Abstract:
A three axis control system employing four flaps is disclosed. The flaps are of uniform design, which decreases machining and manufacturing costs. The flaps are positioned on a vehicle orthogonally, but offset from a vehicle centerline. The system provides not only pitch and yaw control, but also bi-directional roll control with a minimum number of parts and minimal infringement of packaging envelope. The system provides quick response and increased capability for difficult maneuvers and is useable for hypersonic/supersonic applications.

Description:
REFERENCE TO PROVISIONAL APPLICATION 
     This application claims priority of provisional application Serial No. 60/165,920, filed Nov. 17, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of aerodynamics, and more specifically, to a system of deployable control surfaces and an associated control system for effecting yaw, pitch and roll control of a vehicle. The system includes a plurality of deployable flaps whose respective centerlines are offset from the radius of the vehicle. Selective deployment of the flaps will result in a desired pitch, yaw and/or movement of the vehicle. 
     2. Description of the Related Art 
     Aerodynamic control systems for use in rocket launched projectiles are generally known. A particular control situation arises after the motor of a rocket or other such vehicle has fired, and the vehicle continues to move in what is commonly called a “coasting” mode of operation. During vehicle coasting, there is a need for aerodynamic control to guide the vehicle. 
     Prior attempts to provide such a coasting guidance system have employed aero fins to achieve control during coast periods. The use of fins, however, can significantly reduce the number of missiles which can be packaged into a given cross section, such as the payload bay of an airplane or ship. Consequently, designs have been proposed which utilize flaps or panels which lie flush with the skin of the missile when not in use but which can be actuated to extend into the airstream to control the missile. 
     A significant disadvantage to the use of known flap designs is the difficulty of incorporating a design that effectively achieves roll control in addition to yaw and pitch control. Since most guided projectiles require some form of roll control, a system without roll control would likely require steering schemes, such as bank-to-turn, which require more time and therefore limit controllability. 
     In U.S. Pat. No. 5,398,887 to Wassom et al., a control system is disclosed for use with missiles and other projectiles. The control system provides pitch, roll and yaw control by actuating two (2) pairs of flaps disposed at the base of the projectile. Each flap is deployable on command by pivoting about a pivot axis disposed at the leading edge of the flap. The pivot axis is oriented at a first oblique angle to the radial plane of the projectile. Each flap is curved in a shape corresponding to the contour of the projectile body. The oblique angle of the pivot axis of one pair of flaps is opposite the oblique angle of the other pair of flaps. 
     The design of Wassom et al. has several disadvantages. First, the flaps are not of uniform design, which increases machining and manufacturing costs. Second, to obtain pure pitch or yaw control, the angles of the flaps must be paired in precise mirror images in order to avoid creating a roll force, which also increases machining and manufacturing costs. Finally, for hypersonic applications, the oblique hinging may create heating and/or mechanical interface problems and also adversely affect roll control. 
     U.S. Pat. No. 5,211,358 to Bagley describes a plurality of deployable fins which are moveable from a stowed position to a deployed position after launch by inflation of air bags. After launch, or after lowering of a launch platform in the case of aircraft, an actuator opens a valve supplying gas to inflate the bags. After the fin is locked into position, the airbag either rapidly deflates or is decoupled from the missile structure. The fins provide stabilization rather than pitch, roll or yaw control, however. 
     U.S. Pat. No. 5,975,461 to Ullrich discloses a vane control system for a guided missile, in which four vanes are disposed on the fuselage of the missile at a forward portion thereof. The vanes are deployable by actuation of a gear-and-motor arrangement whereby rotation of a gear causes the vanes to extend outwardly from a retracted position. As with the Bagley reference, the vanes appear to provide stabilization as opposed to pitch, roll and/or yaw control. 
     U.S. Pat. No. 5,564,652 to Trimbath describes a body spoiler for yaw control of a supersonic airplane. At the fore body of the fuselage, a plurality of spoilers are disposed, flush with the outer surface in a non-deployed position. The illustrated embodiment describes eight (8) spoilers arranged at equal radially spaced intervals. The patent describes deployment of the spoiler on the same side of the aircraft fuselage as a failed engine. The spoiler generates a shock wave which in turn produces pressure along the fore body thereby creating a yaw moment that substantially counterbalances the yaw moment generated by the malfunctioning engine. It is noted in the patent that a plurality of the radially disposed spoilers can be used to generate other forces, such as by deploying spoilers under the fuselage, an upward pitch movement is generated. 
