Aircraft auxiliary power unit (APU) control system with speed compensation

Abstract

Aircraft auxiliary power unit (APU) control system with speed compensation. An auxiliary power unit (APU) control system for an aircraft is disclosed, the APU control system comprising: an APU drivingly connected to one or more generators; one or more processors; and a memory connected to the one or more processors. The memory stores data including a database and program code, which, when executed by the one or more processors, causes the APU control system to receive one or more environmental signals indicating an air density value and one or more power signals indicating a specific amount of power generated by the APU. The system is further caused to determine a first variable speed of the APU based on the air density value. The APU continues to generate the specific amount of power when operating at the first variable speed. After instructing the APU to operate at the first variable speed, the system receives an electrical load signal.

Description

Technical Field

[0001] The present disclosure relates to an auxiliary power unit (APU) control system for an aircraft. More specifically, the present disclosure relates to an APU control system for compensating the rotation speed of the APU to maintain a substantially constant power output. Background Art

[0002] The aircraft may be equipped with an auxiliary power unit (APU) in the form of a gas turbine. In some large commercial aircraft, the APU provides electrical power as well as exhaust. The exhaust is drawn from the APU's load compressor. However, the APU is limited to providing exhaust on the ground and at very low altitudes. Alternatively, an electric aircraft includes an APU that only provides electrical power. The APU of an electric aircraft does not include a load compressor and therefore does not provide exhaust. Although electric aircraft are discussed, this approach is also used on aircraft that use electricity for all non-propulsion systems.

[0003] Gas turbines are typically operated at a steady-state output speed to provide power and, in at least some aircraft, exhaust. The performance of gas turbines is affected by environmental conditions. Specifically, the power output and efficiency of a gas turbine depend on ambient conditions, such as altitude, air temperature, humidity, and air density. The reduction in gas turbine power output is proportional to the increase in altitude. For example, the air density at sea level is about three times higher than at an altitude of 40,000 feet (about 12,192 meters). The reduction in air density also reduces the mass flow of air entering the gas turbine. Therefore, a gas turbine can only produce about a quarter of the power at an altitude of 40,000 feet compared to sea level.

[0004] Existing APUs found on aircraft are often oversized to accommodate potential surge loads, which occur when load demands increase significantly over a very short period of time. More specifically, the turbine is sized to have sufficient rotor inertia so that surge loads do not cause the rotational speed to drop below a predetermined frequency. However, an oversized APU adds mass to the aircraft.

[0005] It is with respect to these and other considerations that the disclosure herein is presented. Summary of the invention

[0006] According to several aspects, an auxiliary power unit (APU) control system for an aircraft is disclosed. The APU control system includes: an APU drivingly connected to one or more generators; one or more processors; and a memory connected to the one or more processors. The memory stores data including a database and program code, and the program code, when executed by the one or more processors, causes the APU control system to receive one or more environmental signals indicating air density values ​​and one or more power signals indicating a specific amount of power generated by the APU. The system is further caused to determine a first variable speed of the APU based on the air density value and to instruct the APU to operate at the first variable speed. The APU continues to generate the specific amount of power when operating at the first variable speed. After instructing the APU to operate at the first variable speed, the system receives an electrical load signal. The electrical load signal indicates an electrical load demand of the one or more generators. The system is further caused to determine a second variable speed of the APU based on the electrical load demand of the one or more generators and to instruct the APU to operate at the second variable speed.

[0007] According to another aspect of the present disclosure, an aircraft is disclosed, the aircraft comprising an APU control system, an APU, and one or more generators drivingly connected to the APU, wherein only the one or more generators are provided as loads of the APU. The aircraft also includes a memory connected to the one or more processors. The memory stores data and program codes including a database, and the program codes, when executed by the one or more processors, cause the APU control system to receive one or more environmental signals indicating an air density value and one or more power signals indicating a specific amount of power generated by the APU. The system is further caused to determine a first variable speed of the APU based on the air density value and to instruct the APU to operate at the first variable speed. The APU continues to generate the specific amount of power when operating at the first variable speed. After instructing the APU to operate at the first variable speed, the system receives an electrical load signal. The electrical load signal indicates an electrical load demand of the one or more generators. The system is further caused to determine a second variable speed of the APU based on the electrical load demand of the one or more generators and to instruct the APU to operate at the second variable speed.

[0008] According to another aspect of the present disclosure, a method for adjusting the variable speed of an APU in an aircraft is disclosed, the method comprising receiving, by a computer, one or more environmental signals indicating an air density value and one or more power signals indicating a specific amount of power generated by the APU. The method also comprises determining, by the computer, a first variable speed of the APU based on the air density value. The method also comprises instructing the APU to operate at the first variable speed, wherein the APU continuously generates the specific amount of power when operating at the variable speed. After instructing the APU to operate at the first variable speed, the method receives an electrical load signal. The electrical load signal indicates an electrical load demand of one or more generators drivingly connected to the APU. The method also comprises determining, by the computer, a second variable speed of the APU based on the electrical load demand of the one or more generators. Finally, the method comprises instructing the APU to operate at the second variable speed.

[0009] The features, functions, and advantages that have been discussed can be achieved independently in various examples or may be combined in yet other examples further details of which can be seen with reference to the following description and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

[0011] Figure 1 is a schematic diagram of an aircraft including a disclosed auxiliary power unit (APU) according to an illustrative example;

[0012] Figure 2 yes Figure 1 A schematic diagram of an APU is shown, wherein the APU includes a load compressor according to an illustrative example;

[0013] Figure 3 is a schematic diagram of an optional APU for an electric aircraft according to an illustrative example;

[0014] Figure 4 is a graph showing a speed compensation curve based on air density according to an exemplary example;

[0015] Figure 5 It is shown Figure 4 A graph of a speed compensation curve shown, wherein positive and negative compensation of power are shown according to an exemplary example;

[0016] Figure 6 is a graph showing a turbine speed curve for a disclosed APU according to an illustrative example;

[0017] Figure 7 It is shown Figure 4a graph of a speed compensation curve shown, wherein positive and negative compensations for aircraft speed are shown according to an illustrative example;

[0018] Figure 8 is a process flow chart illustrating a method for adjusting an APU rotation speed according to an illustrative example;

[0019] Fig. 9 is a process flow diagram illustrating a method for performing velocity compensation in an electric aircraft according to an illustrative example; and

[0020] Fig.10 According to the exemplary example, Figure 1 Illustration of the computer system used by the APU control system. DETAILED DESCRIPTION

[0021] The present disclosure is directed to an auxiliary power unit (APU) control system for an aircraft, wherein the APU is a gas turbine. The APU control system varies the APU speed while maintaining a substantially constant power output, even if aircraft environmental conditions and operating parameters that affect APU performance change. Specifically, changes in environmental conditions (such as air density, air temperature, humidity, and altitude) may adversely affect the performance of the APU. For example, as the aircraft altitude increases, the air density decreases, which in turn reduces the mass air flow provided to the APU. The APU control system compensates for the decrease in air density by adjusting the APU speed. Therefore, even if the inlet air density decreases, the power output of the APU remains substantially constant. In addition to air density, the APU control system also adjusts the speed of the APU based on other environmental conditions (such as aircraft speed and humidity).

[0022] The APU control system also provides speed compensation based on electrical load demand, thereby improving the efficiency of the APU. Specifically, during full load demand, the APU speed increases, however, lighter load demands allow for a reduction in the APU speed. Thus, the disclosed APU control system may require a smaller APU to perform the same functions as a conventional system that relies on a gas turbine operating at a fixed speed. In one example, the disclosed APU control system may be used in an electric aircraft, or in an aircraft that uses electric power for all non-propulsion systems.

