US3648955A

US3648955A - Method and means for producing divergence of plural ejection seat trajectories

Info

Publication number: US3648955A
Application number: US7679A
Authority: US
Inventors: Fred B Stencel; Walter R Peck
Original assignee: Stencel Aero Engineering Corp
Current assignee: Universal Propulsion Co Inc
Priority date: 1970-02-02
Filing date: 1970-02-02
Publication date: 1972-03-14
Anticipated expiration: 1989-03-14
Also published as: FR2079181A1; GB1342912A; DE2104464A1; FR2079181B1; DE2104464C2

Abstract

A method and means for producing divergence of the trajectories of plural ejection seats as such seats are ejected from an aircraft so that collisions will be avoided. As soon as each seat is ejected upwardly from the aircraft, a first force is applied to the seat at a point offset from the yaw axis of the seat to cause the seat to rotate about its yaw axis at a certain yaw rate. After such rotation at the yaw rate has occurred for a preselected interval of time, an opposed force is applied to terminate the seat rotation and the yaw rate so that the seat is reoriented from its initial trajectory path, but the yaw rate is substantially zero. Thus, when the occupant separates from the ejection seat, he will not be traveling with any yaw rate.

Description

Elnite Stats tet Stencel et a1.

[ Mar. 14, 1972 TRAJECTORIES [72] Inventors: Fred B. Stencel; Walter R. Peck, both of Asheville, N .C.

[73] Assignee: Stencel Aero Engineering Corporation,

Arden, N.C.

[22] Filed: Feb. 2, 1970 21 Appl. No.: 7,679

52 u.s.c1 ..244/122AD 51 lnt.Cl ..B64d25/10 58 Field ofSearch ..244/122,122.13,122.14,122.11,

[56] References Cited UNITED STATES PATENTS 3,186,662 6/1965 Martin ..244/122 3,222,015 12/1965 Larsen et al. ..244/122 X Primary Examiner-Milton Buchler Assistant Examiner--Carl A. Rutledge AttorneyRoylance, Abrams, Kruger, Berdo & Kaul [5 7] ABSTRACT A method and means for producing divergence of the trajectories of plural ejection seats as such seats are ejected from an aircraft so that collisions will be avoided. As soon as each seat is ejected upwardly from the aircraft, a first force is applied to the seat at a point offset from the yaw axis of the seat to cause the seat to rotate about its yaw axis at a certain yaw rate. After such rotation at the yaw rate has occurred for a preselected interval of time, an opposed force is applied to terminate the seat rotation and the yaw rate so that the seat is reoriented from its initial trajectory path, but the yaw rate is substantially zero. Thus, when the occupant separates from the ejection seat, he will not be traveling with any yaw rate.

10 Claims, 12 Drawing Figures PATENTEDMAR 14 m2 SHEET 1 [IF 4 2 m El lNVENTORfi c FRED B. STENCEL WALTER R. PECK ATTORNEY5.'

PATENTEBMAR 14 m2 SHEET 2 0F 4 INVENTQRS'.

FRED B. STENCEL WALTER R. PECK ATTORNEYS.

PATENTEDMAR 14 I972 sum 3 IJF 4 T M F INVENTORS, FRED B. STENCEL WALTER R PECK BY M, /%l

' Z I ATTORNEYS.

.PATENTEUMAR 14 I972 SHEET 0F 4 INVENTORS. FRED B. STENCEL WALTER R. PECK METHOD AND MEANS FOR PRODUCING DIVERGENCE OF PLUIRAL EJECTION SEAT TRAJECTORIES This invention relates to aircraft ejection seats as are customarily used for ejecting occupants from the cockpit of an aircraft under emergency conditions. More particularly, it relates to a means for assuring that plural or multiple ejection seats can be ejected from the aircraft substantially simultaneously but along diverging ejection trajectories so that the seats will not collide with one another and so that the occupants of the seats, when released therefrom, will not collide with each other or with a seat.

