Is thrust to weight ratio based on weight of the copter alone or including payload as well?
For example, say I have copter which ways 1000g and carrying a 200g camera. If i want a 2:1 thrust to weight ratio, should I design it so that the thrust is 2400g or 2000g? Is there an official method for which this value relates to?
design for what is useful for the mission. in this case, include the camera.
btw never use arducopter with T/W < 2. arducopter currently is programmed to sacrifice altitude for attitude when T/W < 2, so the copter will just drop during high attitude outputs. this behavior requires code modification to change. this problem is documented here: AP_MotorsMatrix: prevent altitude loss when thrust-to-weight is less than 2 by 117-MasterChief · Pull Request #23512 · ArduPilot/ardupilot · GitHub
Note that if you design for T/W = 2, be sure its at end of mission (to account for battery drain).
1200g of thrust at 50% throttle minimum. Because thrust is not linear with throttle (there is Expo involved) you would aim for higher than that. Buy a subscription to eCalc and run some scenarios. If the result is 50% linear mid-throttle or less it will work.
Note: You may read some criticism of eCalc as not being accurate. They most likely don’t know how to use it (GIGO). I have been using it for years with I don’t remember how many builds and it has always been very close to actual. So much so that I buy the battery last based on it’s weight to meet the desired thrust/weight.