# Centre of Gravity Track (CGT)

To judge how well (or badly!) the competing aeroplane flies each aerobatic figure we use one or other of two equally important criteria:

• Zero Lift Axis (explained on the next three pages)

The Centre of Gravity Track (refer to it as the CGT) is the imaginary line that the aircraft centre of gravity draws as it flies along.

Note that the longitudinal neutral axis of the aircraft (i.e. the Zero Lift Axis) almost always points away from this track (in pitch and/or yaw) by an amount which depends on the control inputs of the pilot, the wind, and the speed of the aeroplane - usually with larger angles at slower flying speeds.

• Imagine the aeroplane condensed into a dot, and watch the path that the dot takes through the sky.
• This is the Flight Path, or Track of the aeroplanes' Centre of Gravity.
• Judging the flight path involves comparing the observed CGT path against fixed references such as the horizon or the 'A' or 'B' axes of the box.

### Example 1

The aeroplane is required to transit from a vertical down-line to horizontal flight. Although the ZLA remains horizontal after the 90° corner has been completed, the CGT continues to descend below the required horizontal line.

### Example 2

Here the transit is from a vertical up-line to normal horizontal flight. The CGT must remain in a level horizontal line, whilst the aircraft speed will increase from very slow to normal / fast.

Tip - 'screw your eyes up' and just concentrate on the movement of the blurred dot in the sky without moving your head.

You can hold a finger up to where the horizontal line started to give you a fixed reference point for the start of the line. A climbing or descending CGT is usually obvious!

## CGT and Angle of Attack

Judges must always look at the CGT and not be "fooled" by a high angle of attack at low speed – this can be particularly noticeable when the aeroplane is inverted, as the nose will look un-naturally high.