Aircraft Auxiliary Power Unit (APU) Control System with Speed Compensation

Abstract

Aircraft Auxiliary Power Unit (APU) Control System with Speed Compensation. A control system for an auxiliary power unit (APU) of an aircraft is disclosed. The APU control system includes: an APU; one or more processors; and a memory connected to the one or more processors. The memory stores data and program code including a database, 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. Further causes the APU control system to determine a variable speed of the APU based on the air density value and to indicate that the APU operates at the variable speed. The APU continuously generates the specific amount of power when operating at the variable speed.

Description

Technical Field

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

[0002] An aircraft can 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 load compressor of the APU. 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 thus does not provide exhaust. Although electric aircraft are discussed, this approach is also used on aircraft that use electrical power for all non-propulsion systems.

[0003] Gas turbines typically operate at a steady-state output speed to provide power and, in at least some aircraft, provide exhaust. The performance of a gas turbine 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 approximately 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 rate of air entering the gas turbine. Thus, a gas turbine can produce only about one-fourth of the power at an altitude of 40,000 feet compared to sea level.

[0004] Existing APUs found on aircraft are typically oversized to accommodate potential shock loads, which occur when the load demand increases significantly over a very short period of time. More specifically, the size of the turbine is determined to have sufficient rotor inertia so that the shock load does not cause the rotational speed to drop below a predetermined frequency. However, an oversized APU increases the mass of the aircraft.

[0005] In view of these and other considerations, the present disclosure is proposed. 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; 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 variable speed of the APU based on the air density value. The system is further caused to instruct the APU to operate at the variable speed. The APU continuously generates the specific amount of power when operating at the variable speed.

[0007] According to another aspect of the present disclosure, an aircraft is disclosed. The aircraft includes an APU control system, an APU including an output shaft, one or more generators drivingly connected to the output shaft of the APU, one or more processors in electrical communication with the APU, 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 variable speed of the APU based on the air density value. The system is further caused to instruct the APU to operate at the variable speed. The APU continuously generates the specific amount of power when operating at the variable speed.

[0008] According to yet another aspect of the present disclosure, a method of adjusting a variable speed of an APU in an aircraft is disclosed, the method including 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 further includes determining, by the computer, the variable speed of the APU based on the air density value. The method further includes instructing the APU to operate at the variable speed, wherein the APU continuously generates the specific amount of power when operating at the variable speed.

[0009] The features, functions, and advantages discussed can be implemented independently in various examples or combined in other examples, and further details thereof can be seen with reference to the following description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0011] Figure 1Schematic diagram of an aircraft including the disclosed Auxiliary Power Unit (APU) according to an exemplary example;

[0012] Figure 2 is Figure 1 Schematic diagram of the APU shown, where the APU includes a load compressor according to an exemplary example;

[0013] Figure 3 Schematic diagram of an alternative APU for an electric aircraft according to an exemplary example;

[0014] Figure 4 Graph showing the speed compensation curve based on air density according to an exemplary example;

[0015] Figure 5 is showing Figure 4 Graph of the speed compensation curve shown, where positive and negative compensation of power is shown according to an exemplary example;

[0016] Figure 6 Graph showing the turbine speed curve of the disclosed APU according to an exemplary example;

[0017] Figure 7 is showing Figure 4 Graph of the speed compensation curve shown, where positive and negative compensation of aircraft speed is shown according to an exemplary example;

[0018] Figure 8 Process flow diagram showing a method for adjusting the speed of an APU according to an exemplary example;

[0019] Figure 9 Process flow diagram showing a method for speed compensation in an electric aircraft according to an exemplary example; and

[0020] Figure 10 is according to an exemplary example by 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, where the APU is a gas turbine. The APU control system varies the APU speed while maintaining a substantially constant power output, even as 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) can have an adverse effect on the performance of the APU. For example, as the aircraft altitude increases, the air density decreases, which in turn reduces the mass air flow rate supplied to the APU. The APU control system compensates for the decrease in air density by adjusting the APU speed. Thus, the power output of the APU remains substantially constant even as the inlet air density decreases. In addition to air density, the APU control system also adjusts the APU speed 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 periods of full load demand, the APU speed increases. However, lighter load demands allow for a decrease 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 can be used in an electric aircraft or in an aircraft that uses electricity for all non-propulsion systems.

