Background and Summary of the Invention This invention relates to the field of yaw control systems for both model and full-size helicopters, and particularly, to a helicopter yaw (right-left heading) stabilizing system. More particularly, the invention relates to a gyroscopic mechanism mounted on a helicopter tail rotor assembly and configured to vary tail rotor thrust automatically to produce a yaw moment stabilizing the helicopter in flight so that the helicopter is able to fly in a direction (heading) selected by the pilot, whether the pilot is onboard a full-size helicopter or commanding a model helicopter by remote control.

Helicopters are flying machines having the ability to hover and fly forwards, backwards, and sideways. This agility stems from the multiple capabilities of the main rotor system. Since the invention of helicopters in the 1930's considerable effort has been expended advancing helicopter technology, with a substantial percentage of that effort concentrated on main rotor systems. Less effort has been applied to develop better tail rotor systems. While the technology of full-size helicopters progressed for decades, model helicopters remained impractical for lack of suitable engines, radio control equipment, and construction materials. Model helicopter designers often copied the designs of full-size helicopters without understanding the basic differences between full- size and model aircraft. As a result, scaled-down model helicopters were typically unstable in flight and underpowered.

In the 1970's hobbyists developed the first practical model helicopters. Lighter radio control

equipment, more powerful engines, and systematic engineering all contributed to early successes. Much of model helicopter design, however, is rooted in tradition. Even though helicopter technology has advanced considerably since that time, the designs and design philosophies of that era are still in widespread use.

While model and full-size helicopters are mechanically similar, the aerodynamics, operational speeds, and weights of model helicopters are vastly different from those of their full-size counterparts. Model helicopter rotor blades operate within a low speed range where aerodynamic drag due to the thickness of the rotor blade airfoil becomes very important. Early attempts to use the thick airfoils used on full-size helicopters failed in part because engines then available could not overcome the high drag of the rotor blades. The present invention addresses the needs of both model and full-size helicopters for higher efficiency rotor systems.

All single-rotor helicopters require some sort of anti-torque or yaw control system in order to maintain directional control in flight. Several different yaw control devices have been developed (such as blown tail booms and shrouded fans) , but traditional tail rotor systems have remained essentially unchanged for 25 years. In general, maintaining the stable yaw orientation (right-left heading) of a helicopter in hover or low-speed flight can be a difficult business for the pilot. To counterbalance the constantly changing torques on the helicopter fuselage produced by the main rotor blades and atmospheric conditions such as lateral wind gusts, helicopter pilots must continually manipulate the yaw controls of their aircraft. This is especially true for model helicopters because of their small size and low mass, and the resulting tendency to react rapidly to disturbances.

Electronic gyro-stabilizer systems are now widely available for use on the tail rotor controls of both model and full-size helicopters to help pilots cope with yaw instability. These systems, however, are typically heavy and expensive, and often require an additional electric power supply aboard the helicopter. Some existing mechanical stabilization systems interfere with pilot control or forcibly feed back into the pilot's controls.

What is needed is a yaw control and stabilization system for a helicopter which is simple, lightweight, and inexpensive, which requires little power to operate, and which does not unduly inhibit pilot control for normal maneuvering.

One object of the present invention is to provide an improved device for controlling and automatically stabilizing the yaw motion of a model or full-size helicopter.

Another object of the present invention is to provide low-moment, high-lift rotor elements for general application on rotor systems of model or full-size helicopters.

Generally speaking, there is disclosed herein a device for controlling and automatically stabilizing the yaw motion of a model helicopter and/or a full-size helicopter. Such device comprises a primary tail rotor assembly generally mounted at the end of a helicopter tail boom and extending rearward from the helicopter fuselage and a yaw stabilization mechanism generally supported by tail rotor blade pitch varying control elements of the tail rotor and operating as an offset to the pilot's tail rotor thrust-varying controls.

The stabilization mechanism includes gyroscopically actuated rotors and rotor drive means which are generally located at an outboard end of the tail rotor assembly and which automatically vary the thrust generated

by the tail rotor assembly to stabilize yaw motion of the helicopter in flight.

More specifically, on a helicopter, a yaw control and stabilization device includes a primary tail rotor with a plurality of tail rotor blades extending radially from a hollow rotor shaft which is mounted for rotation about a transverse rotor axis, a push-pull rod extending through a hollow shaft and operably connected to the tail rotor blades to manually vary the collective pitch of the tail rotor blades, and a gyroscopic assembly mounted on the tail rotor assembly.

The gyroscopic assembly has a gyro rotor mounted to rotate with the tail rotor, to pivot about a substantially longitudinal pivot axis by and at the outboard end of the push-pull rod, and to vary automatically the collective pitch of the tail rotor blades in response to yaw motion of the helicopter. The gyro rotor has paddles or rotor blades to aerodynamically damp gyro motion and/or augment tail rotor thrust. The gyroscopic assembly further includes a pitch slider operably connected with the push-pull rod, primary tail rotor, and gyro rotor to move generally as a unit with the push-pull rod in order to vary the collective pitch of the tail rotor blades upon manual movement of the push-pull rod relative to the rotor shaft, and to be automatically slid relative to the push-pull rod in order to vary the collective pitch of the tail rotor blades upon precession of the gyro rotor.