[0023] The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.

[0024] refer to Figure 1, shows a schematic diagram of an auxiliary power unit (APU) control system 18 for an aircraft 20. The APU control system 18 includes an APU 22, one or more generators 24 driven by the APU 22, and a control module 28 in electrical communication with the APU 22 and the generators 24. The APU 22 is a gas turbine configured to convert fuel into mechanical energy. Although Figure 1 One or more generators 24 are shown drivingly connected to the APU 22, but it should be understood that other devices may also be driven by the APU 22. For example, devices such as an air compressor or a hydraulic pump may be drivingly connected to the APU 22. The control module 28 is also in electrical communication with one or more other control modules 40 in the aircraft 20. In one example, the control module 40 includes a flight computer control module.

[0025] As explained in more detail below, the APU control system 18 dynamically adjusts the APU 22 rotational speed based on environmental conditions and operating parameters of the aircraft 20. Specifically, the APU control system 18 includes a speed compensation mode that adjusts the rotational speed of the APU 22 to maintain a substantially constant power output. In addition to the speed compensation mode, in examples, the APU control system 18 also includes a variable speed ground mode, a maintenance mode, or both a variable speed ground mode and a maintenance mode. Both the variable speed ground mode and the maintenance mode are executed only when the aircraft 20 is on the ground. However, the speed compensation mode is executed when the aircraft is on the ground or in flight.

[0026] Figure 2 is a schematic diagram of an illustrative example of the APU 22 and the generator 24. Figure 2 In the example shown, the APU 22 includes a power compressor 42, a combustor 44, a power turbine 46, and a load compressor 48 drivingly connected to the APU 22. It will be appreciated that Figure 2 This is merely exemplary in nature, and different configurations of the gas turbine may also be used. For example, in Figure 3 In the illustrated alternative example, the APU 22 does not include a load compressor. As explained below, Figure 3 The illustrated APU 22 is part of an aircraft (e.g., an electric aircraft or an aircraft that utilizes electric power for all non-propulsion systems) that does not provide exhaust air to one or more aerodynamic loads 68 in the aircraft 20. The aerodynamic loads 68 include, for example, an environmental control system (ECS) and main engine starting air for one or more main engines.

[0027] Refer to Figure 2During operation of the APU 22, the power compressor 42 draws ambient air A into the inlet 50, compresses the ambient air A, and supplies the compressed air to the combustor 44. The combustor 44 receives the compressed air from the power compressor 42 and a flow of fuel F from the fuel metering valve 62. The fuel F and the compressed air are mixed and ignited within the combustor 44 to produce combustion gases that are directed to the power turbine 46. The combustion gases expand through the power turbine 46, impinging on turbine blades (not shown), which causes the power turbine 46 to rotate.

[0028] The power turbine 46 includes an output shaft 64 that drives the power compressor 42, the load compressor 48, and the generator 24. The APU 22 is drivingly connected to the generator 24 via the output shaft 64. It will be appreciated that although Figure 2 A single shaft configuration is shown in which the output shaft 64 is coupled to the generator 24, but in another example, a dual shaft configuration may be used instead, in which separate output shafts are used to drive the generator 24 and the load compressor 48. In another example, an accessory gearbox may be used to drive the generator 24 and the load compressor 48. The load compressor 48 draws ambient air A into the inlet 66 through a plurality of inlet guide vanes 67 and compresses the ambient air A. The compressed ambient air is supplied to the pneumatic load 68 through the exhaust valve 70. It will be appreciated that Figure 3 The illustrated example does not include a load compressor 48 for providing exhaust gas to the pneumatic loads 68 in the aircraft 20. In contrast, when the aircraft 20 uses an electric cabin air compressor (i.e., an electric motor drives the air compressor), the load compressor 48 is used. Figure 3 APU 22 shown. Figure 3 In the example shown, only the generator 24 is connected to the APU 22. In one example, instead of a load compressor, an electrically driven compressor 72 is included in the aircraft 20 to provide the air required by the pneumatic loads 68.

[0029] Reference Figure 1 and Figure 2 , the control module 28 is configured to control all operations of the APU 22. Specifically, the control module 28 controls the rotational speed of the output shaft 64 of the APU 22 based on various environmental conditions and operating parameters of the aircraft. As described in detail below. It should be understood that the rotational speed of the output shaft 64 of the APU 22 can be controlled based on the rotational speed signal. However, the rotational speed of the output shaft 64 can also be controlled by other operating parameters. For example, the control module 28 controls the fuel flow to the combustor 44, the electrical power output 38 of the generator 24, or the output power of the output shaft 64 to achieve a specific rotational speed of the output shaft 64.

[0030] The control module 28 receives as input various environmental conditions and data related to the operation of the aircraft 20. Specifically, the control module 28 receives as input one or more environmental signals indicating air density values, an environmental humidity signal, an aircraft speed signal, an electrical load signal 76, a signal indicating the rotational speed of the APU 22 (measured at the output shaft 64), a signal indicating the output torque of the APU 22 (at the output shaft 64), and one or more cockpit commands. The input signals to the control module 28 may be sent from one or more sensors 84 on the aircraft 20 or alternatively through other control modules 40 (e.g., a flight control module).

[0031] In one example, the environmental signal indicating the air density value is a measured value. In other words, the air density value is directly measured by the sensing device. For example, an aneroid barometer can be used to directly measure the air density. Alternatively, in another example, the air density value is a calculated value. Specifically, the environmental signal indicating the air density value is a temperature signal and an altitude signal. The control module 28 receives the air density signal or alternatively the temperature signal and the altitude signal as input. The control module 28 calculates the air density value based on the temperature signal and the altitude signal. In addition, in the example, the measured air density value (i.e., the air density measured by the aneroid barometer) is compared with the calculated air density value (i.e., based on altitude and temperature) for redundancy. For example, if one or more sensors responsible for the measured value of the air density value no longer work, the control module 28 calculates the air density signal based on the temperature signal and the altitude signal.

[0032] The electrical load signal 76 is determined by the control module 28 of the aircraft 20 or alternatively by other control modules 40. The electrical load signal 76 indicates the electrical load demand on the generator 24. Characteristics such as excitation current and control are also included in the electrical load signal 76. In one example, the electrical load signal 76 includes a power transmission pending signal 86 received from the generator control module 78. The generator control module 78 is connected to one or more main generators 80 of the aircraft 20. The main generator 80 is driven by the output shaft 64 of the APU 22. The power transmission pending signal 86 indicates the power transmission from the main generator 80 of the aircraft 20 to the generator 24. It should be understood that the main generator 80 of the aircraft 20 is driven by the main engine 82 of the aircraft 20. During power transmission, the electrical load on the generator 24 can be changed from an unloaded condition (or a relatively light load) to a heavy load. The electrical load signal 76 includes the power transmission pending signal 86. Therefore, the control module 28 is configured to adjust the operation of the APU 22 when a heavier load is expected to be generated by the power transition from the main generator 80.

