react differently to the onset of VRS so ideally an actual KA50 pilot would provide the input.

 

However, for what it's worth, in my experience (with medium Western Twin Turbine helicopters) the initial indicator of possible VRS is the "rumble" felt while decelerating below Effective Translational Lift - it's felt through the seat of the pants as a rumble - not enough to upset the trajectory of the aircraft, but enough to feel through the seat.

 

Once you start to get into VRS the loss of ETL rumble is joined by the vibrations associated with putting a lot of power into the transmission because you're trying to stop the developing descent rate. Once it has developed into VRS the controls feel sloppy (like a fixed wing at the incipient stall phase), the aircraft is still rumbling and pitching and yawing and you're going down. When you demonstrate/practice VRS, the entry to it is all fairly gentle, but a couple of times, I've had it be rather nasty with a lot of yaw and pitching - it can potentially be a rather violent thing once you get in. Often the yawing in a Western style helicopter is because the tail rotor becomes less effective due to being inside the Vortices from the main rotor - so that, coupled with a high main rotor torque demand can lead to quite violent yawing also - given the lack of tail rotor on the KA50 - it may well react differently at this point.

 

To be fair, I think the way it is simulated atm in Blackshark is about right in terms of what you see out of the window - I don't really want to see it modelled significantly differently or make it harder/easier to get - it's about right I'd say (having never flown the KA50!). We are missing the seat of the pants feeling of it though but that's not the fault of the sim.

 

As a side note, you get a similar but shorter lived rumble while accelerating through ETL also.

 

r.

[Rangoon]

ryuzu is right at least from my experience as well. The rumbles from ETL and VRS are different occurrences. Transitioning into and out of ETL is the shuddering due to the disc moving into and out of clean air. Once sitting in a zero airspeed OGE hover, outside of ETL, there is another shudder felt with the onset of VRS. It's generally a deeper vibration, but it can really vary from occurrence to occurrence.

[ericinexile]

I disagree with that method you describe there. That would be distracting and disorienting, and would likely contribute to the problem. When I'm in a low-level, edge of the envelope situation, the last thing I want is the sim jerking my view around.

 

I don't really think the sim needs anything. You just need to have situational awareness....

 

 

Couldn't agree more. Too much is made of the nuances of real flight and how enviromental sensations are missing in entertainment fight simulators. With TrackIr, 95 percent of flight is there, on your display and through your speakers. When done right, as it is with Lomac and DCS, the experience can be incredible. The things that kill you in the sim give you the opportunity to learn how to avoid repeating the same mistakes. that's why simulators exist in the first place. Regarding VRS, I still occasionaly encounter it but now recognize it so early that two really small inputs save the situation before things get interesting.

[Wichow]

I disagree with that method you describe...

 

You disagree but maybe a lot of people will find usefully more helps not exactly as I write but in this way. I f you have it you could activate or deactive it, you choose.

For example, do you use F2 key to watch if one Georgian MI-24 is going to joing to the party in campaign mode? ;)

 

In simulator mode You have helps that are not real but helps, just to cover the lacks of the flight simulators, for example external view, F7, not cockpit view etc. You choose to use or not use it...

 

Anyway,I miss a lot of more vibrations and wind sounds in this sort of situations, tail slip, vortex etc.