 Using this torque curve efficiently is one of the challenges of competitive free-flight rubber flying, and variable-pitch propellers, differential wing and tailplane incidence and rudder settings, controlled by an on-board timeswitch, are among the means of managing this varying torque and there is usually a motor weight restriction in contest classes.Even so, a competitive model can achieve flights of nearly 1 hour.Stored compressed gas, typically carbon dioxide (CO2), can also power simple models in a way similar to filling a balloon and then releasing it.