Advantageously, the aerodynamic paddles or rotor blades included in the gyroscopic assembly in accordance with the present invention are constructed so as to be cyclically pitchable in response to pivoting of the gyro rotor to generate aerodynamic restoring forces in opposition to pivoting of the gyro rotor so as to restore the gyro rotor to a nominal orientation during helicopter

flight at the proper times. Concurrent with the cyclic pitching of the gyro rotor paddles,one variant of the present invention provides for collective pitching of the gyro paddles in response to collective pitching of the tail rotor blades. Simultaneous cyclic and collective pitching advantageously allows the gyroscopic assembly to serve both a stabilizing and thrust producing function.

Further, a helicopter tail rotor in accordance with the present invention includes low pitching-moment rotor blades configured for use with the gyroscopic stabilization system. The blades have an aerodynamic center of pressure located ahead of the pitching axis and mass balancing means to minimize the pitching forces felt at the root of the blades. In addition, a device for operably connecting the tail rotor blades to a tail rotor hub comprises interconnected reach-around grip means operably connecting the roots of the tail rotor blades with each reach-around grip means engaging the side of the tail rotor hub opposite to its respective blade. Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.

Brief Description of the Drawings

The detailed description particularly refers to the accompanying figures in which:

Fig. 1 is a perspective view of a representative helicopter including a tail assembly fitted with an improved yaw control and stability system in accordance with a preferred embodiment of the present invention;

Fig. 2 is an enlarged view of the helicopter tail assembly illustrated in Fig. 1; Fig. 3 is a rear elevation view of the helicopter

tail assembly shown in Fig. 2 (viewed from the rear of the helicopter facing toward the fuselage) fitted with the present invention showing the effect of pilot control input on the pitch (angle-of-attack) of the tail rotor blades, with one tail rotor blade shown in cross section and all details ahead of the tail rotor gearbox omitted for clarity;

Fig. 4 is a view similar to Fig. 3 showing reaction (e.g., change in pitch or angle-of-attack) to yaw motion of the helicopter of one of the tail rotor blades (shown in cross section) and also of the two gyro paddles included in the gyroscopic assembly slidably mounted on an outboard end of the tail rotor assembly;

Figs. 5A and 5B are detailed perspective views of the primary tail rotor blades and tail rotor blade grips in accordance with the present invention prior to assembly (Fig. 5A) and partially assembled (Fig. 5B) , with all other parts of the tail rotor assembly omitted for clarity;

Fig. 6 is a detailed perspective view of tail rotor drive and control elements, and partially assembled tail rotor blades of the current invention, with the tail boom shown in cutaway and all other parts of the tail rotor assembly omitted for clarity;

Fig. 7 is an exploded perspective view of tail rotor and gyro control elements including tail rotor pitch linkages in accordance with the present invention, with all other parts of the tail rotor assembly omitted for clarity;

Fig. 8 is an exploded perspective view of gyro rotor elements in accordance with the present invention, with all other parts of the tail rotor assembly omitted for clarity;

Fig. 9 is a perspective view of tail rotor sub- assemblies in accordance with the present invention showing relative positions of the tail rotor gearbox, tail rotor blades, pitch linkages, and gyro rotor elements;

Figs. 10A and 10B show a comparison view of a symmetrical airfoil section and an undercambered airfoil section;

Figs. 11A and 11F are plan views of different configurations of the tail rotor blades in accordance with the present invention situated next to typical cross sections 11B through HE of selected spanwise stations;

Fig. 12 is a perspective view of an alternative embodiment of a yaw control and stability system in accordance with the present invention showing a fully collective-pitch gyro rotor mounted on a helicopter tail rotor assembly and provided with rotor blades in place of gyro paddles of the type shown in Figs. 1 and 2;

Fig. 13 is an exploded view of the yaw control and stability system shown in Fig. 12 showing the details of each element;

Fig. 14 is a rear-end elevation view of the helicopter tail assembly shown in Fig. 13 (viewed from the rear of the helicopter toward the fuselage) fitted with the additional tail rotor blades shown in Figs. 12 and 13 and a fully collective-pitch gyro rotor and showing the effect of pilot control input on all tail rotor blades, with one tail rotor blade shown in cross section and all details ahead of the tail rotor gearbox omitted for clarity; Fig. 15 is a view similar to Fig. 14 showing reaction (e.g., change in pitch or angle-of-attack) to yaw motion of the helicopter of one of the tail rotor blades (shown in cross section) and also of the two rotor blades included in the gyroscopic assembly slidably mounted on an outboard end of the tail rotor assembly invention;

Fig. 16 shows a drag-reducing turbulator strip appended to the upper surface of an airfoil;

Fig. 17A and Fig. 17B are planform views of tail rotor blades showing various drag-reducing turbulator configurations appended near the leading edges of the

blades; and

Fig. 18A and 18B show the top and rear views of a main rotor blade having airfoiled cross-sections shown as Fig. 18C through 18G, and a turbulator appended to the upper surface near the leading edge.

Detailed Description of the Drawings

Referring to Fig. 1, a helicopter 10 is commonly designed to include a large main rotor 1 which rotates about main rotor axis 5 and lifts helicopter 10 into the air and a smaller tail rotor 2 which rotates about tail rotor axis 1 and is used to counteract the torque produced by main rotor 1 and to steer helicopter 10. Tail rotor 2 is mounted at a rear end of tail boom 16 as shown in Figs. 1 and 2.