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

[0024] Reference 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 that is 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 shows one or more generators 24 drivingly connected to the APU 22, 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 can 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 speed of the APU 22 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 speed of the APU 22 to maintain a substantially constant power output. In addition to the speed compensation mode, in the example, the APU control system 18 also includes a variable speed ground mode, a maintenance mode, or both the variable speed ground mode and the maintenance mode. Both the variable speed ground mode and the maintenance mode are only executed 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 exemplary example of the APU 22 and the generator 24. In Figure 2 the example shown, the APU 22 includes a power compressor 42, a combustor 44, a power turbine 46, and a load compressor 48 that is drivingly connected to the APU 22. It will be understood that Figure 2 this is only exemplary in nature and different configurations of gas turbines may also be used. For example, in Figure 3 the alternative example shown, the APU 22 does not include a load compressor. As explained below, Figure 3 the APU 22 shown is part of an aircraft (such as an electric aircraft or an aircraft that uses electricity for all non-propulsion systems) that does not supply exhaust gas to one or more pneumatic loads 68 in the aircraft 20. The pneumatic loads 68 include, for example, an environmental control system (ECS) and main engine starting air for one or more main engines.

[0027] Referring again to Figure 2 , during operation of the APU 22, the power compressor 42 draws in ambient air A at 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 should be sent 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 through the output shaft 64. It will be understood that although Figure 2Shows a single-shaft configuration in which the output shaft 64 is coupled to the generator 24. However, in another example, a two-shaft configuration may alternatively be used, in which a separate output shaft is 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 understood that Figure 3 the example shown does not include a load compressor 48 for supplying exhaust to the pneumatic load 68 in the aircraft 20. Instead, when the aircraft 20 uses an engine nacelle air compressor (i.e., an electric motor-driven air compressor), the Figure 3 shown APU 22 is used. In the example as Figure 3 shown, only the generator 24 is connected to the APU 22. In one example, instead of a load compressor, an electric drive compressor 72 is included in the aircraft 20 to provide the air required by the pneumatic load 68.

[0029] Referring to 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 a 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 rate to the burner 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 various environmental conditions and data related to the operation of the aircraft 20 as inputs. Specifically, the control module 28 receives 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 as inputs. The input signals can be sent to the control module 28 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 ambient signal indicating the air density value is a measured value. In other words, the air density value is directly measured by a 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 ambient signals indicating the air density value are the temperature signal and the altitude signal. The control module 28 receives the air density signal or alternatively the temperature signal and the altitude signal as inputs. The control module 28 calculates the air density value based on the temperature signal and the altitude signal. Further, 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 are no longer working, 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. For example, the excitation current and the characteristics of the control are also included in the electrical load signal 76. In one example, the electrical load signal 76 includes the power transfer 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 generators 80 are driven by the output shaft 64 of the APU 22. The power transfer pending signal 86 indicates the power transfer from the main generators 80 of the aircraft 20 to the generator 24. It should be understood that the main generators 80 of the aircraft 20 are driven by the main engines 82 of the aircraft 20. During the power transfer, the electrical load on the generator 24 can change from an unloaded condition (or a relatively light load) to a heavy load. The electrical load signal 76 includes the power transfer pending signal 86. Accordingly, the control module 28 is configured to adjust the operation of the APU 22 when expecting a heavier load resulting from the power transition from the main generator 80.

[0033] Now describe the operation of the variable speed ground mode and the maintenance mode. When the APU control system 18 operates 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 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 expected to be used on the aircraft 20. In response to receiving the indication that ground support equipment is to be used, the control module 28 activates the variable speed ground mode. 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 bandwidth speed. The bandwidth speed includes a speed range compatible with the power supply for aircraft ground support equipment (e.g., vacuum cleaners and cargo handling equipment). In a non-limiting example, the bandwidth speed is configured to supply power to ground support equipment operating in a frequency range of approximately 370 Hz to approximately 440 Hz. However, it should be understood that other ranges may also be used.

[0034] When operating in the 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 started. In response to receiving the indication that the maintenance mode is to be started, 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 speed or a speed range determined by an individual (e.g., a maintenance technician). For example, if undesirable vibrations or resonances are experienced at a particular speed or speed range, the user-generated signal can be set to the particular speed or speed range to troubleshoot the APU 22.

