ORIGINAL: allanflowers
I always thought a velocity stack was a tuned length tube that relied on the pressure wave, moving at the speed of sound, to reinforce/supercharge the intake of air into the cylinder. The specific length of the tube causes an increase in intake volume at a particular RPM only. The issue of ram air from the propeller is a separate issue, not to be discounted however.
Allan, you are correct. Especially in relatively still air. But the problem with model aircraft use is that the air around the carb mouth is anything but still... Which is why things work great on one model and terrible on another. Vehicles use large volume airboxes to provide that still air reservoir, but we have no such thing on our planes.
Also, be careful about trying for pressure increases with prop airflow and ram effects.. If you do not vent the reference side of the regulator to the same pressure, you will get mixture changes with the pressure fluctuations. This will cause no end of problems and make tuning impossible. This effect is the very reason so many people vent the diaphragm into the still air inside the fuse to provide stable running.
Mark