Both main rotor 1 and tail rotor 2 are driven by an engine 3 usually located within the helicopter fuselage (body) near the vertical main rotor shaft. Although helicopter 10 shown in Fig. 1 is a model helicopter, it will be understood that man-carrying helicopters (not shown) also include engine-driven main and tail rotors. Illustratively, main rotor 1 rotates about a main rotor shaft aligned with main rotor axis 5 and includes a pair of rotor blades 17 and a pair of shorter subrotor blades 23. A streamlined fuselage shell or canopy 4 covers a front portion of the helicopter 10 as shown in Fig. 1. In use, a radio-controlled command unit (not shown) and other drive mechanisms are contained inside canopy 4. Canopy 4 is designed for use on a model helicopter such as helicopter 10 to protect the radio control unit and to provide the appearance of a pilot-carrying helicopter. Canopy 4 does not extend back to the tail rotor 2 on helicopter 10.

Angular motions of helicopter 10 about main rotor axis 5 are called yaw motions. Forces that tend to rotate

helicopter 10 about main rotor axis 5 and change the azimuthal heading of helicopter 10 are called yaw moments or yaw torques.

Conventionally, the pilot of a full-size helicopter controls the pitch of the tail rotor blade by manipulating foot pedals located within the helicopter cockpit. Cables, push-pull rods, and bellcranks connect the foot pedals to the pitch controls of the tail rotor blades. As the pilot adjusts the pedal position, the change in angle-of-attack (pitch) and associated thrust force of the rotating tail rotor blades results in a yaw moment about main rotor axis 5. This moment is directed to maneuver the helicopter and/or to oppose any destabilizing yaw moment sensed by the pilot. Tail rotors of radio-controlled model helicopters operate in an manner identical to full-size helicopters. The helicopter pilot on the ground manipulates small joysticks on a hand-held radio transmitter (not shown) which in turn sends radio commands to electro-mechanical servo actuators (not shown) located within the flying model helicopter. Push-pull rods and bellcranks connect the servos to the collective pitch controls of the tail rotor blades included in the model helicopter.

In operation, main rotor 1 of helicopter 10 rotates rapidly about main rotor axis 5 in rotation direction 6. As it does so, main rotor blades 17 act like propellers or fans moving large amounts of air downward thereby creating a force that lifts helicopter 10 upward. The torque (reaction force) created by rotating main rotor 1 in rotation direction 6 tends to cause helicopter 10 to swing about main rotor axis 5 in yaw direction 7. When trimmed for steady hovering flight, tail rotor 2 creates enough thrust force in direction 7 to cancel exactly the torque produced by main rotor 1 so that helicopter 10 maintains a constant heading. Decreasing or increasing the

thrust force generated by tail rotor 2 will cause helicopter 10 to turn in yaw directions 7 or 8 respectively. The magnitude of the thrust force generated by tail rotor 2 can be varied using the pilot-controlled system to vary the collective pitch of tail rotor blades 11. The pilot-controlled system for controlling tail rotor 2 will be described in more detail below.

Tail rotor 2 is rotated about transverse tail rotor axis 14 by a drive linkage passing through gearbox 15 and interconnecting engine 3 and tail rotor 2 to generate a thrust force transverse to tail boom 16 and offset from the vertical axis of rotation 5 of main rotor 1. Gearbox 15 is mounted at the end of tail boom 16 and supports pilot- controlled pushrod 20 extending along tail boom 16 between canopy 4 and gearbox 15. Gearbox 15 encloses ball bearings, drive gears, and a drive shaft that support and drive tail rotor 2.

As shown in Fig. 2, tail rotor 2 of the present invention includes a pair of tail rotor blades 11 and a gyroscopic assembly 12 having gyro paddles 59 and gyro hub 62 rotating in rotation direction 13 about tail rotor axis 14. Gyroscopic assembly 12 is mounted on an outboard end of tail rotor 2 and provides a helicopter yaw control and stabilization system for independently changing the pitch of the tail rotor blades 11 in tail rotor 2 automatically and continuously to stabilize the yaw motion of helicopter 10. Thus, the collective pitch of tail rotor blades 11 can be set at a selected pitch angle manually by the pilot using the pilot-control system, while the gyroscopic assembly can vary the collective pitch from the selected pitch angle automatically and continuously in response to yaw motion imparted to helicopter 10 during flight.

Gyroscopic assembly 12 establishes a stabilizing offset supplement or adjustment to the pilot's tail rotor controls. Thus, yaw control signals from the pilot-control

