Abstract:
The invention relates to improvements with regards to the control of VTOL aircraft that use two propellers or fans as the primary lifting devices in hover. More particularly, the invention is a means for effecting control of the aircraft using just the two propellers alone, and comprises the in-flight tilting of them—which are of the conventional, non-articulated type (though they may have collective blade-pitch)—directly and equally towards or away from one another (and therefore about parallel axes) as necessary for the generation of propeller torque-induced and gyroscopic control moments on the aircraft about an axis perpendicular to the propeller tilt and mean-spin-axes. For a side-by-side propeller arrangement, therefore, their (lateral) tilting towards or away from one another produces aircraft pitch control moments for full control of the aircraft in that direction. Unlike the prior art, no cyclic blade-pitch control, slipstream-deflecting vanes, exhaust nozzles, tail rotors or extra propellers or fans, or conventional control surfaces are needed to effect this aircraft pitch control.

Description:
BACKGROUND OF THE INVENTION 
     The requirement for hover stability and control of vertical takeoff and landing (VTOL) rotorcraft other than helicopters—which are limited in forward speed capability—has normally produced solutions which are far from ideal; most being either more complex than a helicopter or compromised in some fashion. 
     Ideally, just two propellers or fans would be all that was required for providing lift and control in hover. However, past VTOL aircraft have not been able to hover in a stable manner and under full control without additional reactive devices, these devices primarily enabling control about the axis joining the two fans. 
     For their stability and control in hover, all past and present VTOL rotorcraft have employed—in their construction or proposal—either: cyclic blade-pitch controls (helicopters and tilt-rotors); more than two propellers or fans; vanes in the propeller/fan slipstreams; or some other secondary, reactive device in addition to the main lifting propellers/fans. 
     Cyclic pitch control is not suitable for small, high speed fans, and so is not-conducive to VTOL aircraft with small footprints. Moreover, this solution results in a duplicity of intricate mechanics when applied to more than one rotor. 
     Aircraft using more than two fans have mechanically complex and heavy drive-trains, and their configurations are usually compromised or restricted by the additional devices. These deficiencies are compounded when the aircraft is intended to transition to airplane mode: either all the fans/propellers must be made to tilt, or some become excess weight and drag. 
     Hover stability and control using just vanes or control surfaces in the propeller/fan slipstreams have been marginalized by the difficulty in obtaining sufficient control moments, since their effectiveness depends on their vertical distance from the aircraft center of gravity and so restricts—or is restricted by—the aircraft configuration. For instance, there may be reduced vane effectiveness in ground proximity. 
     BRIEF SUMMARY OF THE INVENTION 
     By utilizing the gyroscopic properties of dual, counter-rotating propellers or fans, the present invention provides aircraft control without the need for cyclic controls or additional reactive devices. 
     Tilting the two propellers directly towards or away from each other creates gyroscopic and propeller-torque moments about the axis perpendicular to the tilt and mean spin axes, and so provides the required aircraft control about that axis. Specifically, the propeller axes are made to tilt within a common plane as necessary, in opposite directions by an equal amount and rate. The control method is therefore hence referred to as opposed tilting. The unbalanced gyroscopic moments are a result of the tilt rate, and the propeller-torque moments are due to the tilt angle from the aircraft vertical. These effects are independent of the vertical placement of the aircraft center of gravity. 
     For full aircraft control the propellers may also tilt in a direction perpendicular to the opposed tilting direction, providing both horizontal motion and yaw control. They also may be tilted collectively as well as oppositely in the opposed tilting direction. In general, the resulting combined tilting is referred to as oblique tilting. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a side-by-side stick model aircraft representation employing the elements essential for opposed tilting control. 
     FIG. 2 is a perspective view of the stick model&#39;s fans tilting equally inwards, and the resulting pitching of the aircraft. 
     FIG. 3 is a perspective view of a representative fixed oblique tilting arrangement. 
     FIG. 4 is a perspective view of an implementation of fixed oblique tilting for model aircraft. 
     FIG. 5 is a perspective view of an implementation of fixed oblique tilting having a central engine driving the propellers. 
     FIG. 6 is a perspective view of the flight sequence of an aircraft with side-by-side fans employing variable oblique tilting. 
     FIG. 7 is a perspective view of a typical drivetrain and control arrangement that may be used in the aircraft of FIG. 6, with the propeller shafts in the vertical position. 
