Tech Log: electric contra-props
types. However, using piston Twin engine safety and effective endurance of 45 to 90 minutes and gas turbine power plants the classification between charges can be tolerated. arrangement proved incredibly Simple, two-lever thrust In some types, electro-combustion complex and expensive to build, controls and instrumentation hybrid systems may be an as variable pitch propellers and Low impact on flight symmetry appropriate interim solution for gearboxes had to be used to from an ‘engine’ failure longer range. manage the engines’ torque curves No aircraft yaw during power Another issue is the propeller. and speed. Now, developments changes To realise the potential of electric in the automotive industry of Almost instant ‘throttle’ motors requires the design and advanced high-torque, slow speed response manufacture of a completely electric motors, and associated Very powerful reverse thrust new type of fixed-pitch propeller, power electronics set the stage Shortened takeoff and landing capable of providing high thrust for a revolution in light aircraft runs both forward and in reverse, propulsion systems. For the first Improved climb performance and able to absorb the much time fixed-pitch propellers and and higher top speed wider range of power available direct drives can be used, reducing Propulsion system immune to from electric motors. Whilst the the complexity and cost of a CR icing maximum power available from propeller system from a military No weight change during flight a piston engine is rarely more budget to a civil one. Smaller overall propeller disc than thirty per cent greater than diameter its continuous rating, an electric The benefits Extremely simple mechanical motor is often capable of 100- Several benefits are anticipated construction 200% over its continuous rating from an electric contra-rotating, Virtually silent motor operation, for short periods, sufficient to Should we be contrary? fixed pitch propeller propulsion with no exhaust pollution offer huge additional performance system. Thanks to its swirl-free Very high energy efficiency at takeoff and climb out. The thrust, the resulting decrease in No engine warming, shock 125kW (155hp) continuous rated For electric propulsion, one British company is certain the answer is yes airframe drag alone (the drag cooling or spool up time CRPS destined for installation caused by balancing yaw) makes Recharge using ground power, in the Super Cub float/ski plane the CR propeller set-up five to wind or solar sources will be capable of providing well By: Nick Sills* seven per cent more efficient No change in power output at over 250kW (310hp) for takeoff in propelling an aircraft than sea level or high altitude and initial climb, and reducing a single propeller. Maximum No liquid fuel or lubrication drastically the landing run on ew would disagree that top speed compared to a single- 125kW (155hp) single-motor, power can be applied or reduced system water, ice and snow. electric propulsion has propeller driven by an electric or geared system to be tested in a almost instantly with little effect Offers aerobatic aircraft unique a big part to play in the combustion engine of the Super Cub floatplane. Options up on the aircraft attitude or trim, manoeuvring capabilities Propeller development future of aviation−as it same power. to 1,000kW are being explored. considerably enhancing aircraft Negligible vibration The propellers to be used does in the car industry. Driving each propeller with a performance. Very low maintenance and were developed with Hercules Yet,F while a number of developers separate motor further improves Proven technology For the first time in civil aviation operating costs Propellers during an intensive have installed and tested electric performance and confers While contra-rotating propellers it allows a ‘twin engine’ system TBO extended to 5,000 hours eighteen month R&D motors in place of piston engines ‘twin engine’ safety. Certain are commonly used to increase to be mounted on the nose of an System can be retrofitted in programme, assisted by UK in several types of light aircraft, types of electric motor offer an the performance of leisure and aircraft. This is possible because many aircraft. Government and NATEP funding. few have realised the immense extraordinary opportunity to commercial marine craft, the the annulus drive architecture of The research generated computer benefits that the change from construct a simple, inexpensive concept has never been adopted some high power electric motors What’s the catch? algorithms capable of calculating combustion engines to electric contra-rotating propulsion system in civil aviation, at least in allows a coaxial shaft arrangement One downside is the limited the shape, diameter, pitch, blade motors could bring to the design for general aviation. the West. to be installed centrally between endurance and range using area, chord and other parameters and performance of light aircraft One of the companies looking In military aviation it has long two (or more) stacked motors existing battery energy from data inputs such as motor Below: first pass propulsion systems. at this, Contra Electric Propulsion been known that a pair of CR and to directly drive the propeller capacities. Inefficient as they performance, aircraft flight CNC machining Simply swapping a piston Limited, has designed, built and propellers provides significant shafts−no gears are required. are, hydrocarbon-fuelled engine envelope requirements and drag of the wooden engine for an electric motor in an ground-tested a complete twin performance benefits over a same- And, although no legislation yet systems have a range some profile. The software generated propellers aeroplane will not fundamentally motor contra-rotating propeller horsepower single propeller. In exists, it is thought that the coaxial six-to-eight times greater than change the performance: in a system. From the results achieved, the late 1940s, ’50s and ’60s the arrangement of CR propellers will an equivalent power and range propeller-driven aeroplane it is the the company believes that the format was used in many military not require a pilot to hold a twin electric system with the same prop that dictates performance far CR format will become the new aircraft to increase performance rating to operate the system: an total system weight. And it is more than the engine. Exchanging standard for high performance and allow vertical takeoff in some engine failure would cause almost likely to be up to ten years until the traditional single propeller sport and aerobatic aircraft. It no asymmetric flight effect−as electric range exceeds combustion for a pair of contra-rotating (CR) will also be ideal for companies opposed to a wing-mounted range, system weight-for-system ones, however, can significantly operating aircraft from water, engine failure, for which training weight, by which time battery increase the capabilities of snow or ice, and in mountainous has to be undertaken. scientists say they will be able the propulsion system and an terrain, where performance is The number of benefits gained to construct batteries with up to aircraft’s performance. This is often far more important by using a twin electric motor- three times the energy density of because the CR format counteracts than range. powered fixed pitch CR propulsion hydrocarbon fuels. ‘swirl’, giving yaw-free thrust The company plans to install system over a same power single Initially, therefore, adoption (similar to a jet engine), improves and flight-test a pre-production Developed too late to see RAF service, fixed or variable pitch propeller of electric propulsion is most acceleration, and offers a higher 225kW (300hp) twin motor Martin-Baker’s brilliant MB5 was one and piston engine system could likely in aircraft where a huge system in a Falcomposite Furio of the first piston-engine fighters to be almost be seen as too good to be improvement in performance *Technical Director, Contra Electric Propulsion Ltd kitplane, and build an entry level fitted with contra-rotating propellers true. They include: is most beneficial and short
