This invention relates to an in flight refueling apparatus mountable on a tanker aircraft and having a probe receptor coupled with a fuel line, which receptor is arranged to be deployed outboard of the aircraft.

Mid-air refueling is a delicate task requiring extreme pilot skill especially in an adverse environment involving turbulence and night flying.

Although first time link up is common in good conditions and may take only one minute, by contrast in adverse conditions, several passes may be necessary and 10 minutes can be required. An automatic coupling system which cuts down the link-up time is a worthwhile objective of this invention.

A second advantage of automatic link-up is reliability. Such a system could be designed to recognise the parameters and limits which define a bad approach and prevent a dangerous situation occurring or damage to aircraft equipment.

A third advantage is for night operation where

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automatic linkup would not require the illumination of the tanker aircraft as an artificial horizon.

Fourthly, there will be less demand on the pilot skill which could be important for pilots that are tired or injured.

Considerable efforts have already been made to fly the drogue as steadily as possible. The elastic constants of the hose and winding-in mechanism have been controlled to minimise the effects of coupled oscillations to the drogue. The preferred condition occurs when sufficient drag and porosity is introduced to the drogue. In the semi-automatic solution, the drogue is not required to move more than the order of 50 cm up, down, left or right so that in aerodynamic terms this may be regarded as a perturbation of a steady state condition, rather than creating a new flying mode with different aerodynamic characteristics. This requirement distinguishes the application from those of towed bodies with a light weight connection where the towed body is required to move relatively large distances to precise positions with specified orientations.

The present invention seeks to provide a sensing and control apparatus that can be retro-fitted to the existing

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refueling systems in their diverse forms.

According to the invention there is provided an in flight refueling apparatus mountable on a tanker aircraft and having a receptor connected to the end of a fuel line, which receptor is arranged to be deployed outboard of the aircraft, characterised in means for providing a parameter either, in a system termed "the stability mode", representative of deviation of the path of the receptor from a predetermined initial path for actuating control means for changing automatically the position of the receptor relative to said initial path; or, in a system termed "the alignment mode", representative of the relative angular position of the receptor with respect to the probe of an approaching refueling aircraft for actuating control means for changing automatically said relative angular position to achieve alignment of receptor and probe.

A conventional refueling system employed in the United Kingdom has a flexible hose and can have up to three lines from a single tanker; one from the centre section of the fuselage and one from each wingtip. The winding gear and its associated performance parameters are different comparing wing tip and fuselage units. A refinement of the invention for such a U.K. application is that the receptor

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is provided on a drogue trailed by the aircraft, which drogue has at least one flight control surface for altering the flight path of the receptor in dependence upon said parameter.

A conventional system in America is to fly a single, rigid, but manoeuverable boom from the fuselage. A refinement of the invention for such an American application is characterised in that the receptor is provided on a boom and is manoeuverable thereby in dependence upon said parameter.

Different tanker aircraft produce different turbulence patterns at the drogue but in all cases atmospheric turbulence or gusting is less than the effect of the tanker.

There are three classes of Tanker/Receptor aircraft configurations:

i) Tanker to smaller high speed aircraft receptors.

ii) Tanker to receptor aircraft of the same size.

iii) Buddy-Buddy mode of equivalent sized aircraft, but both tanker and receptor are smaller that the tanker in

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( i) or ( ii) .

Often in (i) and (ii) above, tanker and receptor are on opposite extremes of their drag/speed characteristics, i.e., the tanker aircraft is nearly flat out whilst the receptor aircraft is near to stalling.

The invention may provide a servomechanism with a control loop incorporating negative feedback. In the case of a towed drogue, the off axis angle between the link line from refueling probe and the towed drogue is the parameter that must be minimised and an automatic control means in the feedback loop relies on a sensor detecting and continuously updating the system with respect to the off axis angle. All the normal feedback and control mechanism theory is applicable to the described systems.

