Abstract:
A drive system for a helicopter includes a main drive for driving a rotor the helicopter, a flywheel mass battery including at least one flywheel, a first transmission coupling the flywheel mass battery with the main drive such that, during operation of the main drive, output can be transferred from the main drive to the flywheel mass battery, and a second, variable transmission connecting the flywheel mass battery to the rotor of the helicopter such that a predetermined output can be transferred to the rotor through adjustment of a transmission ratio of the variable transmission.

Description:
FIELD OF THE INVENTION 
     Exemplary embodiments of the invention relate to a helicopter with a drive system, a method for operating an auxiliary drive system of a helicopter, and a controller for an auxiliary drive system of a helicopter. 
     BACKGROUND OF THE INVENTION 
     An emergency landing of a helicopter in the event of a turbine failure is performed according to the autorotation method. This method requires comprehensive training of the pilot and can be critical in a given flight situation and environment. 
     For this reason, certain helicopters are equipped with assistance systems based on a battery-powered electrical auxiliary drive. Such an assistance system makes a landing possible that is controlled to a large extent. As a result of the battery and motor technology, the costs and weight of this assistance system are very high. 
     Moreover, auxiliary drives are known that are based on a flywheel. For example, US patent document U.S. Pat. No. 4,609,165 discloses a helicopter in which a flywheel is used as an auxiliary power system. 
     A drive assembly for an automobile in which a detachable flywheel is integrated is described in German patent document DE 102009058695 A1. German patent document DE 102011014098 A1 describes a flywheel generator. A hybrid drive of an automobile with a flywheel as an energy store is known from German patent document DE 102004033039 A1. European patent document EP 1247736 A1 describes an aircraft in which motor output is used to rotate mechanical energy storage means. A power transmission device for helicopters is known, for example, from European patent document EP 0753456A1. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments of the invention improve an auxiliary drive system for a helicopter with a flywheel generator such that a presettable output can be transferred to the rotor. 
     A helicopter according to the invention can comprise a main rotor and a tail rotor or even several main rotors. For example, the helicopter is a single-engine helicopter with a single drive. 
     According to one embodiment of the invention, the drive system of the helicopter comprises a main drive for driving a rotor; a flywheel mass battery comprising at least one flywheel; a first transmission coupling the flywheel mass battery to the main drive such that, during operation of the main drive, power can be transferred from the drive to the flywheel mass battery; and a second, variable transmission connecting the flywheel mass battery to the rotor of the helicopter such that a predetermined output can be transferred to the rotor through adjustment of a transmission ratio of the variable transmission. 
     In other words, the drive of the helicopter comprises a conventional main drive, such as a turbine, for instance, and an auxiliary drive system comprising a fly-wheel mass battery as an energy store and drive element. The flywheel mass battery can be charged with power by purely mechanical means during normal operation of the main drive via the first transmission. If the main drive is to be supported, or if the main drive has failed, the rotational energy stored in the flywheel mass battery can be used to drive the rotor. To enable adjustment of the power transferred from the flywheel mass battery to the rotor, the auxiliary drive system comprises a variable, i.e., adjustable transmission. 
     According to one embodiment of the invention, the variable transmission comprises a continuously variable transmission. For example, the variable transmission is a belt transmission in which a belt or a chain runs between two pairs of cone pulleys. By changing the spacing of the cone pulleys of a pair of cone pulleys, the transmission ratio of the transmission can be adjusted. Other types of continuously variable transmissions are possible, however, such as hydrodynamic transmissions. 
     According to one embodiment of the invention, the variable transmission can be controlled electrically. The variable transmission can comprise actuators (electric motors) by means of which the transmission ratio of the variable transmission can be adjusted using an electrical controller. In this way, the desired transmission ratio can be detected and adjusted by the controller. 
     According to one embodiment of the invention, the first transmission comprises a transmission with a fixed transmission ratio. The first transmission can be a simple pinion gear designed to translate the rotational speed delivered by the main drive during normal operation to a maximum rotational speed of the flywheel of the flywheel mass battery. 
     According to one embodiment of the invention, the drive system further comprises a freewheel unit between the first transmission and the main drive. In other words, the auxiliary drive system is coupled with the primary drive train. In this way, a blocked or slowly rotating main drive can be prevented from drawing energy from the flywheel mass battery. 
     According to one embodiment of the invention, the drive system further comprises a main transmission between the main drive and the rotor. The main transmission has a first input with which the main drive is coupled. Moreover, the main transmission has a second input with which the flywheel mass battery is coupled via the variable transmission. In this way, the operation of the rotor via the main drive and the operation of the rotor via the flywheel mass battery con be completely uncoupled from each other. Mechanical output can be fed in a controlled manner via the controlled variable transmission into the second input of the main transmission. 
