During World War Two, after the 1940 surrender of Belgium to Germany, exiled members of the Belgian Air Force created a new Belgian Section under the control of the Royal Air Force of the United Kingdom. While the RAF ensign most certainly would have been used officially, it is highly probable (and supported by various sources) that members of the Belgian Section altered the colours of the roundel to that of the Belgian flag in an unofficial capacity. It is also extremely likely that the RAF ensign inspired the design of the Belgian Air Force Ensign that was adopted after the war. That ensign can be seen below.
This SVG flag includes elements  from this flag:
Fry1989 eh?