US3014280A - Apparatus for correcting bombsights - Google Patents

Description

Dec. 26, 1961 R. D. WYCKOFF APPARATUS FOR CORRECTING BOMBSIGHTS 2 SheetS-Shet 1 Filed June 19, 1946 ,I 1 ;c' 3'. Z

BOMB RELEASE COURSE OF BQMBING PLANE SIGHTING ANGLE Rn Do lv RALPH D. WYCKOEFF 2 Sheets-Sheet 2 -Ill R. D. WYCKOFF APPARATUS FOR CORRECTING BOMBSIGHTS ,i 1 ';t'c5

ELEVATOR JNTEGRHTOR Dec. 26, 1961 Filed June 19, 1946 RUBBER INTEGRHTO'R RALPH D. WYCKOFF United States Patent 3,014,280 APPARATUS FOR CORRECTING BOMBSIGHTS Ralph D. Wyckolf, Pittsburgh, Pa., assignor to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed June 19, 1946, Ser. No. 677,879 12 Claims. (Cl. 3346.5)

This invention relates to improvements in bombsights, and in particular concerns apparatus whereby the bombing accuracy elicited by such a bombsight is greatly improved when the same is used in conjunction with and for the aiming of dirigible bombs of the high-angle type.

In the bombing of targets from airplanes it has been found possible to greatly increase the accuracy by employing a remotely controllable or steerable bomb. Examples of such bombs are described in copending applications, one of which is entitled Dirigible Bomb by Wyckoif, Molnar, Palmer and Blewett, Ser. No. 673,374, filed May 31, 1946, now Patent 2,466,528, and another entitled Dirigible Bomb by Wyckoif, Fitzwilliam and Salvetti, Ser. No. 673,372, filed May 31, 1946, now Patent 2,495,304. Both of the above described dirigible bombs are remotely steered by radio signals. They are launched at a relatively opportune moment by reference to a conventional bombsight which may for example be of the well-known Norden type. It has been found that for certain types of dirigible bombs it is advantageous to follow the course of the bomb after launching by observation through the bombsight and make the necessary steering corrections by reference to an index in the field of view. This invention comprises the method and apparatus necessary for correcting the bombsight for errors in timing which are introduced as a result of the bombardiers eiforts to steer the bomb after release.

The type of high-angle dirigible bomb, described in the abovementioned Wyckoif, Molnar, Palmer and Blewett application is controlled only in azimuth and is best steered by direct sighting after release from the bombing airplane. Thus, after the release the bombardier dispenses with the sight and follows the bomb with unaided eye, a flare on the tail of the bomb providing adequate visibility throughout its fall to the target. Azimuth errors of the order of ten feet may thus be distinguished even from altitudes of 20,000 ft.

An improved type of high-angle dirigible bomb, which is described in the abovementioned Wyckoif, Fitzwilliam and Salvetti application, may be controlled in both azimuth and range and requires a special bombsight in order to provide a criterion for range steering. It will be clear that in direct observation of the bomb in flight from the vantage point of the bombardier in the bombing airplane, the bomb (flare) appears to travel horizontally along the ground, all impression of vertical perspective having disappeared in the first few thousand feet of fall because binocular vision loses its effectiveness at great distances. Thus the observer, stationed in the bombing airplane in continued straight-line flight directly over the falling bomb, is quite incapable of judging the height of the bomb above the target area and cannot predict the time of impact. Such prediction of the time of impact is essential for an accurate hit.

The obvious remedy and indeed the only means of accomplishing range steering by direct sight without prediction of the moment of impact is to maneuver the bomb, the air-plane, or both so as to eclipse the target by the bomb. If this be accomplished accurately, the bomb must hit the target regardless of just when impact occurs. Unfortunately this method of range sighting requires far more maneuverability of the bomb than is attainable with the usual type of high-angle dirigible bomb. Moreover,

the combined bomb and bomber maneuver requires a maneuver of the bombing plane that is rather drastic for the usual heavy bomber and hence is considered impractical for such airplanes.

The alternative to direct sighting of a range controlled type of bomb is to effect range steering by the use of a special bombsight in which both the bomb and the target are visible and the instant of impact is predicted by a suitable sight mechanism. This invention comprises a method and apparatus for accurately adjusting this prediction in accordance with the direction and amount of steering applied to the bomb during the course of its fall.

From any given altitude the time of flight of a particular type of standard bomb is constant within 0.1-0.2 second, the principal cause of variation being uncertainty in the exact altitude above the target at which the release of the bomb occurs. This fact is used in all standard bombsights. 'For example, in the well-known Norden sight this predicted time of fall is put into the bombsight mechanism by the bombardier as soon as his bombing altitude is determined. Similarly, with a range controlled bomb, suitable ballistic tables may be prepared giving the average time of fall of the bomb for the particular altitude desired. To the extent that such tables provide prediction accurate to a first approximation of the time of fall of the bomb, this time put into the bombsight will provide the criterion for predicting its time of impact. The principle of operation of such a sight can be illustrated by the following considerations and by reference to FIGURE 1 which shows a schematic diagram of a bombing run and the angles and bomb path to be considered. FIGURE 1 also shows diagrammatically the geometrical relationship between bomber, bomb and target at the release of the bomb, at an intermediate instant during the steered fall of the bomb, and at the moment of impact.

It is customary for the bomber airplane to be flying at a constant and accurately known altitude above the target and in a straight line headed directly over the target as illustrated in FIG. 1. At the start of the bombing run the bombardier will sight the target at an angle to the vertical and as he approaches the target this sighting angle will gradually decrease to zero when he is directly over the target. Now suppose that at an appropriate time during this approach the bomb is released and the bomber continues on course with constant speed. Since at release the bomb has the same horizontal velocity as the bomber, and neglecting wind resistance, it will continue its forward motion with undiminished velocity. From the bombardiers viewpoint the bomb will appear to fall in a vertical line directly below him and if he were provided with a vertical telescope the bomb would remain in the field of view of this telescope throughout its fall. In an actual case wind drag on the falling bomb cannot be neglected, and, instead of remaining in a vertical line projected downward from the moving bomber, the bomb will actually trail behind the vertical at a substantially constant angle known as the trail angle. However, for the purpose of this discussion we will continue to ignore the trail angle and assume that the bomb follows a truly parabolic trajectory in space; hence, as seen from the bomber traveling at an equal forward velocity, the bomb will appear to remain vertically below the observer.

