It is a simple answer. The sump is a rather small collection area that pretty much stays full of gas all the time (unless you do inverted flight). Without a sump, as the tanks starts to empty somewhat, the gas can slosh around and unport the fuel collection point. On a Longez, the sumps are towards the rear of the tank and are kind of shaped like the bottom of a football. So even when you have 5 gallons or gas or so aside and do a nose down maneuver where most of the gas rushes to the front of the strakes, the gas in the sumps are still there.
Variezes don't have sumps on the main tanks and kind of have this problem----however they have header tanks which are supposed to help alleviate this problem (varieze guys---please correct me if I am wrong). The Longez and Cozy have sumps----many have moved the sump from bulge underneath the wing to inside the cockpit. I have seen them behind the armrests or underneath the R/C seat. Berkut13.com shows a good interior sump also.
