Theyron’s Flight-Control System

Our Flight-Control has been developed to answer the need for a VTOL UAV with a range, endurance and speed performance equal to or greater than existing helicopter type UAV’s, while providing a cost and operating simplicity closer to that of the electric multirotor. In this, we have exceeded our own expectations and produced a compact, easy-to-manufacture system with the potential to outperform the helicopter UAV flight envelope in some areas.

As with the helicopter UAV, our underlying flight control system is powerplant agnostic, able to be configured for either electric or internal combustion power plant options. Our current development is in the 25kg MTOW class. We are presently utilising internal combustion powerplants with fuel systems adapted for available bioethanol fuel options.

This puts us in a position to produce an electric variant of the same airframe relatively easily if needed. The underlying technology is also scalable to larger vehicles which would benefit from higher Reynolds numbers along with weight savings due to scale.

Advantages

Useful Load Percentage

Useful Load Percentage is typically expressed as a percentage of MTOW. It represents the percentage of useful weight that an aircraft can carry relative to its overall maximum take-off weight.

For a UAV this is typically either fuel or battery weight combined with a commercial payload. Available statistics for Helicopter UAV’s indicate that all systems currently in commercial operation have useful load percentages below 50%, with some considerably below this.

The Theyron system has an inherently lighter vehicle frame and flight operating system. When combined with the potential for lower disc-loadings, the system can be configured to useful load percentages considerably above 50%.

Speed and Endurance

For both helicopter and multirotor type UAV rotorcraft, thrust for forward flight is obtained by a forward tilt of the rotor disc plane. With the rotor tilted forward, a percentage of the total rotor thrust provides a forward thrusting vector. This is considerably less efficient than a traditional fixed-wing type aircraft, where the propellor thrust is aligned with the direction of flight.

Theyron’s flight-control system has the potential to be configured with additional thrust generation for forward flight which can improve forward propulsion efficiency compared to currently available VTOL rotorcraft. When combined with our very small frontal area aspect ratio this translates to a higher speed and speed efficiency potential for a comparable power and weight rated VTOL.

Rotor Efficiency

UAV helicopter rotor blades are necessarily flexible and rely on centrifugal force to remain in a near-flat plane during flight. As a result, they generally require a design employing less efficient symmetrical type aerofoil sections so as, to maintain rotor pitch-control stability. Also, rotor-tip vortexes, require that “washout” be built into the outer sections of a helicopter’s rotor blades.

This means that a degree of blade inefficiency and sub-optimal lift distribution, are necessarily inherent in helicopter rotor design. Larger rotor diameters are therefore needed to provide adequate lift. This causes adverse drag at higher speeds. Also, the open rotor layout, combined with larger retreating-blade pitch angles, limits maximum vehicle speed due to reverse flow and retreating-blade stalling at higher speeds.

Frontal Area Aspect Ratio

Theyron’s system employs lightweight, structurally rigid rotor blades combined with lift-producing intake ducts to eliminate rotor tip vortexes. Rotor rigidity allows for the use of more efficient aerofoil profiles and cambers. Rigidity, combined with the elimination of rotor-tip vortexes, allows for the full optimisation of propellor twist and spanwise lift distribution of the rotor and duct combination.

As a result, the system can operate efficiently with a considerably smaller overall rotor-disc area than a comparable load- and power-rated helicopter-type UAV. This effectively allows for the use of a smaller rotor-disc area while still producing comparable hover-efficiency effects.

The rigid rotor and duct combination also allows for configurations that can delay or eliminate reverse flow and retreating-blade stalls at higher speeds.

Our flight control system also allows for potential configuration to a narrower, longer airframe than would be possible for speed-controlled multi-rotor systems. The overall vehicle height can be configured to as low as 1.3 times the shaft-wise length of the engine, and considerably less than this where configured, to electric power plant options.

Extrapolating the combined effects of the foregoing, a 25kg MTOW vehicle could be optimised with a frontal area of as little as 0.17 square metres, with an electric variant as low as 0.13 square metres.

The utilisation of ducted rotors also allows for the aerodynamic faring of the entire vehicle where desirable along with the potential to design for vehicle-body lift generation during translational flight.

Safety Duplication

In the event of engine failure, a helicopter can enter an “autorotation” mode by lowering the collective rotor pitch to land safely. Most high-end helicopter-type UAV’s have now been successfully configured with the ability to automatically perform an autorotation.

Autorotation can allow for an emergency landing recovery in the case of engine power loss, but not in the case of a failure of the rotor, transmission, or flight-control mechanisms. 

Some UAV helicopters with POR (Payload Over Rotor) capability have been successfully fitted with ballistic parachute systems for use in the event of rotor system failure. Some electric helicopter-type UAV have also been configured with twin electric motors to provide a duplicate power supply.

As pointed out by PAV developers, the multi-rotor system is better suited than the helicopter for complete systems redundancy. Able to be configured with completely duplicate motors, rotors, power supply and flight control systems. Effectively two or more, separate flight control and power systems, are superimposed within the same airframe.

As a result, no single point of failure is able, to cause an uncontrolled landing. This is particularly important with the increasing number of larger UAV’s and human-carrying Passenger Air Vehicles (PAV’s) being tested for commercial operation. 

The Theyron system is powerplant agnostic. Where configured for electric propulsion, the same level of duplication as a multirotor can be employed to the extent of complete flight systems redundancy. 