[0035] Now the speed compensation mode will be described. In the example as Figure 2 shown, the aircraft 20 includes a load compressor (i.e., load compressor 48). Different from the example Figure 3 shown, Figure 2The aircraft 20 therein is configured to provide exhaust to one or more pneumatic loads 68. It should be understood that the speed compensation mode compensates for various environmental and operating parameters of the aircraft 20, which continuously change 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 vary based on the altitude of the airport or terrain where the aircraft 20 is on the ground. For example, compared with a temperature of about 48.9 °C (120 °F) below sea level, the air density at the same altitude with a temperature of -40 °C (-40 °F) is approximately 35 percent higher. When the APU control system 18 operates in the speed compensation mode, the rotational 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 rotational speed of the APU 22 based at least on the air density value.

[0036] Figure 4 is a graph 90 showing an exemplary speed compensation curve 88 that represents the relationship between the variable rotational speed of the APU 22 ( Figure 1 ) and the air density value. As the air density value approaches the minimum value 92 (e.g., 0.3 kg / m 3 ), the compensation of the rotational speed of the APU 22 increases. Similarly, when the air density value approaches the maximum value 94 (e.g., 1.51 kg / m 3 ), the compensation of the rotational speed of the APU 22 decreases. The speed compensation curve 88 includes values that fall between the maximum turbine operating speed 96 and the minimum turbine operating speed 98. The speed compensation curve 88 also includes the 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 varies at a ratio of approximately 5 to 1 between cold days at sea level (1.51 kg / m 3 ) and cold days at an altitude of 40,000 feet (0.3 kg / m 3 ). Therefore, compared with the power output at sea level, a conventional APU operating at a constant speed produces approximately 80 percent less power at high altitudes (i.e., 40,000 feet). However, the disclosed APU control system 18 compensates for the loss of air density at higher altitudes by increasing the rotational speed.

[0037] In a non-limiting example as Figure 4 shown, the speed compensation curve 88 is linear. However, it should be understood that Figure 4 it is actually merely exemplary, and conversely, the APU 22 ( Figure 1) The relationship between the variable speed and the air density value can be non - linear. The specific relationship between the speed of APU 22 and the air density value depends on the type or model of the gas turbine. Therefore, the relationship between air density and variable speed varies based on the specific characteristics of the gas turbine. Additionally, although only a single speed compensation curve 88 is shown in Figure 4 it should be understood that a family of curves or multiple curves can be provided, where each curve corresponds to a specific operating point. Some characteristics of the gas turbine that affect the relationship between air density and variable speed include the geometry or configuration of the air inlet. Additionally, the size, shape, number of rotor blades, number of stator vanes, and the number of stages of the power compressor 42 of the gas turbine may also affect the relationship between air density and variable speed.

[0038] Now, the regulation of the speed of APU 22 based on the air density value will be described. Referring to Figure 1 、 Figure 2 and Figure 4 , the control module 28 receives one or more environmental signals indicating the air density value and one or more power signals indicating the specific amount of power generated by 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 temperature signals and altitude signals. In an example, the power signal includes a speed signal representing the 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 APU 22 based on the speed and 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 outputs of the generator 24.

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

[0040] Referring to 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 burner 44. Alternatively, in another example, equivalent control of the APU 22 is achieved based on adjusting the electrical output of the generator 24 or the electrical 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 an 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 air density. For example, in one method, the speed of the APU 22 is first compensated based on a high altitude of approximately 40,000 feet. However, the electrical load demand on the generator 24 may be relatively low. For example, the generator 24 may only be loaded to approximately twenty percent of its 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 diagram 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 an increasing electrical 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 a decreasing electrical demand.

[0043] Referring to Figure 1 , 2 and 5, in one example, the control module 28 receives an electrical load signal 76 indicating the 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 has increased, the control module 28 increases the variable speed of the APU 22. Specifically, referring to Figure 5, the control module 28 can continuously increase the variable speed of the APU 22 at a given air density value 130 until the variable speed reaches the 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 generated by the APU 22. The maximum speed 132 is positioned along the maximum power regulation curve 120. The maximum speed 132 of the APU 22 is related to the 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, as Figure 5 shown, the control module 28 can continuously decrease the variable speed of the APU 22 at a given air density value 130 until the variable speed of the APU 22 reaches the minimum speed 134. The minimum speed 134 is positioned along the minimum power regulation curve 122 and is related to the minimum power demand threshold. Thus, the control module 28 dynamically adjusts the variable speed of the APU 22 at a given air density value 130 in proportion to the magnitude of the electrical load demand, where 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 electrical output of the generator 24, the power factor of the generator 24, the type of load of the devices powered by the generator 24, and the combination of loads that result in the maximum power demand threshold and the minimum power demand threshold. Specifically, the types of loads that can be powered by the generator 24 include resistive loads and reactive loads (i.e., loads that include motors and require more power to start initially). In one non-limiting example, the maximum power demand threshold ranges from approximately sixty percent to approximately eighty percent of the maximum electrical output of the generator 24, and the minimum power demand threshold ranges from approximately twenty percent to approximately forty percent of the maximum electrical output of the generator 24, but it should be understood that these values are merely exemplary in nature. In yet another example, the maximum power demand threshold and the minimum power demand threshold are determined based on a look-up table. In yet another example, the maximum power demand threshold and the minimum power demand threshold vary based on the flight plan of the aircraft 20.

