VRS innacuracy?

[Shrubbo]

Heh you think that is silly? 25m/s is about the maximum speed I can have it set for and keep it on the ground with only minimal inputs... comparatively. In my earlier versions there was one where it was much higher and you instantly got blown off the ground and had to go to full power just to hold position.:thumbup:

[RvETito]

Without underestimating Vortex'es observations (which I can not test right now), no one is going to try to take-off IRL in those conditions ;)

[Shrubbo]

You clearly underestimate my stupidity :)

[Chizh]

Thanks to all.

Bug is written down.

Edited January 3, 2009 by Chizh

[Yo-Yo]

Copy of PM I've sent to Vortex. :)

 

I am very sorry, but I am on holiday now and can not run BS on my computer. That's why I could not understand the point of your message. If the issue is that you have VRS while hovering with the strong wind it's a certain bug so it will be fixed in the patch and I can only send you warm thanks for the work our testers had to do.

[Reticuli]

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?

 

That's an interesting theory that it's accidentally tied to ground speed instead of airspeed right now. I also find the VRS happening with too much non-vertical motion present and that's not even including any wind...not to mention what I said previously about there seeming to be too much cyclic authority when it does hit. The latter is really hard to "know" for sure, afterall it's a relative thing as to how sufficiently unresponsive a control is becoming using a joystick. The speed thing seems pretty obvious, though. Often when I'm rapidly decelerating by pulling up and dropping collective VRS will hit, even though I haven't even come anywhere close to a stop. So I pretty much just forgo the attempt to keep my altitude level and just accept an increase just to be safe. So much for rapid landings, right?

[Reticuli]

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!

 

Vortex is not complaining about the helo almost tipping over on the ground, I think. Hell, we all love that! Rather that he turned the wind up so high that's what was happening. Right? I think he's just illustrating the point of how high the wind was during one of the tests. And yet VRS still occured in the air...and not just because he was flying downwind at wind speed, either.

[Shrubbo]

Thanks to all.

Bug is written down.

 

:thumbup:

[Vortex]

Thanks for the response. I hope it's an easy fix so you guys can get on with other things.

[geogob]

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

 

Strangely, I would have thought that co-axial rotors would be less likely to enter VRS situations because the wake of one rotor influences the other. At worse, only one of the two rotors would enter VRS, causing a slight loss of lift and loss of yaw stabilization. But that just a guess following my (limited) understanding of VRS. I should take the time to read more on rotary aircraft flying.

[GGTharos]

Yep, your understanding is not correct ... the rotors will both descend through the air at the same speed (being attached to the same heli) so they will fall into vrs more or less together.

 

However, it IS more tolerant - a single rotor heli would enter VRS even faster.

[geogob]

GGTharos, do you have any good reading reference on the subject? I'd like to know more and understand the physics correctly.

 

The way I saw it was that the counter-rotating rotors would create counter-rotating vortex (vortexes?) that would eventually interfere and get disrupted below the lower rotor. So only the top rotor would ever enter a "stable" vortex. Heck, I barely understand VRS with a single rotor... with two rotors, it's way over my head.

 

Reading time :P

[GGTharos]

I don't have a handy one, but basically think of it this way.

The center of your disc spins so slow it doesn't generate lift. As you descent, air flows upwards (instead of downwards) through it - as you descend faster, the area of this upward flow expands until it encompasses your entire rotor - so now your rotor is generating no lift at all.

[geogob]

aw! It's that simple. Thanks for the explanation. Makes sens.

[Weta43]

Not something I know anything about, but I’ve never let that stop me before :-)

 

Seems to me that if the air actually started moving up through the rotor disk – wouldn’t you get more lift not less (like when you lift the nose & increase the angle of attack – more lift = greater load on the rotor, which is why the RPM drops) ?

 

I thought that the problem was that because you have become entrained in a column (the center of the donut) of air that is moving quickly down, you were moving upwards relative to the column of air you were in, but that the whole mass was moving downwards fast enough that your movement relative to the ground was still down ?, and that because you were falling with the column, attempts to climb out of it that weren’t powerful enough just sped the column up & exaggerated the effect ( that and perhaps a loss of efficiency in the lift generated specifically because the airflow is down over parts of the rotor disk – so that there is lower pressure underneath parts of the rotor blade than above it?

 

I'll go & read the manual ...

Edited January 6, 2009 by Weta43

[Reticuli]

GGTharos, do you have any good reading reference on the subject? I'd like to know more and understand the physics correctly.

 

The way I saw it was that the counter-rotating rotors would create counter-rotating vortex (vortexes?) that would eventually interfere and get disrupted below the lower rotor. So only the top rotor would ever enter a "stable" vortex. Heck, I barely understand VRS with a single rotor... with two rotors, it's way over my head.

 

Reading time :P

 

The vortex in VRS are not the micro-vortexes of each individual blade (picturing science shows and micro vortexes of beating wings and dolphin tails amplifying lift), but rather the larger vortex complex that is created like a doughnut rotating airflow outward and then directly back into the center. So it self-sustains itself in a feedback and increasingly the more energy you put into it, the more it just grows, eating up area of the rotor disc, rather than that air pushing against the surrounding air medium...so you suddenly start losing lift. And increasing collective makes it worse. You have to lower collective and build up some kind of lateral (non-vertical) speed to interrupt it and start producing good lift again. It reminds me in old Sunday school of that idiotic admonition against becoming a "law unto yourself". Sinner air, sinner air!

[Aser]

I attach a doc about VRS. :book:

 

Regards

Aser

VRS.pdf

[geogob]

Very nice! Thanks for the PDF, Aser.