Abstract:
A electric taxi system (ETS) for an aircraft may comprise drive units mounted coaxially with wheels of the aircraft and dedicated motor control units for the drive units. The motor control units may be operable independently of one another so that a first one of the drive units can be operated at a speed different from an operating speed of a second one of the drive units. Independent operability of the drive units may provide enhanced maneuverability of the aircraft during taxiing.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to aircraft landing gear. More particularly, the invention relates to landing gear with integrated electric drive systems to propel an aircraft during taxiing. 
     A typical aircraft may taxi on to and from runways with thrust force developed by its engines. A significant amount of fuel may be burned by the engines during a typical aircraft taxi profile before and after each flight. In many cases, the main engines may provide more motive force than is required to complete a successful taxi profile. In that regard, engine-thrust taxiing may be considered inefficient and may contribute to high fuel costs and ground level emissions. 
     Aircraft designers have sought a more efficient method for propelling an aircraft during taxiing. Electric taxi systems (ETS) have been proposed to provide higher efficiency. An ETS may be implemented by using electrical motors to provide the motive force for aircraft taxiing. While this general ETS concept holds promise for improved efficiency, there are practical application problems that need to be addressed in any successful ETS design. For example, it is desirable that an ETS not diminish brake capacity and structural strength of wheels of an aircraft. Also, installation of the ETS should not impact normal take-off and landing procedures or aircraft performance. Additionally, an ETS should not add excessive weight to an aircraft. 
     As can be seen, there is a need for an ETS which may not adversely impact or interact in any way with the aircraft braking system. Additionally there is a need for an ETS which may not interfere with safe aircraft operation during normal take-off and landing cycles. Also, the ETS system should only minimally impact existing aircraft structures and weight, (e.g., landing gear, landing gear doors, and wheel well configuration). 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, an electric taxi system (ETS) for an aircraft may comprise drive units mounted coaxially with wheels of the aircraft; motor control units for the drive units; and wherein the motor control units are operable independently of one another so that a first one of the drive units can be operated at a speed different from an operating speed of a second one of the drive units. 
     In another aspect of the present invention, a drive unit for an ETS may comprise a drive motor positioned coaxially with a wheel of an aircraft; a selectively engageable clutch assembly positioned coaxially with the wheel; and wherein the clutch assembly is positioned internally to the wheel. 
     In still another aspect of the present invention, a method for taxiing an aircraft with an ETS comprising the steps of: producing airflow through rotors of wheel-mounted drive motors to pre-cool the dive motors; and driving the motors to rotate wheels of the aircraft. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an electric taxi system (ETS) in accordance with an embodiment of the invention; 
         FIG. 2  is block diagram of the ETS of  FIG. 1  in accordance with an embodiment of the invention; 
         FIG. 3  is a perspective view of a wheel of an aircraft having an attached drive unit in accordance with an embodiment of the invention; 
         FIG. 4  is a partial sectional view of the wheel and drive unit of  FIG. 3  in accordance with an embodiment of the invention; 
         FIG. 5  is an illustration of airflow circuits through the wheel and drive unit of  FIG. 3  in accordance with an embodiment of the invention; 
         FIG. 6  is an illustration of a fan assembly in accordance with an embodiment of the invention; and 
         FIG. 7  is a flow chart of a method for taxiing an aircraft with an ETS in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Various inventive features are described below that can each be used independently of one another or in combination with other features. 
     The present invention generally provides an ETS for an aircraft. The ETS may employ electric motors mounted directly on axles of landing-gear wheels. The motors may be driven with electric power generated by a starter/generator driven by an auxiliary power unit (APU) of the aircraft. 
     Referring now to  FIG. 1 , an exemplary embodiment of an ETS  10  which may be installed in an aircraft  12  is shown in schematic form. The system  10  may comprise electric drive units  14  mounted on axles of wheels  16 . A power feed  18  may carry power from an APU  32  (see  FIG. 2 ) to an ETS power distribution unit  20 . A pilot interface unit  22  may be connected to the ETS power distribution unit  20  through an interface cable  24 . Upon appropriate commands from a pilot, electric power may be transmitted to the electric drive units  14  through an ETS feeder  26 . 
     Referring now to  FIG. 2 , a block diagram may illustrate some interconnection features of the ETS  10 . The drive units  14  may be mounted directly on outboard ones of the wheels  16 . The drive units  14  may be controlled with dedicated motor control units  28 . The motor control units  28  may be pilot-controlled from the pilot interface unit  20 . Electric power may be supplied to the motor controllers  28  from a starter/generator  30  driven by the APU  32 . Power may be supplied through conventional bus bars  34 , contactors  36  and dedicated AC/DC converters  38 . 
     Because each of the drive units  14  may be controlled through dedicated motor control units  28 , the drive units  14  may be operated independently of one another. For example, a left hand one of the drive units  14  may rotate more slowly that a right hand one of the drive units  14 . This may produce left turning of the aircraft  12 . In other words, the ETS  10  of the present invention may be used to steer the aircraft  12  during taxiing. 
