Patent Application
20210206479
The present invention relates generally to producing improvements in operating efficiency and performance of aircraft engines and specifically to a method for providing extended engine warm-up and cool down time on the ground and improving the operating efficiency and performance of aircraft engines in aircraft equipped with electric taxi systems for autonomous ground movement.
Increasing aircraft engine operating efficiency and performance has many benefits. When aircraft engines operate with optimum efficiency during flight, not only is fuel saved, but engine life may also be extended. The frequency of required engine repairs and the cost of engine maintenance may also be reduced. A primary cause of engine damage that interferes with engine operating efficiency is associated with a reduction in the time available for warming up and cooling down the aircraft's engines when an aircraft is on the ground prior to takeoff and after landing. At busy airports in particular, airlines and/or airport operators try to keep aircraft on the ground for as little time as possible between landing and departure. The result of such pressures may leave less time for aircraft engine warm up and cool down than is desirable for long term engine operating efficiency.
Most commercial passenger aircraft are powered by two or more jet turbine engines with rotor blades mounted on a hub or shaft to rotate within a shroud or like housing structure that may have a static seal. A clearance between the tips of the turbine blades as they rotate within the shroud during engine operation must be maintained to avoid contact between the blade tips and the shroud or static seal. Engine performance parameters, including, for example, thrust, specific fuel consumption, and exhaust gas temperature margin, are dependent on the relationship between blade tips and shrouds or static seals circumferentially surrounding the blade tips. It is important to minimize blade tip clearance while avoiding rubs or other contact between the blade tips and the adjacent shroud or static seal. However, minimizing this clearance presents challenges. During engine operation, the turbine blade tip length from the hub or from an engine axis may increase or decrease at a different rate than the shroud can expand or contract to accommodate the changes in blade tip length. As a result, there may be insufficient blade tip clearance, and the blade tip may contact or rub the shroud or static seal, or there may be excess clearance. In the first instance, rotation of the turbine rotor may be adversely affected and the blade tip and/or shroud or static seal may be damaged, reducing the lives of these engine components. In the second instance, poor engine performance may result.
The clearance gap between the turbine blade tip, which may be referred to as the turbine tip seal gap, may have a dimension that is comparable to the width of a human hair. This clearance gap, which normally varies throughout an aircraft's flight, minimizes airflow losses and cools the turbine blades and shroud or engine casing and sealing structures when maintained at an optimum width. Tip clearance loss may account for a significant amount of engine airflow loss, potentially in the range of about 20% to 40%, depending on the engine type. When an optimum tight tip clearance gap is not maintained, the leakage airflow may induce unsteady heat loads onto the engine rotor casing, resulting in significant thermal stresses at the turbine blade tip.
Clearances between turbine blade tips and the shrouds or housings in which they rotate are affected by differences in the amounts and rates of thermal and mechanical expansion during engine operation. Mechanical expansion or growth is due to centrifugal force that occurs with changing speeds and pressures, with mechanical growth of the turbine blades and engine rotor being greater than that of the engine stator, which experiences greater thermal growth than the blades or rotor. Thermal growth of the turbine blades occurs more quickly than the thermal growth of either the engine stator or the rotor. Ideally, these different growths or expansions of the turbine blades and other engine components should be matched and as tight a clearance as possible maintained between the blade tips and shroud during engine operation.
It has been determined that the length of the turbine blade from the hub to the tip grows in proportion to the square of the rotor angular velocity and linearly with temperature. These results occur when fuel flow is increased, for example, as an aircraft climbs, during portions of its descent and landing sequence, or when the aircraft takes evasive action during flight. Active clearance control may be employed to maintain an optimum clearance between blade tips and the shroud during engine operation. Such processes may involve, for example, bathing the shroud in hot or cold air to cause the shroud to expand or contract as required to match thermal growth or contraction of the turbine blades during engine operation in flight. Control software has been developed to electronically monitor and control blade tip clearance, altering the gap between the blade tip and shroud as required to maintain an optimum clearance during engine operation. For example, U.S. Pat. No. 8,126,628 to Hershey et al. describes an example of an active clearance control system that electronically monitors blade tip clearance during flight and automatically adjusts tip clearance at an appropriate time before an engine command that changes the engine's rotational speed.