[0033] The operation of the variable speed ground mode and the maintenance mode is now described. When the APU control system 18 is operating in any of these modes, the aircraft 20 is on the ground. When operating in the variable speed ground mode, the APU 22 operates to save fuel. First, the control module 28 determines that the aircraft 20 is on the ground and is at a gate or other waiting area where passengers can board the aircraft 20. Then, the control module 28 receives an indication of ground support equipment that is expected to be used on the aircraft 20. The control module 28 initiates the variable speed ground mode in response to receiving an indication that the ground support equipment is to be used. Some examples of ground support equipment include, but are not limited to, vacuum cleaners and aircraft cargo handling equipment. When in the variable speed ground mode, the control module 28 instructs the APU 22 to operate at a wideband speed. The wideband speed includes a speed range compatible with power supplies for aircraft ground support equipment (e.g., vacuum cleaners and cargo handling equipment). In a non-limiting example, the wideband speed is configured to provide power to ground support equipment operating in a frequency range of about 370 Hz to about 440 Hz, however, it should be understood that other ranges may also be used.

[0034] When operating in maintenance mode, the APU control system 18 performs a health monitoring check of the APU 22. The control module 28 first determines that the aircraft 20 is on the ground. Then, the control module 28 receives an indication that the maintenance mode is to be initiated. In response to receiving the indication that the maintenance mode is to be initiated, the control module 28 instructs the APU 22 to operate at a user-defined speed. The user-defined speed is indicated by a user-generated signal. The user-defined speed is a discrete rotational speed or a speed range determined by an individual (e.g., a maintenance technician). For example, if an undesirable vibration or resonance is experienced at a specific speed or rotational speed range, the user-generated signal can be set to the specific speed or rotational speed range to troubleshoot the APU 22.

[0035] The speed compensation mode will now be described. Figure 2 In the example shown, the aircraft 20 includes a load compressor (ie, load compressor 48). Figure 3 The example shown is different. Figure 2The aircraft 20 in the speed compensation mode is configured to provide exhaust to one or more aerodynamic loads 68. It should be understood that the speed compensation mode compensates for various environmental and operating parameters of the aircraft 20, which are constantly changing when the aircraft 20 is on the ground and in flight. For example, when the aircraft 20 is in the climb phase of flight and the altitude increases, the air density and temperature decrease. However, even when the aircraft 20 is not in flight, the air density and temperature may still change based on the height of the airport or terrain of the aircraft 20 on the ground. For example, compared with the temperature of about 48.9°C (120°F) at sea level, the air density at the same altitude is about thirty-five percent higher at a temperature of -40°C (-40°F). When the APU control system 18 is operated in the speed compensation mode, the rotation speed of the APU 22 changes to compensate for the changing conditions to maintain a substantially constant power output. Specifically, the control module 28 adjusts the APU 22 rotation speed based on at least the air density value.

[0036] Figure 4 90 is a graph showing an exemplary speed compensation curve 88 representing the speed of the APU 22 ( Figure 1 ) and the relationship between the variable speed and the air density value. As the air density value approaches the minimum value 92 (for example, 0.3 kg / m 3 ), the compensation of the APU 22 rotation speed increases. Similarly, when the air density value is close to the maximum value 94 (e.g., 1.51 kg / m 3 ), the compensation of the rotational speed of the APU 22 is reduced. The speed compensation curve 88 includes values ​​falling between the maximum turbine operating speed 96 and the minimum turbine operating speed 98. The speed compensation curve 88 also includes a nominal operating point 100 of the APU 22. The variable rotational speed of the APU 22 is inversely proportional to the air density value. In other words, the control module 28 determines the variable rotational speed of the APU 22 based on the inverse proportional relationship between the variable rotational speed of the APU 22 and the air density value. It should be understood that the air density value is cold on a sea level day (1.51 kg / m 3 ) and cold weather at 40,000 feet (0.3 kg / m 3 ) varies by a ratio of about 5 to 1. Thus, a conventional APU operating at a constant speed produces about eighty percent less power at high altitudes (i.e., 40,000 feet) compared to its power output at sea level. However, the disclosed APU control system 18 compensates for the loss of air density at higher altitudes by increasing the rotational speed.

[0037] In such Figure 4 In the non-limiting example shown, the speed compensation curve 88 is linear. However, it should be understood that Figure 4 This is merely exemplary in nature, and instead, the APU 22 ( Figure 1The relationship between the variable rotation speed of the APU 22 and the air density value may be nonlinear. The specific relationship between the rotation speed of the APU 22 and the air density value depends on the type or model of the gas turbine. Therefore, the relationship between the air density and the variable rotation speed varies based on the specific characteristics of the gas turbine. In addition, although Figure 4 Only a single speed compensation curve 88 is shown in FIG. 1 , however, it should be understood that a family of curves or multiple curves may be provided, wherein each curve corresponds to a particular operating point. Certain characteristics of the gas turbine that affect the relationship between air density and variable speed include the geometry or configuration of the air inlet. In addition, the size, shape, number of rotor blades, number of guide vanes, and number of stages of the gas turbine's power compressor 42 may also affect the relationship between air density and variable speed.

[0038] The adjustment of the APU 22 rotation speed based on the air density value will now be described. Figure 1 , Figure 2 and Figure 4 , the control module 28 receives one or more environmental signals indicating air density values ​​and one or more power signals indicating a specific amount of power generated by the APU 22. As described above, the environmental signal is a measured value indicating the air density value (i.e., measured by an aneroid barometer), or, alternatively, the environmental signal is calculated based on the temperature signal and the altitude signal. In the example, the power signal includes a speed signal representing the rotational speed of the output shaft 64 and a torque signal indicating the output torque at the output shaft 64. The control module 28 calculates the specific amount of power generated by the APU 22 based on the rotational speed and the output torque of the output shaft 64. Alternatively, the control module 28 calculates the power of the output shaft 64 based on the voltage and current output of the generator 24.

[0039] The control module 28 determines the variable speed of the APU 22 based on the air density value. Specifically, Figure 4 As shown, the specific rotational speed of the APU 22 corresponds to the air density value on the speed compensation curve 88. Once the specific rotational speed of the APU 22 is determined, the control module 28 instructs the APU 22 to operate at a variable rotational speed. When operating at a variable rotational speed, the APU 22 continues to produce a specific amount of power. In other words, the control module 28 changes the APU 22 rotational speed to maintain a substantially constant power output even as the environmental and operating parameters change. As the air density of the ambient air decreases with increasing altitude, the rotational speed of the APU 22 increases proportionally.

[0040] Reference Figure 1 and Figure 2, the control module 28 instructs the APU 22 to operate at a variable speed based on the adjustment signal 110 sent to the APU 22. In one example, the adjustment signal 110 is configured to adjust the speed of the output shaft 64 of the APU 22. However, in an alternative example, equivalent control of the APU 22 is achieved by controlling the position of the fuel metering valve 62 to adjust the fuel flow to the combustor 44. Alternatively, in another example, equivalent control of the APU 22 is achieved based on adjusting the power output of the generator 24 or the power output through the output shaft 64.

[0041] In addition to the air density value, the variable speed of the APU 22 is dynamically adjusted based on the electrical load demand on the generator 24. The control module 28 receives the electrical load signal 76. The control module 28 adjusts the variable speed of the APU 22 based on the electrical load demand of the generator 24. It should be understood that the speed of the APU 22 is first compensated for the air density. For example, in one method, the speed of the APU 22 is first compensated based on a high altitude of about 40,000 feet. However, the electrical load demand of the generator 24 may be relatively low. For example, the generator 24 may only be loaded to about twenty percent of the rated power. Therefore, the speed of the APU 22 can then be adjusted (i.e., slowed down) based on the reduced load demand of the generator 24, which in turn improves fuel efficiency.

