Abstract:
The compound helicopter is a hybrid combination of a helicopter and a fixed wing aircraft. A conventional helicopter is modified with a nose-mounted tractor propeller to provide thrust for forward flight. Wings are added to provide lift during forward flight. With the propeller providing thrust and the wings lift during forward flight, the helicopter rotor blades are unloaded during cruising flight to allow increased forward speed by avoiding limitations of conventional helicopters, including retreating rotor blade stall and maximum rotor blade tip speeds. A single powerplant drives both the main rotor and the nose-mounted propeller. The compound helicopter employs high aspect ratio wings with large flaps that may be extended to reduce vertical drag during vertical flight and hovering operations.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/484,049, filed Jul. 2, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to vertical take-off and landing aircraft. More specifically, the invention is a compound helicopter having a rotor providing thrust and lift for vertical takeoff, landing, and hovering operations, and a nose-mounted propeller and wings for thrust and lift during forward cruising flight.  
         [0004]     2. Description of the Related Art  
         [0005]     Helicopters provide a valuable and convenient mode of air transportation. Because of their ability to take off vertically from, and descend vertically into, a small area, they can be operated to and from places that conventional aircraft cannot. As city and suburban populations become denser, and land for airports becomes scarce, the ability of a helicopter to operate from rooftops or small heliports within population centers increases their value.  
         [0006]     Despite the advantages of vertical take off and landing capability, conventional helicopters do not perform efficiently in horizontal, cruising flight. The forward velocity of a helicopter is fundamentally limited by various phenomena, including retreating blade stall, a phenomenon in which the airspeed over a retreating rotor blade is decreased as the helicopter&#39;s forward speed increases. The tendency for a retreating blade to stall in forward flight is a major factor in limiting the forward speed of all conventional helicopters.  
         [0007]     Attempts have been made to combine the vertical takeoff and landing convenience of helicopters with the speed and efficiency in forward flight of conventional winged aircraft. Tilt-rotor aircraft employ wing-mounted rotors that can be tilted from a horizontal plane, where they provide maximum lift for vertical or hovering flight, to a vertical plane where they provide thrust for forward flight. Compound helicopters use a conventional helicopter rotor, but add wings and a separate source of forward thrust for level cruising flight. Various configurations of compound helicopters have been attempted, but none have achieved great success.  
         [0008]     U.S. Pat. No. 3,105,659, issued on Oct. 1, 1963 to R. Stutz, discloses a compound helicopter that employs two engine-driven propellers in a conventional twin-engine aircraft configuration, with the engines wing-mounted. The wing-mounted engines drive the helicopter rotor through a shaft coupling. A conventional tail rotor is provided to counteract the torque of the main rotor blades. Because, in this configuration, the propellers are located beneath the main rotor blades, the propellers must operate in the turbulent downwash from the rotors. The mechanical stresses induced on the propellers by the turbulent air prevent the propellers from being operated to produce their maximum thrust. Additionally, the requirement to couple the separately mounted engines to drive the main rotor adds weight to the aircraft. Thus, a compound helicopter of this configuration cannot achieve optimum performance.  
         [0009]     U.S. Pat. No. 2,665,859, issued on Jan. 12, 1954 to P. Papadakos, discloses another example of a compound helicopter that derives forward thrust from twin propellers in a conventional twin-engine configuration.  
         [0010]     U.S. Pat. No. 3,155,341, issued on Nov. 3, 1964 to P. Girard, discloses a compound helicopter that derives forward thrust from a rear-mounted, pusher-type propeller. In addition to providing thrust, the rear-mounted propeller is pivotable to provide a counter-rotational force against the torque of the main rotor, replacing a conventional helicopter tail-rotor. This illustrates one of the difficulties inherent to the use of a rear-mounted propeller. Typically, a tail rotor is used in a helicopter to counteract the torque of the main rotor. When a compound helicopter employs a rear-mounted, pushing-type propeller, it becomes difficult to locate a conventional tail rotor. A more complex scheme, such as the pivotable rear-mounted propeller of the Girard compound helicopter, must be employed, often at the expense of maximized performance.  
         [0011]     U.S. Pat. No. 4,730,795, issued on Mar. 15, 1988 to C. David, illustrates another compound helicopter using a rear-mounted, pusher-type propeller for forward thrust. The David compound helicopter uses, in one embodiment, a pivoted rear-mounted, pusher-type propeller, or, in another embodiment, counter-rotating main rotors, in order to counteract the rotor blade torque.  
