Balancing An Aircraft

Why is center of gravity important? The center of gravity (CG) is the point in the structure where all of the weight can be thought to act.  Aircraft rotation will occur through the axis at the CG.  It has often been said that an aircraft with the CG too far forward will fly poorly, while an aircraft with the CG too far aft will only fly once. 

Optimum location of the CG  Model plans and/or kit manufacturers will almost always specify the optimum location for the CG.  However, for a plane bought at a swap meet or from an individual that information may not be available.  If this information can’t be found on the internet it may become necessary to take matters into your own hands.  In that case the CG should be located at a point between 25% - 33% of the mean aerodynamic cord (MAC).  For a straight wing the MAC is simple; every cord is the same length so the mean is the fore to aft dimension of the wing.  At some point down the road I will update this page with the equations for finding the MAC of more complex wing configurations. 

Measuring the CG   I tried a lot of different balancing techniques before I settled on the following apparatus.  With this technique the CG can be determined within 1/16^th of an inch.  The measurement is repeatable and works on almost any model from a 1/2A up to giant scale. 

Construction I believe the diagram in Figure 1 is self explanatory, but if not, use the wonderful Instantspot "Contact Me" feature and I will be glad to elaborate.  This device can be fabricated in many different forms, from materials that the modeler already has around the shop.   For the vertical portion of inverted “T” I used a 5” piece of 1X2 and for the Cross Member I used a 16” piece of 3/8” fiberglass rod (nice & stiff).   I glued the Cross Member in place and then attached the Wing/Fuse Slings so they can swivel on the Cross Member.  Since I fly a number of large models I used 3/4” nylon tie down straps instead of ropes or string for the Wing/Fuse Slings.

The straps are less likely to produce that ubiquitous hangar rash.  There may be an optimum length for the Wing/Fuse Slings but I'm not sure.  My gut feel tells me the longer the better.  I made my Slings 48” and I am very pleased with the results.   I reverted to my old Boy Scout knot tying experience to make a slip knot for hanging the Plumb Bob.  That makes it real easy to adjust for different wing thicknesses. Since all of the weight of the model is being supported by the Cross Member, the CG will be directly beneath Cross Member and the Plumb Bob points directly at the CG.   

I put a pulley overhead so I can do the rigging on the floor and then lift the aircraft for measuring.  I have also rigged this so I can insert my digital fish scales for weighing my models.

MeasuringThe rigging can be done with the airplane one the floor.  After it is hoisted a few inches off the floor it needs to be adjusted in the Slings until it is in its approximate flight attitude.  Adjust the slipknot on the Plumb Bob until it hangs just above the wing and when everything reaches equilibrium the Plumb Bob will point at the CG.  While still in the Slings you can add trial weights to either the nose or tail (as required) until the Plumb Bob points to the desired CG location (typically 25% - 33% of the MAC).

 

Versatility

This device works equally as well with a low, mid or high wing.  With a biplane, just put the Slings around both wings (Figure 2).  

 

If you have flaps or ailerons that come close to the fuselage, and may not support the weight put on them by the Slings, a couple of things can be done. Make Slings that support the model under the fuselage fore and aft of the wing as shown in Figure 3. With some configurations i.e. a delta wing it may be necessary to place a balsa sheet or boards inside the Slings so the model can be laid on the boards.  As long as the board/s are symmetrical, reasonably homogeneous and light, in comparison to the model, the result will still be accurate.