There are two main systems: the drogue and probe system used mainly in the UK and the boom and probe system used mainly in the USA. The current UK system relies on pilot skill in the refueling aircraft whilst the USA system relies both on the skill of a manual operator in the tanker and also the skill of the refueling pilot.

For both systems there are alternative forms of the

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invention which are: firstly, the "alignment mode" in which the control means moves the drogue or boom end to line up with the approaching probe on the refueling aircraft and secondly the "stability mode" in which the control means moves the drogue or boom end to maintain its position on an initial reference path. In each of these modes there are design options in certain features of the control loop and these options are hereinafter described.

In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, by way of example only, with reference to the drawings, in which:-

Figure 1 is a schematic illustration showing a tanker aircraft and approaching fuel reception aircraft employing apparatus constructed in accordance with the invention.

Figure 2 is a schematic illustration of a system for short range approach sensing prior to coupling.

Figure 3 is a schematic illustration similar to Figure 2 but after coupling.

Figure 4 is a side view of a drogue for a tanker

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aircraft showing sensors and control surfaces,

Figure 5 is a rear view of the drogue of Figure 4,

Figure 6 is a schematic front view of a drogue employing movable vanes to control its flight,

Figure 7 is a schematic side view of the drogue of Figure 6,

Figure 8 is a schematic illustration of a quadrant detector for sensing relative positions of the drogue and probe,

Figure 9 is a graphical illustration of the output of the detector of Figure 8 plotted against displacement of the focused image,

Figure 10 is a block schematic diagram illustrating one possible control arrangement employing a quadrant detector,

Figure 11 is a schematic illustration of the principle of operation of the drogue alignment control system,

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Figure 12 is a schematic illustration of the principle of operation of a stable platform control system,

Figure 13 is a schematic illustration of a boom type refueling system constructed in accordance with the invention,

Figure 14 is an enlarged view of part of the system shown in Figure 13 to illustrate docking,

Figure 15 is a schematic illustration of the principle of operation of a boom control system,

Figure 16 is a schematic illustration of the principle of operation of an alternative boom control system,

Figure 17 illustrates one suitable form of optical source arrangement,

Figure 18 illustrates an alternative suitable optical source arrangement,

Figure 19 illustrates schematically a refinement of the invention in which control in a third direction along the trajectory of the tanker aircraft is provided,

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Figure 20 is a schematic illustration of the principle of operation of the refinement of Figure 19,

Figure 21 is a schematic illustration of an alternative system constructed in accordance with the invention in which the active elements are all provided on the tanker aircraft, and

Figure 22 is a plot of filter response against wavelength for a refinement of the system of Figure 21.

Throughout the specification, the same reference numerals are employed to designate similar parts.

In the drawings of Figures 1 and 2 a tanker aircraft 10 is provided with a fuel line 11 which can be deployed out of the aircraft and which has a drogue 12 at its remote end which has a receptor aperture 13 (figure 2) for a probe 14. The aircraft 10 is provided with a detector 17 which may comprise a quadrant detector for receiving light from a point optical source 15 (e.g. a laser) on fuel receptor aircraft 16. The output of the detector 17 is fed to an interpolator (not shown) which provides a signal representative of relative angle between the probe 14 and

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the aperture 13 and the signal can be used to control the drogue 12 as will be described later. The tanker aircraft 10 is also provided with a camera 19 which may be a charge coupled device camera which is linked to a visual display in the aircraft for aiding the pilot to initially bring the drogue into approximate alignment with the approaching fuel receptor aircraft 16 so that the detector 17 is responsive to light from the optical source 15.

Referring now to Figure 2 there is illustrated the situation where the fuel receptor aircraft 16 is close to the position where its refueling probe 14 engages the receptor aperture 13. The source 15 can here be seen to be offset relative to the probe and an optical detector 17 is correspondingly offset relative to the receptor aperture 13 as is an optical focusing system 18 which directs light onto the detector. This sensor is also coupled with the interpolator (not shown) which produces a signal for controlling the position of the drogue to ensure accurate automatic link up of the two aircraft as can be seen by the illustration in Figure 3.