Now let us equip this same telescope with a split field of view obtained by interposing a suitable moving mirror on its optical axis, such that by adjusting the angle of this mirror the bombardier simultaneously can obtain a line of sight forward of the vertical into the target. At the instant of release of the bomb these two lines of sight (the vertical containing the bomb and the one to the target) will have a certain angle called the dropping angle. If we now provide a motor mechanism to rotate the moving mirror so as to cause this angle gradually to decrease to zero, the bomb and the target can be made to remain in view simultaneously throughout the drop provided the proper goemetrical considerations are introduced and the proper rate of closure of the target angle to the vertical, or bomb angle, is maintained. Moreover, if the total time of closure of the two lines of sight is made exactly equal to the time of fall of the bomb, impact of the bomb will occur at the instant the two lines of sight coincide. Now let us suppose that the bombardier steers the bomb in such a manner that, in his sight, the bomb is always superposed on the target. This assures that the bomb is eclipsing the target at the instant his two lines of sight coincide, and since the timing of this closure has been made to agree with the predicted impact time, the bomb must hit the target.

With such a sight the only factor which could possibly cause an error in the hit would be failure of the impact to occur at the predicted instant that the two sighting angles coincide. With sufficient maneuverability available in the bomb and sufficient skill in manipulation of the steering controls to permit the bombardier to maintain superposition of the bomb and target images in his sight, there are no limitations imposed other than accurate timing. Thus the vertical line of sight need not be actually vertical, the dropping angle need not be that appropriate to an accurate hit, and the bombing plane need not maintain a perfectly constant speed or straight-line course. In fact, the only requirement is that the bombardier be able to keep the bomb and target in view in his sight and have sutficient maneuverability in the bomb to bring the two images into eclipsing position before impact occurs. This will be evident on considering that in effect, the bombardier has two lines of sight, the target being at the end of one line, with the bomb traveling down along the other at such a rate that when the angle between the two lines is closed to zero, the bomb has traveled exactly the vertical distance to the target. In the view through the sight, impact of the bomb will occur exactly when the angle closes to zero and it will be on the target, and to an observer on the ground the actual space trajectory of the bomb will intersect the ground at the target point.

This independence of steering on all factors except timing is, of course, indicative of the practicability of removing sighting or bombing release errors of all kinds and also of producing a hit on a target which has maneuvered out of position after release of the bomb. However, it is also evident that if the bomb has been sighted and released perfectly so that no steering is required, the bombardier would be aware of this fact only if the sight has been designed not only for proper timing of the closure of the bomb and target angle, but also with the proper type of motion in the moving mirror to track perfectly for a normal trajectory.

A modification of the well-known Norden bombsight may be made to fulfill the above requirements. The mechanism of the Norden bombsight through a suitable rotating mirror in effect maintains the objective of the sighting telescope pointed toward the target. The modification required is the addition of an auxiliary bomb-tracking mirror which comprises a fixed partial mirror at the entrance or objective end of the bombsight telescope, this mirror being placed at such an angle so as to provide a fixed vertical" field of view. As is well known, this vertical is maintained substantially in the true vertical by the Norden sight gyro stabilizers.

For the sake of simplicity, I have here spoken of the line of sight to the bomb being in the vertical. This would be true only in a vacuum in which the bomb trajectory would be truly parabolic. In practice, air resistance introduces an aerodynamic drag on the bomb which causes it to trail behind a true parabola by an angle which is almost constant and which is known as the trail angle. This detail, which may be allowed for in the adjustment of th bombsight, is not pertinent to the understanding of my invention and need not be considered further.

The above discussion is presented to permit a proper understanding of the type of bombsight used in steering high-angle dirigible bombs in order that my improvement on this sight may be comprehended.

It has been mentioned that fundamentally all bombsighting techniques are based on timing of the sight to match the predicted time of fall of the bomb. The fact has been stressed that the accuracy of the above described modified bombsight for use with dirigible bombs is very definitely predicated on the degree of precision with which this equality in timing is attained. It should be clear, for example, that if the bombardier has steered the bomb so that it is eclipsing the target when the lines of sight to the target and the bomb coincide, the bomb must have arrived at the ground at exactly that instant of coincidence if a perfect hit in range is secured. If, however, the fall of the bomb has exceeded the predicted time, it will still be in the air over the target at the moment of coincidence and hence will overshoot the target though the bombardier would be unaware that this overshoot was impending. Similarly, if the time of fall is less than that predicted, the bomb must undershoot the target. Evidently the amount of this error will be the total error in timing At multiplied by the horizontal component V or ground velocity of the bomb, namely (ALV In the case of standard bombs or, in fact, dirigible bombs which are allowed to fall uncontrolled, the time of fall of any individual bomb from a given altitude will vary but little from the average time of fall of bombs of identical weight and structure. By using modern radioaltimeters the altitude of the bomber over the target may be determined with an error of the order of :50 ft, which may be ignored. Thus for normal uncontrolled bombs the actual time of fall will usually be within :0.1 second of the value obtained from the ballistic tables for the particular type of bombs involved.

This constancy of the time of fall for a given altitude is, of course, predicated upon a constant drag coefficient or air resistance. Now in the case of dirigible bombs, the drag coefficient has a fixed value only in an uncontrolled drop, for steering the bomb involves aerodynamically derived lift forces which result in increase in wind resistance due to induced drag. Thus the controlled bomb is subjected to the normal drag plus an induced drag due to steering, with the result that such a bomb will, in general, fall slower by an amount depending upon the amount of steering required. 'For example, in actual practice with considerable errors to be corrected, the time of fall may be as much as 2.0 seconds greater than for an uncontrolled bomb and, appearing as a tinting error, would result in overshooting the target 300 ft. or more on a typical 15,000 ft. drop, even though the bombardiers steering appeared perfect in his sight. It is the purpose of the present bombsight improvement to minimize such errors by applying suitable compensating corrections to the timing of the bombsight.