In many aircraft today, two or more ejection seats are provided and it is quite common for such seats to be arranged in tandem, either side by side or one behind the other. It is believed desirable to provide a system wherein such seats can be ejected from the aircraft simultaneously in the event of emergency so that either seat occupant can initiate the ejection operation. In the normal ejection operation, the initial thrust to propel the seat and occupant upwardly out of the aircraft is provided either by means of a rocket or a catapult, or both, and seat rails are customarily provided to guide the upward path of the ejecting seat. The positioning of the seat rails in combination with the aerodynamic forces applied by the airstream and the thrust forces applied by the rocket serve to control the trajectory or path of flight of the seat and the occupant. At some point shortly after the occupant and seat enter the airstream, the occupant is released from the seat automatically. One known method for accomplishing such man-seat separation is to provide snubbing lines which stop the flight of the seat while at the same time opening the lap belt or other coupling means which retains the occupant in the seat. In this fashion, the occupant continues to travel along the already determined trajectory path of the ejection seat and his parachute eventually opens.

It should be apparent and it has been recognized that if two or more ejection seats were simultaneously ejected upwardly without any trajectory correction, there is the possibility that such seats could collide with one another or that the occupants could collide with one another as they released from their respective seats or even that one occupant could collide with the other occupants seat. Additionally, it must be remembered that as the occupants are released from the seats, their parachutes are deployed into the airstream and it is essential that such parachutes do not become fouled or entangled with the seats or with the other occupants parachute lines or with the snubbing lines connected to the seat from the aircraft.

In view of the foregoing, it seems apparent that one method for solving the aforementioned problems is to provide a means where the trajectories of the ejection seats diverge from one another and while this can be most simply accomplished by means of a yaw divergence, such procedure in itself gives rise to certain difficulties and potential problems. The same is also true for a roll or pitch type divergence. These problems are caused by the fact that when yaw, pitch or roll is introduced into the upwardly ejecting seat, it causes movement at a certain rate which tends to continue even after the occupant separates from the seat. The problems which can arise from this rate are perhaps most easily understood when considered in connection with the introduction of a yaw rate, i.e., a rate of seat rotation about its yaw axis. If it is assumed that in order to accomplish divergence of the ejection seat trajectories, a certain force is applied to the ejection seat which causes it to rotate about its yaw axis, this force and this rotation will cause the desired divergence but the force will additionally start the seat spinning about its yaw axis at a certain rate, i.e., so many degrees of rotational spin per unit of time. Thus, because the seat is turning at a particular yaw rate, when the pilot or occupant is released from the seat, he too will be turning at this same yaw rate which means that he will be spinning about as he enters the airstream and as his parachute is deployed. If the pilot parachute on the occupant deploys into the airstream backwardly, which would occur in the event that the occupant 1 was facing backwardly in the airstream due to his spin rate, then there would be the tendency for the pilot chute to wrap around the occupant and prevent satisfactory parachute opening, which could be catastrophic. Further, if the occupant has a significant yaw rotational rate during the period of time wherein the pilot chute firstly deploys and inflates itself in the airstream and then subsequently deploys the main parachute and its suspension lines, the probability of parachute malfunction is greatly increased.

Thus, it is the object of the present invention to provide a means for producing divergence of plural ejection seat trajectories so that the seats or their occupants will not collide with one another, but additionally, it is the object hereof to accomplish such divergence without inducing a yaw rate which is transmitted to the occupant at the time that he is separated from the seat.

Another object of the present invention is to provide both a method and a means for accomplishing a yaw-type divergence of plural aircraft ejection seats so that such seats can be simultaneously ejected from the aircraft without fear of collision.

Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses certain embodiments hereof.

Referring now to the drawings which form a part of this original disclosure:

FIG. 1 is a diagrammatic elevational view showing the manner of producing divergence in accordance with the principles of the present invention.

FIG. 2 is a side elevational view of a typical ejection seat provided with means for applying thereto a yaw producing force;

FIGS. 3, 4 and 5, respectively, are diagrammatic elevational views showing the sequence of operations of the seat of FIG.

FIG. 6 shows in sequential views A, B and C, a different means for applying the yaw producing force to an ejection seat;

FIG. 7 is a diagrammatic view showing still another means for providing a yaw producing force on an ejection seat;

FIG. 8 is a diagrammatic elevational view showing the utility of a drogue parachute in combination with the present invention;

FIG. 9 is a perspective view showing a typical manner in which a drogue parachute can be connected with the ejection seat.

FIGS. 10, 11 and 12 respectively, are diagrammatic elevational views showing the sequence of operations of a seat in accordance with the principles of a modified embodiment of the present invention.

