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The employment of UAVs by various industries and in Defense and Security missions are enabled by their propulsion system to provide them with the necessary power to propel the aircraft for forward flight or hover. Propulsion systems can advance the flight time or endurance of a UAV which is influenced by the propulsion technology used and is dependent on the aerodynamic design and amount of fuel carried. To fulfil the energy requirements of a large variety of UAVs, several variants of piston-engines and electric motors have been designed by the market players.
The type of propulsion depends on the type of UAV and desired performance. UAS are categorized in a variety of ways based on vehicle attributes including the type of aircraft (fixed wing or rotorcraft), flight altitude (high, medium, low), weight, speed, etc. In general, larger aircraft use larger engines that confer higher altitude, longer endurance and more payload capacity than smaller vehicles. The main propulsion system types are electric systems and gas systems. Both systems can be used to drive propellers or ducted fans.
The major types of UAV engines include: turbo-fan engines, turbo-prop engines, piston engines, wankel engines, electrically propelled engines, solar power propelled propulsion systems, and hybrid engines. All types of aircraft (including UAVs), engines and fuel typically account for 40 percent to 60 percent of gross takeoff weight, and the performance of the propulsion system has an enormous effect on air vehicle performance.
Electric propulsion
In addition to thrust, propulsion systems for modern aircraft must provide high fuel economy, low weight, small size (to limit drag), and extremely high reliability. The primary engine performance metrics are minimum total fuel burn (while meeting aircraft performance requirements) and reliability levels commensurate with permissible aircraft loss rate (1 per 108 departures for commercial aircraft).
The need to optimize aircraft performance, decrease operating and maintenance costs, and reduce gas emissions is pushing aircraft industry to explore new concepts including more electric aircraft (MEA), and ultimately an All Electric Aircraft. Electric propulsion, or electric aircraft, describes the range of fixed-wing aircraft and rotorcraft which at least in part rely upon electricity to power their propulsion.
In the context of climate change, electric aircraft may have an advantage commercially going forward as emissions standards and carbon taxes increase the cost of operating a hydrocarbon fleet. Electric propulsion can be powered by rechargeable batteries, fuel cells, or solar energy. Electric plane power is much simpler — batteries power an electric motor that spins a propeller.
However, the specific energy of electric energy storage sources, e.g. batteries, is much lower than that of fossil fuel. As a result, the internal combustion engine (ICE) is preferred for relatively large or long-endurance UAVs, due to its high power and energy density. Efficient energy utilisation on an UAV is essential to its functioning, often to achieve the operational goals of range, endurance and other specific mission requirements. Due to the limitations of the space available and the mass budget on the UAV, it is often a delicate balance between the onboard energy available (i.e. fuel) and achieving the operational goals.
Hybrid electric propulsion system (HEPS)
Industries are now exploring Hybrid-electric propulsion systems that employ two or more distinct types of power, for example power from an internal combustion engine (in the case of an aircraft, this would be using kerosene) and an electric motor. The hybrid-electric propulsion system (HEPS) combines an electric powertrain with a conventional combustion engine to provide propulsion, in other words, being able to have the energy efficiency of an electric propulsion system with the extended range of an ICE.
By taking advantage of both electric motor and internal combustion engine, hybrid-electric propulsion systems provide not only a benefit in fuel saving but also a reduction in takeoff noise and the emission levels. Until electric energy storage systems are ready to allow fully electric aircraft, the combination of combustion engine and electric motor as a hybrid-electric propulsion system seems to be a promising intermediate solution.
The HEPS can also enhance military missions. Hybrid propulsion system has the capability to operate in a mode with the internal combustion engine off, reducing the operating noise of the aircraft until the energy storage system needs recharged using the internal combustion engine. This period of reduced operating noise will improve the survivability of small aircraft in a battle zone by making these aircraft less detectable. Since these aircraft operate at low altitudes, they are subjected to small arms fire, and do not have the weight capacity to carry armor to survive attacks, so reducing the detectability is the only option to improve their survivability.
Hybrid propulsion technology
HEPS can provide better fuel economy and lower emissions without compromising performance. In addition, it can provide on-board electrical regeneration for powering different systems. There are various hybrid powertrain configurations currently in use. The most commonly used configurations are series, parallel, and series-parallel architecture. Among them, the parallel configuration permits a smaller engine or motor to be used as compared to the series one. The parallel architecture for this configuration is also lighter. In addition, to having the ability to select in which mode to operate e.g. electric, combined mode or internal combustion engine mode only. According to Researchers, the parallel configuration is best suited for long-endurance UAVs.
An ECU is used for engine control, and the engine is started using the integrated motor and a dedicated battery. Once the engine started, the integrated motor can also act as a generator to power the aircraft systems (avionics, flight controller, actuators etc.) and the payload via a DC/DC converter. On the other side, the output shaft is connected to the motor/generator through a decoupling device and a reduction gear. An intelligent battery monitoring system monitors the charging/discharging of the battery.
