HIGH-ALTITUDE PSEUDO SATELLITE CONTROL

Abstract

A High Altitude Pseudo Satellite (HAPS) aircraft is disclosed, the aircraft including at least one aeroelastic span loaded fixed wing, an aspect ratio greater than 15 and wing loading less than 6 kg/m.sup.2, where the at least one wing has a plurality of spoilers distributed across the span of the wing and each spoiler being chordwise located adjacent the centre of pressure of the wing. The HAPS aircraft further includes a control system for controlling the spoilers, sensors which allow at least one of the quantity or quantities selected from the group comprising the amount of lift at points or regions along the wing span the pitch and roll at points or regions along the wing span, the bending and torsional strain at points or regions along the wing span, or the net speed and roll and pitch angle of the wing to be determined by the control system, and the spoiler being activatable to reduce the lift experienced by the wing in the location of the spoiler in response to the quantities determined by control system.

Claims

1. A High Altitude Pseudo Satellite (HAPS) aircraft the aircraft having at least one aeroelastic span loaded fixed wing, an aspect ratio greater than 15 and wing loading less than 6 kg/m.sup.2; the at least one wing having a plurality of spoilers distributed across the span of the wing, each spoiler being chordwise located adjacent the centre of pressure of the wing; a control system for controlling the spoilers; sensors which allow at least one of the quantity or quantities selected from the group comprising the amount of lift at points or regions along the wing span the pitch and roll at points or regions along the wing span the bending and torsional strain at points or regions along the wing span the net speed and roll and pitch angle of the wing to be determined by the control system; the spoiler being activatable to reduce the lift experienced by the wing in the location of the spoiler in response to the quantities determined by control system.

2. The A HAPS aircraft according to claim 1 wherein the sensors and control system detects relative differences in lift and/or pitch and/or roll and/or bending and torsional strain along the wingspan which cause the wing to deform, or indicate that the wing has deformed from the optimal shape of the at least one wing, and activates one or more particular spoilers to reduce elevated lift at points or regions and either maintain or reduce the divergence from the optimal wing shape.

3. The A HAPS aircraft according to claim 2 wherein more than one optimal wing shape is stored or generated, and selected according to particular environmental criteria and/or intended manoeuvres.

4. The HAPS aircraft according to claim 1 wherein the sensors and control system detect hazardous errors in the speed or roll angle of the wing and activate a particular spoiler or plurality of spoilers to reduce the speed or roll angle error of the wing.

5. The HAPS aircraft according to claim 1 wherein the control system activates the spoilers to reduce the lift of the HAPS when the HAPS is descending through the troposphere.

6. The HAPS aircraft according to claim 1 wherein the sensors include a plurality of attitude sensors distributed along the wing span.

7. A method of controlling the flight of a High Altitude Pseudo Satellite (HAPS) aircraft, the aircraft having at least one aeroelastic span loaded fixed wing, an aspect ratio greater than 15 and wing loading less than 6 kg/m.sup.2 comprising the steps of sensing a quantity from which lift at points or regions along the wing, and/or local pitch and roll attitude, and/or local bending and torsional strain can be determined detecting relative differences in the lift, and/or pitch and roll attitude that would cause or account for an adverse deformation in the optimal wing shape activating a spoiler on the wing proximal to the where the relative elevated lift is occurring so as to reduce or eliminate the deformation that would otherwise occur.

8. The method according to claim 7 wherein more than one optimal wing shape is stored or generated, and selected according to particular environmental criteria and/or intended manoeuvres.

9. The method according to claim 7 wherein the spoiler may be activated incrementally, the amount of activation being proportional to the relative excess lift determined at the point or region of the spoiler.

10. The method according to claim 7 wherein the spoilers are activated to reduce the lift of the HAPS when the HAPS is descending through the troposphere.

11. A method of controlling the flight of a High Altitude Pseudo Satellite (HAPS) aircraft, the aircraft having at least one aeroelastic span loaded fixed wing, an aspect ratio greater than 15 and wing loading less than 6 kg/m.sup.2 comprising the step of activating one or more spoilers to reduce the lift of the HAPS when the HAPS is descending through the troposphere.

12. The method according to claim 11 wherein there are included the steps of sensing a quantity from which lift at points or regions along the wing can be determined detecting relative differences in the lift that would cause adverse deformation in the wing shape activating a spoiler on the wing proximal to the where the relative elevated lift is occurring so as to reduce or eliminate the deformation that would otherwise occur.

13. The method according to claim 12 wherein the spoiler may be activated incrementally, the amount of activation being proportional to the relative excess lift determined at the point or region of the spoiler.

