The variable pitch propeller optimises traction at low speed (small pitch) and performance at cruising speed (large pitch). In the 1930s, the Ratier company perfected this type of propeller. Its engineers invented a pneumatic mechanism – a rubber bladder inflated on the ground with a bicycle pump – which blocked the blades on small pitch at take-off. At cruising speed, the pilot would deflate the bladder and the blades would switch to high pitch. From 1934 onwards, the aircraft manufacturer Caudron installed Ratier variable-pitch propellers on the C-630, then on the C-635 fitted with a two-blade propeller and 6-cylinder Renault Bengali engine.