Although the receptor is shown as a female aperture 13 for receiving a male probe 14 it is to be understood that the receptor could be a male element on the drogue

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engagement with a female element on the receptor aircraft. Accordingly the term probe is intended to include male and female elements.

Figures 4 and 5 show one possible construction of the drogue 12 in greater detail as having four radially extended fins 20 disposed in a cruciform configuration. The fins are each provided with a control surface 21 on their trailing edge which serves to steer the drogue in dependence upon signals provided from a control circuit board 22 in response to signals detected by the detector 17 on the drogue. The control signals serve to operate motors or actuators for the control surfaces powered by a battery 23.

The control board 22 may also provide a control signal which causes the drogue to be wound in or out from the aircraft to again facilitate engagement.

Although the embodiment described, employs four cruciform fins it will be appreciated that two oppositely disposed radially extending fins will provide two directional control, albeit with less stability of the drogue. Such a construction is considered to be within the scope of this invention.

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Figures 6 and 7 show schematically an alternative construction of drogue which is less susceptible to damage than the construction of Figures 4 and 5. This employs aerofoil vanes 24 which are pivotally mounted at axes 25 which axes extend radially of the fuel line. The number of vanes employed depends upon the total area of aerofoils and the allowable pivotal movement. Although eight are shown in the illustration, more or less could be employed with a minimum of two.

Figure 8 illustrates schematically a simple quadrant detector whilst Figure 9 shows graphically the output of the quadrant detector, on a plot of detector output against displacement of focused image. Each quadrant of the detector gives a voltage output (V., to V ₄ ) proportional to the area of the quadrant that is illuminated by the image having a spacially uniform intensity. There are several functions of these voltages that can be used as a control variable to centre the image on the crosswires but the principle can be appreciated from the error curve shown in Figure 9. A first error signal Ex is the difference V ₂ -v., such that E _χ = V ₂ (max) when the spot is wholly on the second quadrant and E _χ = V,, (max) when the spot is wholly on the first quadrant. Between these extremes is an S-shaped curve determined by the ratio of the illuminated areas on

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quadrants 1 and 2 with E _χ = 0 when the spot is equally distributed left and right in the x-direction (horizontal) . A second error signal E _χ ' is the difference V ₄ -V ₃ between the lower quadrants and similarly E _χ ' = 0 when the spot is equally distributed left and right. Similar considerations occur vertically in the y-direction to give error signals E _y and E ' . The closed loop system operates to move the frame of reference of the detector with respect to the frame of reference of the image spot until all the signals E _χ , E _χ ', E _y and E _y ' are zero when the circular image is central. If the image is not circular the sum of E _χ , E _χ ', E and E _y ' will be zero although the individual error signals will be non-zero.

It should be noticed that the smaller the image spot size relative to the quadrant detector, the steeper the guidance law of the error signal, so that in the extreme for a very small spot size Ex changes directly from V ₂ max to V, max. For a reliable and sensitive system the optics are designed to give an optimised size circular image relative to the detector diameter. The quadrant detector and associated electronics is regarded as a fairly crude control device but for straightforward applications, such as this, it is adequate.

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Such a system is very good when the image is a single circular spot but not so good if there are multiple highlights and spatially distributed noise etc. In these cases more sophisticated systems can be used in the image plane such as image scanning detectors and stairing arrays of n x n detector elements. Such arrangements are considered to fall within the scope of this invention.

Referring again to Figures 8 and 9, the output from the detector is representative of relative probe/drogue offset in a first axial direction e.g. horizontal. The detector also provides another voltage output representative of relative probe/drogue offset in a second axial direction at right angles to the first axial direction e.g. vertical. These two signals are used to control the flight of the drogue as will now be described in connection with Figure 10.