     FIG. 8 is the same as FIG. 7, but with the propellers tilted laterally for pitch control. 
     FIG. 9 is a perspective view of a personal air vehicle with tandem fans employing variable oblique tilting. 
     FIG. 10 is a perspective view of a drivetrain and control arrangement for the vehicle of FIG. 9, with fan shafts in the vertical position. 
     FIG. 11 is the same as FIG. 10, but with the fans tilted longitudinally for roll control. 
     FIG. 12 is a perspective view of an aircraft using opposed tilting in a fixed horizontal plane. 
     FIG. 13 is a perspective of the drivetrain and control arrangement for the aircraft of FIG. 12, with the fan shafts in the longitudinal direction. 
     FIG. 14 is the same as FIG. 13 but with the fan shafts tilted away from the longitudinal direction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a stick-frame representation of a hovering aircraft incorporating the essential, representative elements necessary to the invention. Though this is of a side-by-side fan configuration the discussion to follow applies equally as well to tandem and co-axial configurations. And, although the aircraft employs only lateral opposed tilting, the resulting control in pitch described herein reflects the pitch control obtained with the lateral opposed-portion of more general tilting. 
     Fixed to the airframe  500  are two lift-generating pods  501  and  502  which consist of lift fans or propellers  1  and  2  respectively, and the means to spin them and to tilt them laterally about parallel longitudinal axes x 1 —x 1  and x 2 —x 2  respectively. Pods  501  and  502  are constructed and act identically but opposite—being mirrored in a vertical plane containing the aircraft center longitudinal axis. 
     Considering then just pod  501 , it consists of the lifting propeller or fan  1 , which is-turned about its spin axis by motor/gearbox  3 —which is representative of the drive system—in the direction shown at speed @. Via axles  5  and bushings/bearings  7  the motor/gearbox  3 , and subsequently the fan  1 , can pivot laterally about the local longitudinal axis x 1 —x 1  within yoke  9 , which is representative of the pivot device. Here the yoke  9  is rigidly fixed to the airframe  500 , but it can be made to pivot relative to the airframe about lateral axis y—y by separate means for thrust vectoring, as will be shown in the subsequent embodiments. 
     Fixed to the motor/gearbox  3  is servo actuator  11 , whose output arm  13  rotates in response to input signals coming from the pilot and/or stability augmentation system (SAS). Through linkage  15  the servo arm  13  is connected to the yoke  9 , the attachment point being offset from axis x 1 —x 1 . Therefore, rotation of the servo arm  13  causes the motor gearbox  3  and fan  1  to tilt laterally about axis x 1 —x 1  in proportion. Similar tilting can be obtained if the servo  11  is mounted to the airframe  500  instead, its output arm  13  connected to the motor/gearbox  3 . 
     FIG. 2 shows the spin axes of the two fans  1  and  2  being simultaneously tilted laterally from the aircraft verticals z 1  and z 2  by equal but opposite angles Ψ and at equal but opposite rates Ψ′. Considering again just fan  1 , this tilting causes it to generate a gyroscopic moment M GΨ , whose vector is perpendicular to the fan&#39;s spin and tilt axes, and whose magnitude is equal to the product I R @Ψ′, where I R  is the mass moment of inertia of the fan about its spin axis (ignoring motor/gearbox  3 ). The horizontal component of vector M GΨ , resolved along lateral axis y—y, contributes to the pitching moment M Y  acting on the aircraft. Also contributing to M Y  is the horizontal component of the fan-torque vector Q. M Y  is then the sum of these two effects from both fans:                M   Y     =                2        (         M     G   ψ          cos                 ψ     +     Q                 sin                 ψ       )                   =                2        (         I   R        ω                   ψ   ′        cos                 ψ     +     Q                 sin                 ψ       )                                    
     Pitching moment M Y  as a result of fan lateral tilting causes the aircraft to accelerate in pitch about lateral axis y—y, assumed to pass through the aircraft center of gravity (which, for convenience but not necessary, is placed at the same vertical location as the fan tilt axes). If θ is the aircraft pitch angle, then equating M Y  to the aircraft inertial moment I A θ″—where I A  is the mass moment of inertia of the aircraft about y—y—gives the equation of pitching motion of the aircraft in hover: 
     
       
         ½ I A θ″=I R @Ψ′cosΨ+ Q sinΨ 
       
     
     With this relation it is easily shown that the aircraft is dynamically stable in pitch when-using a simple control model in the SAS such as Ψ=−kθ or Ψ=−kθ′ (θ or θ′ determined by appropriate sensors); also, that the aircraft will respond appropriately (converge to a new pitch angle) to intentional, momentary control inputs. 
