Patent Publication Number: US-2018053991-A1

Title: High altitude aircraft wing geometry

Description:
TECHNICAL FIELD 
     The invention relates to the wing geometry of high altitude aircraft, which deliver information services at altitude, including telecommunications, observation, astronomical and positioning services. 
     BACKGROUND TO THE INVENTION 
     High altitude platforms (aircraft and lighter than air structures situated from 10 to 35 km altitude)—HAPS have been proposed to support a wide variety of applications. Areas of growing interest are for telecommunication, positioning, observation and other information services, and specifically the provision of high speed Internet, e-mail, telephony, televisual services, games, video on demand, and global positioning. 
     High altitude platforms possess several advantages over satellites as a result of operating much closer to the earth&#39;s surface, at typically around 20 km altitude. Geostationary satellites are typically situated in around 40,000 km orbits, and low earth orbit satellites are usually at around 600 km to 3000 km. Satellites exist at lower altitudes but their lifetime is very limited with consequent economic impact. 
     The relative nearness of high altitude platforms compared to satellites results in a much shorter time for signals to be transmitted from a source and for a reply to be received (the “latency” of the system. Moreover, high altitude aircraft are within the transmission range for standard mobile phones for signal power and signal latency. Any satellite is out of range for a terrestrial mobile phone network. 
     High altitude platforms also avoid the rocket propelled launches needed for satellites, with their high acceleration and vibration, as well as high launch failure rates with attendant impact on satellite cost. 
     Payloads on high altitude platforms can be recovered easily and at modest cost compared to satellite payloads. Shorter development times and lower costs result from less demanding testing requirements. 
     U.S. Pat. No. 7,046,934 discloses a high altitude balloon for delivering information services in conjunction with a satellite. 
     US 20040118969 A1, WO 2005084156 A2, U.S. Pat. No. 5,518,205 A, US 2011/0031354 A1 US 2014/0252156 A1, disclose particular designs of high altitude aircraft. 
     However, there are numerous and significant technical challenges to providing reliable information services from high altitude platforms. Reliability, coverage and data capacity per unit ground area are critical performance criteria for mobile phone, device communication systems, earth observation and positioning services. 
     Government regulators usually define the frequencies and bandwidth for use by systems transmitting electromagnetic radiation. The shorter the wavelength, the greater the data rates possible for a given fractional bandwidth, but the greater the attenuation through obstructions such as rain or walls, and more limited diffraction which can be used to provide good coverage. These constraints result in the choice of carrier frequencies of between 0.7 and 5 GHz in most parts of the world with typically a 10 to 200 MHz bandwidth. 
     There is a demand for high data rates per unit ground area, which is progressively growing larger from current levels of the order 1-10 Mbps/square kilometre to many orders of magnitude greater than this over the next decades. 
     To provide high data rates per unit ground area, high altitude unmanned long endurance (HALE) aircraft, or free-flying or tethered aerostats, need to carry large antenna(s) to distinguish between closely based transceivers on the ground. A larger diameter antenna leads to a smaller angular resolution of the system, hence the shorter the distance on the ground that the system can resolve. Ultimately the resolution is determined by the “Rayleigh criterion” well known to those skilled in the art. The greater the antenna resolution, the higher the potential data rates per unit ground area are. 
     However fitting large diameter antenna into the wing or fuselage structures that would normally be used for high altitude aircraft brings significant aerodynamic penalties. 
     To avoid the costs and lack of availability that would be engendered by short flight endurance for HALE aircraft, endurance of many weeks or months rather than hours is necessary. In such aircraft, energy is supplied by solar cells with a battery storage system to provide power overnight, or by Hydrogen fuel. This energy is used for the propulsion system and payload power. Aerodynamic drag consumes energy and reduces the available payload energy, and can curtail the aircraft operating speed; altitude and latitude. It is therefore highly desirable to minimize the aircraft aerodynamic drag. 
     A key problem with such antenna carrying aircraft is therefore to ensure that the aircraft structure can accommodate the relevant antenna geometries whilst having a low aerodynamic drag to minimize energy requirements, as well as an appropriate distributed weight distribution to minimize structural weight. 
     There are various forms of antennas that have advantages when mounted on a HALE aircraft. Of particular utility are phased array antennas and horn antennas. Both forms of antenna can provide low weight, high gain systems that transmit or receive electromagnetic radiation of suitable wavelengths for communication to ground based systems such as mobile phones, computers or base stations. In the context of this invention “ground” includes the surface of water as well as land and so includes the seas. 
     For high data rates to and from the ground, the axis of the beam should normally be approximately vertical to minimize the distance between the plane and the ground-based receivers or transmitters to which it is communicating. Within antenna clusters, composing several distinct antennas pointing in different directions, an individual antenna may transmit or receive at a significant angle to the vertical, but the axis of the clusters will normally be close to the vertical, to ensure the distance between the aircraft and ground based transceivers is minimized. 
