You'll be greeted with a new goal upon logging into your Empire, which will simply have you placing an Air Force Research Lab on your land and whacking it three times to finish its construction. Once that's complete, you can start researching upgrades on the various air units that you've currently unlocked to build in Hangers. That is, even though you can eventually upgrade a Stealth Bomber, for instance, you wouldn't be able to research any upgrades for it until level 47.
You should be pretty familiar with the upgrading process by this point, but if not, here's a recap. You'll first need to research each particular unit type (Vintage Fighters, Zeppelin Airships and Vintage Bombers being the first three) by collecting items like Upgrade Blueprints, Welding Kits, Ejector Seats and so on, with different items being required for different units. Once you have collected all of these items, you'll finally be able to actually upgrade your units, giving them increased damage and accuracy, reducing the time it takes to build them, etc.
You can only upgrade one particular stat at a time, and each upgrade takes some time to complete. It's worth it though, as you'll be able to carry over these upgrades automatically to all units of that type that you currently own, along with any you happen to build in the future. If you're looking for that extra edge against a story-based opponent, or even just looking for a better chance when invading friends' Empires, upgrading your units is a pretty good way to go. Now that you're able to upgrade your Air Force units, on top of ground and naval units, you're ready to build the best army yet, so get out there soldier!
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Do you actively use your other Research Labs? Will you actively upgrade your Air Force units as well, or have you not noticed a difference / improvement in other kinds of troops that you've upgraded? Sound off in the comments.