         [0012]     U.S. Pat. No. 4,928,907, issued on May 29, 1990 to D. Zuck, discloses yet another example of a compound helicopter using a rear-mounted, pusher-type propeller for forward thrust. In the Zuck compound helicopter, the tail rotor is replaced by aileron forces provided by pivotal wings and by a movable horizontal airfoil located on the tail cone or tail boom. U.S. Pat. No. 2,959,373, issued on Nov. 8, 1960, also to D. Zuck, discloses an earlier version of the compound helicopter using a rear-mounted, pusher-type propeller for forward thrust, and using a pivoting rear-mounted propeller to fill the role of the tail rotor.  
         [0013]     Additionally, as with the twin wing mounted propellers, the propeller in a rear-mounted pusher configuration is in an area of turbulent airflow caused by the rotor downwash, as well as by the airframe itself. The propeller cannot be operated to its maximum ability in this environment. An additional limitation of the rear-mounted propeller configuration is in the size of propeller that may be used. A restriction of the propeller size translates to a restriction on the thrust that can be produced. Because helicopters typically operate in a somewhat tail-down pitch during takeoff, landing, and hovering operations, there is a risk of the rear-mounted propeller striking the ground or other obstructions. This risk is increased as the size of the propeller is increased. Ordinarily, a helicopter&#39;s tail rotor is relatively small because it is not required to produce a large amount of thrust. However, when a propeller is relied on for forward thrust to propel the aircraft in cruising flight, it is desirable to use a larger propeller to achieve greater thrust.  
         [0014]     None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed. Thus, a compound helicopter solving the aforementioned problems is desired.  
       SUMMARY OF THE INVENTION  
       [0015]     The compound helicopter is a hybrid combination of a helicopter and a fixed wing aircraft. A conventional helicopter is modified with a nose-mounted tractor propeller to provide thrust for forward flight. Wings are added to provide lift during forward flight. With the propeller providing thrust and the wings providing lift during forward flight, the helicopter rotor blades are unloaded in order to allow increased forward speed by avoiding the limitations of conventional helicopters, including retreating rotor blade stall and maximum rotor blade tip speeds.  
         [0016]     The compound helicopter of the present invention is directed generally to a ten to fourteen seat aircraft in the 12,000-16,000 pound weight category, combining the hover performance of a helicopter with the speed of a fixed wing aircraft. Such an aircraft is suitable for a variety of missions, including a multitude of military and civil applications. The compound helicopter is also suitable to other applications, such as drone aircraft. The compound helicopter offers relative simplicity and a high degree of flight safety. It has the ability to perform touchdown autorotations, as well as to glide to a landing as a fixed wing aircraft. The safety benefit of these lost-power landing modes cannot be overstated.  
         [0017]     The propeller is mounted at the nose of the aircraft, where the propeller operates in clean air, free of the turbulence from the rotors. In the clean air environment, the propeller may be operated at a higher degree of efficiency, producing greater thrust than a propeller mounted elsewhere. The propeller is located slightly forward of the main rotor blade tips to prevent contact under high rotor blade flap angles.  
         [0018]     The nose-mounted propeller and the main rotor may be driven by the same, or by separate, power sources. In a twin engine configuration, twin turboshaft engines are coupled to provide power for both the main rotor and the propeller. A transmission combines output from both engines to provide a single, redundant, powerplant for both the rotor and the propeller.  
         [0019]     The transmission drives two shafts. A first shaft drives the rotor, while a second shaft drives the propeller. Both the rotor drive train and the propeller drive train each include a clutch so that the rotor and the propeller may be independently engaged with and disengaged from the engines. It is thus possible that, during takeoff and landing procedures, the propeller may be disengaged entirely, eliminating a hazard to personnel near the aircraft. Electronic control systems manage the propeller pitch and speed, and collective and cyclic pitch, and speed, of the main rotor in conjunction with a full authority digital engine controller (FADEC) for optimal operation during helicopter and airplane flight modes, and during transition between the modes.  