system pass directly to tail rotor 2 (through elements 20, 21, 22, 15, etc.) to bring about large changes in the azimuthal heading of helicopter 10. At the same time, gyroscopic assembly 12 mounted on tail rotor 2 functions automatically to vary collectively the pitch of rotor blades 11 from the pitch set by the pilot-control system. This "supplemental pitch control input" provided by gyroscopic assembly 12 stabilizes yaw motion of helicopter 10 in flight without unduly inhibiting pilot control. Now refer to Fig. 3 which shows a rear end elevation view of tail rotor 2, gyroscopic assembly 12, and gearbox 15 with helicopter details forward of gearbox 15 removed for clarity. Tail rotor 2 is positioned on tail rotor axis 14 to lie between gear box 15 (mounted on tail boom 16) and gyroscopic assembly 12 as shown in Figs. 2 and 3. Tail rotor 2 (including tail rotor blades 11) rotates rapidly about tail rotor axis 14 and rotates gyroscopic assembly 12 about tail rotor axis 14 by means of delta- drive bars 61 pivotably connected to gyroscopic assembly 12 and slidably connected to spider 46. Gyroscopic assembly 12 is mounted for controlled sliding movement on the outboard straight end of push-pull rod 22 that projects through tail rotor 2 and away from gear box 15 as shown in Figs. 3 and 4. In order to control tail rotor 2 in normal flight, the helicopter pilot can change the pitch, and associated thrust force, of tail rotor blades 11 without affecting gyroscopic assembly 12 using the pilot-control system by moving push-pull rod 22 which extends transversely through the interior of gearbox 15 and tail rotor hub 39 and is fixedly connected to gyro mount 52. For example, to turn in yaw direction 8 (shown in Fig. 1) , the pilot pushes pushrod 20 alongside tail boom 16 which rotates bellcrank 21 (shown in more detail in Fig. 6) and causes push-pull rod 22 to slide axially in the direction

of control input arrow 19. Gyroscopic assembly 12 and tail rotor blade pitch linkages 24 are operably mounted to, and move along with, push-pull rod 22 in the direction of control input arrow 19 thereby pitching tail rotor blades 11 to a higher angle-of-attack as shown by control pitch arrow 25. This new angle-of-attack, and the associated change in thrust force of tail rotor 2 induces helicopter 10 to turn in yaw direction 8.

Generally, wind gusts or changes in the torque of the main rotor system during flight may cause a helicopter suddenly to yaw (rotate about its main rotor axis) . To control this yaw instability, both full-size and model helicopters are frequently equipped with yaw stabilizer systems. Gyro-stabilizer systems can be broadly classified as either mechanical or electro-mechanical. Mechanical systems generally rely on precessional (angular) displacement of a relatively large gyroscopic arm or flywheel mechanism to alter the pitch of the tail rotor blades in opposition to any yaw motion of the helicopter. Electro-mechanical systems sense the precessional (angular) displacement of a relatively small flywheel mechanism, and control the tail rotor blades through electronic amplification and electro-mechanical and/or hydraulic servo actuators. Modern model helicopters frequently carry electro-mechanical gyro stabilizer systems that are electronically mixed into the tail rotor servo control circuit. These gyro systems are relatively expensive and heavy, and draw power from the airborne radio receiver system batteries. An example of an electro-mechanical system designed for full-size helicopters is described in U.S. Patent No. 3,528,633 to Knemeyer.

Some yaw stabilizer systems, especially more sophisticated electro-mechanical systems, disengage whenever the pilot maneuvers the aircraft. Other systems, most notably mechanical systems, act to suppress all yaw

motion of the helicopter including that desired by the pilot. With these mechanical systems the pilot must forcibly override the gyroscopic mechanism in order to control the tail rotor for trimming and normal flight. Since gyroscopic mechanisms tend to resist mechanical displacement, the pilot will feel resistance to control inputs whenever the pilot tries to change the collective pitch of the tail rotor blades manually in order to change the flight direction or heading of the helicopter. This resistance will typically persist as long as the rate of yaw motion is not zero. Generally, these systems tend to increase helicopter stability at the expense of helicopter controllability.

A number of yaw stabilization devices have been developed for use on model and/or full-size helicopters.

See, for example, U.S. Patent Nos. 3,004,736 to Culver and 4,759,514 to Burkam. A mechanical gyro stabilizer mechanism is detailed on page 41 of the March 1973 issue of American Aircraft Modeler magazine (originally located at 733 15th Street N.W. , Washington, D.C. 20005). Recently, the state-of-the-art in yaw stabilization devices has been advanced by development of gyro stabilizers that are connected to the tail rotor and are independent of the pilot-control system so that each of the pilot-control system and the gyro-controlled stabilizer system operates independently to vary the thrust force generated by the tail rotor. Reference is hereby made to U.S. Patent No. 5,305,968 to Paul E. Arlton, which is hereby incorporated by reference herein, for a description of a suitable device for operating a tail rotor to automatically stabilize the yaw motion of a helicopter. Advantageously, the system disclosed in the Arlton '968 patent permits a helicopter pilot to use the pilot-control system to adjust the pitch on the tail rotor blades in order to set the directional heading of the helicopter and

also allows a gyro-controlled stabilizer to change the pitch of the tail rotor blades automatically and continuously (without pilot input) to compensate for unplanned yaw motion without disrupting the pilot (as by increasing resistance to use and operation of the foot pedals) . As noted above, wind or other atmospheric disturbances can impart unplanned yaw motion to a helicopter in flight.

