Free Glider Weight and Balance Calculator

1. What is centre of gravity in a glider?

The centre of gravity, usually abbreviated to CG, is the point at which the complete mass of the glider can be considered to act. For weight-and-balance purposes, every individual mass is combined with its distance from a manufacturer-defined reference plane. That distance is the lever arm. Multiplying mass by lever arm gives a moment.

In flight, CG position affects the relationship between the wing, tailplane and control surfaces. The manufacturer therefore specifies an approved CG range. The aircraft must remain inside that range for the loading condition in question.

2. Why CG position matters

CG position affects longitudinal stability, control forces, trim, low-speed behaviour and spinning characteristics. Even a relatively small change in pilot mass, water ballast or trim ballast can alter the handling of a glider.

3. TrimCalc in practice

TrimCalc provides an immediate and straightforward way to calculate weight and balance. The aircraft-specific information is saved once as a profile. Before flight, only the current masses need to be entered.

• calculate CG and total mass immediately
• display wing loading
• save profiles for several gliders
• share profiles by QR code or file
• load community profiles and verify them against the official documents
• use stored profiles offline
• operate the app in English, German or French

4. Reference plane and lever arm

The lever arm is the distance between a mass and the reference plane defined by the manufacturer. The reference plane may be located ahead of the nose, at a structural point on the fuselage or at another specified location.

Lever arms must never be estimated or copied from a similar glider. They must come from the official documentation for the individual aircraft, such as the flight manual, maintenance manual or weighing report.

5. Weight-and-balance formula

First calculate the moment for every individual mass:

Then add all moments and divide by the total mass:

The calculated value must then be compared with the approved CG limits and maximum permitted mass.

6. Which data are required?

7. Worked example

The following simplified example demonstrates the calculation method only. It must not be used as aircraft data.

Total mass: 448 kg
Total moment: 160.72 kgm

The calculated CG is 0.359 m. Whether that value is permitted depends entirely on the approved limits for the individual glider.

8. Understanding the loading envelope

Many flight manuals use a table or graph rather than a single forward and aft value. The loading envelope shows whether a combination of total mass and CG position is approved. A CG value may look acceptable on its own but still be invalid if the maximum permitted mass is exceeded.

9. Optimum CG range and glide performance

Within the approved limits, many gliders have a range in which they feel particularly well balanced and efficient. This is often described in practice as the optimum CG range.

A forward CG can increase control forces and trim drag. A more favourable CG position further aft may reduce trim drag and improve glide performance, provided it remains within the approved limits.

In practical operation, the correct CG position can improve the glide ratio by more than one point. The exact benefit depends on the glider type, speed, trim setting and flight configuration.

10. Water ballast and CG

Water ballast increases total mass and may alter CG position. The effect depends on the tank locations and lever arms specified by the manufacturer.

Every ballast configuration must be checked for both CG and maximum take-off mass. A loading condition can be inside the CG range and still be overweight.

11. Trim ballast, oxygen, batteries and additional masses

Trim ballast is often used to accommodate different pilot masses. Oxygen systems, batteries, loggers, chargers, brackets and other installed equipment can also affect CG.

Small masses far forward or aft may have a greater effect than larger masses close to the CG. After installation, removal, repair or reweighing, the saved profile should be checked and updated.

12. Flying clubs and QR codes

In club operation, gliders are flown by pilots and instructors with very different body masses. The loading therefore changes frequently, and the calculation often has to be repeated.

TrimCalc allows each glider to have its own profile. The profile can be displayed as a QR code and placed in the cockpit, on the instrument panel or with the aircraft documents.

After scanning, the aircraft-specific data are loaded automatically. Only the current masses then need to be entered.

13. Sharing profiles by file or QR code

TrimCalc profiles can be exported, sent by email or messenger and imported on another device. Profiles can also be displayed as QR codes and scanned directly with TrimCalc.

Every imported profile must be checked against the official documents for the individual aircraft before use.

15. Common weight-and-balance errors

• using the wrong aircraft profile
• copying data from another aircraft of the same type
• mixing millimetres and metres
• using an incorrect sign convention
• estimating pilot mass inaccurately
• forgetting the second occupant, water ballast or baggage
• entering trim ballast in the wrong position
• forgetting oxygen, batteries or installed equipment
• failing to update the profile after modifications
• checking CG but not maximum take-off mass
• using a community profile without verification