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DIY Weight and Balance Spreadsheet
Having a weight and balance spreadsheet on your Pocket PC makes it easy to run
a quick W&B check before launching into the blue yonder. The example spreadsheet
used in this article allows weights to be entered at various stations and then
displays a plain English go/no-go message.
Making your own weight and balance spreadsheet is easy. We'll show you the basics using data for a fictitious aircraft called a SkyFling XL. You will need a spreadsheet application such as Microsoft Excel 
or
OpenOffice (it's free) to
work with the example spreadsheet on your computer. Most Pocket PCs
have Pocket Excel that can also be used to edit the example spreadsheet.
Looking at the SkyFling XL limits chart we note some key points. The max center of gravity (CG) is 47.3 inches, the min CG is 35 inches, max gross weight behind 38.5 inches is 2300 lbs and between 35 and 38.5 inches the max allowable weight varies according to the sloped line. At 35 inches the max allowable weight is 1950 lbs. Time for some math!We need an equation to describe the slope in our chart between CG 35 and 38.5. The linear equation y = mx + b will do this. In this equation, y is weight on the vertical axis, m is the slope, x is CG on the horizontal axis and b is the y-intercept (or where the line crosses the vertical axis when x equals zero). Next we'll find values for m and b.Determine the slope, m: m = (rise)/(run) m = (2300 - 1950)/(38.5 - 35) m = 350/3.5 m = 100 Now solve for b: y = 100*x + b 1950 = 100*35 + b (insert values for y and x ) 1950 = 3500 + b b = -1550 Our equation is then: y = 100x - 1550 Spreadsheet TourThe example spreadsheet allows the user to enter variable information such as oil and fuel quantities and weights at various stations. Try entering some different values for quantities and weights and notice how the calculated fields change. The message in cell B1 will change depending upon our situation.Note: The cells intended for data entry have a solid border. Cell C10 is the calculated total weight, D11 is the calculated CG. When you click on a cell the formula is displayed at the top of the screen. The formula in cell B14 is used to determine the maximum permitted weight at the CG value in cell D11. The formula for this value is: =IF(D11<38.5,(100*D11)-1550,2300) In English this says if our CG (D11) is less than 38.5 use our equation y = 100x - 1550 otherwise our max weight is 2300. The formula in cell B1 gives us an easy to understand message describing our W&B situation. The formula for the message is: =IF(AND(D11>=35,D11<=47.3),IF(C10<=B14,"Let's Fly!","Too Heavy!"),"Outside CG!") Translated this says if the CG range is OK check the weight, else, display "Outside CG!". If we're in the valid CG range then check our total weight in cell C10 against the max allowable weight in cell B14. If the C10 weight is equal or under display "Let's Fly!", else, display "Too Heavy!". Take it From HereTo make a spreadsheet for your aircraft you will need the weight and balance information for that specific aircraft. Many aircraft have a more complex CG limits chart with multiple slopes. For these you will have to solve y = mx + b for each slope and alter the B14 formula to use the correct equation for a given CG. Be sure to test your spreadsheet against data contained within your aircraft's P.O.H. Pay extra attention to boundary points such as min CG, max CG and max weight. You will also have to change the stations and arm values for your type. After altering a cell or formula run a quick test before making the next change. Remember, if you move a cell that is used in a formula make sure that formula references the new cell location.More Ideas LinksDownload the example W&B spreadsheet.More on linear equations. Using Pocket Excel. PTab, an alternative to Pocket Excel. |
