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Old Sep 04, 2009, 09:30 PM
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Idea
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!

Dan

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 ADv1.2.zip.

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 02:40 PM.
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Old Sep 05, 2009, 02:53 AM
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Cool!

I'll run off and have a play with it
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Old Sep 07, 2009, 12:57 AM
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Anybody get to try the calculator? Your thoughts on it? Does it work for you with all the macros and everything?

I had the maiden flight of the plane on the spreadsheet yesterday, and am planning another flight tomorrow. Here's the video from the maiden flight:

Grandpa Bob's Plane (1 min 36 sec)


Surprisingly the average current draw was only about 3.5 amps and only used about 550 mah in each battery in a little more than 10 minutes of flight each.
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Old Sep 07, 2009, 08:12 AM
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Nice looking software ! ... Well done !

I am building a small/mini space shuttle (15" wing span). Your calculator seems to have several interesting features. I will use it for CG calculation and let you know my findings.
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Old Sep 07, 2009, 01:02 PM
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Quote:
Originally Posted by rgt52
Nice looking software ! ... Well done !

I am building a small/mini space shuttle (15" wing span). Your calculator seems to have several interesting features. I will use it for CG calculation and let you know my findings.
Cool! Yeah, let me know how it turns out. Also, if you could do it with and without the fuselage and post how much the neutral point location changes, that would be good. For planes with very large (wide) fuselages I've found that the neutral point is shifted significantly, which makes sense, but I'm not sure how accurate the spreadsheet is in predicting this. It uses a sort of crude approximation.
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Old Sep 07, 2009, 04:43 PM
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Another note - I just got a chance to test out the spreadsheet on a machine that has Excel 2007 installed. It works fine but the charts that were by default hidden get kind of funny looking when you open them up. I should be able to fix that pretty easily though and will add a dedicated Excel 2007 version soon.
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Old Sep 08, 2009, 03:27 AM
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Shuttle numbers

I am using Excel 2003 SP3. I had no issue running macros or anything else (Except for the usual security waring).

I have entered the wing and fuselage measurements for the shuttle project (no tail yet). The overall shape of the craft using the Top View looks good (See attached).

I noticed however that the neutral point location is very different with fuselage than without it:

- With fuselage = 5.28 (length units)
- Without fuselage = 9.27 (length units)

Note: In the attached views the CG values where calculated using aprox 15% static margin.

I have built a small paper model using the top view just to have a feeling for where the CG should be. My tests indicate that the CG should be located around 8 to 10 (length units). So, it seems that the calculator is close enough if the fuselage is not taken into consideration, but far off if the fuselage is put into the equation ... unless I am doing something wrong!
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Old Sep 08, 2009, 02:47 PM
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Quote:
Originally Posted by rgt52
I am using Excel 2003 SP3. I had no issue running macros or anything else (Except for the usual security waring).

I have entered the wing and fuselage measurements for the shuttle project (no tail yet). The overall shape of the craft using the Top View looks good (See attached).

I noticed however that the neutral point location is very different with fuselage than without it:

- With fuselage = 5.28 (length units)
- Without fuselage = 9.27 (length units)

Note: In the attached views the CG values where calculated using aprox 15% static margin.

I have built a small paper model using the top view just to have a feeling for where the CG should be. My tests indicate that the CG should be located around 8 to 10 (length units). So, it seems that the calculator is close enough if the fuselage is not taken into consideration, but far off if the fuselage is put into the equation ... unless I am doing something wrong!
rgt,

Thanks for your input. The shuttle model does indeed look nice. One question, though. Did you make the paper model with a flat representation of the fuselage, so that it looks the same as the first picture you posted? About the fuselage, maybe for now it would be best to just leave that out of the spreadsheet. I can change the formulas very easily to ignore it in the neutral point calculation, but still leave it in for balancing purposes and because it looks nice. It would be nice if someone like Dr. Drela or Don Stackhouse could post here, because I'm sure they would have some knowledge about it. We'll have to see if they find it though.

I'm going to try to find a professor here that might have some knowledge about this sort of thing, and see what they recommend, and also get their opinion on the spreadsheet in general. If I don't have any luck with that, I may be able to get a better approximation for the fuselage effect using a method that is outlined in the first reference listed in the spreadsheet.

