Crabwise · Library
How do you keep moment units and any scale factor consistent?
A 170 lb load at 37 in has a raw moment of 6290 lb-in; typing its chart index 6.29 into the arm field drags the CG from 42.75 to 38.64.
Raw products only
Crabwise computes each station's moment as weight × arm and divides the sum of moments by the sum of weights. It never applies a scale factor. Many aircraft loading graphs instead give a moment index, often moment ÷ 1000, so that numbers stay small. The app has no field for an index: what you type as the arm is multiplied as an arm.
Use pounds with inches or kilograms with millimetres, as the app's unit note says, and convert any index back to a raw moment or arm before entering it.
Worked example
The raw products are 40000 and 8000, giving 48000 total moment. A separately defined index of moment divided by 1000 would be 48, but this worksheet has not applied that scale.
- Total weight in the chosen weight unit
- 1100
- Raw total moment in weight × arm units
- 48000
- CG arm in the chosen arm unit
- 43.636364
Move between index and arm
- Check the loading graph's axis label for words such as index or ÷ 1000.
- Multiply the index by its scale to recover the raw moment.
- Divide the raw moment by the weight to get the arm, or use the arm given in the station table.
- Enter weight and arm, then compare the app's raw total moment with the chart only after rescaling.
An index typed as an arm
Add a 170 lb load at 37 in to the stations 1000 at 40 and 100 at 80. Its raw moment is 170 × 37 = 6290, an index of 6.29 on a ÷ 1000 chart. Entered correctly, total weight is 1270, total moment 54290 and CG ≈ 42.748031. If 6.29 is typed into the arm field, the third moment becomes 1069.3, total moment 49069.3 and the CG ≈ 38.637244.
- Raw moment of the 170 lb load
- 6290
- Chart index (moment ÷ 1000)
- 6.29
- CG with the correct arm
- 42.748031
- CG with the index typed as arm
- 38.637244
Common mistakes
- Comparing the app's raw total moment of 54290 with a chart total of 54.29 and assuming one is wrong.
- Changing a physical arm to make a product match an index.
- Mixing a station list in inches with an envelope drawn in moment index.
Scope
The app does not know which scale your source uses. The FAA weight-and-balance chapter below explains moment indexes; the aircraft's documents state the scale.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.