     Additionally, a left hand one of the drive units  14  may be rotated counterclockwise while a right hand one of the drive units  14  may be rotated clockwise. In this operational state, the aircraft  12  may be propelled in a forward direction even though both the left and right hand drive units  14  may be engaged with outboard ones of the wheels  16  of the aircraft  12 . 
     The drive units  14  may also be controlled to produce reverse movement of the aircraft  12 . In that context, the drive units  14  may be advantageously controlled so that reverse motion of the aircraft  12  is stopped by regenerative braking. By using controlled regenerative braking, the aircraft  12  may be decelerated slowly so that fuel in its tanks does not shift rearward. This may preclude a potential problem associated with reverse movement of aircraft, i.e., a center of gravity of the aircraft shifting rearward if fuel shifts rearward. Such undesirable fuel shifting may cause tilting of the aircraft  12  with a nose wheel lifted from the ground and a tail section resting on the ground. 
     Referring now to  FIG. 3 , an outboard one of the wheels  16  is shown. For purposes of clarity, the wheel  16  is shown without a tire. The wheel  16  may comprise a hub  16 - 1  and rims  16 - 2 . In an exemplary embodiment of the invention, the wheel  16  may have a split-hub configuration. The wheel  16  may have a split line  16 - 3  along which the wheel  16  may be separated for purposes of installing and removing tires. The drive unit  14  may be mounted adjacent an outboard one of the rims  16 - 2  and coaxially with the wheel  16 . Advantageously, the drive unit  14  may have an outside diameter that is no larger than an outside diameter of the rim  16 - 2 . 
     Referring now to  FIG. 4 , a partial cross-sectional view of the wheel  16  may illustrate various inventive features of the drive unit  14 . The drive unit  14  may comprise a drive motor  14 - 1  supported concentrically with a wheel axle  40 . In that regard, the drive motor  14 - 1  may be considered to be a wheel-mounted drive motor. Advantageously, the drive motor  14 - 1  may be a Segmented ElectroMagnetic Array (SEMA) motor. Rotors  14 - 1 - 1  of the drive motor  14 - 1  may be adapted to rotate around the wheel axle  40 . The rotors  14 - 1 - 1  may be connected to drive a clutch assembly  14 - 2 . The clutch assembly  14 - 2  may be selectively engageable with the wheel  16 . In other words, the wheel  16  may be driven by the drive motor  14 - 1  when the clutch assembly  14 - 2  may be engaged. Conversely, when the clutch assembly  14 - 2  may be disengaged, the wheel  16  and the motor rotor  14 - 1 - 1  may be rotatable independently of one another. 
     In an exemplary embodiment the wheel  16  may have a first hub portion  16 - 1 - 1  and a second hub portion  16 - 1 - 2 . The split line  16 - 3  may define a location at which the two hub portions  16 - 1 - 1  and  16 - 1 - 2  may be separated. In  FIG. 4 , the hub portion  16 - 1 - 1  may be shown at a left side of the split line  16 - 3  and the hub portion  16 - 1 - 2  may be shown at a right side of the split line  16 - 3 . A brake assembly  42  may be incorporated into the hub portion  16 - 1 - 1 . The clutch assembly  14 - 2  may be located in the hub portion  16 - 1 - 2 . The motor  14 - 1  may be located outside of the hub portion  16 - 1 - 2  and adjacent the rim  16 - 2 . 
     The relative positions of the motor  14 - 1 , the brake assembly  42  and the clutch assembly  14 - 2  may be advantageous for a number of reasons. First of all, the brake assembly  42  may be located in the wheel  16  at a location that is consistent with conventional locations of brake assemblies in many conventional wheel of existing aircraft. Consequently, such conventional wheels may be retro-fitted for ETS operation without reconfiguration of their brake assemblies. 
     Secondly, conventional aircraft wheels typically have a hollow chamber in their outboard hub portion. In the present embodiment of the invention, the clutch assembly  14 - 2  may be internally positioned in this otherwise hollow hub portion (i.e., the hub portion  16 - 1 - 2 ). This arrangement provides for a reduced axial projection of the drive unit  14 . In other words, the drive unit  14  may extend only a limited axial distance beyond the rim  16 - 2 . In this regard, it may be advantageous to position the drives units  14  in outboard ones of the wheels  16  as shown in  FIG. 2 . When the wheels  16  are retracted after takeoff, the drive units  14  may be oriented in a downward position. As a consequence, an aircraft may be easily retrofitted with the inventive ETS because only a limited modification to landing gear doors (not shown) may be needed to accommodate minimally extending drive units  14 . If drive units were installed on inboard wheels, an extensive fuselage reconfiguration might be required to accommodate drive units when landing gear is stowed. 
     An additional advantage of the present embodiment may be that the motor  14 - 1  may have a diameter larger than an interior of the hub portion  16 - 1 - 2 . Increasing diameter of a SEMA motor may result in increased torque availability. 
     It may also be noted that the drive unit  14  may include a blower motor  14 - 3  which may be operated independently from the drive motor  14 - 1 . Advantageously, the blower motor  14 - 3  may be a SEMA motor. 