The foregoing objects are attained in the preferred embodiment by providing a pair of opposed rotational moment producing means attached to each ejection seat and are attained in a modified embodiment by the main sustainer rockets producing with programmed firing delays opposed rotational moments in conjunction with opposed sideward forces in ejection seats with seat bucket mounted sustainer rockets. As the ejection seat is propelled upwardly out of the aircraft, the first yaw producing force, which is applied offset from the yaw axis of the seat, causes the seat to start rotating about its yaw axis at a certain yaw rate. This yaw-type rotation will, of course, reorient the seat from its initial trajectory path. After a certain interval of time has elapsed, a second and 0pposed rotational force is applied to the seat to, in effect, produce an opposite rotational force about the yaw axis. This second rotational force thus cancels out the first one and stops the rotation of the seat about its yaw axis, thereby reducing the yaw rate to substantially zero. Even at this time, the pilot or occupant still remains attached in the seat and customarily, a main thrust rocket attached to the seat then operates to propel the seat even further away from the aircraft and into the airstream. Thereafter, the coupling means which holds the pilot in the seat is released and. the seat is snubbed or other rhino-I (new means are provided for enabling the occupant to separate from the seat and since the seat no longer has any yaw rate, the occupant will have no yaw rate either as he enters the airstream. This means that when his pilot chute deploys into the airstream, there will be no undesirable tendency for the pilot chute to wrap about the occupant.

It is believed that the principles of the present invention will become more apparent if reference is made to FIG. 1 wherein there is illustrated a fragmentary portion of an aircraft having a cockpit 12 within which two

identical ejection seats

14 and 16 are initially installed. Each of these ejection seats is preferably provided with a main thrust rocket 18 and the initial upward ejection path of each seat is preferably guided by means of upstanding ejection rails 20 coupled with rollers or slide blocks attached to the

seats

14 and 16.

The arrow 22 shown in FIG. 1 represents the axis of forward motion of the aircraft and thus represents the direction in which the aircraft and the seats therein are normally moving. When an ejection operation occurs, both the seat 14 and the seat 16 are simultaneously ejected upwardly out of the cockpit 12 and a first yaw producing force is applied to each of the seats. This first yaw producing force is designated 24 and is illustrated only by means of an arrow which shows the direction in which the force generally acts. The force 24 produces a yaw or rotation about the yaw axis to move each seat away from the axis of forward motion 22. Stated another way, the seat 14 in FIG. I is subjected to a counterclockwise yaw at a particular yaw rate, as shown by the arrow 26 whereas the seat 16 is subjected to a clockwise yaw at a particular yaw rate as shown by the arrow 28. After this yaw rate has been permitted to operate for a preselected interval of time to thus cause a certain rotation of the

seats

14 and 16 about their yaw axes, a second opposed yaw producing force 30 is applied, again as shown by an arrow. The force 30 acts in opposition to the force 24 and thus serves to cancel out the yaw rate and to reduce the same substantially to zero. Thus, after the second force 30 has been applied, each seat will be reoriented from its initial trajectory, but neither seat will thereafter have any marked tendency to rotate about its yaw axis. Actuation of the main rocket 18 on each seat will thus produce a forward force illustrated by the arrow 32 in FIG. 1 which moves each seat forwardly along its reoriented trajectory and further into the airstream. Subsequently, as diagrammatically shown in FIG. 1, the occupant designated P is released from the seat 14 and the occupant designated P is released from the seat 16.

For purposes of the present invention, it is not considered necessary to describe in any detail the manner in which the man-seat separation occurs, since there are several known possibilities for accomplishing such separation. One suitable means for accomplishing the man-seat separation is shown in copending application Ser. No. 728,945, filed May I4, 1968, in the name of Fred B. Stencel. Other suitable man-seat separation techniques include inflatable bladders, yokes which can be pulled taut to propel the occupant forwardly out of the seat, and so on. Ordinarily, for all of these various forms, a releasable coupling means in the form of a lap belt and shoulder belt is provided which holds the occupant in the seat until the separation sequence occurs. At that point, the lap belt is automatically opened and the man and seat are separated from one another, either by snubbing the seat or by propelling the occupant forwardly.

