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Old Sep 04, 2009, 08:30 PM
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Yet another CG Calculator

This CG calculator is meant for airplane design - especially components layout and unusual planform evaluation. If you just want to figure out where to balance a preexisting model, you probably don't need to use all the functionality. However, it will still be very useful for figuring out things like how much nose weight to add, etc.

The CG calculator is designed to work with almost any airplane configuration, including:
  • Standard monoplane and tail configuration
  • Biplanes
  • Triplanes
  • Canards
  • V-Tails
  • Any combination of the above
Unlike other CG calculators, this one allows you to:
  • Include the fuselage contribution to the stability calculations. This is particularly important for wide fuselages that extend well forward of the wing, e.g. jet models.
  • Estimate your actual CG location by assigning weights to airplane components and by adding point masses to represent things like batteries, motor, servos, the fuel tank, or any other weight in addition to the airframe's weight.
  • Compare your actual CG to the neutral point of the plane, before ever building it.
  • Use the "Static Margin Wizard" to move point masses, adjust the weight assigned to point masses, or adjust other quantities to achieve a desired static margin. This is very helpful for deciding where to place components in your plane or for figuring out how much nose weight you need to add, for example.
  • Export your design to AVL for complete aerodynamic and stability analysis.
Before starting to use it, you must change your Excel security settings to enable macros. If you do not enable macros, the spreadsheet will not be functional.

To use the calculator, you simply enter in the dimensions of your plane. Each page shows required input dimensions and displays a chart to show you what it looks like. Additionally, there is a Top View page which draws the completed plane and shows a line for the neutral point and another for the calculated CG. After entering in the dimensions of your plane, you can either assign a weight to each component (wings, fuselage, tail, etc.) or enter in the total "empty" weight and balance point for your airframe. Finally, you can add in point masses to represent the electronics, power system, nose weight, etc. for purposes of balancing. If there are items you don't want to move around, you can add them in to the "empty" weight.

Attached below are versions for Excel 2003 and 2007. Let me know what you think of it!


Edit 7/2/2010
The new version of the CG calculator spreadsheet is out! If you have the old version, you'll definitely want to update. Here are the new features:
  • Completely redesigned and more user-friendly layout
  • Up to 10 panels available to draw all flying surfaces (including stabilizers) - excellent for approximating curves
  • Up to 20 points available to draw the fuselage side -also excellent for approximating curves
  • Most pages have calculations performed behind the scenes - no formulas in cells
  • The majority of cells (including output cells) can be edited manually
  • All charts have "Default Axes" and "Scale Axes" buttons
  • No more entering zeroes for unused panels/fuselage points. Use up/down clickers to select how many panels or points you're using.
  • Wing loading calculator with automatic units conversion is included on the Results page
  • Improved neutral point calculation for biplanes and triplanes (I derived the formula using multiple wings instead of combining the wings into an equivalent single wing)
  • Improved fuselage output algorithm that integrates over each section instead of using only the overall dimensions
  • Automatic detection of intersection between the fuselage and Wing 1 with the option to remove overlapping Wing 1 area from spreadsheet calculations
  • Move the entire fuselage forward or backward by clicking a button (as requested)
  • Wing 1 can be moved forward and backward so that an arbitrary reference point can be used (as requested)
  • There is no separate input box for a V-Tail. A V-Tail is created by using 2 vertical stabilizers with a dihedral angle < 90 degrees.
  • dCm/da (longitudinal stability derivative) is reported for each airplane component

The new version is posted in the first post of this thread and in my blog. There are separate versions for Excel 03 and 07. Let me know if you encounter any bugs.

Note 1/23/2011: For all using the Excel 2003 version: please retain a backup copy of CGCalc_1.04.xls when upgrading to CGCalc_1.05_03.xls. I no longer have a copy of Excel 2003 available to me, and it is possible that saving in that format from a later version introduced errors. If you encounter errors with this, please PM me and I will give you instructions on how to upgrade to v1.05 manually.

Update 3/7/2014: Someone using the CG calculator decided to go ahead an make a modified version including the ability to add control surfaces, specify airfoil sections, and have a more detailed estimate of the inertia. All this information can then be exported to AVL, so this is more like an overall design tool instead of just a CG planner. It is also a bit more complicated than the original CG calculator for the same reason. It is called "Airplane Designer." He gave me permission to post it; it is attached below as

Update 3/16/2014: Added version of CGCalc_1.05 with pages unprotected so you can change cells and see the code if you want. Updated airplane designer to version 1.3.
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Last edited by Montag DP; Mar 16, 2014 at 01:40 PM.
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