U.S. Naval Research Laboratory  

How does it work?

The aerodynamic properties of the F2P propeller blades are tailored to result in a positive-lift blade-pitch trim condition during operation. When operating conditions change and result in new inflow angles, blade pitch adjusts passively to the new trim pitch angle.

What will it accomplish?

By adjusting to operating conditions,
the F2P propeller will operate efficiently over a greatly expanded flight envelope, improving field, climb, and speed performance.

This program investigates, and has demonstrated, a completely self-contained passively-varying pitch propeller concept tailored to complement existing and emerging high-performance power systems for unmanned air vehicles. The propeller is a passive, bolt-on solution, requiring no additional on-board sensing or control, yet results in a wide efficient operating envelope (currently demonstrated static to 200kt).

Widening of the efficient range of propeller operation will allow high-speed and high-altitude vehicles to retain good field performance for conventional take-off and landing, and will improve performance during off-design operation such as climbing (high-power low-speed), descents (low-power high-speed), and gliding (automatic blade feathering).

What is it?

The Multipurpose Ultrasonic Transceiver for Electronic Warfare (MUTE) system is an ultrasonic positioning and communications system that has demonstrated station keeping and autonomous landing on a moving platform. Cooperative autonomous systems use piezoelectric transceivers to control and coordinate maneuvering and mission responsibilities.

How does it work?

Using synchronized clocks and an ultrasonic carrier wave, the receiving platform performs time-of-flight measurements to calculate the relative position of the transmitter. Additionally, communication signal processing extracts transmitted data. 

What will it accomplish?

MUTE is a unique non-radiofrequency-based positioning and reliable communications capability in a GPS-degraded environment.

Objective

The electromagnetic (EM) spectrum is becoming increasingly crowded as demand for bandwidth grows. GPS provides earth-centric ubiquitous low-rate position and velocity information. A platform centric relative positioning solution will enable next generation distributed UAS coordination and local precision maneuvering. Ultrasonic techniques provide accurate navigation information that fills capability gaps GPS was never designed for.

The proposed technology is a non-radiofrequency relative positioning and communications system. The Multipurpose Ultrasonic Transceiver for Electronic Warfare (MUTE) system leverages the ultrasonic spectrum to enable accurate high rate relative position and two-way communications on board, in real time, as a complimentary state estimation sensor; even in GPS-degraded environments. Research focuses on custom ultrasonic transceiver array design for accurate positioning, fast refresh rate and signal processing techniques for communications in an airborne environment.

Technology Advantages

• Accurate relative positioning in GPS degraded environment
• Distributed state estimation and system coordination
• Enables advanced precision maneuvering
• Low power requirements

How does it work?

Natural airflow over its surface provides oxygen delivery, water removal, and cooling. Hydrogen fills the internal volume at low pressure. Cells can be connected in series to create higher operating voltage.

What will it accomplish?

Compared to the stacked fuel cell, the stackless fuel cell is a simpler power system because it has no moving parts. With fewer of the non-power producing components found in a typical (stacked) fuel cell, the stackless fuel cell increases specific power 4x over traditional stacked fuel cells. The increased power enables an unmanned air vehicle to fly longer and faster.

Objective

The stackless fuel cell program aims to construct fuel cells in a novel planar array instead of the conventional stacked format. The array forms part of the skin of the wing or fuselage of an aircraft, e.g., an unmanned air vehicle (UAV). Instead of pumping air and hydrogen through restrictive channels, each membrane electrode assembly (MEA) is exposed to low pressure hydrogen on one face and flowing ambient air on the other face. The natural airflow over the wing or fuselage provides oxygen delivery, water removal, and cooling by evaporation. The bipolar plates in a stacked system are completely eliminated in favor of lightweight conductive/porous flex circuits bonded to the MEA. A number of these cells are then connected in series to create useful operating voltages at low current.

In typical stacked fuel cells, the weight of the MEA membranes (the only power generating component) constitutes less than 1% of the weight of the fuel cell system.  When the stack and support components are eliminated, the fuel cell weight could approach the weight of the membranes alone, substantially reducing the mass and volume while maintaining fuel cell performance.

Technology Advantages

• Increased specific power (1.6 kW/kg)
• Reduced number and complexity of parts
• Elimination of all power-consuming components
• Leveraging of the airflow already existing in flight
• Increased weight budget (i.e., lighter UAV with greater fuel and payload capacity)
• Increased available power (e.g., for faster flight and more sophisticated payload)            

How does it work?