[0042] Figure 5 is a graphical representation of the speed compensation curve 88, an exemplary maximum power adjustment curve 120, and an exemplary minimum power adjustment curve 122. As described below, the control module 28 is configured to increase or decrease the variable speed of the APU 22 based on the proportional relationship between the electrical load demand and the variable speed of the APU 22. The control module 28 is configured to increase the variable speed of the APU 22 relative to the speed compensation curve 88 to accommodate increased electrical power demand. Similarly, the control module 28 is configured to decrease the variable speed of the APU 22 relative to the speed compensation curve 88 to accommodate decreased electrical power demand.

[0043] Reference Figure 1 , 2 In one example, the control module 28 receives an electrical load signal 76 indicating an electrical load demand. In some examples, the control module 28 determines that the electrical load demand on the generator 24 is increasing. In response to determining that the electrical load demand on the generator 24 is increasing, the control module 28 increases the variable speed of the APU 22. Specifically, referring to Figure 5, the control module 28 may continue to increase the variable speed of the APU 22 at a given air density value 130 until the variable speed is at a maximum speed 132. That is, the control module 28 is configured to incrementally adjust the variable speed of the APU 22 at a given air density value to maintain a specific amount of power currently being produced by the APU 22. The maximum speed 132 is located along the maximum power adjustment curve 120. The maximum speed 132 of the APU 22 is associated with a maximum power demand threshold.

[0044] In another example, the control module 28 determines that the electrical load demand on the generator 24 is decreasing. In response to determining that the electrical load demand on the generator 24 is decreasing, the control module 28 decreases the variable speed of the APU 22. Specifically, Figure 5 As shown, the control module 28 can continue to reduce the variable speed of the APU 22 at a given air density value 130 until the variable speed of the APU 22 is at a minimum speed 134. The minimum speed 134 is located along the minimum power adjustment curve 122 and is related to the minimum power demand threshold. Therefore, the control module 28 dynamically adjusts the variable speed of the APU 22 at a given air density value 130 in proportion to the size of the electric load demand, wherein the variable speed of the APU 22 is adjustable between the minimum speed 134 and the maximum speed 132. The minimum speed 134 corresponds to the minimum power demand threshold of the generator 24, and the maximum speed 132 corresponds to the maximum power demand threshold of the generator 24.

[0045] The values ​​of the maximum power demand threshold and the minimum power demand threshold depend on one or more of the following factors: the specific type or model of the aircraft, the size of the generator 24, the maximum power output of the generator 24, the power factor of the generator 24, the load type of the device provided with power by the generator 24, and the combination of loads that cause the maximum power demand threshold and the minimum power demand threshold. Specifically, the types of loads that can be provided with power by the generator 24 include resistive loads and reactive loads (i.e., loads that include electric motors and require more power to initially start). In one non-limiting example, the maximum power demand threshold ranges from about sixty percent to about eighty percent of the maximum power output of the generator 24, and the minimum power demand threshold ranges from about twenty percent to about forty percent of the maximum power output of the generator 24, but it should be understood that these values ​​are exemplary in nature. In another example, the maximum power demand threshold and the minimum power demand threshold are determined based on a lookup table. In another example, the maximum power demand threshold and the minimum power demand threshold vary based on the flight plan of the aircraft 20.

[0046] exist Figure 5In the non-limiting example shown, the maximum power regulation curve 120 and the minimum power regulation curve 122 both follow the speed compensation curve 88. In other words, the values ​​of both the maximum power regulation curve 120 and the minimum power regulation curve 122 change by the same amount as the speed compensation curve 88. However, it should be understood that the values ​​of the maximum power regulation curve 120 and the minimum power regulation curve 122 may also change at different rates compared to the speed compensation curve 88. In other words, the maximum power regulation curve 120, the minimum power regulation curve 122, and the speed compensation curve 88 may each include different slopes. It should also be understood that in another example, the speed compensation curve 88 is non-linear, and the corresponding maximum power regulation curve 120 and the minimum power regulation curve 122 are also non-linear.

[0047] Figure 6 1 is a graph 128 showing an exemplary turbine speed curve 138 for the APU 22. The turbine speed curve 138 is determined based on engine model simulation and test data. It should be understood that Figure 6 The turbine speed curve 138 shown in FIG. 1 is merely exemplary in nature, and the specific profile of the turbine speed curve will vary depending on factors such as, but not limited to, turbine size and number of spools. Figure 6 In the illustrated example, at a relatively low altitude 140 (e.g., about sea level to about 2,000 feet or 609.6 meters), an increase in the rotational speed of the APU 22 results in a greater increase in power than the same increase in the rotational speed of the APU 22 at a relatively high altitude 142. The relatively high altitude includes a maximum altitude value, which includes any altitude above about 35,000 feet (10,668 meters). For example, increasing the rotational speed of the APU 22 from about ten percent to about twenty-three percent at a relatively low altitude 140 results in a twenty percent increase in power output. Conversely, increasing the rotational speed of the APU 22 by about forty percent of the engine speed at a relatively high altitude 142 results in the same power increase (i.e., twenty percent). The turbine speed curve 138 varies based on environmental conditions such as aircraft speed and humidity. Therefore, the turbine speed curve 138 is recalculated based on the environmental conditions.

[0048] In another example, the control module 28 adjusts the variable speed of the APU based on the flight plan of the aircraft 20. In one example, the flight plan is stored in the memory of the flight control module and sent to the control module 28. The control module 28 is configured to calculate the expected or predicted electrical load based on the flight plan, wherein the predicted electrical load is based on the flight control surface demand that modifies the hydraulic load of the aircraft 20. The control module 28 adjusts the variable speed of the APU 22 based on the predicted electrical load. For example, flight control surface commands (such as but not limited to flap extension and retraction, reverse thrust, and automatic clearance flap actuation) increase or decrease the hydraulic load of the aircraft. Automatic clearance flap actuation represents an automatic flap movement function, which is based on the angle of attack, airspeed, and flap position of the aircraft 20.

[0049] It should be understood that an electric aircraft may not include flight control surfaces, such as aerodynamic flaps. Instead, the control module 28 is configured to calculate the expected loads based on the changes in the speed of the plurality of electric motors, where the changes in speed control the attitude or angular velocity of the aircraft 20. Specifically, the electric aircraft is steered based on thrust vectoring, which refers to the direction in which the aircraft manipulates the thrust from the electric motors to control the attitude or angular velocity. Therefore, the control module 28 adjusts the variable speed of the APU 22 based on the demand from the electric motors of the aircraft 20.

[0050] In yet another example, the control module 28 adjusts the variable rotation speed of the APU 22 based on the aircraft speed. In one example, the aircraft speed is compared to the speed of sound, and the aircraft speed is expressed in Mach. Figure 7 1 is a graphical representation of an exemplary speed regulation line 150 showing a linear inverse relationship between aircraft speed and the variable rotational speed of the APU 22. As aircraft speed increases, the variable rotational speed of the APU 22 decreases, but the APU 22 still produces approximately the same power output. When the aircraft speed is at a maximum value 154 of Mach 0.8, the variable rotational speed of the APU 22 remains constant. Likewise, as aircraft speed decreases, the variable rotational speed of the APU increases.

[0051] Reference Figure 1 , 2 In one example, the control module 28 receives a speed signal indicating the speed of the aircraft compared to the speed of sound in the air at a given air density value. The control module 28 adjusts the variable rotation speed of the APU 22 based on the aircraft speed. The relationship between the aircraft speed and the variable rotation speed of the APU 22 (i.e., the speed adjustment line 150) is based on the ratio of the mass flow rate of the intake air of the APU 22 between the minimum value 152 and the maximum value 154 of the aircraft speed. Figure 7As shown in FIG. 1 , the minimum value 152 of the aircraft speed is approximately Mach 0.3, and the maximum value 154 of the aircraft speed is approximately Mach 0.8. In the non-limiting example shown, the ratio of mass flow of the intake air for the APU 22 is 1.96:1. Therefore, the inlet mass flow of the APU 22 at Mach 0.8 is 1.96 times greater than the inlet mass flow at Mach 0.3. It should be appreciated that the ratio of 1.96:1 remains substantially constant even during changes in altitude. Therefore, the speed regulation line 150 remains the same regardless of altitude. It should also be appreciated that the ratio of mass flow of the intake air of 1.96 varies based on the maximum and minimum speeds of the aircraft 20.

