Abstract:
The objective of Project Bhagat was to design an aircraft that would fly unmanned for extended periods of time at high altitudes using solar power. Platform, payload, power sources, long flight duration, unmanned, real-time access, remote guidance and control, and cost were all considered during the design process. An altitude of more then 10,00,00 feet altitude and speed of more then 25mph is kept as target. This can be achieved by triple slotted flap wing because it increases lift and thrust of the air craft. NASA’s Helios aircraft was chosen to be the platform for the mission. The vehicle will be powered by black silicon solar panels and a high-speed flywheel for nighttime flight. The science package will include a Digital Array Scanned Interferometer (DASI), an “Airborne Real-Time Imaging System,” and an acoustic remote sensing system known as SODAR. They will be used for a variety of monitoring purposes, such as storm development, forests, and other large remote expanses to provide early warning of hurricanes, fires, or crop damage. The DASI sensor will also collect, identify, and monitor environmental data for the assessment of global change.
Background:
The first aircraft designed to show the use of solar power for long-duration, high-altitude flight was the Pathfinder. It was constructed using plastics, and foam in order to achieve the desired light weight (around 560 pounds). Solar cell arrays covered about its 98-foot-long upper wing surface. which provided up to 8,000 watts of power at high noon on a clear day. These arrays were used to power the aircraft’s six electric motors as well as its avionics, communications, and other electrical systems. To allow limited-duration flight after dark, the Pathfinder also had a backup battery system that could provide power for around two to five hours. It flew at airspeeds of only 15-20 mph. Pitch control was maintained by using tiny elevators on the trailing edge of the wing. Roll and yaw control were maintained by slowing down or speeding up the motors on the outboard sections of the wing. On September 11, 1995 it set the first altitude record for solar-powered aircraft at 50,567 feet during a 12-hour flight and then on July 7, 1997 it set a new record of 71,530 feet.
The Centurion was designed to reach and sustain an altitude of 100,000 feet for up to two hours. It was considered a demonstrator for a future fleet of solar-powered aircraft that would be able to stay airborne for weeks or months on scientific sampling and imaging missions. It had a wingspan of 206 feet and four, compared to the Pathfinder’s two, underwing pods to support its landing gear and electronic systems. Its solar arrays provided 31 kilowatts of power at high noon and it had 14 electric motors. The Centurion also had a backup lithium battery system. It flew at airspeeds of 17-21 mph. For its final flight it carried a simulated payload of more than 600 pounds, which was about half the aircraft’s empty weight.
The Helios was based on the Centurion design. It has a wingspan of about 250 feet and is designed to operate at up to 100,000 feet. It incorporates a rechargeable fuel cell-based storage system to convert and store excess solar energy absorbed during the day into electrical power in order to maintain flight at night.
Discussion:
Platform Design: The first crucial step in designing a high altitude , high speed, long endurance aircraft was picking a platform that had the desired characteristics. The platform chosen must be lightweight, simple in design, and require a minimal amount of power. For this mission three existing platforms were considered. The first platform that was analyzed was the NASA product called Helios. This platform consisted of six wing sections each 41 feet in length joined together to form one massive wing. The unique single wing design created a large amount of lift, which correspondingly required less power to remain aloft at high altitudes. Another platform considered was a twin boom aircraft that was studied under the HAPP program in the mid-1980s. This design consisted of a single engine glider design. The twin boom design offered increased structural stability as a result of a dual rudder attached to the second boom. The last design that was considered was the Proteus produced by Scaled Composites Incorporated. This design involved a conventional airframe design that was powered by two turbofan turbines. The turbines provided enough power to allow for large payloads at high altitudes.
