A diorama can be telling a story, showing a vehicle 'doing it's thing' or simply a small base to give an idea of where the vehicle is operating (Photo 1 sees an A7-E shown on a tarmac base, which looks much better than just sitting on a bare table)
up to more complex scenes with multiple buildings or vehicles. (photo 2 is a combination of five Verlinden building kits with a Churchill Bridgelayer about to 'repair' a demolished bridge, while photo 3 shows a simple scene with a P-51D in a natural setting)
Aircraft dioramas are a little more limited in that an aircraft 'doing it's thing' is flying and it is very difficult to convincingly portray an aircraft in flight. Aircraft invariably come back to earth either sitting on a tarmac being serviced (photo 4, is an Israeli F-4E in final preparation for another raid into Lebanon) or by crashing (photo 5).
Vehicle dioramas are virtually unlimited in the use of terrain, the number of vehicles, figures or buildings etc. as anything is possible.
If you are building your first diorama start with a simple scene (the Verlinden range is ideal for this) before moving on to larger and more complex scenes as your skills and confidence increase.
Ideas and Planning