Figure 1-79. Turboshaft engine.  be larger in area. The result is that the fan develops most of  the thrust. The thrust produced by the fan more than makes  up for the decrease in thrust of the core (gas generator) of  the engine. Depending on the fan design and bypass ratio,  it produces 80 percent of the turbofan engine’s total thrust.  free turbine rotates independently. This principle is used  extensively in current production of turboshaft engines. The  turboshaft engine’s output is measured in horsepower instead  of thrust because the power output is a turning shaft. The turbofan gas turbine engine is, in principle, the same as  a turboprop, except that the propeller is replaced by a duct- enclosed axial-flow fan.  [Figure 1-80]  The fan can be a part  of the first-stage compressor blades or can be mounted as a  separate set of fan blades. The blades are mounted forward  of the compressor. Two different exhaust nozzle designs are used with turbofan  engines. The air leaving the fan can be ducted overboard by  a separate fan nozzle  [Figure 1-43] , or it can be ducted along  the outer case of the basic engine to be discharged through  the mixed nozzle (core and fan exhaust together). The fan air  is either mixed with the exhaust gases before it is discharged  (mixed or common nozzle), or it passes directly to the  atmosphere without prior mixing (separate nozzle). Turbofans  are the most widely used gas turbine engine for air transport  aircraft. The turbofan is a compromise between the good  operating efficiency and high thrust capability of a turboprop  and the high speed, high altitude capability of a turbojet. The principle used by a gas turbine engine as it provides  force to move an airplane is based on Newton’s Third Law.  This law states that for every action there is an equal and  opposite reaction; therefore, if the engine accelerates a mass  of air (action), it applies a force on the aircraft (reaction).  The general principle of the fan engine is to convert more  of the fuel energy into pressure. With more of the energy  converted to pressure, a greater product of pressure times  area can be achieved. One of the major advantages is turbofan  production of this additional thrust without increasing fuel  flow. The end result is fuel economy with the consequent  increase in range. Because more of the fuel energy is turned  into pressure in the turbofan engine, additional stages must  be added in the turbine section to provide the power to drive  the fan. This means there is less energy left over and less  thrust from the core exhaust gases. Also, in a mixed-exhaust  nozzle (where fan air and core air mix in a common nozzle  before entering ambient conditions) the exhaust nozzle must  Turbine Engine Operating Principles $^{ }$Turbofan Engines 1-57