Your VRS Escape Plan

[damoncooper]

This came up in another thread but I think it's important enough to warrant it's own.

While VRS (Vortex Ring State) is most often caused by descending straight down too quickly, and it's true that firmware 3.0.4 limits descent to 2m/s to help prevent this condition, VRS nonetheless continues to claim Phantoms.

If you are a new Phantom pilot and don't know what VRS is or how to avoid it, you may risk losing your investment to a mysterious type of crash that will appear like your quad just dropped from the sky for no good reason.

Wind speed and direction, stick inputs (rapid yaw has been implicated on multiple occasions) and a host of other factors make VRS a possibility even when running 3.0.4. The results are frequently a crash or loss, especially if flying long distance or over water.

I think it's critical that pilots (new and old alike) be able to recognize the early signs of VRS and have a VRS "escape plan" they are prepared to execute immediately.

(I'll update this post based on recommendations/changes suggested).

Recognizing VRS
If flying line of sight, it can look like a "wobble", or oscillations around one or more axis. On FPV, the signs are usually the landing gear oscillating in and out of view and/or uncontrolled yaw movements. The first signs can quickly be following by a rapid uncontrolled descent/drop from the sky.

More on VRS mechanics here: http://www.copters.com/aero/settling.html

Escaping VRS
From advice I've gathered here, at the first signs of VRS something like the following make a good escape plan:

1) Flip to ATTI mode (if in GPS mode).
2) Release throttle.
3) If uncontrolled yaw/rotation is occurring, apply opposite yaw to stabilize heading.
4) Apply elevator or aileron (forward or side) stick to get clean air and try to "punch out" of the vortex. If possible, punch out into or across the wind (upwind or crosswind).

Newcomers to the hobby are usually pretty perplexed and dismayed when they experience VRS and I think it's the responsibility of folks who have suffered through it to help educate and help folks who haven't yet experienced VRS know how to recognize it and escape it when it happens.

Avoiding VRS
Of course, none of this guarantees you WILL escape VRS, so it's highly preferred to avoid VRS altogether if possible. Tips I've gathered from extensive reading on this topic as well as my own sad experience include:

1) Don't fly when wind speed is >13-17mph. DJI is explicit about this in the manual. Beaufort scale 4 for those of you who prefer that sort of thing. Remember that winds aloft can differ greatly from wind speed on the ground. A quick flip to ATTI mode aloft and observing drift or observing the OSD horizon line in GPS mode can help gauge wind speed and direction at your flight altitude.
2) If you aren't on 3.0.4 or later which limits descent speeds to 2m/s, limit your "straight down" descent speed.
3) Maintain lateral movement when descending to keep the quad in "clean air" (zig zag side-to-side, circle, etc).
4) Avoid fast yaw movements where possible and avoid fast yaw combined with descent. You can enter your own prop wash and provide the conditions for VRS.
5) There have been many reports of users with prop guards and heavily laden quads encountering VRS. Many observers appear to agree that using prop guards appears to increase the chances of invoking VRS.

Thoughts, other ideas, corrections, example videos, etc welcome.

Update: DJI apparently is taking VRS losses so seriously they have not only limited descent speed in 3.0.4 and above but they are also about to start shipping a new propeller with greater lift characteristics called the 9450 to help battle VRS as well. More on the new DJI 9450 prop here: viewtopic.php?t=20292

 

[Pull_Up]

Good post. If possible (if you have the awareness) moving upwind or crosswind out of the wash will clear it quicker as the column of turbulent air will be moving downwind.

 

[damoncooper]

Good post. If possible (if you have the awareness) moving upwind or crosswind out of the wash will clear it quicker as the column of turbulent air will be moving downwind.

Noted and added. Thx!

 

[Dadcat]

Avoiding VRS
Of course, none of this guarantees you WILL escape VRS, so it's highly preferred to avoid VRS altogether if possible. Tips I've gathered from extensive reading on this topic as well as my own sad experience include:

1) Don't fly when wind speed is >13-17mph. DJI is explicit about this in the manual. Beaufort scale 4 for those of you who prefer that sort of thing. Remember that winds aloft can differ greatly from wind speed on the ground. A quick flip to ATTI mode aloft and observing drift or observing the OSD horizon line in GPS mode can help gauge wind speed and direction at your flight altitude.

Good post Damon. Thanks for pulling all this together. I didn't know what Vortex Ring State was until I started reading this forum, despite 2 years of flying RC helis and quadcopters. I knew these things would wobble if you descend too fast vertically, but I just thought it was prop wash and didn't know it could have the disasterous effects it can. I must have been lucky. Since I also have a background as a full size airplane pilot, I tend to think of these things from the point of view of the aircraft. Because of that, I have one minor quibble with the paragraph I've quoted here:

DJI's windspeed limitation is not about VRS. The only way I can see that flying with a wind speed above 17 mph would be a cause of VRS is if you are descending while flying in the same direction as the wind at about the same groundspeed as the wind. Descendimg straight down while in GPS position hold would actually be safer in this situation, because the aircraft is moving through the air at that speed. In other words, it has airspeed, which is what matters.

This applies to any descent while flying downwind at any wind speed. For example, if you have flown upwind from home, in a 10 mph wind, you had better not return directly to home at a 10 mph ground speed in a rapid descent. That is effectively a "straight down" descent relative to air from the Phantom's point of view. Your options in this case are to fly much faster, zig-zag across the wind or step down with a vertical descent in GPS mode when close to or at the homepoint. Airspeed is what matters, not the groundspeed the GPS is telling you.