Since, in general, the effect of steering is always to increase the time of fall it is possible to ameliorate the resulting error by using ballistic tables in which an increment of time has been added to the time of fall for an uncontrolled bomb, the added increment being such as to compensate for an average amount of steering. However, bombing errors encountered in practice, or the bombing of a maneuvering target, may involve a wide diversity in the amount of steering required to afiect a hit. Hence it is impossible to derive an average time increment applicable to all cases. On the other hand, I have found that there is a reasonably accurate relation between the amount of steering and the increment added to the time of fall which permits reasonably accurate prediction of the time increment which may then be applied as a correction to the timing of the bombsight.

When this correction is made by the method and apparatus of this invention a very high degree of bombing accuracy is obtained.

It will be further evident that if up-elevator is applied to a dirigible bomb, the left brought into play has an upward component which opposes the acceleration of gravity, thus slowing up the fall. This is an effect distinct from the induced drag and is additive thereto. On the other hand, down-elevator will produce a lift force having a downward component aiding gravity and hence speeding the fall. Therefore, in the case of down-elevator the downward lift component and the induced drag are opposite in sign, thereby requiring a smaller net correction. Thus, in the case of elevator applications, in addition to the induced drag there is an additional effect which is additive in the case of up-elevator and subtractive for down-elevator. Thus the correction required is one which varies also with the direction of steering applied to the bomb.

Having discovered that a predictable relation exists between the duration of rudder and/or elevator applications and the increment of time added to the normal time of fall, it is accordingly an object of this invention to provide means for adjusting the bombsight whereby this predicted time increment may modify the bombsight timing to eliminate the range error normally caused by steering effects which modify the normal ballistic time.

It is a further object of this invention to provide means for introducing into a bombsight a time increment correction in proportion to a predicted time increment caused by steering elfects which modify the normal ballistic time.

It is a further object of this invention to provide means whereby the timing of a bombsight may be continually adjusted while the bomb is falling for time errors introduced by steering of a dirigible bomb.

It is a still further object of this invention to provide means whereby the timing of a bombsight may be adjusted while the bomb is falling, such adjustment being made at a rate which varies with the direction of steering applied to the bomb.

It is a still further object of this invention to provide a bombsight for use with a dirigible bomb, such that when used together with a bomb controllable in both range and azimuth allows attainment of a bombing accuracy many times greater than has heretofore been attainable.

In the dirigible type bombs described in the abovementioned copending applications, control is achieved by yawing or pitching the entire bomb under control of the rudder or elevator flaps, in order to produce the necessary aerodynamic lift forces. The simplest form of control described therein is an on-oif type of control, so that with control applied the bomb responds with its full and constant trim angle of attack. Thus the induced drag accompanying a control is repeated in full magnitude with each application. This type of control is advantageous over proportional type of control for purposes of making the timing corrections. Moreover, since the drag iricrements due to yaw or pitch are equal, and because the curve of induced drag vs. yaw angle is known to follow a square law, the drag eifects of yaw and pitch will be additive if both are applied simultaneously. Actually, because of somewhat complex considerations this simple relation does not apply exactly throughout the trajectory. However, in an average drop the steering is usually applied during approximately the same part of the flight and as a result statistical data has shown that for a given dirigible bomb a numerical constant b may be derived such that when multiplied by the total duration of rudder application the incremental time of fall is obtained with reasonable accuracy. Similarly, this same constant would apply to elevator applications insofar as induced drag is concerned, but there are other factors involved in the elevator application.

Again statistical data provides a numerical constant a which multiplied by the total time of up-elevator application gives the total increment of time added to the fall. Similarly the effect of down-elevator may be expressed by a third coefiicient c which, depending on the relative magnitude of the induced drag and the downward lift component, may be either slightly positive or negative in value depending on the particular type of bomb.

The derivation from statistical ballistic data of the coefficients a, b, and c and their significance have been shown. Thus:

ue ue At =CT At =bT (3) where:

At =total added time of flight due to up-elevator application.

At =total added time of flight due to down-elevator application.

At =total added time of flight due to rudder applications.

T T T =total duration of up, down, or rudder applications, respectively.

In principle, therefore, I have found that, to a close approximation the incremental time of fall affecting a dirigible bomb may be represented as a product of the integrated time of application of a particular control and a numerical coefiicient.

In a simple embodiment of a correcting device a small motor may be caused to rotate at a speed a revs/sec. during the time an up-elevator control is applied. The total revolutions of this motor will then be proportional to At the total time added to the bombs flight due to tip-elevator applications. Similarly the total revolutions of another motor of speed b revs/sec. operating in like fashion when rudder is applied will provide a measure of the rudder effect. A third similar device will handle the down-elevator effect. Since the control applications are transmitted from the bombing plane to the bomb by radio and the control signals originate in the bombing plane, it is a simple matter to couple the bombardiers control stick to the corrector device so that the separate corrector elements are actuated during the control operation.

Having thus described in principle the method of deriving a measure of the correction time desired, the method by which this quantity may be applied to the bombsight to effect the proper correction in sighting the bomb will now be described.

The properties of the bombsight used in dirigible bombing have been outlined and to those skilled in the art, the manner of accomplishing these properties will be clear. The mechanism of the sight provides known means for synchronizing the motion of the line of sight to the target so that the cross hair remains at least approximately on the target during the bombing run. Means are also provided whereby for any given altitude a certain timing, termed by those versed in the art as the disc speed, may be set into the sight by the bombardier, this speed being a function of the type of bomb used and determined from suitable ballistic tables. With these essential quantities put into the sight and other operational adjustments made, the bomb is then released automatically by the sight at the proper moment. The angle between the vertical and the line of sight to the target is called the dropping angle. Now, while this dropping angle is variable and will depend upon various factors such as altitude, air speed, windage, etc., it is a property of the ordinary sight mechanism that the time from the instant of release to closure of the target line of sight into the auxiliary line of sight to the bomb will be a constant and equal to the desired time set into the sight as the disc speed.

