TECHNICAL FIELD

[0001] The present invention relates to a method and system of controlling an internal combustion engine of an unmanned aerial vehicle (UAV).

BACKGROUND ART

[0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0003] The Applicant’s International Publication W02020160625A1 , the contents of which are hereby incorporated herein by reference, discloses a method of controlling an internal combustion engine of a UAV, having a fuel delivery means operable to deliver a metered quantity of fuel to a combustion chamber of the engine, and an air flow control means for regulating air flow to the combustion chamber. The method comprises controlling the engine through control of fuelling by way of the fuel delivery means independently of the air flow control means. This provides for fuel-led control of the engine system with a fuelling requirement for the engine being determined and implemented, and with the corresponding air requirement then being determined contingent on the fuelling requirement.

[0004] During operation of a UAV, a condition known as “windmilling” can arise. As the term suggests, windmilling occurs when a propeller of a UAV is rotated due to prevailing airflow or wind speed. This may cause an over-speed condition in which the engine speed rises above its demanded set-point, the degree of windmilling being a function of, amongst other things, the altitude of the UAV, wind speed and/or the rate of descent of the UAV.

[0005] Currently, increased engine speed resulting from such a windmilling condition is sought to be minimised or reduced by transitioning the engine to a minimum fuelling condition. However, this minimum fuelling condition can potentially cause poor run quality with significant engine speed excursions and vibration issues arising on some engines. “Run quality” here is understood to primarily relate to the performance, stability, and smoothness of engine operation, where performance relates to the ability of the engine to deliver consistent power output, stability relates to the ability of the engine to maintain a steady and reliable running condition without sudden unexpected fluctuations, and smoothness is effectively the absence of noticeable variations or irregularities in engine speed. Stable combustion and smooth engine operation hence typically provide for reliable engine speed and torque, with minimal or no misfires and no unexpected or excessive NVH (i.e. noise, vibration, harshness) issues.

[0006] Issues relating to poor run quality are also worsened in the absence of electrical load. Often, and depending on the rate of descent of the UAV and the altitude (which directly impacts the air density seen by the engine of the UAV), the minimum fuelling condition applied is unable to completely address the increased engine speed resulting from windmilling as it is not possible to simply continue reducing the minimum fuelling condition as the engine would get to a point where it would inevitably stall.

[0007] Furthermore, even though the reduction to a minimum fuelling condition helps the engine speed to fall below the windmilling speed threshold, the flight controller may continue to reduce the engine speed demand input to a minimum value even though this is not required for efficient engine control and, below a certain engine demand threshold, is typically unlikely to affect engine speed in any event. In such a situation, when engine speed demand is once again increased, there may appear to be an apparent lack of response until the engine speed demand once again rises above the windmilling speed threshold.

[0008] The present invention aims to provide a method and system to ameliorate the above-described UAV engine control problems.

SUMMARY OF INVENTION

[0009] In a first aspect, the present invention provides a method of controlling an internal combustion engine of a UAV, the engine having a fuel delivery means operable to deliver a fuel to a combustion chamber of the engine, the method comprising: controlling the engine through control of fuelling by way of the fuel delivery means, said control of fuelling including the steps of: determining whether the engine is operating in a first condition; and determining a minimum fuelling requirement (FPCmin) for the engine; wherein, when the engine is determined to be operating in the first condition, the minimum fuelling requirement is increased from FPCmin to FPCmin+ AFPCfirst.

[0010] Typically, the fuelling requirement for the engine is determined based on information received from a flight controller. The flight controller effectively demands an increase or decrease in engine torque which corresponds to a change in propeller speed for the UAV engine in the known manner. That is, the flight controller requests an engine demand, which is effectively an engine speed demand based on the propeller curve for the UAV engine, which leads to a fuelling change for the engine that is effected by the fuel delivery means.

[001 1 ] The first condition typically equates to a windmilling condition in which engine speed is increased due to prevailing airflow increasing the propeller speed of the UAV. Where the fuelling requirement, based on a request from the flight controller, is a minimum fuelling requirement (FPCmin), this is increased by AFPCfirst to increase engine speed. The increase in fuelling requirement is that determined to be sufficient to improve run quality and reduce engine speed excursions to an acceptable speed range, as observed empirically, for example, from a plot of engine speed against time. For example, where the minimum fuelling requirement is set to say 0.4 FPC, the increase in fuelling requirement could be such as to change the FPC setting to say 0.42 or 0.44 FPC. The effect of AFPCfirst on the engine speed is expected to be small, for example a few hundred rpm, but nonetheless sufficient to improve run quality without losing the advantages of fuel-led engine control.