This really reinforces the other points you made in this paragraph: Know the speed and direction of the wind. Putting the aircraft in ATTI to judge windspeed and drift,as you recommend, is great advice. If you can't see the craft, then the OSD horizon combined with the groundspeed reading are your best sources of knowing if you have airspeed. If you're in ATTI mode, just keeping the stick pressed a decent amount should keep it safe, at least from VRS if not from trees.

 

[cosmonaut]

Thanks for the post. I learned the hard way with a broken landing gear.

 

[FLGulf]

@GreenMarine also wrote a great post about VRS. Just scroll below the beginning information and look for:
----------------------------------------------------------
** Aerodynamics lesson for the day - VORTEX RING STATE (Settling with power) **
viewtopic.php?f=29&t=16749#p153247

 

[ElGuano]

My most effective VRS escape plan is a bit of a cop-out to the question asked, and that's to actually plan my flights and know when I expect to be coming back, or descending from height. That gives me some insight into how much time I'll need to descend and keeps me from planning flights in which I find I *need* to be back down within 1 minute, such as coming down from altitude at the end of the flight on the last dregs of battery power. That makes my flights a lot more leisurely, which in turn naturally makes for slower, more controlled descents.

If I ever think I'm getting close to VRS, I do as pull_up suggests and change directions to move up- or cross-wind, without attempting to increase the throttle (aileron change only).

 

[damoncooper]

DJI's windspeed limitation is not about VRS. The only way I can see that flying with a wind speed above 17 mph would be a cause of VRS is if you are descending while flying in the same direction as the wind at about the same groundspeed as the wind. Descendimg straight down while in GPS position hold would actually be safer in this situation, because the aircraft is moving through the air at that speed. In other words, it has airspeed, which is what matters.

This applies to any descent while flying downwind at any wind speed. For example, if you have flown upwind from home, in a 10 mph wind, you had better not return directly to home at a 10 mph ground speed in a rapid descent. That is effectively a "straight down" descent relative to air from the Phantom's point of view. Your options in this case are to fly much faster, zig-zag across the wind or step down with a vertical descent in GPS mode when close to or at the homepoint. Airspeed is what matters, not the groundspeed the GPS is telling you.

Duly noted. I was thinking about the situation where a yaw + fast descent maneuver may put the top or bottom of the props at a high angle to the wind might result in instability. But you're correct: that's not VRS, that's bad juju

 

[damoncooper]

@GreenMarine also wrote a great post about VRS. Just scroll below the beginning information and look for:
----------------------------------------------------------
** Aerodynamics lesson for the day - VORTEX RING STATE (Settling with power) **
viewtopic.php?f=29&t=16749#p153247

Awesome. Thanks for posting that.

 

[damoncooper]

Updated with link to VRS mechanics

 

[damoncooper]

Updated to add link to the 9450 prop info as well

 

[Original poster]

Good post.

I wasn't aware of the symptoms of VRS so definitely worth knowing how to beat it if it starts to happen!

 

[J.James]

Great post. I recently had to do the above on the fly when I had a very bad VRS occurrence right after switching to vision props on my fC40 and not having the gains all set just right. It was pretty hairy as it was just wobbling side to side and dropping like a rock from a few 100 feet up and not able to control it at all. and only by sheer luck by trying any thing I could to regain control I was able to pull out of it by going forward and flying back out with about 20 feet to spare before it would of been smashed on the pavement. then it was still hard to land because it would start rocking from side to side again when I would try to descend but was able to just bump it down a few feet at a time very slowly till I had her down safely on the ground. Even if I tried to let it land its self in fail safe it would start doing it. But if I were still a newbie flyer I'm not so sure I would of been able to on the fly get out of it and would of just had to helplessly watch my phantom bite the pavement. I could also picture some people just panicking and freezing up and just saying oh **** oh **** oh **** and bang.

 

[gunslinger]

Great post. Thanks for taking the time. It's now a habit of mine to keep moving horizontally in any direction during any decent, unless I'm coming down slowly while filming. Moving upwind makes a lot of sense, though I hadn't considered that until reading this topic, as the dirty air will move away from you quicker. Good point...

I've seen the dreaded wobble on a few occasions when I first started flying a P2V, but was lucky enough to avoid a crash...

Has anyone mentioned prop guards as being a suspect in VRS crashes?

-slinger

 

[Monte55]

I have the Phantom 1 and never decend fast vertically. If I want to come down fast, I do it at an angle and into the wind. Works every time plus it's fun to do.

 

[Bathy]

I fly on 3.04 descent rate is slow but at least it's safe!!! One thing I have noticed though , is when coming down at full pace you can't move in any other direction anyway!

 

[Monte55]

I fly on 3.04 descent rate is slow but at least it's safe!!! One thing I have noticed though , is when coming down at full pace you can't move in any other direction anyway!

What? explain please

 

[MrC]

Holding full left stick down for 3secs+ while flying will initiate landing procedure. It will lock-out movements except the throttle. Raise above 10% throttle to regain movement.

 

[damoncooper]

Bump. Just saw another VRS crash posted

 

[damoncooper]

Updated to add prop guard caution in there under "avoidance"