Ever been flying your drone and find yourself in a situation where your drone is in a free fall and no matter what you do, you can't gain altitude? I'm betting you thought the same thing I did when it happened to me over and over again. “What's wrong with my drone?” “Is it busted?”
You might have thought to yourself that the explanation lies with faulty manufacturing. Or maybe your drone's battery isn't that great because no matter how hard you try to gain altitude, it just keeps descending, even at full power. It turns out, that there isn't anything wrong with your drone at all. This crazy situation can be explained by one thing: science!
This descent into aerial madness can be explained by a little thing called Vortex Ring State (also known as the “wobble of death” or “settling with power”). “Wait, what? That has got to be made up.” Nope, this is not Science Fiction, but Science Fact.
Before we get into what Vortex Ring State is, let's just real quick break down what happens when you fly a drone up and down. When a drone lifts up off the ground and climbs up into the air, it is taking air from above it and pushing it down through its rotors, creating a turbulent air cushion, to achieve a state of lift. Are you with me so far? Great! The more you ascend, the harder it gets to climb and maintain altitude.
So what happens if you then decided to go down? If you descend too fast, you descend into turbulent air beneath the drone, the air that you were once just using to push the drone UP. Once this happens, that turbulent air that was being used to lift your drone, creates a ring around the blades, called the Vortex Ring State. This means that your rotors will stall, and your drone will start to free fall towards the ground. Once this occurs, it's nearly impossible to reverse the situation and gain altitude before hitting the ground. Needless to say, you want to avoid this situation at all costs.
To put it simply, quickly descending straight down can cause your drone to enter vortex ring state and ultimately result in a crash because your drone won't respond when you increase the throttle in the attempt to gain altitude. I can't tell you how many times I crashed my mini-drone because of this and had no idea of the cause.
I bet you're wondering right now “so what am I supposed to do if my drone is in this state?” The best way to avoid it is the obvious: do not to descend too fast. I'll cover descent and landing in a moment, but first I'll address how to recover from vortex ring state.
If you are flying your drone and realize that you've entered this state, try moving forward with forward pitch as fast as you can with full power and hope that it moves out of it (in other words, push forward on both the left and right control stick – this will increase throttle while hopefully steering your drone forward and out of the vortex). By doing this, it creates a bit of a counteraction to the turbulent air around the rotors.
Alright, now that you know what's going on, what can happen when you descend too fast, and how to recover once you've entered it, how's a pilot supposed to avoid this entirely?
You can prevent vortex ring state by slowly descending at an angle instead of coming straight down. Meaning, you can pitch the “nose” up and descend at a longer length rather than straight down, and once you're close to the ground, hover and descend. You could also do it circularly, having it essentially fly down until it's close to the ground, and land.
Picture gliding your drone down sort of like a plane. Notice how when a plane lands, it picks its nose up and glides down. The difference here, of course, is that you're not landing it like a plane, but bringing it down to a low point where you can safely settle it back on the ground.
Next time you see a helicopter flying around your area or in a movie, watch how it moves. If you get the opportunity, watch how it lands. Notice that it doesn't just drop from the sky, it instead either comes around in a circular motion, slowly losing altitude, to a hovering stop a few feet above its destination, or it hovers well above and slowly moves down until it touches the ground.
Hopefully you now know a little something about the vortex ring state, and how best to avoid it. Here is a great video showing the “wobble of death” in action and demonstrating how to escape it: