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Javelin Aerodynamics | |
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The javelin and the discus first appeared in competition in the ancient games in 708 B.C. Originally there were two events for the javelin: target throwing and distance throwing, using a sling. By 1780, the javelin was an event in the Scandinavian games. The sling and the target were no longer in use. The current one-handed throwing style, while on the run, was in use. In 1953, a hollow javelin was invented by Franklin Held. Since the javelin had to match a standard weight the surface area was increased. This, in turn, increased the javelin's flight capability. Another characteristic of this javelin was that it landed horizontally (flat on the ground). This was a problem in competition because the exact landing spot of the javelin was hard to determine. In 1984 the International Amateur Athletic Federation (IAAF) adopted new rules to make sure future javelins were designed to land point-down, so the "touch-down" point would be known exactly. This also tended to shorten flight time. Aerodynamic Forces By examining the aerodynamics of the javelin, we will begin to understand how a javelin might be designed to limit the time of flight and land point down. When a javelin is thrown, air travels around the shaft. The air flow tends to separate on the upper surface. One normally thinks of flow separation increasing the drag force. That is what happens here, only the direction of the force is opposite that of the gravitational force. Therefore, the separation of the flow from the upper surface of the javelin actually increases the flight time. So, how is the flight time actually "decreased" ? Every javelin designed for competition has the same "center of gravity" or balance point. Add to this a "center of pressure" while the javelin is in flight. This causes a "nose-down pitching moment" (the point is always slanted toward the ground). Even though the javelin varies in its flight path, the center of pressure remains behind the center of gravity, keeping the point slanted toward the ground. The point-down position is safer, more accurate and shortens the flight time. The javelin also experiences a spin about its longitudinal axis during flight. This spin can be as high as 25 revolutions per second. This spin tends to stabilize {steady} the javelin in flight. A problem in flight is oscillation (vibration). This oscillation has a frequency of about 25 hz. The oscillation disturbs (bothers) the flight of the javelin and needs to be minimized by the thrower. Summary
The javelin is another field event which relies heavily on aerodynamic
forces. The current javelin is designed to limit flight distance to
approximately 90 meters (295 ft 4 in). With the current world record of
95.66 meters (313 ft 10 in.) will the javelin be redesigned again?
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