3D aerobatics setup

Model Airplane News, Jul 2003 by Somenzini, Quique

Editor's note: with multiple Tournament of Champions (TOC) and F3A U.S. National Champion titles to his credit, Quique Somenzini is extremely well known in aerobatics circles. Model Airplane News is pleased to present Quique's second article in his series on freestyle aerobatics. The first article, on the "Roller Coaster," appeared in our April 2003 issue.

Although specific flying techniques are important in 3D aerobatics, setting up your airplane correctly will greatly help you to perform maneuvers better and more easily. This month, let's take a look at the airplane and radio setup.

RUDDER, ELEVATOR

AND AILERON AREA

To perform most 3D acrobatic maneuvers, including the Roller Coaster described in the April issue, your plane must have large control-surface areas. In fact, the rudder, elevator and aileron surfaces are the heart of 3D aerobatics. How large should they be? Well, as a percentage, the elevator should be at least 40 percent of the total horizontal stabilizer area, the rudder at least 50 percent of the fin area and the ailerons at least 20 percent of the total wing area. The deflection is as important as the area. The elevator should deflect at least 45 degrees each way, the rudder at least 45 degrees and the ailerons at least 35 degrees. The combination of these large surface areas and deflections allows you to control the airplane even when less air flows over the control surfaces.

During the Roller Coaster, for instance, the airplane is flying at such a low airspeed that the airflow generated by the propeller (the propwash) is the only thing that allows the plane to continue to perform such a large change in attitude. At the top of the Roller Coaster, when the airplane is at zero airspeed, the propwash blows over the tail surface and the inside area of the ailerons. To maneuver in this situation, your plane must have large control surfaces that will be able to use this very small airflow. Extra long and wide ailerons help considerably to better maneuver the airplane at zero airspeed.

Even with extra-large control surfaces that have enough deflection, you must be able to move those surfaces safely and with authority. That's why a good linkage system and powerful digital servos are important. Digital servos have a greater holding power than standard servos do; it is so important to avoid any chance of flutter, which is significantly increased when you enlarge the control surface and its deflection!

CENTER OF GRAVITY

Your plane's center of gravity (CG) is also a key factor in performing wild 3D aerobatics. As you move the CG rearward, the plane becomes less stable, but pitch and yaw control become more responsive and more sensitive. Most acrobatic airplanes have CGs that are around 32 to 33 percent of the wing's mean aerodynamic chord (MAC). If you move the CG farther back, you will gain even more pitch and yaw authority, but the airplane will simply be too unstable. It depends on the airplane, but the best balance between acrobatic and unstable is usually achieved when the CG is about 37 to 40 percent of the MAC.

When you fly maneuvers such as the Roller Coaster, flat spin, Cobra (Harrier), waterfall, Pendulum, etc., your plane's CG is very important; simply having large control surfaces isn't enough to allow the plane to pitch properly. Note: when you do a maneuver that's like a torque roll, the CG is not a key element. For that, a plane with a standard, aerobatic CG of about 32 percent MAC will be fine, and you won't gain any more maneuverability by moving the CG rearward.

As you know, the best CG position for 3D aerobatics is a compromise; the real challenge is to find the CG that will give the airplane the best overall balance for most maneuvers. A freestyle program should be balanced with a good mixture of 3D and precision acrobatic maneuvers, and your performance should be smooth and show control at all times. If you move the CG rearward, your plane will more easily perform wild 3D maneuvers, but it won't fly as precisely or solidly. On the other hand, if the airplane is nose-heavy, you'll have a solid airplane with show precision, but you won't be able to execute wild 3D aerobatics! You need to end up with a CG that's between these extremes. For their freestyle routines at TOC, most pilots move the CG rearward by adding weight at the tail.

This point may differ for various planes and pilots. I like to fly a plane that has a CG that's at 35 percent of the MAC for precision maneuvers and then move it back to about 38 percent for the freestyle portion of the competition.

The illustration shows how to find the MAC and then the CG. I like to use these numbers as a reference, but I adjust the final CG by flying the airplane. I've found this is the best way to correctly set my airplanes, so they all "feel" the same and have the same trim characteristics.

FLIGHT-TESTING THE CENTER OF GRAVITY

It's easy to fine-tune the CG for precision maneuvers: fly the airplane upright and level, pull to 45 degrees (at full throttle), and do a Vi roll to inverted. The airplane should fly inverted by itself in a straight, 45-degree line for 2 to 3 seconds and then start to fall toward the canopy (you should barely have to push the elevator stick forward to keep the airplane flying In a straight line).