TURBULENCE ON AIRCRAFT

Technical Field

[0001] The present invention relates generally to methods and systems for attenuating the effects of turbulence on aircraft.

Background of Invention

[0002] Turbulence can impose various detrimental effects on aircraft. In particular, turbulence can cause random large accelerations of the aircraft. These turbulent events may increase dynamic loading thus leading to shorter airframe life due to fatigue; degrade performance of on-board avionics and payload sensors; induce attitude and flight path deviations; and induce changes in altitude resulting in a risk of collision with other aircraft or obstacles during critical manoeuvres such as landing. In commercial passenger aircraft turbulence can also displace objects or passengers causing sensations of discomfort and a risk of injury to passengers and crew.

[0003] There are four main types of turbulence which can cause problems for aircraft: clear air turbulence, convective turbulence, wake turbulence and atmospheric boundary layer turbulence. Clear air turbulence typically results from wind shear and is non-convective. This turbulence type occurs at high altitude close to jet streams. Convective turbulence occurs inside or close to clouds. In particular, severe turbulence may occur in storm clouds, in which rapid vertical currents in opposite directions may coexist. Wake turbulence, on the other hand is created by the passage of an aircraft or the interactions between the wind and buildings and other obstructions in low altitude environments. Atmospheric boundary layer (ABL) turbulence can arise from thermal effects generated from the solar heat coming off the ground coming into contact with cooler air that is higher up from the ground and can exist in relatively low wind conditions. Turbulence from thermal effects is particularly manifest for low altitude aircraft on landing approach in hot environments. ABL turbulence can also arise from the mechanical mixing of the roughness elements on the surface of the Earth (eg vegetation, naturally occurring topographical roughness or man-made roughness such as buildings) and is usually prevalent under higher winds. [0004] Turbulence poses a particular challenge to the attitude stability of micro air vehicles (MAVs) due to their relatively small size. Since MAVs tend to operate in low altitude environments they are particularly subject to wake and both types of ABL turbulence. Larger aircraft tend to be challenged by wake turbulence induced by a preceding aircraft during an approach to land. The induced wake turbulence is sufficiently significant that aircraft must be sufficiently spaced on approach to allow the turbulence to decay between approaches. Enabling closer spacing between aircraft has the potential to improve the capacity of an airport to accommodate larger numbers of aircraft thereby increasing revenue.

[0005] Attempts to mitigate the disruptive effects of turbulence on aircraft are classified as either active or passive. Passive methods involve aircraft design considerations to ensure the aircraft has inherent tolerance to perturbations. For example, increasing wing dihedral for lateral stability or the use of flexible membrane wings on MAVs to alleviate wing loads and minimise the transfer of perturbations to the aircraft's centre of gravity. Turbulence induced perturbations can also be attenuated by increasing aircraft mass, inertia and wingspan. However, increasing the aircraft's inertia, mass and/or wingspan can also degrade response to control inputs. That is, highly stable aircraft will lack agility and the manoeuvrability required for navigation since passive attitude control cannot be disabled on demand. Accordingly, a balance must be struck between increasing inherent mitigation tolerance using aircraft design and maintaining optimal control response times.

[0006] Active approaches to mitigating the effects of turbulence involve electronic systems that sense the effects of the perturbation on aircraft movement by measuring accelerations and sometimes angular rates. Active approaches typically either warn the aircraft operator or pilot, (for example, windshear warning systems used in large passenger aircraft), or attempt to actively suppress perturbations by actuating various flight control surfaces of the aircraft, such as ailerons, flaps, spoilers, slats, elevators, rudders, elevons, etc, to counteract the perturbation. However, the lag time for actuation of flight control surfaces by control systems in response to the detection of a perturbation means that known attitude control systems are slow to respond to flow disturbances, in requiring a change in attitude to actually occur before any action is taken to suppress the effects of a turbulence induced event. Accordingly, known devices and methods employed for attenuating the effects of turbulence are largely unsatisfactory.

[0007] The present invention proposes a method and a sensory system for attenuating the effects of turbulence and/or other flow perturbations experienced by aircraft more effectively than known prior art methods and devices.