After considering each of the three platforms the decision was made to use the Helios platform for this mission. The crucial design driver that determined the selection of Helios was the gross weight. Helios had an extremely lightweight frame that was constructed out of composite materials such as carbon fiber, graphite-epoxy, and Kevlar. These materials provided adequate strength for the given loading requirements and would enable a gross weight for the aircraft of 2048 pounds. In addition to this weight, the Helios would be capable of storing an additional 200 pound payload. The twin boom aircraft would weigh more without a significant improvement in payload capacity as a result of the airframe structure. The addition of two rudders and another boom made the weight of this aircraft too heavy for this mission. The Proteus would be able to carry the
highest amount of payload weight. However, the structural airframe and high payload capacity made the conversion to solar-power impossible. The reason for this was that the weight requirements necessitate more power than any electrical motor could supply.
There were many other advantages in choosing Helios. First of all, the single wing design was relatively simple and would enable easy reproduction of the aircraft. In addition, the Helios design was already in prototype form therefore, development and testing costs would be kept to a minimum. Figure 2 shows the Helios platform.
Energy Storage System:
One of the most crucial design drivers for a high altitude long endurance solar powered aircraft was a high capacity energy storage system. The current energy storage systems proposed rely heavily on conventional electrochemical fuel cells. However, current electrochemical fuel cells are too heavy and do not have a high enough energy conversion efficiency. To make high altitude long endurance aircraft a reality, energy storage systems would need improved power output, weight, and energy conversion efficiency. An alternative to conventional batteries has been proposed involving the use of a flywheel energy storage system. Flywheel energy storage systems have long projected significant potential for improved energy storage capability over existing electrochemical battery storage systems. Yet, conventional flywheel systems have energy conversion efficiencies of less than 90% and although this is an improvement to electrochemical batteries (efficiency around 67%), the useable energy capacity is still limited. For high altitude long endurance aircraft a new type of flywheel energy storage system may provide sufficient power. The new system involves a high-power, ultra low loss, ironless generator concept as part of a flywheel energy storage module. The ironless generator contains a supersonic magnet array with 20 Neodymium-Iron-Boron magnets on a 12 inch diameter rotor. This flywheel configuration would increase energy conversion efficiencies to 99% and provide more than 2.9kW of useable energy. The actual flywheel system is displayed
Flywheels have conventionally not been used on aircraft because of the impact of their highly gyroscopic motion on the flight of a plane. A single flywheel could create instability in the lift of a plane by generating a sideways force imbalance. However, this effect can be corrected by having a pair of flywheels rotating in opposite directions. By having a set of counter rotating flywheels any centrifugal force they might create is canceled by the motion of one another
one of the greatest limitations in the development of flywheels has been the ability to maintain the integrity of the magnet rotor at high speeds. When the rotor reaches high speeds the magnets tend to break as a result of high tension stresses. This problem has typically limited the power output of flywheels as the generators could not attain high enough speeds to produce appreciable amounts of energy. The new flywheel design contains an almost zero tension magnet rotor, which eliminates this problem. The rare earth metals used for the magnets are very weak in tension but can withstand high compressive stresses (even greater than most alloy steels). To take advantage of this property of the magnets, a rotor has been developed that creates a uniform compression stress at the interface between the magnet rotor and the interface disk. As the speed of the magnet rotor increases, the hub of the rotor expands due to centrifugal loading. The hub expansion creates a radial compression at the inner diameter of the magnet rotor, which eliminates almost all of the tension stresses between the rotor and the interface disk. This allows the magnet rotor to obtain supersonic speeds that enables the flywheel to produce sufficient power for a high altitude long endurance aircraft.
