posted 7 years ago
The separate air intake at the base of the feed tube is generally an idea of people who haven't built RMHs for themselves. It defeats one of the first principles of the J-tube, the downward-rushing air that pulls flames down and keeps the entire fuel load from burning at once and making a strong competing chimney that would burn up out of the feed opening.
There is a common secondary air supply practice, called a P-channel (for Peter van den Berg who developed it), which consists of a way for about 5% of the cross section to be separated and protected from fuel blockage. It is generally implemented as a sheet of metal at the edge of the feed tube, extending down about 1/4-3/8" below the burn tunnel roof. This creates added beneficial turbulence and fresh air at a critical spot.
Operators generally find that at full combustion rate, the top of the feed tube can be blocked about 2/3 to achieve the best combustion.