Referring now to FIG. 2, the seat 14 is shown for purposes of illustration ejecting upwardly from the seat rails 20. In FIG. 2, guide rollers 34 mounted along the sides of the seat cooperate with channels in the seat rail 20 as the upward ejection motion illustrated by the arrow 26 takes place. Along the outside of each ejection rail 20, a rod or cable 38 is provided with the lower end thereof attached at or adjacent the floor 40 of the aircraft and the upper end thereof attached to a stop or projection 42. A ring 44 is slidable along the rod or cable 38 until the ejection seat 14 just clears the rails 20 whereupon the ring 44 will contact the stop 42 and further upward movement will terminate. The ring 44 is attached to a stabilizing line 46 which connects to a cooperating device 48 attached to the seat so that a slack portion 50 of the line is provided on the other side of the cooperating device. Preferably, the stabilizing line 46 is of the energy absorbing type and the line 46 and cooperating device 48 can be of the type shown and described in copending application Ser. No. 778,244, filed Nov. 22, 1968, in the name of James E. Haile. If so, then the stabilizing line 46 can be regarded as a rendable web and the cooperating device 48 can be regarded as the pin or element which causes the rending to occur. Alternatively, the stabilizing line can be of the type shown and described in U.S. Pat. No. 3,l03,33 l is sued Sept. 10, 1963, in the name of Fred B. Stencel. In either event, it will be understood that as the seat 14 continues to move and the ring 44 is no longer able to move, a pulling force will be applied to the stabilizing line 46 which will tend to pull the line past the cooperating device 48, whether such device be a pin element, a die, or whatever. This force thus corresponds to the force 24 in FIG. 1 and since it is applied to the seat in a manner offset from the yaw axis of the seat, it tends to cause the seat to rotate about its yaw axis. The components of the force applied by the line 46 can be broken into components C1 and C2, as shown in FIG. 2, with the larger of these components C1 being the yaw inducing component while the smaller of the components C2 is the roll and pitch inducing component.

If reference is now made to FIGS. 3, 4 and 5, the seat shown in FIG. 3 corresponds to the seat as it is initially provided within the aircraft cockpit. As such, it can be seen that the rollers 34 are positioned within the channels of the guide rails 20 and both the seat 14 and the occupant P are facing straight ahead. As the ejection operation starts and the seat moves upwardly to the position of FIG. 2, thus clearing the ejection rails, the line 46 pulls taut and continues to pull the slack portion 50 thereof past the cooperating device 48, thus producing a force in the direction of the arrow 24 shown in FIG. 4. Since this force is offset from the yaw axis of the seat, it produces a rotation about this yaw axis at a particular rate and in the direction of the arrow 26 of FIG. 4. Meanwhile, another similar stabilizing line 52 and cooperating device 54 are provided on the opposite side of the seat 14, but the line 52 is of such a length that it does not pull taut at the same time as the line 46 pulls taut. This necessarily means that there will be a time delay after the force 24 is applied until the line 52 pulls taut and during this time delay, the seat is undergoing yaw in the direction of the arrow 26 at a particular yaw rate. When the line 52 pulls taut as shown in FIG. 5, and the line 46 runs out, then the force will be applied in the direction of the arrow 30 of FIG. 5 to oppose and substantially cancel out the yaw rate of the seat 14. In one embodiment of the invention, the seat will be displaced through a yaw angle of approximately 25 in the interval of time between application of the force 24 and application of the force 30. Then, when the force 30 is applied, the seat will move through slightly more of an angle due to inertial effects and when the yaw rate is finally canceled out, the seat will have moved through a yaw angle of approximately 30. The line 52, of course, as shown in FIGS. 4 and 5, terminates adjacent the ejection seat rails 20 with a ring 44 cooperative with the rod or cable 38 in the manner previously described in conjunction with the line 46. After the seat has been reoriented to the position shown in FIG. 5 and is no longer experiencing any yaw rate or change of yaw angle, the continuing thrust of the rocket 18 will displace the seat forwardly until the snubbing lines cause man-seat separation.

As an alternative to the use of stabilizing lines or lanyards such as the

lines

46 and 52, it is possible to use opposed yaw rockets, as shown in FIG. 6. The rocket with its nozzle 62 is disposed on one side of the yaw axis Y while the rocket 64 with its nozzle 66 is disposed on the opposite side of the yaw axis. In FIG. 6A, neither of the rockets has yet fired. In FIG. 6B, the yaw rocket 60 has fired to produce an effective rotational force about the yaw axis as previously designated 24. In FIG. 6C, the other rocket 64 is fired to produce the counteracting yaw force 30 which stops the yaw rate and change of yaw angle.

