Models can be flown as high-speed kites. Why do this? It's fun. It's different. It may be a better alternative for some kinds of models, such as models of airplanes that are streamlined and fly fast. And actually, kites are very simple, with no moving parts such as motors, control surfaces, wheels, etc. They aren’t big, because when they fly fast, they pull hard.
How do they work? Think of a kite line as replacing gravity as one of the forces acting on the model. Wherever you attach the line is the equivalent of the CG on a free flight plane. On small, hand-flown models, two lines attach to tabs on the bottom of the wing between the leading edge and the high-point of the rib camber.
Another force acting on the model as it flies is the stabilizer pushing the tail down so that the wing flies at a high, but not stalling, angle of attack. Without this down-force on the tail, wings like those on airplanes are hard to stabilize. Of course, having a tail with an elevator and rudder looks normal on airplanes,, but kites such as deltas or parafoils don’t usually have this aerodynamic stabilizer. They rely on bridles that hold their wings at a stable angle of attack relative to the line. Sailboats use hydrodynamic forces to keep the sail moving at the best angle to the wind. For built-up models, though, an airplane-like tail works very well.
Now we get to the wing. It’s going through the air at the highest angle of attack that it can maintain without stalling. Sounds like a freeflight when gliding, doesn’t it? That’s right, except that the lift that it generates isn’t just enough to hold the plane in the air, it’s being used to move the plane forward, like a sail moves a sailboat. And the amount of lift created increases as the square of the wind speed. This means that when the wind blows a little faster, the wing creates more lift. This extra lift pulls the plane forward, increasing the speed of the wind hitting the model, or apparent wind. This extra wind speed then creates more lift, and on and on.
Of course, there is another force acting on the model, drag. Most drag comes as a byproduct of lift, so the wing is the chief culprit. The better the lift to drag ratio of the model, the faster it can go. This is why airplane-shaped kites can go much faster than conventional deltas or parafoil kites. An airplane, even a small model such as a 2-3 foot span P51, has a lift to drag ratio perhaps twice that of a parafoil or delta kite, so the airplane-shaped kite will go much faster, since for the same amount of lift generated, it has only about half the drag. Since the lift increases as the square of the wind speed, and the lift to drag ratio remains constant, small improvements in L/D result in big increases in speed.
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