Photos from wind tunnel tests showing airflow on top and bottom of flying disc.

Remembering this from Physics 101 - aerodynamic drag is the transfer of energy from a moving object to the air though which it moves. This means that most of the energy imparted on a disc by a thrower is lost to air it encounters during its flight. Let's build upon that.

A simple measure of the air accelerated by object motion is given by determining the object "frontal area." For a disc, imagine placing a disc flat on a table. Then turn on a flashlight and lay it on the table facing the disc. The shadow cast by the disc is a representation of its frontal area in flight. This area then can be considered as a "piston" which the thrower is pushing through the resisting air. To continue the analogy, the "stroke" of the piston is the distance that the moving object advances in one second. Thus, the rough volume of air disturbed is frontal area multiplied times speed in feet per second (fps).

How much pressure does the air exert against the front of a flying disc? If we replace our disc with a flat piece of plywood of the same area, and assume that all the air hitting its flat surface is stopped dead (actually it is accelerated to the speed of the disc), then the force created is the so-called "dynamic pressure." This is the pressure that results from full conversion of air velocity into pressure, and it proportional to the square of the speed. At 100 fps (68 mph - a medium hard throw) this is about 12 pounds per square foot, at 150 fps (100 mph - probably few players can throw this hard) the dynamic pressure is 27 lbs/sf, and so on. A disc sized flat frontal area of 1/12 of a square foot (a lid, 1" high by 12" wide approximately), moving at 100 fps would produce a drag force of 1 lbs/sf. A golf disc with a flat frontal area of 1/48 of a square foot (3/4" high by 8.5" wide), moving at 150 fps would produce a drag force of .56 lbs/sf.

Discs do much better that a flat piece of plywood of course. The rounded front edge, beveled rim and sloping top surface means that the air is not completely stopped against it converting all its energy into pressure. Instead, airflow will curve gently around, to flow off the edge of the disc almost parallel with the disc's direction of motion. Discs will convert some of the air's velocity (relative to movement) into pressure, but because this is no longer 100 percent conversion, the dynamic pressure on the rounded front will be less than it was against the flat plywood. Drag is lessened. This kind of frontal streamlining cuts drag to less that half that of a flat plate. A final word about drag resulting from dynamic pressure, when a thrower launches a disc "with the wind", the increase in distance expected as opposed to throwing into the wind, is achieved not because the wind is pushing the disc from behind, but because there is less drag due to dynamic pressure. The drag is lessened because the disc is moving slower relative to the air when thrown down wind

The smooth disc shape guides the air around and the then the air must be put back together behind the disc. This wake region is at low pressure because it's surrounded by high-velocity, low-pressure airflow. Crudely put, drag is the difference between the dynamic pressure action on the disc's front surface and this low-pressure acting on the rear. The flow streaming past the edges of a disc is pushed into this low-pressure wake region by the surrounding 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 wake region. Finally hitting the flow on the opposite side, 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 disc's wake becomes a trailing series of such alternating vortices, carrying away energy in their whirling motion. Anyone who has ridden close behind a big truck has felt the side-to-side buffeting of these forces known as "von Karman" vortices.

The flow tries to come back to together behind our disc smoothly, rather than leaving a mess of energy robbing vortices. Ideally, drag becomes zero when the disc merely displaces air molecules from their original positions momentarily, and then puts them all carefully back exactly where the were originally and leaving them with no extra energy. Think of the shape of an airplane wing, the round front with a gentle taper in the rear. This reduces drag because instead of a partial vacuum in the wake region we now have whatever pressure the flowing decelerating air exerts pushing on the tapered tail. The recovered pressure helps to offset the pressure of air hitting the front of the wing.

The problem with streamlining discs is that since they rotate, they have no front and back per se. Unlike an airplane wing (or even a fish), they don't have a tail end to be tapered to smooth the passage of the air (water) from front to rear. A tapered tail would keep the airflow attached as long as possible, keeping the turbulent wake small. The rear inside face of the rim is catching air, rather than allowing its smooth passage from the rear of the disc. The reason why throwers should keep the front edge slightly "nose-down" on long throws.

The other element of drag to consider is "skin drag" which is the energy transfer resulting from the collision of air molecules with the moving surface of the disc. Smooth surface, less skin drag (harder to grip to throw - your choice).

We can think about the perfect disc thrown without drag forces. Disc designers move toward it, but can't reach it. What a drag.

Physics Rules!