In FIG. 7, there is shown still another possible embodiment of the present invention, and although this embodiment is not preferred, it is nevertheless considered operative. That is in FIG. 7, the first force 24 is applied by means of the lanyard 46, as previously described. The second or counteracting force along the line 30 is provided by canting the nozzle 68 on the main rocket so that it acts along an angular line of thrust designated 70. The components of the thrust force produced by this canted nozzle 68 will serve to provide the force 30 which counteracts the yaw force 24 induced by the stabilizing line 46.

In FIGS. 10, II and 12 there is shown an embodiment of the present invention applicable to all ejection seats using dual seat bucket mounted sustainer rockets. The advantage of this modified embodiment of the invention lies in its use of the existing sustainer rockets to achieve the desired objective of lateral seat displacement and motion without yaw rotational rates which continue beyond the time of man-seat separation. Referring now to FIG. it is seen that the seat bucket mounted sustainer rocket 90 is ignited prior to ignition of its matching sustainer rocket 91, preferably concurrently with the booster catapult separation to assure thrust continuity, and such ignition generates a horizontal component of thrust 92 which is directed forward and sideward. This force 92 passes forward of the ejected mass center-of-gravity, designated C.G., producing a clockwise moment which causes the seat and pilot to rotate about the yaw axis in the direction of the arrow 28, and additionally accelerates the seat forward and to the right. After a short time delay the sustainer rocket 9I ignites as shown in FIG. 11 to produce a thrust force 93 which intersects the force 92 forwardly of CG. Then, for another short time period, both sustainer rockets are thrusting with approximately zero net moment acting about the yaw axis. With these conditions existing the seat continues to rotate clockwise and to accelerate to the side under the action ofthe thrust of both sustainer rockets.

In FIG. 12, after the sustainer rocket 90 which was ignited first burns out, the sustainer rocket 91 continues to burn for a time equal to its ignition time delay. During this period a counterclockwise moment from the thrust force 93 exists which will reduce the seat rotation in yaw to a very negligible value but will not appreciably reduce the sideward velocity of the seat and pilot.

It should thus be apparent that it is possible to use stabilizing lines of the energy absorbing type, yaw-type rockets, or a canting of the main rocket nozzle, or any combination thereof as the means for providing the two separate and opposed yaw inducing forces necessary for the present invention. Further, it should be apparent that it is possible to use a time delay of one sustainer rocket in those ejection seats which have seat bucket mounted sustainer rockets to achieve two separate and opposed yaw inducing forces necessary for the present invention without the addition of any other force generating devices.

In FIG. 8, the seat 14 is shown with a drogue parachute 72 connected by a line 74 to the main rocket 18 which is mounted centrally on the back of the seat 14 and the seat 16. The purpose for utilizing such a drogue parachute in conjunction with the present invention is to provide a neutral stability so that the aerodynamic forces will not interfere with the position of the seat. This would be particularly true if the center of gravity, designated CO. in FIG. 8, is displaced slightly from the center of the seat. In such event, this displacement would produce a moment arm X which tends to provide a couple which either adds to or detracts from the yaw rate, depending upon the direction in which the center of gravity is offset. The provision of the drogue parachute 72, however, provides a force which counteracts the aerodynamic forces acting through the moment arm X and which thus tends to give the seat a more or less neutral stability, regardless of the air speed at which ejection occurs. In FIG. 9, it is shown that the line 74 from the drogue parachute terminates in a loop '76 which passes around the main rocket 18 mounted within the back of the housing between the upper and

lower braces

80 and 82. The vertical position of the loop 76 can be maintained by attaching the same to guide

webs

84 and 86 which extend between the upper and lower braces and 82.

After reading the foregoing detailed description, it should be apparent that the objects set forth at the outset of the specification have been successfully achieved by the present invention. Accordingly,

What is claimed is:

l. A method for producing divergence of plural ejection seat trajectories comprising the steps of:

ejecting plural ejection seats substantially simultaneously upward out of an aircraft cockpit in which such seats are initially installed;

applying a first yaw producing force to each seat to start each seat rotating at a yaw rate and in opposed directions;

applying a second yaw producing force to each seat at a predetermined time after said first yaw producing force is applied;

said second yaw producing force being applied in opposition to said first yaw producing force to substantially terminate said yaw rate and said seat rotation.

