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Vortex Ring State (Stall); A video demo and explanation

Discussion in 'General Discussion' started by StumbleBee, Aug 27, 2015.

  1. StumbleBee

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    Here's a video I found by a knowledgeable and skilled pilot. He demonstrates VRS, what causes it and how to avoid it. Most remarkable is his demonstration of how to get out of it if it happens to you. I knew about fixed wing stall and didn't fully realize that VRS is the copter version of stall. If the acronym's meaning were changed to Vortex Ring Stall it would be more apparent.
    I've read on this forum that it isn't a big problem with P3's, or maybe not even a problem at all. But it was good tech information and I think anyone who likes to know how our birds 'fly' would be interested.
    Edit: Darn! I just 'searched' the specific video URL and I see that this video has been mentioned before. Previously I had searched VRS and not seen it. Sorry for the rehash.

     
  2. Bob Denny

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    Well it's like a stall (separation of flow around an airfoil) but it is not stalling the rotor blades on the quad. The problem is that the air flowing out below the rotor disk is being forced right around the outside of the rotor disk and is being sucked right back into the top. Thrust comes from accelerating and moving mass (air) through the rotor disk. When the mass is simply circulating around in a circle, the rotor isn't acting against anything new, and isn't producing thrust.

    It is the excessive descent rate through the air that forces the down flow to turn sharply around and up, only to be sucked right back into the rotor disk, beginning the vicious cycle.

    It can only happen in a vertical or near vertical descent. So to get out of it you have to achieve some horizontal velocity and leave the vortex behind. It will cost you altitude (like a stall recovery).

    PS to be more precise, a second vortex ring forms around the rotor drive shaft/mast from the same thing, air rushing upward through the center of the rotor disk and getting sucked back in to the disk outward of the center.
     
    #2 Bob Denny, Aug 27, 2015
    Last edited: Aug 27, 2015
  3. LUISMARTINEZ

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    I just descend slowly, never in a hurry, unless a mean old chopper pilot is trying to crash me.
     
  4. Bob Denny

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    Luis is right, better to prevent the problem!
     
  5. Ezookiel

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    I almost never descend straight down, I always have some directional motion, either rocking backwards and then forwards as I come down, or I come down in reasonable size "spiral" so that the bird is always moving out of it's own prop-wash as it descends. So far I've never had more than a tiny bit of wobble develop, and never been unable to stop the descent.
     
  6. Meta4

    Meta4 Moderator
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    DJI's designers have had to work this out because the Phantoms get sold to all kinds of pilots.
    Originally the P2 series could descend at 6 metres/sec and VRS was possible.
    Later firmware upgrades slowed this to 3 m/s and then 2 m/s and VRS incidents disappeared.
    The P3 has offset motors which reduce the potential and the designers allow the P3 to descend 50% faster than the P2 at 3 m/s.
    No VRS incidents have been reported for the P3 and it is quite safe to descend at full left stick down in still air.
    But you still hear plenty of pilots afraid to try it.
    DJI have done the thinking for you - there's no need to come down slowly.
     
  7. StumbleBee

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    Since I like to be ready for any emergency I'll give that a go. I have some tall grass and some blankets I can spread around, just in case. It would be good to know I can safely land full stick straight down if I were in a dicey situation. I'm thinking rapid battery power loss, bird attack, zombies, all the usual.

    PS Meta4 (like the name!), you have some very nice photos on your website. Good job.
     
    #7 StumbleBee, Aug 28, 2015
    Last edited: Aug 28, 2015
  8. Meta4

    Meta4 Moderator
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    No need to do the "just in case" stuff.
    Plenty of users have tried it and use it routinely.
    I have tested in very still air from a significant height multiple times.
    The designers are pretty smart and they got it right.
     
  9. StumbleBee

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    OK then, thanks. I will admit to having been overly cautious on landings since I can't spiral in, there are too many obstacles.