It will now be evident that the time corrector device which is the subject of the present invention is intended to modify the normal timing of the sight by the incremental time as derived by the corrector mechanism in accordance with the integrated steering operations. For example: In an uncontrolled drop the time of fall of a particular dirigible bomb from 15,000 ft. is 31.9 seconds and coincidence of the lines of sight to bomb and target will occur 31.9 seconds after release in a properly adjusted bombsight. However, in order to correct a large aiming error and cause the bomb to eclipse the target in the sight as required suppose the bombardier applied, say, 10 seconds of up-elevator to the bomb. The resulting steering effect will add some 2.0 seconds to the time of fall with the result that, as explained earlier, the bomb will be directly over the target 2 seconds before impact and will be some 340 ft. beyond the target at the moment of impact if the horizontal component of velocity is, say, 170 ft./sec. On the other hand, had the bombsight coincidence been delayed the required 2 seconds by a suitable correcting device, coincidence and bomb impact would have been simultaneous, and the hit would occur precisely in accordance with the relative positions of bomb and target as seen in the bombsight. Thus the method of this invention is to modify the relative motion of the bombsight lines of sight by an amount equal to the incremental time induced by steering. This may be done by a simple displacement of the relative angular position of the auxiliary bomb-tracking mirror previously described and the target-tracking mirror. The required motion may be put into either mirror for purposes of this invention. However, it is preferable for reasons of expedient design to apply the correction to the moving target-tracking mirror. This may be accomplished by any suitable differential gear or similar device whereby both the normal mirror motion and the desired correcting motion may be introduced independently. I do not limit myself to the details of the correcting means for carrying out the method of my invention here disclosed by way of example. It is merely necessary that the proper incremental time correction be superimposed on the angular tracking rate or disc speed.

In order to illustrate how the objects of my invention may be accomplished, and one means whereby my method of bombsight correction may be applied, reference is made to the following figures forming a part of this invention specification:

FIG. 1 already referred to, shows a schematic diagram of a bombing run and the dual lines of sight which are superimposed in the bombsight, showing also the geometrical relationship between the bomber, bomb and target at the release of the bomb, at an intermediate time representing an instant during the steering operation, and at the moment of impact;

FIG. 2 shows schematically one type of integrating mechanism which may be used to derive a rotational motion the total number of revolutions of which is proportional to the summation of the Ats required in carrying out the objectives of my invention in the manner described;

FIG. 3 shows schematically the principle of the bombsight and one method of injecting the time correction into the sight mechanism according to my invention;

FIG. 4 shows a schematic electrical Wiring diagram of a preferred embodiment of my invention which may conveniently be applied to a conventional Norden bombsight;

FIG. 5 shows a schematic diagram of the mechanical system of FIG. 4 illustrating the manner in which the purpose of my invention may be accomplished on the Norden bombsight; and

FIG. 6 shows a speed vs. voltage characteristic curve of one of the motors shown in FIG. 4.

Referring to FIG. 2, numeral 1 indicates the frame of an integrating device of a well-known type in which a smooth-faced disc 2 is continuously rotated about an axis normal to the plane of the figure at a constant predetermined speed by any suitable type of drive such as a constant speed governor-regulated D.-b. motor operating from the airplane power source. A suitable planimeter wheel 3 is carried by a carriage 9 sliding on guide rods 9a. The position of the carriage 9 may be adjusted through lead screw 8 by a manually or otherwise driven dial 7, such that the radial position of the planimeter wheel contact on the driving disc 2 may be varied from zero to any desired radius. The purpose of this adjustment will be brought out later.

The rotation of the planimeter wheel is transmitted through a suitable reduction gearing 4, 5, to an output shaft 6. Shaft 6 has a keyway throughout its length and the shaft gear has a key which engages the keyway so as to transmit rotation to the shaft 6 at any position of the carriage 9. Interposed in shaft 6 is a magnetic clutch 59, so that the output shaft 60 is stationary except when the magnetic clutch 59 is energized, in which case, it rotates with shaft 6.

In operation, the driving disc 2 is rotating continuously at a predetermined constant speed, and this together with the radial position of the planimeter wheel adjusted by dial 7, comprises an adjustable constant-speed drive to the shaft 6. Thus, if the magnetic clutch 59 is energized each time, and for the duration of, the up-elevator control U, connected mechanically or electrically to the bombardiers control stick, the total number of revolutions of the shaft 60 will be an integrated measure of the total elevator application applied to the bomb. Output shaft 60 is connected through a suitable bevel gear 10 to shaft 11 into a differential gear box 12, thence through differential gear box 15 to the final output shaft 18.

Into differential gear 12 through shaft 14 and magnetic clutch 13 is fed the output of a second integrator identical with the up-elevator integrator 1, except that, by a suitable selection of gear ratios, the speed of shaft 61 in relation to that of shaft 6 is determined by the ratio of the up-elevator coeflicient a to the down-elevator coeflicient c referred to in the earlier discussion. Magnetic clutch 13 serves to engage shaft 14 with 61 whenever the down-elevator control D is applied. Differential gear 12 thus delivers to its output shaft 63 the sum of rotation of shafts 11 and 14. Similarly, the output 62 of a third integrator of identical type, whose clutch is actuated by application of either R or L rudder controls through magnetic clutch 16 to shaft 17, feeds through differential gear 15. The speed of this third or rudder integrator is designed, by proper gear ratios, to be in the ratio of coefficients a and b referred to earlier.

Finally it will be evident that, with the proper selection of disc speeds and/or gear ratios for the three integrators, the total number of revolutions of shaft 18 will be proportional to the summation of the incremental Ats due to all rudder and elevator control applications, and is therefore proportional to the displacement of the mirror from that normally required in the bombsight, that is, to the correction required to delay coincidence of the two lines of sight.

FIG. 3 shows schematically the essential features necessary in the application of the corrector mechanism to the bombsight. Here 19 is the sighting telescope which in the Norden type sight is gyro-stabilized so that the line of sight 22 through the partial mirror 20 is held substantially in the vertical or alternatively backward along the trail angle of the falling bomb 23. Partial mirror 20 is pivotally mounted on an axis 65 normal to the plane of the figure so that the angle between the line of sight 24 to the target and the vertical 22 may be varied. Partial mirror 20 is connected by drive 26 through a differential gear box 27 to the shaft 28 which is driven by the usual bombsight mechanism and which provides a motion to the mirror 20 suitable for continuous tracking of the target in the conventional manner.