     In the case at hand the axis y—y about which My acts is the aircraft pitch axis, but in general terms it is the axis perpendicular to the propellers&#39; (parallel) tilt axes and to the mean of their spin axes. 
     Pitch control alone will not sustain forward motion of the aircraft in hover mode if the aircraft is to remain essentially horizontal. For this, some longitudinal thrust vectoring is required, and a simple way of obtaining it is to skew the fan or propeller tilt axes such that a portion of the tilting is in the longitudinal direction. FIG. 3 shows such an arrangement, where yokes  19  and  20  are oriented so that the respective tilt axes u 1 —u 1  and u 2 —u 2  are at equal but opposite angles λ from the longitudinal axis. Tilting the propellers  1  and  2  simultaneously at equal rates Ψ u ′—and in the same longitudinal direction—about axes u 1 —u 1  and u 2 —u 2  respectively creates aircraft pitch control moments as before (due to the lateral component of the tilting) and now also a longitudinal thrust component. This method of control is referred to as fixed oblique tilting. 
     An implementation of fixed oblique tilting for electric model aircraft is the pod shown in FIG. 4, where the electric motor  22 , driving a toothed pinion gear  24 , is encased in a motor cap  26 . Integral to motor cap  26  are the mounting plate  28  for the servo  12 ; a fixed or embedded output shaft  30 —about which the internal-tooth reduction gear  32  and attached propeller (not shown) spin—and the yoke  34 . The entire foregoing tilt collectively about the axis of axle  36 , which is fixed-to spindle  38 —which, in turn, is fixed to the airframe via mounting block  40 —and engages the motor cap yoke  34 . As before, tilting is controlled by servo-arm rotation; in this case servo arm  14  is connected to arm  44  of the spindle  38  via linkage  42 . Alternately, the servo  12  can be mounted to the airframe and connected to an arm similar to  44  but part of the yoke  34 . As well, other reduction gear arrangements are possible, but in all cases it is preferable that the motor  22  and propeller turn in the same direction, so that the motor armature adds to rather than subtracts from the gyroscopic effect. Other types that accomplish this include two-stage conventional gearing and planetary gearing. 
     An implementation of fixed oblique tilting for use with a central drive engine is shown in FIG.  5 . The central drive engine and its bevel pinion (not shown) turn the common shaft  56  through bevel gear  55 . Considering just one of the identical but opposite pods, bevel pinion  57  is fixed to the end of the common shaft  56 —which is supported in the airframe-affixed yoke  59  by frictionless bearings—and drives bevel gear  63 , attached propeller shaft  67 , and propeller  1  through bevel idler  61 . The propeller shaft  67  rotates in frictionless bearings contained within the vertical cylinder-portion of the t-shaped spindle  65  (for a better view of this spindle see item  66  of the opposite pod). The axis of the horizontal shaft-portion of spindle  65  is coincident with the tilt axis of the pod, having the same orientation as axis u 1 —u 1  of FIG.  3 . Cantilevered in frictionless bearings contained in the yoke  59 , the spindle  65  can swivel about this axis and is controlled in doing so—as was the motor/gearbox  3  of FIG.  3 —by the servo mechanism attached to it and connected to the yoke  59 . Bevel idler  61  floats on frictionless bearings about the horizontal shaft-portion of the spindle  65 . 
     More suitable and flexible for horizontal motion control—including transition to airplane mode—is variable oblique tilting, where the lateral and longitudinal components of propeller or fan tilting are independently controlled. FIG. 6 depicts an aircraft using such control; its fan axes essentially vertical in hover, then tilted longitudinally for transition but also laterally to maintain pitch control during said transition and to counter any pitching moments resulting from the ensuing forward motion. This control via fan tilting may also be used in airplane mode, where the fan axes are essentially or nominally horizontal, and therefore can supplant conventional control surfaces. 