[0046] In Figure 5In the non-limiting example shown, both the maximum power adjustment curve 120 and the minimum power adjustment curve 122 follow the speed compensation curve 88. In other words, the amount of change in the values of both the maximum power adjustment curve 120 and the minimum power adjustment curve 122 is the same as that of the speed compensation curve 88. However, it should be understood that the values of the maximum power adjustment curve 120 and the minimum power adjustment curve 122 can also change at different rates compared to the speed compensation curve 88. In other words, the maximum power adjustment curve 120, the minimum power adjustment curve 122, and the speed compensation curve 88 can 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 adjustment curve 120 and minimum power adjustment curve 122 are also non-linear.

[0047] Figure 6 FIG. 128 is a graph showing an exemplary turbine speed curve 138 of the APU 22. The turbine speed curve 138 is determined based on engine model simulations and test data. It should be understood that Figure 6 the turbine speed curve 138 shown therein is only exemplary in nature, and the specific profile of the turbine speed curve varies depending on factors such as but not limited to turbine size and number of scrolls. In an example such as Figure 6 that shown, at a relatively low altitude 140 (e.g., from approximately 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 compared to the same increase in rotational speed of the APU 22 at a relatively high altitude 142. The relatively high altitude includes a maximum altitude value that 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 the relatively low altitude 140 results in a twenty percent increase in power output. In contrast, increasing the engine speed of the APU 22 by about forty percent at the relatively high altitude 142 results in the same increase in power (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 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 an expected or predicted electrical load based on the flight plan, where the predicted electrical load is based on flight control surface demands that modify 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 deployment and retraction, reverse thrust, and automatic gap flap actuation) increase or decrease the hydraulic load of the aircraft. Automatic gap flap actuation represents an automatic flap movement function that 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 an expected load based on changes in the speed of multiple electric motors, where the changes in speed control the attitude or angular velocity of the aircraft 20. Specifically, an electric aircraft is maneuvered based on thrust vectoring, which refers to the aircraft's manipulation of the direction of thrust from the electric motors to control the attitude or angular velocity. Thus, the control module 28 adjusts the variable speed of the APU 22 based on the demands from the electric motors of the aircraft 20.

[0050] In yet another example, the control module 28 adjusts the variable 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 represented in Mach. Figure 7 FIG. is an illustration of an exemplary speed adjustment line 150, which shows a linear inverse relationship between the aircraft speed and the variable speed of the APU 22. As the aircraft speed increases, the variable speed of the APU 22 decreases, but the APU 22 still produces approximately the same power output. When the aircraft speed is at the maximum value 154 of 0.8 Mach, the variable speed of the APU 22 remains constant. Similarly, when the aircraft speed decreases, the variable speed of the APU increases.

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

[0052] As Figure 7 shown, the maximum turbine speed factor 156 is related to the minimum value 152 of the aircraft speed, and the minimum turbine speed factor 158 is related to the 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 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. Thus, 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, where there is an inverse relationship between the absolute humidity and the power output of the APU 22. In the example, the humidity signal indicates the presence and amount of precipitation (such as 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 8is a process flow diagram showing an exemplary method 200 for adjusting the variable speed of the APU 22 when operating in a speed compensation mode. It should be understood that blocks 208, 210, 212, 214, 216, and 218 of method 200 are optional and may be omitted in some examples. Generally referring to Figure 1 , 2 , 4, and 8, method 200 begins at block 202. In block 202, the control module 28 receives as inputs 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. As described above, the environmental signal is a measured value (i.e., measured by an aneroid barometer) or a calculated value (i.e., based on altitude and temperature). In one example, the measured air density value is compared with the calculated air density value for redundancy. Then, method 200 may proceed to block 204.