I'll soon be posting an updated version of the spreadsheet for Excel 2003 and 2007, most likely ignoring the fuselage in the neutral point calculation. I hope to eventually be able to use it again, though.

Dan

P.S. It looks like you still have the default vertical stabilizer in there, and that's why your top view is zoomed out so much, fyi. If you say "No vertical stabilizer" on the first page, it will close that tab for you and it won't show up on the top view.
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Old Sep 08, 2009, 03:28 PM
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Yes, the paper model is just like the flat representation of the airplane seen from above and it looks the same as the first picture I posted including the fuselage portion.
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Old Sep 08, 2009, 11:08 PM
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Quote:
Originally Posted by rgt52
Yes, the paper model is just like the flat representation of the airplane seen from above and it looks the same as the first picture I posted including the fuselage portion.
Alright, for now I've uploaded new files (one for Excel 2003 and one for 2007) where the fuselage is only used for the CG calculation, and not the neutral point. Eventually I hope to get a better approximation of the fuselage effect, but I'm not sure when that will happen.

So rgt52, if you use the new file and copy your data into it with the fuselage there, you should get the same neutral point that you did before without the fuselage.
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Old Sep 09, 2009, 06:33 AM
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I have copied the data for the Shuttle to the new sheet. The results seem more logical (at least to me) after removing the fuselage part from the equation.

Based on my own (limited) experience, the fuselage does have an effect but we may not think of the fuselage as just another wing panel. In fact, most airplanes have cylindrical fuselages which behave more like arrows (with little effect) than actual wings.

A few additional comments/suggestions:

-Autoscale feature or a scale option in the wings/tail/stab sheets would be nice to have.
-Adding the “wing loading” factor to the results sheet wold be useful too.

Regardless, the calculator looks better now and I really like your weight balancing feature. In the past I used to plan the location of components by “gently placing” them on several locations of the aircraft before actually fixing them. This is a cumbersome process with small foamy airplanes as the different components normally “refuse to stay still in place” until you actually “fix them in place”. Your calculator allows for placement simulation and I plan to use it during the construction of the Shuttle project. I will let you know at a later stage how accurate it turned up to be.
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Old Sep 09, 2009, 10:28 PM
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Thanks for your input again. Glad to hear you're enjoying the spreadsheet. The balancing thing should be very good (exact) for a flat foamie.

As far as wing loading goes, the only reason I didn't include it is because I don't have defined units. I would have to have a unit selection and conversion thing on the Results page, which wouldn't be hard, but would take some effort.

Your other idea about putting a scale axes option on the other charts is probably a good idea too, but once again will take a little time. Classes are starting to pick up now, so it might be a little while, but I will eventually get an update done. I found a reference here that has a fairly simple but I'm sure much more accurate empirical formula for estimating the fuselage contribution. I will probably add that in there (hopefully have a professor look at it too) and include that in the update as well.

Anyone else get to play with the spreadsheet at all?
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Old Sep 10, 2009, 06:19 AM
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When time allows, a simple solution for the unit of measure is to add in the initial/general sheet an option to select the units of length and weight. Then use them next to the results to clarify what the numbers mean. You do not have to make any conversions. It would still be up to the user to ensure that he/she uses the selected units of measure consistantly across all sheets.
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Old Sep 10, 2009, 10:02 AM
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Quote:
Originally Posted by rgt52
When time allows, a simple solution for the unit of measure is to add in the initial/general sheet an option to select the units of length and weight. Then use them next to the results to clarify what the numbers mean. You do not have to make any conversions. It would still be up to the user to ensure that he/she uses the selected units of measure consistantly across all sheets.
That's an idea, but I really prefer to stay unitless for most of the sheets, because wing loading would be the only result that is a combination of units (and thus potentially require conversion). All other results are just lengths, areas, angles, or unitless. I don't want the user to feel restricted by units throughout most of the spreadsheet when they're really not.

I think a better idea would be to just, in the results page, ask what units of length and weight have been used and to ask which units of area and weight he/she wants the output wing loading to be. That way if, for example, I've been using ounces and inches (which is common) and want the wing loading in oz/sq. ft. (also common), the spreadsheet will convert the units automatically.
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Old Oct 03, 2009, 03:36 PM
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Thanks for sharing your new program. looks very nice. I'm sure I will be useing it alot.
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