Abstract:
A rotor aircraft has a fuselage with a rotor mounted above by a rotor shaft. An arm is pivotally engaged with a lower portion of the rotor shaft and pivotally engaged with the fuselage, enabling the rotor to move with little restriction vertically and horizontal in all directions relative to the fuselage as the rotor rotates in order to isolate rotor oscillations. An infinitely variable air spring is used to counter vertical and fore and aft loads. Damping in the form of elastomeric materials, piston seal friction, and fluid flow through an orifice may be added as required.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of provisional application Ser. No. 60/628,371 filed Nov. 16, 2004. 

   FIELD OF THE INVENTION 
   This invention relates in general to rotor aircraft, and in particular to an assembly for isolating the rotor vertical, horizontal and side-to-side oscillations from the airframe. 
   BACKGROUND OF THE INVENTION 
   One type of rotor aircraft, referred to as a gyroplane, has a fuselage, wings, rotor and a propeller. For a jump takeoff, the rotor is pre-rotated to an overspeed while the aircraft is still on the ground. The propeller also is rotating. The pilot releases the brakes and increases the collective pitch on the propeller and rotor to cause the aircraft to lift from the ground. As the aircraft picks up forward speed, the wings assume more of the load and the rotor is tilted forward, decreasing the rotor speed and decreasing the load assumed by the rotor. At cruise speed, the rotor turns slowly while the wings supply most of the lift. 
   Changing the responsibilities of the rotor from primary lift source to passive lift source has great application and speed because the wings have much less drag than a fast turning rotor at high speeds. This transfer, however, can be responsible for significant ride vibrations or disturbances felt by the passengers. The oscillations, which occur twice per revolution for a two blade rotor, are due to the fact the rotor produces more lift and drag when the rotor is perpendicular to the airflow than when it is parallel to the airflow. Even when the rotor is substantially unloaded, these oscillations will be observable and made even more noticeable due to the reduced rotor RPM at the higher airspeeds. For large transport aircraft the rotor may be slowed down to 25 RPM during high speed cruise 
   One way of reducing the disturbances felt by the passengers is by increasing the oscillation frequency by increasing the number of blades, because higher frequencies are not as disruptive to passengers. Typical rotor aircrafts such as helicopters do not slow down the rotor during cruise conditions, rather the blades remain rotating at a high speed. The gyroplane described above, however, is able to obtain high cruising speeds only by slowing down the rotor speed. 
   Rotor aircraft can increase oscillation frequency and reduce the amplitude by increasing the number of rotor blades. However, in addition to the increased complexity and weight from using more than two blades, there is another significant drawback to using more than two blades on the rotor, which is the inability to have compact storage. When using a three or four bladed rotor, it becomes necessary to fold the blades to store the aircraft compactly. This increases weight and complexity. A two bladed rotor has the ability to store compactly by arranging a rotor blade fore and aft over the aircraft. This eliminates the weight and complexity associated with folding, and allows for a more compact rotor hub housing. This advantage is particularly significant aboard an aircraft carrier where space is limited. 
   SUMMARY OF THE INVENTION 
   The rotor aircraft of this invention has a fuselage with a rotor shaft extending upward from the fuselage. A rotor is mounted to the rotor shaft. A drive mechanism in the fuselage rotates the rotor shaft. The drive mechanism is configured to isolate rotor horizontal and vertical oscillations/vibrations from the airframe by allowing the rotor to move in all directions with little restriction. Damping in the form of elastomeric materials, piston seal friction, fluid flow through an orifice, etc may be added as required. 
   Preferably, the rotor shaft extends upward from a gear box, which is operatively coupled to a shaft of the engine. A support arm is pivotally connected to the fuselage and to the gear box. The support arm allows vertical movement of the gear box and the rotor shaft about a pivot member. In addition the rotor shaft may pivot near the gearbox in the fore and aft direction and, to a lesser extent, from side to side. 
   One method of accomplishing this movement is to use elastomeric bearings with good dampening characteristics located on either side of the gearbox on the crossover shaft connecting a propeller on each side of the fuselage. The elastomeric bearings mount to the support arms described above and allow the rotor shaft to pivot fore and aft and tilt side to side due to the softness of the elastomeric bearings, allowing the rotor to move horizontally and vertically as required to isolate rotor oscillations/vibrations. 
   Preferably the vertical damper assembly comprises a piston and cylinder coupled between the support arm and the fuselage. The cylinder preferably contains air to support, isolate and dampen as required the oscillations of the gear box and arm. The fore and aft support, isolation and damper assembly could use a similar arrangement. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top view illustrating a rotor aircraft constructed in accordance with this invention. 
       FIG. 2  is an exploded isometric and schematic view of the propulsion source, rotor and propeller drive assemblies of the rotor aircraft of  FIG. 1 . 