[0052] like Figure 7 As shown, a maximum turbine speed factor 156 is associated with a minimum value 152 of the aircraft speed, and a minimum turbine speed factor 158 is associated with a maximum value 154 of the aircraft speed. In one example, the control module 28 determines that the aircraft speed is less than the maximum value 154. In response to determining that the aircraft speed is less than the maximum value 154, the control module 28 adjusts the variable speed of the APU 22 based on a linear inverse proportional relationship between the aircraft speed and the variable speed of the APU 22. Conversely, in another example, the control module 28 determines that the aircraft speed is at the maximum value 154. In response to determining that the aircraft speed is at the maximum value 154, the control module 28 determines that the variable speed of the APU 22 remains unchanged. In other words, the variable speed of the APU 22 is adjusted based on the minimum turbine speed factor 158 of 1.0. Referring to Figure 1 and Figure 2 In yet another example, the variable speed of the APU 22 is further adjusted based on the ambient humidity. There is an inverse relationship between the ambient humidity and the power output of the gas turbine. Therefore, as the ambient humidity increases, the power output of the APU 22 decreases proportionally. It should be understood that the ambient humidity represents the absolute humidity, which is the total mass of water vapor present in a given volume or mass of air. The control module 28 receives an ambient humidity signal that indicates the absolute humidity of the air at a given air density. The control module 28 determines the absolute humidity based on the ambient humidity signal. Then, the control module 28 adjusts the variable speed of the APU 22 based on the absolute humidity, wherein there is an inverse relationship between the absolute humidity and the power output of the APU 22. In an example, the humidity signal indicates the presence and amount of precipitation (e.g., rain, ice, or snow). The control module 28 also adjusts the variable speed of the APU 22 based on the presence and amount of precipitation.

[0053] Figure 82 is a process flow diagram illustrating an exemplary method 200 for adjusting the variable speed of the APU 22 when operating in the speed compensation mode. It should be understood that blocks 208, 210, 212, 214, 216, and 218 of the method 200 are optional and may be omitted in some examples. Figure 1 , 2 , 4 and 8, the method 200 begins at box 202. In box 202, the control module 28 receives as input one or more environmental signals indicating air density values ​​and one or more power signals indicating a specific amount of power generated by the APU 22. As described above, the environmental signals are measured values ​​(i.e., measured by an aneroid barometer) or calculated values ​​(i.e., based on altitude and temperature). In one example, the measured air density value is compared to the calculated air density value for redundancy. Then, the method 200 can proceed to box 204.

[0054] In block 204, the control module 28 determines a variable speed of the APU based on the air density value. As an example, Figure 4 The speed compensation curve 88 shown in FIG. 8 is used to determine the variable speed of the APU based on the air density value. Then, the method 200 can proceed to block 206 .

[0055] In block 206, the control module 28 instructs the APU 22 to operate at a variable speed, wherein the APU 22 continues to produce a specific amount of power when the APU 22 operates at the variable speed. In one example, the method 200 may then terminate, and the APU 22 may then operate in a ground maintenance mode. However, in some examples, the control module 28 also performs speed compensation based on the electrical load demand on the generator 24, the aircraft speed, the humidity, or any combination of load, aircraft speed, and humidity. It should be understood that although Figure 8 The variable speed of the APU 22 is adjusted in a particular order, but no particular order is required. Thus, in one example, the method 200 may then proceed to block 208 .

[0056] In block 208 , the control module 28 receives the electrical load signal 76 , which indicates an electrical load demand of one or more generators 24 . The method 200 may then proceed to block 210 .

[0057] In block 210, the control module 28 adjusts the variable speed of the APU 22 based on the electrical load demand of the one or more generators 24, as described above and in Figure 5 The method 200 may then proceed to block 212 .

[0058] In block 212 , the control module 28 receives a speed signal indicating the speed of the aircraft 20 compared to the speed of sound in air. The method 200 may then proceed to block 214 .

[0059] In block 214, the control module 28 adjusts the variable speed of the APU 22 based on the speed of the aircraft 20, as described above and in Figure 6 The method 200 may then proceed to block 216 .

[0060] In block 216 , the control module 28 receives an ambient humidity signal indicative of ambient humidity. The method 200 may then proceed to block 218 .

[0061] In block 218 , the control module 28 adjusts the variable speed of the APU 22 based on the ambient humidity, as described above. The method 200 may then terminate or return to block 202 .

[0062] Now refer to Figure 1 , 3 9, the compensation of the variable speed of the APU 22 for an electric aircraft or an aircraft that uses electric power for all non-propulsion systems will now be described. Specifically, if the aircraft 20 is electrically propelled, the variable speed of the APU 22 is first compensated for the air density value. Then, the variable speed is adjusted based on other conditions such as the speed of the aircraft 20 and the ambient humidity. Then, the variable speed of the APU 22 is adjusted based on the electrical load demand on the generator 24. This is because the values ​​of environmental conditions such as air density values, the speed of the aircraft 20, and humidity do not usually change or change suddenly and significantly. For example, when the aircraft is in the climbing phase of flight, the air density will gradually decrease. In contrast, the electrical load demand of the generator 24 may suddenly increase. Therefore, compared with a fixed speed APU, by first compensating for environmental conditions that tend to change more gradually, the variable speed of the APU 22 (and the associated power capacity) can more easily adapt to sudden changes in load. If the control module 28 predicts the electrical load based on the flight plan, the control module 28 determines the variable speed of the APU 22 based on the predicted electrical load.

[0063] The control module 28 first compensates for the air density value by instructing the APU 22 to operate at a first variable speed. Then, after instructing the APU 22 to operate at the first variable speed, the control module 28 adjusts the first variable speed of the APU 22 based on one or more environmental conditions of the aircraft 20. Specifically, the environmental conditions of the aircraft 20 include, but are not limited to, the speed of the aircraft 20 and the ambient humidity, the speed of the aircraft 20 compared to the speed of sound in the air. After compensating for the environmental conditions of the aircraft 20, the control module 28 compensates for the electrical load demand on the generator 24 by instructing the APU 22 to operate at a second variable speed. In other words, the control module 28 determines the second variable speed of the APU 22 only after adjusting the first variable speed of the APU 22 based on the environmental conditions of the aircraft 20. Therefore, the second variable speed is based on the air density value, the environmental conditions of the aircraft, and the electrical load demand on the generator 24.

[0064] Now refer to Fig. 9 , an exemplary process flow diagram is shown that illustrates an exemplary method 300 for regulating the rotational speed of the APU 22 in an electric aircraft. Figure 1 , 3 9, the method 300 may begin at block 302. In block 302, the control module 28 receives as input one or more environmental signals indicating an air density value and one or more power signals indicating a specific amount of power generated by the APU 22. The method 300 may then proceed to block 304.

[0065] In block 304 , the control module 28 determines a first variable speed of the APU based on the air density value. The method 300 may then proceed to block 306 .

[0066] In block 306 , the control module 28 instructs the APU 22 to operate at a first variable speed, wherein the APU 22 continues to produce a specific amount of power when the APU 22 operates at the variable speed. The method 300 may then proceed to decision block 308 .