Description

[0037] The invention will now be described, by way of example, with reference to the drawings, of which

[0038] FIG. 1 is a front elevation and plan view of a HAPS aircraft according to an embodiment of the invention;

[0039] FIGS. 2a and 2b are a lateral sections of the wing of the HAPS aircraft

[0040] FIG. 3 is a front elevation view of a HAPS aircraft with an example of an incoming air velocity field;

[0041] FIG. 4 is a front elevation view of a HAPS aircraft with an example of a different incoming air velocity field;

[0042] FIG. 5 depicts the process for flight envelope protection—Spanwise Shape Control;

[0043] FIG. 6 depicts the process for flight envelope protection—Speed or Roll Control;

[0044] FIG. 7 is a front elevation view of a HAPS aircraft with three examples of different span wise shapes representing different desirable dihedral conditions for different phases of flight; and

[0045] FIG. 8 presents HAPS use case examples for selecting optimal dihedral shape demand as a function of operating altitude.

[0046] The premise of the invention is a new gust alleviation control system for maintaining desired aerodynamic derivatives for high aspect ratio and low wing loading, fixed wing HAPS and is based on actively controlling the span wise lift distribution during turbulence, using a system of distributed spoilers, which are used to maintain or change the span wise shape of an aeroelastic structure, and control speed and wing torsion when combined with a system of one or more fuselages with supporting elevators.

[0047] An example of the invention is depicted in FIG. 1. which presents a span loaded HAPS wing 1 supporting a distributed avionics and power generation and storage system. The HAPS wing 1 has a distributed propulsion system 2 for generation of thrust in cruise, climb, and when multiple propulsion systems are used, has the potential to generate differential thrust to augment flight control and navigation. The HAPS wing 1 has one or more fuselages 3 with attached elevator(s) 4 to control pitch, speed, and total torsion in the wing. A single tail can be used, or multiple tails 18 used as shown here to minimise total exposed torsion to within the ultimate torsion limit of the aerostructure, or to provide increased speed envelop, or permit the use of a lighter weight structure which is less stiff in torsion. Each fuselage 3 also supports a vertical fin and rudder 5 to provide lateral stability and augment yaw and roll control.

[0048] The main feature of this aircraft is that the span loaded wing 1 is also fitted with a distributed system of spoilers 6, which are located near the aerodynamic centre of pressure, so that when deployed, they achieve a lift modification with minimum effect on wing pitching moment and longitudinal stability. This is as opposed to trailing edge devices, which for example, impart a torsion and pitching moment into the wing. The spoilers 6 are used to modify the spanwise lift distribution to protect the aerostructure when encountering turbulence. They are used in conjunction with a distributed system of attitude sensors 7, which are installed near each spoiler and which allow the spoilers to be used to maintain aircraft stability and speed range through maintaining the span wise shape of the aircraft. Local Angle of Attack (AoA), Yaw and Airspeed are measured using a local air data probe 19 installed near the front of each fuselage.

[0049] Referring to FIG. 2. a typical chord wise lift distribution on the upper surface of an example aerofoil section of the wing 1, illustrated schematically is the lift distribution 9 in cruise with associated centre of pressure 8. Also depicted is the change in chord wise lift distribution 10 for slower speeds, or higher AoA where the centre of pressure moves 8 forward, and similarly lift distribution 11 for higher speeds, or lower AoA, the centre of pressure moves 9 rearwards. Of particular interest is the high-speed case where the movement of the centre of pressure rearwards inputs a chord wise torsion which pitches the wing downwards. In large wingspan aircraft especially highly flexible structures, the compounding torsional deflections lead to change in lift distribution, possibly negative lift at tips which can be divergent and result in structural failure or ‘tuck under’. Even if divergent failure does not occur, a likely reduction in dihedral shape of the wing 1 can contribute to a loss of roll stability and control, increasing the likelihood of further turbulence causing a departure from the flight envelope.

[0050] The approximate location of the spoiler 6 on the wing 1 is also indicated in FIG. 2, this should be located on or near the centre of pressure in cruise. Activation of this spoiler in the overspeed case can alleviate the pitching moment to assist in speed control, reducing torsion and restore the correct dihedral shape of the aircraft.

[0051] The spoiler 6 itself can be a front hinged spoiler type or fence spoiler, with exact dimensions specific to the implementation on the selected aerofoil(s), wingspan and aeroelasticity of the design solution. Actuation is through an electromechanical servo causing the spoiler to pivot along arrow a.