As noted, the time pressures exerted by airlines and airport operators to keep aircraft moving into and out of gates and taking off after a relatively short engine warm up time may result in adverse effects on engine operation in flight. An observed adverse effect that may affect all aircraft engines, and particularly affects models of engines that require a longer than typical or average warm up time, is known as rotor bow. This condition results from the differential thermal and mechanical expansion of the blades and rotors discussed above and causes the blade tips to rub against the shroud or casing walls because thermal bowing of the rotor has occurred. Rotor bow, or thermal bowing, may occur as a result of asymmetrical cooling after shut down of an engine on the previous flight. Differential thermal deformation of the material of the engine shaft section supporting the rotors as a result of temperature differences across the shaft causes the rotor axis to bend. The result may be an offset in the center of gravity of the bowed rotor and the clearance between the blade tips and the compressor or shroud wall. It has been recognized that all engines may display some degree of rotor bow. As discussed above, maintaining blade tip clearance as closely as possible to an optimal minimum clearance may have a critical impact on engine operating efficiency. When sufficient time is provided for an aircraft engine to be thoroughly warmed up prior to take off and slowly cooled down after landing, the rotor, shaft, and other engine components may be heated and cooled evenly so that differential thermal and mechanical expansion may be controlled and rotor bow is minimized and, ideally, eliminated.
There is clearly a tension in current airport and airline operations between moving aircraft as quickly as possible between landing and take off to minimize time aircraft spend on the ground so that airport use and aircraft flight cycles are maximized and providing adequate time on the ground for aircraft engines to be optimally warmed up prior to take off and cooled down after landing to maximize engine operating efficiency during flight. The art has not provided a solution to this dilemma.
It is primary object of the present invention, therefore, to provide a method that provides optimal time required for aircraft to warm up and cool down engines with long warm up and cool down time requirements without increasing aircraft time on the ground or negatively impacting airport ground operations.
It is another object of the present invention to provide a method for moving an aircraft during ground travel that allows adequate time for controlled aircraft engine cool down and warm up after landing and prior to take off implemented in an aircraft with engines requiring long cool down and warm up times that are powered for ground movement by electric taxi systems.
It is another object of the present invention to provide a method for maximizing engine operating efficiency during flight by providing sufficient time to warm up and cool down aircraft engines so that differential thermal and mechanical expansion of engine components is controlled.
It is an additional object of the present invention to provide a method for extending aircraft engine run times before requiring take off thrust without increasing total aircraft time on the ground between landing and take off.
It is a further object of the present invention to provide a method for extending aircraft engine run times prior to take off and after landing that enables engines to be designed for and operated with longer warm up and cool down times than are currently required for most aircraft engines without adversely affecting airport operations.
It is a further object of the present invention to provide a method for the slow steady cool down of aircraft engines after landing that controls thermal properties of engine components while the aircraft is driven on the ground.
It is yet another object of the present invention to provide a method for minimizing or avoiding rotor bow in aircraft engines that are cooling down after landing that cools rotors with air supplied by an aircraft air source.
It is yet another object of the present invention to provide a method for providing adequate time for controlled aircraft engine cool down and warm up after landing and prior to take off without extending time required at a gate or for ramp operations.
In accordance with the foregoing objects, a method is provided for moving aircraft equipped with flight engines requiring long warm up and cool down times and with pilot-controllable electric taxi systems to drive aircraft during ground travel that increases engine operating efficiency during flight without increasing the time an aircraft spends on the ground between landing and take off. The equipped aircraft are driven only with the pilot-controllable electric taxi systems operating, simultaneously with both the electric taxi systems and the aircraft flight engines operating, or only with the aircraft engines operating at different times during aircraft ground travel after landing and prior to takeoff to ensure sufficient time for optimal aircraft engine warm up and cool down while the aircraft is kept moving on the ground.