     It is therefore desirable to have lightweight large diameter horizontal antenna structures located in the aircraft in such a fashion as to minimize drag. Conventionally, with lower altitude aircraft, if an antenna is sufficiently large not to fit into the aircraft structure, they are externally mounted on the aircraft fuselage. See U.S. Pat. No. 6,844,855. Elaborate folding structures have been proposed to allow antenna or antennas to be deployed and drag increased only when needed. See U.S. Pat. No. 5,357,259A. If the antenna is sufficiently small to fit into the aircraft structure, then an enclosing structure transparent to the required electromagnetic radiation can be designed to minimize aerodynamic drag as for example referred to in U.S. Pat. No. 3,953,857. 
     However, for high data rates and or high resolution between mobile user equipment possessing transmitters or receivers, for example mobile phones, computers, equipment carried on vehicles, there is a need for a wing design that provides low aerodynamic drag and weight for a suitably large antenna enclosure, with large wing spans particularly for wing spans of greater than 30 m and more particularly for still larger wing spans of 50 m or more. 
     A similar need arises for connection to fixed user equipment where for particular reasons such as cost or location it is impractical to connect to fibre networks. Communication to user equipment on aircraft and satellites can also require such large antennas. This invention enables these large antennas to be carried by HALE aircraft in a more efficient manner in these characteristics than prior art. 
     Wing tips can be provided that are upwards or downwards orientated. In this work all wing lengths and chord calculations exclude the contribution of the wing tip length and width. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the invention relates to an unmanned high altitude aircraft operating above 15 km altitude with transmitting and/or receiving antennas enclosed or substantially enclosed in a wing structure where the longest chord length of the wing enclosing the antenna or antennas, the “encumbered section,” is at least 30 percent greater than the mean wing chord length of the “transition” and “unencumbered” sections which do not enclose the antenna. 
     Such a design reduces aerodynamic drag. 
     Preferably the transmitting and/or receiving antennas comprise one or more phased arrays and/or horn antennas. Preferably the transmitting and/or receiving antennas may comprise quadridge horn, log periodics, individual Vivaldi, patch antennas, dipoles, quarter wave whip, bow tie etc. 
     However in a second aspect it has been discovered that aerodynamic drag can be reduced for such an aircraft carrying an antenna by maintaining a comparable “circulation” around the wing enclosing the antenna or antennas and the wing adjacent to this wing section. 
     In such a wing design, three parts to the wing can be defined, firstly, the antenna “encumbered” section or sections, containing the antenna or antennas in all vertical cross-sections orientated parallel to the direction of flight, secondly various “transition” sections connecting the enclosure section(s) with thirdly, the “unencumbered” sections whose design is dominated by conventional aerodynamic and structural considerations and not primarily affected by the design of the “encumbered” section. 
     The concept of “circulation” referred to above is known to those skilled in the art of aerofoil and wing theory, and is defined as the line integral of the velocity field around the relevant aerofoil sections: see H Glauert “The elements of aerofoil and airscrew theory.” CUP 1986 p  34 . Minimum induced aerodynamic drag of a planar wing is achieved by an elliptic distribution of circulation over the wing span at a particular dynamic loading or airspeed for a specified operating altitude. Normally this airspeed will be chosen to be the cruising speed of the aircraft. 
     By a suitable choice of local aerofoil shape and local effective angle of attack of both the “encumbered” section, and the “transition” section, even for large antenna sizes, circulation can most preferably be kept elliptical to within twenty percent, preferably less than within ten percent, over the “encumbered” section, the “transition” section and the edge of the “unencumbered sections” adjacent to the “transition section.” Calculation of aerofoil and wing circulation is familiar to those skilled in aerofoil aerodynamics, see for example, Schlichting, Truckenbrodt “Die Aerodynamik des Flugzeuges Bd II.” Springer-Verlag 1969, p 9. 
     By maintaining a relatively elliptical circulation around the wing in this manner, the impact on the aircraft aerodynamic drag of an antenna or antennas can be minimized, where because of the size or required orientation of the antenna, it is not possible to wholly enclose the antenna or antennas within a conventional wing. Such large antenna or antennas would hitherto have resulted in a large mean wing chord length if the antenna or antennas were enclosed or substantially enclosed in the wing, or mounted externally. With a large mean wing chord length, the aerodynamic drag is increased—as will be shown below with a less “slender” wing with a lower aspect ratio than in the invention. If the antenna(s) are not substantially, preferably 90% but in general more than half enclosed within the wings or fuselage, the extra obstruction will increase aerodynamic drag as for example, in the well-known externally mounted radome of AWACS aircraft. 
     As is known from lifting line theory, the induced drag coefficient of a untwisted wing with elliptic planform is a function of wing lift coefficient and aspect ratio. Thomas (F Thomas, Fundamentals of Sailplane design, College Park Press 1989, page 40) describes this result specifically, 
         C   D   =C   D0   +C   L   2 /(π· e ·AR)
 