[0052] The aircraft employing this system is scalable with span in accordance with the principles of span loaded design, that is to maintain total bending loads and torsion loads over the entire flight envelope, as well as permitted gust envelope, to within the specification of the aero-structural limits. In this way, a smaller span HAPS aircraft using distributed spoilers to control spanwise shape, could be designed with just one fuselage and elevator to control pitch and speed, while maintaining the aerostructure within its torsion envelope. A larger span aircraft might require two or more fuselages with elevators to limit the total exposed torsion along the wingspan. When turbulence is encountered, the spoilers are used to maintain span wise shape and alleviate torsion build up due to excessive high, or low, speeds. Maintaining the designed span wise shape in turn maintains stability in flight control and flight control authority, while the elevators maintain local angle of attack and ultimately control the speed of the HAPS.

[0053] The aircraft offers an exceptionally light weight, simple and efficient sensing, and actuation system, to protect the shape of the aircraft when turbulence or gusts are encountered. Controlling the spanwise shape of the HAPS wing to the optimum shape, ensures the design's intended stability characteristics and flight control authority is always present. If the shape is not maintained during turbulence or on encountering a gust, and the span wise shape were to deform into a less aerodynamically stable shape, then the result is likely to be insufficient flight control authority to prevent or limit the build-up of speed, or excessive roll angle. This would ultimately cause catastrophic departure from the flight envelope and structural failure in flight. The mass and simplicity of the spanwise lift modification system, based on spoilers, is an important aspect of the innovation. Since the purpose of this configuration is to protect the HAPS aircraft during exposure to turbulence, without forcing the aircraft to carry a significant weight, drag or complexity penalty throughout all other phases of flight. The stratosphere is in general a very benign environment and this aircraft ensures the system level efficiency is not compromised under such calm conditions i.e. the spoilers are only used to save the aircraft from severe turbulence effects most likely encountered in the troposphere during ascent to the stratosphere or during descent for recovery.

[0054] The configuration can be considered a flight envelope protection system for fixed wing HAPS, activating when the absolute attitude of the aircraft e.g. conventional angle of bank, or speed, exceed a given threshold for the design, or if the relative attitudes of sections of wing deviate beyond a specified threshold e.g. the combinations of bending loads and torsional loads resulting from flight envelope manoeuvres or environmental turbulence, result in the shape of the wing deviating from the idealised shape by more than a predefined threshold.

[0055] To further explain the function of the spoiler configuration of the aircraft, FIG. 3. depicts an incoming air velocity field 12, as might be associated with a thermal dead ahead of the aircraft and with dimensions approximately equal to or less than the span of the aircraft. Without the spoiler actuated gust alleviation system, the fixed wing HAPS 1 would encounter a higher AoA and increased lift associated with the rising air affecting the centre of the aircraft, this would cause the wing to deform into the shape depicted by the dotted line 13, with lower dihedral and a significant departure from the idealised shape. Instead, the gust alleviation system senses the commencement of wing deformation and activates the spoilers in the centre of the wing 14, and which modify the span wise lift distribution to counter the effect of the air velocity field and preventing excessive deformation during the encounter and limit the maximum bending loads at critical points. The shape of the aircraft and thus lateral stability modes are maintained through reducing lift in the centre of the wing.

[0056] In the case of asymmetric gusts, FIG. 4. illustrates an approaching gust velocity field 15. presenting raising air such as a thermal, but which only affects the port wing 16. In this scenario, the port wing experiences a higher AoA and increased lift associated with the rising air causing the port wing to lift, invoking a bank to the right and in the case of flexible large span HAPS, deforming the aerostructure into adopting more dihedral and causing both bending and torsional loads in the structure to increase. The gust alleviation system responds by having detected the building errors in roll and span shape as reported by the distributed system of pitch and roll sensors and through a Proportional, Integral and Differential (PID) control loop, activates the spoilers appropriately, in this case, those on the port wing 16. Port wing spoiler activation serves to reduce local lift, resisting the rolling moment caused by the gust. The port fuselage elevator 17. responds to the local increase in AoA through its PID control loop with elevator actuation to reduce torsion in the wing and maintain the demanded AoA and therefore also maintain speed.

[0057] Referring to FIG. 5. in summary the process of gust alleviation comprises the steps of taking measurements 20 from the HAPS sensors, using this data to determine the amount of deformation (or predicted deformation), for example by calculating 22 an error value from the optimal wing shape. When this error value exceeds some threshold value or values, the spoilers are activated to compensate for this and bring the wing shape back within the acceptable profile. Different spoilers may be activated to different degrees in a co-ordinated manner, depending on the distribution of the error values along the wingspan. This process is continual, so that the spoiler activation is responds as forces on the wing that would deform it change, until no longer required.