2. A method as defined in claim 1 wherein said first yaw producing force is applied to each seat to rotate each seat away from the axis of forward motion of said aircraft.

3. Means for producing divergence of the trajectories of plural ejection seats which are initially housed in the cockpit of an aircraft, said means comprising:

a pair of opposed rotational force producing means connected to each seat;

means for actuating one of said pair of opposed rotational force producing means substantially when said seat is ejected upwardly out of the aircraft cockpit to start said seat rotating about its yaw axis at a certain yaw rate; means for actuating the other of said pair of opposed rotational force producing means at a predetermined time after said first rotational force producing means has been actuated; said other force producing means, when actuated, causing said seat to substantially stop its rotation about its yaw axis;

at least two such ejection seats being provided, the first actuated of said rotational force producing means acting in one rotational direction on one seat and acting in the opposite rotational direction on the other seat to cause divergence of said seat trajectories.

4. Means as defined in claim 3 wherein releasable coupling means are provided for keeping the occupant of said ejection seat in the seat for a predetermined time after ejection and until said other rotational force producing means has been actuated so that the yaw rate will be substantially zero when said occupant is released from said seat.

5. Means as defined in claim 3 wherein one of said pair of opposed rotational force producing means comprises a stabilizing line connected to the seat offset from the yaw axis thereof so that when said line pulls taut, the seat is caused to rotate about its yaw axis.

6. Means as defined in claim 5 wherein said stabilizing line is of the energy absorbing type.

7. Means as defined in claim 3 wherein one of said pair of opposed rotational force producing means is a rocket connected to said seat with its nozzle offset from the yaw axis thereof so that when said rocket fires, the seat is caused to rotate about its yaw axis.

8. Means as defined in claim 3 wherein each seat includes a main thrust producing rocket attached to the rear thereof and wherein the nozzle of the main rocket is canted to provide an offcenter thrust which provides one of said pair of rotational force producing means.

9. Means as defined in claim 3 further including a drogue parachute connected centrally to the rear of each seat to cancel any seat instability caused by aerodynamic moments.

10. Means as defined in claim 3 wherein each seat includes dual seat bucket mounted sustainer rockets wherein the ignition of one sustainer rocket is delayed to provide initially an offcenter thrust in the desired direction and to provide subsequently an equal offcenter thrust in the opposite direction.

nu unn'a-r Inc:

Claims

1. A method for producing divergence of plural ejection seat trajectories comprising the steps of: ejecting plural ejection seats substantially simultaneously upward out of an aircraft cockpit in which such seats are initially installed; applying a first yaw producing force to each seat to start each seat rotating at a yaw rate and in opposed directions; applying a second yaw producing force to each seat at a predetermined time after said first yaw producing force is applied; said second yaw producing force being applied in opposition to said first yaw producing force to substaNtially terminate said yaw rate and said seat rotation.

3. Means for producing divergence of the trajectories of plural ejection seats which are initially housed in the cockpit of an aircraft, said means comprising: a pair of opposed rotational force producing means connected to each seat; means for actuating one of said pair of opposed rotational force producing means substantially when said seat is ejected upwardly out of the aircraft cockpit to start said seat rotating about its yaw axis at a certain yaw rate; means for actuating the other of said pair of opposed rotational force producing means at a predetermined time after said first rotational force producing means has been actuated; said other force producing means, when actuated, causing said seat to substantially stop its rotation about its yaw axis; at least two such ejection seats being provided, the first actuated of said rotational force producing means acting in one rotational direction on one seat and acting in the opposite rotational direction on the other seat to cause divergence of said seat trajectories.

8. Means as defined in claim 3 wherein each seat includes a main thrust producing rocket attached to the rear thereof and wherein the nozzle of the main rocket is canted to provide an off-center thrust which provides one of said pair of rotational force producing means.

10. Means as defined in claim 3 wherein each seat includes dual seat bucket mounted sustainer rockets wherein the ignition of one sustainer rocket is delayed to provide initially an off-center thrust in the desired direction and to provide subsequently an equal off-center thrust in the opposite direction.