In the final assembly of the sight correcting device the output of the corrector shaft 18, FIG. 2, is put into 9 the differential 27, FIG. 3, through shaft 66, FIG. 3. The mechanical connection from the corrector output 18, FIG. 2 to 18 FIG. 3 may be by flexible shafting or other convenient means. A manually operated clutch 30 or alternatively a magnetically operated clutch is interposed between shafts 18 and 66 and serves to disengage the entire corrector mechanism of FIG. 2 beyond shaft 18 and locks shaft 66 so as to permit normal adjustments of the bombsight prior to release of the bomb. After release, clutch 30 is engaged (or a magnetic type clutch may be connected to automatically engage whenever steering controls are applied) and thereafter any operation of the bomb steering controls will superpose on the normal motion of the tracking mirror 20 a lag displacement calculated to delay coincidence by the appropriate incremental time necessary to achieve correction of the bombardiers sighting angles in accordance with the delayed fall of the bomb.

Evidently the relative speeds of the component parts of the corrector mechanism compared with the normal bombsight mechanism must be properly selected in order that the proper amount of correction be applied. However, to one skilled in the art this is a detail of design not pertinent to the principles of my invention. It should be sufiicient to say that the corrective angular motions of the mirror that are required to correct the timing are small and over the duration of a fully controlled drop, may never exceed about 2 total angular motion. Thus, a 1 mirror correction would correspond to an ap parent shift of the target of 35 mils or 535 ft. ground displacement as viewed from 15,000 ft.

In FIG. 2, the dial 7 serves to vary the position of the planimeter wheel 3 on the radius of the main driving disc 2. Similar adjustments exist on the other component correctors and they serve to vary the output speed of the mechanism in accordance with the setting of the dial. Now it may be shown that the total correction applied to the bombsight should be proportional to the T.D.A. (tangent of the dropping angle) of the sight plus the trail angle set into the sight. Thus, while the coeflicients a, b and c which represent the time delays in the fall of the bomb have a constant value for a given type of bomb, the correction introduced into the sight for a T.D.A. of, say, 0.5 should be /2 that required if the indicated T.D.A. of the sight were 1.0. The tangent of the dropping angle (T.D.A.) is related to the ground speed of the bombing plane and therefore to the ground speed of the bomb. Hence the dial 7 is calibrated in T.D.A. units such that at a T .D.A.=0, the planimeter wheel is at zero radius on the disc 2' (except for the trail angle correction) and at any other T.D.A. it will be at a proportionately greater radius so that the speed of the output shaft 6 will be proportional to the T.D.A. setting as required. The T.D.A. is a quantity whose value may be obtained from the bombsight in conventional manner. The correction for the fixed trail angle may be introduced by offsetting the Zero of the scale on dial 7 in such manner that with the T.D.A. dial set at zero, the planimeter wheel 3 is not exactly at zero radius on disc 2 but is at a point corresponding to the trail angle. Thus any setting of the T.D.A. dial will provide a speed of the output shaft 6 proportional to the T.D.A. plus trail angle. The trail angle has a fixed value for any given type of dirigible bomb and may be set with the permanent adjustments of the sight when such type bombs are being used.

In practice, the T.D.A. dials of all three units are coupled together so that only one dial need be adjusted to the proper value. This value and the setting of the corrector T.D.A. dial is made by the bombardier immediately after release of the bomb when the T.D.A. of the drop is indicated by a suitable index on the Norden sight. Moreover, since this indicated value is repeated exactly by the angular position of one of the adjusting knobs on the sight (ordinarily manually set by the bombardier in his synchronizing adjustment) the T.D.A. dials of the corrector may be connected directly to his synchronizing adjuster of the bombsight by means of a suitable coupling shaft or other obvious means, Whereby the proper setting of the corrector is automatically changed to correspond with the T.D.A. of the sight. This method is preferred since it eliminates one manual operation in the bombsight adjustment.

While the above described corrector mechanism is illustrative of one embodiment of the bombsight time corrector principle, it represents only one apparatus for accomplishing the desired result. Evidently many mechanical modifications may be made without departing from the principles of this invention. A preferred embodiment of my invention, shown in FIGS. 4 and 5, has advantages of convenience in being simply an attachment which may be added to the Norden bombsight mechanism Without modification of any details of the sight itself. 'In this embodiment adjustable but relatively constant speed electric motors are used to perform the integration of the steering effects, one such electric motor taking the place of each of the units 1 of FIG. 2. This effects considerable simplification as will be apparent.

FIGURE 4 shows a schematic electric wiring diagram of the simplified device adapted for use on the Norden sight. Switches 35, 36, 37 are connected to the bombardiers bomb control stick, either through mechanical connections or by auxiliary relays, and are respectively "closed whenever the bombardier imparts to the bomb left-rudder, right-rudder or up-elevator steering control. The switches 35, 36, 37 control the electric motors whose properly integrated revolutions give a measure of the steering corrections applied to the bomb and hence also of the resulting timing correction required to be made on the bombsight.

A further simplification may be effected by omitting, as in FIG. 4, the provision for correction of the sight mechanism timing in accordance with down-elevator applications. This may be done because in most cases the additional downward acceleration of the bomb due to down-elevator application is approximately equal to the deceleration resulting from the added induced drag. Thus it is true that within the overall approximations involved in the time correction method here used, the effect of down-elevator may be ignored in practice. However. it will be evident that in case the down-elevator factor should be important in certain types or designs of dirigible bombs, an additional corrector unit may be superimposed on the system of FIGS. 4 and 5 in the same manner as the rudder and elevator units described.

In FIG. 4, 31 and 32 represent two small permanent magnet type D.-C. motors Whose speed vs. voltage characteristics are linear or substantially so. This linear characteristic, shown as curve 49, FIG. 6, has an intercept 50 which represents the voltage which must be applied to overcome friction in the motor. Thus, if a residual bias voltage equal to this intercept is permanently applied to this type motor, the speed vs. voltage characteristics may be made to pass through the Zero point as curve 51. Hence with a permanent bias of this kind applied, the speed of the motor will be linearly proportional to any additional voltage applied at the motor terminals, that is its speed vs. voltage characteristic will then be similar to curve 51. The necessity for this characteristic Will become evident later.

In FIG. 4 motors 31 and 32 are connected to relays 33 and 34 respectively and in such fashion that on closing either switch 35 or 36 motor 32 will operate and similarly, the closing of switch 37 causes motor 31 to operate. Switches 35 and 36 close in response to the bombardiers left and right rudder controls while switch 37 closes on application of up-elevator, these switches being connected to the bombardiers bomb control stick either mechanically or by relays.