[0058] Functionally the invention is a flight envelope protection system which maintains the desired aerodynamic derivatives set optimal for the phase of flight in question. When implemented optimally, the spoilers should not activate when performing normal navigational manoeuvres during flight in calm conditions in the stratosphere. It is permissible, but undesirable to use the spoiler system during routine banking manoeuvres, since they present significant drag in contrast to rudder or differential thrust only actuation and the spoilers represent excessive roll control authority in calm conditions. Their purpose is to correct span wise shape errors, roll errors and speed errors resulting from environmental disturbances and remain undeployed when not required. The threshold for activation is determined specifically for each aircraft design and is based on the definition of the normal manoeuvre envelope and aircraft span wise shape used in calm conditions, relative to thresholds in speed error, roll error and shape error for which there is a statistically acceptable drag penalty during flight in mild turbulence (this is analogous to an acceptable false alarm rate), contrasted with the critical bending and torsional thresholds for which structural failure will occur. When flying at its overnight stratospheric altitude, when energy constraints are most acute, the optimum implementation will typically be one in which the spoiler gust alleviation system only actuates when passing through hazardous turbulence.

[0059] Referring to FIG. 6. the process for errors in speed or roll control is similar to that for gust alleviation, and comprises the steps of taking measurements 25 from the HAPS sensors, and comparing the measured states with the flight control system speed and roll demands to determine speed and roll error. When this error value exceeds some threshold value or values, the spoilers are activated to reduce the speed and roll error. Generally, the normal flight control system actuators e.g. elevator, rudder and sometimes differential thrust if available, will be actuated to correct speed and roll errors, however the control can be supplemented using this system when it is clear the system is entering a hazardous state.

[0060] The spoilers in this system are used in a distributed manner across the entire span of the wing to not only support roll control, but to maintain stability through controlling the span wise shape of the wing, without imparting significant pitch moment or chord wise torsion.

[0061] There is an alternative mode of operation offered by the configuration described here which is of benefit to the operation of fixed wing HAPS. During the descent phase of a mission, it is advantageous to descend as fast as possible from the stratosphere. This is to minimise duration of exposure to potential turbulence in the troposphere, but also the potential downwind drift distance resulting from descent through the Jetstream or other strong wind layers in which the aircraft's True Air Speed (TAS) is less than the wind speed at the altitude in question. In the descent phase, efficient low drag flight only serves to extend the duration of the descent and so it is advantageous to use the distributed system of spoilers to speed up the rate of descent through deliberate increase in drag and/or speed, this operating mode is referred to as the rapid descent mode. In the rapid descent mode, all or almost all spoilers are initially deployed, maximising drag to maximise rate of descent while maintaining operation within the normal speed envelope. In this condition, to combat hazardous turbulence, shape errors are corrected by actuating the reduction in spoiler deployment and hence local increase in lift generation as the restoring force acting to reduce the span wise shape error. This is the exact opposite to normal operation of the spoiler actuated gust alleviation system during climbing or cruising phases where minimising drag is a priority and spoilers are retracted most of the time, deploying only to reduce local lift.

[0062] FIG. 7 and FIG. 8 depict the standard (optimal) implementation of the invention with spoilers not deployed 18 during normal manoeuvres while cruising in the stratosphere. At very high altitude, typically above the intended normal over night cruise height for the HAPS, the standard dihedral shape resulting in the baseline aerodynamic derivatives may present too much roll stability resulting in the risk of oscillatory modes such as Dutch roll, making climb and flight at higher cruise altitudes hazardous or non-optimal for certain imaging payloads. The distributed system of spoilers can be set to reduce lift on the outboard wing sections having the effect of reducing the dihedral and reducing roll stability to avoid oscillatory modes 19. Conversely, during climb or descent through the troposphere, where greater roll authority is advantageous in mitigating hazardous turbulence, spoilers in the centre of the wing reduce lift to promote flight with increased dihedral and greater spiral stability 20.

[0063] The highly flexible nature of the HAPS wing therefore permits different wing profiles to be adopted, which is not possible in convention fixed wing aircraft having relatively stiff wings. When the measurement of the amount of deformation (or predicted deformation) or other similar value is calculated from the sensed data, for example an error value from the optimal wing shape, the optimal wing shape may change depending on the altitude, manoeuvres to be carried out, efficiency desired, and so on. Several different optimal wing shape profiles may be stored and selected for particular environmental criteria and/or intended manoeuvres. Alternatively, the optimal wing shape at a particular time can be calculated according to the particular environmental criteria and/or intended manoeuvres, that is, the optimal wing shape may be selected or calculated in a scheduled manner, such as when a change in altitude is required, or by energy constraints (such as night-time flying when solar charging is unavailable), or in a reactive manner, such as when turbulence is encountered and it is desirable for the wing shape to adopt a more stable but less energy efficient profile.