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

[0055] In block 206, the control module 28 instructs the APU 22 to operate at a variable speed, where the APU 22 continuously generates a specific amount of power when operating at the variable speed. In one example, method 200 may then terminate, and then the APU 22 may 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 the load, aircraft speed, and humidity. It should be understood that although Figure 8 includes adjusting the variable speed of the APU 22 in a specific order, a specific order is not required. Thus, in one example, method 200 may then proceed to block 208.

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

[0057] In block 210, the control module 28 adjusts the variable speed of the APU22 based on the electrical load demand of one or more generators 24, which has been described above and is shown in Figure 5 . Method 200 may then proceed to block 212.

[0058] In block 212, the control module 28 receives a speed signal that indicates the speed of the aircraft 20 relative to the speed of sound in the air. The method 200 can 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, which has been described above and is shown in Figure 6 The method 200 can then proceed to block 216.

[0060] In block 216, the control module 28 receives an ambient humidity signal that indicates the ambient humidity. The method 200 can 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, which has been described above. The method 200 can then terminate or return to block 202.

[0062] Now referring to Figure 1 、 3 and 9, the compensation for the variable speed of the APU 22 for an electric aircraft or alternatively an aircraft that uses electricity 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 the air density value, the speed of the aircraft 20, and the humidity generally do not change or vary suddenly by large amounts. For example, when the aircraft is in the climb phase of flight, the air density will gradually decrease. In contrast, the electrical load demand on the generator 24 may suddenly increase. Therefore, compared to an APU with a fixed speed, by first compensating for the environmental conditions that tend to change more gradually, the variable speed (and the associated power capacity) of the APU 22 can more easily adapt to sudden changes in the 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, and 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 referring to Figure 9 , an exemplary process flow diagram is shown, which illustrates an exemplary method 300 for adjusting the speed of the APU 22 in an electric aircraft. Generally referring to Figure 1 , 3 and 9, method 300 may begin at block 302. In block 302, the control module 28 receives as inputs one or more environmental signals indicating the air density value and one or more power signals indicating the specific amount of power generated by the APU 22. Then, method 300 may 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. 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 speed, where the APU 22 continuously generates a specific amount of power when operating at a variable speed. Then, method 300 may proceed to decision block 308.

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

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

[0069] In block 312, the control module 28 adjusts the first speed of the APU 22 based on the speed of the aircraft 20, which has been described above and inFigure 6 is shown. Method 300 can then proceed to block 314.

[0070] At block 314, control module 28 receives an ambient humidity signal indicative of the ambient humidity. Method 300 can then proceed to block 316.

[0071] At block 316, control module 28 adjusts a first speed of APU 22 based on the ambient humidity, as described above. Method 300 can then proceed to block 318.

[0072] At block 318, control module 28 receives an electrical load signal 76 indicative of the electrical load demand of one or more generators 24. Method 300 can then proceed to block 320.

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

[0074] At block 322, control module 28 instructs APU 22 to operate at the second speed. Method 300 can then return to block 302.

[0075] Generally referring to the drawings, the technical effects and benefits of the disclosed APU control system, as compared to a fixed-speed APU, include a smaller and lighter gas turbine that requires less fuel. Specifically, the disclosed APU compensates for environmental conditions that adversely affect power output. Thus, a smaller-sized gas turbine can be used to perform the same function as a heavier fixed-speed gas turbine. Additionally, 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 referring to Figure 10 , APU control system 18 is implemented on one or more computer devices or systems (such as exemplary computer system 1030). 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. Computer system 1030 is operatively connected to one or more external resources 1042 via network 1026 or I / O interface 1038. External resources can include but are not limited to servers, databases, mass storage devices, peripherals, cloud-based network services, or any other suitable computer resources that computer system 1030 can 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 operation 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 disk 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 residing in the memory 1034. The operating system 1046 manages computer resources such that computer program code embodied as one or more computer software applications (e.g., the application 1048 residing in the memory 1034) can have instructions executed by the processor 1032. In an alternative example, the processor 1032 can execute the application 1048 directly, in which case the operating system 1046 can be omitted. One or more data structures 1049 also reside in the memory 1034 and can 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 in operation to other devices and systems, such as the network 1026 or the external resource 1042. The application 1048 thus works in cooperation with the network 1026 or the external resource 1042 by communicating via the I / O interface 1038 to provide various features, functions, applications, processes, or modules including the examples of the present disclosure. The application 1048 also includes program code executed by one or more external resources 1042, or otherwise depends on functions or signals provided by other systems or network components external to the computer system 1030. Indeed, given the almost infinite possibilities of hardware and software configurations, those of ordinary skill in the art will understand that the examples of the present disclosure can include applications located external to the computer system 1030, distributed across multiple computers or other external resources 1042, or provided by computing resources (hardware and software) offered as a service (e.g., cloud computing service) on the network 1026.

