January 21, 2017
When it comes to RC airplanes, many say that ΄΄two sets of wings are better than one΄΄. For any model to perform properly and be safe to fly, it has to be properly balanced. This is especially true for multi-wing models. As a basic rule, you first find the MAC or ΄΄Μean Aerodynamic Chord΄΄ of your biplane, and then establish the CG Range from there. If you are building a kit or assembling an ARF, start where the instructions indicate. If you don’t have the information or it looks weird somehow, compute the range and see for yourself.
Manufacturers have been known to get the numbers wrong. Why find the range? The CG balance works in both directions. You model can be too nose heavy or too tail heavy. Nose heaviness leads to less elevator authority and increases landing speeds while making it harder to flare to bleed off speed. Tail heaviness lessens stability and increases pitch responsiveness, sometimes to the point where you lose control. So find the limits for your model’s 25% to 33% spot range and then work from there by doing test flights to check how your biplane responds.
The Pitts Model 12 is an aerobatic biplane with a swept back top wing.
Illustration ΄΄Α΄΄ shows how to find the MAC and thus your 25% and 33% MAC locations for the CG range. Finding the CG of a biplane with a swept top wing like a Pitts Model 12, is best done with a top view of both wings and then treating the drawings like a single tapered wing.
Project the Tip and the Root chord lines and then draw reference lines. Where the lines intersect, is the MAC. Then place your 25% and 33% locations for the balance range. Illustration ΄΄Β΄΄ shows the side view illustration for finding the model’s Center of Lift (CL). Placing the CG forward or aft of the CL affects the model’s performance.
Illustration ΄΄C΄΄ shows the quick and easy way to balance a biplane with two equal upper and lower wing chords. For biplanes with constant and equal wing cords, use the horizontal distance from the top wing΄s leading edge (LE) to the bottom wing΄s trailing edge (TE) for the MAC. Use the 25% and 33% MAC positions for the CG range.
These techniques for finding the center of lift and the MAC and CG locations for biplanes have worked on several different models. They are meant to give you the necessary knowledge so you can perform a safe and controllable test flight. After you fly your model, you can make fine adjustments to dial in the balance point that you are most comfortable with.
No matter the size of your biplane it has to have the proper CG balance point to maximize its performance. It goes for both being too tail heavy as well as too nose heavy. Don’t let this happen to your multi-wing airplane. Proper balance is the most important thing for safe flights.