I refer to an earlier answer of mine, which in part deals specifically with the the German Airforce in Russia, to be more precise: The German army established a secret airbase in Lipetzk. This was made possible by the 1922 Treaty of Rapallo which established a friendly atmosphere between Russia and Germany.https://www.reddit.com/r/AskHistorians/comments/63rrpw/did_fokker_willingly_sell_aircraft_to_the_germans/dfwh72a?utm_source=share&utm_medium=web2x

Between 1925 and 1933, in defiance of the Versailles treaty, the Germans operated Lipetzk as a training and research facility. They were able to develop aerial warfare tactics there and trained aircrews, far away from the eyes of the western nations. These pilots and aircrew formed the kernel around which Nazi Germany was able to expand its Luftwaffe so rapidly after they openly started to defy the Versailles treaty.

For a broader answer to your question I hope someone else will pitch in.

In addition to the source mentioned in my earlier post:

Heinrich Beauvais et al, Die Deutsche Luftfahrt Flugerprobungsstellen bis 1945 (27), pp 54-65James S. Corum, The Luftwaffe: Creating the Operational Air War 1918-1940