Many things can contribute to the drone loosing control and having an unstable flight. If you calibrate the drone properly (See this guide) and fly in good conditions it should be difficult to loose control. I describe 2 common scenarios that can occur when flying the H109s.
Circle of Death
When CoD happens the drone will start to move in circles, as it progresses the circles will become bigger and bigger. As they become bigger the drone will loose altitude and become more and more unstable. It can happen at any time and many things can increase the chances of it happening. It happens because the drone is trying to stabilise its self but can’t because of either the conditions (wind, etc) or because its getting conflicting data from the flight control sensors such as a un-calibrated compass. If you have ever tuned a PID algorithm you can get the same symptoms if one or two of the parameters are out.
To regain control put the drone into altitude hold mode, push up and to the right on both sticks putting the drone into a corkscrew climb. Once control has been regained land the drone (in either Alt or GPS mode depending on if it will stabilise in GPS mode) to see what caused the failure. It can happen if the drone is not flat when power is applied or if the compass calibration is not carried out properly. To stop this from happening ensure that you start the drone and calibrate the compass properly. Check the compass readings before flying the drone. Go to our startup and compass calibration guide for more information. If it happens regularly then the FC, magnetometer or GPS receiver could be unserviceable.
CoD has happened to me twice in windy conditions and where I didn’t (due to inexperience) calibrate the compass properly the first time I crashed and the second time I regained control by using the above technique. Be aware that magnetic anomalies occur around ground that contains a high metal content. Ensure that you calibrate the drone away from metal structures such as ISO containers or metal masts. If you get noisy data back from the sensors the chance of this happening is increased.
Toilet Bowling / Vortex Ring State
This is bad, when this happens if you don’t regain control the drone will crash. Toilet bowling or vortex ring state as its known is when one or more props stall and the drone tries to stabilise itself. It can be caused by overly windy conditions, prop wash, a loose prop or a slow running motor.
If the drone is falling vertically due to prop stall then increasing the power may make the problem worse (compare it to a car skidding more brake will make the skid worse). In this scenario you should put the drone into altitude hold mode, leave the throttle stick and move either forwards or backwards to try and get out of the dirty air. Once you have moved away from the dirty air give the drone more throttle to gain altitude.
If this happens you will have seconds to react; acceleration in free air is around 9.8 meters per second so if your 50 meters up you’ve got less than 10 seconds before your drone is in the dirt.
Using carbon fibre or fibreglass props will lessen the chances of PSVRS occurring as they are stiffer than the stock plastic props and less prone to stalling.
You could land at an angle rather than vertically to decrease the chance of the props stalling due to dirty air.
Videos of drones loosing control
Not sure what caused this drone to flip but it looks nasty. The parachute deployed and probably caused more issues. The drone would have been too low for the parachute to of had any affect.
This is a classic case of CoD;
The below video describes VRS and what to do if it occurs;
Please, Please, Please be aware of VRS and the fact that drones are inherently unstable by design. With this in mind don’t fly irresponsibly. If you do and crash your drone there is a chance that you could do some serious damage that you will be liable for. Always check the laws where your flying and never take any risks like flying over motorways or populated areas.