Innovating UAS project R^2

If in a civilian/commercial UAS application the aircraft is the most visible part, it is not always well understood how critical its design can be for the intended missions of the system. The R^2 based on the rhomboid wing concept started as the answer for a very specific requirement. After the first prototypes flew successfully it became obvious that this type of aircraft was opening some amazing capabilities for commercial and civilian applications as an aerial platform. It was decided to push the envelope to its limits and try to combine the exceptional aerodynamic and structural performances with the best available propulsion to offer a small but very capable and versatile aircraft for maritime surveillance.

The rhomboid wing, beside its inherent low drag characteristics, offers the same performances as a classical cantilever wing aircraft of twice its wing span, for a similar total wing area. Furthermore its speed range and low speed / high angle of attack performances are comparable with a delta wing of similar wing span. The overall drag gain is further improved by the total absence of horizontal and vertical stabilizing surfaces.

However, this is the theory and to get to the final design (aerodynamic loft) it was a rather lengthy and tedious process involving months of computation, analysis with CFD software and, later, 3D modeling with the structural design. The reward was that success was achieved from the first flight of the first prototype.

During the following flight test campaigns close to a dozen different aircrafts were experimented with different type of propulsion. At first an electric motor / pusher propeller configuration was used to open the flight envelope and validate the aerodynamic stability. Then an electric duct fan was used to confirm that the pusher configuration was not the main factor in yaw stability. A micro turbine version was ultimately tested to achieve the higher flying speeds.

Structurally the “bracing” of the wings in the 3 dimensions allows for up to 30% mass saving. The compactness of the structure gives the aircraft excellent maneuverability while enhancing its ruggedness.

The only concession for complexity comes with the flight control surfaces as they are needed on the forward and rearward wings to fully utilize the exceptional speed range and maneuverability of the rhomboid configuration. This requires the use of an autopilot capable of handling up to 8 independent control surfaces with multiple mixings.

Aero Composites Innovations proprietary manufacturing processes and technics have led to not only a very efficient industrialization, but a very light airframe that is only 1/5th of the complete aircraft MTOW. This means that, at 1.5 meter wing span, the smallest version of the R^2 is capable of carrying 5 kilos of fuel with a payload capacity, including the communication system, of 3.5 kilos.

But, as always, even the exceptional performances and capabilities of this small aircraft are not enough for the always demanding customers and Aero Composites Innovations got numerous requests to offer a larger aircraft based on the same design. It was rapidly discovered that the rhomboid wing, contrary to a conventional cantilever design, doesn’t allow for any direct homothetic design, requiring again a lot of tedious and time consuming designs and CFE simulations.

Because of this unique line of products, Aero Composites Innovations got the “Young Innovative Company” label from the French authorities and the ONERA (the French National Aerodynamic Lab) is an active partner for the validations of the bigger designs following the earlier validation process of its smaller sibling. This exciting partnership is giving a new dimension to the innovative design capabilities and professionalism of Aero Composites Innovations.

For any aircraft to perform up to its full potential, regardless of the airframe capabilities, the overall performances for range, endurance and speed are largely depending of the propulsion unit. To achieve maximum endurance the choice today is a petrol engine with a customized propeller. The pusher configuration is also a critical design driver with the added constraint of optional launch and recovery systems.

Engine

At the early design stages of the production aircraft it became obvious that a rotary engine would offer the best compromise between mass, volume and good RPM range. AEI responded very positively to the challenge and came with a very unique design proposal to answer the stringent requirement.

An engine was specifically designed for the R^2 airframe while keeping within the mass and space constraints.

The R^2 project presented AIE with the very rare and exciting opportunity to design and develop a completely new propulsion unit in parallel with the development of the platform itself.

The potential benefits of running vehicle and engine development in parallel with the constant interaction between both project teams are enormous.

