Electronic – accelerometer/gyro combo question

From a control system perspective it really does not matter where you place your sensors. The most important thing is that you know where they actually are!!! I mean this with regards to knowing where the sensor is relative to the centre of the rotary wings. As long as you know the offsets you can apply translations to calculate what your vehicle is doing.

From an electrical and performance perspective away from motor wiring and h bridges, away from vibrations sources and suitably mounted and damped. The dampening is important for the accelerometers to ensure that they do hit mechanical rails from high frequency vibration (do the math to work out the maximum displacement your accelerometer can with stand at various frequencies and you will understand this point). Similar for, but not as important for the gyros.

You can integrate acceleration to obtain velocity data, and you can further integrate velocity to get position. Integration is just the process where you just cumulatively add something up. For example, if you get a new acceleration reading every 0.1 second, you assume the acceleration was constant over the last 0.1 second; you then find the change in velocity over that same time interval by adding 0.1 seconds * acceleration to the current velocity estimate, and repeat this every time a new acceleration reading comes in.

This is what's known as 'dead reckoning', and it has some down sides. Most important is that because you basically are just adding up measurements across time, any errors (e.g., noise) in that data also accumulates, so your approximation of where you are, where you're headed, and how fast you're going all degrade over time.