       FIG. 3  is a top view of the drive assembly shown in  FIG. 2 . 
       FIG. 4  is an enlarged isometric view of the vertical support, isolation and damper assembly for the rotor aircraft of  FIG. 1 . 
       FIG. 5  is a side schematic view illustrating the vertical support, isolation and damper assembly of  FIG. 4 . 
       FIG. 6  is similar to  FIG. 5  except it illustrates the fore and aft support, isolation and damper assembly. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , an example of a rotor aircraft  11  is shown. Aircraft  11  has a fuselage  13  and two fixed wings  15 . Each wing  15  has an aileron  17  in this embodiment. Aircraft  11  also has two vertical stabilizers  19 , each located immediately forward of a movable rudder  21 . A movable horizontal stabilizer, referred to as a stabilator  23 , is located between vertical stabilizers  19 . 
   A rotor  25  is mounted to the upper side of fuselage  13 . Rotor  25  has two blades  27  in this embodiment, but it could have more than two. Each blade  27  has tip weights  26  for providing inertia for jump takeoff and stabilization at slow rotor speeds. Aircraft  11  has two propellers  28  in this embodiment, each mounted to one of the wings  15 . Preferably propellers  28  provide differential thrust to counter the torque created by rotor  25  when driven during flight, particularly when hovering. Aircraft  11 , alternately, could have a single propeller mounted along the longitudinal axis of fuselage  13 . In that instance, rotor  25  would not be driven during flight, rather it is only driven prior to leaving the ground for jump takeoffs. 
   Referring to  FIG. 2 , the drive assembly for aircraft  11  includes in this embodiment two engines  29 , which could either be internal combustion or jet engines. Engines  29  are stationarily mounted in fuselage  13 . Each engine  29  is connected by splined coupling and constant velocity joint  30  to a drive shaft  31 , which in turn is connected to a gear box  33 . Each gear box  33  is capable of limited vertical movement relative to fuselage  13 . Each gear box  33  has a right angle output that couples to a propeller shaft  35  extending lengthwise along one of the wings  15 . The outer ends of propeller drive shafts  35  connect to 90 degree gear boxes  37  that are stationarily fixed to wings  15  ( FIG. 1 ). Gear boxes  37  connect to planetary gear boxes  39 , which in turn drive propellers  28 . The lengths of propeller drive shafts  35  and the type of coupling between drive shafts  35  and gear boxes  33  enable some flexing of drive shafts  35  to occur as gear boxes  33  oscillate vertically. The connections between constant velocity joints  30  and drive shafts  31  enable the forward ends of driven shafts  31  to pivot upward and downward with gear boxes  33  relative to the aft ends of drive shafts  31 . 
   A pair of support arms  41  is mounted between and generally parallel to drive shaft  31 . Support arms  41  extend in this embodiment farther forward than drive shafts  31 . Support arms  41  are connected to a pivot support  43  at their aft ends. Pivot support  43  is pivotally mounted to fuselage  13  ( FIG. 1 ). Pivot support  43  can pivot relative to fuselage  13  but cannot move in translational movement, such as vertical or horizontal. 
   A rotor 90 degree gear box  45  is mounted to and between support arms  41  for movement therewith. Rotor 90 degree gear box  45  is mounted in alignment with propeller drive shafts  35 . Each gear box  33  has an output coupling  44  ( FIG. 3 ) that couples to rotor 90 degree gear box  45  for driving rotor gear box  47 . Rotor 90 degree gear box  45  is able to oscillate vertically in unison with gear boxes  33 . Output couplings  44  extend through elastomeric bearings  46  mounted to arms  41  so that the rotor 90 degree gear box  45 , planetary gear box  47 , and rotor shaft  51  can tilt fore and aft and to a lesser extent side to side to isolate horizontal movements, oscillations, or vibrations of rotor  25  as it rotates. Likewise arms  41  pivot about pivot support  43  to isolate vertical movements, oscillations, or vibrations of rotor  25 . 
   In this embodiment, a planetary gear box  47  mounts to the upper end and output of rotor 90 degree gear box  45 . A conical transmission housing  49  is shown extending upward from planetary gear box  47 . A rotor shaft  51  extends through transmission housing  49  into engagement with planetary gear box  47 . A hub  53  ( FIG. 2 ) is located at the upper end of rotor shaft  51 . Hub  53  connects blades  27  to rotor shaft  51  for rotation therewith. Preferably, hub  53  has two halves that are able to twist relative to each other. Each half is integrally joined with one of the blades  27  for collective pitch changes. Rotor  25  can also be tilted relative to rotor shaft  51  for cyclic pitch control. 
   A forward crossbar  55  extends between the forward ends of support arms  41 . A vertical oscillation isolation assembly  57  is mounted to forward crossbar  55  for supporting the rotor lift and for isolating the vertical oscillations caused by the rotation of blades  25 . 
