Question: How do I know where the CG (Center of Gravity) is to be located on my model?
Answer: Always refer to the instruction manual and documentation that came with your model. There is usually a diagram and/or explanation of where the CG lies in each manual. Any changes after the manual is printed are reflected in documents included with the kit. If the documentation has been misplaced and/or you want to double check, use our Knowledge Base and search on CG or look for your specific model in the listing of products.
Information: In the event that you cannot obtain any documentation related to your model and wish to locate the CG here is a procedure that you may wish to try. The CG is generally located just forward of the thickest part of the wing. Setting the CG slightly further forward will not hurt, it will limit the aerobatic capabilities of the model if the CG is too far forward but it will not cause loss of control. the CG too far back makes the model unstable, difficult to control and will often result in a crash. If you are stuck with no solid information at all we suggest setting the CG 3/4" forward of the thickest part of a non-swept wing for initial flights and then gradually moving it back to about 1/4-1/2" forward of the thickest part of the wing but only if the model remains stable in flight.
This procedure works reasonably well for non-swept wings. For swept wings, the thickest part of the wing also tends to sweep back with distance out from the fuselage. To roughly set the CG on a swept wing model we suggest going with a location that represents 25% of the area of the wing. This is the location from which a line running at right angles (perpendicular) from the fuselage will disect the wing such that 25% of the wing area is forward of this line and 75% of the wing area is aft of this line. After initial flights you may wish to move the CG back towards about the 30% mark but only if the model remains stable in flight.
Use of "CG Machine": Unfortunately, our comments in this regard are not very positive. Over many years we have noticed that when we get questions from modelers about CG, that 99% of the time they are using a "CG Machine" of some sort and get totally confused about how to use it and in many cases have erroneous information about the CG location of their model.
In our opinion, for the vast majority of RC Aircraft applications, such "CG Machines" are tools looking for a purpose. Yes, if you have a swept wing biplane with non-symmetric mass in all three axis and you are entering the Tournament of Champions and know little about CG in model airplanes, you might get some use out of a "CG Machine" IF IF IF you took a few hours to study the device carefully and read the instructions thoroughly.
However, if you are not in this rather august group, there are more practical, faster, easier and cheaper ways to set up the CG on your RC model airplane without spending money, getting yourself confused or frustrated.
1) Check the documentation that came with your model and any support information that may be on the suppliers web site or knowledge base. From this determine "where" the CG is supposed to be. We will call where the CG is supposed to be, the "CG Datum Point".
2) Mark the CG Datum Point on the model.
3) For a high wing model extend the CG Datum Point out to the wing tips. If the leading edge (LE) of the wing is straight, use that as your reference point. If, for example, the CG Datum Point is 3" back from the LE of the wing then mark this on the wing tips. In any event the CG Datum Point should be extended out to the wing tips at right angles (90 degrees) to the thrust line running from the spinner to the tail.
4) Put your fingers on or under the wing tips where the CG Datum points are. Lift the model. If it is balanced, you are good to go. If the tail drops, move your battery forward or add small amounts of weight to the nose until it balances. If the nose drops you can leave it slightly nose heavy for training and first flights and then shift your battery back or add a bit of weight to the tail to balance it later on. If the nose drops a great deal, you are very nose heavy and should fix this before flying by shifting weight aft until the model is balanced.
5) For a low wing model, you can extend the CG Datum Point out to the wing tips in the same manner as for a high wing model OR better yet, invert the model and screw a cup hook ($.05 cents at the hardware store) into the wing center joint at the CG Datum Point. Hang the model from the cup hook using a piece of string. If it is balanced, you are good to go. If the tail drops, move your battery forward or add small amounts of weight to the nose until it balances. If the nose drops you can leave it slightly nose heavy for training and first flights and then shift your battery back or add a bit of weight to the tail to balance it later on. If the nose drops a great deal, you are very nose heavy and should fix this before flying by shifting weight aft until the model is balanced.
The "cup hook" method can also be used for high wing models if you can put a cup hook into top of the fuselage at the CG Datum Point. The cup hook method offers the advantage of also being able to balance the model across the wing span. The wings should also be level when hanging. If not, add weight to the high wing tip by pushing nails or lead pellets into the bottom of the high wing tip and using CA or Epoxy to retain them. You can also use stick on weights.
This is not intended to be a treatise on CG Datum Points or adjusting CG locations but it is a good practical simple approach that works for most trainers, fun-fly, sport, scale and pattern airplanes. If you are into something more exotic there are additional considerations.
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