Fig. 4 is a view similar to Fig. 3 and illustrates the effect of helicopter yaw motion on gyroscopic assembly 12. In use, yaw motion of helicopter 10 in direction 7 (shown in Fig. 1) applies yaw forces to gyro spindle 51 and gyroscopic assembly 12 causing gyroscopic assembly 12 to precess (tilt) in precession direction 27. Tilt of gyroscopic assembly 12 actuates tail rotor pitch linkages 24 to slide along push-pull rod 22 in the direction of stabilizing input arrow 28 thereby pitching tail rotor blades 11 to a higher angle-of-attack as shown by stabilizing pitch arrow 26. This new angle-of- attack, and the associated change in thrust force of tail rotor 2 resists the initial yaw motion of helicopter 10. Maximum control pitch from pilot control inputs (as illustrated by control pitch arrow 25 in Fig. 3) generated manually using pushrod 20 is usually greater than maximum stabilizing pitch (as illustrated by stabilizing pitch arrow 26) generated automatically by gyroscopic assembly 12 so that the pilot has control authority to override the gyro stabilizer 12 in order to set the azimuthal heading of helicopter 10 in flight. The limits of stabilizing pitch generated by gyroscopic assembly 12 can be set in many ways. In the preferred embodiment, gyroscopic assembly 12 is allowed to precess in one direction until tail rotor pitch linkages 24 butt against gyro mount 52, and in the other direction until the inner surfaces of gyro spindle 51 contact gyro

mount 52. Once the mechanism reaches a mechanical limit, no further precession is possible, and so no further gyroscopic input to vary the collective pitch of tail rotor blades 11 is possible. Once gyroscopic assembly 12 has tilted, some

"mechanism" is needed to return it to a nominal vertical orientation. As gyroscopic assembly 12 tilts in direction 27, delta-drive bars 61 operating within angled drive bar holes 66 formed in delta paddle grips 60 (see Figs. 8 and 9) cause gyro paddles 59 to pitch cyclically (twice per revolution, once upward and once downward) so as to produce an aerodynamic moment opposite to direction 27. This aerodynamic moment restores gyroscopic assembly 12 to a nominal vertical orientation after a short time lag, such time lag being adjustable by varying the size and weight of gyro paddles 59 and delta drive angle 65 (shown more clearly in Fig. 8) . When in a nominal vertical orientation, gyro paddles 59 are set to a positive pitch angle into the oncoming airflow and can be used to trim tail rotor 2.

Functionally, gyroscopic assembly 12 establishes a stabilizing offset or adjustment to the pilot's tail rotor controls. Continuous pilot yaw control inputs necessary to swing helicopter 10 to a new heading cause gyroscopic assembly 12 to precess to a preset limit after which point the mechanism no longer acts to counter the yaw motion. Instantaneous control inputs displace (i.e., slide) gyroscopic assembly 12 on the outboard straight end of push-pull rod 22 in a direction substantially parallel to axis of rotation 14. Such linear displacement has no precessional effect on gyroscopic assembly 12, and so the gyroscopic mechanism described herein does not impede such inputs. This means that gyroscopic assembly 12 stabilizes yaw motion of the helicopter 10 in flight, but does not unduly inhibit pilot control of tail rotor 2.

To develop a detailed understanding of the present invention, it is easiest to view isolated elements of the invention separated from the whole mechanism. Referring to Figs. 5A and 5B, in the preferred embodiment of the present invention, tail rotor blades 11 are fixedly secured to reach-around blade grips 32 which also support timing weight bolts 34. Tail rotor blade assembly 36 shown in Fig. 5B comprises two opposing tail rotor blades 11 pivotable about tail rotor blade pivot bolts 35 extending through reach-around grips 32 at the base of each tail rotor blade 11 and through the reach-around blade grip 32 of the opposing tail rotor blade 11. Thrust washers 37 abut the inner faces of reach-around grips 32 and are manufactured of a low friction material such as Teflon. In Fig. 6 which shows certain tail rotor elements before assembly, tail rotor pushrod 20 extends rearward along tail boom 16 from the pilot controls of helicopter 10 and is operably connected to bellcrank 21. Push-pull rod 22 is also operably connected to bellcrank 21 and extends transversely through the interior of gearbox 15 and tail rotor hub 39 to exit tail rotor hub 39 through hole 41. Tail rotor assembly 36 is operably connected to tail rotor hub 39 by tail rotor blade pivot bolts 35 which extend into hub pivot holes 40 located on opposite faces of tail rotor hub 39. A power transmission system is provided having bevel gear 69 fixedly connected to drive shaft 70 located within tail boom 16 to transmit power from engine 3 to tail rotor 2. In the preferred embodiment, tail rotor 2 is driven at a rotational speed 2 to 3 times that of main rotor 1. Higher speed ratios will increase the thrust and gyroscopic effectiveness of the present invention at the cost of increased power consumption.

Now referring to Fig. 7, tail rotor pitch linkages 24 (shown in Fig. 3) comprise slider eyelet 45, spider 46, spider separator washer 47, and spider slider

48. Spider eyelet 45 extends through spider 46 and is forcibly pressed into spider slider 48 thereby securing spider 46 and spider separator washer 47 for rotation against spider slider 48. Gyro spindle 51 is pivotably supported on gyro mount 52 by gyro pivot pins 53 which pass through spindle pivot holes 54 and are pressed into gyro mount pin holes 55 formed in opposing faces of gyro mount 52. Spider slider 48 is operably connected to gyro spindle 51 by slider link pin 56 which extends through spindle link holes 57 and is held within slightly undersized slider link hole 58. Gyro mounting screw 50 is provided to secure gyro mount 52 to the end of push-pull rod 22 (shown in Fig. 6) .

