Flight has always captured man’s imagination, which is evidenced by the great variety of aerial vehicles that exist today. Everything from fixed-wing to helicopters; sailplanes to spaceships; mono-wing to quadrotor. However, despite the wide variety of flying vehicles, not one of them has attained eternal flight. Accomplishing this feat is one of the great challenges still facing the aviation community.

Motivation  

Satellite Demand

Satellites have had a profound impact on our society by connecting the world through telecommunications, delivering precise locations with geopositional navigation, and enabling imaging and sensor systems to significantly increase our understanding of our planet as well as the entire universe.  

But there is still a need for even greater capabilities while delivered at a lower cost.

Society has developed an increased appetite for omnipresent communications, surveillance capabilities, and instant access to information. More recently, the expectation is for these services to be achievable in real-time as on-demand services.

These new capabilities are not well met by existing satellites, and will require a new solution with several orders of magnitude lower signal latency, greater operational flexibility, and dramatically lower costs. 

Classes of Satellites

Various types of satellites are currently in practice, each having its own strengths and weaknesses. Geosynchronous satellites offer a continuous coverage capability with a single satellite. Low Earth Orbit satellites are fifty times closer for improved single latency, power requirements, and imaging resolution; but require a fleet to be operational. Atmospheric satellites are another fifty times closer for even better latency, power, and resolution; and are reusable with extremely low capital costs.

Atmospheric Satellites

  • Operate ~60k ft
  • 50 times closer than LEO Sat
  • Broadband transmission speeds
  • Takeoff and landing reduces costs

Geosynchronous Satellites

  • Fixed point for continuous coverage
  • Farthest distance, greatest area
  • Large transmission delay
  • Significant power loss to transmit

Low Earth Orbit

  • 50 times closer that Geo Sat
  • Requires a constant orbit, no fixed point
  • Need a fleet for complete coverage
  • Still not capable of two-way transmission

Limitations

An eternal flight capability is needed before Atmospheric Satellites become a reality.  However, current aircraft designs have pushed the boundaries of structural material capabilities. Even with cutting-edge exotic composite materials, current designs are unable to incorporate enough strength and stiffness to employ the long and slender wings needed to achieve better aerodynamic efficiency.

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Concept

Attaining eternal flight requires a paradigm shift in the vehicle design.  By taking the best features of both glider and helicopter design methodologies, a new concept vehicle is able to incorporate each of their benefits while minimizing their detriments.

Glider Vs Helicopter

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Helicopter Attributes

Helicopter rotor blades, which bend under their own weight, are stiffened through centrifugal forces.  Unfortunately, rotorcraft are not aerodynamically efficient vehicles.  

PROS

  • Vertical takeoff and landing (VTOL)

  • Centrifugal forces stiffen thin rotor blades

CONS

  • Not aerodynamically efficient

  • Triangular span loading

  • Complex rotor mechanism

  • Must withstand extremely large forces

Glider Attributes

Gliders demonstrate extreme aerodynamic efficiency, because they employ long slender wings to reduce drag.  But this design approach is limited by structural material properties.

PROS 

  • Aerodynamic vehicles with minimal drag

  • Utilize long slender wings (high aspect ratio)

CONS

  • Thin wings lead to bend and twist

  • Physical constraints limit the aspect ratio

  • Requires additional material to mitigate bending

  • Cannot hover or takeoff vertically

Best of Both Worlds

A new concept vehicle combines the best features of both glider and helicopter design methodologies.

  • Apply centrifugal stiffening within a glider wing to further increase the aspect ratio and aerodynamic efficiency

  • Eliminate wasted inboard material from a traditional rotor to achieve better aerodynamics

Introducing the Tethered Uni-Rotor Network (TURN)

Without further ado, lets unveil this revolutionary new concept vehicle. This research introduces a novel UAV a called the Tethered Uni-Rotor Network (TURN).  The system utilizes a unique alternative approach, which exceeds existing capabilities, outpaces current research efforts, and may be the answer to attain eternal flight. 

