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Towards the end of a flight with a quadcopter, I started a vertical descent to come in and land. There was no wind on that particular day so I just came straight down.

Shortly after starting to descend, the quadcopter started falling faster. I increased the throttle to compensate, but it only seemed to fall faster.

After a rather unceremonious landing, I checked the quad over and found it still had power enough to take off and hover, so I don't think it outright lost power. It came down level with the motors going.

What would have caused the rapid loss of vertical thrust?

14

It sounds like you've encountered what's called "vortex ring state", when a rotorcraft descends into its own wake -- essentially, a self-created downdraft.

There are two ways to escape from such a situation:

  • Power out of it. Climbing out of the craft's wake vortex requires about twice as much power as hovering, so while most full-sized helicopters can't do this, most quadcopters and other small-scale craft can.
  • Move forwards (or backward, or sideways) out of the craft's wake. This is the standard procedure for helicopters.

You can also avoid it entirely by not descending too fast, or by not descending straight down.

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  • I suspect this was the case. Although it is difficult to prove because it all happened so fast. It was an rather under powered brushed quad that it happened to. My initial instinct with descending too rapidly was to just push the throttle further up. In that case it only exasperated the problem due to the lack of power. I've learnt my lesson to never descend rapidly straight down! – Daniel Ballinger Apr 14 at 21:51
  • This is much more common in low power craft, such as micro brushed quads. The higher the power to weight ratio, the less likely you are to encounter it. Primarily the faster the motors and props can respond to the commands from PID controller (the more authority the quad has) the better it can handle the issue, and power out of it without exceeding the limits of the PID controller. On low power quads that react more slowly the PID controller lacks the authority to correct the unstable state and gets stuck in a control loop that can't react fast enough or with enough power to stabilize. – QuadMcFly Apr 15 at 12:57
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If the battery was low enough, voltage sag during flight could be enough to cause the loss of thrust. When the quad has enough time to sit, the voltage may bounce back enough to take off for a brief period of time. Lipo Batteries are near empty when they are around 3.3v per cell. And to keep from damaging them, they should not be brought down even that far.

Descending directly down quickly into prop-wash may also affect the amount of thrust due to the air being turbulent. Turbulent air may cause the propeller to be less efficient than without turbulence.

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Yes as others have stated, it sounds like a vortex ring state. The copter is descending into its own downward thrust. Try to only descend while moving into 'fresh air', and keep the rate of descent gentle to manage the descending weight inertia of the craft. Never descend vertically. The heavier your craft, the more this can occur and if you're low on power, you could be looking at a disaster. A very real problem for all rotor driven aircraft.

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3

It does sound like a vortex ring it's never a good idea to descend vertically with a rotary aircraft. I have seen one on a large RC Heli I was flying on a misty day the mist formed a perfect wring around the tips so I gently pushed forward to fly out of it with an underpowered quad this could well be what you experienced.

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