VRS innacuracy?

[Vortex]

I was playing around with wind speeds set at 25m/s and turbulence set at maximum (6m/s) after reading this thread http://forums.eagle.ru/showthread.php?t=35978

 

Now it's been bugging me that it's a little easy to enter VRS, given my ex military instructors said that sometimes they simply could not demonstrate it when they tried.

 

But one thing is for sure it's very odd that with an air speed of at least 36 knots I can enter VRS. I had zero ground speed at the time so can anyone from ED confirm that the VRS model is not based on AS but GS?

[Yo-Yo]

I am not sure if the VRS diagrams was inserted to the DCS manual... I think Chizh can ckear up this item. VRS diagram show the Vx and Vy (pay attention please at the subsripts - x and y in local helicopter axes). If rotor AoA is small you can consider vx = AS (airspeed?) and vy = VS but if you try to decelerate hard your vx will be built up for

 

AS*sin (AoA) (It's for rough estimation - the accurate formulas are well known for axes rotation).

 

DCS:BS is matched for the VRS diagrams so if you VS is lrss than 5-6 m/s you never get VRS.

[Vortex]

Yep there is no VRS diagrams in the English manual, and surprisingly also no H/V diagram either.

 

I understand the aerodynamics behind VRS and have had training entering and exiting it. It just seems odd that I entered VRS at an air speed of over 36 knots, which shouldn't happen.

Edited December 31, 2008 by Vortex

[Aser]

I was thinking the same about VRS, too easy, I didn't look at the numbers but in any real aircraft you need to be in a descent near the air downwash speed and below translational speed to enter fully developed vrs.

The magical number of less than 300'/min. used worldwide in the schools is just too far in the safe side, another myth like LTE, usually overpitching (no enough power to hover) is confused as VRS.

I'd would like to see the numbers if anyone can provide.

Also a performance section in the manual would be nice even just a scan from the real manual.

Anyway BS is like nothing before as helicopter simulators.

 

Regards

Aser

[Acedy]

I am not sure if the VRS diagrams was inserted to the DCS manual... I

 

Yep there is no VRS diagrams in the English manual, and surprisingly also no H/V diagram either.

 

Both diagrams will be in the extended manual.

[bradmick]

Anytime you're operating in an enviroment where these conditions occur simultaneously you're sitting real likely to (>90%) get into this condition:

 

A vertical or near vertical descent of at least 300 feet per minute. Actual critical rate depends on gross weight, rotor RPM, density altitude, and other pertinent factors

 

Slow forward airspeed (less than ETL), the reason being you're still very much in that prime zone for recylcying your own rotor downwash. Once you're through ETL the rotor disc is operating in clean, undisturbed air.

 

Rotor system must be using 20 to 100 percent of the available engine power with insufficient power remaining to arrest the descent. Low rotor RPM could aggravate this. The italicized portion there is the kicker that most folks forget about (and what IP's like to nail you on check rides for). If you don't have the power to stop that descent, and you're falling even faster get some kind of airspeed and fly out of it. It can be forward, rearward, sideward, whatever just as long as you avoid obstacles and get you're but out of that column of nasty air.

 

Now, all that being said, if you're out there flying along and the enemy takes some shots at you to which you respond with a sharp bank in the opposite direction and a dive to the deck for speed and cover, and you maneuver yourself outside of the threat radius and find that perfect little hidey hole, it is possible to do a rapid decel to a hover and not get yourself into this situation. My usual trick is this: First, I know what my hover power requirements are. In the BS I tend to go off of the blade pitch indicator because its the most like Torque to me. I know that for an OGE hover I usually have to have around 9-10 degrees of blade pitch or so. I lead my deceleration with an aggressive drop in collective as I aggressively pull aft on the cyclic, as the airspeed begins to bleed off and the first signs of a descent come in I bring the power back in to catch it, usually this is followed by the nose wanting to drop as well. As the airspeed continues to bleed off, the nost is gradually going to want to settle (in the aircraft you'd actually feel it, but you just have to go off of visual cues here). Around 80 km/h or so you should be at about 75% towards your goal of the required blade pitch angle and still sitting at a 0 feet per minute rate of descent. If you have a positive one, that's okay so long as you're not balooning yourself. By the time you reach 0 km/h you should have the full required blade pitch for and OGE hover and shouldn't have gained or lost a bit of altitude. It's all about being smooth, controlled, and staying ahead of the aircraft. You have to meet that power requirement prior to even thinking about stabilizing yourself or fly out of it, keep slow, and try a less aggressive maneuver.

 

Hope that helps.

 

Brad

[Vortex]

Thanks for taking the effort to make the post, but I think you missed my point or didn't read my posts.

 

I already know about VRS. What I'm talking about is that in a hover over an airfield (zero ground speed) with a 25m/s wind and gusts +-6m/s I was able to get into VRS when descending 2m/s with power applied.

 

By my calculations I only had 2 out 3 conditions for VRS.

[RvETito]

Could you post a track?

