by Ollie Brower
In still air, the ideal performance is to get the displaced air to converge behind the tail and come to rest, as still as it was before it was moved by the nose. If the air is moving when it leaves the tail, it has lost energy, by setting this air in motion. This energy loss is called drag, and it causes a retarding force slowing down the racer. In order to close at the tail of the racer, the air must follow the racer's surface. Air separates for all soap box racers. Separation occurs because the boundary layer of air next to the surface experiences friction, and looses speed, and moves along with the racer. This friction in itself may be minor, but it can control the whole flow pattern. If air in the boundary layer stays with the racer's surface, then it blocks the path of the air your racer is going through next, and deflects that air away from the surface. A separated flow creates a large wake of disturbed air at the rear of the racer, which trails along behind the racer, instead of leaving it cleanly and coming to rest (relative to the surrounding air). Pressure in the wake is lower than in an unseparated boundary layer where it closes at the tail, so the energy invested at the nose of the racer is not returned. This wake is the major source of drag for unstreamlined racers. Air that doesn't move with the racer causes friction with the air on the surface of the racer and helps prevent turbulence. The principal task of streamlining is to expose the boundary layer to a sufficient amount of this friction. Stagnation and separation are most likely to occur around projections (axle trees, cables, headrest, and wheel pins) and sharp curvatures, where the boundary layer is sheltered from the outer stream of air. The need to keep the boundary layer out in the wind is what determines the characteristic smooth curvatures and gentle tapers of streamlined shapes. The air flow works exactly as does a wheel going over a bump (Derby Tech, June 1984, p3). The air must return energy on the back side of of the airfoil (car body), as the wheel regains energy on the backside of a bump.
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