         [0020]     The wings are optimized for high altitude cruising, and have a relatively high aspect ratio for a reduction of induced drag during high speed cruise. The high aspect ratio wings, because they are relatively narrow, present a reduced flat plate area, decreasing vertical drag during vertical or hovering operations. Additionally, the wings feature rather wide flaps that can be deployed during hovering and vertical flight operations to further reduce the flat plate area of the wings. A wing with a 35.3% chord single-slotted flap, deflected reduces vertical drag 40-50% compared to a bare wing. A more complex wing, with a leading edge flap and 30% chord plain flap of an “umbrella” design, reduces drag 70-75% as compared to a bare wing, but at the expense of the complexity of the mechanism required to operate the flaps.  
         [0021]     Accordingly, it is a principal object of the invention to provide a compound helicopter with a nose-mounted propeller.  
         [0022]     It is another object of the invention to provide a compound helicopter with a nose-mounted propeller for increased forward thrust during cruising flight.  
         [0023]     It is a further object of the invention to provide a compound helicopter with a nose-mounted propeller having the propeller located in a clean-air area for improved performance.  
         [0024]     Still another object of the invention is to provide a compound helicopter with a high aspect ratio wing to reduce the flat plate area beneath the main rotor.  
         [0025]     Yet another object of the invention is to provide a compound helicopter with a high aspect ratio wing with flaps designed to further reduce the flat plate area beneath the main rotor.  
         [0026]     It is an object of the invention to provide improved elements and arrangements thereof for the purposes described which is inexpensive, dependable and fully effective in accomplishing its intended purposes.  
         [0027]     These and other objects of the present invention will become readily apparent upon further review of the following specification and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1  is a perspective view of a compound helicopter according to the present invention.  
         [0029]      FIG. 2  is a top view of a compound helicopter according to the present invention.  
         [0030]      FIG. 3  is a front view of a compound helicopter according to the present invention.  
         [0031]      FIG. 4  is a side view of a compound helicopter according to the present invention.  
         [0032]      FIG. 5  is a diagrammatic cut-away side view of a compound helicopter according to the present invention, showing the rotor and propeller drive trains of an embodiment incorporating a twin engine powerplant.  
         [0033]      FIG. 6  is a schematic diagram of the rotor and propeller driving mechanism of the compound helicopter shown in  FIG. 5 .  
         [0034]      FIG. 7  is a schematic of the engines, gear box, and rotor driving mechanism of the compound helicopter shown in  FIG. 5 .  
         [0035]      FIG. 8  is a perspective view of a twin-engine powerplant according to a preferred embodiment of the compound helicopter shown in  FIG. 5 .  
         [0036]      FIG. 9  is a top view of the twin-engine powerplant shown in  FIG. 8 .  
         [0037]      FIG. 10  is section view of the transmission assembly for the twin-engine powerplant shown in  FIG. 8 .  
         [0038]      FIG. 11  is a diagrammatic cut-away side view of a compound helicopter according to the present invention, showing the rotor and propeller drive trains of an alternate embodiment wherein the powerplant is located in the nose of the compound helicopter.  
         [0039]      FIG. 12  is a diagrammatic cut-away side view of an alternate embodiment of a compound helicopter according to the present invention, configured for use as an unmanned aerial vehicle, showing the rotor and propeller drive trains.  
         [0040]      FIG. 13  is a perspective view of a single-engine powerplant for the compound helicopter of  FIG. 12 .  
         [0041]      FIG. 14  is a side view of the single-engine powerplant shown in  FIG. 13 .  
         [0042]      FIG. 15A  is a section view of a wing for the compound helicopter of the present invention, having an umbrella flap and a slotted leading edge.  
         [0043]      FIG. 15B  is a section view of a wing for a compound helicopter of the present invention, having an umbrella flap and a slotted leading edge, with the flaps extended.  
         [0044]      FIG. 16A  is a section view of a wing for a compound helicopter of the present invention, having a conventional flap.  
         [0045]      FIG. 16B  is a section view of a wing for a compound helicopter of the present invention, having a conventional flap, with the flaps extended.  
         [0046]      FIG. 17  is a block diagram of a control system for a compound helicopter of the present invention. 
     
    
       [0047]     Similar reference characters denote corresponding features consistently throughout the attached drawings.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0048]     The present invention is a compound helicopter, designated generally as  10  in the drawings. Referring to  FIGS. 1-4 , the compound helicopter  10  is, generally, a conventional helicopter modified by the addition of a nose-mounted tractor propeller  30 , to provide thrust for forward flight, and wings  36  to provide lift during forward flight. A conventional helicopter main rotor  32  provides for vertical flight, such as during takeoff and landing, and hovering operations. A tail fan  34  or tail rotor functions to counteract torque from the main rotor  32 .  
