Weight and balance calculations to ensure that the center of gravity (CG) is within the design limits and that the maximum gross weight allowed by the design is not exceeded are critical to safe flight. Simple gliders, such as the BG 12 series, are much easier to keep within limits than more complex powered craft (no fuel, oil, hydraulics, extra passengers, baggage, etc.). There is really little mystery about adding up a couple columns of numbers and then dividing one by the other. The problem lies in the fact that to do several "what if" calculations takes time and is error prone if using a pencil and calculator alone. The program BG12WB.exe is designed to be specific for the BG series of gliders and allows automatic calculations of many different possibilities.
Some of the mystery that may surround weight and balance calculations may be due to the fact that sometimes information from the designer of a homebuilt design gets lost along the way. Also, designs mature and change along the way. There may be confusion that a flying gross weight is the maximum gross weight or that a CG imposed gross weight is the maximum gross weight for a design. Use what ever authoritative resources are available to determine exactly the limits for your flying ship and then work with the numbers to understand what they are telling you.
Some terms that I learned nearly 30 years ago in my primary ground school still serve me today. Basic Empty Weight is pretty much that - the aircraft without anything not required installed such as pilot, parachute, etc. Gross Weight is the Basic Empty Weight plus the other stuff that goes into the craft, like the pilot, parachute, and baggage. (For gliders we don't have to worry about the gas and oil that powered pilots have to take into consideration.) Maximum Gross Weight is determined by the designer and is a major factor in determining stresses and speeds at which those stresses are allowed. The Datum is a measuring point from which the designer has chosen to measure things, such as the center of gravity limits. The Center of Gravity (CG) limits are usually designed as a range of percentage of the Mean Aerodynamic Chord but expressed as a range of measurement from the Datum. Inches are commonly used. Measurements from the Datum are commonly called Arms. Arms can have plus or minus values depending upon where the Datum is located. If the Datum is the leading edge of the wing (like the BG 12) then items forward of the wing have a minus arm and items aft of the leading edge have a positive arm. A Moment is a Weight multiplied by an Arm. Moments will have negative values if the Arm is negative.
An aircraft has a specific Basic Empty Weight and CG (a specialized Arm) and Moment. This is determined by an official weighing. The plane is leveled according to the design plans (or factory information if it is a certified craft) and weights are taken at the wheels (and or skids). The distance from the Datum is determined for each wheel - it's Arm. The weights are added up and the moments are calculated. The total moment is divided by the total weight and the Basic Empty Weight CG is the result. This is the information that is used to now find out how much of what can be put where in the aircraft.
An Arm for the pilot and parachute is found. (The BG 12B final assembly instructions state that the pilot Arm is from the Datum to the pilots navel.) Anything else installed in the plane after the initial weighing (instruments, oxygen tank, etc.) is also measured and weighed. The weights and moments for what is in the plane is added to the officially weighed Basic Empty Weight and moment. Again the total moment is divided by the total weight and the operating CG is the result. Two things need to be checked - 1) is the operating gross weight less than the maximum design gross weight? and 2) is the CG within the allowable range? If both questions are answered YES then, and only then, is it time to go flying.
The screen shot at the top of this page shows the features of BG12WB. The information that is the basis for this program came from the BG 12B Final Assembly, Final FAA Inspection and Test Flying instructions (part of the plans) and plan drawing 12046, Placards. The gross weight note on plans sheet 12046 states: NOTE: GROSS WEIGHT FOR MODEL "B" THROUGH "D" SHALL NOT EXCEED 900 POUNDS. There have been reports that other gross weight information given for other ships is DIFFERENT. Do NOT use this program if you have ANY doubts as to weather this specific information is correct for your application!
Along left side of the program screen are columns of numbers for the weights, arms, and moments. Only weights and arms are entered into the program - the moments are automatically calculated. Along the bottom left of the screen is a graphical representation of the CG limits and boxes for the Gross Weight and CG. If a calculation results in an answer within the allowable range, the answer box for it turns green. If the result is outside the allowable range, the box turns red. Along the right side of the screen is a series of four buttons. These buttons will take information stored in a small text file (BG12WB.dat) and allow quick switching between different gliders or different fixed loading conditions (oxygen tank installed or not...). The only button that is active in the screen shot is the first one for N7438. Pilot weight, with parachute, is typed in and the result shows that the Gross Weight (828 pounds) and the CG (13.24 inches) are both within the allowable limits. (828 pounds was the test weight of the glider described in the SOARING June 1970 article "Polars of Eight".)
The screen shot above shows the minimum pilot weight allowed for the empty weight and CG of N7438.
The screen shot above shows the maximum pilot weight allowed for the empty weight and CG of N7438. Notice that the Gross Weight (868 pounds) is LESS than the Maximum Gross Weight allowable.
In order to get to maximum gross weight at the current empty weight and CG of N7438, weight will have to be added to the tail. In this case a heavier pilot can be carried (who the heck is going to fit 285 pounds of pilot into a BG??) but not 286 pounds. This put the Gross Weight over the Maximum Allowable by 1 pound, and the Gross Weight box indicates this by turning red.
As you can see from the screen shots, a few keystrokes will allow the modeling of different loadings with direct visual evidence of the result of that loading condition. I have found this to be a valuable tool in determining if a safe loading condition is to be flown. It also helped in determining the weight placards for minimum and maximum pilot weights. Six additional spaces are given for other equipment besides the pilot. Find out how much radio or battery you can put and where it will best help your CG loading. In my opinion, flying at the very forward or very aft limits of the CG range is inviting trouble in the form of stability problems. I am fortunate to fly much closer to the first screen shot loading than any of the other extremes.
BG12WB.exe can be downloaded below.
This is a Visual Basic 6.0 program that should run under Windows 95 on up to WinXP. I have not tried it on Vista or Win7. Most computers will have the auxiliary files to run this program. If there is a need for other files by users (xxx.dll) let me know.
WARNING! LARGE FILE! 1.8 Mb
Here is the full install version as a zip file.
WARNING! LARGE FILE! 1.8 Mb
If you feel that this program is valuable and useful to you, please email me. Also, consider sending a $5+ donation to encourage me to do more glider programming.
Created 7 April 2002.
Updated 18 November 2011
(C) Copyright 2002-2011 by Paul R. Jorgenson
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