When the electric taxi systems-equipped aircraft are cleared for departure, the electric taxi systems are activated and controlled to drive the aircraft forward, or reverse as required to push back, from a parking location to an engine start and warm up location where the flight engines may be safely started without the risks posed by jet blast or engine ingestion. The aircraft is driven during a period of hybrid taxi with the flight engines operating at a lowest throttle setting simultaneously with the electric taxi systems continuing operate to a threshold before a takeoff runway, where the hybrid taxi period ends. The electric taxi systems are then inactivated, and the aircraft continues to be driven with the flight engines set to operate at a higher throttle setting for take off. The hybrid taxi period provides sufficient time for the engines with longer warm up time requirements to warm up so that all for engine components are heated evenly. The hybrid taxi period enables an extended period of controlled engine warm up before the higher engine throttle setting is required, and the engine and engine components will be in an optimal thermal state prior to take off.
After landing, the aircraft flight engines continue to operate at a very low throttle setting selected to promote optimal cooling of the engines, and the electric taxi systems are simultaneously activated and controlled to drive the aircraft during a hybrid taxi period until the engines are cooled down and may be shut off. The aircraft is then driven with only the electric taxi systems to an airport parking location. Alternatively, the aircraft engines may be kept turning over slowly at a no throttle setting or while they are not operating and cooled with air supplied to the engine rotors from a source of air on the aircraft.
The present method not only accommodates the longer warm up and cool down requirements of some existing aircraft flight engines, but also enables aircraft engines to be designed to have longer warm up and cool down times than are presently required without increasing total time the aircraft spend on the ground, or otherwise adversely affecting airport operations.
As discussed above, current airport ground operating procedures and the pressures resulting from minimizing aircraft time on the ground between landing and take off may not provide an optimum, or even a sufficient, amount of time to adequately warm up aircraft engines. An adequate engine warm up time prevents damage to engine components, for example rotor bowing, where distorted rotors cannot maintain optimum rotor blade tip clearance during flight. Engine operating efficiency during flight may be reduced when aircraft engine components are damaged or distorted as a result of an improperly warmed up or cooled down engine. Aircraft engine designers must presently design and configure aircraft engines to conform to engine start times imposed by airport operators. At most airports, the time between when both aircraft engines are started prior to take off and the subsequent application of a take off throttle setting is currently about 2.5 to 2.67 minutes (150 to 160 seconds), although engine start times may be shorter. While reduced engine start times may reduce aircraft time on the ground, this may be achieved at a cost in the form of uneven heating and differential thermal expansion of aircraft engine components.
When an aircraft engine is heated unevenly, one result of the differential thermal expansion of the rotor blade is rotor bowing and adverse contact between the blade tip and the engine casing or shroud. In addition, the optimum clearance gap between the blade tips and the casing or shroud that promotes airflow and enhances engine operating efficiency cannot be maintained. Consequently, engine performance suffers during flight and engine components, particularly rotors, casings, and seals, may need increased maintenance and frequent replacement. Providing sufficient time for an engine to optimally warm up and cool down in a manner that minimizes differential thermal expansion would both improve engine operating efficiency during flight and reduce engine maintenance and repair requirements.
When engines can be kept running on low throttle settings or off while aircraft are being moved to a terminal parking location after landing, engine cool down occurs more evenly so that rotor bow and other thermally-induced structural deformations may be avoided. The even cooling of aircraft engines that have been turned completely off after landing may be accomplished with cooling air from a source of air on the aircraft directed at the engine rotors to keep the rotors turning while the engines are off. Rotor bow may be avoided by keeping the rotors turning over slowly, even very slowly, while the rotors are cooled with airflow directed at them while they turn. One source of cooling air useful for this purpose may be bleed air from the aircraft's auxiliary power unit (APU). Other aircraft air sources may also be employed to provide cooling air, or a dedicated engine cooling air source may be provided.
