Air Start
On the RB211 powered B747 Classic the QRH clearly indicates Air Starts at below 250 KIAS will require Starter Assistance. As an indication of what windmilling RPM's you should get on the RB-211 @ FL200 $ 250 KIAS. N1 22% - N2 9% and N3 13%. For a normal Ground Start the Start Lever is moved to Rich at 15% N3 and Starter Cut-out is at 50% N3. Windmilling RPM should not cause a problem for engaging the Starter.