     According to one embodiment of the invention, the drive system further comprises a drive shaft by means of which the main drive is coupled with the first input of the main transmission. The first transmission can also be coupled with this drive shaft in order to transfer a portion of the output of the main drive to the flywheel mass battery. 
     According to one embodiment of the invention, the drive shaft is coupled via a freewheel unit with the main transmission. The freewheel unit ensures that a slowly rotating main drive is uncoupled from the main transmission. As a result, it is also ensured that a defective, blocked main drive does not impede the rotor. 
     According to one embodiment of the invention, the drive system further comprises an auxiliary drive shaft by means of which the variable transmission is coupled with the second input of the main transmission. The auxiliary drive shaft can be coupled via a freewheel unit with the main transmission. This freewheel unit can be used to ensure that only output from the flywheel mass battery to the rotor can be transferred, but not in the other direction. 
     In other words, the helicopter comprises a drive system comprising a mechanical auxiliary drive system in which energy from the main drive can be transferred in a purely mechanical manner from the main drive to the auxiliary drive system and from the auxiliary drive system to the rotor. 
     According to the invention, the method for operating an auxiliary drive system of a helicopter comprises the steps: determining whether a main drive of the helicopter is providing the desired output; if the main drive is not providing the desired output, connecting a flywheel mass battery to a rotor of the helicopter by means of a variable transmission, flywheel mass battery having been charged mechanically by the main drive during normal operation; and controlling a transmission ratio of the variable transmission such that a desired output is transferred to the rotor. 
     If it is determined that the main drive, due to a defect, for example, is no longer providing the desired output or has failed completely, the energy stored in an auxiliary drive system can be used to continue driving the rotor. The auxiliary drive system comprises a flywheel mass battery, a first transmission for charging the flywheel mass battery via the main drive, and a second variable transmission for coupling the flywheel mass battery with the rotor. 
     The auxiliary drive system and particularly the flywheel mass battery can be charged by the main drive via the first transmission at startup. During normal operation of the main drive, small power losses of the main drive can be compensated. 
     Upon failure of the main drive, both the main transmission and the auxiliary drive system are uncoupled from the still or braking main drive by means of freewheel units. 
     The auxiliary drive system can be used as an emergency landing system for a helicopter. For this purpose, it can support the pilot while landing the helicopter with a defective or failed main drive. 
     According to one embodiment of the invention, the method further comprises the steps: determining whether the rotor is being operated in autorotation; and if the rotor is being operated in autorotation, separating of the flywheel mass battery from the rotor. The operation of the rotor with the auxiliary drive system can be interrupted when the rotor is being operated in autorotation. 
     According to one embodiment of the invention, the method further comprises the steps: determining whether a landing procedure is being initiated; if a landing procedure is being initiated, connecting the flywheel mass battery to the rotor. Right before the helicopter touches down, the auxiliary drive system can be used to (automatically) introduce additional rotational energy into the rotor in order to slow the descent of the helicopter. 
     The controller according to the invention for an auxiliary drive system of a helicopter is designed to execute the method as described above and below. The controller can be an electronic controller, for example. 
     For example, the controller can receive information from a control system about the current performance of the main drive or at least its rotational speed in order to deduce from this whether the main drive is providing the desired output or is defective. The controller can be regarded as a control system for the output control of the helicopter in several phases of flight, such as normal operation, autorotation or landing. 
     The flywheel mass battery can also comprise sensors that detect its current rotational speed and then report it to the controller. Using these data, the controller can determine the required transmission ratio of the variable transmission. 
     Moreover, the controller can receive information from the control system of the helicopter as to whether the pilot has put the rotor in autorotation and/or whether the pilot has begun with the landing procedure. Using these data, the controller can then detach the variable transmission from the rotor or couple it appropriately with the rotor as desired. 
     Exemplary embodiments of the invention are described in detail below with reference to the enclosed figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a schematic view of a helicopter according to an embodiment of the invention. 
         FIG. 2  shows a schematic view of a drive system of a helicopter according to an embodiment of the invention. 
         FIG. 3  shows a flowchart for a method for operating an auxiliary drive system of a helicopter according to an embodiment of the invention. 
     
