While our industry is working towards developing a workable set of BVLOS and other safety regulations, the elephant in the room that has largely been ignored is endurance. Currently, typical heavy lift and/or high endurance rotary UAVs have flight times measured in the tens of minutes. This has to change if we are to perform many of the most valuable tasks we, as an industry, are promoting.

How did we get here?

The use of electric propulsion systems for UAVs greatly simplified their construction and control and reduced their costs, making them generally affordable. Improvements in battery technology, especially with lithium batteries, have led to the widespread adoption of hobbyist and light industrial UAVs. However, the stored power versus weight (called the “gravimetric energy density” by scientists) of even the best of today’s batteries severely limits the flight times of UAVs.

For heavy lift, or high endurance, UAVs this is an even greater issue. Parcel delivery, LIDAR mapping of large areas, inspections of pipelines and/or hydro lines, search and rescue operations and many other potentially high benefit uses of UAVs are curtailed or prohibited by the limited range and flight times imposed by the use of batteries as the UAV’s power source.

Hybrid Power

One solution to this problem is the use of a hybrid powertrain for UAVs. A hybrid powertrain is one which includes an internal combustion engine which burns a fuel to operate a generator to make the electricity to operate the propulsion motors of the UAV.

As a combustible fuel (such as gasoline, kerosene, etc.) has a much higher gravimetric energy density than even the best batteries, flight times of six, or even ten, times longer duration than with batteries are possible – making many of the desired use cases for UAVs practical for the first time.

While a hybrid powertrain seems like a simple idea (after all, it’s just a motor attached to a generator) implementing it in a lightweight, durable, failsafe and reliable manner is actually quite challenging. The system has to operate over a wide range of potential operating conditions (altitude, temperature, etc.) and should simplify the operational logistics and costs of operating the UAV platform.

What’s Required?

A hybrid drive powertrain should be completely automatic, placing no additional burden on the UAV pilot. Ideally, the hybrid powertrain can self-start, and will enable a pilot to stop the motor anytime during flight (if desired) and subsequently restart it. The powertrain should also self-tune, to adjust its operation to flight parameters such as altitude, temperature, humidity, etc., to ensure reliable performance with optimum fuel consumption. To integrate with existing flight controls, speed controllers, propulsion motors and payload packages, the electrical power from the powertrain should be “clean”, looking like power from a battery to the UAV’s systems and payload and the powertrain’s control system should include battery management services to manage the charge and maintenance of the onboard battery (provided for self-starting capabilities and redundant power for safety). Lastly, for reliability in operation and for maintenance, the powertrain should provide comprehensive diagnostic information for the operator to monitor the status and operating conditions of the UAV.

Range Extenders

Examples of available hybrid powertrains include the Pegasus Aeronautics GE 35 Range Extender, which can continuously output up to 2000 Watts of electrical power, and the Pegasus Aeronautics GE 70 Range Extender, which can continuously output up to 4000 watts of electrical power.

The Pegasus Range Extenders have been engineered to the highest aeronautical standards for reliability and for efficiency. An integrated , sophisticated computer system controls all operations of the Range Extender, including the operation of the motor, the generator and all ancillary systems, making the upgrading of an existing UAV easier and allowing for the design of purpose built UAVs with amazing capabilities. Generally, a Range Extender with its fuel supply weighs less than a battery pack which would otherwise provide only 1/6 of the flight time that is possible with the Range Extender . In other words, the hybrid powertrain yields up to six times the flight time, with the same, or less, gross takeoff weight.

How Do We Get There?

With a hybrid powertrain system such as that described above, you do not need to get rid of your existing UAV platform. In many cases, a hybrid powertrain can be retrofitted to existing UAV platforms to increase their flight times.

Some of the design considerations for integrating a hybrid powertrain include the amount of fuel to be carried (a trade-off between maximum flight time, useful payload and maximum takeoff weight); the size of the onboard battery; and the location of subsystems such as the fuel tank, cooling radiators, etc.

A purpose built platform, designed to maximize the potential of the hybrid propulsion system, can provide even more impressive results. Rotary UAVs with 10kg payloads, and three hour flight times have been obtained by Pegasus customers.

Go Where No UAV Has Gone Before

In addition to extended flight times, a properly design hybrid powertrain can allow for operations under conditions where batteries cannot be used, such as in extreme cold. A Pegasus powered UAV is scheduled to perform mapping operations in Antartica in the near future, where battery powered UAVs cannot operate due to the cold.

The harsh landscape in Antarctica presents tremendous challenges to battery powered vehicles. Where batteries struggle, however, is where the advantage of hybrid power becomes evident.

Logisitcs

With a hybrid powertrain, operational logistics are simplified in the field. For example, by emptying the fuel tank, a UAV equipped with hybrid powertrain can be shipped via air freight (unlike lithium batteries).

In the field, providing for the “recharging” of a hybrid powered UAV just means having a gas can at hand, rather than having multiple sets of expensive batteries and/or some means to recharge those batteries. In most use cases, a hybrid powertrain saves significant amounts of money.

Hours (and even days) worth of UAV work can be achieved with a single fuel container of gasoline. Recharging of batteries in areas with no access to infrastructure are an issue of the past using hybrid technology.

The Future Is Clear

Heavy Lift and/or High Endurance UAVs need increased power and payload capabilities. The battery technology available now, and even that projected for the future, cannot match the weight/performance of a properly designed hybrid powertrain.

Contact Pegasus at info@pegasusaero.ca to learn more.

THE FLIGHTPATH AHEAD