STALLING
What's a stall? A STALL is a loss of lift caused by the airflow becoming chaotic over the wing. This happens when an airplane travels at too slow a speed, when the airplane's wings are slanted too high against the airfow (known as a high ANGLE OF ATTACK), or a combination of those two things. When the air can no longer follow the shape of the wing to generate sufficient lift, a stall is the result.
GLIDE RATIO
Glide ratio is a number that's used to describe how far forward an aircraft can travel before losing a standard unit of altitude. Usually, this is expressed as a ratio like 4:1 or 50:1. In those examples, the aircraft would travel 4 feet forward before losing 1 foot of altitude, and 50 feet forward before losing 1 foot of altitude. Needless to say, 50:1 is a better glide ratio than 4:1.
SINK RATE
Equally or even more important than glide ratio, at least when it comes to some aeronautical competitions, is SINK RATE. Sink rate is a measure of how much altitude is lost over a standard amount of time. If it's light enough, a plane with a low glide ratio may still have a low sink rate meaning it may be able to stay aloft longer than a heavier plane with a higher glide ratio. For example, if a plane has a glide ratio of 1:1 but falls at only 1 foot per second, then it will stay in the air longer than a plane with glide ratio of 5:1 that loses 2 feet per second. The key measurement for a duration flight is the number of seconds the plane takes to hit the ground. A good glide ratio can really help, but the lowest sink rate will be the winner, assuming the planes start from the same height.
CONTROL SURFACES
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.
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