1.NounA device that alters the aerodynamics of an airplane, glider etc, allowing it to descend quickly but safely, especially when caught in a thermal""One of the most bittersweet joys in model aviation is an out of sight flight by a Free Flight model. While we all strive for our models to fly as well as we can, it is possible to have a model fly too well. Well trimmed, and with a bit of luck, a model can be caught in a thermal and climb to beyond the limits of our vision. It is the purpose of a dethermalizer to effect a change in the geometry of a model, such that it descends more rapidly, yet still safely. Getting the DT to activate after a target time is the job of a DT timer." ~ Thayer Syme. Reprinted with permission."