Patent Publication Number: US-10766614-B2

Title: Method and system for improving transition lift-fan performance

Description:
CROSS REFERENCE 
     This application is a continuation of commonly owned U.S. patent application Ser. No. 13/311,295, filed on Dec. 5, 2011, by Robert Parks, which is hereby incorporated by reference. 
    
    
     FIELD 
     The present disclosure relates to a system and method for use with a lift-fan aircraft. Specifically, the present disclosure relates to systems and methods for reducing power consumption and momentum drag in a lift-fan aircraft. 
     BACKGROUND 
     Over the years, gas turbine-powered aircraft have used a variety of configurations to achieve vertical and/or short take-off and landing (V/STOL). One method is to vector the engine exhaust from one or more turbojet engines downward to create lift. Once airborne, this type of aircraft gradually transitions the thrust from downward to aft until a forward airspeed sufficient to support wing-borne flight of the aircraft is achieved, at which point conventional aerodynamics may take over. Other configurations use remotely located lifting fans powered by compressor stage bleed air from the turbojet engines to create lift. Alternatively, a ducted mixture of compressor bleed air and turbojet exhaust gas may be ducted to remotely located nozzles that discharge a downward thrust, thereby creating reaction-lifting forces that lift and control the aircraft. While these configurations often use very high disc loading fans, they are still more efficient than pure jet variants. 
     An exemplary high disc loading lift-fan aircraft is the Ryan XV-5, which was developed during the 1960s and flown successfully in 1968. The XV-5 used a pair of General Electric J-85 turbojet engines and three lift fans to provide controlled flight. Specifically, installed in each wing is a 62.5-inch diameter fan configured to provide collectively a majority of the vertical thrust, with a smaller fan in the nose to provide lift and pitch trim. For vertical liftoff, jet-engine exhaust is diverted to drive the lift-fan tip turbines via a diverter valve. The core engines provided a total thrust of 5,300 pounds in forward flight mode, but could generate a total lift thrust of 16,000 pounds via the lift fans in hover mode. Using the lift fans provides a 200% increase in the total thrust, a clearly advantageous feature for vertical takeoff and landing (“VTOL”) aircraft. 
     Unfortunately, a disadvantage associated with ducted lift-fan aircraft is momentum drag, which causes an aircraft to require much higher power levels. Momentum drag is generally caused by a directional change of the airflow going through the lift fans. For instance, the lift fan airflow initially has horizontal momentum (relative to the aircraft and due to the forward speed of the aircraft), but exits vertically, with no relative horizontal momentum. This change in momentum results in a horizontal force towards the back of the aircraft (i.e., momentum drag), which has an effect similar to normal aerodynamic drag. This momentum drag is a function of the mass flow rate of air through the lift fans times the forward speed of the aircraft through the air. Often the goal of lift-fan aircraft design is to increase the mass flow of air by using larger diameter fans with a smaller lift per unit area. This reduces the power needed to produce the required lift; however, the associated higher mass flow can greatly increase the momentum drag. As momentum drag can become quite large, forward flight thrusters require a large amount of power to enable the aircraft to accelerate fully to wing-borne flight. 
     Therefore, a need exists for a system and method to both increase efficiency and reduce the momentum drag. More specifically, a need exists for a system and method for increasing efficiency and reducing the momentum drag of a light or low disc loading lift-fan aircraft, such that its peak power requirements are not increased greatly from its hover power requirements. 
     SUMMARY 
     The present application discloses a system and method for increasing efficiency and reducing the momentum drag of a low disc loading lift-fan aircraft, such that its peak power requirements are not greatly increased from its hover power requirements. 
     According to a first aspect, a method of operating a propulsion system in a VTOL aircraft includes controlling a forward thrust propulsion system to generate a forward thrust during a transition from vertical flight to forward flight; selectively allocating, during the transition from vertical flight to forward flight, each of a plurality of ducted lift fans into a first operation group configured to operate at substantially full throttle, a second operation group configured to operate at intermediate throttle, or a third operation group configured to operate at substantially zero throttle; and selectively reallocating, during the transition from vertical flight to forward flight, one or more of the plurality of ducted lift fans from one of the first operation group, the second operation group, or the third operation group to a different one of the first operation group, the second operation group, or the third operation group to generate a second desired total thrust, where each of the plurality of ducted lift fans is configured to be controlled independently and has an axis of rotation that is fixed and substantially vertical; the first operation group, the second operation group, and the third operation group are configured to generate, in aggregate, a first desired total thrust. 
     According to a second aspect, a system for operating a VTOL aircraft includes a forward propulsion system; a plurality of ducted lift fans; and a controller. The controller is configured to control the forward thrust propulsion system to generate a forward thrust during a transition from vertical flight to forward flight; selectively allocate, during the transition from vertical flight to forward flight, each of the plurality of ducted lift fans into a first operation group configured to operate at substantially full throttle, a second operation group configured to operate at intermediate throttle, or a third operation group configured to operate at substantially zero throttle, selectively reallocate, during the transition from vertical flight to forward flight, one or more of the plurality of ducted lift fans from one of the first operation group, the second operation group, or the third operation group to a different one of the first operation group, the second operation group, or the third operation group to generate a second desired total thrust; control the forward thrust propulsion system to generate a forward thrust during forward flight; and allocate each of the plurality of ducted lift fans to the third operation group during forward flight, where each of the plurality of ducted lift fans is configured to be controlled independently and has an axis of rotation that is fixed and substantially vertical; the first operation group, the second operation group, and the third operation group are configured to generate, in aggregate, a first desired total thrust, and a minority of the plurality of ducted lift fans are maintained in the second operation group relative to the first operation group and the third operation group during the transition from vertical flight to forward flight. 
     In certain aspects, a minority of the plurality of ducted lift fans are maintained in the second operation group relative to the first operation group and the third operation group during the transition from vertical flight to forward flight. 
     In certain aspects, the second operation group is configured to operate at intermediate throttle by varying a rotation per minute (RPM) of one or more of the ducted lift fans of the second operation group; varying a pitch of one or more of the ducted lift fans of the second operation group; or a combination thereof. Such throttling may also be used, for example, to trim the aircraft&#39;s pitch and roll. 
     In some aspects, the inactive lift fans may be covered after being shut down to decrease aerodynamic drag on the VTOL aircraft. 
     In certain aspects, at least one of the plurality of lift fans may have adjustable vanes to direct the lift fan exhaust flow aft, thereby providing thrust and reducing drag. 
     In certain aspects, the plurality of lift fans may comprise six or more lift fans. 
     In certain aspects, the VTOL aircraft uses a hybrid of turbine and electric thrusting. 
    