[0012] The increased fuelling requirement, AFPCfirst, for the first condition may be a fixed value. However, with potential further benefit in terms of improving run quality, AFPCfirst may be made a function of one or more parameters, advantageously including altitude of the UAV, said altitude including the case where the UAV is on the ground. Temperature, although typically related to altitude, may also be a parameter. As barometric pressure is a function of altitude it is also convenient for AFPCfirst to be a function of barometric pressure. AFPCfirst may also be made a function of other engine or flight operating parameters, such as the rate of descent of the UAV, or for example electrical or other load(s) on the engine, as the fuelling requirement can be determined taking the beneficial effects of such loads on engine run quality into account.

[0013] Where lag in engine speed response to increased engine speed demand is an issue, in particular, during exit from the windmilling condition, such lag may conveniently be reduced through the following steps: determining whether the engine is exiting from the first condition; and if the engine is exiting from the first condition and a change in engine speed demand input is detected, increasing the engine speed setpoint in proportion to the detected change in engine speed demand input.

[0014] Typically, there is an upper limit engine speed at which the above-described fuel-led engine speed control strategy can no longer be implemented and a fuel-led open loop control strategy is necessary. As the adjusted engine speed setpoint approaches the upper limit engine speed, the degree of adjustment of the engine speed setpoint or scaling factor is curtailed to maintain the engine speed setpoint below the upper limit engine speed. The scaling factor may be varied within a range of 1 to 1 .6, with a scaling factor of 1 indicating that the engine speed setpoint is not adjusted. This allows improvement in engine speed response as the engine exits the first condition, in particular the windmilling engine operating condition. Further, if the windmilling speed is greater than the upper limit engine speed, the strategy cannot be implemented because the system transitions to open loop fuel-led control.

[0015] As lagging and run quality issues are typically encountered at the same time, the increase of fuelling and controlled increase of engine speed input are advantageously implemented in combination with the controller configured to achieve this. However, it is possible that run quality could be deemed acceptable but lag in engine response is deemed unacceptable. In such an embodiment, the present invention provides - in a second aspect - a method of controlling an internal combustion engine of a UAV, the engine having a fuel delivery means operable to deliver a fuel to a combustion chamber of the engine, the method comprising: controlling the engine through control of fuelling by way of the fuel delivery means; determining whether the engine is exiting from a first condition; and if the engine is exiting from the first condition and a change in engine speed demand input is detected, increasing the engine speed setpoint in proportion to the detected change in engine speed demand input.

[0016] Preferably, the internal combustion engine includes an air flow control means for regulating air flow to the combustion chamber and the method includes determining an air flow requirement based on or with reference to the fuelling requirement. The control of the engine is therefore preferably fuel-led with a fuelling requirement for the engine being determined and implemented, typically by an electronic control unit (ECU), independently of air flow. The corresponding air requirement is then determined contingent upon the determined fuelling requirement. Fuel-led control of the engine provides certain advantages over throttle or air-led control, including those described in International Publication W02020160625A1 , incorporated herein by reference for all purposes.

[0017] The method of the present invention would typically comprise an engine speed feedback loop. The engine speed feedback loop may be configured to adjust fuelling to achieve a target engine speed as required. The engine speed feedback loop may comprise a controller operable to adjust fuelling to reduce engine speed error and/or maintain engine speed at a target speed or set-point speed. Conveniently, the controller includes PI control loop(s). A first PI control loop may control the abovedescribed scaling factor for engine demand speed setpoint. A second PI control loop is conveniently the engine speed feedback loop mentioned above. The first PI control loop may preferably sample more slowly than the second PI control loop.

[0018] In a third aspect, the present invention provides a UAV engine system comprising a combustion chamber, flow control means for regulating air flow to the combustion chamber, and fuel delivery means operable to deliver fuel into the combustion chamber, the engine system comprising: a controller for controlling the engine through control of fuelling by way of the fuel delivery means, said controller being configured to implement the steps of: determining whether the engine is operating in a first condition; and determining a minimum fuelling requirement (FPCmin) for the engine; wherein, when the controller determines that the engine is operating in the first condition, the controller increases the minimum fuelling requirement from FPCmin tO FPCmin+ AFPCfirst.