Summary of Invention

[0008] According to an aspect of the present invention, there is provided a method for attenuating the effects of turbulence on an aircraft, the method including the following steps: providing at least one sensor on or forward of a leading surface of the aircraft; sensing a flow disturbance caused by turbulence approaching the leading surface to generate sensed input; transmitting the sensed input to a control system; and actuating one or more flight control surfaces to counteract an aircraft perturbation that is anticipated to occur in response to the sensed flow disturbance; wherein the flow disturbance is sensed ahead of the leading surface of the aircraft so that the flow disturbance is detected substantially before the aircraft perturbation is initiated by the flow disturbance.

[0009] The at least one sensor may sense or extend ahead of the leading surface of the aircraft.

[0010] In one embodiment, the leading surface of the aircraft is a leading edge of at least one wing.

[0011] In one form of the invention, at least two sensors are provided with at least one sensor being provided on the leading edge of each wing. The sensors may be placed in a position on the leading edge of the wing which will provide the most significant response to flow disturbances.

[0012] In one embodiment, the flow disturbance is sensed as a variation in angle of attack.

[0013] In an embodiment, the control system is configured to permit independent actuation of the flight control surfaces to control of six degrees of freedom in response to the flow disturbance. [0014] According to another aspect of the present invention, there is provided a system for attenuating the effects of turbulence on an aircraft, the system including: at least one sensor provided on a leading surface of the aircraft, the at least one sensor configured to sense a flow disturbance caused by turbulence approaching the leading surface to generate sensed input; a control system for receiving the sensed input and actuating one or more flight control surfaces to counteract an aircraft perturbation expected to occur in response to the flow disturbance; wherein the sensors sense the flow disturbance ahead of the leading surface of the aircraft so that the flow disturbance is detected substantially before the aircraft perturbation is initiated.

[0015] The at least one sensor may comprise a pressure sensor. In one particular form of the invention, the pressure sensor is a MEMS based pressure sensor. In another form of the invention, the at least one sensor comprises a LIDAR sensor or other optically based systems. Alternately, the sensor may comprise a SODAR sensor.

[0016] The at least one sensor may be provided on the leading edge of each wing.

[0017] According to an embodiment, the control system is a remote control system. Alternatively, the control system may be an on-board attitude control system.

[0018] It is to be understood that the method and system of the invention described herein is applicable to various aircraft types including but not limited to micro aerial vehicle (MAV), commercial passenger aircraft, and the like.

Brief Description of Drawings

[0019] The invention will now be described in further detail by reference to the accompanying drawings. It is to be understood that the particularity of the drawings does not supersede the generality of the preceding description of the invention.

[0020] Figure 1 shows a flow chart showing generally the steps of a method embodying the present invention.

[0021] Figure 2 is a schematic diagram showing advection of a turbulent flow disturbance over a segment of the leading edge of an aircraft wing. [0022] Figure 3 is a schematic diagram showing examples of pressure variations caused by variations in the angle of attach of the approaching flow disturbance and the consequential effects on the attitude of the aircraft.

[0023] Figure 4 is a schematic of a pressure sensor used to sense flow disturbances in accordance with an embodiment of the present invention.

[0024] Figure 5 is a schematic of a pressure based roll angle tracking controller.

[0025] Figures 6A to 6C show more detailed schematics of an exemplary control architecture using an inertial sensing controller together with a feed-forward sensing controller in accordance with an embodiment of the present invention.

[0026] Figure 7A is a boxplot showing roll angle displacements for different control architectures.

[0027] Figure 7B is a boxplot showing roll rate displacements for different control architectures.

Detailed Description

[0028] As used herein, the term "turbulence" is intended to include various types of turbulence including, by way of example, clear air turbulence, convective turbulence, wake turbulence and ABL turbulence together with other flow perturbations that may be experienced by an aircraft during flight.

[0029] The term "aircraft" as used herein, is intended to encompass various flying craft including fixed-wing aircraft, rotary-wing aircraft (including single and multiple rotor), flapping-wing aircraft and other airborne vehicles of various scales.

[0030] Referring firstly to Figure 1 , there is generally shown a series of steps for a method for attenuating the effects of turbulence on an aircraft. At step 1 10, at least one sensor is provided on a leading surface of the aircraft. The at least one sensor measures a flow disturbance caused by turbulence approaching the leading surface of the aircraft at step 120, to generate sensed input. At step 130, the sensed input is transmitted to a control system. The control system actuates a flight control surface or surfaces in order to counteract an aircraft perturbation that is anticipated to occur in response to the sensed flow disturbance. Such flight control surfaces include ailerons, flaps, spoilers, slats, elevators, rudders, motors and the like.