Moving aircraft on the ground without thrust from main engines by using electric taxi systems has been proposed. Such systems employ electric motors that may be mounted to drive aircraft landing gear wheels during taxi and move the aircraft on the ground between landing and take off without operation of the aircraft's main engines, or autonomously. Typically, after an aircraft equipped with one or more electric taxi systems lands, the aircraft engines are shut down as soon as possible, the electric taxi systems are activated and controlled, and the aircraft is driven to a terminal gate or parking location with only the electric taxi systems. When the aircraft is cleared for departure, the electric taxi systems are activated and controlled to push back the aircraft, or move the aircraft forward out of a parking location, without external tow vehicles or operating engines, and the aircraft is driven with only the electric taxi systems to a take off location. The current state of the art is to start the aircraft main engines of an electric taxi systems-driven aircraft as late as possible before take off. When an aircraft is equipped with electric taxi systems, the art demonstrates that the goal is to shorten aircraft engine run times, primarily to avoid the adverse effects of jet blast and engine ingestion, as well as to minimize damage to engines from foreign object debris (FOD). Other benefits of shortened engine run time may be reductions in fuel use, noise and atmospheric pollution. However, when the running time of aircraft engines is shortened as described, especially the running time of the types of engines that may require longer warm up and/or cool down times, the likelihood of uneven heating of engine components and the differential thermal expansion of those components described above is guaranteed, as are the resulting deviations from the required optimal blade tip clearance.
The method of the present invention is most effectively performed using aircraft equipped with electric taxi systems, such as, for example, the electric taxi systems developed by the present inventor. Such systems are identified by the name WHEELTUG® and are described, for example, in commonly owned U.S. Pat. Nos. 7,975,960; 9,022,316; 9,033,273; and 9,511,853, among others. The disclosures of the foregoing patents are fully incorporated herein by reference. The present method may also be used by aircraft equipped with other electric taxi systems known in the art. These electric taxi systems may move aircraft efficiently during ground travel and operations and provide significant turnaround time savings compared to aircraft moved during ground travel only with aircraft engines. The present method incorporates the significant time savings possible when aircraft are driven on the ground with electric taxi systems with simultaneously providing sufficient time for aircraft engines, especially those with longer than average warm up and cool down time requirements, to warm up and cool down in a manner that prevents engine damage from adverse thermal stresses.
The terms “aircraft engines,” “aircraft main engines,” and “aircraft flight engines” are used interchangeably to indicate an aircraft's engines, such as engines 32 and 34 on aircraft 30 in
Referring to the drawings, which are not drawn to scale,
The aircraft may additionally have one or more electric taxi systems 36, such as those referred to in the above-listed patents, that include electric drive motors mounted within one or more nose or main landing gear wheels. The electric drive motors may be activated and controlled to move the nose or main landing gear wheels in which they are mounted at a desired speed or torque and, therefore, to drive the aircraft on the ground with operation of and without operation of the main engines or external tow vehicles. In
The use of electric taxi systems, such as the nose landing gear wheel-mounted electric taxi systems 36, to drive aircraft during ground operations has heretofore been suggested to reduce aircraft fuel use and to reduce the time aircraft engines may be required to operate during aircraft ground travel. One of the significant benefits of employing an electric taxi system to move aircraft during ground travel is the time savings that are possible. Moving an aircraft on the ground, particularly into and out of a congested airport ramp area, may be done more quickly, efficiently, and safely with electric taxi systems than with thrust from aircraft engines. In addition, since external tow vehicles are not required to move electric taxi system-driven aircraft during push back or at other times, the time required to attach and detach a tow vehicle can be eliminated from the time these aircraft spend on the ground between landing and take off. Reducing the time aircraft spend on the ground may amount to significant cost savings for airlines. Reducing aircraft time on the ground, which increases time when aircraft can be in the air, saves time, a core driver for why people fly. Aircraft engines can operate at a zero throttle setting and idle safely and for a longer time during hybrid taxi with engines operating simultaneously with electric taxi systems in accordance with the method of the present invention. Overall aircraft operating costs may be reduced. Paradoxically, running aircraft engines at a zero throttle setting to idle for longer times than is currently done reduces both costs attributed to engine wear associated with thermal issues and costs attributed to higher aircraft utilization.
The present method enables airlines to simultaneously realize the benefits of moving an aircraft with an electric taxi system and of extending the available warm up and cool down time for the aircraft's flight engines. The time when aircraft engines are running during ground operations may be extended with the present method, rather than shortened as with previously proposed electric taxi systems, so that sufficient time is available for the engines to optimally warm up and cool down more slowly and evenly than when engine running times are shortened.
At the engine start location 48, while the electric taxi systems remain operative and the aircraft continues to move, the aircraft main engines are turned on to a lowest throttle setting for the engine to start a hybrid taxi period. The risk of damage from FOD at the engine start location 48 is likely to be minimal, since vortices are not produced at the lowest engine throttle settings used during the hybrid taxi period. A throttle setting of about 30% or higher is generally required to produce vortices.