[0067] In decision block 308, if the variable speed of the APU 22 is compensated for the additional ambient conditions, the method proceeds to block 310. Otherwise, the method 300 proceeds to block 320 to compensate for the electrical load demand of the generator 24.

[0068] In block 310 , the control module 28 receives a speed signal indicating the speed of the aircraft 20 compared to the speed of sound in air. The method 300 may then proceed to block 312 .

[0069] In block 312, the control module 28 adjusts the first variable speed of the APU 22 based on the speed of the aircraft 20, as described above and in Figure 6 The method 300 may then proceed to block 314 .

[0070] In block 314 , the control module 28 receives an ambient humidity signal indicative of ambient humidity. The method 300 may then proceed to block 316 .

[0071] In block 316 , the control module 28 adjusts the first variable speed of the APU 22 based on the ambient humidity, as described above. The method 300 may then proceed to block 318 .

[0072] In block 318 , the control module 28 receives the electrical load signal 76 , which indicates an electrical load demand of one or more generators 24 . The method 300 may then proceed to block 320 .

[0073] In block 320, the control module 28 determines a second variable speed of the APU 22 based on the electrical load demand of the one or more generators 24. In other words, the control module 28 first adjusts the variable speed of the APU 22 for environmental conditions (e.g., air density, humidity, speed of the aircraft 20), and then adjusts the variable speed of the APU 22 for the electrical load demand. The method 300 may then proceed to block 322.

[0074] In block 322 , the control module 28 instructs the APU 22 to operate at the second variable speed. The method 300 may then return to block 302 .

[0075] Referring generally to the accompanying drawings, technical effects and benefits of the disclosed APU control system include a smaller and lighter gas turbine requiring less fuel than a fixed speed APU. Specifically, the disclosed APU compensates for environmental conditions that adversely affect power output. Thus, a smaller size gas turbine can be used to perform the same functions as a heavier fixed speed gas turbine. In addition, the disclosed APU control system also provides speed compensation based on electrical load demand, which in turn improves the efficiency of the APU.

[0076] Now refer to Fig.10, the APU control system 18 is implemented on one or more computer devices or systems, such as an exemplary computer system 1030. The computer system 1030 includes a processor 1032, a memory 1034, a mass storage device 1036, an input / output (I / O) interface 1038, and a human-machine interface (HMI) 1040. The computer system 1030 is operatively connected to one or more external resources 1042 via the network 1026 or the I / O interface 1038. The external resources may include, but are not limited to, servers, databases, mass storage devices, peripheral devices, cloud-based network services, or any other suitable computer resources that the computer system 1030 may use.

[0077] The processor 1032 includes one or more devices selected from a microprocessor, a microcontroller, a digital signal processor, a microcomputer, a central processing unit, a field programmable gate array, a programmable logic device, a state machine, a logic circuit, an analog circuit, a digital circuit, or any other device that manipulates signals (analog or digital) based on operating instructions stored in the memory 1034. The memory 1034 includes a single storage device or multiple storage devices, including but not limited to read-only memory (ROM), random access memory (RAM), volatile memory, non-volatile memory, static random access memory (SRAM), dynamic random access memory (DRAM), flash memory, cache memory, or any other device capable of storing information. The mass storage device 136 includes a data storage device such as a hard disk drive, an optical drive, a tape drive, a volatile or non-volatile solid-state device, or any other device capable of storing information.

[0078] The processor 1032 operates under the control of an operating system 1046 resident in the memory 1034. The operating system 1046 manages computer resources so that computer program code embodied as one or more computer software applications (e.g., the application 1048 resident in the memory 1034) may have instructions executed by the processor 1032. In an alternative example, the processor 1032 may directly execute the application 1048, in which case the operating system 1046 may be omitted. One or more data structures 1049 also reside in the memory 1034 and may be used by the processor 1032, the operating system 1046, or the application 1048 to store or manipulate data.

[0079] The I / O interface 1038 provides a machine interface that connects the processor 1032 to other devices and systems when in operation, such as the network 1026 or external resources 1042. The application 1048 thus works in conjunction with the network 1026 or external resources 1042 by communicating via the I / O interface 1038 to provide various features, functions, applications, processes, or modules that include examples of the present disclosure. The application 1048 also includes program code executed by one or more external resources 1042, or otherwise relies on functions or signals provided by other systems or network components external to the computer system 1030. Indeed, given the possibility of almost unlimited hardware and software configurations, it will be understood by those of ordinary skill in the art that examples of the present disclosure may include applications that are located external to the computer system 1030, distributed among multiple computers or other external resources 1042, or provided by computing resources (hardware and software) provided as a service (e.g., cloud computing services) over the network 1026.

[0080] The HMI 1040 is operably connected to the processor 1032 of the computer system 1030 in a known manner to allow a user to interact directly with the computer system 1030. The HMI 1040 may include a video or alphanumeric display, a touch screen, speakers, and any other suitable audiovisual indicators capable of providing data to the user. The HMI 1040 also includes input devices and controls, such as an alphanumeric keyboard, a pointing device, a keypad, buttons, control knobs, microphones, etc., which are capable of accepting commands or input from a user and transmitting the typed input to the processor 1032.

[0081] The database 1044 may reside on the mass storage device 1036 and may be used to collect and organize data used by the various systems and modules described herein. The database 1044 may include data and supporting data structures for storing and organizing the data. In particular, the database 1044 may be arranged using any database organization or structure, including but not limited to a relational database, a hierarchical database, a network database, or a combination thereof. A database management system in the form of a computer software application executed as an instruction on the processor 1032 may be used to access information or data stored in records of the database 1044 in response to a query, wherein the query may be dynamically determined and executed by an operating system 1046, other application programs 1048, or one or more modules.

[0082] In addition, this disclosure includes examples according to the following clauses:

[0083] 1. An auxiliary power unit (APU) control system (18) for an aircraft (20), the APU control system comprising an APU (22) drivingly connected to one or more generators (24), the APU control system comprising:

[0084] one or more processors (1032); and

[0085] a memory (1034) coupled to the one or more processors, the memory storing data including a database (1044) and program code which, when executed by the one or more processors, causes the APU control system to:

[0086] receiving one or more environmental signals indicating an air density value and one or more power signals indicating a specific amount of power generated by the APU;

[0087] determining a first variable speed of the APU based on the air density value;

[0088] instructing the APU to operate at the first variable speed, wherein the APU continuously generates the specific amount of power when operating at the first variable speed;

[0089] receiving an electrical load signal (76) after instructing the APU to operate at the first variable speed, wherein the electrical load signal indicates an electrical load demand of the one or more generators;

[0090] determining a second variable speed of the APU based on the electrical load demand of the one or more generators; and

[0091] The APU is instructed to operate at the second variable speed.

[0092] 2. The APU control system (18) of clause 1, wherein the one or more processors (1032) execute instructions to:

[0093] receiving a temperature signal and an altitude signal; and

[0094] The air density value is determined based on the temperature signal and the altitude signal.

[0095] 3. The APU control system (18) of clause 1, wherein the one or more processors (1032) execute instructions to:

[0096] determining whether the electrical load demand of the one or more generators (24) is increasing or decreasing;

[0097] In response to determining that the electrical load demand on the one or more generators has increased, increasing the second variable speed of the APU (22) based on a proportional relationship between the electrical load demand and the variable speed of the APU; and

[0098] In response to determining that the electrical load demand on the one or more generators has decreased, the second variable speed of the APU is decreased.

