For the calculation of the centre of gravity envelope (Center of Gravity Envelope), VFRnav provides two different input modes.
Which mode is used depends on the complexity of the centre of gravity data given in the flight manual.
Simple Mode
For many – in particular two-seat ultralight aircraft or experimental aircraft – the manufacturer only specifies the following limits:
- Maximum takeoff mass
- Forward centre of gravity limit
- Aft centre of gravity limit
These limits must not be undercut or exceeded when loading.
In this case the simple input mode should be selected.
Only the specified limits are entered here, from which VFRnav automatically derives the permissible centre of gravity envelope.
Advanced Mode
For larger and multi-seat aircraft the flight manual often includes a loading diagram (envelope diagram) that shows the permissible weight and centre of gravity limits in relation to each other.
For these cases the advanced input mode is available.
The input is made by transferring the corner points of the envelope diagram in text form.
Each point is specified in the following scheme:
\u003cArm in mm\u003e \u003cGewicht in kg\u003e
- one pair of values per line
- order corresponding to the progression of the diagram
Example: Cessna 172N
Below is a simplified example, based on the centre of gravity data of a Cessna 172N.
The input begins with the forward centre of gravity position in the empty condition:
- Arm: approx. 890 mm aft of the reference datum (upper scale)
- Weight: approx. 680 kg (right-hand scale)
The second point is at:
- Arm: 890 mm
- Weight: approx. 890 kg
This yields the following input values, which are entered line by line into the corresponding text field in VFRnav:
890 680
890 890
Further points of the envelope diagram are then added in the same way until the complete centre of gravity envelope is represented.
Note:
The entered values should be taken exactly from the flight manual.
If the arms and weights in the flight manual are not given in millimetres (mm) and kilograms (kg), they must be converted accordingly before being entered into VFRnav.