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Sunday, October 26, 2014

Vortex Ring State (The Wobble of Death)

Has this ever happened to you? You start descending your brand new quadcopter from altitude, with its quite advanced (and expensive) camera and gimbal system. You try to slow the descent by applying some throttle. But instead of slowing, it starts to wobble in air. You apply more throttle to stop the descent, but that just makes it wobble and drop even faster. You're finally at full throttle as it smashes into the ground (or submerges in some lake, river, or ocean). You curse the maker of the quadcopter for selling you such an obviously defective product. But the product was not defective. You've just encountered a common aerodynamic phenomena that is particular to all rotorcraft. 

As mentioned, Vortex Ring State (VRS), aka Power Settling, aka Blade Stall, is common to all rotorcraft. It's encountered when a rotorcraft descends vertically too quickly. From this point on, let's assume that the rotorcraft is a quadcopter. The quadcopter's propeller blades may descend into the turbulent downwash beneath the craft. If this occurs, the blades lose some lift, causing an even faster descent into the downwash. A vortex also starts to form in a circular ring (the Vortex Ring) around the blade's path of rotation. This vortex suck turblent air from beneath the blades to the top of the blades. Applying throttle just increases the vortex ring, eventually causing total loss of lift from the blades. 

This is obviously an extremely dangerous situation to encounter for manned aircraft, such as helicopters and other multirotor aircraft. But there are ways to avoid it, and if it can't be avoided, there are ways to attempt to recover from it. The following video discusses VRS in more detail, along with avoidance and recovery procedures for your quadcopter. 


13 comments:

  1. Replies
    1. small rotors and low weight make them less susceptible, but as long as it has rotors it can enter vrs.

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    2. It has happened to me once. A small, very light, overpowered micro quad smashed in to the ground even when I applied full throttle for at least 20 meters. That throttle would have normally pushed it to orbit :) I was standing close and heard that the motors were operating at max.

      Usually it's just heavy shaking though on very small quads.

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  3. This makes sense even to a novice who knows the basics of 'lift' (pressure under a wing greater than above it) in an airplane wing, kite, or blades of a copter. The 'vortex ring' is turbulence right where your copter blades are ... so to get air under the blades that is not turbulent, glide away to some side (any direction) into undisturbed air. To me, it's somewhat like what you should do in 'undertow' that takes you down the beach. Gradually glide with it to save energy and think straight but to an angle to get out of the water current 'stream' that is towing you away. I hope this helps.

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  4. Looking into this a bit more, I found that rotating blades are just one of many things that can produce Vortex Rings. Sea mammals, volcanos, humans, cannons, etc. can also make them. Here's an interesting video titled: Extraordinary Toroidal Vortices (aka Vortex Rings). https://www.youtube.com/watch?v=mHyTOcfF99o
    From what I understand, the only difference is that a rotating blade can produce a sustained vortex.
    More info on Wikipedia: http://en.wikipedia.org/wiki/Vortex_ring
    Thanks for an very interesting chapter.
    Doug in AZ

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  5. Brilliant, that's why that happened to my helicopter! Thank you for sharing that.

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  6. Is this possible on just 1 or 2 props while turning causing a flip instead of a decent?

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    Replies
    1. it can, but shouldn't be very common.
      you'd probably have some very unfortunate wind conditions though, or if you're descending while moving forward at just the right speed to let the "rear" rotors get caught in the turbulence of the "front" rotors.

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  7. Excellent explanation!

    I wanted to ask for permission to copy and translate this video to spanish in order to reach the audience at my country. Not sure if through subtitle or a spanish audio. The credits would go to you and your site as I would only be the translator and if someone wnatsto see any other of your videos and feels fluent at english he could come here and watch any of the other videos. Would this be ok?

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  8. Part of a vortex ring state condition involves upwash near the hub (inner portion) of the rotors. This results in a stalled condition near the hub, and combined with the vortex ring at the outer edge of the props, there's insufficient lift to stop the descent.

    http://en.wikipedia.org/wiki/Vortex_ring_state#Description

    In the case of high power to weight ratio 3d models (or a full scale Sky Crane helicopter), there's enough excess power so that a high powered strong induced downwash at the outer portions of the props combined with viscosity result in the downwash quickly moving inwards, overcoming the upwash, eliminating the stalled condition by literally powering their way out of vortex ring state. This is why high powered 3d models can perform rapid collective transitions without any lateral movement.

    However as pointed out in the video, many models don't have sufficient power to power out of vortex ring state, so lateral motion is needed to reduce the induced downwash related tip vortices so there's enough lift to overcome the stalled condition at the hub of the prop.

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