[0064] This system adds minimal control surface mass and does not impart a significant pitching moment or chordwise torsion into the wing structure during activation. Such a system would support minimising total aero-structural mass by permitting the use of highly flexible, span loaded structures, with extremely low bending strength and exceptionally low torsional stiffness. Such a system would permit an extremely light weight span loaded aircraft, robust to the effects of turbulence but able to maximise performance in the stratosphere. This would ensure the delicate day-night energy budget can be positively closed on existing battery technology, or performance enhancements from future battery technology can be used to provide more power to the payload or project the capability to even higher latitudes in winter months.

[0065] Though the use of spoilers reduces lift and increases drag so that continual use during cruise in the stratosphere would represent an undesirable inefficiency, the use of a distributed system of spoilers is an advantageous solution to mitigating catastrophic turbulence, and such hazardous turbulence is typically short lived and transient, normally only associated with tropospheric flight. The priority in the solution to mitigating turbulence on a span loaded wing is therefore to minimise mass in the implementation of the actuation system and supporting aerostructure needed to give the aircraft extra-control authority when encountering hazardous turbulence. In this regard, a spoiler located on or near the centre of pressure achieves minimal additive aero-structural mass, since its activation imparts little or no torsion into the wing and it can be used with other spanwise located spoilers to modify the entire span wise lift distribution in a particularly mass effective way. During flight in calm conditions or mild turbulence, e.g. stratospheric flight, the spoilers are not routinely used for normal navigational manoeuvres and so do not contribute significant additional drag.

[0066] During flight in the troposphere either climbing or descending, when energy budgets are far less restricted, the distributed system of spoilers can be used to modify the lift distribution to deliberately increase banding load to enhance dihedral and increase spiral stability, pre-emptively of encountering gusts which are potentially unforecastable. During daytime once the battery storage system is full and when there is typically excess electric power generation, this can be stored as gravitational potential energy prior to night fall, by climbing higher in the stratosphere. The system of spoilers can be used to reduce dihedral to create a Dutch-roll stable shape at high altitude.

[0067] As previously described, the system of distributed spoilers can also be operated to deliberately increase drag and permit a rapid descent mode. This is to allow recovery of HAPS while minimising tropospheric exposure time to turbulence or downwind drift conditions when passing through strong wind layers. A further benefit of deploying all spoilers to reduce lift is during ground handling associated with taxing and launch process to reduce susceptibility to hazardous gusts lifting the structure prematurely and results in a wider operating envelope for ground handling.

[0068] The aircraft uses multiple span distributed spoilers and attitude sensors with one or more fuselages and associated elevators and permits control of bending and torsional forces resulting from gusts and turbulence, through modification of the span wise lift distribution for the maintenance of the desired spanwise shape and to provide high control authority to reduce hazardous roll errors or speed errors. The use of this configuration in the design and operation of fixed wing HAPS enables: [0069] Expansion of the operating envelope of HAPS to permit statistically more launch and recovery opportunities than systems not equipped with this technology. [0070] Lower loss rate due to catastrophic aero-structural failure resulting from un-forecast turbulence encounters. [0071] Designers of HAPS can further reduce the airframe mass fraction of fixed wing HAPS through extremely light weight span loaded airframe which can be highly flexible in bending and torsion. [0072] Lower airframe mass fraction can allow a smaller span aircraft which is less vulnerable to gusts, to carry a larger mass payload, which would normally only be possible from a larger span, more gust vulnerable aircraft.

[0073] In the rapid descent mode with all spoilers deployed, increase in rate of descent is variable based on specific aircraft factors. The predicted improvement in reducing tropospheric exposure time is up to a factor of 10. For example, in the case of an efficient HAPS able to cruise year-round on current battery technology in the stratosphere, the descent time from 60 kft, without a rapid descent mode, is about 20 hrs. This configuration applied to a HAPS can reduce this to approximately 2- to 3 hrs. Some existing HAPS aircraft have demonstrated descent from the stratosphere in less than 6 hrs without a rapid descent mode, however, this is a reflection of their poor system level efficiency and higher wing loading in that such aircraft can't sustain stratospheric flight, day and night, outside of summer conditions where days are long and nights are short and do not represent commercially viable HAPS systems.

[0074] The system as described above relies on attitude sensors to detect deformation of the wing, whereupon the spoilers are activated to locally reduce lift and maintain an acceptable wing shape. However, it will be realised that other quantities could be used instead of or in addition to attitude in order to determine the lift or deformation, such as strain gauges, air pressure etc.