The relay armature springs 70b, 70d of relay 33 and 71b, 71d of relay 34 are shown in FIG. 4 in the deenergized position against the upper set of contacts 70a, 700 of relay 33 and 71a, 710 of relay 34 respectively. It is apparent that on closing switch 35 or 36, current from the battery 69 energizes relay coil 34, and springs 71b, 71a are drawn down to the lower set of contacts. Motor 32 is thereupon supplied with power from the positive side of the battery 69 as follows: wire 72, resistor 40, wire 73, wire 74, contact 71c, spring 71d, wire 75, motor 32, wire 76, wire 78 and return to battery 69. Similarly on closing switch 37 current from battery 69 energizes relay coil 33 and spring 70b, 70d are drawn down to the lower set of contacts. Motor 31 is thereupon supplied with power from battery 69 as follows: wire 72, resistor 40, wire 73, wire 74, contact 70e, spring 70d, wire 77, motor 31, wire 76, wire 78, and return to battery 69.

It will be noted in FIG. 4 that when the motor actuating relays 33, 34 are not energized, the armatures of the respective motors 31, 32 are short circuited. Since these motors have permanent magnet fields, this short-circuit elicits electrodynamic braking and thus prevents any coasting of the motors. This feature is desirable since the total number of revolutions of the motor is in this embodiment used as an integration measurement. Upon de-energizing relay 34, spring 71d returns to contact 710 and a short circuit is thus immediately placed on the motor armature 32 via wire 75, spring 71d, contact 710, wire 78 and wire 76. Similarly de-energizing relay 33 allows spring 70d to return to contact 700 and places a short circuit on motor armature 31 via wire 77, spring 70d, contact 700, wire 78 and wire 76. It is apparent that the operations of motors 31 and 32 may be carried out independently or simultaneously as is dictated by the bombardiers steering effects.

In FIG. 4 resistances 40 and 41 are connected in series as a voltage divider across a D.C. power supply 69, the adjustable tap on resistor 40 permitting any desired voltage to be applied to the motors. In order to avoid any fluctuations of motor voltage when one or the other motor is turned on or off, the resistors 38 and 39 are placed in the circuit as dummy loads which replace the motor loads when the relays are not energized. They are adjusted to match the normal motor loads and thus prevent any serious change in the measured voltage drop across the voltage divider 40, 41 when the motor are actuated. Thus from wire 74 there exists either a load through resistor 39, contact 70a, spring 70b to wire 78 or else through the motor 31 armature as previously traced, and similarly for resistors 38 or motor 32.

A voltmeter instrument 43 calibrated in T.D.A. (tangent of the dropping angle) from -1.1 or 1.2 in effect measures the voltage applied to the motors minus the voltage drop across the small adjustable resistor 41. Thus, the motors are always subjected to voltage greater than that read by the T.D.A. meter by an amount equal to the voltage drop across 41. This is the bias voltage previously referred to whereby the speed of the motors may be made proportional to the T.D.A. voltage over the complete range of from 0 to full-scale values.

It will thus be evident from FIG. 4 that on closing switch 37 actuated when the bombardier applies up-control in steering a bomb, motor 31 will rotate at a speed directly proportional to the T.D.A. meter reading 43. Moreover, if the tap on divider 40 is adjusted so that the T.D.A. meter reading 43 is identical with the T.D.A. of the bombsight at release, the total number of revolutions made by this motor will be proportional to the incremental time of fall of the bomb due to up-elevator applications multiplied by the T.D.A. of the drop as required of the corrector mechanism. Similarly, applications of right or left rudder will actuate motor 32 with a similar integration of the incremental time lag of the bomb caused by rudder applications.

This information, in the form of total number of revolutions of the respective motor shafts, must be applied to the bombsight additively to alfect the desired delay in the sight coincidence. In this embodiment it was found most expedient to apply the correction directly to the rate knob of the Norden sight. (See US. Army Air Forces publication entitled Bombsights-M-Series Types M-9A and M9BModification and Operation, Technical Order No. ll3073 of June 29, 1945.) To those familiar with the art, this expedient means will be readily understood. It is sufficient here to explain that the rate knob or synchronizing knob of the Norden sight affects an adjustment of the angular speed of the target-mirror 20, FIG. 3. The effect of applying the correction to the rate knob as compared to a direct displacement of the mirror as in the method of FIG. 3, is that the Norden sight mirror mechanism already comprises an integrating device. Thus, a correction applied to the rate knob changes the mirror rate throughout the remaining time of flight of the bomb. Accordingly, the coefiicients a, b, and c for this type corrector must be derived from the empirical data with this integrating mechanism in mind, and they will have not only different numerical values but mathematical functions of different character than in the case of a direct displacement corrector as in FIG. 3.

However, in either the displacement method or the rate knob method of application of correction, I have found that suitable correction coefiicients can be derived.

The mechanical connection between the corrector motors 31 and 32 of FIG. 4 is shown schematically in FIG. 5. Here the motors are shown connected through individual reduction gears 44 and 45 to a differential gear 46 such that the rotation of the output shaft 48 represents the summation of the speeds of the individual motors. Output shaft 48 connects to the rate knob of the N01- den sight by a suitable gear with a manually operated friction clutch 47 interposed to permit normal manual adjustment of the rate knob during the bombardiers target-synchronizing operation. At release of the bomb the corrector mechanism is clutched in and automatic time correction proceeds.

As an example of the numerical factors involved, in the specific embodiment of the device illustrated in FIG. 5 designed for a specific dirigible bomb (VB-3), the speeds at which the rate knob should be turned are:

For up-elevator- Rate knob speed=514.3(T.D.A.-l-trail)a=degrees/sec.

For 1'udder-- Rate knob speed=514.3 (T.D.A.-I-trail)b=degrees/sec. where a and b are coefficients determined from actual drop data as already mentioned, and the trail is the rearward tilt of the line of sight to the bomb (a normal value of which is 45 mils).

Examples of typical values of the coefiicients a and b are: a=0.0088 and b=0.0035

At a T.D.A. setting of 0.70, the rate knob speeds are therefore:

For up-elevator corr.=514.3 (0.70+0.04S)0.0088=3.38

sec. For rudder corr.=514.3(0.70+0.045)0.0035=1.34/sec.