[0031] Flow disturbances are sensed ahead of the leading surface of the aircraft to enable the flow disturbance to be detected substantially before the aircraft perturbation is initiated by the flow disturbance. Accordingly, the flight control surfaces can be actuated at or before any change in the attitude of the aircraft which would not be sensed until it occurs by conventional inertial-based attitude control systems. By "substantially before", it is intended that the flow disturbances are sensed in sufficient time to permit at least some attenuation of the perturbation.

[0032] The method is based on the premise that by detecting a flow disturbance or gust before a perturbation is initiated, a countering or nullifying response can be induced by the control system to ameliorate the effect of turbulence on the aircraft. The method, has the potential to reduce the detrimental effects of turbulence on a wide range of aircraft, and has been demonstrated to significantly improve the in-flight stability of MAVs which are particularly susceptible to turbulence due to their small mass.

[0033] MAVs are considered to represent a worst-case scenario in terms of their vulnerability to turbulence, due to both their physical design and operational requirements, which require them to operate at altitudes that are significantly lower than larger aircraft. The low altitude range is characterised by a higher density of obstacles and increased levels of turbulence due, at least in part, to mechanical mixing induced by ground roughness. The MAVs small size relative to the turbulence makes attenuating the effects of turbulence on MAVs particularly challenging. Accordingly, it will be understood that if the methods and systems disclosed herein improve the attitude stability of MAVs, then a larger scale system applied to larger aircraft will provide comparable or even improved effects, since larger aircraft are inherently easier to stabilize than MAVs.

[0034] By sensing the flow disturbances, which exist ahead of a leading surface of the aircraft, typically the leading edge of a wing, it is possible to compensate for the time lag inherent in causing a control system to actuate flight control surfaces in response to sensed input. Considering the example of a MAV, a typical time lag within which a control response to a perturbation sensed using inertial sensors is initiated is >0.52 seconds. Therefore, even a seemingly small time-forward in sensing will improve attempts to attenuate the effects of turbulence on aircraft attitude.

[0035] Sensing ahead of the leading surface of the aircraft can be achieved by physical extension of the sensors, for example in the case of a pressure-based multi- hole sensor probe by or other rapidly responsive velocity sensors such as LiDAR or SODAR, or other optically or acoustically based sensors.

[0036] Whilst a single sensor provided on a leading surface of the aircraft will suffice to provide some attenuation of a turbulence generated aircraft perturbation before it occurs, providing a greater number of sensors, increases the redundancy in the sensing system and enhances the prediction of oncoming flow disturbances. Accordingly, it is anticipated that a plurality of sensors be provided on or in front of relevant aerodynamic surfaces for optimal results.

[0037] Optimal positioning of the sensors on a wing surface involves identifying regions over the wing which result in the highest magnitude of lift fluctuations due to turbulence. Due to the random nature of oncoming turbulence, the relative incidence angle of oncoming flow will generally not impinge the wing at the same incidence angle across the entire wingspan. This variation in flow angle across the span, is recognised as being more detrimental to MAV attitude stability than variations in velocity magnitude. Wing surface pressure fluctuations tend to correlate with the pitch angle of the upstream flow, rather than with yaw flow angle or the overall velocity magnitude.

[0038] Ideally, the flow angle variation forward of the entire span of the wing's leading edge is measured and a weighted average of its effects on roll input used as an input to the control system. However, in the case of small aircraft such as a MAV, it is not practical to implement a large number of sensors and only one sensor per wing achieves good results. In this case, the sensor is most effectively positioned along the wingspan in a region that has been shown to attain the highest correlation between upstream flow pitch angle (i.e., surface pressure fluctuation) and roll acceleration. Regions in the vicinity of the wing's leading edge have been demonstrated to have the highest correlation with oncoming turbulence. [0039] Referring now to Figure 2, there is shown a schematic representation of the advection of a turbulent flow disturbance over a segment of the leading edge of a wing. A sensor 210 in the form of a multi-hole pressure probe extends ahead of the leading edge of the wing 220. The sensor 210 measures the transient flow angles and velocity (magnitude and direction) upstream of the wing's leading edge 220. Gust 230 represents the incident flow disturbance sensed as a variation in angle of attack and flow magnitude due to turbulence. Ordinarily, this scenario would be expected to cause a significant perturbation, shown as a roll perturbation 240 which would then be sensed by inertial-based sensors, which measure the change in the attitude of the aircraft in response to the gust. For example, attitude sensors such as accelerometers, gyroscopes, inertial measurement units (IMUs), which measure variations in roll acceleration and the roll rate respectively. Tilt sensors, optical flow sensors and horizon sensors measure angular displacement and divergence from the flight path is measured by position and localization sensors such as GNSS. Using predictive sensing in accordance with the method of the present invention, which senses a flow disturbance before it impacts the attitude of the aircraft, it is possible to minimise the perturbations that would otherwise be sensed as input to an inertial- based control system.