[Vortex]

Simply do what I did after reading shrubbo's post. Create a wind of 25m/s and gusts at their maximum. Then try and hover over the runway with all the autopilot aids switched off.

 

I can re-create it everytime.

 

edit, okay here is a quick and dirty track

 

http://www.filefactory.com/file/aff3g83/n/Vortex_ring_state_trk

 

 

At first I wondered if it was simply overpitching, but notice how I only go from zero to 1/3 pitch just before I enter VRS. My ROD goes from 2m/s to 8m/s in 50 foot.

Edited December 31, 2008 by Vortex

[bradmick]

Sorry about that, I just realized I meant to post this elsewhere. Got click happy and didn't pay attention.

 

Brad

[Sinner6]

Remember, the speed indicators of the Ka-50 are in km/h, not knots. A signifiicant difference that may be influencing your observations...

[Vortex]

There is no observations to be made. The wind is set at a such a speed the KA50 is damn near tipping over while sitting on the ground.

[crazysundog]

Perhaps the reason is that the wind is perpendicular to your aircraft/you are not facing the wind. If your track is set up the same as Shrubbo's, and the wind is moving to your right side (east if you are facing north) and you are pulling a lot of power and dropping it back off...when you pull that power your blades still go through gyroscopic precession; do they not??

 

Maybe an experienced RL rotary wing pilot could explain better...

 

This is a curiosity, I can hardly wait for an informative answer!

[Shrubbo]

NAh I think I am still too hung over to understand much of this atm hah

[Vortex]

Perhaps the reason is that the wind is perpendicular to your aircraft/you are not facing the wind. If your track is set up the same as Shrubbo's, and the wind is moving to your right side (east if you are facing north) and you are pulling a lot of power and dropping it back off...when you pull that power your blades still go through gyroscopic precession; do they not??

 

Maybe an experienced RL rotary wing pilot could explain better...

 

This is a curiosity, I can hardly wait for an informative answer!

 

It shouldn't matter what direction you are facing, airspeed is airspeed. You can escape VRS in any direction, left, right or backwards, it doesn't matter.

 

Anyone looked at the track yet?

[bradmick]

As I think about this, the only thing I can think of is this, and I came to this conclusion after looking at the airflow patterns in a hover diagrams for IGE/OGE. While airspeed is airspeed, sure, at a hover the induce flow velocity is far greater than it would be in actual forward flight, as you begin to pick up more and more airspeed, you require less power to keep the aircraft aloft because the rotor system is becoming more efficient. Any time the wind is anything but directly off your nose, you're still going to require a significant amount of power to keep it aloft, so the vortices aren't exactly being blown as they would with you being in forward flight. The aircraft is still operating in largely disturbed air. A way to test this, pick the aircraft up to a hover on a no wind day and see where you're holding for power/blade pitch. Next, put about a 24-30 knot head wind in, and see if the power requirement is lessened. It should be. I know my dad was out flying gunnery one night and had winds in excess of 24 knots, he pulled up to a hover and was registering 24 knots (outside of ETL and was requiring something around 30-ish percent torque to hover, typically a loaded out 64 will hover around 75-ish percent). So...yeah. My rambling thoughts (shouldn't do this at 1:20 in the a.m.)

 

Brad

[Vortex]

See that is what I mean, it's not something that is easily explained and should not happen in the real world as far as I know. I've escaped from VRS (sure, it was only an R22) going sideways and backwards while training.

 

I greatly appreciate the guys from ED for having a look at my track. It's what makes ED such a great company :thumbup:

 

If it's something oddly specific to the KA50 and co-axial design it would be great to know what it is. But I have a feeling winds at 25m/s and gusts at their max simply stretch the physics engine too far and something odd is happening.

[Shrubbo]

Just had a look at your trak, interesting. Disregard what I said about loading up the blades too much, that is something different and certainly appears to be VRS or what the sim thinks is VRS at any rate. You fell out of the sky lol.

 

I have not encoutered that before as I have only used that mission type to hover around in ground effect area.

[Groove]

Wild guess here but is it possible that co-axial rotor configurations are much more VRS "sensitive" than single rotor configurations?

[GGTharos]

Patience Vortex, Yo-Yo will get around to it. They have big holidays for new year's in Russia though so it might be some time :)

[Vortex]

Actually I am impressed by the fact I've gotten response's from you guys, let alone over the holiday's.

 

Great game, and great support from you guys :thumbup:

 

I'm happy to wait and if you guy's go missing for the next few days it won't bother me.

 

I just thought I'd draw attention to something I thought was odd, so if it is an issue, it can be identified and allocated some time to fix for the new engine upgrade.

[GGTharos]

No problem, the devs are glad to examine such things and fix them up if an issue exists :)

[RvETito]

Well first of all at 25 m/s you won't get a take-off clearance as it's beyond helicopter's wind limitations:P

[Shrubbo]

Yah true but that didn't stop me and now Vortex has found something odd:music_whistling:

 

or has he? Seems odd.

[GGTharos]

For the patch we'll just have your heli automatically explode if you try to do such silly things. :P