The generator charges the battery if the state-of-charge (SOC) is low. This setup enables the engine to operate in the highest efficiency region with the motor/generator providing the additional power required and also the ability to provide instant power to cover rapid transient demands in power. The energy management strategy can optimize the amount of motor/generator power and the time depending on the operating conditions, the SOC, and the flight mission. Moreover, this architecture also permits the motor, in the event of an engine failure or ‘stealth mode’, to act as the primary powerplant and drive the propeller.
Hybrid propulsion demonstrations
Skyfront multirotor drone sets a new world record for drone flight time.
California-based drone manufacturer Skyfront announced in March 2021 that its lightweight, powerful, and efficient Perimeter 8 gasoline-electric hybrid drone achieved the record-breaking world flight. The long-endurance drone stayed airborne for 13 hours and 4 minutes and traveled a distance of 205 miles (330 km).
The drone is powered by Skyfront’s proprietary fuel-injected hybrid gasoline-electric powertrain. The powertrain replaces the battery and dramatically extends flight times by a factor of twenty. Without a payload, it can fly more than five hours at a time and carry up to 11 lb. (5 kg) payload for 2 hours. The Perimeter 8 has a range of 110 miles (177 km) and can be set up in only five minutes. The secret behind its long flight times is a proprietary fuel-injected G2K hybrid-electric power source that allows the drone to convert gasoline into electricity in flight.
This unprecedented flight signals a new era in aviation: Skyfront’s drones can now fly longer than manned helicopters, replacing them for many dull, dirty, and dangerous missions, from search and rescue to police overwatch. The official Guinness world record for a multicopter is held by South Korea’s Metavista Inc, whose hydrogen-fuel-cell-powered quadcopter flew for 12 hours, 7 minutes, and 5 seconds.
Energy-Harvesting Hybrid Tiger Soars for Long Distance, Long Endurance: 24–48 Hours, reported in April 2021
A long-endurance solar/soaring/fueled UAV, designed and built to demonstrate synergistic range and multi-day endurance benefits through an integrated power management system and optimal autonomous path planning, gave a public demo at the Naval Research Laboratory’s (NRL’s) Aberdeen Proving Grounds, Maryland. Hybrid Tiger is a Group 2 UAV (typically 21-55 pounds, operate below 3,500 feet AGL at speeds less than 250 knots), 16 kg (34 pounds), 5.7 m (18.7 feet) wingspan autonomous air vehicle that is launched from a tether. It employs two energy-harvesting techniques:
• auto-soaring, autonomously finding and exploiting thermal updrafts to harvest energy from them by rising;
• integrated solar, with unique co-cured solar panel integration process employing high-efficiency photovoltaics, enabling nearly “free” flight during daylight.
The two techniques are backed by an onboard high-pressure hydrogen fuel tank and fuel cell system that provide nighttime power in an integrated power management system. Such overnight endurance has previously been very difficult for tactical vehicles to sustain.
This was Hybrid Tiger’s first 24-hour flight. Its NRL forerunner, Ion Tiger, had unofficial flight endurance records of 26 hours (with a gas hydrogen fuel source) and 48 hours (with a liquid hydrogen fuel source).
“The flight was effectively a performance test in worst-case conditions: temperatures falling below zero degrees Celsius, winds gusting to 20 knots, and relatively little solar energy as we approached the solar solstice Dec. 21,” Stroman said. “Despite all of that, Hybrid Tiger performed well.” The Aberdeen flight demo was conducted on Nov. 18, 2020, but only recently announced by the NRL. NRL researchers have developed energy-aware power management algorithms, which vary operational modes and generate a vehicle navigation strategy based on weather forecasts and locally observed opportunities for energy harvesting
Undefined claims 4.5-min flight for its “silent” ion-propulsion drone in Sep 2022
Florida-based startup Undefined Technologies has announced that its ion-powered Silent Ventus drone has passed outdoor testing. So it’s closer to commercial release in 2024, reports New Atlas.
Silent Ventus drone does not use propellers for flight. Instead, its entire broad structure contains two stacked arrays of electrodes designed to generate high-voltage electric fields that can ionize oxygen and nitrogen molecules in the air. By releasing electrons to give them a positive charge, the drone can propel them downward to create an “ion wind,” which in turn creates thrust.
Undefined Technologies uses Air Tantrum ion thrust technology, which the startup claims produces 150% more thrust than other current ion thruster technologies.
The drone manufacturer also claims to have achieved a noise level below 75 decibels. Earlier in 2022, the company released footage of 2.5 minutes of indoor flight testing, saying the drone produced 85 decibels of noise. Now, Undefined Technologies claims that the prototype drone flew for 4.5 minutes, although the company has only released one minute and 16 seconds of video. “This 4+ min flight required advances in the chemical composition of the batteries that can now provide us with higher energy densities,” says Undefined’s Lead Aerospace Engineer Thomas Benda Jr. in a press release. “This improvement is part of our efforts to target lighter weights.”