[0080] The HMI 1040 can be operatively connected to the processor 1032 of the computer system 1030 in a known manner to allow a user to directly interact with the computer system 1030. The HMI 1040 can include a video or alphanumeric display, a touch screen, speakers, and any other suitable audio-visual 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, a microphone, etc., which are capable of receiving commands or inputs from the user and transmitting the typed inputs to the processor 1032.

[0081] The database 1044 can reside on the mass storage device 1036 and can be used to collect and organize data used by the various systems and modules described herein. The database 1044 can include data and supporting data structures for storing and organizing the data. In particular, the database 1044 can 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 instructions on the processor 1032 can be used to access information or data stored in the records of the database 1044 in response to queries, where the queries can be dynamically determined and executed by the operating system 1046, other applications 1048, or one or more modules.

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

[0083] 1. An auxiliary power unit (APU) control system (10) for an aircraft (20), the APU control system including an APU (22), the APU control system comprising:

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

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

[0086] 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;

[0087] determine a variable speed of the APU based on the air density value; and

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

[0089] 2. The APU control system (10) according to Clause 1, wherein the one or more processors (1032) execute instructions to:

[0090] Receive a temperature signal and an altitude signal; and

[0091] Determine the air density value based on the temperature signal and the altitude signal.

[0092] 3. The APU control system (10) according to Clause 1, the APU control system includes one or more generators (24) driven by an output (64) of the APU (22), wherein the one or more processors (1032) execute instructions to:

[0093] Receive an electrical load signal (76), wherein the electrical load signal indicates an electrical load demand on the one or more generators; and

[0094] Adjust the variable speed of the APU based on the electrical load demand on the one or more generators.

[0095] 4. The APU control system (10) according to Clause 3, wherein the one or more processors (1032) execute instructions to:

[0096] Determine whether the electrical load demand on the one or more generators is increasing or decreasing;

[0097] In response to determining that the electrical load demand on the one or more generators is increasing, increase the 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 is decreasing, decrease the variable speed of the APU.

[0099] 5. The APU control system (10) according to Clause 3, wherein the one or more processors (1032) execute instructions to:

[0100] Dynamically adjust the variable speed of the APU (22) at a given air density value in proportion to the magnitude of the electrical load demand, wherein the variable speed of the APU is adjustable between a minimum speed (134) and a maximum speed (132).

[0101] 6. The APU control system (10) according to clause 5, wherein the minimum speed (134) corresponds to a minimum power demand threshold of the one or more generators (24), and the maximum speed (132) corresponds to a maximum power demand threshold of the one or more generators.

[0102] 7. The APU control system (10) according to any one of clauses 1 to 6, wherein the memory (1034) stores a flight plan of the aircraft (20), and wherein the one or more processors (1032) execute instructions to:

[0103] calculate a predicted electrical load based on the flight plan, wherein the predicted electrical load is based on flight control surface demands that modify the hydraulic load of the aircraft; and

[0104] regulate the variable speed of the APU (22) based on the predicted electrical load.

[0105] 8. The APU control system (10) according to any one of clauses 1 to 6, wherein the one or more processors (1032) execute instructions to:

[0106] receive a speed signal that indicates an aircraft speed compared to the speed of sound in air at a given air density value;

[0107] determine that the aircraft speed is less than a maximum value; and

[0108] in response to determining that the aircraft speed is less than the maximum value, regulate the variable speed of the APU (22) based on a linear inverse relationship between the aircraft speed and the variable speed of the APU.

[0109] 9. The APU control system (10) according to any one of clauses 1 to 6, wherein the one or more processors (1032) execute instructions to:

[0110] receive a speed signal that indicates an aircraft speed compared to the speed of sound in air at a given air density value;

[0111] determine that the aircraft speed is at a maximum value; and

[0112] in response to determining that the aircraft speed is at the maximum value, determine that the variable speed of the APU (22) remains unchanged.