In the preferred embodiment of the present invention, gyro spindle 51 is made of an aluminum alloy and is hard-coat anodized to provide a good wear surface for gyro assembly 12. Spindle pivot holes 54 and spindle link holes 57 are formed of, or lined with, a wear and vibration resistant material such as nylon.

Moving on to Fig. 8, which is an exploded view of gyroscopic assembly 12, gyroscopic assembly 12 has gyro paddles 59 extending through finger holes 63 formed in the arms of gyro hub 62 and grip through-holes 64 formed in delta-drive grips 60. Gyro paddles 59 are fixedly secured to delta-drive grips 60 and thereby secured for rotation relative to gyro hub 62. Delta-drive bars 61 operate within drive bar holes 66 and are thereby pivotably connected to delta-drive grips 60. In the preferred embodiment of the present invention delta drive angle 65 is 62.6 degrees so that a 15 degree tilt of gyroscopic assembly 12 results in approximately a 12 degree change in pitch of gyro paddles 59.

Delta-drive components are so-named because delta drive angle 65 couples the tilt of gyroscopic assembly 12 to the pitching motion of gyro paddles 59 in a manner conceptually similar to delta hinges on a main rotor

(helicopter rotor hinge-axes are generally referred to with the Greek letters Alpha, Beta, Gamma and Delta) . While the definition of a delta hinge on a rotor is widely known, the concept of coupling gyro tilt with gyro paddle pitch, and the mechanism for doing so, is entirely novel and disclosed herein for the first time. This concept is also a key distinction between the current invention and that disclosed in the Arlton '968 patent which used springs to restore the weighted arms of the gyro rotor to a nominal orientation.

Aerodynamic centering using gyro paddles 59 is especially useful in applications where the tail rotor (and consequently the gyro assembly) must operate throughout a wide range of rotational speeds. While return-springs always produce the same restoring force regardless of tail rotor rotational speed, gyroscopic forces causing a gyroscopic assembly to tilt and aerodynamic forces produced by cyclically pitching paddles are directly proportional to rotational speed. As a result, gyro paddles 59 produce restoring forces of the same magnitude as the gyroscopic tilting forces felt by gyroscopic assembly 12. This means that the time-response and overall effectiveness of the entire stabilization system is essentially invariant even though tail rotor rotational speed may be constantly changing.

As illustrated by Fig. 9, gyroscopic assembly 12 is supported for rotation on gyro spindle 51. Delta-drive bars 61 extend through and are slidably supported within spider drive holes 44 in spider drive arms 43 of spider 46. Push-pull rod 22 extends through the center of slider eyelet 45 and is fixedly secured to gyro mount 52 by means of gyro mounting screw 50. Blade pitch pins 31 extend through pitch pin holes 30 in reach-around grips 32, and are pressed into spider pitch pin holes 42 in spider 46. An important design consideration when using

mechanical yaw stabilizer systems is the force required to actuate tail rotor blades 11. The actuating power of a mechanical gyro stabilizer is determined by the rotational speed, size, and mass of the gyro rotor (e.g., gyroscopic assembly) . Given a particular tail rotor rotational speed, high actuation forces necessitate large and/or heavy gyro rotors. Rotor blade actuation forces, therefore, must be kept low to minimize the size and weight of the gyro rotor. In light man-carrying helicopters the mechanical force required to actuate (e.g. , change the collective pitch) tail-rotor blades 11 is produced by the pilot's legs pushing against foot peddles. In heavier helicopters, bearing friction and aerodynamic forces developed by the tail-rotor blades can be too high for the pilot to overcome. Heavy helicopters, therefore, are frequently fitted with power-assist devices such as hydraulic systems to boost the pilot's controls. The weight of power-assist devices is roughly proportional to the forces they must develop. In all helicopters, minimizing control loads is desirable since pilot work load, linkage forces, and actuator weight can all be reduced.

For reasons of structure and simplicity, and to minimize actuation forces, helicopter tail rotor blades have traditionally employed symmetrical airfoils such as section 74 shown in Fig. 10A, and simple constant chord

(rectangular) planforms. While easy to build, these rotors do not use power very efficiently. Since helicopters have relatively low payload capacities, even small increases in rotor performance can result in large percent changes in payload capability.

Cambered airfoils, such as airfoil 75 with curved camber line 76, shown in Fig. 10B, can substantially increase the lifting potential of a rotor blade. Cambered airfoils, however, have a drawback: the curvature of the airfoil causes the airfoil to pitch downward toward

negative angles of attack. This pitching tendency can cause the rotor blade to twist and to exert high loads on the rotor pitch control linkages (and consequently to the pilot's legs) . One tail rotor design successfully employing cambered airfoils is described in a technical paper entitled "Increasing Tail Rotor Thrust and Comments on Other Yaw Control Devices" by Frank Robinson published in the Journal of the American Helicopter Society in 1970. In this design, centrifugal force acting through the blade center-of-gravity balances the unwanted steady-state aerodynamic forces of the cambered airfoil. A disadvantage of Robinson's approach is the need to control blade chordwise center-of-gravity. Some blade manufacturing techniques such as plastic injection molding are not well suited to this restriction.