         [0049]     A fuselage  20  has a somewhat extended nose  24  so that the propeller  30 , mounted on the nose  24 , is located forward of the main rotor  32 , eliminating the possibility of contact with the main rotor  32  at high rotor flap angles. This forward location of the propeller  30  places the propeller  30  in relatively undisturbed air, ahead of turbulence from the rotor downwash and turbulence caused by the airframe itself. A major advantage of the nose-mounted propeller  30  is that relatively low stresses are induced in the propeller  30  as it rotates in the relatively undisturbed air, allowing the propeller  30  to be operated more efficiently to produce greater thrust. Tail section  22  of the fuselage  20  carries, in addition to the tail fan  34  or tail rotor, the empennage of a conventional airplane, including a vertical fin  26  and a horizontal stabilizer  28  located aft of the main rotor  32 . The fuselage  20  includes, in a passenger carrying embodiment, a cockpit or cabin area  21  suitable for pilot, crew, and passengers. An unmanned aerial vehicle (UAV) is also conceived wherein the cockpit or cabin area  21  is eliminated or made available for payloads.  
         [0050]     Wings  36  are affixed to each side of the fuselage  20 , and are optimized for high altitude cruising. The wings  36  are positioned, and droop slightly toward their tips, to maintain clearance from the main rotor  32  for at least a 30° rotor flap angle. The wings  36  have a relatively high aspect ratio for reduced induced drag during high speed cruise. The high aspect ratio wings  36  also, because they are relatively narrow, present a reduced flat plate area beneath the main rotor  32 , decreasing vertical drag during vertical or hovering operations.  
         [0051]     The compound helicopter is powered by a single powerplant that drives both the main rotor  32  and the propeller  30 . The single powerplant may be of a single engine or a twin engine configuration. Turning now to  FIG. 5 , the compound helicopter is illustrated with a twin-engine powerplant. The powerplant comprises twin engines  38  of any suitable type, such as turboshaft engines, coupled by a combining gearbox  40 . An advantage of this arrangement is that if one of the engines  38  fails, the other continues to provide power for both the main rotor  32  and the propeller  30 . For weight and balance considerations, the powerplant is best located generally amidship, in an upper part of the fuselage  20  behind the cabin area.  
         [0052]     A twin-engine drive train arrangement is shown in greater detail in  FIGS. 6 and 7 . The combining gearbox  40  combines the output of the engines  38 , using a freewheeling clutch  42  connected to each of the engines  38  to accommodate single engine operation in the event of an engine outage. Reduction gears  44  reduce the engine output speed to about 6,000 rpm. The combining gearbox  40  provides power output on an upper and a lower output shaft  46  and  48 , respectively. The combining gearbox  40  also provides some accessory drive pads for supporting the power requirements for the accessory devices, such as the hydraulic pumps and electrical generators.  
         [0053]     Two identical clutches  50  and  52  are connected directly at the combining gearbox  40  output shafts  46  and  48 . The upper clutch  50  drives the main rotor  32 , while the lower clutch  52  drives the nose-mounted propeller  30 . With a separate clutch provided for the propeller  30  and the main rotor  32 , either can be separately engaged with, or disengaged from, the powerplant. This allows the propeller  30  to be disengaged during takeoff and landing procedures, and during other situations in proximity to the ground, providing greater safety for personnel near the aircraft. During cruising operation, the main rotor  32  is disengaged from the powerplant and allowed to auto-rotate.  
         [0054]     From the output of the upper clutch  50 , power is transmitted to a multi-speed transmission  54 . A gear selector allows selection of the transmission output at full speed, or at a reduced speed. The gear selector is operated from a gear selector control switch located in the cockpit or cabin area  21 .  
         [0055]     The transmission  54  drives a main rotor gearbox  56 . The main rotor gearbox  56  includes reduction gearing to produce a suitable main rotor speed. This type of gearbox is common to most helicopters. A main rotor shaft  58  extending from the main rotor gearbox  56  drives the main rotor  32 . Additionally, the transmission  54  provides a tail rotor output  60  that connects to a tail rotor shaft  62  to drive the tail fan  34 .  
         [0056]     From the output of the lower clutch  52 , power is transmitted to an upper pair of bevel gears  64  that drives a vertical shaft  68 . The vertical shaft  68  extends downward to near the bottom of the fuselage  20 .  