Autonomous soaring uses onboard sensors and a custom guidance algorithm to locate and fly in regions of rising air in which the UAV consumes less propulsion power. NRL’s multi-junction photovoltaic technology enables lightweight, flexible, and highly efficient solar cells to be integrated into the wings to generate onboard power.

What will it accomplish?

Solar-Soaring will demonstrate the potential for atmospheric energy extraction over the ocean and will integrate NRL-developed multi-junction and flexible photovoltaic cells to create a highly efficient solar array. The aim is to enable multi-day endurance by using soaring to help close the 24-hour energy balance.

This program harnesses energy from the sun in two distinct ways:

1. Directly converting solar radiation to electricity for charging batteries and providing propulsion power.
2. Using solar-driven atmospheric convection to stay aloft without fuel expenditure.

A primary motivation for the research is to investigate solar-electric capability at a tactical altitude with small aircraft.  A key enabler is integrating flexible, high-efficiency solar cells into an airframe with wider speed range and structural margin than traditional solar-electric airplanes.  This higher structural margin gives the ability to handle and exploit localized regions of rising air, similar to soaring birds.

NRL has integrated multiple PV array technologies into interchangeable wing panels for direct comparison purposes.  These solar wings were built in-house by co-molding the array during the aerodynamic structure fabrication.  This technique could be applied to other aircraft for a highly-integrated solar solution.

NRL-developed autonomous soaring algorithms sense, identify, and guide a UAS into areas of positive vertical air motion caused by convective thermal updrafts.  In this program, NRL is investigating open-ocean environments, where bird and cloud evidence suggests sufficient updrafts may exist. 

The PV-SBXC aircraft demonstrated dawn-to-dusk endurance (10.5hr) in October 2016 at a latitude near Washington, DC using a 14% efficient monocrystalline silicon PV array and basic autonomous soaring methods.  The same aircraft with a 19% efficient single-junction GaAs array flew again dawn-to-dusk (11hr) in April 2017 without autonomous soaring mode active, confirming the combination of solar and soaring is effective in compensating for lower performing solar arrays.

Possible applications include:

• Increasing endurance of existing UAS over land.
• Communications relay missions are well-suited for soaring flight patterns.

How does it work?

This research examines overlapping principles of aerodynamics and hydrodynamics in design and performance of unmanned electric aircraft and underwater gliders.

A multi-modal design optimization process was developed to select the vehicle configuration and sizing for a specific notional mission.  The wings support flight loads in air and enable efficient underwater gliding.

Airborne propulsion is battery-electric with a folding propeller.  Underwater propulsion is accomplished via a large-volume buoyancy engine and hydrodynamic gliding.

What will it accomplish?

The Flying Sea Glider research project will develop techniques and quantify the design trade space for long-range air delivery of UUVs as well as investigate vehicle configurations optimized for both flying and underwater gliding.

Objective

The U.S. Naval Research Laboratory (NRL) is merging two distinct research areas — unmanned undersea vehicles (UUVs) and unmanned air vehicles (UAVs) — in order to improve the tactical availability of UUVs in time-critical situations. The Flying Sea Glider research program is focused on developing an underwater glider that delivers itself by flying above the water (patent pending). Flying emplacement offers several benefits over traditional deployment: unmanned aircraft typically fly two orders of magnitude faster than sea-gliding UUVs, and flying emplacement reduces the need to task a ship for glider deployment.

The NRL Flying Sea Glider builds on previous UUV development focused on novel propulsion concepts (WANDA/Flimmer programs).  Much of the current work focuses on the trade-space of a flying and sea-gliding vehicle, which operates in much the same manner yet in drastically different environments.  The resulting design is a product of that trade space analysis, yielding a vehicle with the desired mix of flying range and swimming endurance.

This hardware is undergoing glide testing to verify the underwater electronics and controller. Fabrication of a second article is nearing an end, with flight testing slated for spring 2019 and demonstration of full mission profile expected in mid 2019. NRL is seeking industry partners interested in cooperative research or commercializing the Flying Sea Glider design or concept.

Applications

• Searching for downed aircraft
• Rapid-reaction measurements of environmental disasters
• Emplacing underwater gliders upstream in high-current areas
• Environmental sampling in difficult-to-reach areas