     Although the aforementioned patents describe the use of control surfaces to achieve certain aerodynamic effects, a continuing need exists for a relatively simple structure for effecting pitch, yaw and/or roll movement in a vehicle. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved three axis flap control system. 
     It is a further object of the present invention to provide pitch, yaw, and roll control with a minimum number of parts and minimal infringement of packaging envelope. 
     It is a further object of the present invention to provide a control system that is variable to allow for a variety of vehicle loading conditions. 
     It is a further object of the present invention to minimize machining and manufacturing costs associated with a three axis flap control system. 
     The present invention provides a vehicle control system that provides pitch, yaw, and roll control. The control system includes a sensor, a controller, actuators, and four flaps. The flaps are positioned on the vehicle orthogonally, but offset from the vehicle centerline. By engaging various pairs of the flaps, any desired vehicle orientation may be achieved. The flaps are independently controlled. The flaps may be engaged to any desired angle of engagement, from none to the maximum possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is described with reference to the accompanying drawings, in which like reference characters reference like elements, and wherein: 
     FIG. 1 is a perspective view of a vehicle having the control flaps according to the present invention; 
     FIG. 2 is a rear, schematic view of the vehicle of FIG. 1, showing four flaps in a deployed position; 
     FIG. 3 is a rear schematic view of the vehicle of FIG. 1, with a first pair of the four flaps deployed to effect positive pitch, and wherein the flaps of the deployed pair are offset away from each other; 
     FIG. 4 is a rear schematic view of the vehicle of FIG. 1, with a second pair of the four flaps deployed to effect negative pitch, and wherein the flaps of the deployed pair are offset away from each other; 
     FIG. 5 is a rear schematic view of the vehicle of FIG. 1, with a third pair of the four flaps deployed to effect positive yaw, and wherein the flaps of the deployed pair are offset towards each other; 
     FIG. 6 is a rear schematic view of the vehicle of FIG. 1, with a fourth pair of the four flaps deployed to effect negative yaw, and wherein the flaps of the deployed pair are offset towards each other; 
     FIG. 7 is a rear schematic view of the vehicle of FIG. 1, with a fifth pair of the four flaps deployed to effect negative, or counter-clockwise, roll, and wherein the flaps of the deployed pair are diametrically opposed, with centerlines of the flaps disposed on opposite sides of a plane of symmetry; 
     FIG. 8 is a rear schematic view of the vehicle of FIG. 1, with a sixth pair of the four flaps deployed to effect positive, or clockwise, roll, and wherein the flaps of the deployed pair are diametrically opposed, with centerlines of the flaps disposed on opposite sides of a plane of symmetry; and 
     FIG. 9 is a schematic diagram showing a flap and actuator structure, coupled to a controller. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a vehicle  10  is shown in the form of a projectile of the type that can be propelled by a rocket (not shown) at hypersonic speeds. The vehicle  10  includes a body  12  having a pointed forward end  14  and an aft end  16 . The body  12  between the opposite ends  14  and  16  is substantially conically shaped, except for four planar sections  18 ,  20 ,  22  and  24 . As is usually the case with a projectile, in order to maintain stable flight, the center of gravity CG is forward of the moment of resistance. 
     Four control surfaces or flaps  26 ,  28 ,  30 , and  32  are pivotally mounted by their respective leading edges on the body  12  at respective planar sections  18 ,  20 ,  22  and  24 . Each flap is pivotally mounted for movement between a stowed, non-deployed position, and an un-stowed, deployed position. When in the stowed position, the outer surface of each flap is flush with the respective planar section surface to thereby avoid drag; at this position, the flap is in a zero degree of deployment. When deployed, each flap is moved by an actuator to a desired degree of deployment. The full limit of deployment can be limited by the particular application, meaning the type of vehicle, its aerodynamic characteristics, and the projected flight characteristics and control requirements. The flaps illustrated in FIG. 1, flaps  26  and  28 , are shown in approximately a 45 degree deployment. If that is the selected full extent of deployment, the control system described herein can operate the flaps to any position between 0 and 45 degrees, depending on the desired movement. Any position beyond “0” is considered a deployed position. 