    
     
       DRAWINGS 
       The foregoing and other objects, features, and advantages of the devices, systems, and methods described herein will be apparent from the following description of particular embodiments thereof, as illustrated in the accompanying figures; where like reference numbers refer to like structures. The figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the devices, systems, and methods described herein. 
         FIG. 1 a    illustrates an XV-5 lift-fan aircraft. 
         FIG. 1 b    illustrates the engine and lift fan of the XV-5 lift-fan aircraft. 
         FIG. 2  illustrates an exemplary contemporary lift-fan aircraft having multiple low disc loading fans. 
         FIGS. 3 a  through 3 c    illustrate typical airflow patterns around a lift fan, such as a lift fan of the aircraft shown in  FIG. 2 . 
         FIG. 4  illustrates a graph of the transitional power requirements of a high disc loading lift-fan aircraft. 
         FIG. 5  illustrates a graph of the transitional power requirements of low disc loading lift-fan aircraft using fan throttling methods. 
         FIG. 6  illustrates a graph of the transitional power requirements using the systems and methods of an embodiment of the present disclosure. 
         FIG. 7  illustrates an overlay comparison of the total power curves from  FIGS. 5 through 6 . 
     
    
    
     DESCRIPTION 
     Embodiments of the present disclosure will be described hereinbelow with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail because they would obscure the invention in unnecessary detail. The present disclosure discloses a system and method for increasing efficiency and reducing the momentum drag of a low disc loading lift-fan aircraft, such that its peak power requirements are not greatly increased from its hover power requirements. 
     Lift fans are commonly used in VTOL aircraft, which can include both fixed-wing aircraft and non-fixed-wing aircraft. However, lift fans can enable VTOL aircraft to operate in other modes as well, including, for example, CTOL (conventional takeoff and landing), STOL (short takeoff and landing), and/or V/STOL (vertical and/or short take-off and landing). 
     An example of an early lift-fan aircraft is the Ryan XV-5 (a/k/a XV-5a and XV-5b), which was developed during the 1960&#39;s and flown successfully in 1968. As depicted in  FIGS. 1 a  and 1 b   , the XV-5  100  used a pair of General Electric J-85 turbojet engines  110  (see  FIG. 1 b   ) and three lift fans  102   a ,  102   b ,  102   c  to sustain controlled flight. Specifically, installed in each wing  104   a ,  104   b  was a 62.5-inch diameter fan  102   a ,  102   b  configured to provide the majority of the vertical thrust, with a smaller 36-inch diameter fan  102   c  in the nose  106  to provide some lift as well as pitch trim. Each wing fan  104   a ,  104   b  has a disc area of approximately 21 square feet with a thrust of approximately 6,800 pounds, yielding a high disc loading (thrust divided by disc area) of approximately 320 pounds per square foot. 
     A safety feature of the gas-driven XV-5 lift-fan concept  100  was the robustness of the lift fans  102   a ,  102   b ,  102   c . The absence of drive shafts, shaft bearings, gearboxes, and the attendant-pressure-lubrication systems resulted in relatively low maintenance and high pilot confidence. In fact, the only on-board indicators associated with the three lift fans  102   a ,  102   b ,  102   c  installed in the XV-5  100  were rotations per minute (RPM) and fan cavity temperature. Pilot monitoring of fan machinery health was thus reduced to a minimum, which was highly desirable for a single-piloted aircraft. Such lift fans have also proven to be highly resistant to ingestion of foreign objects, which is a positive factor for remote site operations. 
     As seen in  FIG. 1 b   , to operate the XV-5  100  lift fans  102 , exhaust from the jet engines  110  is diverted via a diverter valve  112  via tip turbines  114 . The core engines  110  provide a total thrust of 5,300 pounds in forward flight mode but can yield a total lift thrust of 16,000 pounds via the lift fans  102   a ,  102   b ,  102   c  in hover mode. This provides a 200% increase in the total thrust, a clear advantage for a VTOL aircraft. 
     Another lift-fan aircraft is disclosed by U.S. Pat. No. 4,022,405 to John M. Peterson entitled “Fan Lift-Cruise V/STOL Aircraft” (the “&#39;405 patent”). The &#39;405 patent discloses an aircraft for making vertical and short field takeoffs powered by a bypass turbofan engine having a core turbojet and a bypass fan. For vertical takeoffs, the efflux from the bypass fan is deflected downward to create lift. Exhaust from the core turbojet is ducted to turbine-driven lifting fans at remote locations to create supplemental lifting forces, which also provides aircraft control and stability. For cruise, the bypass fan efflux is discharged aft until sufficient forward airspeed is attained that exhaust gas flow to the lifting fans may be shut off, and then exhaust gas flow may be discharged aft. The &#39;405 patent also discloses a number of suitable lift-fan throttling and vane controller methods that may be used in conjunction with the present application. 
     However, more recently, the design trend has been to move away from these configurations by decreasing disc loading, thereby reducing peak power. An added benefit of this new configuration is that the resulting reduced downwash speed also minimizes ground erosion. In addition, multiple fans may be used to allow redundancy in case of a lift-fan failure. 
     In certain aspects, lift fans may be modular units with integrated electric power where stored electric power may be used to handle the very high power burst needed for takeoff, without increasing the size of the main cruise jet engines. This configuration may promote greater efficiency. For instance, an aircraft may store the required takeoff power in the form of a primarily electric fan engine, and secondarily in the form of an internal combustion engine. As a result, the combined power of the electric fan and internal combustion engines may cause the VTOL aircraft to take off using substantially less time and space as compared to other VTOL aircraft. In addition, using a hybrid configuration, the transition from vertical to horizontal thrust may be carefully executed to rise rapidly from the takeoff position to a forward flight position, thereby minimizing the necessity for a larger electric fan engine. 