     where the terms (defined by Thomas) are as follows: C D  is the drag coefficient of the aircraft, C D0  is the drag coefficient at zero lift, C L  is the wing lift coefficient, π=3.14 . . . , e is the Oswald span efficiency factor which depends on the wing planform induced drag, but also includes profile drag and parasitic drag, AR is the aspect ratio of the wing which is the square of the wingspan divided by the projected wing area. 
     Non-elliptical wing circulations can be used if the wing has twist or winglets to provide low drag. In this case it is important that by a suitable choice of local aerofoil shape and local effective angle of attack of both the “encumbered” section, and the “transition” section, even for large antenna sizes, circulation can be kept constant to within twenty percent, preferably less than within ten percent, over the “encumbered” section, the “transition” section and the edge of the “unencumbered sections” adjacent to the “transition section.” 
     For typical HALE aircraft designs it has been found that the induced drag of the wing has a significant contribution to the overall aerodynamic drag, and slender wings of high aspect ratio are to be preferred to minimize aerodynamic drag. Lift to drag ratios at operating altitude are typically over 25:1, more typically over 35:1, and can with suitable aerofoil designs, large wingspans, and high aspect ratios, be much higher. Wingspans are typically greater than 20 m, more typically greater than 25 m. The Helios aircraft wingspan was 75 m and even higher wingspans have been contemplated. Payloads vary substantially, from a few kg for the early Zephyr aircraft to much higher values for the Helios aircraft or the Global Observer of more than 100 kg. 
     Modest antenna sizes do not give a drag problem: if the antenna or antennas can be fitted into slender wing aerofoil sections with no elongation of the aerofoil chord, and the aerofoil cross section is of sufficient depth, then a conventional wing design is possible without an aerodynamic drag penalty with the antenna position being determined primarily by structural considerations. 
     Two separated antenna groups can be desirable to allow the aircraft transmitter and receiver functions to be separated resulting in a greater sensitivity of signal reception and/or transmission, and a more distributed load on the wing minimizing the structural loads on the wing and its weight. 
     Introducing one or more antenna or antenna groups into, or substantially into, the wing of the aircraft in this fashion whilst maintaining relatively elliptic circulation rates around the wing as described above, allows the additional drag to be minimized for a given size of antenna when the antenna dimensions are greater than the wing chord length would be in a rectangular or near rectangular or elliptical design. 
    
    
     