Where arranged for internal combustion power supply, the Theyron system is readily configurable for dual engine, dual transmission, and power distribution. Combined with a contra-rotating rotor configuration, along with duplicate flight control systems and electronics, this can provide a complete redundancy of systems comparable with that of a high-end multi-rotor.

Eliminating Helicopter Flight-Envelope Hazards

Helicopter rotor blades rely on centrifugal force to remain in a near-flat plane during flight. The inherently large inertial mass of the main rotor, combined with rotor flexibility creates flight envelope hazards during ground handling such as the potential for entering a destructive ground resonance cycle.

Rotor tip vortexes, combined with the sub-optimal spanwise lift distribution inherent to the helicopter rotor system, increase the potential for entering a ring-state-vortex condition as a flight envelope hazard in helicopter descents and other near-ground conditions. The inherent height of the helicopter rotor system also lowers the inclination at which dynamic rollover can occur.

Theyron’s Rotor Control System

Theyron’s rotor control system lends itself to the application of lightweight rotor blades contained within carbon fibre ducts. The low inertial mass of the rotors, combined with their rigidity and comparatively low rotor tip speeds, eliminate the risk of ground resonance, also enabling them to be shut down very rapidly at the point of landing.

Rigid ducted rotors, eliminate rotor tip vortexes and allow for the optimisation of aerofoils, cambers and spanwise lift distribution. This delays the potential onset of rotor-blade stall during flight. Delayed rotor stall, combined with the elimination of rotor tip vortexes, has the potential to reduce or eliminate, the ring-state-vortex condition as a flight-envelope hazard for the Theyron system.

The potential to configure to a very low airframe height, combined with a swift rotor shut-down capacity, raises the threshold for the onset of dynamic rollover. Sturdy propellor ducts and the absence of a tail rotor along with a very small vehicle area footprint, reduce risks to ground personnel. 

Vehicle Footprint Noise

Efforts have previously been made to develop VTOL UAV for cinema use with a focus on noise signature reduction. Two areas have been found to have had the greatest beneficial effect.

1 – Ducted rotors

Most of the noise produced by an open-rotor UAV comes from propellor tip “buzz” caused by vortexes at the rotor tips. This is exacerbated where the vortexes of the different rotors of a multi-rotor interfere with one another, or in the case of a helicopter, where the tail rotor vortex interferes with that of the main rotor.

A ducted rotor configuration eliminates the rotor tip vortex. A lengthened rotor duct has also been found to have the further effect of reducing lateral noise propagation. 

2 – Disc Loading

For a VTOL UAV, the same maximum take-off weight can be achieved with less power by an increase in the combined rotor disc area. The resultant lowered rotor disc loading also has a considerable effect on noise reduction, particularly, when combined with a ducted-rotor configuration.

Considerable resources have previously been applied to develop for the film industry, a large electric multirotor with a reduced noise signature. A ducted-rotor configuration with lengthened ducts, combined with a reduced disc loading ratio was developed based on the speed-control multi-rotor system.

Effective noise signature reduction was reported as having been achieved with this development. However, the lowered disc-loading, combined with the duct shrouding was said to have had an adverse effect on control and stability that the rotor speed-control system was unable to overcome. The development has so far been shelved, as having been unable to provide adequate stability for filming purposes. 

Theyron have produced a variant with a focus on noise signature reduction and have achieved good results so far. A duct length of ½ duct diameter was found to be optimal for further reducing outward noise propagation. Our rotor-control system provides better control authority at lower disc loadings and encounters no adverse effects when employing, lengthened rotor ducts.

For this variant, we remained with the present internal combustion powerplant, configuration but adopted a quieter exhaust silencer system. We also achieved promising initial results from redirecting exhaust silencer output at various angles into the downstream airflow within the ducts. 

The best results for the greatest noise reduction would ultimately be achievable with a low disc-loading electric variant of the Theyron system. A low disc loading configuration would also create the potential for longer flight times when compared to currently available electric multi-rotors adapted for film use. 

Vehicle Footprint – Area and Volume

The Theyron system employs optimised ducted rotors able to be configured to a compact and narrow planform combined with a very low vehicle height. Based on our current development, a 25-30Kg MTOW vehicle can be produced with a ready-to-fly planform area of less than 1.5 square metres along with a volume of less than 0.3 cubic metres.

These real-world figures can be directly extrapolated to larger vehicle weights to produce machines of a footprint and performance hitherto unachievable by available VTOL technology.

Conclusions

Theyron’s new and unique rotor control system, combined with its economy and adaptability, has the potential to revolutionise currently available VTOL UAV air platforms for several reasons.

ENDURANCE & PAYLOAD

SPEED & AGILITY

SAFETY & PAV POTENTIAL

SIMPLICITY OF MANUFACTURE AND OPERATION

FOOTPRINT AND SCALABILITY

Worldwide, an increasingly large number of commercial UAV operations are being performed by electric multi-rotors. Many of these find themselves restricted by the energy density limitations of the multi-rotor system, however, due to economic and technical barriers they are unable to progress to currently available long-endurance machines such as helicopter UAV’s.

Theyron’s new and unique rotor control system has the real potential to drastically reduce costs and operations difficulties while providing range, speed and endurance combinations, previously only achievable by high-end helicopter type UAV.

This creates the potential for our technology to enter and disrupt numerous established commercial UAV operations. Flight-envelope advantages along with a reduced vehicle footprint, create a further potential for completely new commercial applications based on, our system.