     Referring now to  FIG. 5 , a partial sectional view of the drive unit  14  and the wheel  16  may illustrate cooling features of an exemplary embodiment of the invention. A series of arrows may represent a motor cooling airflow circuit  46 . Another series of arrows may represent a brake cooling airflow circuit  48 . 
     The blower motor  14 - 3  may be operated at a desired rotational speed irrespective of the speed at which the drive motor  14 - 1  may be operated. Consequently, a positive airflow may be induced along the motor cooling airflow circuit  46 . The motor cooling airflow circuit  46  may pass between elements of the rotor  14 - 1 - 1  of the drive motor  14 - 1 . As a result of positive cooling, the drive motor  14 - 1  may be operated with a high torque output even when it may have a low rotational speed. Indeed, the drive motor  14 - 1  may be safely operated in an overcurrent condition for extended time periods because of cooling produced with the blower motor  14 - 3 . This feature may allow for use of a relatively small and readily stowable drive motor, even though there may be high torque requirements associated with moving the aircraft  12 . 
     The brake cooling airflow circuit  48  may pass through the brake assembly  42  and past the drive motor  14 - 1  to exit from the drive unit  14 . Airflow through the circuit  48  may be positively produced by operation of the drive motor  14 - 1 . A fan blade assembly  50  (See  FIG. 6 ) may be adapted to rotate with the rotor  14 - 1 - 1  of the motor  14 - 1 . This arrangement may be particularly useful after an aircraft has arrived and stopped at a gate following a landing. The brake assembly  42  may be hot as a result of braking during landing. When the aircraft stops, the clutch assembly  14 - 2  may disengage the drive motor  14 - 1  from the wheel  16  and the drive motor  14 - 1  may then be operated at a high speed. High speed operation of the drive motor  14 - 1  may propel the fan blade assembly  50  at a corresponding high speed. Induced cooling airflow may advantageously pass through the brake assembly  42  (along the circuit  48 ) thereby cooling the assembly. 
     Referring back now to  FIG. 3 , it may be seen that a shroud  16 - 4  may be attached to the wheel  16  and may overlie a portion of the drive unit  14 . The shroud  16 - 4  may be adapted to rotate at the same rotational speed as the wheel  16 . The shroud  16 - 3  may be useful to protect against tire damage during high speed taxiing turns of the aircraft  12 . In some high speed turns, a tire (not shown) may deflect so that its sidewall (not shown) may protrude beyond the rim  16 - 2  of the wheel  16 . Moreover, the protruding portion of sidewall may project below the rim  16 - 2 . In this event, the sidewall may come into contact with the shroud  16 - 4 . Contact between the sidewall and the drive unit  14  may be precluded by presence of the shroud  16 - 4 . Without such a feature, the tire might be damaged because the drive motor  14 - 1  may rotate at a speed different from that of the wheel  16 . Contact between the tire and the motor  14 - 1  may result is damage to the tire. 
     Referring now to  FIG. 7 , a flow chart  700  may illustrate an exemplary method which may be employed to taxi an aircraft with ETS. In a step  702 , a blower motor may be started (e.g., a pilot may operate the pilot interface unit  22  to start the blower motor  14 - 3  of the drive unit  14  to initiate cooling airflow across the rotor  14 - 1 - 1  of the drive motor  14 - 1  to pre-cool the drive motor). In a step  704 , the drive unit may be engaged with the wheels by engaging the clutch assembly (e.g., the clutch assembly  14 - 2  may be engaged so that rotation of the rotors  14 - 1 - 1  may impart rotation to the wheels  16 ). In a step  706 , current may be applied to drive units to move the aircraft (e.g., the pilot may operate the pilot interface unit  22  to apply electrical power from the motor control units  28  to the drive units  14  to produce desired rotation of the wheel  16  and movement of the aircraft in either a forward or reverse direction). In a step  708 , the aircraft may be taxied to a take-off position. In a step  710 , the drive units may be disengaged from the wheels (e.g., the pilot may operate the pilot interface unit  22  to disengage the clutch assembly  14 - 2 ). In a step  712  takeoff of the aircraft may be performed in a conventional manner. 
     In a step  714 , landing of the aircraft may be performed in a conventional manner (e.g., landing may be performed with the clutch assembly  14 - 2  disengaged so that the wheels  16  do not produce rotation of the drive motors  14 - 1 ). In a step  716 , the drive unit may be engaged with the wheels after the aircraft has stopped on a landing runway (e.g., the pilot may operate the pilot interface unit  22  to engage the clutch assembly  14 - 2 ). In a step  718 , current may be applied to drive units to move the aircraft (e.g., the pilot may operate the pilot interface unit  22  to apply electrical power from the motor control units  28  to the drive units  14  to produce desired rotation of the wheels  16  and movement of the aircraft). In a step  720 , the aircraft may be taxied to a gate. In a step  722 , the clutch assembly may be disengaged. In a step  724 , the drive motor may be operated to cool the brake assemblies (e.g., the drive motor  14 - 1  may be operated at a high speed so that the fan assembly  50  may produce cooling air flow through the brake assemblies  42 ). 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.