Figure 10 illustrates one possible control arrangement in which a quadrant detector 17 provides four output voltages V_ _j , V ₂ , V ₃ and V ₄ one from each quadrant. These voltage quadrants can also be seen in Figure 8. The magnitude of these voltages is proportional to the area of the quadrant that is illuminated by the image and depends upon the position of the focused image. The voltage is a

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maximum for a particular one of the outputs if the focused image falls wholly within its related quadrant. The voltages V., and V ₂ are summed in adder 26, V ₃ and V ₄ are summed in adder 27, V ₁ and V ₃ are summed in adder 28 and V ₂ and V ₄ are summed in adder 29. The resultant voltages V ₁ + V ₂ are representative of content of the focused image above a desired position and V ₃ + V ₄ are representative of the content of the focused image below the desired position. The voltages V ₁ + V ₂ and V ₃ + V ₄ are fed to a comparator 30 which provides an output which is positive or negative dependant upon which voltage is highest or 0 if both voltages are identical. The output voltage drives a servomotor 31 which controls the flaps or veins of the drogue to cause the drogue to adopt the correct vertical disposition when V., + V ₂ = V ₃ + V ₄ . A similar arrangement is employed to control the drogue in the horizontal plane this time summing the voltages v., + V ₃ and V ₂ + V ₄ , comparing them in comparator 32 and using the output of the comparator to control a servomotor 33 which controls the appropriate flaps or veins of the drogue which cause horizontal displacement. Instead of adding the voltages they could of course be averaged to provide a voltage for the comparator.

The drawing of Figure 11 illustrates schematically the

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operation of the drogue alignment control mode previously described. Light from the source 15 coming from a particular angular direction hits the optical sensor which provides output error information which is interpreted by a processor to provide a correcting signal. The correcting signal is fed to an actuator which controls the appropriate aerofoil to correct the drogue position. This is illustrated by the dotted line which is a notional angle correction value which provides a corrected angle of incoming light from the source. This illustrates that the system is effectively a closed loop correcting system. Although the system previously described is an alignment control mode, an alternative stable platform control system or "stability mode" is also considered to fall within the scope of this invention. Both systems are closed loop but the alternative system is intended to provide a more stable platform for linking rather than providing guidance of linking. Sensors on the drogue such as accelerometers will give an output relating to the departure of the drogue from a reference ideal trajectory. The control system will then operate to restore the drogue to that trajectory. The pilot of the receptor aircraft then flies the probe into the drogue without the fear of the drogue suddenly slipping sideways in the last metres due to the bow wave from the nose, for example.

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In the stability mode accelerometer sensors may be used to detect movement of the drogue from a reference axis. The error signal is then used to correct the drogue position by actuating the control surfaces or vanes in the manner previously described. The pilot of the refueling aircraft must align his aircraft for linking fuel lines. The drawing of Figure 12 illustrates schematically this so called stability mode for stable platform control.

Whilst the previous description of the invention has been concerned with the control of a drogue, it is also applicable to the control of a boom type refueling system in which the position of the probe (normally female) is controlled either in the alignment mode or stability mode system based on the previously described principles. The drawings of Figures 13 and 14 show this system schematically. A boom 34 is mounted on a tanker aircraft and is controlled by a hydraulic or electrical servo system (not shown) in accordance with signals derived from the sensor 17 e.g. quadrant detector which receives a signal from the source 15 e.g. an optical source on an approaching refueling aircraft. The refueling aircraft is provided with a receptor probe 14 which has a fuel receiver tube 35 the walls of which are divergent towards its outer end to

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facilitate entry of a filter pipe on the boom of a tanker aircraft. The receptor probe has a forwardly projecting spike 37 for engagement with a docking latch 38 provided on the boom 34. It will be appreciated that the geometrical configuration of the fuel receiver tube, projecting spike 37 and offset optical source will be common for all refueling aircraft that need to be refuelled.

The drawing of Figure 14 illustrates the link up configuration where a refueling aircraft is docked with the tanker aircraft. When docked a telescopic extension is actuated to project into the receiver tube 35. This actuation is normally effected under the control of tanker aircraft but could be automated upon detection of latching.

