*Puts on blockbuster trailer voice* "In a world of helicopters, one DLC had... *pause for explosion* ...more helicopters. It's time to experience the helicopter experience of the year. From the studio that brought you Take On Helicopters; it's Helicopters—a DLC for Arma 3. Coming this right now." *Fade to black*
Okay, so its not loads more helicopters. In fact, two: the CH-67 Huron and the Mi-290 Taru. In addition to that, there's a new showcase and some time trials. The DLC costs 11 / $16.
Perhaps more interesting are the free additions being made to the base game. Accompanying Helicopters is a platform update that adds a new multiplayer mode, helicopter VR training, and optional alternative flight dynamics. These extras are being made available to all Arma 3 players, regardless of whether they buy the Helicopters DLC.