The concept of high-altitude, long-endurance solar-powered aircraft has been demonstrated by a number of air vehicle research projects in the past. In 1974, AstroFlight built the first solar-powered drone, Sunrise I. The promising results of the 32 foot span, Sunrise I, led to the Sunrise II, which with 4480 solar cells was theoretically capable of attaining a service ceiling of 75,000 feet. Sunrise II flew successfully, but broke up in flight at 22,000 feet due to a suspected aeroelastic problem. The next advance in solar-powered flight occurred in 1980 with AeroVironment's Gossamer Penguin, which performed the first solar flight carrying a human, followed by the Solar Challenger, which reached an altitude of 12,000 feet on its flight across the English Channel. NASA's High Altitude Solar (HALSOL) project in 1995 saw the flight of the Pathfinder, which reached an altitude of 50,000 feet. This was followed by the Pathfinder-Plus which, with its new 19% efficient silicon solar cells, was able to reach 80,201 feet. The Pathfinder aircraft then led directly to the Centurion. The Centurion was aimed at creating an aircraft that would have a real world scientific application. The Centurion had a span of 206 feet with 62,120 bi-facial solar cells.
Under NASA's Environmental Research Aircraft and Sensor Technology Program, 1998-2003, the Centurion was modified to become Helios. The Helios prototype was designed as a proof of concept high-altitude unmanned aerial vehicle that could fly on long endurance environmental science or telecommunications relay missions lasting weeks or months. Helios made use of 19% efficient silicon based solar cells on the upper wing and lithium batteries. Helios had a constant 8 foot chord and was assembled in six 41-foot sections with under-wing pods at the juncture of each section. Helios reached record-setting altitude of 96,000 feet on solar power. In subsequently testing, Helios broke up in-flight. The in-flight break-up was caused when a gust-induced aeroelastic wing shape change led to control system instability. The resulting pitch oscillation resulted in excessive speeds which caused failure of the wing covering. The wing spar actually withstood deflections of 150% of the design configuration. In 2005, AC Propulsion developed the SoLong aircraft. With the energy storage advances made with Li-Ion batteries (220 Whr/kg), SoLong was able to stay airborne for two half nights, starting with a charged battery at midnight and flying to midnight the next day. This initial 24 hour flight was followed a few months later with a full 48 hour flight. In 2007, the English company Qinetiq flew the Zephyr 54 hours. This aircraft has taken advantage of both 25% efficient solar cells and 350 Whr/kg Lithium Sulfur batteries.
The best example of previously built and flown state of the art is the AeroVironment aircraft, culminating in the Helios. Much of this is described in U.S. Pat. No. 5,804,284, to Hibbs, et al. (hereinafter, the Hibbs patent). The Hibbs patent shows a very large wingspan aircraft, with the solar collection and other mass distributed along a very high aspect ratio wing. This allowed the use of a very light wing spar, and the simple, clean design consumed very low power during the night. As discussed in great detail below, night time power usage is especially critical, because the storage system is quite heavy, and there is a storage “round trip” efficiency. This means that a large amount of solar energy must be collected to provide even a small amount of power at night. In the example given in the Hibbs patent, 2.5 Watt hours of electrical power had to be collected during the day to provide 1 Watt hour at night.
However, a significant limitation of the airplane disclosed in the Hibbs patent is that it is poor at collecting energy during the winter time at high latitudes. For example, London, England is approximately 51.5 degrees latitude. At winter solstice, the peak elevation of the sun above the horizon is only 15 degrees, and the horizontal solar collector, as shown in the Hibbs patent, will collect at most 25% of the energy it would collect with the sun overhead. Another significant limitation is that at high latitudes, the aircraft must fly predominantly towards the west, so the sun, at peak elevations, will be predominantly off the left wing tip. Thus the normal flexing of the wing, such as shown in flight on Helios, aims much of the wing panels away from the sun, while also putting some of the remainder of the wing in the shadow of the left wing tip. Thus, the net collection capability is likely only about 15% of what it could optimally collect with the sun overhead. The poor collection geometry of the airplane disclosed in the Hibbs patent (i.e., the horizontal solar panels), combined with short days and long nights makes it very difficult for the Hibbs' airplane to collect sufficient solar energy.