     For further information on hybrid turbine electric thrusting techniques, see, for example, commonly owned U.S. Pat. No. 7,857,254 (“&#39;254 patent”) to Robert Parks, entitled “System and Method For Utilizing Stored Electrical Energy for VTOL Aircraft Thrust Enhancement and Attitude Control.” The &#39;254 patent also discloses a suitable programmable controller for controlling, inter alia, a plurality of electrically-driven fans in an aircraft. Specifically, the &#39;254 patent explains that the aircraft control systems, and any and all components thereof (e.g., a main flight control computer, attitude control circuit, and/or thrust control circuit, among other components) can employ any suitable type of processor and memory, which can be programmed using conventional techniques known to those having ordinary skill in the art of computer programming (whether in actual source code or object code). The processor and memory can be used, for example, to perform some or all of the functions of the aircraft control systems. 
     Due to scaling laws for fans and motors, employing multiple smaller fans can be lighter than using fewer large fans to provide the same total thrust. In addition, multiple smaller fans also provide more flexibility to the configuration designer. For example, as seen in  FIG. 2 , an aircraft  200  may employ a forward thrust propulsion system  108  and multiple smaller lift fans  102  may be used to create a vertical takeoff vehicle. As illustrated, a forward thrust propulsion system  108  may be positioned on each side of the fuselage  116  and oriented to direct thrust toward the aft end, thereby resulting in forward thrust for the aircraft  200 .  FIG. 2  also illustrates an arrangement of multiple smaller lift fans  102  to provide vertical thrust. Specifically, three lift fans  102  are positioned near the distal end of each of the two aft wings  104   c  and eight lift fans  102  are positioned above the fuselage  116  (and arranged to provide vertical thrust straddling each side of the fuselage  116 ). As illustrated in Detail A of  FIG. 2 , each of the lift fans  102  may employ adjustable vanes  118  (e.g., on the top and/or bottom of the lift fans  102 ) to direct airflow in and/or out of the lift fans  102  (e.g., during operation). The vanes  118  may be fully shut to reduce drag during forward flight when the lift fans  102  are not in operation. As noted above, using multiple smaller fans permits redundancy, thereby providing backup fans in the event of one or more fan malfunctions. Nevertheless, momentum drag can remain an issue when ducted lift fans are used. 
     To better understand momentum drag, basic lift-fan operation and its associated physics will now be described in greater detail. The fundamental concept behind ducted lift fans is that they accelerate air downwards to produce the lift needed for vertical takeoff. This lift results from the change in vertical momentum of the airflow that occurs when the airflow, which initially has little or no vertical momentum, accelerates downward at high speed, increasing the vertical momentum. 
     For solid objects, momentum is simply the mass of the object multiplied by its velocity relative to a reference point. However, for a fluid (e.g., air), momentum is the mass flow rate (e.g., kg or slugs per second) multiplied by the velocity. For the following examples, the VTOL aircraft will be used as the velocity reference point (e.g., either in a static hover, with no airspeed, or at a constant forward speed). The force produced on a given axis is simply the change in momentum of the airflow along that axis. 
     Turning now to  FIG. 3 a   ,  FIG. 3 a    shows the airflow (illustrated as arrows) for an example lift fan  102  during hover operation (i.e., zero horizontal speed). The airflow is pulled into the lift fan  102  from a full hemisphere above the lift fan  102 . The airflow accelerates as it goes into and through the lift fan  102 , and exits straight out the bottom of the lift fan  102 . The vertical thrust is the change in vertical axis momentum. Since the initial speed is zero, the vertical lift (or thrust) is simply the mass airflow rate times the exit airflow speed. Conversely,  FIG. 3 b    shows the same lift fan  102 , with the aircraft flying horizontally at a relatively high speed, such as it would when transitioning into wing-borne forward flight. Now the airflow that enters the lift fan  102  is coming from the front (nose-end of the aircraft), with an initial airspeed equal to the flight speed of the aircraft. The air is pulled into the lift fan  102 , and again exits out the bottom, moving relatively straight out the bottom, as seen in the frame of reference of the moving aircraft. As before, the vertical lift is the change in vertical axis momentum, and since the vertical relative speed of the air is initially zero, the vertical force is the mass airflow rate times the vertical exhaust speed. In this case, however, there is an initial horizontal relative speed of the air, due to the forward speed of the aircraft, but the lift fan  102  and its duct force the airflow to exit nearly vertical, relative to the aircraft, with near zero relative horizontal speed. This change of horizontal speed of the air, and thus a change in horizontal momentum, means there is a force to the aft of the aircraft, i.e., drag. Finally,  FIG. 3 c    shows the same lift fan  102 , again with the aircraft flying horizontally at a relatively high speed. However, in this instance, there are deflected vanes  118  under the lift fan  102  that deflect the exhaust airflow stream aft. This reduces the change in horizontal momentum, thereby reducing the drag. However, such vanes  118  have a practical limit with respect to their ability to turn the air, and thus they usually cannot fully eliminate the drag. In practice, the vanes  118  turn the airflow about 30 to 40 degrees, without excessive complexity and/or energy losses. 
     Assuming the aircraft is in a static hover, with no vertical speed, airflow starts out at rest relative to the aircraft, yielding no momentum. A lift fan  102  may be used to accelerate the air by sucking air from the top of, for example, the wing, and expelling it at high speed out the underside. A characteristic of a ducted lift fan, having a fully subsonic exit flow, is that the air exits the duct at a pressure equal to the ambient pressure, and thus exits in a cylindrical stream tube of constant area. Contrary to a non-ducted rotor (e.g., a propeller), where the wake contracts and increases in speed downstream of the rotor, the airflow speed at a ducted fan exit is equal to the free wake. Therefore, the exit momentum of the air is equal to the mass flow rate at the exit multiplied by the velocity at the exit. Since the pressure at the exit is equal to the ambient pressure, the density of the air would be the same as ambient density, where the airflow rate is equal to the density multiplied by the cross section area of the lift fan exit multiplied by the airflow speed. Assuming a static hover condition, Equation 1 is a suitable equation for fan thrust (T) where ρ is the air density, A is the cross section area of the lift fans, and s is the exit flow speed.
 
