Andy,

Your 50:1 math is right and wrong at the same time. What is missing
is the fact the the centre of the 3 km finish cylinder is located at
the opposite end of the 4000 ft landing runway. This means that when
you cross the finish line at about 200 ft agl you are about 1.8 km
from the touchdown point (and 3 km from the other end of the runway).
This gives L/D of 30:1 to touchdown which is sufficient for the ships
flying in the 15/18/ Open Classes.