   In this embodiment, preferably the entire rotor shaft  51  can tilt fore and aft relative to fuselage  13  ( FIG. 1 ) to selected positions for controlling the pitch of the aircraft. A tilt and damping cylinder  59  extends parallel to support arms  41  and from the rotor mast assembly to tilt cylinder  59  pivot support  48 , which is attached to fuselage  13 . The forward end of tilt and damping cylinder  59  is pivotally connected to part of the drive assembly, such as schematically shown in  FIG. 2 . Preferably, tilt and damping cylinder  59  is pneumatic, but it could be a combination pneumatic and hydraulic cylinder. As shown in  FIG. 6 , by increasing or decreasing the air pressure in tilt and damping cylinder  59 , drive shaft  31  can be tilted a significant amount, such as up to 25 degrees. 
   By maintaining an average air pressure, tilt and damping cylinder  59  will hold the desired angular position, except for fore and aft oscillations occurring around that desired angular position due to rotation of rotor  25 . The spring force due to the pneumatic pressure in tilt cylinder  59  is lighter when rotor shaft  51  is upright, thus providing less restriction to oscillating movement of the piston within tilt cylinder  59  and greater fore and aft movement of rotor shaft  51  during high speed cruise flight. The spring force in cylinder  59  due to the air pressure is light enough to allow fore and aft movements or vibrations of rotor  25  as it rotates regardless of the tilt position. 
   The dotted lines in  FIG. 5  illustrate the oscillations or vibrations that occur due to the upward and downward movement of rotor  25 , and  FIG. 6  illustrates the oscillations or vibrations that occur due to the fore and aft movement of rotor  25 . Note that fore and aft oscillations of rotor  25  do not cause arm  41  to pivot and do not affect damper assembly  57 . Preferably as little damping as possible is used to dampen the fore and aft oscillations of rotor shaft  51  to avoid transferring forces absorbed by the damping into fuselage  13  ( FIG. 1 ). If damping is required, it can be accommodated by the addition of hydraulic fluid to tilt cylinder  59 , by increasing friction in tilt cylinder  59  or by other means. To make tilting rotor shaft  51  also function to isolate and dampen fore and aft movements or vibrations of rotor  25  as it rotates, a key element in this design is to also size the air volume on either side of the piston in cylinder  59  so that the natural frequency of this air spring/mass combination will always be less that the rotational speed of rotor  25 . As in the vertical isolation system  57 , the air pressure in tilt cylinder  59  will be the greatest when the rotor lift and RPM is the greatest, which makes it easier to keep the natural frequency of this system below the rotor RPM throughout the rotor operating range. The spring force due to the pneumatic pressure in tilt cylinder  59  is lighter when rotor shaft  51  is upright, thus allowing greater fore and aft movement of rotor shaft  51  during high speed cruise flight. 
     FIGS. 4 and 5  illustrate one embodiment of vertical oscillation isolation assembly  57 . A piston  65  has its lower end pivotally connected to forward crossbar  55 . Piston  65  is part of air cylinder  63 , which contains air pressure on the upper side of piston  65 . In this embodiment, the lower side of cylinder  63  below piston  65  is open to the atmosphere. Air cylinder  63  is connected by a line  67  to an air accumulator  69 . A connector  66  on the top of air cylinder  63  pivotally connects air cylinder  63  to fuselage  13  ( FIG. 1 ). The air pressure is supplied by an air source  73 , which typically is an air compressor. Sensor  64  senses the position of piston  65  and provides a signal through a controller (not shown) for operating valves  83  and  85  as required to keep piston  65  essentially centered in cylinder  63 . Air pressure on top of piston  65  supports the lift of rotor  27 . Opening valve  83  will reduce the air pressure by venting some of the air, while opening valve  85  will increase the air pressure. The pressure in accumulator  69  is charged by air source  73  to that required to support the maximum gross weight of the aircraft. The area of the piston and the pressure required is a basic design optimization. 
   A key element of this design is to size the air volume in the upper chamber of support cylinder  63  so that the natural frequency of this air spring/mass combination is always less than and never exceeds the rotational speed of rotor  25 . This prevents a potential destructive resonate natural frequency from occurring. Note that when the rotor lift is the greatest, the rotational speed of rotor  25  must also be high to keep blades  27  from stalling. This means that the resulting higher vertical natural frequency of the system due to the higher air pressure has a tendency to track and always stay below the rotational speed of rotor  25 . 