 T=ρ×A×s   2    Equation 1
 
     The power needed to accelerate the airflow is similar, but can vary as the kinetic energy of the exit flow changes. However, again assuming a static hover condition, there will be zero initial kinetic energy. Thus, Equation 2 is an equation for power (P) where {dot over (m)} is the mass flow rate and v is the velocity. 
     
       
         
           
             
               
                 
                   P 
                   = 
                   
                     
                       
                         
                           m 
                           . 
                         
                         × 
                         
                           v 
                           2 
                         
                       
                       2 
                     
                     = 
                     
                       
                         ρ 
                         × 
                         A 
                         × 
                         
                           v 
                           3 
                         
                       
                       2 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     Using these two relations, it is evident that, for a given thrust ( 7 ), high exhaust speeds require high power levels. However, having large cross sectional areas, lower speeds and/or larger mass flows can be used to minimize power consumption. In fact, unlike the higher gas velocities, these lower exhaust speeds can serve the added benefit of minimizing ground erosion. While ground erosion can be problematic for proper runways, it has a more significant impact on a semi-prepared strip and may lead to potential foreign object damage (“FOD”) hazards. 
     Despite the benefits of lift-fan aircraft, an issue remains with respect to the momentum drag generated when the air exiting from the lift fan  102  is directed downward, relative to the aircraft frame of reference. When the aircraft is moving (e.g., when transitioning from hover to forward flight), incoming air typically has a horizontal speed relative to the aircraft. However, the duct around the lift fan  102 , particularly the inlet, directs the horizontal airflow to enter the lift fan to be redirected downward. This is mainly from a large suction force on the upstream inlet lip. Thus, the air initially has horizontal momentum relative to the aircraft, but it exits vertically, with no horizontal momentum. 
     This change in momentum results in a horizontal force toward the back of the aircraft, known as momentum drag—exhibiting forces similar to normal aerodynamic drag. The initial momentum is equal to the forward speed of the aircraft times the mass flow rate. The mass flow rate would be equal to the mass flow exiting the lift fan  102 . Thus, Equation 3 represents the equation for momentum drag (D), where S is the forward flight speed of the aircraft.
 
 D=ρ×A×S   2    Equation 3
 
     Thus, a lift-to-drag ratio for a lift fan may be defined as lift divided by momentum drag, which, using the above equations, can be reduced to: 
     
       
         
           
             
               
                 
                   
                     Ratio 
                   
                   = 
                   
                     
                       FanLift 
                       D 
                     
                     = 
                     
                       s 
                       S 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
               
             
           
         
       