Hover Operation

During flight, the vehicle operates in a perpetual state of rotation. Each of the satellite bodies drive the rotation of the system with their respective propulsion systems, which have an optimum twist and pitch designed for the rotation rate. This propulsion technique is opposite that of a conventional helicopter rotor, where a torque is applied to the central shaft, which requires a counter-torque to prevent the helicopter body from spinning in the reverse direction. Since this is a tip driven system, there is no torque transmitted back to the central hub. Because the system is spinning, centrifugal forces keep the tethers taught and mitigate bending moments commonly present within thin wing sections. As the satellite bodies move through the air, lift is generated on each of the winged airfoil sections, which is enough to counteract the weight of the satellite, and a distributed portion of the weight of the central hub. 

Vehicle Layout

The TURN system has a central hub which stores the vehicle payload. Several high tensile, small diameter cable tethers radiate outwards from the central hub. At the end of each tether, a satellite body is attached at the outboard position. Each satellite contains all the components typically found on an aircraft system, including: an airfoil lifting section, small propeller for propulsion, several control surfaces, and a fuselage containing batteries, hardware, and sensors. Each satellite resembles a flying-wing aircraft, which provides all the lift, propulsion and control for the TURN system. The propeller is mounted on the leading edge of the wing, located near the outer wingtip. Immediately behind the prop, stabilizers and control surfaces are located directly in the prop wash. The concept was named Tethered Uni-Rotor Network because a network of aircraft systems are tethered together to form a much larger singular rotor system. 

Flight Operation

Individual satellites are controlled through their propeller and control surfaces, and the central hub is controlled by coordinating the tether forces imparted from the satellite vehicles. Two types of translation are considered, vertical and horizontal, which have a parallel in helicopter terminology as “collective” and “cyclic” commands. Each type of translation has two associated control modes which can be implemented through different control inputs. 

HORIZONTAL TRANSLATION

Horizontal translation is achieved with “cyclic” commands, where the control inputs are changed in a sinusoidal fashion throughout the rotation. The first cyclic command is applied to the vertical control surface, which manipulates the amount of tension on the tether. At one point in the rotation there is maximum pull from the tension, and exactly opposite that point there is a minimum tension force. This imbalance causes the TURN system to translate horizontally. Another approach uses cyclic elevator commands to achieve horizontal translation. At one point in the rotation a satellite passes a low elevation, and at the exact opposite point the satellite traverses a high elevation. This essentially tilts the plane of rotation, which inclines the thrust vector coming from the airfoils, and yields a horizontal force component. This type of thrust vectoring is identical to how traditional multirotors translate horizontally. 

VERTICAL TRANSLATION

Vertical translation uses “collective” commands where each satellite adjusts its settings in unison. Adjusting throttle increases or decreases the velocity of the satellite, and thus the angular rate of the TURN system. This changes the amount of airflow over the wing, which increases or decreases the total amount of lift generated, and causes the vehicle to ascend or descend. Alternatively, adjusting the pitch of each satellite through the elevator control surface, causes each satellite to nose up or down, thus the entire system will climb or fall as each satellite moves through a spiral trajectory.

System Benefits

Centrifugal Stiffening

  • Mitigates bending moments and flexibility typically found on fixed-wing aircraft.

  • Reduces the amount of structural material needed to add stiffness within the wing section.

  • Allows for much larger aspect ratios and greater efficiency without additional material.

High Aspect Ratios (AR)

  • Typical aircraft have AR of 10 where 10-20% of the vehicle mass resides in the wing.

  • HALE aircraft target AR of 25, but require 40-50% of the vehicle mass in the wing to add stiffening.

  • Helicopters have AR of 50, while the rotors only account for 2-3% of the total vehicle mass.

Accomodate Bulky Payloads

  • Bulky payloads on traditional aircraft feel aerodynamic drag from the cruise speed required to produce lift.

  • A TURN payload is housed in the central hub which stationary, and independent of the lift generating components.

  • Thus, large bulky payloads do not incur the same drag penalty seen with traditional aircraft.

High Lift-to-Drag Ratios

  • Centrifugal stiffening allows for very thin airfoils with low thickness-to-chord (t/c) ratios.

  • Large diameter means the system can cruise at low airspeeds and low Reynolds number.

  • Rotors are not operating in the downwash field of their neighbor which helps the aerodynamics.

  • Larger swept area reduces the disk loading on the system.

