Moderator: Moderator
my italicsThe work has shown that the presence of fillets can decrease the overall drag of the fin, even though the viscous drag is increased due to the added surface area, the smoothing of the sharp edges between the fin and the board causes pressure distributions to be spread over a larger area.
However, this decrease in drag is restricted to double-foiled fins such as the middle fin, and there is very little noticeable difference when fillets are added to the side fins, possible due to the fact that the whole inside edge is flat, so the effect on drag of filleting the base is less pronounced than for a double foiled fin (middle fin).
However, even for the middle fin the total drag decrease is so marginal that it is unlikely to be noticed in practise, even by expert surfers, but the fillets do create a substantial increase in vertical lift, which may partially explain the more buoyant, lighter feel to glassed-on fins, and
their preference in use by surfers on the professional circuit.
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Oddly enough, with the AOA on a toed in fin you might expect the board to turn in the other direction to what it does ie towards the stringer (or would you?
A. The wing shape is designed to create relatively lower pressure above the wing as it meets the oncoming airstream, according to the Fort Worth Division of the General Dynamics Corporation in Fort Worth, Tex., and the key word is relatively.
If you hold a piece of paper at the bottom edge, let it flop away from you and blow hard across it, the paper rises, company spokesmen explained, because you have created lower pressure above the curved surface it forms. The same thing would work upside down, because the pressure differential would still exist. The question is how to get it to work for you rather than against you.
According to Joe Bill Dryden, senior experimental test pilot for General Dynamics, many factors affect flying ability, but one key to flying upside down is attaining the right angle of attack, one of the major contributors to lift.
Loosely speaking, he said, the angle of attack is the angle at which the wind is coming at the plane. If you draw a line from the leading edge to the trailing edge of the wing's airfoil, or cross section, it must always form a small angle toward the direction at which the wind approaches the plane.
"Upside down, if you do nothing, the airplane wants to fall to the ground," he said. "But if you push the plane so it is at the same angle as right side up, you develop enough lift so the plane flies upside down."
The pilot feels his way to that angle. When sufficient lift is achieved, Mr. Dryden said, "the airplane doesn't know if it is flying upside down or not."
The Pitt Special, a biplane made especially for acrobatics, has a symmetrical airfoil that presents exactly the same profile to the oncoming airstream both top and bottom, to make flying upside down easier, Mr. Dryden said. The plane also has oil and fuel systems designed for the gravitational condition of flying upside down, a limiting factor for other planes. A jumbo passenger jet, for example, could perhaps fly upside down only 15 or 20 seconds before its internal systems would be disrupted.
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