     
     The maximum airspeed that the lift fans  102  need to operate would be equal to the wing-borne speed, where the aircraft is fully supported by its wings and the lift fans  102  can be shut down. This speed may be determined by the aircraft design and its unique mission requirements. For example, an aircraft with a high cruise speed would typically employ a relatively small wing to minimize the cruise drag; however, a small wing would then have a relatively high wing-borne speed. Thus, to minimize momentum drag, a high exit flow speed and a high fan disc loading are desirable. Ironically, this is counter to the above-mentioned goal of a low exit speed and low fan disc loading to reduce hover power and reduce ground erosion. 
     During transition (e.g., from hover to forward flight or vice versa), as the aircraft speed increases, the amount of lift from the wing also increases and the thrust needed from the lift fans decreases. This can be beneficial because the power needed for the lift fans  102  also decreases, allowing more engine power to be used for forward propulsion. However, a problem remains that traditional methods of reducing lift-fan thrust are to reduce the air velocity through the lift fan  102  by simply decreasing fan RPM, or by decreasing fan blade pitch angle. This lower exit flow speed can mean that the lift fan  102  lift-to-drag ratio also decreases. At the same time, the power required to overcome drag is the flight speed multiplied by the drag force, resulting in a large drag at higher speeds with a large power requirement. The result is that the momentum drag can become quite large, and require large amounts of power to the forward flight thrusters to allow the aircraft to accelerate to fully wing-borne flight. 
     Thus, a situation arises where the light disc loading lift fans used to reduce hover power are actually causing an increase in the peak power required for takeoff and transition to wing-borne flight. In fact, this peak power may actually be comparable to the peak power needed by a much higher disc-loading fan, rendering the whole effort fruitless. 
     Thrust reduction of a lift fan  102  may be accomplished by a combination of reducing fan blade pitch angle and/or reducing fan RPM. Thrust reduction may be performed simultaneously for all lift fans  102 , or, as described herein, a subset or operation group of the lift fans  102 . For example, half thrust for the aircraft may be achieved by operating half of the lift fans  102  of the aircraft at zero throttle, while maintaining the other half of the lift fans  102  at full throttle. Correspondingly, one-quarter thrust could be achieved by operating one quarter of the lift fans  102  at full throttle and remaining three quarters of the lift fans  102  at zero throttle. Intermediate values are also contemplated and could be obtained by operating some lift fans  102  at zero throttle, some at full throttle, and a nominal amount (e.g., one or two) of lift fans  102  at an intermediate, partial throttle. Thus, the lift fans  102  can be controlled independently, but allocated and operated as separately controllable operation groups (i.e., one or more lift fans per operation group). 
     Operating a lift fan  102  at zero throttle may be accomplished by shutting off the lift fan  102  (e.g., no longer supplying power/fuel to the lift fan  102  such that is not driven) or otherwise adjusting the lift fan  102  such that it no longer provides vertical thrust (e.g., by adjusting the fan blade pitch angle to 0 degrees). A processor-based controller may be programmed to operate the various lift fans  102  and vanes  118  (as applicable) in accordance with programming instructions (e.g., software) to achieve a desired total thrust and to allocate the lift fans  102  to one of the separately controllable operation groups. An example programmable controller for performing the various processes and flight-control techniques disclosed herein is described above and by the &#39;254 patent. 
     In other words, the momentum drag and power required may be reduced by maintaining only a preset number/subset of lift fans (an operation group) at a continuous full throttle setting, thereby only reducing the throttle on a smaller number of the lift fans  102  (e.g., another operation group of one or two of the lift fans  102 ), and having any remaining fans fully shut down (e.g., an operation group where the lift fan(s)  102  will be off). The lift fans  102  to be throttled back (e.g., have their power reduced), or shut off, may be selected in a manner that trims the pitching and rolling movements of the aircraft (e.g., in accordance with the controller). For example, depending on the layout of lift fans, a front left fan and a back right fan may be shut down, thus maintaining zero moments about the center of mass of the aircraft. 
     To illustrate the beneficial results from the above-mentioned techniques, a number of tests were performed and evaluated using a VTOL aircraft performance simulation. For the following analysis, a number of relations and assumptions were established. First, at hover, all of the lift will be generated from the lift fans. Second, the lift fans for each test have a specified disc loading value—the total fan thrust divided by the disc area of the lift fan, as measured at full throttle for each fan. However, the disc loading value could be varied to show different cases for different aircraft. Third, the lift-fan thrust and power was calculated according to the equations described above, but may differ for different disc loadings because the calculation is based on the actual fan thrust required at that instant in the flight. Fourth, the aircraft has at least one wing that operates a constant lift coefficient throughout the transition from hover mode to wing-borne flight. For instance, at the wing-borne speed, the wings, and not the lift fans  102 , carry all of the lift. The wing lift will also vary as the square of the airspeed (i.e., constant lift coefficient). Similarly, since the lift coefficient CL is constant, the drag coefficient is constant (i.e., the aerodynamic lift-to-drag ratio is constant), thus the basic aerodynamic drag coefficient is also constant. 