   In operation, aircraft  11  may take off conventionally on a runway, utilizing propellers  28  for thrust and wings  15  and rotor blades  27  for lift. Alternately, it may take off in a vertical or hover manner by driving rotor  25  and using propellers  28  to provide the differential thrust to counter the rotor torque. The pilot increases the collective pitch, which causes aircraft  11  to leave the ground. As aircraft  11  moves forward, air flows over wings  15 , causing the wings to assume more of the lift. The pilot, or an automatic controller, reduces the collective pitch of rotor blades  27  and increases the pitch of propellers  28  as the aircraft accelerates. 
   At some speed the pilot or an automatic controller begins to tilt rotor  25  and rotor shaft  51  forward to reduce the rotor lift. If rotor  25  was powered for a hover type take-off, at some forward speed a clutch (not shown) disengages rotor shaft  51  from engines  29  during forward flight. As the forward speed and wing lift continues to increase, Rotor  25  can slow down and will continue to freewheel or auto-rotate. The lower the rotor RPM, the lower its drag, however at very low RPMs, the reduction in drag reaches a point of diminishing returns and further RPM reduction is not justified by a rapid reduction in rotor stability and control response. 
   As blades  27  rotate, either by freewheeling during forward flight or when driven, the difference in lift and drag between when rotor  25  is perpendicular to the air stream and parallel to the air stream causes oscillations of the rotor both vertically and horizontally. Rotor shaft  51  is free to move vertically a limited distance in unison with the oscillations. Gear boxes  33  and  45  move in response to the oscillations of rotor shaft  51  along an arcuate or curvilinear path which has a radius extending to pivot bar  43  and gear box couplings  44 . Arms  41  pivot about pivot bar  43  as indicated by the dotted lines in  FIG. 5 . Damper and isolation assembly  57  smoothes and reduces the amplitudes of these oscillations. As arm crossbar  55  moves upward, piston  65  moves upward, but this upward movement is resisted by air pressure contained in cylinder  63 . 
   While in flight, sensor  64  monitors the relative position of piston  65  to detect the average position of piston  65  within cylinder  63  and adjusts the air pressure as needed. If the average position is above the centerline of cylinder  63 , air is added to the system through valve  85 , which increases the pressure on top of piston  65 . The opposite is true if the average position is lower than the centerline. In that case, valve  83  is opened and air is vented, which reduces the air pressure and force on top of piston  65  and allows it to move up. 
   At takeoff, the lift of rotor  25  increases, tending to pull rotor shaft  51  upward. The forward ends of arms  41  move upward to a maximum position stop. Position sensor  64  registers the average position of piston  65  as being too high in cylinder  63 , and opens valve  85  to add air and increase the pressure. After landing and as the rotor lift decreases, the air pressure will be decreased to keep piston  65  centered, but at some point there will not be enough lift to keep the piston centered even with no air pressure and the piston will drop down to a low position stop. 
   During hover or initial lift, the pressure in accumulator  69  may rise up to five times the pressure that it will have at high speed cruise flight with rotor  25  unloaded. The spring rate of the system and its natural frequency will be higher during hover because of the higher pressure, yet if the volume of air on top of piston  65  is sized correctly, the higher natural frequency will still be less than the high rotor RPM required for hover. 
   High speed flight is the point at which the average rotor lift is the lowest and the dissimilar lift and drag by rotor  25  is the greatest. Less air pressure is needed to keep piston  65  centered, and as a result the lower air spring rate allows rotor  25  and transmission  45  to have more vertical travel relative to the aircraft. This helps isolate the dissimilar rotor forces from the airframe. The more rotor travel available both vertically and horizontally, the more these dissimilar rotor forces can be isolated from the airframe. Again proper sizing of the air volume above piston  65  will keep the system natural frequency less than the low rotor RPM associated with high speed cruise. 
   As illustrated in  FIG. 6 , tilting of rotor shaft  51  fore and aft with cylinder  59  does not affect damper assembly  57 . Tilt cylinder  59  attaches to the fuselage at a point where cylinder  59  is parallel to arms  41  and the distance between attach points on cylinder  59  is the same as the distance between pivot points  43  and  44 . This parallelogram arrangement keeps the rotor shaft  51  from tilting fore and aft as rotor shaft  51  moves vertically ( FIG. 5 ). In addition to tilting rotor shaft  51  as required to control the aircraft pitch, tilt cylinder  59  also isolates the dissimilar fore and aft forces of rotor  25  at high cruise speeds and low rotor RPMs in the same manner as cylinder  63  and piston  65 , 
   The invention has significant advantages. The drive mechanism is configured to isolate rotor horizontal and vertical oscillations or vibrations from the airframe by allowing the rotor to move in all directions with little restriction. This arrangement allows a two bladed rotor to operate at a low RPM during high speed forward flight. The vertical and horizontal rotor isolation and damper mechanism does not interfere with tilting of the rotor. This rotor isolation and damper works when lift conditions are high on the rotor, such as at takeoff and landing, as well as when they are low on the rotor, at cruise conditions. 
   While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention.