Im looking for some placement help for 2 sensors, A gyro and an accelerometer both are going into a helicopter. my guess is to put the gyro and the accelerometer right below the main rotor, getting it as close to the center as possible, but on the rc receiver (has been removed) the gyro was mounted to the front. The data sheets do not give any placement suggestions either. any help would be appreciated.
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.
If the device has a tendency to rotate, I'd try to place them as close to the rotation centre(s) as possible and keep axes aligned to "natural" horizontal/vertical as much as possible.
So for a helicopter, I'd also say anywhere directly below the rotor axle, maybe at the height of the tail. Or wherever is convenient.
It probably mostly only matters to how you intend to use the readings. There may also be differences like the Z axis being less sensitive than X/Y in 3d accelerometer that you may want to consider.