         [0057]     A lower pair of bevel gears  66  translates power from the vertical shaft to a propeller drive shaft  70  that extends forward to the nose  24  of the fuselage  20 . Flexible universal couplings  72  join sections of the propeller drive shaft  70 , accommodating bends in the propeller drive shaft  70  required to reach the nose  24  of the fuselage  20 . A propeller gearbox  74  (shown in  FIG. 5 ) is mounted in the nose  24  of the fuselage  20 , and is driven by the propeller drive shaft  70 . The propeller gearbox  74  is a single stage reduction gear that drives the propeller  30 . The propeller gearbox  74  also provides a mounting pad for a propeller control unit, an over-speed governor and a high-pressure oil pump for operation of the propeller control unit, as well as gear lubrication.  
         [0058]     Turning now to  FIGS. 8-10 , a preferred twin-engine powerplant and transmission is illustrated. This embodiment features a transmission assembly in connection with a pair of turboshaft engines  38 , the transmission assembly having a plurality of output shafts for driving the propeller  30 , the main rotor  38 , and the tail rotor, respectively. Each of the turboshaft engines  38  has a freewheeling clutch  100  connecting the engine  38  to a first drive shaft  102 , which is, in turn, connected by a reducing bevel gear assembly  104  to a second drive shaft  106 . The freewheeling clutch  100  allows for single engine operation in the event of an engine outage. The bevel gear assembly  104  provides for about a 3:1 reduction in shaft rpm between the first drive shaft  102  and the second drive shaft  106 .  
         [0059]     A main gearbox module is driven by the twin turboshaft engines  38 . The main gearbox module comprises a multiple concentric shaft coupling a hydrodynamic coupling  120  and a planetary gearbox  130 . An outer shaft  122  is driven by a main spiral bevel gear  110 , which is driven by each of the engines  38  via second drive shafts  106 . The outer shaft  122 , in turn, drives a hydrodynamic coupling  120 . The hydrodynamic coupling  120  comprises a radial impeller  124 , driven by the outer shaft  122 , and a radial turbine  126 , coupled to an inner shaft  128 , the inner shaft being located concentrically within the outer shaft  122 . The hydrodynamic coupling  120  is a fluid coupling. Engine torque, applied by the second drive shaft  106  of each engine  38  to the outer shaft  122  by spiral bevel gear  100 , is converted within the hydrodynamic coupling  120  by the impeller  124  into moving fluid energy. The fluid energy is transformed back into mechanical energy by the turbine  126 , driving the inner shaft  128 . Thus, hydraulic fluid within the hydrodynamic coupling  120  functions to transfer energy from the outer shaft  122  to the inner shaft  128 . Emptying the hydraulic fluid from the hydrodynamic coupling  120  prevents the turbine  126  from rotating even as the impeller  124  rotates, thereby disengaging the inner shaft  128 . The hydrodynamic coupling  120  preferably includes a mechanical clutch to engage and cause a positive no-slip coupling between the impeller  124  and the turbine  126 , the mechanical clutch beginning to engage as the turbine  126  “catches up” with the impeller  124 , reaching approximately eighty-eight percent of the impeller&#39;s speed.  
         [0060]     The inner shaft  128  drives both the main rotor, through a planetary gear system  130 , and the tail rotor. A bevel gear set  134  is driven by the inner shaft  128  to provide a power take off for the tail rotor shaft  62 . The planetary gear system  130 , driven by the inner shaft  128 , provides a further reduction in rpm to drive the main rotor shaft  132 . The main rotor shaft  132  is disposed concentrically within the inner shaft  128 , and extends upward through the hydrodynamic coupling  120  to drive the main rotor. It can be recognized that the hydrodynamic coupling  120  provides a mechanism to engage and disengage both the main rotor and the tail rotor from the engine power.  
         [0061]     A hydrodynamic coupling  142  is also employed in the propeller drivetrain, to allow the propeller  30  to be engaged and disengaged from the powerplant independently from the main rotor  32 . A propeller drive takeoff shaft  113  has a bevel gear  112  engaged with the main spiral bevel gear  110 . The propeller drive takeoff shaft  113  in turn drives an input to the hydrodynamic coupling  142 . The hydrodynamic coupling  142  drives a first propeller drive shaft  144 , that engages with the vertical shaft  68  to drive the remainder of the propeller drive train generally as shown in  FIG. 6 . Note that, in the case of a UAV, it is unnecessary to employ flexible couplings  72  to route the propeller shaft  70  clear of a passenger cabin or cockpit area  21 . Instead, the propeller shaft  70  may be a single, straight shaft length.  