The permanent-magnet fields of the two motors 31 and 32 may be adjusted so that they have equal speeds at any given voltage. Then in FIG. 5, the gear train of motor 31 through 44 and 46 may be designed so as to compare with the gear train of motor 32 through 45 and 46 in the ratio of the coeflicients a and b. As an example, to fit the values of a and b above given, one may use a reduction of 10,7116/1 for motor 31 and a reduction of 26,733/1 for motor 32. Moreover, the actual speed of the motors at the voltage corresponding to a T.D.A. meter reading of 0.70 may be adjusted to:

3.38 x 10,716/360: 100.6 rev./sec.=6036 r.p.m. approx.

Under these conditions, the sight corrector will then apply to the rate knob an appropriate rotation to delay the motion of the target mirror into the vertical by an incremental time equal to the total time lag induced in the fall of the bomb due to the applications of rudder and upelevator controls during steering.

It will be noted from the above equations that the desired rate knob speed involves a factor: (T.D.A.+trail) the trail being a fixed value equal to the trail angle previously mentioned. (The trail angle and the tangent of the trail angle are substantially equal since this angle is always small.) Since this is a constant value in any given sight for dirigible type bombs this factor is introduced in the T.D.A. meter 43 of the corrector by merely calibrating the meter so that its mark is to the right of the no-current point of the meter. The meter may then have T.D.A. calibrations to the right of the O and trail readings to the left of the O. The trail value may then be adjusted if necessary by adjusting the no-current zero position of the meter in the customary way, in this way setting the no-current zero of the meter needle to the appropriate trail value. Consequently, when the resistor 40 is adjusted to give a T.D.A. meter indication on the meter dial, the actual voltage used will then be the T.D.A. voltage plus the trail voltage as required. This voltage will exist across the left-hand part of resistor 40 of FIG. 4, and is the voltage abscissae of curve 51 of FIG. 6. The actual voltage applied to the motors (between wire 74 and 78) will be this voltage plus the voltage drop across resistor 41, FIG. 4, this total being represented by the abscissae of curve 49, FIG. 6.

An additional detail concerns the purpose of the adjustable resistor 42 in series with the T.D.A. meter 43 of FIG. 4. This is an expedient method of introducing minor corrections as, for example, the effect of altitude on the incremental time lags. It has been found that the numerical value of the corrector coeflicients changes slightly with altitude. This effect may be compensated adequately by proper adjustment of resistor 42, whereby the effect of a change in the numerical value of the corrector coefiicients may be made conveniently. The dial on this resistor may be calibrated to read altitude.

I do not limit the scope of my invention to the particular apparatus embodied in the above disclosures. For example, in FIG. 2 it will be evident that the dial 7 may be used as an adjustment to match the output speed to any particular value of correction factor a, with similar adjustments in the several corrector units to match coefficients 1: and 0. Alternatively, in the embodiment of FIG. 2, the required speed vs. T.D.A. adjustment may be accomplished by simultaneously varying the speed of the discs 2 in accordance with the T.D.A. value. These and other modifications of the mechanical or electrical details may be made without departing from the spirit of my method of time correction of dirigible-bomb sights.

What I claim as my invention is:

1. In a bombsight having a time-controlled mechanism for closing the lines of sight to the bomb and to the target and adapted to be used for range control of dirigible bombs, apparatus for correcting the prediction of the closing time of the line of sight to the bomb with the line of sight to the target such that said closing time will coincide with the impact time of the bomb which comprises means for generating a motion which is proportional to the change in dropping time of the bomb induced by steering thereof, means actuated by said generating means for transmitting said motion whenever steering influences are imparted to the dirigible bomb and means combining said transmitted motion with the normal motion of the bombsight line-of-sight-closure mechanism whereby there is introduced into the closure time of the lines of sight a time change substantially equal to the change in the dropping time of the bomb induced by steering thereof.

2. In a bombsight having a time-controlled mechanism for closing the lines of sight to the bomb and to the target and adapted to be used for range control of dirigible bombs, apparatus for correcting the prediction of the closing time of the line of sight to the bomb with the line of sight to the target such that said closing time will coincide with the impact time of the bomb which comprises means for generating a motion which is proportional to the change in dropping time of the bomb induced by steering thereof, means for adjusting said generating means so that said motion is proportional to the sum of the tangent of the dropping angle at release of the bomb and the trail angle setting of the bombsight, means actuated by said generating means for transmitting said motion whenever steering influences are imparted to the dirigible bomb and means combining said transmitted motion with the normal motion of the bombsight line-of-sight-closure mechanism whereby there is introduced into the closure time of the lines of sight a time change substantially equal to the change in the dropping time of the bomb induced by steering thereof.

3. In a bombsight having a time-controlled mechanism for closing the lines of sight to the bomb and to the target and adapted to be used for range control of dirigible bombs, apparatus for correcting the prediction of the closing time of the line of sight to the bomb with the line of sight to the target such that said closing time will coincide with the impact time of the bomb which comprises means for generating a motion which is proportional to the change in dropping time of the bomb induced by upward steering thereof, means for generating a motion which is proportional to the change in dropping time of the bomb induced by downward steering thereof, means for generating a motion which is proportional to the change in dropping time of the bomb induced by right and left steering thereof, means actuated by said generating means for transmitting said motions whenever said steering influences are imparted to the dirigible bomb and means combining said transmitted motions with the normal motion of the bombsight line-of-sight-closure mechanism whereby there is introduced into the closure time of the lines of sight a time change substantially equal to the change in the dropping time of the bomb induced by steering thereof.

4. In a bombsight having a time-controlled mechanism for closing the lines of sight to the bomb and to the target and adapted to be used for range control of dirigible bombs, apparatus for correcting the prediction of the closing time of the line of sight to the bomb with the line of sight to the target such that said closing time will coincide with the impact time of the bomb which comprises means for generating a motion which is proportional to the change in dropping time of the bomb induced by upward steering thereof, means for generating a motion which is proportional to the change in dropping time of the bomb induced by downward steering thereof, means for generating a motion which is proportional to the change in dropping time of the bomb induced by right and left steering thereof, means connected to said generating means for adjusting said motions whereby they may be made proportional to the sum of the tangent of the dropping angle at release of the bomb and the trail angle setting of the bombsight, means actuated by said generating means for transmitting said motions whenever said steering influences are imparted to the dirigible bomb and means combining said transmitted motions with the normal motion of the bombsight line-of-sightclosure mechanism whereby there is introduced into the closure time of the lines of sight a time change substantially equal to the change in the dropping time of the bomb induced by steering thereof.