Greater Energy Storage and Density

  • The vast majority of fixed-wing aircraft can only allocate 40% of vehicle for battery mass.

  • Reduced structural material within TURN concept, means nearly 80% of the vehicle mass resides in batteries.

  • Most unmanned aircraft rely on Li-Po batteries because they require the high discharge capacity.

  • Because the TURN system is a low-power concept, it can utilize Li-Ion or Li-S batteries, which nearly doubles the energy storage for the same battery mass.

Slow System Rotation Rate

  • Helicopters have 97% of the mass suspended from the swashplate, so their rotors must spin at very high angular rates.

  • The TURN concept has large masses concentrated at each of the wingtips (predominantly from batteries).

  • This substantially reduces the angular rate, where the largest system takes over 30 seconds to complete a single revolution.

  • Thus, each wing moves more slowly (lower power), and beyond other downwash fields (better aerodynamics).

Applications

An eternal flight capability has a wide variety of applications across a number of different industries.  But the unique TURN design also allows for alternative embodiments, including small scale commercial drone and airborne wind energy applications.

Eternal Flight Uses

Placing a black-box sensor payload within the stratosphere, can provide all the services as our existing and aging satellite networks, but at vastly reduced cost, complexity, and signal latency. Furthermore, pseudo-satellites offer capabilities that satellites cannot deliver, like atmospheric monitoring and broadband speed transmission data rates for global Internet connectivity.

Government Applications

  • Forrest Fire Monitoring

  • Crop Disease Tracking

  • Search and Rescue

  • Border Patrol

  • Global Mapping

  • Disaster Assessment

Civil Applications

  • Agricultural Health Monitoring

  • Wireless Broadband

  • Satellite Phone

  • High Fidelity Imagery

 

Atmospheric Observations

  • Extreme Weather Monitoring

  • Hurricane Origination and Development

  • Tornado Monitoring

  • Pollution and Air Quality Studies

  • Glacier Ice Dynamics

  • Volcano Monitoring

Internal Combustion Engine Embodiment

The TURN concept was awarded a Small Business Innovation Research (SBIR) grant sponsored by the US Air Force. Research investigated a variation of the TURN system, utilizing an internal combustion engine. While not an eternal flight solution, the predicted flight endurances vastly exceed what other current unmanned systems can attain.

Mission Requirements

  • Payload: 250 lb, 2000 W

  • Medium Altitude: 10k-15k ft

  • Wind Tolerance: 50 knots

  • Best Available: 5 day flight

Research Investigation

  • Fuel burn alters TURN dynamics

  • Investigated full flight envelope

  • Revised struct/aero models

  • Geometric programming optimization

TURN Capabilities

  • Satisfies mission requirements

  • Wingspan: 45 feet

  • Can withstand hurricane wind speeds

  • Standard conditions exceeds 30 days

Smaller Scale Commercial Drone

The primary benefit of the TURN concept is its extreme aerodynamic efficiency.  That feature holds even at smaller scale systems, while still offering hover and vertical takeoff and landing (VTOL), which would yield a formidable competitor within the existing fixed-wing commercial drone industry.

Potential Customer

  • Price sensitive consumer

  • Hover capability is desirable

  • Doesn't require eternal flight

  • Does require longer flight endurance

TURN System

  • Remove solar cells to reduce cost

  • Same payload capacity

  • TURN is 4X more efficient

  • 6.0+ hr flight endurance

Existing Fixed-Wing

  • 10 foot wingspan

  • 5 pound payload

  • Needs a launch site

  • 60-90 min flight endurance

Airborne Wind Energy (AWE) Platform

Existing wind turbines are only applicable for 15% of the Earth's surface, and larger scale systems are not cost effective.  New interest has focused on AWE devices which can collect more energy by tapping into stronger and steadier winds.  These systems have some shortcomings which can be directly mitigated by a TURN design methodology. 

AWE Approach

  • Eliminate tower structure

  • Energy kites fly higher

  • Collect more energy

  • Greater number of locations

AWE Limitations

  • Long tether subject to drag

  • Bending loads on kite wing

  • Limited trajectory control

  • Inefficient multirotor for hover

TURN Improvements

  • Rotates around stationary central hub

  • Eliminate primary tether drag

  • More aerodynamically efficient

  • Increased directional control