[0040] Detailed characterisations of turbulence relevant to MAVs have identified roll-axis attitude instabilities to be the most significant detrimental factor to small fixed wing aircraft operating outdoors. As a gust impacts on the wing's leading edge, the flow angle and velocity of the gust is altered, inducing variations in the pressure distribution. Asymmetric fronts lead to uneven lift distribution over the wings, including a rolling motion as depicted in Figure 3. Symmetric gusts, also depicted in Figure 3, tend to induce a "heaving" motion. Since turbulence structures are highly three dimensional in nature, instability to rolling motion forms the most significant detrimental factor to MAVs. Large turbulence structures can be considered to be quasi-static and are more easily compensated for than smaller eddy type turbulence structures.

[0041] Sensors that are capable of detecting phenomenon early in the gust perturbation process are referred to herein as phase-advanced sensors. Phase- advanced sensors do not measure attitude directly, but sense the disturbance itself, for example, by sensing the incident flow variation (magnitude and angle), pressure/velocity variation, e.g. flow sensors, and structural stress, e.g. strain sensors. Phase-advanced sensors are capable of providing time forward information to the control system for early mitigation of associated perturbations. Phase- advanced sensing offers a control system with the capacity to compensate for the time-lags inherent in inertial-based attitude control systems, and the accordingly, the potential to eliminate perturbations completely. The use of phase-advanced sensors can reduce attitude perturbations in all vehicular axes. Further beneficial effects of phase advanced sensing are accurately measured relative turbulence; less flight path divergence, reduced drag and reduced adverse yaw.

[0042] The method for attenuating the effect of turbulence on an aircraft is based largely on Taylor's hypothesis, which assumes that turbulence structures remain frozen during advection. In reality, turbulence structures constantly vary in all three dimensions. However, under certain conditions Taylor's hypothesis is sufficiently valid, particularly over small distances.

[0043] Referring now to Figure 4, there is shown an example of a pitch probe or flow sensor 400 suitable to be provided on or forward of the leading edge of the wing. In this example, a multi-holed lightweight tube 410 with chamfered leading edges 420 is used to form a pressure probe. The probes directional tubes are chamfered at substantially 45° allowing an angle measurement ran ge of substantially 90°. The internal tubes of pitch probe 400 are connected to a pressure sensor, such as, for example, a MEMS differential pressure sensor. Typical MEMS accelerometers are based on capacitive, piezo-resistive or tunnelling-current mechanisms.

[0044] The pressure probe length (L) is selected to provide the maximum time- forward advantage as possible within physical constraints. For an exemplary MAV exhibiting a >0.52 second time lag for actuating a control response to a sensed perturbation, travelling at a cruise velocity of 10ms ^"1 , a probe length of 5.2 metres would be desirable. However, due to practical constraints associated with the use of long probes, e.g. increased mass, increased susceptibility to vibration and bending, dynamic response of the tubes (i.e. increased phase lag within the tubes), and degraded correlation due to misalignment between the sensing location and the aircraft an end-to-end probe length (L) of 0.15m is considered to provide optimal time- forward advantage within practical constraints, i.e. 15 ms when the MAV is flying at 10ms ^"1 .

[0045] Various types of suitable flow sensors may be employed including thermal, strain or capacitance-based sensors. Flow sensors may further be laboratory-based such as: Particle Image Velocimetry (PIV), Laser Doppler Anemometry (LDA), Light Detection and Ranging (LiDAR), Sonic Detection and Ranging (SODAR) and Radio Detection and Ranging (RADAR). The suitability of flow sensor types will depend on the respective application, i.e. MAV vs a commercial airliner, for example.