[0019] Typically, the fuelling requirement for the engine is determined based on information received from a flight controller. The flight controller effectively demands an increase or decrease in engine torque, which corresponds to a change in propeller speed for the UAV engine, in the known manner. That is, the flight controller requests an engine demand, which is effectively an engine speed demand based on the propeller curve for the UAV engine, which leads to a fuelling change for the engine that is effected by the fuel delivery means.

[0020] In a fourth aspect, the present invention provides a UAV engine system comprising a flow control means for regulating air flow to the combustion chamber, and a fuel delivery means operable to deliver a fuel to a combustion chamber of the engine, the engine system comprising: a controller for controlling the engine through control of fuelling by way of the fuel delivery means, said controller being configured to: determine whether the engine is exiting from the first condition; and if the engine is exiting from the first condition and a change in engine speed demand input is detected, increasing the engine speed setpoint in proportion to the detected change in engine speed demand input.

[0021] In a fifth aspect, the present invention provides a UAV powered by an internal combustion engine controllable in accordance with a method according to the first aspect of the present invention.

[0022] The fuel delivery means may comprise a dual-fluid fuel injection system or a single-fluid fuel injection system.

[0023] The fuel delivery means may be operable to deliver fuel entrained in a gas directly into the combustion chamber. [0024] The method and system for controlling an internal combustion engine of a UAV during, or on exit from, windmilling helps to provide several advantages. Firstly, through implementation of an increase to a minimum fuelling requirement - when such is set in response to the UAV entering a windmilling condition - run quality of the UAV is markedly improved, even with no electrical or other load(s) acting on the internal combustion engine. Secondly, through increase of engine speed by a controlled adjustment in response to engine demand, apparent lags in engine response when a UAV is exiting a windmilling condition are favourably addressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of a UAV incorporating an internal combustion engine controlled by a system and method according to one embodiment of the present invention addressing UAV operation during a windmilling condition;

Figure 2 is a graph of engine speed with time for the UAV of Figure 1 when the UAV is operating under a windmilling condition;

Figure 3 is a graph of actual engine speed (RPMA) with demand engine speed (RPMD) for a UAV exiting a windmilling condition;

Figure 4 is a graph of engine speed and engine demand with time for the UAV of Figure 1 when exiting from the windmilling condition; and

Figure 5 is a block diagram for a control system for the UAV of Figure 1 according to one embodiment of the present invention operating during windmilling.

[0026] The drawings shown are not necessarily to scale with emphasis instead generally being placed upon illustrating the principles of the present disclosure. [0027] The drawings depict an embodiment exemplifying the principles of the present disclosure. The embodiment illustrates a certain configuration, however, it is to be appreciated that the inventive principles can be implemented by way of many different configurations as would be obvious to the person skilled in the art, whilst still embodying any of the inventive principles. These configurations are to be considered within the embodiment described herein.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0028] Referring now to Figure 1 , there is schematically shown a UAV 100 incorporating an engine system 101. The engine system 101 comprises a small, single-cylinder reciprocating piston two-stroke engine 102 operating under the control of an electronic control unit (ECU) 105. The engine 102 is arranged to drive a propulsion element provided in the form of a propeller 103. An air intake system (not shown) is provided to deliver combustion air to a combustion chamber (not shown) of the engine 102. The air intake system includes an air intake path (not shown) incorporating an air flow control means in the form of a throttle assembly (not shown) operable under the control of the ECU 105 in the known manner.

[0029] The engine system 101 further comprises a fuel injection system (identified schematically by the group 123 in Figure 1 ) comprising a fuel delivery means (not shown) by means of which fuel is delivered directly into the combustion chamber of the engine 102. The fuel injection system 123 operates under the control of the ECU 105 which includes a controller (identified schematically by the group 133 in Figure 1 ). The fuel injection system 123 may comprise a dual-fluid direct fuel injection system facilitating an air-assist fuel delivery process wherein fuel entrained in air is delivered directly into the combustion chamber of the engine 102. Other forms of fuel injection systems are however also contemplated for use with the present invention.

