Losing a propeller in flight is a nightmare situation. If you intend to fly another day you’ve got to think quickly because you only get one go at it.
In this article, we look at causes and solutions. If you have the height you should be able to land when you have an engine failure or lose a prop and If you’re like me your emergency landings are always your finest landings. If, on the other hand, you’re a long way from home then you have few options, but it’s highly likely you’ll be ditching somewhere along the way. In this hobby, it is a certainty that you’re going to have an engine or motor failure. It is possible to reduce the chances but you cannot stop it from happening as there are inherent issues with internal combustion engines but we should do everything within our power to reduce the risk.
Stopping Propeller Coming Loose
Let’s look at each cause and a possible solution.
1. Loose Prop Nut
Running a large heavy prop such as a glass fiber reinforced prop that is designed for a glow or gas motor can be problematic when combined with the brake on an electric motor. It is possible the prop may come loose and this is due to Newton’s First law of motion. Every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. In layman's terms, the prop would prefer to keep spinning and the motor is trying to stop. The resultant force can loosen the prop and nut and you may even see the prop break completely free of the motor. 
One solution may be to apply a thread locking product so that the nut stays in position without breaking free. This is not a permanent bond and one that can be broken should you need to remove the nut to replace the propeller. Another option is to use a lock nut as this can provide enough friction to reduce the chance of the propeller coming loose. If you don’t have a thread lock handy yet you still would like to add a layer of protection, try a little nail polish, plumbers tape, and even dental floss. You can reduce the chance of the brake causing an issue by adjusting the type and force of braking. For example, the YEP line of ESC allows for 5 different brake settings, and of most interest would be the soft setting. https://hobbyking.com/en_us/yep-esc-programming-card.html
2. Losing a Propeller in Flight
If you are losing propellers in flight it is highly likely you're using the wrong type of prop. There are normal rotation (Left hand) and reverse rotation (Right hand) propellers and if you are using a normal rotation prop in a pusher configuration then it's highly likely you’re going to lose a propeller. This was a common issue in the early days of multirotors when props would fly off. 
When force is applied to a normal rotation prop in a pusher configuration it will naturally loosen the prop and most likely fall off completely. When force is applied to a reverse rotation prop in a pusher configuration it will naturally tighten the prop and even whilst braking. The more force and breaking the tighter the nut and prop. The only solution here is to use a reverse rotation prop.
3. Propeller Collet Adapter Becomes Loose
There are two issues that could be at play. The motor shaft is too smooth and is allowing the collet adapter to slip on the shaft. The collet adapter is too large and cannot get adequate purchase on the shaft. If the shaft is too smooth you can rough the shaft up with wet and dry sandpaper. 400 grit should be sufficient to score the shaft. Be sure to tape up the motor so that no grit can enter the motor or the bearings as it will destroy the bearings in little to no time. If the collet is too large you can pack the shaft with paper to see if that is sufficient to provide enough fill. Start by wrapping the paper neatly around so that there is just enough paper so the collection can still slide onto the shaft without the paper bunching up. Best of Luck and Happy Landings. Written by Gozarian
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