Thanks all for you info on this ...

... and hey, thanks for discovering my FAQ draft, forgot I had it up. John
Seaborn improved on it (with nice pictures) at www.ssa.org/UsTeam/adobe
pdf/pr pdf/BR Soaring FAQ V4 04.pdf , along with other gems in the US Team
Press Room, www.ssa.org/UsTeam/ust_press.htm .

My reading of many old books suggests that segelflugzeug - prueflung -
zoegling (sailplane - secondary - primary) was a breakdown set at 20:1 and
10:1 as of the early days of the RRG and Wasserkuppe flight school ca 1923.
In the USA, builders preferred lumping primary and secondary categories
together as the "utility" glider, reflected in the 1930 Franklin PS-2
("Primary and Secondary, too") or some Schweizer gliders up through 1968
(e.g. SGU 2-22E, SAC Glider Utility model #22, vs. SGS 2-8). This despite
Col. Evans and JC Penney Jr. having imported one of all 3 kinds of german
glider in 1928 for display the Detroit Air Show (including the D-1 as the
"sailplane").

Good point about Otto and Gustav. "Birdflight as the basis for Aviation" is
in reprint and evidences the fascination of those brothers with soaring
flight. The quote I liked came early in their program:
"We returned home, after these experiments, with the conviction that sailing
flight was not the exclusive prerogative of birds" - Otto Lilienthal, 1874.
However, their devices were hardly sailplanes (either in performance,
control, or launch means), more like parachutes.