Yaw and Pitch Control of Air Vehicles at High Angles of Attack

The invntion relates generally to the control of air vehicles at high angles of attack and more specifically concerns controlling the vortex patterns around forebodies to control the yaw and pitch during high angle of attack maneuvering of air vehicles, when conventional control surfaces become ineffective due to massive flow separation on lifting surfaces.

The current trend toward enhanced post-stall maneuverability of tactical aircraft demands control capability at angles of attack well above the range of conventional aerodynamic control surfaces. A pertinent example is the aft rudder, which rapidly loses effectiveness when immersed in the low-energy separated flow from the wing, requiring consideration of alternate means of yaw control.

An approach of considerable interest is to exploit well-known vortex asymmetry occurring on pointed forebodies at high angles of attack, which usually results in sizeable side forces acting well forward of the center of gravity. The prior art has investigated the use of air injection near the forebody nose to control the vortex asymmetry. An effective yaw power was demonstrated at high angles of attack on a particular forebody whose flattened cross-section favored the generation of strong vortices; the jet injection in this concept presumably acts as a fluidic switch to control the direction and degree of vortex asymmetry.

The question arises in the use of the jet injection technique whether a comparable level of yaw power would be obtainable when the prevailing forebody vortex system was not strong (e.g., on forebodies of shorter length or those having a circular or upright oval cross-section). The jet injection technique also was found somewhat limited in terms of the linearity control and effectiveness at large sideslip angles. In addition, to ensure jet fluid supply at high angles of attack when engine bleed air cannot be relied on the jet injection technique recommended a solid propellant blowing system. Such a system, however, has inherent control complexity, duration limitations, and servicing requirements.

Narrow, sharp-edge strakes have frequently been applied to pointed slender forebodies for modifying the high-alpha vortex shedding characteristics, as a solution to the zero-sideslip asymmetric problem as well as to improve lateral-directional stability. Unfortunately, fixed strakes have shown deleterious side effects such as high-alpha pitch up, diminished yaw damping, and marked reductions in dihedral effect.

It is an object of this invention to utilize a simple technique, exploiting the vortex asymmetry occurring on pointed forebodies at high angles of attack, for controlling yaw at hight angles of attack, that does not have the complexities and disadvantages of prior techniques.

Another object of this invention is to utilize a vortex technique for controlling pitch at high angles of attack.

A further object of this invention is to utilize deflectable strakes on the forebodies of air vehicles to control yaw and pitch at high angles of attack.

Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings. For example, a symmetrical pair of strakes (later disclosed) alleviates the zero-sideslip force and yawing moment from the forebody at high angles of attack.

The invention is a method and apparatus for controllng the yaw and pitch of air vehicles at high angles of attack. Two symmetrical strakes are longitudinally hinged along hinge lines near the outer surface of the forebody of an air vehicle. The strakes are symmetrically hinged on opposite sides of the forebody. A control is located on the air vehicle for controlling the deflections of the two strakes independently. When one of the strakes is deflected with the other one remaining undeflected, or when one is deflected upward and the other one is equally and simultaneously deflected downward the vortex patterns around the forebody are altered resulting in yawing moments. Also, when both strakes are equally and simultaneously deflected either upward or downward in synchronism pitching moments result.

FIG. 1 is a drawing of the embodiment selected for illustrating the invention;

FIG. 2 is a drawing showing two strakes;

FIG. 3 is a cross-section of a forebody of an air vehicle showing strakes for generating a yaw control of the air vehicle at high angles of attack;

FIG. 4 is a graph demonstrating the yaw control of FIG. 3;

FIG. 5 is a graph demonstrating the yaw control of forebodies having triangular cross-sections;

FIG. 6 is a schematic cross-sectional drawing of a forebody with strakes showing anti-symmetric deflections of the strakes for the purpose of controlling the yaw at high angles of attack;

FIG. 7 is a schematic cross-sectional drawing of a forebody with strakes showing symmetric deflections of the strakes for the purpose of controlling the pitch at high angles of attack; and

FIG. 8 is graphs demonstrating the yaw control of FIG. 2.

Turning now to the embodiment of the invention selected for illustration in the drawings the number 11 in FIG. 1 designates a strake that is attached to the forebody 12 of an air vehicle 13. Strake 11 is attached to forebody 12 by means of a hinge 14 such that strake 11 can be deflected through an angle of approximately 180 degrees. A second symmetrical strake (not shown in FIG. 1) is symmetrically attached to the opposite side of forebody 12 by means of a hinge such that the second strake can also be deflected through approximately 180 degrees. A strake controller 15 located on board air vehicle 13 controls the deflections of the two strakes. The manner in which the deflections of the two strakes are controlled will be disclosed later in the specification. Strake controller 15 can be any conventional type controller and can be operated electrically, hydraulically or pneumatically. Controls that can be used as strake controller 15 are obvious to one having ordinary skill in the art and are therefore not disclosed in detail in this specification.