The drawing of Figure 15 illustrates schematically the operation of the boom control system as applied to either the alignment mode or the stability mode as a retrofit for an existing manual control system. The manual control system is illustrated in which a man 42 operates a joystick 43 which acts upon transducers 44 to provide control information which is fed via a combiner 45 to actuators 46 and 47. The actuator 46 controls an aerofoil 48 via a servomotor (not shown) to cause the boom to move up or down. The actuator 47 is a hydraulic ram device which moves the

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boom from side to side. In dependence upon the joystick movement an operator in a refueling aircraft can adjust the position of fuel feed for assisting coupling with an aircraft to be refueled. The retrofit replaces or provides an alternative automatic control system in which an alignment sensor 49 i.e. quadrature detector or accelerometer detector actuates the control transducers 44 via a signal processor 50 and hydraulic rams 51. Whilst as shown the rams act directly on the control transducers it will be understood that the rams could be arranged to act on the joystick.

A more elegant solution is schematically illustrated in Figure 16 in which the output of the processor is arranged to provide control signals directly to the combiner 45.

Comparing the alignment mode and stability mode, the following is initially apparent. Since the modes are of similar complexity it is normally more advantageous to have the extra advantage of the automatic alignment system to avoid the effects of pilot error.

In the alignment mode modulation of the transmission beam is particularly advantageous. The advantages of

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modulation are well known to those versed in the art. The essential feature is that the modulated signal is easily distinguished from the background clutter and system noise. Noise is proportional to bandwidth and as the modulation bandwidth is kept very low the noise is correspondingly reduced. These benefits are quantified in a low signal to noise ration and a low signal to clutter ratio. In addition it is possible to encode the modulation so that different transmission beams can be separately identified. This feature would be useful when a tanker aircraft is refueling three aircraft simultaneously. The refueling pilot would switch the transmitter module to the predetermined code according to the choice of port, starboard, or central refueling station on the tanker aircraft.

As shown in Figure 17, a preferred optical source is an electronically controlled light emitting diode or laser diode 52 either of which is optically coupled into a fibre optic cable 53 as is well known to those versed in the art. The end of the fibre optic cable is suitable terminated to produce a lens 54 so as to produce an optical beam with the required divergence and in the required position at a predetermined distance from the probe of the refueling aircraft.

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In Figure 18 an alternative simpler and cheaper optical souce comprises a filament bulb or a miniature fluorescent tube 55 which is modulated mechanically by a chopper such as a rotating mask 56, which is limited in bandwidth by an optical bandpass filter 57 and which is collimated into a beam 58 by a lens 59 or curved mirror.

Figure 19 illustrates an independent closed loop system to control the third dimension (i.e. along the trajectory of the tanker aircraft) which can be applied to the drogue and probe system only but equally well to alignment mode or stability mode control.

There are strict limitations to the change in relative velocity that can be achieved by winding in or letting out of fuel hose 11. However, in principal a very powerful motor could be applied to fuel hose drum 60 to achieve the requirement. In most practical cases the relative velocity of the refueling aircraft approach is low by pilot control so that the effects produced by the hose drum can be significant. Also the detection of dangerously large relative velocities can trigger a warning system.

If a Doppler Radar (this being well known to those versed in the art) is used to determine the relative

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velocity and distance to go then it is more convenient to house the radar in the tanker aircraft as shown at 61 where the derived control voltages can be applied directly to hose drum controller 62. However if the doppler radar module is attached to the drogue as shown at 63, the relative velocity can be directly determined and an optical link 64 can be made from the drogue to the drum control in the tanker for signal communication. The schematic link is illustrated in Figure 20.

