Get your CG Correct Early in the Build

Nov 26, 2011 No Comments by

I build a lot of planes each year.  One of the best things I have available to me not just after I build, but while I build, is the CG Machine.  I say this because many times instructions are arbitrary in the fact they suggest components, and the variability of these components may cause issues with CG.  As an example, I recently completed a flying wing with a suggested battery range from 800 mAh to 1500 mAh.  The location of this battery was up front so one can imagine how much the CG could be impacted by my battery choice.  In the end it was a 1200 mAh battery at exactly 121 grams that balanced the model.  To sort this out I committed to my electronics, their location and the servo placements.  But realize how much those electronics vary with regard to the weight of the receiver, ESC, and most importantly the motor with the largest moment from the CG.

So my suggestion to you is sit tight on your placement of any items that have optional placement.  Servos on many models are pre-defined especially in balsa and lite-ply models with servo cut0outs.  But think about the weight of a metal gear servo versus a plastic gear version.  Make your servo selection and place them correctly.  Your motor has a huge impact on the CG and too big or heavy a motor can create a nose heavy plane.  Of course better than a tail heavy plane.  In the case of being tail heavy, your servos, receiver and receiver battery pack (now use of LiFe or Li-Ion adds less weight) and in some cases tail servos to control either the rudder or elevator or dual elevator controls.  These servo are an off-set to the motor and if these are not placed correctly they may cause a tail heavy plane.

So here is my suggestion, build your plane and place the items that have no choice for movement.  Next, put the plane on a CG Machine and start adding items.  I use double stick tape to place those tail servos until I get them just right for perfect CG.  MOve your receiver battery and receiver too to get everything right.

Your fuel tank is something to think through.  Balance with the tank empty and use your best judgement for moving it to the firewall or to the CG point.  As the tank draws down the CG will move to the tail.  In other words you start with a slightly nose heavy plane and after fuel burn you move to the CG point or a very slightly tail weighted plane.  Be careful that if you move your tank too far back you may need a fuel pump.

So spend a little more time during the built with your plane on the CG Machine.  Move things around to make sure before you complete the build you will have a balanced aircraft.  Believe me this will greatly reduce frustrations and result in a great flying airplane you will enjoy flying throughout the flight envelope.

 

David Vaught

About the author

A frequent contributor to Model Airplane News and Electric Flight, Dr. Dave is a true RC enthusiast with over 40 years of flying experiences as well as a private pilot license. He flies and writes about everything from ornithopters to giant-scale aircraft, building and flying an average of 20 planes a year.
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