Nevertheless, improved collection geometry has been suggested in the prior art. An example is shown in U.S. Pat. No. 4,415,133, issued in 1983 to Phillips (hereinafter the Phillips patent). This configuration is also shown in NASA Technical Paper 1675, “Some Design Considerations for Solar-Powered Aircraft,” published in June 1980, also by Phillips. The cruciform configuration shown is capable of flying in any desired roll attitude, and thus can have its solar panel track the sun in elevation. While the cruciform configuration disclosed in the Phillips patent provides improved solar energy collection than the configuration shown in the Hibbs patent, it has twice as much wing area as is needed to produce lift, and thus incurs a significant penalty in drag and thus energy required to fly, especially during the night (when no solar radiation energy collection can occur).
Another NASA study published in 1983, Contractor Report CR-3699 by Hall, Dimiceli, Fortenbach and Parks, entitled “A Preliminary Study of Solar-powered Aircraft and Associated Power Trains” (hereinafter the 1983 NASA C. Report) looked at, among other things, a wide range of configurations that attempted to combine both low power consumption at night with good solar radiation energy collection geometry during the day. Some of these configurations are shown in FIGS. 46 and 47 of the report, on pages 120 and 121 respectively. Configurations 2 and 3 in FIG. 46 show aircraft that have pointable collectors, but exhibit high drag both during days and nights. FIG. 4 of the report shows an early attempt to combine improved solar energy collection with good night time power efficiency. As those of ordinary skill in the art can appreciate, however, only one of the elevated wing panels has good solar energy collection. For westward flight with the sun off the left wing tip, the left wing has poor solar energy collection, as mentioned above, and can shadow the right wing.
Variable geometry designs are shown in FIG. 47 of the 1983 NASA C. Report, particularly in configurations 14, 17, and 18. All of these have a large wing span, and all of the wings provide lift for low night time energy consumption. Configurations 17 and 18 are symmetric in both day and night modes, but require solar cells on the bottom of one tip and on the top of the other. This is good for typical westerly winds, but for the occasional easterly winds, cells would be needed on both sides of both tips, which is both a mass and cost penalty. Configuration 14 of FIG. 47 provides solar cells on top of both tips, but is not symmetric, and it was believed that the control systems of the time would not be able to fly the airplane. Furthermore, in configurations 14, 17, and 18, the wing tips were only able to be oriented vertically or horizontally. Thus, while they were pretty good at solar radiation energy collection with the sun on the horizon or overhead, their solar radiation energy collection is significantly reduced when the sun is at 30° to 40° elevation angle with respect to the horizon.
A significant shortcoming of all three configurations shown in FIG. 47 of the 1983 NASA C. Report is that when the wing tips are vertical, they cannot support their own weight. As a result, a large downwardly directed load is brought upon the tips of the center section. To enable the aircraft to support such large load factors, a large structural mass is designed into the aircraft. Because the tips cannot support their own weight, the fraction of the span that could be pivoted up is limited.
In U.S. Pat. No. 7,198,225, issued in 2007, to Lisoski and Kendall (hereinafter referred to as the Lisoski patent), which also relates to the Helios type aircraft, a variant of Helios is proposed with variable wing angles to improve solar radiation energy collection, as shown in FIGS. 6E and 6F. However, as those of ordinary skill in the art can appreciate, the configurations shown in FIGS. 6E and 6F of the Lisoski patent are essentially the same concept shown in FIG. 46 of the 1983 NASA C. Report, configuration 4.
All of the above concepts have some problems with either solar collection at low sun elevation angles, sun collection at medium sun elevation angles, night time energy requirements or excessive structural mass. Thus, there is a need for a solar aircraft configuration that can effectively adapt to a wide range of sun angles, does not carry collectors that are not useful at some sun angles, has very low drag for low night time energy requirements, and also does not require excessive structural mass, and thus can allocate a large mass to the energy storage system.
While the historical solar-powered aircraft have increased flight duration and altitude over time, none have exhibited the ability to fly at high latitudes; none have any shown greater duration than perhaps a day or two. Thus, historical solar-powered aircraft all have limitations due to poor high latitude solar collection efficiency, due to the horizontal nature of their arrays and insufficient energy storage to fly through a long winter night; therefore there is also a need for an improved energy storage system. Thus, a need exists for a solar-powered aircraft that can overcome the deficiencies of the prior art, by operating at high latitudes and during long periods of darkness.