       This is illustrated in the following figures and examples. 
         FIG. 1  shows in plan and side elevation an aircraft with two circular phased arrays with an approximately constant chord length for some distance from the aircraft fuselage. The wing design is similar to the design of high performance modest Reynolds number aircraft for high performance manned gliders. The Reynolds number—familiar to those skilled in the art—is a measure of the ratio of turbulent to viscous forces concerning the relevant fluid flow. The plane thrust is provided by a plurality of propellers ( 1 ), supported by a long thin wing ( 105 ). The main wing section is of a chord length sufficiently great to accommodate the two antennas ( 2  and  3 ): it can simplify the antenna electronics and improve signal processing discrimination to have one antenna transmitting and one antenna receiving particularly if both transmission and reception are required at the same time. 
         FIG. 2  shows in plan and side elevation an aircraft with two circular antennas ( 4 ,  5 ) utilizing the invention, where the diameter of the antennas is much greater than the average wing chord length. In this case, the vertical cross section where the antennas are located is also considerably greater than the average vertical cross section of the wing. There are two substantial “transition” sections (T) in addition to the encumbered (E) and unencumbered UE) wing sections. 
         FIG. 3  shows in plan and side elevation an aircraft with four circular antennas ( 4 , 5 , 6 , 7 ) utilizing the invention, where the diameter of the antennas is much greater than the average wing chord length. In this case, the vertical cross section where the antennas are located is also considerably greater than the average vertical cross section of the wing. There are also as in the aircraft shown in  FIG. 2 , two substantial “transition” sections (T) in addition to the encumbered (E) and unencumbered UE) wing sections. 
         FIG. 4  shows in plan and side elevation an aircraft with two circular antennas ( 8 , 9 ) utilizing the invention, where the diameter of the antennas is much greater than the average wing chord length and the transition section is short. In this case, the vertical cross section where the antennas are located is also considerably greater than the average vertical cross section of the wing. 
         FIG. 5  shows an aircraft with square antennas ( 10 , 11 ) utilizing the invention, rather than circular antennas otherwise similar to the aircraft shown in  FIG. 4 . 
       In  FIG. 6  the relatively thin phased array ( 61 ) sits just below the bottom of the wing spar ( 62 ), which can be made of conducting materials being above the main electromagnetic radiation field entering or leaving the phased array ( 61 ). The wing surface ( 64 ) defines the aerofoil shape and should be of sufficiently low conductivity when situated below the phased array if the array is communicating downwards to avoid significant interference with the electromagnetic radiation transmitted or received by the antenna(s). The top of the wing spar ( 63 ) sits just below the upper surface of the wing. 
         FIG. 7  shows an aircraft with two separated antennas ( 73 , 74 ) to provide a more uniform mass distribution and reduce structural loads on the aircraft and/or to allow reduced electromagnetic interference between the antennas. 
         FIG. 8  shows a plane with a large pair of antennas ( 82 , 83 ) and a small pair of antennas ( 81 , 82 ). Such an arrangement can be optimal if the communication to small antennas on the ground is carried out at much lower frequencies than the backhaul frequencies—communication to larger antennas on the ground linking the aircraft to a core ground based network. 
         FIG. 9  shows an example of a multiple antennas arrangement designed to allow an individual aircraft to communicate with a much larger area on the ground than would be possible with a flat almost horizontal phased array antenna(s). Typically, flat phased arrays only project and receive within a cone of around 60 degrees to axis of the array; normally the axis is at right angles to the plane of the array. Therefore communication to transmitters or receivers, or transceivers based at an angle more than sixty degrees to the axis of the array begins to be inadequate. This problem is exacerbated if the plane pitches or rolls and continuous communication is required. The arrangement shown is mirrored on both sides of the fuselage; the centerline of the plane ( 92 ) is shown horizontally. 
     
    
    
     A plan view and three sections (AA, BB, and CC) are shown. The encumbered (E) section ( 91 ) encloses all the antennas. 
     There are three sets of antennas: a single horizontal antenna ( 94 ) pointing directly down, a pair of antennas ( 95 ) allowing better communication from side to side, and a pair of antennas ( 93 ) allowing better communication forward and backwards. The antennas need usually to be sited to avoid significant interference with one another. Round, ellipsoidal or more complex shapes can be envisaged as well as an “inverted saucer” shape. The angles can be varied and larger or smaller numbers of sets of antennas can also be used. 
     For a given antenna projected size—the area of the antenna when viewed normally to the main plane of the antenna—to minimize aerodynamic drag, the entire antenna should usually be enclosed by the wing structure. However in some instances, the design will benefit from a modest portion of the antenna or antenna casing being outside the aerofoil cross section of the wing rather than going to the expedient of increasing the aerofoil chord length(s) in the “encumbered” section(s) of the wing. This may be because of the particular antenna shape not readily fitting in with the aerofoil section, being for example square rather than elliptical or circular, or for particular attachments to pods containing other equipment or access points or for a variety of other reasons. Usually the encumbered section will enclose a “substantial” fraction being at least 50%, preferably 80% and more preferably all of the projected area of the antenna(s). 
     High altitude long endurance planes fly quite slowly: typically at speeds lower than 100 m/s and more usually below 50 m/s and sometimes as slow as 15 m/s. At these velocities with the cold, low density, relatively viscous air encountered at high altitude, the wing Reynolds number is much lower than that encountered in conventional aircraft: gliders or powered vehicles. However, aerofoil sections designed for low Reynolds numbers are common in low altitude unmanned aerial vehicles, in wind turbines and other applications. Examples of such an aerofoils have been designed by for example Selig (see “New Airfoils for Small Horizontal Wind Turbines,” Giguere and Selig, Trans ASME, p 108, Vole 120, May 1998): particularly the aerofoils SG 6040, SG 6041, SG 6042, SG 6043, with thicknesses of respectively 16%, 10%, 10%, and 10%. 
     The aircraft designs described below in Table 1 show the advantages of utilizing the invention. 
     All cases tabulated are for the same weight of wing per unit wing area with the addition of a constant spar weight per unit width of wing. The aircraft design is for operation at a latitude of within 15 degrees of the equator, and the powers and speeds are calculated on the basis of mid—winter conditions to allow station holding throughout the year. In the base case utilizing the invention, the “encumbered” section is designed on the basis of an SG 6040 cross section with a 16% thickness to chord length, two antenna of 1.6 m diameter with a weight of less than 6 kg/m 2  (total weight of antenna+electronics=30 kg), can be fitted into the encumbered sections having a chord length of 2 m. The unencumbered sections are designed on the basis of an SG 6043 cross section. 
     Utilizing the invention results allows a plane of the same wingspan to either support a heavier payload and larger antenna with a similar operating speed (necessary for station-holding in many applications) than a conventional plane, or with a similar payload weight, the maximum operating speed is significantly increased. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Comparison of “classical” wing and “novel” wing designs 
               