[0030] The fuel injection system 123 is operable under the control of the ECU 105 in response to a flight request (identified schematically by block 124a in Figure 1 ). The throttle assembly is also operable under the control of the ECU 105 (via a throttle control module (not shown)). However, operation of the fuel injection system 123 is not linked to operation of the throttle assembly. That is, the engine 102 is controlled as a fuel-led system instead of a throttle or air-led control system. [0031] Engine torque/power and speed can be selectively increased or decreased by control of the fuel injection system 123, regulating fuel delivery to the combustion chamber of the engine 102 by the fuel delivery means. The throttle assembly is then operated in response to the fuel requirement under the control of the ECU 105. In this way, the air fuel ratio can be chosen for any demand fuelling level providing for advantageous control and operation of the UAV engine 102.

[0032] In response to the flight request 124a sent via the onboard flight controller (identified schematically by block 124 in Figure 1 ), a signal representative of the request is transmitted to the ECU 105. The ECU 105, by way of the controller 133, compares the current engine speed to a target engine speed commensurate with the flight request 124a. If there is a determination by the ECU 105 that the flight request 124a entails a change in engine speed (or torque) which necessitates a variation in fuelling, the ECU 105 determines the necessary fuelling requirement with reference to fuelling maps 125a and 125b which provide fuelling rates as a function of engine speed (or torque).

[0033] In the arrangement shown, the two fuelling maps 125a and 125b are provided for selection dependent upon altitude of the UAV 100 (i.e. high and low altitude fuelling maps respectively). The fuelling rate selected by the ECU 105 is typically interpolated from the two fuelling maps according to the prevailing altitude. The ECU 105 then controls the fuel injection system 123 to provide the required fuelling for the engine 102. Further description of determination of fuelling rate with respect to the two fuelling maps 125a and 125b is provided in the Applicant’s International Publication No. W02020160625A1 , incorporated herein by reference for all purposes. Other fuelling map arrangements could also be implemented depending on specific operating and vehicle requirements.

[0034] The ECU 105 features an engine speed feedback loop incorporating a controller (identified schematically by block 135 in Figure 1 and blocks 201 -211 of Figure 5) based on PI control methodology. With the controller 135, a proportional and integral constant algorithm restores actual engine speed to desired (target) speed in an optimum way with a fuel-led control strategy as described in the Applicant’s International Publication No. W02020160625A1 incorporated herein by reference.

[0035] Air flow to the engine 102 is determined as a function of the fuelling rate determination from fuelling maps 125a and 125b which is delivered as an input to a throttle position map with further description of the relationship between fuelling rate and throttle position settings also being provided in the Applicant’s International Publication No. W02020160625A1 , incorporated by reference for all purposes. The ECU 105 then controls the throttle assembly (not shown) as necessary via the throttle control module (not shown) to provide the required intake air for the combustion chamber of the engine 102.

[0036] In operating the UAV 100 remotely, a user can issue operational commands/signals via a remote controller, including flight requests 124a which demand certain engine operating conditions such as, for example, a particular engine speed or engine torque/power. When the user issues a flight request (via the remote controller), the request is received by the on-board flight controller 124 and communicated to the ECU 105 which electronically assesses the request. If the ECU 105 makes a determination to implement the flight request in a manner which would involve a change in the fuelling requirement, the ECU 105 would operate the fuel injection system 123 by way of the controller 133 as further described below.

[0037] In order to achieve a change in engine speed or engine torque/power, the ECU 105 operates the fuel injection system 123 to bring about a rapid (i.e. an almost immediate) change in fuelling (fuel per engine cycle or FPC) and engine speed. If the change necessitates a change in air flow and thus a change in throttle position, there is still a need for physical movement of the throttle valve of the throttle assembly, which is effected by the air based system referred to as AIR CONTROL in Figure 1 and described in the Applicant’s International Publication No. W02020160625A1 .

[0038] When the UAV 100 enters a first engine operating condition - here a windmilling condition - with propeller speed being increased above setpoint due to prevailing wind and airflow conditions, the controller 133 of Figure 1 detects that the windmilling condition has been entered and reduces engine demand with fuelling also reducing in an initial phase.