     As seen in FIG. 2, a longitudinal axis “A” of the vehicle  10  is defined by the intersection of a first plane of symmetry “B-B-” and a second plane of symmetry “C-C.” The two planes of symmetry B-B and C-C divide the body  12  into four quadrants I, II, III and IV. Each flap has a centerline  26   c ,  28   c ,  30   c , and  32   c , each of which is in a plane offset from but parallel to one of the planes of symmetry. The centerlines are also offset from the longitudinal axis A of the vehicle  10 . The centerlines are substantially perpendicular to their respective pivot axes, and the pivot axes are substantially perpendicular to the longitudinal axis A of the vehicle  10 . 
     The offset of flap  26  positions a major portion of the control surface of flap  26  into quadrant IV, whereas the diametrically opposed flap  30  has a major portion of its control surface in quadrant II. Thus, opposite flaps  26  and  30  are diametrically offset with respect to each other. Similarly, the major portion of the control surface of flap  28  is in quadrant IV, while the major portion of the control surface of flap  32  is in quadrant II. Thus, opposite flaps  28  and  32  are diametrically offset with respect to each other. Preferably the amount by which the centerline  26   c  of flap  26  is offset from the plane of symmetry B-B is the same amount by which the centerline  30   c  of flap  30  is offset from the plane of symmetry B-B, but the offset is in an opposite direction. 
     A similar relationship exists with respect to flaps  28  and  32  relative to the plane of symmetry C-C. In particular, the amount by which the centerline  28   c  of flap  28  is offset from the plane of symmetry C-C is the same amount by which the centerline  32   c  of flap  32  is offset from the plane of symmetry C-C, but the offset is in an opposite direction, meaning to the opposite side of the plane of symmetry C-C. The result is that quadrants I and III are minor surface portions of the flaps while quadrants II and IV have major portions of the flaps. This offsetting of flaps creates eccentric forces that lead to the generation of pitch, roll and yaw moments, depending on which flaps are actuated into deployed positions. 
     FIG. 3 illustrates an example of how the flaps are actuated to achieve a positive pitch movement in the body  12 . To create positive pitch, flaps  26  and  32  are deployed while the other two flaps remain stowed. The deployment of flaps  26  and  32  must be to the extent that equal moments are generated on opposite sides of the vehicle longitudinal axis, thereby creating a positive pitch moment which causes the nose of the body  12  to pivot upwardly about the center of gravity. By simple addition of force vectors, flap  26  generates a force V 1  having downward and sideward components. Flap  32  generates equal downward and sideward components in force V 2 , but the sideward component is opposite the sideward component generated by flap  26 . When the force vectors are added, the combine force vector V 3  has only a combined downward force. Since the force vector V 3  is aft of the center of gravity, the nose of the body  12  pivots upwardly about a horizontal pivot axis passing through the center of gravity, thus producing positive pitch. If the flaps were disposed forward of the center of gravity, an opposite effect would occur, such that the pitch would he negative. 
     FIG. 4 illustrates how, with the center of gravity forward of the flaps, deployment of flaps  28  and  30  generate a negative pitch. Each of the flaps  28  and  30  produces a force vector V 1  and V 2 , having equal upward components, but opposite sideward components that cancel each other when the force vectors are added to produce the upward force vector V 3 . Since the flaps are aft of the center of gravity, the upward force vector causes the body  12  to pivot about a horizontal pivot axis passing through the center of gravity, thus causing the nose of the body to move downwardly and thereby generating negative pitch. 
     FIG. 5 illustrates a flap deployment that generates positive yaw, i.e., nose movement from left to right. By deploying flaps  30  and  32 , while leaving flaps  26  and  28  stowed, force vectors V 1  and V 2  are generated. In this case, the vertical components of vectors V 1  and V 2  cancel each other out so that the net force vector V 3  produces a sideward force. Since the flaps are aft of the center of gravity, this sideward force causes the nose to move to the right. FIG. 6 shows the opposite or negative yaw movement, generated when flaps  26  and  28  are deployed. The vector addition described above produces a combined, sideward or horizontal force vector V 3  that causes the body  12  to rotate about a vertical axis passing through the center of gravity, thus causing the nose to move from right to left and thereby generating negative yaw. 
     FIGS. 7 and 8 illustrate a flap deployment scheme in which negative (counterclockwise) roll and positive (clockwise) spin is imparted in the body  12 . Referring to FIG. 7, diametrically opposed, but offset flaps  26  and  30  produce, respectively, force vectors V 1  and V 2 , each having vertical and horizontal components. The horizontal and vertical components cancel each other out, thus resulting in no pitch or yaw moments, i.e., no rotations about axes passing through the center of gravity. However, since the force vectors V 1  and V 2  do not pass through the center of the body, a spin is imparted, thus causing the body  12  to spin in the counter-clockwise direction. 