     For purposes of this analysis, the wing-borne speed was set to 100 knots for each aircraft and the aerodynamic lift-to-drag ratio was set at 6:1. While these values may be low for a cruising aircraft, a transitioning VTOL aircraft tends to have open doors and other drag-increasing devices exposed. As the wing lift increases with increasing speed, the lift fan  102  throttle may be reduced, such that the total lift is constant but wherein the lift fans  102  are operated via their separately controllable groups. 
     When using a single group, as in prior attempts, the thrust of all fans must be simultaneously reduced by the same fraction. However, as disclosed herein, multiple fans may be combined into multiple separate groups, such that as the required thrust is reduced. For example, the various lift fans  102  (e.g., those illustrated in  FIGS. 1 and 2 ) can be controlled independently to adjust pitching and rolling movements of the aircraft. In certain aspects, some of the lift fans  102  (e.g., a subset of the total number of lift fans  102  may be arranged into independently controllable groups of lift fans  102 . 
     Using this arrangement, one group (e.g., a first operation group of lift fans  102 ) may be kept at full throttle (e.g., full power), one group (e.g., a second operation group of lift fans  102 ) may be proportionally throttled down (e.g., as wing-borne lift increases, fan power decreases), and another group (e.g., a third operation group of lift fans  102 ) may be fully shut down (e.g., at zero throttle). During operation, the number of lift fans  102  operated in, or as part of, an operation group may adjusted. In other words, one or more lift fans  102  may be operationally reallocated from one operation group to another operation group to achieve a desired total thrust or thrust profile (e.g., pitch, trim, etc.). For instance, when transitioning from hover to wing-borne flight, all groups of fans may be initially at full throttle. At mid-transition, each group of fans may have a different throttle level, and at the end (e.g., wing-borne flight), all groups of fans are in the fully shut-down group. 
     For the following analysis, eight fan groups were considered. The power required for each fan group was calculated according to the previously described methods. Similarly, the momentum drag of the lift fans was calculated according to the previously described methods. As expected, in the case of multiple fan groups, each group was calculated separately. As described above, the lift fans  102  may be constant RPM with variable pitch to change the throttle, or they may be fixed pitch with variable RPM. The only requirement is that each group of fans can be controlled independently of other groups. The lift fan groups may be arranged on the aircraft in whatever locations the designer deems appropriate for the best packaging, power distribution, attitude control and safety. 
     For this calculation, the airflow is understood to exit from the outlet of the lift fans moving vertically in the frame of reference of the aircraft. Turning vanes  118  were not used in the analysis; however, the use of turning vanes  118  will be discussed below. The aircraft was set to accelerate at a constant rate throughout the transition where thrust was produced as required to achieve this acceleration. Thus, the net thrust after subtracting all drag is constant. Due to the large variation in total power versus flight speed during transition, constant acceleration may not always be the case, but it provides a simplified calculation for the purpose of this analysis, and the results can still be realistically compared and applied to other cases. 
     For purposes of this analysis, a transition time of 30 seconds was used for all cases. While other methods may be used, ducted thrust fans were used to produce the horizontal thrust for propelling the aircraft. The disc area of the lift fans  102  was set to 20% of the area of all the lift fans—a realistic value based on prior designs, such as the aircraft of  FIG. 2 . 
     An advantage of thrust fans, for this analysis, is that they require input shaft power, so it is easy to combine the thrust fan power with lift-fan power to obtain total engine power. The thruster fan analysis uses a similar type of mass flow, momentum, and energy analysis as described previously for lift fans. However, the thrust fan analysis includes the effect of the flight speed, which is not a zero initial flow energy and momentum. As the forward thrust requirement is increased for a given airspeed, the shaft power required increases faster than linearly due to the increase in fan disc loading. Each of the lift fans  102  is assumed to have 100% energy conversion efficiency—the ability of the lift fan  102  to convert shaft power into energy in the lift fan exit flow. While a 100% energy conversion may not be possible, it is consistent in all the comparative analysis. 
