By Steve Belknap
There is an outstanding website, created by a man named Adam, called: https://rcplanes.online/index.html. He has given me permission to reference his work in this article. Within his website is a very thorough and easy to understand explanation on why your model airplane gets off the ground, flies well, and is so much fun. If you are interested in your model’s aerodynamics, give it a look. I have chosen to display his calculator for determining your model’s Center of Gravity (CG). Included on the above website are several other calculators as well.
We all know that sometimes model manufacturers do not get the CG location correct for your model. For example: My 59″ Kyosho Hawker Hurricane ARF had the CG location placed about 5/8″ too far aft, making it nearly impossible to fly. After a simple calculation, the correct CG was determined and the plane flies like a dream.
The CG calculator below very accurately determines the optimum location for your CG. It takes into account both the wing and horizontal stabilizer. It also gives you a lot more information about your plane. Below is a picture of the real calculator. Click on it, or the link below, to go to the real one.
CG Calculator: https://rcplanes.online/cg2_calc.htm
Using the calculator: To determine your CG you need to first find your MAC (Mean Aerodynamic Chord). If your wing has no taper and no sweep (rectangle) the MAC is simply your chord. For all others, this calculator is very useful. The calculator locates your MAC and the CG as a percentage of the MAC.
The MAC for a typical biplane is found here near the bottom of the page.
All you need to do is take some measurements and the calculator does all the work. The notes presented with the calculator are pretty self explanatory. Just a few helpful tips: For the “Stabilizer Efficiency”, use “std” or “low” for pretty much everything that we fly. Where is says “Enter Static Margin”, start with 15% for beginners and maybe 10% for experienced pilots, 5% is reserved for highly aerobatic planes (3D flying).
You will note there are other calculators listed at the bottom of the notes for aircraft with multi-paneled wings.
Use your common sense! If the CG you calculate seems far from where you think it ought to be, stop and re-check your measurements and inputs. You can always email me and I will be happy to help. Editor@sefsd.org.
To calculate the “Wing Loading” above, (both area and cubic) you need the Wing Area, calculated by the CG calculator, and the measured “Aircraft Weight”. It is doubtful you will have the “Lift Coefficient” the Chinese used for your model’s airfoil. The “CL” is based on many factors that are far too complicated for this simple exercise. Just leave it at 1.0.
Area wing loading is most commonly used because it is widely understood. Unfortunately, the acceptable range of area wing loading changes with the size of the plane. An 100″ plane can tolerate a much higher area wing loading than a 50″ version of the exact same plane. Cubic wing loading is the same regardless of the size of the plane. It makes it easier to compare wing loadings between similar planes of different sizes. For more information see: Cubic Wing Loading.