            
           
           
               
               
               
               
            
               
                   
                 Novel 
                 Constant 
                 Constant 
               
               
                   
                 design 
                 speed 
                 payload 
               
               
                   
                 (with 
                 (classical 
                 (classical 
               
               
                   
                 invention) 
                 design) 
                 design) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Design variables 
                   
                   
                   
               
               
                 Payload power (W) 
                 350 
                 350 
                 350 
               
               
                 Overall power train efficiency 
                 70% 
                 70% 
                 70% 
               
               
                 Battery energy (Whr/kg) 
                 350 
                 350 
                 350 
               
               
                 Aircraft altitude (m) 
                 18000 
                 20000 
                 18850 
               
               
                 Wing span (m) 
                 35 
                 35 
                 35 
               
               
                 Wing area (m 2 ) 
                 43.5 
                 56 
                 70 
               
               
                 Average wing chord length (m) 
                 1.2 
                 1.6 
                 2.0 
               
               
                 Overall lift to drag coefficient 
                 43 
                 38 
                 37 
               
               
                 Reynolds number of average 
                 280,000 
                 280,000 
                 340,000 
               
               
                 wing chord length 
               
               
                 Outcomes 
               
               
                 Payload weight (kg) 
                 30 
                 20 
                 30 
               
               
                 Aircraft speed (m/s) 
                 28 
                 28 
                 23 
               
               
                 Reduction in payload weight 
                 — 
                 32% 
                 — 
               
               
                 Reduction in speed 
                 — 
                 — 
                 18% 
               
               
                   
               
            
           
         
       
     
     It can be seen that an aircraft utilizing the invention has a significantly higher payload weight (32%) than a conventional design with the same cruising speed, or a significantly higher cruising speed (18%) than planes of the same wing-span with conventional design and the same cruising speed. 
     This is a result of higher “induced drag” caused by the lower aspect ratio for wings of classical design, which reduces the energy available for the payload or results in lower aircraft speeds than would be desirable. The operating altitude has been optimized to reflect the different characteristics of the different designs. 
     It may also be desirable to maintain a similarity of circulation over a variety of airspeeds if for example low drag performance is necessary for high flying speeds as well as low. 
     In a third aspect of the invention additional wing flaps are provided in one or more of the “encumbered,” “transitions” or “unencumbered” sections that allow the circulation to maintained at a more elliptical level over the sections for a greater range of aircraft speeds. 
     In a fourth aspect of the invention the flap sections are of variable relative chord length along the wing allowing a more elliptical circulation and lower drag along the length of the wing. The relative flap chord length is defined as the distance from the leading edge of the flap to the trailing edge of the aerofoil referenced to the chord length of the aerofoil at a particular distance from the fuselage centerline. It is familiar to those skilled in aerofoil aerodynamics that deflection of an aerofoils flap results in a change to the effective local angle of attack, see Schlichting, Truckenbrodt “Die Aerodynamik des Flugzeuges Bd II.” Springer-Verlag 1969, p 439. 
     In a fifth aspect there are two main frequencies used on the plane: a relatively low frequency of between 0.5 and 5 GHz with large phased arrays which can provide uplink and down link to ‘user equipment’ with a suitably long wavelength such that transmission and reception can be through rain and building walls of a reasonable thickness and secondly a higher frequency than the uplink/downlink utilizing a much larger bandwidth and smaller arrays that is used for backhaul to and from the plane. These phased arrays can have beam axes that are approximately vertical, or be made up of clusters of arrays whose axes are approximately vertical, or be clusters some of whose axes are approximately vertical and some of whom which are not.