My background is Mechanical Engineering. I had spent the better part of 12 years working at a rocket ranch (Hughes Missile Systems). I am also lucky enough to say I have not missed an archery Elk hunt in 28 years. I am even luckier to say it has been with the same group of friends. My thoughts regarding bodies in motion come from crunching the numbers. My thoughts regarding arrow flight comes from hunting and the frustration of getting a broadhead to fly. So in a nut shell.....I am a "hillbilly" engineer and proud of it.
The 3 basic principals to the flight of the FOB are; drag, spin and lift. For my thoughts on spin and drag please see thread https://archerynation.forumotion.com/t32-spin-and-drag
As we all know, the top of an airplane wing has more surface area than the bottom. The air that travels under the wing has pretty much a straight shot to the back while the air that travels over the top has a longer way to go. Both the air on the top and bottom has to meet at the back of the wing at the same time so the air on the top of the wing is forced to go faster than the air on the bottom. Based on the laws of fluid dynamics, the faster a fluid is moving the lower the pressure. So the top of the wing has a lower pressure and creates lift.
The FOB lift is a different animal all together, but it does follow the laws of fluid dynamics. The air that enters the FOB is funneled through the ring wing, hits the fins like a fletch (causing rotation) but also induces circumferential lift (360 degree lift) the ring wing acts like an airplane wing even though it is not shaped like a conventional wing. This is caused by the fact that the air traveling inside the FOB is going slower than the air traveling outside the ring wing. This pressure differential causes the lift (true lift created by pressure). So in effect, the FOB is trying to pull itself apart. It is basically an air gyro. It does not lift the back of the arrow. It provides as much lift up as it does down and side to side. All we have done is taken the drag component (momentum transfer of the air hitting the fins) and used it for good. This effect is substantial and greatly reduces the oscillation of the arrow = more KE downrange.
If you look carefully at the fin structure you will see that the fin is a double back wedge shaped. The volume of the internal structure of the FOB is therefore reduced as you move through the FOB causing the compression. A closer look at the fin structure will reveal that the last 1/4 of the fin profiles back to a sharp edge, basically a reverse wedge. This is where the decompression takes place. This structure is a kind of Ventura as the fins are wedge shaped but angle back to a sharp trailing edge. So the volume decreases and then opens back up. We found that this decompression induced a greater torque (spin) without having to increase to angle of the wedge thus keeping drag at a minimum. The fact that the air is being compressed does take energy, however, the fact that the air increases in mass makes the air contacting the fin more efficient. Heavier air has more bang for the buck causing a higher torque for a quick spin up. Not faster spin, just getting up to terminal spin quicker.
It is the Lift that makes the FOB work. That is how we get so much bang for the buck out of something that is only 1/2 inch long. This is also why some folks think the FOBs make more noise. But that is for another thread all together!
Well…..Those are my thoughts of how a FOB works from a proud “hillbilly” engineer.