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.
Interesting! So basically a (good designed) velocity stack acts like sort of a "tuned intake pipe" using waves to force air inside?
Tuned pipes are actually better than using a free exhaust on 2-strokes, so this sounds like there could be some potential improvements. In fact, I now remember a small trimmer engine I saw at my university. It had a telescopic tube fitted into the carb's intake. By varying it's lenght, you could hear the rpm change, so it makes sense to think that.
And yes, that is actually a very different thing oposed to a ram air system. Even if you put the intake right behind the prop, I'm sure there would be some fluctuations (apart from those cause by varying rpm). Cowl shape (inside & outside), plane attitude, will probably cause the device to work partially or even decrease performance. So I'm kind of unpassionate about trying to make one. Making a thought design is a nice excuse to learn aerodynamics, but if I want to try something fast I'll just have to experiment. Maybe a tube of varying lenght, and several shapes. That is, for a plane and a certain rpm (more likely top rpm). I guess if I still found a design that helped with top rpm, it could still degrade midrange performance. So unless someone has a radically different point of view, I'll make some experiments when I have the time and lack of projects to work on.
Mark, could you further explain that venting issue?