public void flipBackward() {
        timeline.stop();
        //keyValueFlap = new KeyValue(rotateFlap.angleProperty(), -180, Interpolator.SPLINE(0.5, 0.4, 0.4, 1.0));
        keyValueFlap = new KeyValue(rotateFlap.angleProperty(), -180, Interpolator.EASE_IN);
        keyFrame     = new KeyFrame(Duration.millis(getSkinnable().getFlipTime()), keyValueFlap);
        timeline.getKeyFrames().setAll(keyFrame);
        timeline.play();
    }