[0039] Beyond a certain point, even though the controller 133 reduces fuelling to a minimum fuelling requirement (FPCmin, of say 0.4 FPC) set for the engine 102, the engine speed no longer falls, and instead the engine speed “hangs up” or fluctuates within a range, effectively independent of engine demand which continues to be reduced to a minimum engine demand as set by the controller 133. [0040] At the same time, and in a situation exacerbated by low load on the engine 102 and fuelling at FPCmin, run quality for engine 102 and hence UAV 100 deteriorates as can be observed from the “bumpy” or erratic average engine speed (RPM Avg) trace portion A of Figure 2. The excursions or fluctuations from engine speed setpoint, which could be used as a threshold criterion for implementing control over engine 102 operation during windmilling, are both uneven and significant, preventing smooth, low vibration operation of the engine 102. Without intervention from the ECU 105, this poor run quality would continue until the engine 102 exits the windmilling condition.

[0041 ] In contrast, in this embodiment, the minimum fuelling requirement FPCmin set for engine 102 and stored in the ECU 105 is increased by an increment AFPCfirst, which may be determined through trial and error and stored in the ECU 105 as apparent from the following. If - for example - the ECU 105, during windmilling, raises the target engine fuelling requirement (i.e. (FPCmin + AFPCfirst)’ to 0.42 FPC, the increase in target fuelling requirement increases the engine speed slightly as shown for trace portion B of the engine speed against time plot of Figure 2. Run quality is thereby significantly improved as evidenced by lesser and more uniform engine speed fluctuation and engine speed fluctuation within a slightly narrower range.

[0042] Further benefit is achieved by increasing the target fuelling requirement for the windmilling condition (FPCmin + AFPCfirst)’ from 0.42 FPC to a slightly higher value (FPCmin + AFPCfirst)’, here being 0.44 FPC which increases engine speed still further - and to a greater degree than shown in the adjustment from 0.4 to 0.42 FPC to provide trace portion B of the engine speed against time plot of Figure 2 - as shown for trace portion C of the plot. Run quality is significantly improved over that shown in trace portion B and substantially improved over that shown in trace portion A.

[0043] It is to be understood that the values of FPC given above are provided as examples only and are not in any way intended to be limiting. As has been described above, the determination of an optimal FPCmin + AFPCfirst target fuelling could take into account a range of parameters of which barometric pressure, altitude, and the rate of descent, as taken into account in this embodiment, as will be described further below, are selected. However, other engine and flight operating parameters could be taken into account and the assessment of engine run quality is, necessarily subjective, though the person skilled in the art would understand trace portions B and C of Figure 2 to show improved run quality over trace portion A where minimum fuelling is not adjusted to account for windmilling behaviour.

[0044] The effect of the fuelling increment AFPCfirst on the engine speed of UAV 100 is expected to be small, for example a few hundred rpm sufficient to improve run quality, but not typically high enough to take engine speed over an upper limit above which the fuel-led strategy can no longer be implemented and would need to be replaced with a fuel-led open loop strategy which cannot provide any benefits for the control of engine 102 during windmilling as described herein. Controlling the fuelling to the engine 102 to only increase the engine speed by a few hundred rpm as described also has the advantage of not making the engine speed so high that the engine 102 loses engine braking ability (i.e. whereupon the UAV cannot descend safely because the resulting propeller speed is too high). Control of the fuelling in this way hence provides an acceptable trade-off between engine run quality and not increasing the engine speed by too much which would be detrimental for other reasons.

[0045] The fuelling increment, AFPCfirst, as described could be a fixed value stored in ECU memory, the fixed value being determined by a trial-and-error process for a particular engine and propeller arrangement as described above. However, with potential further benefit in terms of improving run quality and, observing that windmilling behaviour is - in any event - affected by a number of parameters, AFPCfirst may advantageously be made a function of barometric pressure which itself is a function of altitude. The AFPCfirst could also be modified to a different value if, for example, a sensed electrical load is acting on the engine 102.

[0046] Propeller loading may also usefully be taken into account in the calculation of AFPCfirst. Determination of AFPCfirst could also depend on the detected range of fluctuation in engine speed, the degree of fluctuation providing a measure of run quality and being mapped to a sufficient AFPCfirst to provide acceptable run quality, if required. It will be understood that an option when run quality is acceptable, even in a windmilling condition, is to maintain the engine speed control methodology of the Applicant’s International Publication No. W02020160625A1 , incorporated herein by reference.