     The lift fans and thrust fans have similar conversion efficiency, so the efficiency can be applied to the total power with reasonable accuracy. Thus, it is reasonable to do this analysis without factoring in fan conversion efficiency. 
     All analyses were done at sea-level air density. At higher altitudes, even more power would be needed, and the engine power is reduced, so an even greater increase in engine size would be needed. Additional power and thrust would also be needed to allow for trim and maneuver margins. However, an objective of this analysis is to merely demonstrate the advantages of the present disclosure as compared to the prior art as the analysis could apply equally to all the cases. 
     Referring now to  FIG. 4 , a graph of the transitional power requirements of a high disc loading lift-fan aircraft is shown. The graph represents an aircraft having a high disc loading, 300 psf, comparable to the XV-5 of  FIGS. 1 a  and 1 b   . As seen in the graph, the hover lift-fan shaft power is very high, about 177 ft-lb/s/lb (0.322 hp/lb) and the initial transition thrust, with acceleration power, is somewhat higher at about 203.5 ft-lb/s/lb (0.37 hp/lb). 
     The peak power requirement occurs when the lift-fan power is still large, while the momentum drag is near peak, at 50% of wing-borne speed. As seen in the graph, this power level is equal to about 256.85 ft-lb/s/lb (0.467 hp/lb). For the purpose of this illustration, the power values do not take into account fan or drivetrain efficiency losses. Therefore, the actual power values would be even higher and even more power margin is needed to provide for aircraft control (e.g., a peak power could be over 400 ft-lb/s/lb). 
     Referring now to  FIG. 5 , a graph of the transitional power requirements of a low disc loading lift-fan aircraft using traditional fan throttling methods is illustrated. The graph represents an aircraft having a low disc loading, 40 psf, with all fans being throttled together as a single group. Using Equation 2, it is evident that reducing the disc loading by decreasing the area A will result in a lower power requirement P. While the hover lift-fan power is greatly reduced, about 60 ft-lb/s/lb (0.11 hp/lb), roughly ⅓ of the high disc loading equivalent illustrated in  FIG. 4 , the momentum drag is now much larger. For instance, the peak power now occurs at 75% of wing-borne speed and is 234.3 ft-lb/s/lb (0.426 hp/lb), which is about 91% of the first case, significantly worse than the reduction in hover power. 
     Referring now to  FIG. 6 , a graph of the transitional power requirements using the systems and methods of an embodiment of the present disclosure is shown. The graph represents an aircraft having a low disc loading, 40 psf, with fans arranged and controlled in eight separate groups in accordance with the present disclosure. While a low disc loading of 40 psf was used in this demonstration, other low disc loading values may be used. The lower the disc loading in hover, the greater the advantages of the present disclosure. 
     While the hover power is the same as the case shown in  FIG. 5 , 60.5 ft-lb/s/lb (0.11 hp/lb), the reduction in momentum drag causes the peak power to now occur at a much lower airspeed, 60% of wing-borne speed, similar to the high disc loading case of  FIG. 4 ; wherein the peak power level is now 195.8 ft-lb/s/lb (0.356 lb/hp), or 83% of case of  FIG. 5 , and 76% of the high disc loading case shown in  FIG. 4 . In this analysis, only one of the eight groups of lift fans was at partial throttle at any given time. The remainder were either full throttle or shut off. For example, to obtain a fan thrust of 55% of hover thrust, four of the eight groups were at full throttle (50% of hover thrust), three groups were shut off and the remaining group was at 40% throttle (0.05/0.125). 
     Referring now to  FIG. 7 , an overlay comparison of the total power curves from  FIGS. 4, 5, and 6  is shown. The chart clearly shows that arranging the lift fans into multiple groups of independently controlled fans with a low disc loading (i.e., 40 psf) yields a 17% reduction in the peak power compared to the traditional singular lift-fan group having the same disc loading. Even more remarkable is the 24% reduction in the peak power compared to the traditional singular lift-fan group having a high disc loading (i.e., 300 psf). 
     Several factors help determine the procedure for throttling back or shutting down a lift-fan as the airspeed (and thus wing lift) increase during transition to forward flight. While this description applies to transitions that occur in straight and level flight, the method may be extended to maneuvering flight paths. 
     The first factor is that the total vertical force should equal to the weight of the aircraft, thus wing lift plus fan lift should equal weight. One or both of the wing lift or the lift fan thrust may be varied, but the ultimate goal of transition is to end up in fully wing-borne flight, so the fan thrust must be eventually reduced. 
     A second factor to consider is that the lift created by the lift-fans must be distributed across the aircraft such that the total moment about the pitch axis is zero. Accordingly, a reduction of lift on fans or surfaces ahead of the center of mass must be balanced by a reduction of lift aft of the center of mass. Otherwise, the aircraft will pitch. 
     Third, and similarly, the total moments about the roll axis should also be zero. Accordingly, a reduction on the left side must be balanced by a reduction on the right side. 