During the hybrid taxi period, the electric taxi systems are operating simultaneously with the aircraft main engines at a lowest throttle setting and moving the aircraft toward a take off location 50 on a runway 46. The lowest throttle setting for engines is designated zero (0). This is sufficient to produce idle thrust and will not produce vortices. The engines remain at the zero throttle setting and idle thrust during the hybrid taxi period as the aircraft 40 is driven simultaneously with the electric taxi systems and the engines at the zero throttle setting until the engines are warmed up. With the newer generation engines, the throttle cannot be advanced until the engines are warmed up. The length of the hybrid taxi period will depend on the warm up time for the specific type of engines on the aircraft. During the hybrid taxi period, the aircraft engines should warm up steadily, evenly, and safely as the aircraft is moved along the ground travel path with the simultaneously operating electric taxi systems and main engines.
The aircraft is driven during the hybrid taxi period along a path, represented by arrow c, to the take off location 50. At the take off location 50, the engines should be optimally warmed up, the electric taxi systems may be inactivated, the engine throttle setting may be increased, and the aircraft is moved on the ground with only the warmed up engines. If the aircraft is to take off at location 50, the engine throttle setting may be increased through a breakaway throttle setting of 40% to a take off throttle setting of 90% to 100%. It the aircraft needs to move past location 50 to a different take off location, the throttle setting may be increased to up to the 15% throttle setting for normal taxiing until the take off location is reached where the throttle setting will be increased to the take off throttle setting.
The distance between the engine start location 48 and the take off location 50, represented by the arrow c, may be determined by the time required to sufficiently warm up the specific kind of turbofan engines on aircraft 40 to ensure that these engines are steadily and evenly warmed up prior to increasing the throttle setting. As noted, the optimal engine warm up time may vary for different turbofan engine designs, and the distance between engine start location 48 and the takeoff location 50 will also vary accordingly.
When the aircraft 40 takes off from the take off location 50 on runway 46, the engine should be optimally and evenly warmed up without differential thermal or mechanical expansion of rotor blades, blade tips, and other engine components. As a result, rotor bow and blade tip rubs due to differential thermal expansion of these structures, as shown and described in connection with
The present method may also be employed to ensure that the kinds of turbofan aircraft engines described herein are evenly cooled down after landing during taxi-in. Engines remain on at a zero throttle setting while the pilot simultaneously activates and controls the electric taxi systems and drives the aircraft to a terminal parking location, such as parking location 42 (
When aircraft equipped with electric taxi drive systems and are moved simultaneously with aircraft main engines during hybrid taxi-out and taxi-in encounter ground travel conditions in which it is unsafe for the main engines to be operating, even at a zero throttle setting, the engines may be shut down and the aircraft can continue to be driven with the electric taxi systems. This may occur, for example, at an airport terminal or within safety margins beyond the terminal.
It is possible during hybrid taxi-out or hybrid taxi-in when the aircraft main engines are operating at a zero throttle setting simultaneously with operation of the electric taxi systems that a ground travel condition may be encountered in which the electric taxi system and the engine zero throttle setting are not able to keep the aircraft moving. This might occur if the aircraft must stop during ground travel, for example to move out of a runway depression or other ground surface condition. In that instance, the engine throttle setting may need to be increased to as high as 40% to move the aircraft out of the runway depression. Once the aircraft is moving, the throttle setting may be reduced to zero, and hybrid taxi can continue.
An alternative approach to cooling engines, as noted above, may be employed during taxi-in when the aircraft engines may be set to a zero throttle setting or turned off completely, and engine rotors and/or other rotating components may be rotated by directing a flow of cooling air at these structures to keep them rotating while the electric taxi system is driving the aircraft after landing during taxi-in. For example, bleed air from the APU aimed at rotating engine components can keep these components rotating so that they keep turning, even at a very slow rate, while cooling.
