Determination of specific fuel consumption of air-breathing engines is one of the problems of modeling their performance. As a rule, the estimation error of the specific fuel consumption while calculating air-breathing engine performance is greater than that of thrust. In this work, this is substantiated by the estimation error of the fuel-air ratio, which weakly affects thrust but significantly affects the specific fuel consumption. The presence of a significant error in the fuel-air ratio is explained by the use of simplified methods, which use the dependence of enthalpy as a function of mixture temperature and composition without taking into account the effect of pressure. The developed method to improve the calculation accuracy of specific fuel consumption of air-breathing engines is based on the correction of the fuel-air ratio in the combustor, determined by the existing mathematical models. The correction of the fuel-air ratio is made using the dependences of enthalpy on mixture temperature, pressure and composition. The enthalpy of the mixture is calculated through the average isobaric heat capacity obtained by integrating the isobaric heat capacity, depending on mixture temperature, pressure and composition. The calculation accuracy of the fuel-air ratio was verified by comparing it with the known experimental data on the combustion chamber of the General Electric CF6-80A engine (USA). The average calculation error of the fuel-air ratio does not exceed 3 %. The developed method was applied for correcting the specific fuel consumption for calculating the altitude-airspeed performance of the D436-148B turbofan engine (Ukraine), which made it possible to reduce the estimation error of the fuel-air ratio and specific fuel consumption to an average of 3 %
For supersonic cruising, combined power plants can be used, in which a gas turbine engine reaches the cruising mode, and a ramjet is used for cruising. Supersonic transoceanic flights are characterized by a long cruising segment, which is decisive in terms of required fuel mass. Therefore, the selection of cruising and engine operation parameters is an important task. As a rule, when selecting the cruising mode, the range parameter is used, which depends on the flight and engine operation modes. To take into account the influence of the ramjet operating mode on the range parameter, dimensionless relationships of engine parameters with control factors were obtained. Using the obtained relationships together with the equations of aircraft motion in steady horizontal flight, it is shown that the values of the engine control factors and the range parameter do not change at the altitudes of 11...20 km. This made it possible to conclude that the range parameter can be increased only by selecting the cruising and engine parameters that provide the minimum specific fuel consumption. The variable cruising parameters are speed and initial altitude. A method for selecting the cruising and ramjet operation parameters was developed, based on the analysis of the relationship between the range parameter and the flight speed and initial altitude at the most advantageous values of the engine control factors. The obtained relationships allow selecting the cruising parameters and the engine operating mode, taking into account the restrictions. It is shown that the specific fuel consumption decreases by 0...30 %, depending on the engine operating mode, when the control program is optimized
There are two aircraft concepts for transoceanic flights at supersonic speed. The first is the aircraft with moderate supersonic cruising speed M = 1.7...2.2, and the second is the aircraft with cruising speed M ≥ 3. Propulsion with a ramjet engine (ramjet) is preferable for high supersonic speeds. However, a ramjet has no starting thrust and is uneconomical at subsonic flight speeds. Combined propulsion with a turbojet duct for takeoff and a ramjet duct for supersonic cruise flight are used to overcome this contradiction. There are known propulsion with turboramjet engines, as well as various propulsion schemes with turbofan engines (afterburning and duct-burning ones), in which the outer contour together with the afterburner and nozzle can be considered the ramjet duct. When the turbojet duct is turned off, the operating process of such turbofan engines is practically the same as the ramjet, which allows using the advantages of a ramjet in supersonic cruise flight. Because flight at supersonic cruising speed can be provided by different propulsion compositions, the choosing problem of the composition and parameters of the propulsion for an aircraft with supersonic cruising speed is relevant. A calculation-analytical method was used to select the composition and parameters of the propulsion, which is based on the maximum relative mass of the payload criterion. Using the aircraft mass balance equation this criterion can be represented as a minimum conditions for the relative mass of the fuel and propulsion. Using this method, for an aircraft with a given mass, aerodynamic characteristics and a given flight profile, for the indicated propulsion compositions, the change patterns of the minimum relative mass of fuel and propulsion in the range of M = 2.5...4 are established. The established patterns allow choosing the propulsion composition and parameters depending on the speed of the supersonic cruising flight or choosing the flight speed of the aircraft at a given propulsion composition. The dependance of the propulsion working process optimal parameters for takeoff and cruising mode on the speed of supersonic cruising flight according to the criterion minimum of relative mass of fuel and propulsion were obtained.
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