Differential thrust is not a good way to control the yaw of an airplane. For one thing thrust control has to be extremely quick, if it takes even a second between a control input and the thrust response then you won't have adequate control over the airplane. For another you are entirely dependent on having engine power available for stability, if you lose your electrical system or your battery goes dead then so are you!
Another reason differential thrust is not a good idea is that at full power the only way to create differential thrust is to reduce power on one side. If I'm on climbout and I need to yaw I will lose climbing power, this means I would have to reduce my flight path angle to maintain airspeed. If I'm close to stall speed a sudden reduction in power could make me enough airspeed to stall. Not good. The only way to really mitigate against this is to increase motor power and restrict the maximum power a pilot can get. This is extra weight and cost.
Lastly, the X-57 has the engines all down the wings, this means that when the engines are going they will be producing airflow over the wing. Differential thrust means that the engines are forcing more air over one wing than the other, the wing getting more airflow will create more lift, this has the following undesirable characteristics:
- A yaw input will have to be coordinated with reverse aileron to maintain level flight, a strong yaw input would need a stronger aileron change. This means badly coordinated controls and would not be very intuitive
- At high angles of attack, for instance close to stall speed, sudden reduction of thrust over one wing could stall it while the other wing keeps flying, this would lead to a snap-roll and possible loss of control. In short it's dangerous.
Comparing quadcopter technologies to the technology going onto the X-57 is a bit of a misnomer:
- Quadcopter motors are small and the blades are small compared to what will be used on the X-57, the ones on an airplane will be much bigger and will have a much larger response time as a result due to the longer moment arm
- A quad has a higher thrust to weight ratio than the X-57 will, so less power available for differential thrust