Slats were first developed by Gustav Lachmann in 1918. A crash
in August 1917, with a Rumpler C aeroplane on account of
stalling caused the idea to be put in a concrete form, and a
small wooden model was built in 1917 in Cologne. In 1918,
Lachmann presented a patent for leading edge slats in Germany.
However, the German patent office at first rejected it as the
office did not believe in the possibility of increasing lift by
dividing the wing. Independently of Lachmann, Handley-Page Ltd
in Great Britain also developed the slotted wing as a way to
postpone stall by reducing the turbulence over the wing at high
angles of attack, and applied for a patent in 1919; to avoid a
patent challenge, they reached an ownership agreement with
Lachmann. That year a De Havilland D.H.9 was fitted with slats
and flown. Later a D.H.4 was modified as a monoplane with a
large wing fitted with full span leading edge and back ailerons
(i.e. what would later be called flaps) that could be deployed
in conjunction with the leading edge