    
    
     In principle, identical or similar parts are provided with the same reference symbols. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  shows a helicopter  8  comprising a drive system  10  with a conventional main drive  12 . The main drive  12  can comprise a combustion engine, such as a turbine or a piston engine, for example. 
     The main drive  12  is coupled via a main transmission  14  with a rotor shaft  16  or a rotor mast  16  to the end of which a rotor  18  of the helicopter  10  is attached. In  FIG. 1 , the helicopter  10  is depicted as a helicopter with a main rotor  18  and a tail rotor  20 . It will readily be understood that the helicopter  10  can have more than one lift-generating rotor  18  and also need not comprise a tail rotor  20 . 
     The helicopter  10  has a mechanical auxiliary drive system  22  designed to feed output to the main transmission  14  in the event the main drive  12  fails. For this purpose, the helicopter  10  has a controller  24  designed to detect a failure of the main drive  12  and to control the auxiliary drive system  22  appropriately. 
       FIG. 2  shows a section from  FIG. 1  with the drive system  10  and shows, in particular, the auxiliary drive system  22  in more detail. 
     The main drive  12 , for example a turbine  12 , is coupled via a shaft  30  with a first input  32  of the main transmission  14 . Located between the input  32  and the shaft  30  is a freewheel unit  34 , which can transfer torque from the turbine  12  or the shaft  30  to the main transmission  14  but, conversely, prevents torque from being transferred from the transmission to the main drive  12 . 
     The auxiliary drive system  22  comprises a flywheel mass battery  36 , which is coupled via a shaft  30  with an auxiliary transmission  38  that is coupled at its other end with the shaft  30  and hence with the main drive  12 . The auxiliary transmission  38  can be a transmission with a fixed transmission ratio and comprise two pinions  40 , for example. 
     A freewheel unit  42  is arranged between the auxiliary transmission  38  and the shaft  30 , the freewheel unit  42  is designed to transfer torque from the shaft  30  or the main drive  12  to the auxiliary transmission  38  and hence to the flywheel mass battery  36  but, conversely, prevents torque from being transferred from the flywheel mass battery  36  to the main drive  12 . 
     In this way, output can be transferred from the main drive  12  to the flywheel mass battery  36  when the main drive  12  is running. During normal operation of the helicopter  10 , a first portion of the output generated by the main drive  12  is transferred via the freewheel unit  34  to the main transmission  14  and used to drive the rotor  18  of the helicopter  10 . Another, second portion of the output (which is usually smaller than the first portion) is transferred via the auxiliary transmission  38  to the flywheel mass battery  36  in order to cause the flywheel masses to rotate, that is, to charge the flywheel mass battery  36 , and also to compensate for frictional losses in the flywheel mass battery  36 . 
     If the main drive  12  fails, the flywheel mass battery  36  can continue to rotate without output being transferred back to the main drive  12 . 
     To store the energy, the flywheel mass battery  36  can have one or more flywheels  44 . It is possible for the flywheel mass battery  36  to merely comprise one flywheel  44 , but several and/or counter-rotating flywheels  44  are also possible. 
     The auxiliary drive system  22  further comprises a variable transmission  46  by means of which the flywheel mass battery  36  is coupled with the second input  48  of the main transmission  14 . An input of the variable transmission  46  is connected to the shaft  30 , which is rigidly connected to the flywheel mass battery  36 . The other input of the transmission  46  is connected to a shaft  50 , which is coupled via a freewheel unit  52  with the second input  48  of the main transmission. 
     The freewheel unit  52  is designed to transfer torque from the variable transmission  46  to the main transmission  14  but to prevent torque from being transferred from the main transmission  14  to the variable transmission  46  and hence to the flywheel mass battery  36 . 
     By virtue of the two freewheel units  34  and  52 , either the main drive  12  or the fly-wheel mass battery  36  (or both) can input torque into the main transmission  14 . Conversely, through the freewheel units  34  and  52  the main drive  12  and the fly-wheel mass battery  36  are prevented from mutually impeding each other, for example if the main drive  12  has failed. 
     The variable transmission  46  can be a continuously variable transmission  46  comprising cone pulleys  56  mechanically connected by means of a chain  54  or belt  54 , for example. By changing the spacing of the cone pulleys  56 , the transmission and transfer ratio of the transmission  46  can be adjusted in a stepless manner. 
     The adjustment of the transmission ratio of the transmission can be done by means of the controller  24 , which, controls corresponding actuators of the variable transmission  46 , for example. This controller can also be designed to detect the current rotational speed of the flywheels  44  and the operating mode (defect/normal operation) of the main drive  12 . 
       FIG. 3  shows a method for controlling the auxiliary drive system  22  which can be carried out from the controller  24 . 
     In step  100 , the controller  24  detects a failure or a defect of the main drive  12 . A corresponding signal can be provided by a main drive controller of the drive  12 , for example. 
     In step  102  output is introduced from the auxiliary drive system  22  in a transition phase between the detection of the defect and the beginning of autorotation. For this purpose, the controller  24  adjusts the transmission ratio of the variable transmission  46  such that the desired output is transferred from the flywheel mass battery  36  to the rotor. For this purpose, the controller  24  can determine the current rotational speed of the flywheel mass battery  36 . 
     If the main drive is not rotating or not rotating with sufficient speed, the main drive  12  is uncoupled by means of the freewheel unit  34  from the main transmission  14  and by means of the freewheel unit  42  from the flywheel mass battery  36 . Continuing to step  102 , the flywheel mass battery  36  is coupled via the variable transmission  46  with the main transmission  14  and is adjusted by the controller  24  such that the desired output is introduced into the second input  48  of the main transmission  48 . 
     In step  104 , the rotor  18  transitions to autorotation and the auxiliary drive system  22  is adjusted so as not to introduce any output. For this purpose, the pilot sets the rotor  18  to autorotation, which is detected by the controller  24 , for example by means of a corresponding signal from a control system of the helicopter  8 . Upon detection of the signal, the controller  24  adjusts the variable transmission  46  such that no more output is transferred to the rotor  18 . 
     In step  106 , the landing procedure of the helicopter  8  is initiated and the auxiliary drive system  22  is set so as to introduce output again. The landing procedure is initiated by the pilot by moving the rotor  18  out of the autorotation position into a landing position in which the energy stored in the rotor and transmission is used to brake the helicopter  8 . This switchover can be detected by the controller  24 , for example through a corresponding signal from the control system of the helicopter  8 . After that, the controller  24  sets the variable transmission  46  (analogously to step  102 ) again such that the desired output is introduced into the second input  48  of the main transmission  14  in order to additionally drive the rotor  18 . 
     To enable estimation of how a flywheel mass battery and its flywheel could be dimensioned, the following formulas can be used: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Kinetic rotational energy 
                 
