Hi kararan,
Please be aware that the free plane, the Cessna 172, has only one propeller so it will tend to pull slightly left due to the rotation. You can reset the calibration at any point during a flight by tapping the pause icon (top right corner) and going into the settings screen. The re-calibrate button is there. The app can be pretty sensitive to movement, so it might just take a little getting used to the controls. We are working hard to get a new update out that will make the flight model more realistic.