In the Gathering Storm expansion, the Biplane requires no Oil to train but 1 Oil per turn to maintain.
The Biplane is the first air unit that becomes available, and the only one that doesn't require Aluminum to build. It also becomes available before any form of anti-air defense, which will allow it to provide support to nearby land units without fear of retaliation until other civilizations reach the Atomic Era.
Although its Ranged Strength of 75 is respectable (and allows it to attack all ground units without retaliation), the unit suffers from its extremely limited range of operation. Even when deployed, its effective range from base is 6 tiles (3 of which must be on your territory!), which is usually not enough for reaching targets for offensive operations. This means that unless you have a squad of Military Engineers ready to build Airstrips on your frontiers, you will have to use Biplanes in defense only.
If air units are an important part of your strategy, be sure to research Advanced Flight and upgrade Biplanes to Fighters as quickly as possible - this will make them much more useful. Even so, though, be mindful that the real offensive air weapon in the game is the Bomber.
Civilopedia entry Edit
Before WW1, the airplane was a clever curiosity for getting grown men excited. Airplanes were fragile, underpowered (gliding as much as flying), clumsy, limited in distance and payload, expensive, and generally biplanes, since two wings served better than one for maneuverability and lift in the face of all these limitations. By the end of WW1, the airplane was a dreaded machine of war, able to spot enemy movement at a distance and then rain destruction down upon it. The biplanes of the 1914-1918 conflict became ever-more specialized as the war progressed, from the nimble fighters to the dependable recon planes to the lumbering bombers soon flying over enemy cities dropping explosives on civilians. By the end of the war, however, the wood-and-fabric biplanes were disappearing, being replaced by metal covered, single-wing craft capable of better speed and faster climb. But by then, the exploits of heroic, gallant men in their brightly-painted biplanes had already entered the psyche of war.