                   
                     
                       
                         
                           W 
                           rot 
                         
                         = 
                         
                           
                             1 
                             2 
                           
                           · 
                           J 
                           · 
                           
                             ω 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                   
               
               
                 Torque of a flywheel as a massive cylinder (a cylindrical shell has twice the torque.) 
                 
                   
                     
                       
                         J 
                         = 
                         
                           
                             1 
                             2 
                           
                           · 
                           m 
                           · 
                           
                             R 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                   
               
               
                 Energy density of the flywheel 
                 
                   
                     
                       
                         
                           
                             W 
                             rot 
                           
                           m 
                         
                         = 
                         
                           
                             
                               
                                 R 
                                 2 
                               
                               · 
                               
                                 ω 
                                 2 
                               
                             
                             4 
                           
                           = 
                           
                             
                               ( 
                               
                                 
                                   R 
                                   · 
                                   ω 
                                 
                                 2 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                 
               
               
                   
               
               
                 Circumferential speed 
                 v u  = R · ω 
               
               
                   
               
               
                 Energy density 
                 
                   
                     
                       
                         
                           
                             W 
                             rot 
                           
                           m 
                         
                         = 
                         
                           
                             v 
                             u 
                             2 
                           
                           4 
                         
                       
                     
                   
                 
               
               
                   
               
             
          
         
       
     
     This results in the following values for various materials. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                 Specific 
                 Maximum 
               
               
                   
                   
                 Density 
                 strength 
                 energy density 
               
               
                   
                 Strength (MPa) 
                 (kg/m 3 ) 
                 (MPa/kg) 
                 (MJ/kg) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Steel 
                 1700 
                 7800 
                 0.22 
                 0.11 
               
               
                 Aluminum 
                 600 
                 2700 
                 0.22 
                 0.11 
               
               
                 Titanium 
                 1200 
                 4500 
                 0.27 
                 0.13 
               
               
                 CFRP 
                 3200 
                 2000 
                 1.6 
                 0.8 
               
               
                   
               
             
          
         
       
     
     Despite their low mass density, high-strength materials such as CFRP (carbon fiber-reinforced plastics), for instance, can be used as the material for the flywheel. Metals and metal alloys are also suitable materials. 
     By comparison, lithium batteries have an energy density of 200 Wh/kg, which corresponds to 0.72 MJ/kg. 
     An EC120-type helicopter has a power input of about 100 kW. For 30 s, this corresponds to an energy requirement of 3 MJ. The energy density of CFRP is 800 kJ/kg, which yields a flywheel weight of 3.75 kg. 
     Assuming a factor of 4 for additional components (bearings, housing), one obtains a mass for the flywheel mass battery of about 15 kg. For the other components of the auxiliary drive system as well (first transmission, variable transmission, actuators, control, etc.), a weight of 15 kg can be assumed. For the auxiliary drive system, this results in a total weight of 30 kg. 
     In addition, it should be pointed out that “comprising” does not exclude any other elements or steps, and “one” or “a” does not exclude the plural. Furthermore, it should be noted that features or steps that have been described with reference to one of the above exemplary embodiments can also be used in combination with other features or steps of other exemplary embodiments described above. Reference symbols in the claims are not to be regarded as a restriction. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.