This article discusses the features of very high bypass ratio turbofans and open rotor engines. Each of these engine options has its pros and cons to consider. The very large bypass ratio turbofan engine maintains that the proven capability of containment of blade failures is inherently quieter due to ability to incorporate acoustic treatment in the fan duct and is not subject to high fan tip losses associated with direct exposure to higher cruise level flight speeds. The duct does not come for free, however, and installed weight becomes a primary concern as the increased bypass ratio drives up the engine diameter. Additionally, the fan is subject to higher local airfoil incidence when the fan nozzle un-chokes at low flight speed. The open rotor engine can achieve potentially greater improvements in propulsive efficiency than a turbofan but lacks the containment and noise reduction benefits of a duct. The rotor is also exposed to flight speed, driving up tip losses at today's accepted cruise flight speeds.

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Engine manufacturers and commercial airlines are Looking to the future for the next step in the evolution of gas turbine engines for the industry. Conservation of fuel and reduction in noise are of primary importance in their pursuits. The key to both objectives is improving the fundamental propulsive efficiency of the engine. With subsonic transport applications, this is best achieved by thrusting large quantities of air flow at velocities close to the aircraft travel speed. Two options are to increase the current state of the art bypass ratio level of turbofans (the ratio of air mass flow rate ducted by the fan around the engine core to the engine core airflow) or to eliminate the duct and use an open rotor (advanced prop fan). In either option, the pressure ratio of the fan element has direct influence on the velocity of the air propelled by the engine. Figure 1 illustrates how this fan pressure ratio reduction enhances propulsive efficiency at two levels of flight Mach number. It is evident that reducing exit air velocity has most benefit at lower flight speeds.

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Each engine option has its pluses and minuses to consider. The very large bypass ratio turbofan engine maintains the proven capability of containment of blade failures, is inherently quieter due to ability to incorporate acoustic treatment in the fan duct and is not subject to high fan tip losses associated with direct exposure to higher cruise level flight speeds. The duct does not come for free, however, and installed weight becomes a primary concern as the increased bypass ratio drives up the engine diameter. Additionally, the fan is subject to higher local airfoil incidence when the fan nozzle un-chokes at low flight speed. The open rotor engine can achieve potentially greater improvements in propulsive efficiency than a turbofan but lacks the containment and noise reduction benefits of a duct. The rotor is also exposed to flight speed, driving up tip losses at today's accepted cruise flight speeds.

Different engine manufacturers are currently developing advanced turbofan and/ or open rotor engines, and making claims to be the correct choice for the future. Without specific details on any particular engine geometry or technology, it is difficult to assess the individual options, but it is possible to do a “first cut” comparison. To do this, we will confront the major performance issue of each engine. For the turbofan, it is the installed weight and drag associated with the large duct. For the open rotor, it is the performance loss at higher flight speeds.

This evaluation considers two engines of similar technology and thrust size. The turbofan engine incorporates a gearbox to enable the optimum combination of low rotor speed of the fan and high rotor speed for engine core components. It also includes the latest state of the art compact variable pitch mechanism to adjust fan blade incidence at low flight speed. Based on installed performance evaluation, this engine optimizes at a fan pressure ratio of 1.3 and a bypass ratio in the range of 12 to 13. The comparable open rotor engine has an effective “bypass ratio” level in excess of 20. Each engine will be compared with today's nominal turbofan engine having a bypass ratio of6 and a fan pressure ratio of1.67 on the basis of fuel burn at different flight speeds.

The results of this assessment are provided in Figure 2 [Note that the open rotor engine fuel burn does not include a debit for installation effects associated with the weight of required local hardening of the aircraft for blade loss which is difficult to evaluate for this level of comparison.] As one might anticipate, the open rotor engine is a clear winner at low flight speeds, but loses this advantage as flight speed is increased beyond a flight Mach number of0.7.With current cruise speeds in the range of Mach 0.8 and higher, the very high bypass ratio engine the scale tips in favor of the very large bypass ratio turbofan. In either case, whether the application is for short cruise segments with extended loiter where the open rotor fits best or for extended high speed cruise where the very high bypass ratio engine excels, the future engines offer substantial fuel savings over today's engines.

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