If you’ve ever lost control of your multirotor during a descent, it’s likely you entered a dangerous phenomenon known as a Vortex Ring State. Even if you haven’t experienced it personally, there’s a good chance you’ve seen one of the countless YouTube videos documenting the poor fate of various multirotors. In order to prevent this from ever happening to you, I’m going to explain how to recognize a vortex ring state, recover from it, and actively avoid it from occurring in the first place.
What is a Vortex Ring State?
A vortex ring state, also known as “settling with power”, can occur when you enter a vertical or near-vertical descent. While descending, your propellers lose the ability to produce lift because they are traveling directly through the turbulent downwash they are creating. As you can see in the picture below, the vortex rings that are created recirculate the turbulent airflow across the propellers. Because turbulent air is not conducive to producing lift, your multirotor will begin to sink rapidly because it has essentially entered into a stall.
How to Recognize VRS
Early recognition is key to preventing a crash. The earliest warning signs include increased vibrations / shuttering / wobbling of your multirotor and an increased sink rate (rate of descent). The worst thing you can do in this situation is try and increase your power output. While increasing power may seem very intuitive, it will actually increase your descent rate by increasing the size of the vortices that are recirculating the turbulent air flow over your propellers.
To recover from a VRS, the objective is to get your multirotor’s props back into clean (non-turbulent) air. To find clean air, you need to get out of the vertical descent by pitching forward or moving laterally to break free from the propellers downwash. If you can imagine a column of turbulent air directly beneath your multirotor, your job is to “punch” through it by pitching forward or rolling any direction in order to escape it. In addition to this, it’s helpful to reduce your power output by decreasing throttle input. If you remember, more power = larger vortices while in the same manner less power = smaller vortices which are more manageable to get out of. In some cases of VRS, the airflow you’re descending through can be so disrupted that you will lose all control of your stick inputs (the multirotor won’t respond to pitch/roll/yaw inputs). This is a late stage and is generally irreversible which is why early recognition is so important! To recap, once you notice that you’ve entered into a VRS, do the following:
- Pitch forward (or roll to either side) to break out of the propellers’ downwash
- Reduce power to decrease the size of the vortices (if the above doesn’t work) and repeat #1
REMEMBER: NEVER INCREASE POWER WHEN IN A VORTEX RING STATE
Prevention is the Best Medicine
To prevent a vortex ring state all you have to remember is to never descend through your multirotor’s downwash. If you choose to descend vertically, you must do so slowly. If you want to descend quickly, do so in a zig-zag or circular/corkscrew pattern. This technique is effective because you’re maintaining forward air speed while avoiding the turbulent air flow created by your propellers. You can be creative with how you descend as long as you are flying ahead of your props downwash and not through it.
Hopefully this post has given you the tools to recognize, recover from, and most importantly avoid ever entering a vortex ring state. Leave some comments to share your own experiences with VRS!