[0113] 10. The APU control system (10) according to any one of clauses 1 to 6, wherein the one or more processors (1032) execute instructions to:

[0114] Receive an ambient humidity signal indicating the absolute humidity of air at a given air density;

[0115] Determine the absolute humidity of the air based on the ambient humidity signal; and

[0116] Adjust 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.

[0117] 11. The APU control system (10) according to any one of clauses 1 to 6, wherein the one or more processors (1032) execute instructions to:

[0118] Determine that the aircraft (20) is on the ground; and

[0119] In response to determining that the aircraft is on the ground, instruct the APU (22) to operate at a bandwidth speed, wherein the range of the bandwidth speed is compatible with the power supply for aircraft ground support equipment.

[0120] 12. The APU control system (10) according to any one of clauses 1 to 6, wherein the one or more processors (1032) execute instructions to:

[0121] Determine that the aircraft (20) is on the ground; and

[0122] In response to determining that the aircraft is on the ground, instruct the APU to operate at a user-defined speed, wherein the user-defined speed is a discrete rotational speed or speed range determined by an individual.

[0123] 13. An aircraft (20), the aircraft comprising an auxiliary power unit APU control system (10), the aircraft comprising:

[0124] An APU (22) including an output shaft (64);

[0125] One or more generators (24) drivingly connected to the output shaft of the APU;

[0126] One or more processors (1032) in electrical communication with the APU; and

[0127] A memory (1034) connected to the one or more processors, the memory storing data and program code including a database (1044), the program code when executed by the one or more processors causing the APU control system to:

[0128] 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;

[0129] Determining a variable speed of the APU based on the air density value; and

[0130] Instructing the APU to operate at the variable speed, wherein the APU continuously generates the specific amount of power when operating at the variable speed.

[0131] 14. The aircraft (20) according to clause 13, wherein the one or more processors (1032) execute instructions to:

[0132] Receive a temperature signal and an altitude signal; and

[0133] Determine the air density value based on the temperature signal and the altitude signal.

[0134] 15. The aircraft (20) according to clause 13 or 14, wherein the one or more processors (1032) execute instructions to:

[0135] Receive an electrical load signal (76), wherein the electrical load signal indicates an electrical load demand on the one or more generators; and

[0136] Adjust the variable speed of the APU (22) based on the electrical load demand of the one or more generators.

[0137] 16. A method for adjusting a variable speed of an APU (22) in an aircraft (20), the method comprising:

[0138] 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;

[0139] Determining, by the computer, the variable speed of the APU based on the air density value; and

[0140] Instructing the APU to operate at the variable speed, wherein the APU continuously generates the specific amount of power when operating at the variable speed.

[0141] 17. The method according to clause 16, the method further comprising:

[0142] Receiving a temperature signal and an altitude signal; and

[0143] Determining the air density value based on the temperature signal and the altitude signal.

[0144] 18. The method according to clause 16 or 17, the method further comprising:

[0145] Receiving an electrical load signal (76), wherein the electrical load signal indicates an electrical load demand on one or more generators (24) drivingly connected to the APU (22); and

[0146] Dynamically adjusting the variable speed of the APU based on the electrical load demand on the one or more generators.

[0147] 19. The method according to clause 18, the method further comprising:

[0148] Determining whether the electrical load demand on the one or more generators (24) is increasing or decreasing;

[0149] In response to determining that the electrical load demand on the one or more generators is increasing, increasing the variable speed of the APU (22) based on a proportional relationship between the electrical load demand and the variable speed of the APU; and

[0150] In response to determining that the electrical load demand on the one or more generators is decreasing, decreasing the variable speed of the APU based on the proportional relationship between the electrical load demand and the variable speed of the APU.

[0151] 20. The method according to clause 16 or 17, the method further comprising:

[0152] Receiving a speed signal that indicates an aircraft speed compared to the speed of sound in air at a given air density value;

[0153] Determining that the aircraft speed is less than a maximum value; and

[0154] In response to determining that the aircraft speed is less than the maximum value, adjusting 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.

[0155] The description of the present disclosure is exemplary in nature only, 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 (10) for an aircraft (20), the APU control system including an APU (22), the APU control system comprising: one or more processors (1032); and a memory (1034) connected to the one or more processors, the memory storing data and program code including a database (1044), the program code, when executed by the one or more processors, causing 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; determine a variable speed of the APU based on the air density value; and indicate that the APU operates at the variable speed, wherein the APU continuously generates the specific amount of power when operating at the variable speed.