Figure 21 illustrates an important alternative construction operating in the alignment mode. Instead of the combination of a transmitter source on the refueling aircraft and a detector module on the receptor this deploys the combination of a source 15 and detector module 17/18 both on the receptor, side by side on a short baseline. A third element to make this system work is a retroreflector 65 on the refueling aircraft in the same position as the transmitter used to be, such that the retroreflector is a standard offset distance from the refueling probe. Thus an energy beam radiates from the transmitter and is reflected from the retroreflector to the detector module, such that the distance from the detector centre to the image of the retroreflector is proportional to the off axis angle of the retroreflector from the alignment axis. The control

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mechanism then operates in the same way as for previously described alignment mode. A complication of this method is that some energy will be reflected back from corners, curves and discontinuities other than the retroreflector on the refueling aircraft and the following technique is an example of the means by which the background clutter can be removed. A high resolution detector is required for this technique such as a staring array with at least 30 x 30 elements. The transmitter is a dual band optical source switching alternatively between wavelengths LI and L2. The retroreflector is convered with a filter 66 which is absorbing for the wavelength LI but transmitting for wavelength L2 as is illustrated in Figure 22.

At the detector sequential images at wavelengths LI and L2 will correlate with the switching transmitter. The background clutter will be constant at both wavelengths but the retroreflector image will be present for L2 but absent for LI. By subtracting the images the clutter can be removed. Variations of Dual Band Systems are well known to those versed in the art.

Such a system is designed to operated only at short range in the closing stages of link up when the refueling aircraft has been piloted near to the required alignment,

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and the illuminating beamwidth angle is chose to be compatible with the likely misalignment at a predetermined short range.

In all other respects the systems described in connection with Figures 21 and 22 are similar to the systems previously described and may incorporated any of the feature therefrom.

In some of the embodiments described a flexible hose is used to transfer fuel from the tanker aircraft and the drogue is designed to have sufficient drag and porosity to provide a stable aerodynamic system with minimal oscillations of the hose and minimal vortices which cause rotation of the drogue.

There are many types of sensor systems that can be used including different forms of energy transfer between source and detector, and also different geometrical configurations. A preferred type, for economy, deploys a modulated visible source near the receptor aircraft probe and an imaging system on to a quadrant detector or other image analysing system. The quadrant detector system has been described. In the alternative matrix detector the distance of the imaged optical source from a central datum

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would yield the relative angular separation of drogue and probe.

An advantageous refinement of the invention could deploy an IR source and and IR detector array. Both the source and detector array could be on the drogue whilst a form of retro-reflector could be placed near the probe on the receptor aircraft.

There are many types of control mechanisms that could be used to control the drogue including a variety of aerodynamic control surfaces operated by transducers or a set of reaction jets.

The power required to operate these control mechanisms can be provided by an electrical cable from the tanker aircraft's electrical system attached to the fuel hose, or by a battery attached to the drogue, or by a wind vane electrical generator attached to the drogue or by a ram jet.

The alternatives for design features and components of the control loop are as follows:

(a) The transmission energy between the source on the refueling aircraft and detector on the receptor can be

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millimetric, infra red, or visible light.

(b) The optical sources, for infra red or visible radiation, can be electronically controlled light emitting diodes or electronically controlled laser diodes, or filament bulbs or miniature fluorescent tubes.

The electronic control leads to simple intensity modulation techniques whereas other sources would require mechanical chopping to provide modulation.

(c) the detector module must produce an image which can be analysed to determine the angle of the transmitter source from the alignment axis when viewed from the detector. The quadrant detector has been described in detail. An alternative detector is a focal plane staring array which is well known to those versed in the art, and is most applicable to an infra red system. There are many image analysers employing scanning techniques also well known to those versed in the art which may be employed. The image can be formed by various mechanical scanning means. The image formed on the elements of a deflector array can be electronically scanned and electrical output from each element identified such that the distance of an image point, corresponding to the transmitter point source, from the

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array centre can be determined. Hence the required off-axis angle is obtained. A staring array falls into this class.

(d) Several possibilities exist for the number and type of controllable aerodynamic surfaces (or aerofoils) designed into the drogue.

The diagrams are illustrative of just two examples. For the simplest system a single aerofoil controls the movement of the drogue in one dimension. A preferred concept is for two or more aerofoils to control the drogue in two dimensions, as previously described.

An additional independent system can be applied to the motion of the drogue system only, in the third dimension i.e. along aircraft trajectory.

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