As opposed to the vehicle developer having to “bolt on” a standard engine to a new vehicle design with all the compromises which that entails (to both vehicle and propulsion unit), by developing in parallel, both components can be designed to fit together perfectly with no compromises on engine mounting, ancillary positioning, mass location and aerodynamics. This ultimately maximizes the performance of the vehicle as a whole.

After the initial discussions between Aero Composites and AIE it was quickly realized that in order to meet the R^2 vehicle goals, a radically new rotary-engine design was required, one that was highly compact, lightweight and yet still able to deliver sustained high-power levels and return the TBOs required

While this requirement slotted in perfectly with AIE’s own core development goals, and on the surface was a great fit for its revolutionary new SPARCS* (Self Pressuring Air-Cooled Rotor Cooling System) Cooling technology, the extremely small packaging requirements of the R^2 meant that AIE could not utilize its already developed “Compact SPARCS*” cooling system with its internally integrated heat exchanger.

With this in mind, and after further analysis and concept-design development, AIE determined that the core SPARCS* technology could be adapted to utilize an external, gas heat exchanger, allowing the core engine to remain extremely compact in size and still deliver the required performance and service life.

From these concept designs the AIE 40S engine was developed with the following result: a single rotor 40cc Wankel type engine with the potential to deliver 5hp at 9600rpm in a 125mm diameter x 130mm length form factor and with a core weight of less than 2kg.

The full R^2 40S engine package also takes advantage of state-of-the-art engine management systems which, in addition to controlling both the fuel injection and ignition systems, also allows full variable control over the engine’s oil delivery and liquid cooling systems.

This full control over the engine’s ancillary systems ensures the engine will operate at peak performance even in the most demanding of conditions, while allowing fine-tuning of the engine’s parasitic electric power requirements and, more importantly, oil consumption.

To complete the propulsion package, the 40S is equipped with a bespoke starter-generator unit to allow remote, safe engine-starting in addition to delivering the electrical power required by the vehicle’s avionics and payload during its mission. This remote-starting capability is a requirement of the R^2 vehicles’ launch concept, but also produces a much more elegant on-vehicle solution than those of traditional starter systems.

*SPARCS (Self-Pressurised-Air Rotor Cooling System). The patented SPARCS technology is a sealed, self-pressurising rotor cooling system that utilises blow-by gases from the combustion process, which are continually recirculated through the engine’s rotor and subsequently passed through an external intercooler. This process enables rejection of heat directly from the engine core, independent of the engine’s liquid cooling system.

Propeller

For the R^2 UAV, a specific propeller was designed by Hélices E-Props (French carbon propeller manufacturer) to maximize the overall performances of the engine at the various required UAV air speeds.

The propeller designed for the R^2 UAV belongs to the 3rd generation of propellers. Due to mechanical performances of the carbon fiber, new aerodynamic designs have become possible: high CL profiles, narrow chords, big diameters, special positions of the blades…

Those type of propellers present a strong ESR effect (Extended Speed Range effect). The term ESR effect is used to define a fixed pitch propeller (or ground adjustable pitch one) which behavior is near from the behavior of a variable pitch propeller.

This ESR effect has the following characteristics: it causes very small gap between the static RPM and the flight RPM, this allows to keep a strong power at take-off. It seems that the max throttle RPM stays nearly constant.

The ESR effect can improve the performances at take-off, because then the engine is running very near of its max RPM, and provides all its power during take-off.

Thanks to their geometries and their profiles, the E-Props propellers as the one developed for the innovative UAV R^2 have a very strong ESR effect, which allows to improve the performances of the aircraft at take-off and in cruise, in comparison with a standard fixed pitch propeller.

Due to the assisted launch and recovery options, the maximum propeller diameter was a design driver as well as the necessity of having a folding propeller.

Seven innovations can be found on the propeller designed for the R^2 UAV, compared with a standard propeller. The obtained performances are exceptional.

The combination of these state of the art airframe, engine and propeller results in a unique and optimal airborne platform for UAV applications.