[0099] 4. The APU control system (18) of clause 1, wherein the one or more processors (1032) execute instructions to:

[0100] After instructing the APU (22) to operate at the first variable speed, adjusting the first variable speed of the APU based on one or more environmental conditions; and

[0101] After adjusting the first variable speed of the APU based on the one or more environmental conditions, determining the second variable speed of the APU, wherein the second variable speed of the APU is based on the air density value, the one or more environmental conditions, and the electrical load demand of the one or more generators (24).

[0102] 5. The APU control system (18) of clause 1, wherein the one or more processors (1032) execute instructions to:

[0103] The first variable speed of the APU (22) is dynamically adjusted at a given air density value in proportion to the magnitude of the electrical load demand, wherein the first variable speed of the APU is adjustable between a minimum speed (134) and a maximum speed (132).

[0104] 6. The APU control system (18) of clause 1, wherein the memory (1034) stores a flight plan for the aircraft (20), and wherein the one or more processors (1032) execute instructions to:

[0105] calculating a predicted electrical load based on the flight plan, wherein the predicted electrical load is based on changes in speed of a plurality of electric motors, and wherein changes in speed of the plurality of electric motors control the altitude or angular velocity of the aircraft; and

[0106] The second variable speed of the APU (22) is determined based on the predicted electrical load.

[0107] 7. An APU control system (18) as described in any one of clauses 1 to 6, wherein the one or more processors (1032) execute instructions to:

[0108] receiving a speed signal indicative of a speed of the aircraft (20) compared to a speed of sound in air at a given value of air density after instructing the APU (22) to operate at the first variable speed;

[0109] determining that the aircraft speed is less than a maximum value; and

[0110] In response to determining that the aircraft speed is less than the maximum value, the first variable rotational speed of the APU is adjusted based on an inverse linear relationship between the aircraft speed and the first variable rotational speed of the APU.

[0111] 8. An APU control system (18) as described in any one of clauses 1 to 6, wherein the one or more processors (1032) execute instructions to:

[0112] receiving a speed signal indicative of a speed of the aircraft compared to a speed of sound in air at a given value of air density after instructing the APU (22) to operate at the first variable speed;

[0113] determining that the speed of the aircraft (20) is at a maximum value; and

[0114] In response to determining that the aircraft speed is at the maximum value, determining that the first variable rotational speed of the APU remains unchanged.

[0115] 9. The APU control system (18) of any one of clauses 1 to 6, wherein the one or more processors (1032) execute instructions to:

[0116] After instructing the APU (22) to operate at the first variable speed, receiving an ambient humidity signal indicative of an absolute humidity of air at a given air density;

[0117] determining the absolute humidity of the air based on the ambient humidity signal; and

[0118] The first variable speed of the APU is adjusted based on the absolute humidity, wherein there is an inverse proportional relationship between the absolute humidity and the power output of the APU.

[0119] 10. The APU control system (18) of any one of clauses 1 to 6, wherein the one or more processors (1032) execute instructions to:

[0120] determining that the aircraft (20) is on the ground; and

[0121] In response to determining that the aircraft is on the ground, the APU (22) is instructed to operate at a bandwidth rotational speed, wherein the range of bandwidth rotational speeds is compatible with power supplies for aircraft ground support equipment.

[0122] 11. An APU control system (18) as described in any one of clauses 1 to 6, wherein the one or more processors (1032) execute instructions to:

[0123] determining that the aircraft (20) is on the ground; and

[0124] In response to determining that the aircraft is on the ground, the APU is instructed to operate at a user-defined speed, wherein the user-defined speed is a discrete rotational speed or speed range determined by an individual.

[0125] 12. An aircraft (20), comprising an auxiliary power unit (APU) control system (18), the aircraft comprising:

[0126] APU (22);

[0127] one or more generators (24) drivingly connected to the APU, wherein only the one or more generators are provided as loads to the APU;

[0128] one or more processors (1032) in electrical communication with the APU; and

[0129] a memory (1034) coupled to the one or more processors, the memory storing data including a database (1044) and program code which, when executed by the one or more processors, causes the APU control system to:

[0130] receiving one or more environmental signals indicating an air density value and one or more power signals indicating a specific amount of power generated by the APU;

[0131] determining a first variable speed of the APU based on the air density value;

[0132] instructing the APU to operate at the first variable speed, wherein the APU continuously generates the specific amount of power when operating at the first variable speed;

[0133] receiving an electrical load signal (76) after instructing the APU to operate at the first variable speed, wherein the electrical load signal indicates an electrical load demand of the one or more generators;

[0134] determining a second variable speed of the APU based on the electrical load demand of the one or more generators; and

[0135] The APU is instructed to operate at the second variable speed.

[0136] 13. An aircraft (20) according to clause 12, wherein the aircraft is an electrically propelled aircraft.

[0137] 14. The aircraft (20) of clause 12 or 13, wherein the one or more processors (1032) execute instructions to:

[0138] determining whether the electrical load demand of the one or more generators (24) is increasing or decreasing;

[0139] In response to determining that the electrical load demand on the one or more generators has increased, increasing the second variable speed of the APU (22) based on a proportional relationship between the electrical load demand and the variable speed of the APU; and

[0140] In response to determining that the electrical load demand on the one or more generators has decreased, the second variable speed of the APU is decreased.

[0141] 15. The aircraft (20) of clause 12 or 13, wherein the one or more processors (1032) execute instructions to:

[0142] After instructing the APU (22) to operate at the first variable speed, adjusting the first variable speed of the APU based on one or more environmental conditions; and

[0143] After adjusting the first variable speed of the APU based on the one or more environmental conditions, determining the second variable speed of the APU, wherein the second variable speed of the APU is based on the air density value, the one or more environmental conditions, and the electrical load demand of the one or more generators (24).

[0144] 16. A method of regulating a variable speed of an APU (22) in an aircraft (20), the method comprising:

[0145] receiving, by a computer (1030), one or more environmental signals indicating an air density value and one or more power signals indicating a specific amount of power generated by the APU (22);

[0146] determining, by the computer, a first variable speed of the APU based on the air density value;

[0147] instructing the APU to operate at the first variable speed, wherein the APU continues to generate the specific amount of power when operating at the first variable speed; and

[0148] receiving an electrical load signal (76) after instructing the APU to operate at the first variable speed, wherein the electrical load signal indicates an electrical load demand of one or more generators (24) drivingly connected to the APU;

[0149] determining, by the computer, a second variable speed of the APU based on the electrical load demand of the one or more generators; and

[0150] The APU is instructed to operate at the second variable speed.

[0151] 17. The method according to clause 16, further comprising:

[0152] receiving a temperature signal and an altitude signal; and

[0153] The air density value is determined based on the temperature signal and the altitude signal.

[0154] 18. The method according to clause 16, further comprising:

[0155] determining whether the electrical load demand of the one or more generators (24) is increasing or decreasing;

[0156] In response to determining that the electrical load demand on the one or more generators has increased, increasing the second variable speed of the APU (22) based on a proportional relationship between the electrical load demand and the variable speed of the APU; and

[0157] In response to determining that the electrical load demand on the one or more generators is reduced, the second variable speed of the APU is reduced based on the proportional relationship between the electrical load demand and the variable speed of the APU.

[0158] 19. The method according to any one of clauses 16 to 18, further comprising:

[0159] After instructing the APU (22) to operate at the first variable speed, adjusting the first variable speed of the APU based on one or more environmental conditions; and

[0160] After adjusting the first variable speed of the APU based on the one or more environmental conditions, determining the second variable speed of the APU, wherein the second variable speed of the APU is based on the air density value, the one or more environmental conditions, and the electrical load demand of the one or more generators (24).