2. The APU control system (10) according to claim 1, wherein, The one or more processors (1032) execute instructions to: receive a temperature signal and an altitude signal; and determine the air density value based on the temperature signal and the altitude signal.

3. The APU control system (10) according to claim 1, the APU control system including one or more generators (24) driven by an output (64) of the APU (22), wherein the one or more processors (1032) execute instructions to: Receiving an electrical load signal (76), wherein, the electrical load signal indicates an electrical load demand on the one or more generators; regulate the variable speed of the APU based on the electrical load demand of the one or more generators; determine whether the electrical load demand of the one or more generators increases or decreases; in response to determining that the electrical load demand on the one or more generators increases, increase the variable speed of the APU 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 decreases, decrease the variable speed of the APU.

4. The APU control system (10) according to claim 3, wherein, The one or more processors (1032) execute instructions to: dynamically regulate the variable speed of the APU (22) at a given air density value in proportion to the magnitude of the electrical load demand, wherein the variable speed of the APU is adjustable between a minimum speed (134) and a maximum speed (132), and wherein the minimum speed corresponds to a minimum electrical demand threshold of the one or more generators (24), and the maximum speed corresponds to a maximum electrical demand threshold of the one or more generators.

5. The APU control system (10) according to claim 1, wherein, The memory (1034) stores a flight plan of the aircraft (20), and wherein the one or more processors (1032) execute instructions to: calculate a predicted electrical load based on the flight plan, wherein the predicted electrical load is based on flight control surface requirements that modify the hydraulic load of the aircraft; and regulate the variable speed of the APU (22) based on the predicted electrical load.

6. The APU control system (10) according to any one of claims 1 to 5, wherein, The one or more processors (1032) execute instructions to: Receive a speed signal indicative of an aircraft speed relative to the speed of sound in air at a given air density value; Determine that the aircraft speed is less than a maximum value; And In response to determining that the aircraft speed is less than the maximum value, adjust the variable speed of the APU (22) based on a linear inverse relationship between the aircraft speed and the variable speed of the APU.

7. The APU control system (10) according to any one of claims 1 to 5, wherein, The one or more processors (1032) execute instructions to: Receive a speed signal indicative of an aircraft speed relative to the speed of sound in air at a given air density value; Determine that the aircraft speed is at the maximum value; and In response to determining that the aircraft speed is at the maximum value, determine that the variable speed of the APU (22) remains unchanged.

8. The APU control system (10) according to any one of claims 1 to 5, wherein, The one or more processors (1032) execute instructions to: Receive an ambient humidity signal indicative of the absolute humidity of air at a given air density; Determine the absolute humidity of the air based on the ambient humidity signal; and Adjust 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.

9. The APU control system (10) according to any one of claims 1 to 5, wherein, The one or more processors (1032) execute instructions to: Determine that the aircraft (20) is on the ground; and In response to determining that the aircraft is on the ground, instruct the APU (22) to operate at a bandwidth speed, wherein the range of the bandwidth speed is compatible with the power supply for aircraft ground support equipment.

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

11. A method of adjusting a variable speed of an APU (22) in an aircraft (20), the method comprising: Receiving, by a computer (1030), one or more ambient signals indicative of an air density value and one or more power signals indicative of a specific amount of power generated by the APU; Determining, by the computer, the variable speed of the APU based on the air density value; And Instructing the APU to operate at the variable speed, wherein the APU continuously generates the specific amount of power when operating at the variable speed.

12. The method according to claim 11, the method further comprising: Receiving a temperature signal and an altitude signal; And Determining the air density value based on the temperature signal and the altitude signal.

13. The method according to claim 11 or 12, the method further comprising: Receiving an electrical load signal (76), wherein the electrical load signal indicates an electrical load demand on one or more generators (24) drivingly connected to the APU (22); and Dynamically adjusting the variable speed of the APU based on the electrical load demand on the one or more generators.

14. The method according to claim 13, the method further comprising: Determine whether the electrical load demand on 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 is increasing, increase the 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 is decreasing, decrease the variable speed of the APU based on the proportional relationship between the electrical load demand and the variable speed of the APU.

15. The method according to claim 11 or 12, the method further comprising: Receiving a speed signal that indicates an aircraft speed compared to the speed of sound in air at a given air density value; Determining that the aircraft speed is less than a maximum value; And In response to determining that the aircraft speed is less than the maximum value, adjusting 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.

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