Details of Drag
Details of drag
TDC
Kevin Cameron
REMEMBER THIS FROM PHYSICS 101? Aerodynamic drag is the transfer of energy from a moving vehicle to the air through which it moves. Let's build upon that.
A simple measure of the air accelerated by vehicle motion is given by "frontal area." This is measured by pointing the vehicle straight at a distant light source (for example, the sun, just after dawn) and then tracing around its shadow, cast onto a large piece of paper hung behind it. This area is then the "piston" which the vehicle pushes through the resisting air. The analogy of "stroke" is the distance that the moving machine advances in one second. Thus, the rough volume of air disturbed is frontal area, multiplied times speed in feet per second.
How much pressure does the air ex ert against this moving frontal area? If we replace our real motorcycle by a flat piece of plywood of the same area, and assume that all the air hit ting its flat surface is stopped dead there (actually, accelerated to the mo torcycle's speed), then the force creat ed is the so-called "dynamic pres sure." This is the pressure that results from full conversion of air velocity into pressure, and is proportional to the square of speed. At 100 feet per second (68 mph) this is about 12 pounds per square foot, and at 200 fps (136 mph) it becomes 48 pounds per square foot, and so on. While this drag force increases as the square of speed, the horsepower consumed-because it is force times speed-increases even faster, as the cube of speed. A motor cycle-sized flat frontal area of 6 square feet, moving at 136 mph, would produce a drag force of roughly 6 x 48, or 288 pounds, requiring 105 horse power to overcome. Too much.
We can do better. If we place a rounded front on our flat plate, the air will no longer be completely stopped against it, converting all its energy into pressure. Instead, airflow will curve gently around, to flow off the edge of our shape almost parallel with the ve hicle's direction of motion. Our vehicle will still convert some of the air's ye locity (relative to our moving shape) into pressure, but because this is no longer 100 percent conversion, the dy namic pressure on the rounded front will be less than it was against the flat plate. Drag will fall. This kind of frontal streamlining may cut drag to about half that of a flat plate of the same frontal area. Sportbikes and high way trucks are like this-bodies round ed in front, but chopped off at the rear.
Our smoothly rounded frontal fairing guides the air around our vehicle more gently, but nothing puts the flow back together again behind the flat rear, or base, of our shape. This wake region is at low pressure because it's surrounded by high-velocity, low-pressure airflow. Crudely put, drag is the difference be tween the dynamic pressure acting on the vehicle's front surface, and this low pressure acting on its base. The flow streaming past the edges of our roundfronted shape is pushed into this lowpressure wake region by the surround ing still air, forming vortices. Because no flow is completely symmetrical, some part of this vortex-forming action will grow faster than others, getting larger until it fills the base or wake re gion. Finally hitting the flow on the op posite side of the wake, the vortex is detached, or "shed." A new vortex forms on the opposite side, rotating the opposite way, hits the flow on the other side, and is also shed. The vehicle's wake becomes a trailing series of such alternating vortices, carrying away en ergy in their whirling motion. Anyone who has ridden close behind a big truck has felt the side-to-side buffeting of these so-called "von Karman" vortices. We refine our shape further by try ing to put the flow behind our vehicle back together smoothly, rather than leave a mess of energy-robbing vor tices. Ideally, drag becomes zero when the vehicle merely displaces air mole cules from their original positions mo mentarily, and then puts them all care fully back exactly where they were originally-leaving them with no extra energy. Perfection is impossible, but we can achieve some success. Just as we put a rounded front on our flat plate to guide the flow around it, we now add a long tapering tail behind it. If we taper this tail too quickly, the flow sep arates from it and again becomes tur bulent. But a gentle taper worksthe air remains attached most of \the way to the tip, just as it does in flowing over a wing. This re duces drag because instead of a partial vacuum in the wake region, we now have whatever pressure the flowing, decelerating air exerts on our tapered tail. This recovered pressure helps to offset the pressure of air hit ting the front of the vehicle.
You can see this effect at work in the Daytona 200, as riders raise their sloping backs on the straightaways until they can feel their fluttering leathers lie down as the previously separated flow over them attaches and becomes smooth. This small drag re duction can be worth an extra 300 rpm on top end.
Motorcycles are too short to carry full tapered tails. To get some benefit from pressure recovery, their thickest cross section is located near the front, to leave as much as possible of their length to be used in tapering inward, keeping airflow attached as long as possible, making the turbulent wake small. Then the shape is chopped off. The round-fronted, tapered and then chopped-off shape of such a semistreamlined motorcycle may drop to 40 percent of the flat plate's drag, but a full-length streamlined shape could cut this by half again. Yet even shaping a motorcycle to resemble a fish on wheels (excellent streamlining!) would not eliminate "skin drag," which is en ergy transfer resulting from the colli sions of air molecules with the moving surface of the vehicle. We can think about perfection, we can move toward it, but we can't reach it.