Tail rotor blade 11 of the present invention balances the aerodynamic moment of a cambered airfoil with other forces, and does not require chordwise control of the blade center-of-gravity, or place restrictions on blade coning or flapping. Tail rotor blade 11 balances the pitching moment of the cambered airfoil with a positive aerodynamic lift force located in front of blade pitching axis 81. Tail rotor power requirements and pilot workload are both reduced through the use of cambered airfoils and balanced aerodynamic and centrifugal forces.

To understand the operational details of tail rotor blade 11, it is easiest to develop the blade shape in steps. The planform of tail rotor blade 11 shown in Fig. HA can be generated from a standard, rectangular planform by performing a number of design modifications. Blade leading edge 82 is swept forward in rotation direction 13 in order to position blade center-of-pressure 80 (where all aerodynamic lift forces are assumed to be concentrated) ahead of pitch axis 81. Blade thrust forces acting through

offset center-of-pressure 80 tend to oppose the nose-down pitching moment of the cambered airfoil sections as long as the airfoil sections are operating at a positive angle-of- attack. These thrust forces, which increase with increased angle-of-attack, can also help to offset the natural mass- feathering (or "tennis racquet") effect which is well known to those skilled in the art to which the present invention relates.

Without further modification, tail rotor blade 11 could exhibit pitch divergence (angle-of-attack instability) and tend to flutter or transition rapidly between positive and negative pitch. To reduce the potential for flutter, blade tip 83 (after approximately the 70% semi-span location) is swept back. This swept tip may also be advantageous on blades with tips designed to operate at high Mach numbers (as will be understood by one skilled in the art) . Blade tip 83 is tapered and the entire blade is twisted linearly eight to ten degrees to reduce induced drag. Fig. 11F shows that a conventional straight tail rotor blade 87 can be modified to move the blade center of pressure forward by adding a leading-edge cuff 88.

Since the aerodynamic force acting on center-of- pressure 80 is actually the lift force produced by tail rotor blade 11, and since lift is proportional to angle-of- attack, the force opposing the airfoil pitching moment is dependent upon blade angle-of-attack. Small balancing weights 33 are added to blade grips 32 to balance blade pitching forces throughout a large angle-of-attack range. Actual sizing and location of balancing weights 33 is dependent upon the construction and mass distribution of tail rotor blades 11.

Tail rotor blade 11 need not be fixedly secured to reach-around grips 32, and such grips need not be of the reach-around variety. As an alternative embodiment of the

present invention, tail rotor blades 11 can be hinged at the root to lead or lag with blade center of gravity 29 being so placed as to induce tail rotor blade 11 to fly in the desired orientation when rotating at flight speed. In addition, standard tail rotor blade grips having multiple ball bearings can be substituted for reach-around grips 32, but at additional cost.

Note that blade cross sections need not be undercambered to benefit from the force balancing method of the present invention. The blade configuration of the present invention can also be used to balance the mass- feathering ("tennis-racquet") forces felt by blades with symmetrical airfoils, and can also be used on rotor systems other than tail rotors such as main rotor systems with undercambered airfoils.

The airfoils shown in Fig. HA through HE are meant for use on very small model helicopters operating at Reynolds numbers below 50,000 (as would be understood to one skilled in the art) . The cross sections labeled as Fig. HB through HE at 15%, 4.7%, 4.6% and 6.8% thick respectively (measured as a percentage of chord) , and pitched to 20 deg., 17 deg. , 13 deg., and 10 deg. respectively (measured from the horizontal) . Airfoils meant for larger applications should be selected on the basis of the particular Mach and Reynolds numbers associated with the application (as would be familiar to one skilled in the art) .

Thin airfoils such as those in Fig. HC-E, and airfoils intended for use at low speeds (such as on very small helicopters) can benefit from turbulator strips such as turbulator 150 in Fig. 16 shown appended to the top surface of airfoil 75. Turbulators are thin, raised strips of material running spanwise near the leading edge of an airfoil (top and/or bottom) that induce the airflow over the surface of the airfoil to become turbulent and to stick

more readily to the airfoil. Turbulators can reduce the drag produced by some airfoils operating at certain (especially low) speeds where the airflow tends to separate from the surface. Whereas turbulators have found applications on some fixed wing aircraft, turbulators on helicopter rotor blades are novel since most helicopters currently available utilize relatively large, high speed rotors where turbulators are not beneficial. Figs. 17A-C show various turbulator configurations on tail rotor blade 11. Straight turbulator 156 and zig-zag turbulator 157 are positioned approximately 10% to 20% back from leading edge 82 of tail rotor blade 11, are 1% in width, and 0.2% in height (where percentages refer to local airfoil chord length) . The actual position of turbulators 156 and 157 depend upon the airfoils, operating angle of attack and operating speed of tail rotor blade 11.

Fig. 18A and 18B show main rotor blade 17 with blade root 160 and blade tip 161. Various airfoiled cross- sections of blade 17 are shown in Fig. 18C through 18G at selected spanwise stations. Arlton, in U.S. Application No. 08/233,159, disclosed airfoils for use on a main rotor blade on a helicopter. The preferred embodiment of the present invention advantageously employs selected airfoils and a turbulator strip to provide a superior rotor blade to operate with a tip Reynolds number of approximately 100,000 and above.