It says it achieved a noise level below 75 dB, and it’s calling for further investment toward a cargo delivery drone product it says will fly for 15 minutes and make less than 70 dB by the end of 2023.Undefined says its “silent” 70-dB drones will cause far fewer noise complaints in urban cargo delivery services than propeller-forced designs.
Hybrid propulsion Military demonstrations
Orbital UAV, an Australian designer and manufacturer of propulsion systems for unmanned aerial vehicles (UAVs), has signed a deal with Northrop Grumman to design and develop a hybrid system for a vertical take-off and landing (VTOL) UAV, the company announced in April 2020. The system will combine an electric motor, which has yet to be selected, with the company’s flight-proven heavy-fuel engine. A hybrid system will allow greater payload to be carried. Under the contract Orbital UAV will develop, supply, and support two initial hybrid propulsion systems for integration into what the company described as Northrop Grumman’s small UAV development platform. Work will be performed at Orbital’s facility in Perth, Western Australia.
The Air Force is investing in LiquidPiston’s X-Engine technology to create a hybrid-electric propulsion system to power emerging technologies like unmanned aircraft systems (UAS) and orbs, the company announced on March 2021. The Small Business Technology Transfer (STTR) contract worth $150,000 was awarded through AFWERX to support Agility Prime, a program developing electric vertical take-off and landing (eVTOL) aircraft for commercial and military use.
UAS and eVTOLs are being developed with battery-powered propulsion systems which have limited their range of flight. The X-Engine technology would use fuel to power a generator and charge the aircraft’s batteries extending its flight time and range, according to the company. “Today’s solutions for power and energy are held back by a lack of technological innovation; gasoline engines are inefficient, diesel engines are big and heavy, and while the world wants to go electric, batteries lack significantly compared to the energy density of fuel,” Alec Shkolnik, CEO and co-founder of LiquidPiston, said in a statement. “The X-Engine solves these challenges, and with this contract, we look forward to showcasing the value a hybrid-electric configuration can bring to unmanned flight.”
The X-Engine runs on JP-8, diesel, and other heavy fuels but is 30 percent more fuel-efficient than a diesel engine, according to LiquidPiston. It is also five to 10 times smaller and lighter than a diesel engine and is two to four times more fuel-efficient than a small turbine. The Army also awarded LiquidPiston a Small Business Innovation Research (SBIR) contract in December 2020 to develop the X-Engine platform for small tactical generators.
Hybrid Electric UAV from Advanced Aircraft Systems: HAMR UAVs Selected by AFWERX, reported in Fen 2022
The United States Air Force (USAF) AFWERX program has selected hybrid-electric unmanned aircraft systems developer Advanced Aircraft Company (AAC) for the development of Small Unmanned Aerial Systems (SUAS) as part of the Small Business Innovation Research (SBIR) program Open Topics 21.2/B Cohort.
In collaboration with the Air Force Research Lab (AFRL) and the National Security Innovation Network (NSIN), AFWERX developed the SBIR Open Topics to improve upon the effectiveness and transition rate of the SBIR program. Using a competitive awards-based program, the SBIR program allows small businesses to explore their technological potential and offers an incentive to profit from its commercialization.
AAC’s flexible HAMR UAS uses a multi-rotor configuration with a hybrid fuel-electric propulsion system for extended endurance and multiple, simultaneous payload capabilities, and is able to be optimized for a wide range of defense applications and mission profiles. The HAMR also works as a force multiplier, granting a major increase in ISR capabilities corresponding to DoD’s incumbent tactical ISR UAS.
Elbit Systems
Hybrid Propulsion Small Tactical UAS reported in Feb 2022
The Skylark 3 Hybrid is equipped with a hybrid propulsion system, both an electric and an internal combustion engine, tripling endurance and offering up to 18 hours of operations, with no change to size or weight, significantly increasing mission effectiveness and cost efficiency.
The Skylark 3 Hybrid uses its combustion engine to fly rapidly to the Area Of Interest (AOI) and switches to the electrical engine while operating above the AOI. The twin engine architecture of the new Skylark enables one to back up the other, providing greater reliability and safety. The significantly higher endurance of the Skylark 3 Hybrid provides forces with greater capacity to hover above AOI and requires fewer platforms to execute the same mission.
Skylark 3 Hybrid is based on the Skylark family of STUAS that have been ordered by 27 countries, to date. Skylark 3 Hybrid has a 4.7m wingspan, a maximum takeoff weight of 50kg, service ceiling of 12,000ft and a range of 120km. It features dual payload capacity with a “plug and play” interface for a quick replacement of sensors in the field. The Skylark 3 Hybrid is capable of integrating a range of payloads including high-resolution Electro-Optical gimbaled payload, ELINT, COMINT, laser designators and others. It is deployed and operated by a crew of two, launched via a pneumatic launcher and can be mounted on a vehicle or vessel. Two Skylark 3 Hybrid STUAS can be assigned to the same mission simultaneously managed by a shared Ground Control Station.
