Many FPV pilots run into the issue of their drone motors spinning faster and faster by themselves.
Most of the time, this happens when testing motors after you build a drone, or before a flight with the propellers off.
All of a sudden, the motors start spinning faster and faster. They stop when you disarm the drone, but whenever you arm it, they start spinning faster again.
There’s no need to worry, your drone isn’t going to get damaged and nothing is wrong.
How a Flight Controller Works
To understand why your motors are spinning up, first we need to understand how a flight controller works on a basic level.
A flight controller runs something called a feedback loop, that measures proportional, integral, and derivative gain. This is commonly referred to as a PID loop.
In other words, the flight controller measures position, and based on the position and how often it updates, tells the motors what to do.
If a gust of wind hits the back of your drone and pushes it down, the flight controller senses a position change, and tells the motors how much power to use to bring the back of your drone back up to where it was.
Your flight controller does that calculation, predicts where the drone is going to be, how fast it will move, and many more calculations thousands of times per second.
Why Drone Motors Spin Up Faster
Without propellers on your drone, there’s no way for the commands that the flight controller gives the motors to actually move the drone.
Drones are very capable and notice the slightest bump, and try to correct its position. With propellers on, the drone would fix its position so fast that you probably wouldn’t be able to notice it.
However, when the drone doesn’t have propellers on it thinks it’s commands aren’t working.
It’ll keep trying, and send more and more power to the motors to try to get the drone back in its original position but without propellers, the drone obviously can’t obey the instructions, so the motors just spin faster and faster.
Here’s a quick video showing the motors spinning up.
What if I haven’t bumped my drone?
If a drone is armed and doesn’t move at all, the motors will spin at a stable speed. If you haven’t bumped or moved your drone, why is it
The sensors used in your drone have to control a drone going 100mph through turbulent air, so they need to be super sensitive to notice the smallest amount of drift. Even walking past your table can sometimes be enough for the drone to sense a tiny bit of movement.
Sadly, drones can’t be 100% perfect and the sensors aren’t always accurate, and can drift.
When the drone’s flying, it can usually correct a little bit of sensor error but if it’s sitting on a table it won’t be able to correct it, and the motors will spin up.
Fix: Disable Airmode
Brushless motors usually need to be running all the time, so when you arm your drone the throttle is still technically at 0%.
In normal acro mode, the feedback loop doesn’t run at 0%, and only starts once you apply some throttle and start flying.
However, since the feedback loop isn’t running at 0%, whenever you lower your throttle in the air you can start to notice the drone drifting around, and feeling not locked-in.
To counteract this drift, there’s a setting called airmode that is usually turned on in Betaflight. This setting allows your PID loop to run all the time, even at 0% throttle to give you a nice and controlled flight even if you’re just dropping straight down!
When your drone is armed and the motors spin faster and faster, that means that airmode is turned on, and the PID loop is running at 0% throttle. If you don’t want your motors to spin up without propellers, you can turn airmode off.
Will it Damage my Drone?
Even though your drone motors can get spinning pretty fast, without a prop on there’s no load on the motor or the ESC, and running even for a couple minutes shouldn’t hurt it.
When you’re testing your motors before a flight and the motors spin up, it’s not going to hurt it whatsoever.
Is it a good idea to leave your motor spinning at max RPMs for a long time? Probably not.
Arming your drone without propellers is like trying to do math with a calculator that always gives you random numbers.
It’s a little confusing, worrisome, and a bit goofy but it won’t hurt anything.
Let us know if you’ve had this happen to you in the comments! We’re more than happy to answer any questions.