Balancing the four forces we discussed at the beginning of this chapter is the first part of making a good fying machine. Causing or correcting maneuvers is the next step. This is where CONTROL SURFACES come into play. Control surface is the term used to describe the moving part of any fying surface, which on an airplane includes the rudder, elevator, and ailerons. Elevators control the up and down movement of a plane, rudders control the right and left movement, and ailerons control rolling. All of these control surfaces will be discussed in the sections that follow. In the name of aerodynamic research only, and never just for fun, put your hand out the window of a moving car. Do not extend your reach past the side-view mirrors. Position your hand horizontally (so, flat and parallel to the ground) with your thumb facing the direction the car is moving. The smallest move of your hand can force it up, down, left, or right. Air bounces off the fat side of your hand that's facing the direction of travel. The defecting air forces your hand in the opposite direction. If you twist your wrist to point your thumb downward, air is now hitting the back of your hand and bouncing upward. The air bounces up, and your hand is pushed down, Aircraft control surfaces follow this same principle. Air bounces against them, forcing the plane to maneuver left, right, up, down, or to roll. Flaps and ailerons are found at the rear of the main wing on conventional airplanes. But for paper airplanes, the elevator and rudder are the most important control surfaces. Technically, most paper airplanes employ a BLENDED WING, meaning there's no separation between the main wing and the tail. On blended wings, some control functions are combined; for example, the elevator also acts as the aileron, and is called an ELEVON. Don't sweat that. A paper airplane has few moving parts and therefore fewer controls. This makes it a great way to learn how control surfaces work.
RUDDERS
Let's look at a rudder turn. The pilot operates controls that cause the rudder to start deflecting air. In Figure 11, the rudder has been caused to stick out to the right side of the tail. Air will hit that rudder and get deflected to the right. That will push the tail to the left. Here's the key: The aircraft will rotate around the center of lift in flight. If the tail goes left, the nose will go right. It's like a seesaw, with the center of lift in the middle. The left rudder pushes the nose left because the air gets deflected left, which pushes the tail to the right.
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