     In FIG. 8, flaps  28  and  32  are deployed to generate positive spin, i.e., spin in the clockwise direction. As in FIG. 7, force vectors V 1  and V 2  have horizontal and vertical components that cancel each other to thus prohibit rotation about an axis passing through the center of gravity. The spinning motion causes the body  12  to rotate about the longitudinal axis of the body  12 . 
     Any number of standard actuator means can be employed to move the flaps from their stowed positions to their deployed positions. As seen in FIG. 9, flap  26  is pivotally mounted at its leading edge for pivotal movement about mounting pin  34 . The pivot axis defined by the mounting pin  34  is transverse the longitudinal axis of the body  12 . Preferably, the pivot axes of all four flaps lie in a common, transverse plane. In the non-deployed position of the flap  26 , the outer surface of the flap  26  is flush with the surface of the planar section  18 . The lower surface of the flap  26  is connected to an actuator  36  which responds to a control signal issuing from controller  38 . In the illustrated embodiment, the actuator is a linear actuator that has an extendable arm which is pivotally connected to the flap  26 . The arm is extendable to whatever extent is commanded by the controller  38 . For example, the full extent of deployment may be 30 degrees, as shown by the phantom line drawing of the flap. The controller  38  may command that the degree of deployment is any amount between zero (non-deployed) to 30 degrees (fully deployed). 
     The linear actuator  36  can be hydraulic, pneumatic or electric, depending on space and weight requirements. Also, the mechanism for actuation can be any other type other than linear, including gear driven, inflatable bladders, etc. Each flap is required to have its own actuator. Redundant actuators of the same or different types may also be employed as a safeguard against failure. 
     The controller can be any of a variety of standard control technologies. For example, the controller  38  may include a programmed microprocessor capable of executing pre-programmed telemetry data to effect desired pitch, roll and/or yaw movements. The controller may include feedback circuitry whereby flight data is fed to the controller, and the programmed telemetry data is changed on the fly to compensate for real flight data. The feedback data may be from sensors (not shown) provided on or in the projectile, or data may be transferred via radio frequency transmission. In that case, the controller would include receiver circuitry and a receive antenna for receiving signals from either a ground station, air or space-based station, or from orbiting satellites, such as the GPS constellation. 
     FIG. 9 shows control signal coupling lines  40 ,  42  and  44  for coupling the controller  38  to the other actuators (not shown) of the other three flaps. The coupling lines may be hard wired connections, either copper or fiber optic, or they could be wireless couplings, in which case the lines schematically represent the connections between a transmitter in the controller  38  to receivers in the actuators. 
     As noted previously, the flaps may be positioned along other axial positions with respect to the body  12 , and can be, for example, forward of the center of gravity. If forward of the center of gravity, the flap movements will generate opposite effects, i.e., in pitch and yaw, negative pitch and yaw rather than positive. It should also be noted that the flaps could be deployed on any number and variety of vehicles. While the illustrated embodiment is of a hypersonic reentry vehicle, which in effect is the payload of a rocket booster and motor, the flaps could be disposed on the rocket booster, rather than the payload, so that control movements are made before separation of the payload from the booster. In general, the flaps can be mounted on any part of a vehicle. However, the configuration of four flaps operates best with axi-symmetric bodies, meaning those of nearly circular cross-section. Also, while the invention has been described with reference to bodies traveling at hypersonic speeds, it is expected that positive results will be achieved for supersonic vehicles as well. 
     It is additionally seen from the above that the flaps are not equally spaced around vehicle  20 . Flaps  26  and  28  are offset toward each other, as are flaps  30  and  32 . This balancing of flap offset allows vehicles employing the present control system to achieve pure pitch, yaw, and roll movements. Thus, use of flaps provides not only pitch and yaw control, but also bi-directional roll control with a minimum number of parts and minimal infringement of packaging envelope. Since the flaps are relatively small, mutual interference between the flaps is negligible. Small flap size also mitigates the rotational effect of a crosswind. The flaps allow for roll capability without the necessity of bank-to-turn flight, and thus the flaps provide some level of control in a fourth dimension—the axial translational dimension. 
     The present invention provides quick response and increased capability for difficult maneuvers such as the necessity to bleed off speed, navigate to a target, and meet angle of obliquity (flight path relative to the ground) and angle of attack (body orientation relative to flight path) requirements. 
     While the preferred embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not of limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus the present invention should not be limited by the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.