While the method described may require more fuel than shutting down the engines entirely when using only an electric taxi system to drive the aircraft during ground travel, it allows the airline to balance additional fuel consumption costs against engine maintenance costs. Further, the present method produces a reduction in overall fuel requirements, since the zero throttle settings used in conjunction with operation of the electric taxi system consume less fuel than when only one or more of the engines are propelling the aircraft. Engines given maximum time to optimally warm up prior to being set to a taxi, breakaway, or take off throttle setting and to optimally cool prior to being shut down completely operate with greater efficiency and produce corresponding maintenance savings. As discussed above, when thermal expansion of engine components occurs steadily and evenly, thermal and mechanical expansion can be matched, and an optimum clearance gap can be maintained between blade tips and the casing or shroud while the engine is operating during flight. Rotor bow can be eliminated, or at least significantly minimized.
With the present method of providing longer than currently available engine warm up and cool down times without increasing aircraft ground time, aircraft engines can be designed specifically to rely on simultaneous operation of electric taxi systems for hybrid taxi ground travel so that they may function optimally with these longer warm up and cool down times. Engines may be designed to require the longer warm up times possible with the present method, so that warm up time requirements on the order of 3 to 6 minutes or even longer may be implemented. Engine cool down times may also be extended to ensure a cooling time period that produces optimal cooling of engine components. Both of these outcomes may be achieved, moreover, without increasing total aircraft time on the ground or negatively impacting airport or airline operations. It is anticipated that the improvements in engine operating efficiency and maintenance reduction possible with the present method will extend beyond the rotors and casings or shrouds to rotor hubs and other related engine components.
It is additionally anticipated that aspects of the present method may be controlled automatically. For example, the determination of when to begin the engine warm up may be made using data that includes engine type, optimum blade tip clearance gap, engine warm up time requirements for the engine type, the distance between the engine start location and the aircraft take off location, engine cool down requirements for the engine type, the distance between an airport touchdown location and the taxiway location where engines are turned off, the ground speed of the aircraft driven by the electric taxi system, the travel route to or from the aircraft take off or airport touchdown location, and the like. Other aspects of the method may also be automated using appropriate data. Further, engine operating software may be modified to incorporate the longer warm up and cool down periods possible with the present method.
With the present method, aircraft engine designers may design engines specifically to rely on the capability provided to extend both warm up and cool down time when aircraft are equipped with pilot controllable electric taxi systems to power ground movement with and without simultaneously operating engines. Engine design and use may rely on the engine's integration with a simultaneously functioning electric taxi system to move an aircraft while the engine is running, but at a specifically designed low or very low throttle setting that facilitates the even, steady application or removal of heat from engine components. An engine may be designed with rotor blades and blade tips specifically configured within a casing or housing sized to promote symmetrical heating and cooling while maintaining an optimum clearance gap during engine warm up and cool down while the aircraft is driven on the ground with only the electric tax systems during push back or taxi-in. Other engine components, including, for example, the rotor shaft, that are subject to thermal deformation when temperatures of these components fluctuate and are not maintained at a steady level during engine operation may also be designed to conform to tolerances that may be different from those required in engines that must warm up or cool down quickly. Other aspects of aircraft engine design that avoids uneven or asymmetrical thermal expansion or contraction of engine components when extended engine warm up and cool down periods are available are also contemplated to be within the scope of the present method. When an engine's operating envelope is changed, which is the case with the required longer engine warm up time periods addressed by the present method, corresponding changes in engine hardware and software may be needed to ensure optimum efficiency of operation.
While the present invention has been described with respect to preferred embodiments, this is not intended to be limiting, and other arrangements, structures, and steps that perform the required functions are contemplated to be within the scope of the present invention.
The present invention will find its primary applicability where it is desired to improve aircraft engine operating efficiency during flight without adversely impacting airport and airline ground operations, particularly when a longer than average or typical time period is required to warm up and/or cool down aircraft engines, and to ensure the optimal engine warm up and cool down needed to prevent engine damage from adverse thermal stresses.
This application is a continuation-in-part of U.S. patent application Ser. No. 15/441,555, filed 24 Feb. 2017, which claims priority from U.S. Provisional Patent Application No. 62/299,475, filed 24 Feb. 2016, the disclosures of the foregoing applications being hereby fully incorporated herein by reference in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| 62299475 | Feb 2016 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15441555 | Feb 2017 | US |
| Child | 17199151 | US |