Airfoil section 164 is a thick section designed to connect blade root 160 and airfoil section 165. Illustratively, sections 165 and 166 are 7.1% thick Sokolov airfoils with 6% camber and turbulator strip 158 appended to the upper surface at the 10% chord location (as described by Dieter Althaus in his book "Profilpolaren Fur Den Modellflug" published by C.F. Muller in Germany). It will be understood that sections 165 and 166 can be other

similar thin (4%-8%) sections developed for use at low Reynolds numbers, for example, below 60,000.

Turbulator 158 extends over the inboard portion of main rotor blade 17 where airflow tends to separate from the surface (since the inboard section of a rotating rotor blade travels more slowly than the outboard section) . Outboard portion 159 of blade 17 is traveling at a higher speed, and has an airfoil section chosen to reduce the likelihood of airflow separation. In the preferred embodiment, rotor blade 17 is made of molded plastic and turbulator 158 is a molded detail of blade 17.

Section 166 transitions to section 167 in outboard region 159 of blade 17 where section 167 is illustratively a 9.2% thick SD7037-PT airfoil with 3% camber developed by Michael Selig et al. for use at low Reynolds numbers (described in Soar Tech 8 published by H.A. Stokely of Virginia Beach, Virginia, USA) . It will be understood that section 167 can be another similar section developed for use in the low Reynolds number range of, for example, 50,000 to 100,000.

Section 166 transitions to section 168 which is a 4% to 9% airfoil with low camber for use as a low drag tip. The planform of rotor blade 17 is tapered, and the blade twisted (washed-out) 10 to 14 degrees from root to tip for higher aerodynamic efficiency.

In the preferred embodiment, tail rotor blades 11 for model helicopters are molded from plastic such as nylon, and in full-size helicopters can be molded with fiber composite skins such as Kelvin/carbon over a foam or honeycomb core and bonded with epoxy. Angled cross-fibers can be used to distribute torsional loads across the blade span.

Many variations of the present invention are contemplated. Figs. 12-14 describe an alternate embodiment with gyro rotor 94 having gyro rotor blades 91 in place of

gyro paddles 59 and ball-drive links 92 in place of delta drive bars 61. Fig. 12 is similar to Fig. 2 and shows the alternate tail rotor of helicopter 10. The primary differentiating features of this alternate embodiment are gyro rotor blades 91 which pitch collectively in unison with tail rotor blades 11, and thus form a secondary, thrust-producing rotor system.

It can be seen by examining Fig. 13, which is an exploded view of the embodiment shown in Fig. 12, that many elements of the present invention are unchanged in form and function in the alternate embodiment. Spider 99 is similar to spider 46 but without spider drive arms 43. Reach- around grips 101 have drive arms 100 receptive to ball- drive links 92. Gyro rotor hub 93 engages gyro rotor blades 91 in blade recesses 96 at the root of each gyro rotor blade 91. Gyro rotor blades 91 are pivotably secured to gyro rotor hub 93 by gyro pitch pins 95 extending through blade pin holes 97 at the root of gyro rotor blades 91 and through hub pin holes 98 in arms of gyro rotor hub 91.

Referring now to Fig. 14, which is similar to Fig. 3 and illustrates the effect of pilot tail rotor control inputs, gyroscopic assembly 94 and tail rotor blade pitch linkages 24 are operably mounted to, and move along with, push-pull rod 22. Gyro rotor 94 is driven by ball drive links 92 connected to grip drive arms 100. Operation of push-pull rod 22 in the direction of control input arrow 19 pitches tail rotor blade 11 to a higher angle-of-attack as shown by control pitch arrow 25. Simultaneously, grip drive arms 100 act upon ball-drive links 92 to pitch gyro rotor blades 91 collectively to a higher angle-of-attack. Both the primary tail rotor blades and the secondary gyro rotor blades thereby create useful thrust for maneuvering. Proceeding to Fig. 15 which is a view similar to Fig. 4 illustrating the effect of yaw motion on gyroscopic

assembly 94, yaw motion of helicopter 10 applies yaw forces to gyroscopic assembly 94 causing gyro rotor 94 to precess (tilt) in precession direction 27. Tilt of gyro rotor 94 causes tail rotor blades 11 to pitch to a new angle-of- attack as shown by stabilizing pitch arrow 26. Grip drive arms 100 act upon ball-drive links 92 to pitch gyro rotor blades 91 collectively to a new angle-of-attack. This new angle-of-attack of both primary and secondary rotor systems, and the associated change in thrust force, resists the initial yaw motion of helicopter 10. Because gyro drive balls 90 are offset slightly from the tilt axis of gyro rotor 94 (defined by gyro pins 53 shown in Fig. 7) , gyro rotor blades 91 also pitch cyclically (twice per revolution) when gyro rotor 94 is tilted. The resulting aerodynamic moment restores gyro rotor 94 to a nominal vertical orientation.

Note that the thrust forces produced by gyro rotor blades 91 are transmitted to push-pull rod 22 and all associated tail rotor control linkages. Such linkages must therefore be designed to accommodate a thrust load. Note also that one ball-drive link 92 may require ball joints on both ends to prevent linkage binding.

Alternative embodiments are contemplated wherein gyro rotor blades 91 are replaced by paddles similar to gyro paddles 59. Said replacement paddles can be pitched to minimize aerodynamic drag and any thrust forces transmitted to push-pull rod 22.

Although the invention has been described and defined in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.