A propelling nozzle converts a gas turbine or gas generator into a jet engine. Energy available in the gas turbine exhaust is converted into a high speed propelling jet by the nozzle. Turbofan engines may have an additional and separate propelling nozzle which produces a high speed propelling jet from the energy in the air that has passed through the fan. In addition, the nozzle helps to determine how the gas generator and fan operate as it acts as a downstream restrictor.