         [0062]     While a twin engine powerplant generally as described is a preferred power source for a manned, passenger-carrying embodiment of the compound helicopter, unmanned and remotely piloted embodiments of the compound helicopter are conceived that may benefit from a smaller overall size and may be best suited to a single engine power plant configuration.  
         [0063]     Referring to  FIG. 11 , an alternate configuration of the compound helicopter is shown with the powerplant located in the nose  24  of the fuselage  20 . This arrangement, particularly using a single engine powerplant, may be suitable for some applications but presents weight and balance challenges. It is also possible, in a further embodiment not illustrated, to locate a first engine in the nose to drive the propeller  30  and a second engine amidship to drive the main rotor  32 .  
         [0064]     Turning now to  FIGS. 12-14 , a single engine configuration of the compound helicopter is illustrated that is well suited for use as an unmanned aerial vehicle (UAV).  
         [0065]     The powerplant is located generally amidship, just behind the main rotor shaft  58 , comprising a single turbine engine  38  and a transmission assembly having a plurality of output shafts for driving the propeller  30 , the main rotor  38 , and the tail rotor, respectively. The single engine  38  turns an output shaft  102 , connected to a bevel gear set  150 . A first output of the bevel gear set  150  is a vertical shaft connected to a hydrodynamic coupling  154 . A second output of the bevel gear set  150  connects to the propeller shaft  70 .  
         [0066]     The hydrodynamic coupling  154  has dual concentric shafts, consisting of an input shaft  152  and an output shaft  156 . The input shaft  152  is the inner shaft of the dual concentric shaft arrangement, the output shaft  156  being the outer shaft. The input shaft  152  is driven by the bevel gear set  150 . Within the hydrodynamic coupling  154 , impeller rotors connected to the input shaft  152  drive turbine blades connected to the output shaft  156  through a hydraulic fluid medium. Emptying the hydraulic fluid from the hydrodynamic coupling  154  prevents the turbine blades from rotating even as the impeller blades rotate, thereby disengaging the input shaft  152  from the output shaft  156 . A hydraulic oil pump is provided to pump oil into and out of the hydrodynamic coupling  154 . Additionally, the hydrodynamic coupling  154  includes a one-way clutch that allows the outer shaft  156  to turn faster than the input shaft  152 , thereby allowing the main rotor to autorotate in the event the engine  38  is shut down during flight. Gear  158 , disposed on the output shaft  156 , drives gear  160  disposed on the main rotor shaft  58 . Gear  160  also drives the tail rotor shaft  62  by way of bevel gear set  162 .  
         [0067]     A propeller gearbox  74  (shown in  FIG. 12 ) is mounted in the nose  24  of the fuselage  20 , and is driven by the propeller drive shaft  70 . The propeller gearbox  74  is a single stage reduction gear that drives the propeller  30 . The propeller gearbox  74  also provides a mounting pad for a propeller control unit, an over-speed governor and a high-pressure oil pump for operation of the propeller control unit, as well as gear lubrication.  
         [0068]     The wings  36  will provide about 70% of the required lift for the compound helicopter  10  during cruising flight, also referred to as the airplane flight mode. The main rotor  32  provides the remaining lift while auto-gyrating in cruising flight. The cantilevered high wings are installed above the cabin on each side of the fuselage. The wings  32  have a relatively high aspect ratio, and are optimized for cruising flight. Desirable aerodynamic features of the wings  32  include low drag, high coefficient of lift, gentle trailing edge stall characteristics, and a sufficient maximum thickness and favorable chord-wise thickness distribution for structural efficiency and low weight. An aerofoil section, such as the NACA 653-218 aerofoil, provides these characteristics.  
         [0069]     The relatively narrow, high aspect ratio wings  32  present a reduced flat plate area, decreasing vertical drag during vertical or hovering operations. Wing flaps may be deployed to further reduce vertical drag.  