[0046] A variety of control architectures may be employed to provide the requisite stabilisation. In the case of an MAV or similar craft the control system for receiving the sensed input and actuating one or more flight control surfaces to counteract an aircraft perturbation expected to occur in response to the flow disturbance may be provided by way of a control system off board the aircraft, with corrective actuation commands calculated off board the aircraft and transmitted to the aircraft via a communications link. In other embodiments, the control system is provided on board the aircraft. In either case, the pitch probe or flow sensor 400 signals comprise dynamic input to the flight control system. The flight control system can vary from a programmable closed loop control system with no GPS, to an autopilot system depending on the requisite application.

[0047] Referring now to Figure 5, a generic feed forward control system 500 shows how the disturbance turbulence 510, is sensed by sensors on the leading edge of the left and right wings, 520. Any signal processing or multiplexing technique may be used to merge the signals of the plurality of sensors into a hybrid signal. The sensed input is transmitted to a flight control system 530. The flight control system 530 causes actuation of one or more flight control surfaces to counteract an aircraft perturbation that is anticipated to occur in response to the sensed flow disturbance. Actuation of aerodynamic surfaces is effected, for example, by actuation techniques involving the use of servo motors, piezoelectric actuators and the like. For optimal attenuation of perturbations, the control system should be configured to permit independent actuation of the flight control surfaces to control of six degrees of freedom in response to the flow disturbance. [0048] Referring now to Figures 6A to 6C, there is shown an exemplary control architecture using active stability augmentation based on an inertial sensing controller together with a feed-forward component in accordance with the invention. In Figure 6A, the lateral controller includes a feed-forward path with input from left wing flow sensor 610 and right wing flow sensor 620. These inputs result in actuation of individual servos to actuate flight control surfaces of the respective wings 630, 640. In accordance with this architecture, the wings are capable of uncoupled deflection, i.e. each wing servo actuates independently of the other. Therefore, deflection of the flight control surface is based on the sensed oncoming flow disturbance over each respective wing. In Figures 6B and 6C, the longitudinal and directional controllers which control the pitch and yaw rates respectively, are controlled by inertial-based sensors.

[0049] There are numerous control architectures can be used to calculate the required actuations. Examples include simple feed forward arrangements,

Proportional-Integral-Derivative (PID) controllers, and model predictive control systems. For example, a Proportional-Integral-Derivative (PID) controller such as used in many commercial autopilot stability augmentation systems is an inertial-based controller using an attitude tracking architecture, with an IMU as the only sensor for feedback. The attitude-tracking controller has a Proportional-Integral-Derivative (PID) outer-loop and uses angular-rate tracking inner loops on each of the roll and pitch axes. The directional controller includes a yaw angular rate-tracking mode.

[0050] Referring now to Figures 7A and 7B, a MAV was equipped with two probe sensors, 400 where its output signals were used to mitigate the perturbations expected to occur during flight within high turbulence. This led to a comparison between the attitude mitigation performance of a control systems employing flow sensors, to the performance of a control system employing no flow sensors.

[0051] Improvement has been shown in the roll stability of an MAV with use of control system employing flow sensors when compared with the control system which has no flow sensors. Use of the phase-advanced flow sensors improves the effectiveness of the control system in reducing the magnitude of perturbations. Referring now to Figures 7A and 7B, there are shown boxplots overlayed on top of the probability density function, to visually show roll angle and roll rate variation respectively, for control systems employing flow sensors and control systems without flow sensors. From the results, it is clearly apparent that pressure-based phase- advanced sensing offers superior results, where roll perturbations are significantly mitigated.

[0052] MAVs operate in challenging turbulence environments, which requires novel sensory approaches to attenuate the detrimental effects thereof. Time forward sensing promises to minimise perturbations by enabling the actuation of stabilising flight control surfaces before the effects of turbulence cause an undesirable level of perturbations in the aircraft.

[0053] The present invention is equally applicable to larger aircraft, including larger unmanned aircraft through to commercial passenger jets which could all benefit from systems for minimising the effects of turbulence.

[0054] While the invention has been described in conjunction with a limited number of embodiments, it will be appreciated by those skilled in the art that many alternative, modifications and variations in light of the foregoing description are possible. Accordingly, the present invention is intended to embrace all such alternative, modifications and variations as may fall within the spirit and scope of the invention as disclosed.

[0055] The present application may be used as a basis or priority in respect of one or more future applications and the claims of any such future application may be directed to any one feature or combination of features that are described in the present application. Any such future application may include one or more of the following claims, which are given by way of example and are non-limiting in regard to what may be claimed in any future application.