A few suggestions on aircraft techs, then. Also have a look at my post on the previous page, listing my suggestions for aircraft models, and on page 4 of this thread for lighter-than-air techs.
1. Principles of Flight Includes the techs Primitive Aerodynamics then Gliders. No benefits, just pre-requisites for later techs. Second-rank industrial powers would be at this level at the start of the game.
2. Powered Flight Includes the techs Primitive Aero-Engine (needs Internal Combustion Engine as a prerequisite), Airscrew, Primitive Airframe. A classic example/picture would be the Wright Flyer. Again, no direct benefits. Most first-rank powers would be at this level at the start of the game.
3. Primitive Aircraft Allows the aircraft unit Reconnaissance Plane to be built. (As with standard HoI, I'd suggest you first have to develop a prototype, then research the actual aircraft model.) Represents aircraft such as the French Farman or British Shorthorn and Longhorn. Germany, France and Britain should be at this level at the start of the game; maybe Russia, not sure about Austria and the USA.
4. Armed Aircraft Includes the techs Bombs (which may need a prerequisite Aerial Fuses) and Machine Guns (may need prerequisites from the Infantry tech tree). Gives an AA factor to the Recce Plane, and allows the unit Basic Bomber to be built. Represents the first armed aircraft. Italy should have developed the gold tech at the start of the game (they were the only country in 1914 to have already used aircraft in combat).
5. Basic Combat Aircraft Design Includes the (expensive) tech Synchronised Machine Guns. Also includes Basic Engine, Basic Airframe, Basic Aerodynamics.
6. Basic Combat Aircraft Development Allows the units Basic Scout Fighter, Basic Two-Seat Fighter, Basic Seaplane. Classic example: the Fokker Eindecker.
7. Multi-engined Aircraft Requires Engine Synchronisation, Improved Aerodynamics, Improved Structural Materials. Allows the units Basic Strategic Bomber, Basic Escort Fighter. Classic example: the Gotha.
8. Improved Combat Aircraft Design Includes Improved Engine, Improved Airframe.
9. Improved Combat Aircraft Development Allows the units Improved Scout Fighter, Improved Two-Seat Fighter, Improved Escort Fighter, Improved Seaplane, Improved Bomber. Classic examples: Albatros, Sopwith Triplane, Nieuport 17.
10.Ground Attack Aircraft Includes Advanced Structural Materials, Advanced Engine, Armour Plate. Allows units Improved Strategic Bomber, Ground Attack Aircraft. Classic example: Handley Page 0/400.
11. Advanced Combat Aircraft Design Includes Advanced Aerodynamics, Advanced Airframe.
12. Advanced Combat Aircraft Development Allows the units Advanced Scout Fighter, Advanced Two-Seat Fighter, Advanced Escort Fighter, Advanced Seaplane, Advanced Bomber, Advanced Strategic Bomber, Improved Ground Attack Aircraft. Classic examples: Fokker D.VII, Sopwith Camel, Vickers Vimy.