         [0070]     An “umbrella” flap design in conjunction with a slotted leading edge, shown in  FIGS. 15A and 15B , provides the maximum reduction in vertical drag. Upper and lower leading edge slats  76  and  78  can be extended to create a leading edge slot, or retracted to form a smooth and continuous leading edge profile. A flap  80  is pivotally mounted at the trailing edge of the wing, with upper and lower trailing edge slats  82  and  84  providing a smooth trailing edge surface while the flap  80  is retracted. The flap width is about 30° of the wing chord. When the flap  80  is extended, pivoted downward up to 80°, the trailing edge slats  82  and  84  extend to provide a trailing edge slot in front of the flap  80 . With the flap  80  and slats  76 ,  78 ,  82 , and  84  fully extended, vertical drag is reduced 70-75% as compared to a bare wing. However, it has a penalty in its complexity of the mechanism to move the flaps  80  and slats  76 ,  78 ,  82 , and  84 .  
         [0071]     A simpler flap configuration is a conventional single slotted flap  86  shown in  FIGS. 16A and 16B . The single slotted flap  86 , about  350  of the wing chord, can be deflected to 80°, providing a 40-50% reduction in vertical drag.  
         [0072]     The use of lightweight materials, composites, fly-by wire controls, advanced digital display systems, digital navigation, communication and flight control systems, coupled with highly advanced composite propeller design, provides a breakthrough in compound helicopter design performance and technology. Turning to  FIG. 17 , a control system provides for engine control, synchronization of the propeller  30  and main rotor  32 , and, in the case of unmanned or UAV embodiments, operation of all flight control systems by remote control. Elements of the control system communicate with one another over a common communication bus  1001 , or over independent communication links where appropriate.  
         [0073]     Referring to  FIG. 17 , a helicopter flight control system (HFCS)  1000  includes control elements for cyclic and collective inputs to the main rotor  32 , yaw inputs to the tail fan  34 , engine  38  power/speed, as well as control surfaces of the wings  36  and vertical stabilizer  26 . The HFCS  1000  is driven by inputs from flight deck controls  1010  (or, in the case of the unmanned aerial vehicle, a remote control receiver  1020 ) and an autopilot  1030 . The autopilot  1030  provides at least three axes of control. Fourth and fifth axes are rotor collective and engine power control, which are primarily relevant for missions requiring automation in the low speed regime. An air data computer  1050  gathers information from a pitot static system  1052 , along with temperature information, and provides inputs to the autopilot  1030 .  
         [0074]     A full authority digital engine control (FADEC) module  1060  controls the engines  38  based on inputs from the pilot, the HFCS  100 , and other control systems. The FADEC  1060  works in conjunction with a propeller control unit (PCU)  1070  and a main rotor control unit (MRCU)  1040  to optimize engine power settings, propeller pitch, and main rotor collective and cyclic settings. The FADEC  1060 , MRCU  1040 , and PCU  1070  cooperate to optimize control of the compound helicopter  10  through a helicopter flight mode wherein the main rotor  32  is the primary source of lift, an airplane flight mode wherein the wings  36  are the primary source of lift, and transition between helicopter and airplane flight modes. The FADEC  1060 , MRCU  1040 , and PCU  1070  work together to manage power sharing between the main rotor  32  and the propeller  30 .  
         [0075]     During operations in the helicopter flight mode, power is directed primarily to the main rotor  32 . During operations in the airplane flight mode, power is directed primarily to the propeller  30 . The main rotor  32  is controlled by the MRCU  1040  to achieve minimum drag during the airplane flight mode as the main rotor  32  autorotates due to the forward movement of the compound helicopter  10 .  
         [0076]     During the transition between helicopter flight mode and airplane flight mode, the FADEC  1060  and MRCU  1040  function to facilitate the transfer of lift between the main rotor  32  and the wings  36 . The MRCU  1040  varies the speed of the main rotor  32  by varying the collective pitch of the main rotor  32 . As main rotor speed and engine speeds are synchronized, such that the rotations of the hydrodynamic coupling&#39;s  120  outer shaft  122  and inner shaft  128  are approximately matched, the MRCU  1040  commands the hydrodynamic coupling&#39;s  120  mechanical clutch to engage, in transition from airplane mode to helicopter mode, or disengage in transition from helicopter mode to airplane mode.  
         [0077]     With its inherent ability to auto-rotate as a conventional helicopter or glide as an airplane, the compound helicopter  10  offers a high degree of flight safety and crash survivability compared to other non-conventional helicopter technologies, such as tilt rotors. The simple nose-mounted propeller configuration makes the compound helicopter  10  maintenance- and pilot-friendly. The configuration also provides outstanding maneuverability and speed.  
         [0078]     It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.