In FIG. 2, a view of forebody 12 in which the left strake 11 is conformed and the right strake 16 is deployed. Strakes 11 and 16 are fitted into slots cut into the forebody.

FIG. 3 shows a cross-section of forebody 12 containing strake 11 and the second strake 16 mounted for deflection on a hinge 17. The angle that a line 18 through the center of the cross-section and perpendicular to the line of hinge 14 makes with the vertical axis 19 of the cross-section is the strake location angle. Note that hinge 16 has the same strake location angle as hinge 14 except in the opposite direction. During one mode of operation of this invention strake controller 15 deflects hinge 16 while not deflecting strake 11. This forces a stable, asymmetric vortex pattern on the forebody at high angles of attack, thereby generating a side force component 20. The side force 20 acting considerably forward of the air vehicle 13 center of gravity produces a yawing moment. Reliance on controlled asymmetry of forebody vortex separation allows this yawing moment to continue unabated to high angles of attack, unaffected by flow separation on wings and aft surfaces. Variations in the deflection of strake 16 changes the magnitude of side force 20. Deflecting strake 11 with strake 16 undeflected produces side forces in the opposite direction to that of side force 20.

The symmetric placement of strakes 11 and 16 at different strake angles on forebody 12 will produce different results. Each air vehicle that utilizes this invention should be tested at different strake location angles to determine which strake location angle produces the most desired results. A delta-wing-fuselage-vertical tail model similar to air vehicle 13 has been wind tunnel tested to determine that 60.degree. to 120.degree. locations of the strakes offer desirable results. The data of this test and tests of other modes of operation of this invention are disclosed in a paper AIAA-86-0333 by D. M. Rao and D. G. Murri, "Exploratory Investigation of Deflectable Forebody Strakes for High-Angle-of-Attack Yaw Control," presented at the AIAA 24th Aerospace Sciences Meeting on Jan. 6-9, 1986 at Reno, Nev.

FIG. 4 shows a graph of .DELTA.C.sub.n, where C.sub.n is the yawing moment coefficient, versus .alpha., the angle of attack, for the mode of operation in FIG. 3 for two different strake location angles and for a 30.degree. rudder. Note that angles of attack above 20.degree. the rudder begins to lose its effectiveness and at angles of attack above 40.degree. the rudder becomes completely ineffective. Yet the strakes start to be effective at 20.degree. and remain effective through 70.degree..

A cross-section of forebody 12 as disclosed is circular; however, it can have different shaped cross-sections without detracting from this invention. For example, FIG. 5 discloses a graph of .DELTA.C.sub.y, where C.sub.y is side force coefficient, versus angle of attack for a typical rudder and different triangular forebody orientations utilizing this invention. Note that at angles of attack above 40.degree. when the rudder becomes ineffective the triangular shapes remain most effective.

Thus far a mode of operation has been described in which one strake is not deflected and the other strake is deflected through different angles to provide yaw control at high angles of attack. Another mode of operation is to initially deflect both strakes to some angle, say 0.degree. (strakes are perpendicular to the forebody), however other angles can be used. Then the two strakes are deflected equally and anti-symmetrically as shown in FIG. 6. Both of the strakes 11 and 20 are shown being deflected counter-clockwise to produce a yaw force in one direction. If both of the strakes are deflected in the clockwise direction, a yaw force in the opposite direction will be produced. The term "anti-symmetrically controlling" as used in the claims is intended to cover both the mode of operation in FIG. 6 and the mode of operation in FIG. 3.

In a further mode of operation, both of the strakes 11 and 16 are deflected equally and symmetrically as shown in FIG. 7. That is, the two strakes are deflected in unison, one in the clockwise direction and the other in the counter-clockwise direction. This mode of operation varies the normal force on the forebody and thereby provides a pitch control without inducing a yawing moment.

Normally without strakes the vortex causes an unwanted yaw side force at high angles of attack due to an unsymmetrical vortex acting on the forebody. With the double strake modes of operation, the unwanted yaw forces are substantially reduced. That is, with the anti-symmetric deflections of the strakes in FIG. 6 and the symmetric deflections of the strakes in FIG. 7, the unwanted yaw side forces will be decreased.

FIG. 8 is a graph of experimental results for a delta wing configuration as shown in FIG. 1 and with strake location angles of 105 degrees as shown in FIG. 2. Both single strake and dual strake operations are shown.

The advantages of this invention are that it provides a simple and easy to install yaw and pitch control for air vehicles at high angles of attack while avoiding the system complications of jet techniques. Also in the pitch control mode of operation the symmetrical strakes prevent side force development at high angles of attack.

Although a specific embodiment of the invention has been described herein, variations in the specific embodiment will be apparent to those skilled in the art in light of the above teachings without departing from the spirit and scope of the appended claims. For example, strake location angles greater than or less than 90.degree. can be used. Although this invention has been disclosed with a delta wing configuration, it can be used with other configurations, for example a trapezoidal wing configuration without departing from the invention.