     On lift-fan VTOL vehicles, it can be difficult to maintain trim and control at low to medium speeds (10% to 50% of wing-borne airspeed). The amount of lift and control moment available from the forward flight control surfaces is minimal. Moreover, there can be large upsetting moments due to aerodynamic interactions, both fan-to-fan (a fan downstream of another fan has reduced lift) and fan-to-wing (where a fan flow can either increase or decrease the lift of a nearby wing or stabilizer). 
     As was shown in the prior momentum drag analysis, only a small amount of power is needed to overcome momentum drag at low speeds. The drag itself is proportional to flight speed/fan exhaust speed, so it is small at low forward speeds. In addition, the power needed to overcome drag is drag times velocity, so low velocity means low power required. This means that at low speed, it is reasonable to throttle all the lift fans  102  in whatever way is best to maintain control of the aircraft, and reduction of drag is only a minor factor, so, it is likely that all fans will be operating at partial thrust at low speed. 
     However, as the power analysis shows, the power required to overcome momentum drag is very large at higher speeds, such as 50 to 90% of wing-borne speed. At these speeds, the aerodynamic control surfaces have substantial authority, and the wing can produce almost enough lift to support the airplane. Thus, it is now reasonable to control the lift fans  102  according to the methods disclosed herein. The various lift fans can be operated either at full throttle, or shut down as rapidly as possible, thus minimizing the peak transition power required. The wing lift can be easily adjusted to compensate for the sudden variations in total fan lift. 
     Notably, there are no apparent drawbacks or required hardware modifications apart from software required to implement the system and method. Accordingly, there is minimal expense involved in employing the power-efficient method, wherein the lift fans  102  are combined into separately controllable groups. 
     As those skilled in the art will know, lift-fan aircraft may use vanes  118  (e.g., turning vanes  118 ) underneath the lift fans. Lift fans may have adjustable vanes  118  to direct the lift fan exhaust flow aft, thereby providing thrust and reducing drag. In general, the vanes  118  are in the vertical position when hovering, but can rotate to turn the lift-fan flow aft and to provide the desired thrust for transition. If the vanes  118  turn the airflow, such that the aft component of the velocity equals the flight speed, then the momentum drag is exactly cancelled. 
     As mentioned, vanes  118  were not included in the above analysis because they can add unnecessary complexity, both in the calculations, but also in the determination of the way the vane angles and throttle settings are changed as the airspeed varies. Thus a proper comparison including vanes  118  would require determining the optimal combination of vane angles and throttle settings for each fan group, for each airspeed in the transition. 
     However, this complexity is not needed to show the merits of the present disclosure. For instance, the turning vanes  118  have a limit in how much they can turn the airflow without large losses. For practical vanes  118 , the limit is around 35 to 40 degrees. For the high disc loading case, the lift fan exhaust speed is about 355 ft/second. With 35 degree turning, this gives an aft component of about 200 ft/sec, which is greater than the 100 knot (169 ft/sec) wing-borne speed. Thus, the momentum drag can be fully compensated. 
     In practice, the power required by an aircraft like this would be nearly constant throughout the transition region, but the details depend on the exact forward propulsion system, and how power is allocated to the various systems and the scheduling of both lift-fan throttle and turning vane angles. 
     For the case of a light loading fan, such as the 40 psf fans in the above example, vanes  118  can be useful, but are not as effective as at high disc loadings. For a 40 psf fan, the hover fan exhaust speed is about 130 feet per second (ft/sec). When turned 35 degrees by vanes  118 , the aft component is only 75 ft/sec, or well below the 169 ft/sec wing-borne speed. Thus, the momentum drag is only partly compensated, and thruster power is needed to overcome the remaining drag. 
     When all fans are throttled back, the result is that the lift fan exhaust speed is reduced, and the aft component would be reduced even more. Thus, very low aft component speed, low vertical component speed, and high power is needed. By keeping operating fans at the highest possible throttle setting, the momentum drag is reduced, as shown in the example, and the thrust available from vanes  118  is increased. 
     Although various embodiments have been described with reference to a particular arrangement of parts, features, and the like, these are not intended to exhaust all possible arrangements or features, and indeed many other embodiments, modifications, and variations will be ascertainable to those of skill in the art. For instance, although the various embodiments are directed to airfoils, the concepts may be applied to primary wing airfoils, tail airfoils, and other wake-forming aircraft surfaces. 
     All U.S. and foreign patent documents, and all articles, brochures, and all other published documents discussed above, are hereby incorporated by reference into the Detailed Description.