A disc is suitable for flight if its air resistances in different directions are of an appropriate relationship, allowing its flight path to be more efficient in the direction in which it is thrown; and it can therefore be thrown farther than a ball. It must be made of an adequately dense material such that its velocity change due to air resistance is low and the force exerted by gravity is low enough for air flight. Additionally, the shape of the disc must be suited to the throwing action: a flat disc would be inappropriate due to the lack of grip that the thrower could get from the disc, resulting in accidental release.
Air Resistance:
The two major types of flying disc are the conventional design used in both Ultimate and Disc Golf and the aerobie ring. Although these designs are very different, they both use air resistance in the same way to fly.
Effects on Flight:
When a spherical ball flies through the air, it has the same cross-sectional profile causing air resistance in all directions, while a flying disc does not. Its shape causes it to have much more vertical air resistance (assuming horizontal flight) than horizontal air resistance. This is partly due to the large circular cross-sectional area vertically, but is enhanced by the rim around the edge of the disc which encloses air inside the disc in the same way as a parachute.
Effects on Flight Path:
The angle at which the disc is thrown alters the ratios of air resistances in different directions, affecting the direction of flight.
At low speed this can have the opposite effect, and a disc that is pitched upwards will usually fall backwards on its path as it falls through the plane of low resistance. The diagram below shows the path of a pitched disc that is thrown downwards.
The tilt of the disc can also cause the disc to ascend vertically if the disc is tilted upwards, without the need for the disc to be thrown in the upwards direction. This is because of the lift effect caused by the rim of the disc. This is the same principle that is used in the wing of an aeroplane. A disc that is thrown downwards at a tilted angle, and then ascends, performs a movement known as the air-bounce (or air-hop). The diagram below shows the airflow acting upon the disc.
Spinning the Disc:
The spin of the disc helps to keep the disc stable in flight and prevent unplanned tilting. This is because a large angular momentum stabilizes the disc in the same way that it keeps a gyroscope steady, with the angular force forcing the mass of the disc away from the centre of mass, perpendicular to the axis of rotation. Any unequal force acting on a particular area of the disc is quickly redirected to be equal over the circumference of the disc. As the disc spins faster, it becomes more stable due to the rapidity of the equalisation of forces.
At low speed this can have the opposite effect, and a disc that is pitched upwards will usually fall backwards on its path as it falls through the plane of low resistance. The diagram below shows the path of a pitched disc that is thrown downwards.
The tilt of the disc can also cause the disc to ascend vertically if the disc is tilted upwards, without the need for the disc to be thrown in the upwards direction. This is because of the lift effect caused by the rim of the disc. This is the same principle that is used in the wing of an aeroplane. A disc that is thrown downwards at a tilted angle, and then ascends, performs a movement known as the air-bounce (or air-hop). The diagram below shows the airflow acting upon the disc.
The spin of the disc helps to keep the disc stable in flight and prevent unplanned tilting. This is because a large angular momentum stabilizes the disc in the same way that it keeps a gyroscope steady, with the angular force forcing the mass of the disc away from the centre of mass, perpendicular to the axis of rotation. Any unequal force acting on a particular area of the disc is quickly redirected to be equal over the circumference of the disc. As the disc spins faster, it becomes more stable due to the rapidity of the equalisation of forces.
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