Another important aspect of the new flywheel design is the energy conversion efficiency. The improvement of energy conversion efficiency in this flywheel system is related to the dual axial gap electrical machine design. The high efficiency energy conversion characteristics of the flywheel energy storage system are achieved through the use of a dual axial flux permanent magnet. The design consists of laminated rotor backirons that permit use of an ironless stator core in the machine design. Typical iron core stator losses are on the order of 10% to as much as 25% for conventional motor designs and by eliminating the iron core these losses become negligible. The conventional stator ferromagnetic core used to concentrate the magnetic field across the stator gap is replaced by a ferromagnetic backiron that rotates synchronous with the magnet rotor. The backiron has multiple laminations that decrease the eddy current loss. .Triple Slotted Flap Wing:
This type of wing is used to increase the pace of the air craft to over come the basic draw back of the Helios’s air craft. What this wing real does is that it allows the air to flow through the gap between the slots hence the force over the air craft decreases
Results and Conclusion:
Platform: The final design of the platform was based on the Helios aircraft. The mission required Helios to fly at an altitude of 70,000 feet with a cruising speed of approximately 22 mph. The atmospheric conditions required that Helios have 16kW of power available. This power was provided by 10 electric motors that each supplied 1.5 kW. The calculations of the necessary output power are displayed
Propulsion System:
The propulsion system for the aircraft was based on the electric motors that were used on Helios. These motors were the most practical for this application because they were lightweight and very efficient. A total of 11 direct-current electric motors that each provided 1.5 kW were used. The motors each drove two-blade, 79 inch diameter propellers that were specifically designed for high altitude use.
Solar Panels:
The solar panels used in this mission were single flat crystal silicon cells such as the ‘Pegasus’ used on the Helios with the addition of the black silicon treatment. Black silicon is produced by scanning it with ultra short, ultra-intense laser pulses. The energy in a single pulse approximates focusing all the sunlight hitting Earth at one time onto a space the size of a fingernail. After more than 500 pulses, the silicon turned black. This was because its surface had been etched by the heat and gas into a forest of billions of minute needlelike spikes (see Figure 4). If a light is shone on such a surface, it repeatedly bounces back and forth between the spikes in a way that most of it never comes back out again. The spiky surface also absorbs infrared radiation (heat), making it an excellent detector of clouds, pollution, water vapor, and specks of dirt and liquid that change the quality of the air and influence global climate. Treated correctly, the black variety absorbs 96 to 98 percent of the light that hits it. Commercial solar cells, usually made from silicon, are only about 17 to 23 percent efficient at converting sunlight directly into electricity. Using black silicon will reduce the weight of the aircraft without losing any power; thus, leaving more allowable weight for payloads.
The maximum output of the Pegasus was 42 kW with 83,639 standard cells and an area of .024 ft2 (use EQ. 1); this output using the black silicon can be reached using only 4,345 standard cells with an area of .024 ft2 thus taking up a wing area of 102.7 ft2
Energy Storage System:
To make a high altitude long endurance solar-powered aircraft a reality a new energy storage system must be used to supply enough power. A new flywheel energy storage system is proposed as the solution. The two main components of the flywheel configuration are an ironless generator and a supersonic magnet array. The generator combines a graphite composite storage rotor into a double axial gap electrical machine structural design. The rotor serves to maintain the structural integrity of the high-speed magnet array as well as providing energy storage. The magnet array consists of 20, 1/2 pound Neodymium-Iron-Boron magnets on a 12 inch diameter rotor that is located directly across from the graphite interface disk. The magnet array is designed to operate at a peak speed of 32,000 rpm which translates to a Mach number greater than 1.5.
The flywheel energy storage system proposed will make high altitude long endurance solar-powered aircraft feasible. The flywheel will be activated when the solar panel power output drops below the necessary minimum. The flywheel system must produce a total 16kW. The flywheel described can produce a total of 2.9kW of energy for a maximum of 10 hours. The target power will be generated by using a total of six flywheels in conjunction with one another. The six flywheels will be divided into pairs with each one rotating in opposite directions. This configuration eliminates any impact the motion of the flywheels could have on the flight of the aircraft. The flywheel system will produce a total of 17.4 kW of useable power. This allows up to 1.4 kW to be provided to the electronics and science package onboard. The weight of a single flywheel is 120 pounds so the total weight of the system will be 720 pounds. Although this value is still high, the reduction in weight of the solar panels compensates for this load. In addition, further research is continuing in the area of flywheels and within the next couple of years an even lighter flywheel should become available.