[0047] It will be understood that wind conditions and incoming air flow are variable and windmilling effects may dissipate during a flight. As the UAV operator looks to exit the windmilling operating condition for the UAV 100, the fuelling at FPCmin, or even FPCmin + AFPCfirst dictated by engine control during windmilling, typically creates a further issue in that - during recovery - engine speed response may unacceptably lag engine speed demanded by the flight controller. This lag is apparent from a consideration of Figure 3.

[0048] If, during exit from windmilling of UAV 100, the operator increases the demand engine speed (RPMD) as shown by engine speed trace 31 a, it can be seen from schematic Figure 3 that - under conventional engine control methodology - sensed or actual engine speed (RPMA) remains the same, i.e. it is unresponsive to the engine demand increase, as shown by engine speed trace 33a. It is only when RPMD is increased further by a step change 31 b to a higher engine speed as shown by engine speed trace 31 c, where RPMD exceeds windmilling speed RPMw, that RPMA starts to increase at 33b to the demand engine speed at 33c where (RPMA=RPMD) > RPMw. This lagging response - and, in particular, the non-response at 33a to engine demand 31 a (i.e. effectively a “dead zone”) - can be perceived as a lack of response by the operator. Embodiments of the presently described method and system for controlling internal combustion engine operation of a UAV address this issue as described below with reference to Figures 3 to 5.

[0049] In Figure 3, the increase in RPMD to demand speed 31 a, using conventional control methodology, resulted in no increase in sensed engine speed 33a. The method and system described herein, in contrast, adjusts the target engine speed setpoint (RPMs) to an adjusted engine speed setpoint RPMs in proportion to the change in engine speed demand, i.e. change in RPMD. Thus, contrary to the Figure 3 example, the increase of RPMD to 31 a of the present embodiment would result in an increase in the target engine speed setpoint with this being described with reference to Figure 4.

[0050] In Figure 4, engine speed demand RPMD is increased from minimum engine speed demand M by the flight controller by a step change in RPMD at ti. However, unlike the unresponsive scenario described with reference to Figure 3, the ECU here adjusts RPMs’ to RPMs as a proportion of the demanded increase in RPMD. In this embodiment, RPMD is multiplied by a scale factor of between 1 and 1 .6 to provide an increased RPMs at Y1 , i.e. ARPMs is not zero as in conventional practice. This can be seen at point X of Figure 4 where the change in RPMs is a proportion of the increase in RPMD. RPMS is then input to the controller 133 with the consequence that fuelling requirement FPC is increased; engine speed consequently increases as shown in the increasing value of RPMA trace Z in Figure 4. It will be noted that, consistently with the engine speed control methodology described above, FPC and RPMA rapidly, i.e. almost immediately, respond to the increase in RPMs.

[0051] As Figure 4 shows, the operator may require a still higher engine speed for UAV 100 and may implement a further step change in RPMD at Y2. In this case, the ECU - determining that RPMD and the associated RPMs will be less than an upper limit engine speed, T, for controlling the engine 102 with a fuel-led control strategy, allows the step change in RPMD while applying a scale factor of between 1 and 1.6 It will be understood however that the ECU could also have applied a scale factor of 1 to the step change in RPMD at Y1 . However, if the FPC requirement is already more than the FPC target that would be applied by the windmilling control strategy to raise the fuelling requirement from a minimum of say 0.4 FPC to improve run quality, then there would be no job for the windmilling control strategy to do, and so in this case the scale factor is kept at 1 at all times so the scaler does nothing as it has no impact on the target RPM.

[0052] As can been seen in Figure 4, a final step change in RPMD is demanded at Y3 with RPMs levelling out at Z which is less than the upper limit engine speed T. If RPMs were to exceed T, the ECU would switch to a fuel-led open loop air led strategy as described in the Applicant’s International Publication No. W02020160625A1 , incorporated herein by reference. That is, the windmilling control strategy cannot be implemented for engine speeds higher than T because the system transitions to open loop fuel-led control at that point. The control strategy described with reference to Figure 4 hence provides the operator of the UAV 100 with an acceptable response in RPMs and RPMA when engine speed demand RPMD is increased on exit from windmilling.

