Weight management and how everything affects everything
The most important thing when it comes to multi-copters is to do a lot of systems engineering. Every component affects multiple other components. There are just so many parameters that it is nearly impossible to design the optimal system without having custom parts (which I don’t). The only way is to find the best possible compromise.
Weight directly affects the most important attributes of a UAV. These are flight time, size, hazard/insurance and maximum possible payload. While a lot of things define the weight, weight in return also defines a lot of the components. You need to make sure that the motor-propeller & battery combination reaches its highest efficiency at the given load. A bigger battery will not always result in more flight time. Bigger batteries result in higher weight and thus cause more load on the motors. If the motor/propeller is not operating in the sweet spot it will draw much higher currents which dramatically reduces flight time.
To get an idea of how much the different components add to the total weight I made a chart showing weights of my first calculations. This must be the first step in development! Note: Some of the weights are estimated (plastics, screws).
To get the weight approximation you need to have an idea about what components you want to use and how the frame should look like. So in the early stage, it is more like a “do it all at once” approach. Once you got that you start refining. This can take a lot of iterations until you get to a good result.
The above chart gives you some idea of what to expect when you want to build an action cam based drone. The weight will be around 1150 g if you try to keep it low. If you decide to use a retail frame the weight might be 200-500 g higher since those are a lot heavier.
The estimated 1200 g of overall weight will be the starting point for the following powertrain calculations.