Before you start flying your RC Airplane and
sadly seeing your RC Airplane broke to pieces in
front of you, it is important to check your RC
Airplane for strength.
For most trainer model RC Airplane, you should design your wing and structure to carry at least 2g of loads in maneuvers. What it means is that your wings should be able to carry twice the weight of the aircraft. Yes right, if your rc airplane weight is 4 lb. 2g will mean that your airplane wing should be able to carry at least 8 lb of loads. For aerobatic models, the requirements may be high e.g. 4g or 6g loads.
But don't start adding more wood in your RC Airplane in making it stronger. Remember, the more structure you add in your RC Airplane, the more weight you are adding up in to your RC Airplane. So, use better design techniques to make your RC Airplane structure with a lesser weight.
Most of the time, new modelers don't care about the grain direction of the balsa wood and ply wood when making there RC Airplanes. If you see the strip of balsa wood and ply wood you would have seen a grain structure on it. You should that in mind, what kind of loads the structure is taking and in which direction the load is acting. you should ask is the load compression or expansion? With proper direction of the grain structure of balsa wood and plywood on your RC airplane, you will not only save weight but will also make a stronger structured aircraft.