The coaxial blades make it fly to the left. You should also notice the nose dipping to the left, especially with agressive forward flight. The top blades don't get exactly the same airflow.

All you have to do to get straight fight is give it a little right aileron when you are moving forward. Pretty easy once you have tried it a few times. You just need to move the stick a little diagonally ( towards about 2 o'clock) instead of just straight forward.