[0161] 20. The method according to any one of clauses 16 to 18, further comprising:

[0162] calculating predicted electrical loads based on a flight plan stored in a memory of the computer, wherein the predicted electrical loads are based on changes in speed of a plurality of electric motors, and wherein changes in speed of the plurality of electric motors control the altitude or angular velocity of the aircraft (20); and

[0163] The second variable speed of the APU (22) is determined based on the predicted electrical load.

[0164] The description of the present disclosure is merely exemplary in nature, and variations that do not depart from the gist of the present disclosure are intended to fall within the scope of the present disclosure. Such variations should not be regarded as departing from the spirit and scope of the present disclosure.

Claims

1. An auxiliary power unit (APU) control system (18) for an aircraft (20), the APU control system comprising an APU (22) drivingly connected to one or more generators (24), the APU control system comprising: one or more processors (1032); and a memory (1034) coupled to the one or more processors, the memory storing data including a database (1044) and program code which, when executed by the one or more processors, causes the APU control system to: receiving one or more environmental signals indicating an air density value and one or more power signals indicating a specific amount of power generated by the APU; determining a first variable speed of the APU based on the air density value; instructing the APU to operate at the first variable speed, wherein the APU continuously generates the specific amount of power when operating at the first variable speed; receiving an electrical load signal (76) after instructing the APU to operate at the first variable speed, wherein the electrical load signal indicates an electrical load demand of the one or more generators; determining a second variable speed of the APU based on the electrical load demand of the one or more generators; and The APU is instructed to operate at the second variable speed.

2. The APU control system (18) according to claim 1, wherein: The one or more processors (1032) execute instructions to: Receive temperature signal and altitude signal; determining the air density value based on the temperature signal and the altitude signal; determining whether the electrical load demand of the one or more generators (24) is increasing or decreasing; In response to determining that the electrical load demand on the one or more generators has increased, increasing the second variable speed of the APU (22) based on a proportional relationship between the electrical load demand and the variable speed of the APU; and In response to determining that the electrical load demand on the one or more generators has decreased, the second variable speed of the APU is decreased.

3. The APU control system (18) according to claim 1, wherein: The one or more processors (1032) execute instructions to: After instructing the APU (22) to operate at the first variable speed, adjusting the first variable speed of the APU based on one or more environmental conditions; as well as After adjusting the first variable speed of the APU based on the one or more environmental conditions, determining the second variable speed of the APU, wherein the second variable speed of the APU is based on the air density value, the one or more environmental conditions, and the electrical load demand of the one or more generators (24).

4. The APU control system (18) according to claim 1, wherein: The one or more processors (1032) execute instructions to: The first variable speed of the APU (22) is dynamically adjusted at a given air density value in proportion to the magnitude of the electrical load demand, wherein the first variable speed of the APU is adjustable between a minimum speed (134) and a maximum speed (132).

5. The APU control system (18) according to claim 1, wherein: The memory (1034) stores a flight plan for the aircraft (20), and wherein the one or more processors (1032) execute instructions to: calculating a predicted electrical load based on the flight plan, wherein the predicted electrical load is based on changes in speed of a plurality of electric motors, and wherein changes in speed of the plurality of electric motors control the altitude or angular velocity of the aircraft; and The second variable speed of the APU (22) is determined based on the predicted electrical load.

6. The APU control system (18) according to any one of claims 1 to 5, wherein: The one or more processors (1032) execute instructions to: receiving a speed signal indicative of a speed of the aircraft (20) compared to a speed of sound in air at a given value of air density after instructing the APU (22) to operate at the first variable speed; Determining that the aircraft speed is less than a maximum value; as well as In response to determining that the aircraft speed is less than the maximum value, the first variable rotational speed of the APU is adjusted based on a linear inverse proportional relationship between the aircraft speed and the first variable rotational speed of the APU.

7. The APU control system (18) according to any one of claims 1 to 5, wherein: The one or more processors (1032) execute instructions to: receiving a speed signal indicative of a speed of the aircraft compared to a speed of sound in air at a given value of air density after instructing the APU (22) to operate at the first variable speed; determining that the speed of the aircraft (20) is at a maximum value; and In response to determining that the aircraft speed is at the maximum value, determining that the first variable rotational speed of the APU remains unchanged.

8. The APU control system (18) according to any one of claims 1 to 5, wherein: The one or more processors (1032) execute instructions to: After instructing the APU (22) to operate at the first variable speed, receiving an ambient humidity signal indicative of an absolute humidity of air at a given air density; determining the absolute humidity of the air based on the ambient humidity signal; and The first variable speed of the APU is adjusted based on the absolute humidity, wherein there is an inverse proportional relationship between the absolute humidity and the power output of the APU.

9. The APU control system (18) according to any one of claims 1 to 5, wherein: The one or more processors (1032) execute instructions to: determining that the aircraft (20) is on the ground; and In response to determining that the aircraft is on the ground, the APU (22) is instructed to operate at a bandwidth rotational speed, wherein the range of bandwidth rotational speeds is compatible with power supplies for aircraft ground support equipment.

10. The APU control system (18) according to any one of claims 1 to 5, wherein: The one or more processors (1032) execute instructions to: determining that the aircraft (20) is on the ground; and In response to determining that the aircraft is on the ground, the APU is instructed to operate at a user-defined speed, wherein the user-defined speed is a discrete rotational speed or speed range determined by an individual.

11. A method of regulating a variable speed of an APU (22) in an aircraft (20), the method comprising: receiving, by a computer (1030), one or more environmental signals indicating an air density value and one or more power signals indicating a specific amount of power generated by the APU (22); determining, by the computer, a first variable speed of the APU based on the air density value; instructing the APU to operate at the first variable speed, wherein the APU continuously generates the specific amount of power when operating at the first variable speed; receiving an electrical load signal (76) after instructing the APU to operate at the first variable speed, wherein the electrical load signal indicates an electrical load demand of one or more generators (24) drivingly connected to the APU; determining, by the computer, a second variable speed of the APU based on the electrical load demand of the one or more generators; and The APU is instructed to operate at the second variable speed.

12. The method according to claim 11, further comprising: Receive temperature signal and altitude signal; as well as The air density value is determined based on the temperature signal and the altitude signal.

13. The method according to claim 11, further comprising: determining whether the electrical load demand of the one or more generators (24) is increasing or decreasing; In response to determining that the electrical load demand on the one or more generators has increased, increasing the second variable speed of the APU (22) based on a proportional relationship between the electrical load demand and the variable speed of the APU; as well as In response to determining that the electrical load demand on the one or more generators is reduced, the second variable speed of the APU is reduced based on the proportional relationship between the electrical load demand and the variable speed of the APU.

14. The method according to any one of claims 11 to 13, further comprising: After instructing the APU (22) to operate at the first variable speed, adjusting the first variable speed of the APU based on one or more environmental conditions; as well as After adjusting the first variable speed of the APU based on the one or more environmental conditions, determining the second variable speed of the APU, wherein the second variable speed of the APU is based on the air density value, the one or more environmental conditions, and the electrical load demand of the one or more generators (24).

15. The method according to any one of claims 11 to 13, further comprising: calculating predicted electrical loads based on a flight plan stored in a memory of the computer, wherein the predicted electrical loads are based on changes in speed of a plurality of electric motors, and wherein changes in speed of the plurality of electric motors control the altitude or angular velocity of the aircraft (20); and The second variable speed of the APU (22) is determined based on the predicted electrical load.

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