[0053] Referring to Figure 5, there is shown a block diagram for the controller 133 which, while compatible with the engine control system described in the Applicant’s International Publication No. W02020160625A1 , is significantly modified. This significant modification, involving a combination of an additional relatively slow outer PI controller 201 -204 sampling at 20ms intervals combined with the previously described fast PI controller 207-211 sampling each engine cycle, conveniently at Top Dead Centre (TDC), achieves improved run quality during windmilling and improved engine speed response when exiting windmilling as described hereinbefore with reference to Figures 3 and 4. [0054] Considering Figure 5 further, a target fuelling is first set for the windmilling condition. When windmilling is detected, with engine “Demand” (RPMD) to RPMs lookup map 209 at a minimum, the controller 133 sets a target fuelling requirement FPCmin + AFPCfirst, with AFPCfirst being dependent on barometric pressure (BAP) and located by the controller 133 in a table of AFPCfirst against BAP as part of the base fuelling loop. This target fuelling requirement together with a filtered actual FPC (FPCt) are inputs to the slow PI controller 201 -204 sampling at 20ms intervals. The actual FPC is filtered to reduce noise on the sensed FPC value. The PI controller 201 -204, with engine speed demand RPMD at a minimum, and after an initial decline independent of engine speed, during windmilling, is not required to proportion “Demand” (RPMD) as this is already at a minimum (hence the PI controller 201 -204 is in effect providing integral only control). RPMAvg is at the windmilling speed with the influence of the PI controller 201 -204 on the engine speed diminished to nil because, due to windmilling, it sees an RPMAvg that is higher than the target set point (i.e. it has wound the fuelling integral term out to the maximum allowed negative value). Feedback control is implemented to control the actual FPC to FPCmin + AFPCfirst. This mode of control is maintained throughout windmilling mode of operation for the UAV 100.

[0055] On exit from the windmilling condition, controller 133 operates differently. Here “Demand” or RPMD is increased and a new initial setpoint engine speed RPMs’ is obtained from demand engine speed setpoint look-up map 209 in response to RPMD. Provided that RPMs’ is lower than upper limit engine speed T, the slow PI controller 201 -204 sets a scaling factor between 1 to 1 .6 to be applied at block 205 to RPMs from look-up map 209 to provide the engine speed setpoint RPMs. The scaling factor is set as a function of target fuelling and actual fuelling. More specifically, the scaling factor raises the RPM setpoint from map 209 and then, in the fast-acting PI controller 207-211 , it adds or subtracts fuel in proportion to the difference between the actual RPM and the RPM setpoint that has already been modified by the scaling factor. A scaling factor of greater than 1 may be applied where the increased demand RPMD and setpoint cannot exceed T as shown for the change Y2 in RPMD as shown in Figure 4. As RPMs approaches T, as determined from actual engine speed RPMA or RPMAvg, the scaling factor may approach 1 though, where possible, it is maintained high enough to provide a perceptible engine speed response. Consequentially, fuelling requirement is obtained from look-up map 210 and delivered to engine 102 through the fuel injection system 123 at block 21 1. This control regime, providing feedback control over engine speed (RPMAvg or RPMA) may be maintained for so long as RPMA is less than RPMw and T and, irrespective of the number of changes in RPMD demanded by an operator.

[0056] If RPMs exceeds T, then - in this embodiment - a fuel-led open loop strategy is implemented. The fuel-led multi-PI control loop implemented engine control strategy described above ends at this point.

[0057] The subject method and system for controlling an internal combustion engine of a UAV during, or on exit from, windmilling provides a number of advantages. Firstly, through implementation of an increase to a minimum fuelling requirement - when such is set in response to the UAV entering a windmilling condition - run quality of the UAV is markedly improved, even with no electrical or other load(s) acting on the internal combustion engine. Secondly, through increase of engine speed by a controlled adjustment in response to engine demand, apparent lags in engine response when a UAV is exiting a windmilling condition are also suitably addressed.

[0058] Those skilled in the art will appreciate that the method and system of controlling an internal combustion engine of a UAV as described herein is susceptible to variations and modifications other than those specifically described. The invention includes all such variations and modifications. The invention also includes all of the steps, features, formulations and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

[0059] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.