5. In a bombsight having a time-controlled mechanism for closing the lines of sight to the bomb and to the target and adapted to be used for range control of dirigible bombs, apparatus for correcting the prediction of the closing time of the line of sight to the bomb with the line of sight to the target such that said closing time will coincide with the impact time of the bomb which comprises means for generating an angular motion functionally related to the duration and direction of steering influences imparted to the bomb and proportional to the sum of the tangent of the dropping angle at release of the bomb and the trail angle setting of the bombsight and means actuated by said generating means for introducing said angular motion into the normal motion of the bombsight line-of-sight-closure mechanism whereby there is introduced into the closure time of the lines of sight a time change substantially equal to the change in dropping time of the bomb induced by steering thereof.

6. In a bombsight having a time-controlled mechanism for closing the lines of sight to the bomb and to the target, said mechanism having an adjustment for its rate of closure, and adapted to be used for range control of dirigible bombs, apparatus for correcting the prediction of the closing time of the line of Sight to the bomb with the line of sight to the target such that said closing time will coincide with the impact time of the bomb which comprises means for generating a motion which is proportional to the change in dropping time of the bomb induced by upward steering thereof, means for generating a motion which is proportional to the change in dropping time of the bomb induced by right and left steering thereof, means actuated by said generating means for transmitting said motions whenever said steering influences are imparted to the dirigible bomb to the rate-ofclosure-controlling adjustment of the bombsight.

7. In a bombsight having a time-controlled mechanism for closing the lines of sight to the bomb and to the target, said mechanism having an adjustment for its rate of closure, and adapted to be used for range control of dirigible bombs, apparatus for correcting the prediction of the closing time of the line of sight to the bomb with the line of sight to the target such that said closing time will coincide with the impact time of the bomb which comprises means for generating a motion which is proportional to the change in dropping time of the bomb induced by upward steering thereof, means for generating a motion which is proportional to the change in dropping time of the bomb induced by right and left steering thereof, independent speed-adjusting means connected to each of said motion-generating means for adjusting said motions whereby they may be made proportional to the sum of the tangent of the dropping angle at release of the bomb and the trail angle setting of the bombsight, means actuated by said generating means for transmitting said motions whenever said steering influences are imparted to the dirigible bomb to the rate-of-closure-controlling adjustment of the bombsight.

8. In a bombsight having a time-controlled mechanism for closing the lines of sight to the bomb and to the target, said mechanism having an adjustment for its rate of closure, and adapted to be used for range control of dirigible bombs, apparatus for correcting the prediction of the closing time of the line of sight to the bomb with the line of sight to the target such that said closing time will coincide with the impact time of the bomb which comprises a source of electric power, a constant speed electric motor, an electric switch means connecting said motor to said source of power whenever a steering influence is imparted to the dirigible bomb, a gear train transmitting the total motion of said motor to the rate-ofclosure-controlling adjustment of the bombsight in a ratio related to the change in dropping time of the bomb induced by steering thereof.

9. In a bombsight having a time-controlled mechanism for closing the lines of sight to the bomb and to the target, said mechanism having an adjustment for its rate of closure, and adapted to be used for range control of dirigible bombs, apparatus for correctingthe prediction of the closing time of the line of sight to the bomb with the line of sight to the target such that said closing time will coincide with the impact time of the bomb which comprises a source of electric power, a constant speed electric motor, an electric switch means connecting said motor to said source of power whenever a steering influence is imparted to the dirigible bomb, an adjustable gear train including differential gear means for transmitting the total motion of said motor to the rate-of-closurecontrolling adjustment of the bombsight in a ratio related to the change in dropping time of the bomb induced by steering thereof and with said ratio adjusted to be proportional to the sum of the tangent of the dropping angle at release of the bomb and the trail angle setting of the bombsight.

10. In a bombsight used for range control of dirigible bombs, apparatus for correcting the prediction of the closing time of a line of sight to the bomb with a line of sight to the target such that said closing time will coincide with the impact time of the bomb which comprises means for generating a motion proportional to the duration of a component of steering influence imparted to the bomb, means for adjusting said motion-generating means whereby said motion may be made proportional to the sum of the tangent of the dropping angle at release of the bomb and the trail angle setting of the bombsight, means actuated by said motion-generating means for adjusting the angle between the bombsight lines of sight by an amount proportional to said motion and in a ratio related to the change in dropping time of the bomb induced by steering thereof.

11. In a bombsight used for range control of dirigible bombs, apparatus for correcting the prediction of the closing time of a line of sight to the bomb with a line of sight to the target such that said closing time will coincide with the impact time of the bomb which comprises means for generating a motion proportional to the duration of a component of steering influence imparted to the bomb, means controlled by said motion-generating means for reducing the rate of decrease of the angle between the bombsight lines. of sight by an amount proportional to said motion and in a ratio related to the change in dropping time of the bomb induced by steering thereof.

12. In a bombsight used for range control of dirigible bombs, apparatus for correcting the prediction of the closing time of a line of sight to the bomb with a line of sight to the target such that said closing time will coincide with the impact time of the bomb which comprises means for generating a motion proportional to the duration of a component of steering influence imparted to the bomb, means for adjusting said motion-generating means whereby said motion may be made proportional to the sum of the tangent of the dropping angle at release of the bomb and the trail angle setting of the bombsight, and means controlled by said motion-generating means for reducing the rate of decrease of the angle between the bombsight lines of sight by an amount proportional to said motion and in a ratio related to the change in dropping time of the bomb induced by steering thereof.

References Cited in the file of this patent UNITED STATES PATENTS 1,114,705 Boykow Oct. 20, 1914 2,118,041 Estoppey May 24, 1938 2,162,698 Chafee et al June 20, 1939 2,404,746 Rylsky et a1. g July 23, 1946 2,428,678 Norden et al Oct. 7, 1947 2,438,532 Barth Mar. 30, 1948 2,459,919 Clark Jan. 25, 1949

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