Microturbine Propulsion for UAVs

August 1, 2019

Seventy years ago, military aviation moved from reciprocating engines to vastly more reliable turbo jets and turboprops. Shortly after, the commercial air transport industry followed suit, enabling modern air transport. Today, virtually all large aircraft rely on turbine propulsion, yet small aircraft, both manned and unmanned, have not exploited the advantages of turbines for propulsion.

While UAVs have become integral for both commercial and military aerial missions, the continued use of reciprocating engines has limited the UAV market’s ability to reach its true growth potential. The industry has been slow to innovate and develop turbine propulsion systems for small aircraft because it is far more difficult to design and produce high performance small turbines than large ones.

Today’s engine market is riddled with inefficiencies and hazards that jeopardize the safety and reliability of UAVs. Current engines powering drone delivery are unreliable and run on highly volatile and dangerous fuel, minimizing the impact of a drone fleet and raising costs. A major percentage of vehicle losses with their payloads are attributed to engine failure. Plus, with the need for frequent overhauls, customers have to purchase multiple engines for a single vehicle so that, when one engine is being worked on, they can continue operating with an alternate engine. This severely limits the potential for the entire industry.

Designing a Solution

After spending five years working to design and manufacture a world-class gas turbine engine small enough to fit into Group 3 (max gross take-off weight less than 1320 lbs) and Group 4 (MGTOW of more than 1320 lbs) the team at UAV Turbines has achieved this engineering feat with the recent launch of its Monarch propulsion system. The Monarch turboprop was carefully designed to outperform conventional reciprocating engines in several ways:

Monarch propulsion systems will provide more time in the air and less time being serviced on the ground with upwards of a 2,000 hour increase in operation time between overhauls when compared to available Class 3 engines.
The Monarch system’s variable pitch propeller will enable UAVs to climb faster and reach greater dash speeds, enabling greater performance and efficiency in both commercial and military aircraft.
The reliability of Monarch systems eliminates the need for extra engines for a single aircraft.
The flexibility to run efficiently on all types of heavy fuels, such as jet fuel, makes Monarch propulsion safer and more convenient than engines running on volatile aviation gasoline.
Monarch RP generates useful onboard electrical power that is 2-3× greater than what is produced by conventional engines.

In designing this system, the team at UAV Turbines had to overcome two key issues facing the UAV engine industry: designing a small, powerful engine that was both reliable and ran on safer fuel. There are several fundamental problems that had to be overcome, starting with the fact that the size and weight of these systems are major constraints. A little over thirteen hundred pounds really isn’t much.

How It Works

Artist’s illustration of a Monarch RP microturbine engine.

To achieve significant thrust, the heavy fuel (typically JP-8 or Jet A) burns in a very small space at similar temperatures as the larger engines. The internal distances, however, are much smaller, so managing thermal gradients and the resulting stress becomes more difficult. The low air flows in these engines call for the use of very small air passages and very high-speed turbomachinery.

The core rpm in the first system UAV Turbines is releasing for flight test is right around 100,000 rpm at cruise speed. UAV Turbines built even smaller experimental engines with rotor speeds of 200,000 rpm. Managing tolerances and designing to process capabilities becomes critical. New engineering approaches were essential to deal with the extreme internal thermal stresses and tight tolerances.

One solution to the thermal issue was to separate the hot section from the engine’s bearing cavity by placing the turbine rotor at the end of the shaft, cantilevered, so that the bearings are located in a cold section of the engine core. Overhung systems have been used before but there are critical fits between the components that have to provide intimate contact from zero cold to full speed hot - and all transients in between. The designers ended up balancing on the limits of what can be made with what will operate.

Impeller and turbine for a 197,000 RPM turbogenerator, one of several 10 HP electrical generator prototypes built and operated in the UAVT development program.

Another factor is that this is not just an engine, but a propulsion system. To put that power to work, it’s necessary to step down from 100k core rpm to 6k rpm via an extremely efficient, lightweight gearbox. This drives a variable pitch propeller so that the power can be efficiently converted to thrust through each mission stage, from take-off to landing. The engine itself runs at essentially constant speed, so pilot throttle changes result in pitch changes to the propeller blades via a variable pitch mechanism as calculated by the engine FADEC (Full Authority Digital Engine Control). The propeller itself was designed with small pusher UAV installations in mind. The propeller was designed with three blades to reduce noise from the propeller interacting with the wing wakes, increase ground clearance, and reduce the chance of ground strikes during takeoff or landing.

The entire system must then be integrated into a flyable package, a classic systems challenge.

Today’s military and commercial organizations are urgently in need of a more reliable and safer propulsion system for their UAVs. Reliable, lightweight, fuel efficient microturbine engines may be the answer to provide propulsion and power generation in small to medium-sized UAV propulsion systems.

This article was written by Kirk Warshaw, CEO, UAV Turbines (Miami, FL). For more information, visit here .