     FIG. 7 shows the drivetrain and control system that may be used in the aircraft of FIG. 6, which is considered to be the best mode for carrying out the invention. Again, the apparatus consists mainly of two identical but opposite pods, here  601  and  602 . Considering pod  601  as representative of both, its propeller or fan  1  is fixed to and driven by propeller shaft  83 , which in turn is driven by bevel gears  85 ,  87 , and  89 , the latter of which is fixed to and driven by the common horizontal shaft  91 . Driving common shaft  91  is gear  93 , which meshes with the output gear of the centrally-located drive engine(s) (not shown). 
     Common shaft  91  rotates freely about its axis (y—y) on frictionless bearings contained within torque-tube  105 , which itself—being mounted in bearings fixed to the airframe (not shown)—can rotate about axis y—y in a controlled manner relative to the airframe as will be discussed. 
     Propeller shaft  83 , fixed to bevel gear  85 , rotates freely about its own axis in frictionless bearings contained in the vertical portion of the t-spindle  103 . Idler bevel gear  87 , meshing with bevel gears  85  and  89 , rotates freely about local axis x 1 —x 1  on frictionless bearings placed over one side of the horizontal portion of t-spindle  103 . The other side of the horizontal portion of t-spindle  103  is supported by—and can rotate about local longitudinal axis x 1 —x 1  in a controlled manner within—frictionless bearings contained within yoke  107 , which is rigidly fixed to torque-tube  105 . 
     Lateral tilting of the propeller  1  about axis x 1 —x 1  is prescribed by servo  113 —which is fixed to the bottom of the t-spindle  103 —through rotation of its servo-arm  115 , which is connected to yoke  107  by linkage  117 . 
     Longitudinal tilting of propeller or fan  1  about axis y—y is prescribed by servo  133  (which is fixed to the airframe), through rotation of its servo-arm  135 , the associated movement of linkage  137 , and the ensuing rotation of the control horn  139  fixed to torque-tube  105 . Similarly, longitudinal tilting of propeller or fan  2  is prescribed by servo  134 . 
     FIG. 8 shows the servo arms of the lateral tilt-servos rotated and the propellers correspondingly tilted laterally for aircraft pitch control. 
     With variable oblique tilting any combination of longitudinal and lateral tilting is possible, giving full aircraft control (except perhaps roll in hover or yaw in airplane mode, which are obtained separately by using differential fan thrust-control via either their speeds, collective blade-pitch angles, or other means) in the hover, transition and airplane modes, and the means for achieving said transition. It is possible that the control linkages be replaced by jack screws or other actuation devices without departure from the scope of the invention. 
     FIG. 9 shows a personal air vehicle with tandem fans, and its drivetrain and control system shown in FIG. 10 represent another method of implementing the invention. Here, the counter-rotating fans  201  and  202  can tilt in any direction via the constant velocity (CV) joints  211  and  212  incorporated in the shafts driving them. Considering just the one fan  201  it is controlled in doing so by two servos (not shown) that are linkaged to the balls  217  and  219  of the semi-swashplate  215 . The latter is non-spinning relative to the vehicle airframe, being fixed to the outer race of frictionless bearing  221 —its inner race fixed to fan  201  itself or the short piece of shafting between the CV joint  211  and the fan  201 —and prevented from spinning by pin  223  which engages a vertical groove or slot (not shown) in the airframe. FIG. 11 shows the fans  201  and  202  being tilted longitudinally away from one another, thereby creating lateral gyroscopic and fan-torque moments for roll control of the vehicle. 
     FIG. 12 shows another side-by-side fan arrangement, where the fan ducts  303  and  304  are fixed relative to the airframe in the longitudinal direction. FIG. 13 shows the drivetrain and control system for this arrangement, with FIG. 14 showing the axes of the fans  301  and  302  being tilted oppositely in the horizontal plane, thus providing pitch control of the aircraft. This tilting of the fans within the ducts can be accomplished in a manner similar to that for the tandem-fan-vehicle of FIG. 9, and provides pitch control whether the aircraft is in hover or in forward flight. Hovering is achieved by deflecting the fan airstreams downwards via the cascades of turning vanes  306  or similar turning devices contained within—or part of—the ducts  303  and  304 ; forward motion control, and transition to airplane mode, is achieved by rotating the vanes  306  about their local, lengthwise axes.