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‘3-D’ models that could be directly shape. The outer portion of each is possible with a ‘single-stage landing run performance, it was input to a purpose-designed blade was given a symmetrical propeller’. Thus a single propeller necessary to design and build CNC machine to manufacture aerofoil allowing the propellers driven aircraft designed for high a complete electric propulsion propeller sets. A ‘loss’ process to generate both forward and speed ‘maxes-out’ at about 0.5 system and install it on a mobile of propeller manufacture was reverse thrust. Mach whilst a similar aircraft test rig. chosen over other methods used Particular attention was paid to with same power contra-rotating The unit has a coaxial propeller for composites or metals, where the manner in which two fixed- propellers maxes-out at about shaft assembly and motor cooling expensive tooling is generally pitch contra-rotating propellers 0.6 Mach, some 0.1 Mach system. Considerable care had required. Laminated beech wood work together to generate thrust higher (0.1 Mach = 75mph). to be taken in designing the blocks (the ‘original carbon fibre’ and in establishing their ideal This is the reason the Tupolev components to enable a workable as Rupert Wasey from Hercules spacing. The propellers are 95 ‘Bear’ aircraft is one of the assembly sequence, as the coaxial puts it) were used as the base designed to be efficient over a world’s fastest propeller driven shaft assembly and bearings had Thrust (blue) & torque (orange), front motor Thrust & torque, rear motor material from which the two- wide range of speed from 1,800 aeroplanes. to be designed to be installed bladed propeller pairs were (cruise) to 2,700rpm (max through the motors and take Below: test vehicle machined and finished by hand. power) in order to absorb the Proving the design axial loading in both directions with full CRPS The two propellers were motor’s power delivery curve and In order to prove the propeller during forward and reverse system installed. The technician is designed to be driven at the same offer a wide airspeed range. design and establish how they thrust tests. calibrating system speed and to absorb the same A significant benefit of contra performed in a real world The completed unit was sensors array prior power, but as a contra-rotating rotation is that as a two stage airfield environment, including mounted to a purpose-designed, to conducting pair they differ significantly in system it can raise the velocity ground operations, taxying fully instrumented test frame. static tests pitch, diameter, blade area and of air significantly higher than and simulated takeoff and The test frame was integrated into an electric vehicle with duplicated power packs, inverters, power electronics, wiring and controllers. Potenza Technology Ltd., a Coventry based high tech automotive company specialising in electric vehicle propulsion, was awarded the CRPS detailed design, engineering and commissioning contact. Two 125kW permanent magnet axial flux motors were bolted together in series and a coaxial contra rotating shaft assembly installed to transfer Combined motors - net torque close to zero power from the rear motor to the front propeller and from the front motor to the rear propeller. Test Results thrust. (The difference in forward An annular splined coupling In tests the 1,200kg vehicle and reverse thrust is due to within each motor delivers power assembly (including occupants) the bias in propeller design to directly to a spline on each shaft. was accelerated from standstill to maintain maximum forward There are no gears. 63kt IAS in 200m using 138kW propulsive efficiency.) The vehicle and propulsion (180hp) total power or 69kW The first two graphs above system were extensively (90hp) from each of the motors. show that torque/yaw (orange instrumented and real time This test was undertaken to curve), induced by the CRPS data recorded for thrust, torque compare the performance of a propellers running separately, (yaw), rpm, temperature and Furio aircraft fitted with a single increases with power and thrust power consumption during Lycoming IO-360 engine rated (blue curve). This would cause static tests and additionally at 180hp and three-bladed VP an aircraft to yaw left or right acceleration and speed during propeller with a takeoff weight depending on the direction of mobile test series on the runway. of 1,200kg to that with an electric propeller rotation. The third Video recordings were made CR system of the same power. graph shows that the torque, to show the independent control (The Furio’s rotation speed, Vr is induced by the same two of the propellers forward and under 63kt.) propellers running in contra backwards, in contra-rotation Static thrust tests from the rotation, is effectively zero at any and each propeller separately piston powered Furio showed power setting. The aircraft would and also to show the braking a 310kg forward thrust at therefore experience no yaw. effect of reverse thrust to stop maximum power. With this The test program revealed the vehicle and then back it thrust, takeoff run at 1,200kg many other important down the runway−much to the (MTOW) to Vr is above 300m. characteristics of the CRPS amazement of passing aviators. Static performance tests from design. No initial warming The static and mobile test the CRPS-equipped vehicle cycle was required prior to program was undertaken at showed 470kg forward thrust operating the power/thrust level Gloucestershire Airport. and, interestingly, 360kg reverse to maximum power or other
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settings. No cooling or idling the design. There are only two period was required to avoid rotating components (two shaft/ shock cooling. Between tests propeller/drive ring assemblies) the CPRS was simply turned other than the bearing off and on, and switched to assemblies, which are sealed forward or reverse as required. self-lubricating components. In As a true ‘twin engine’ system addition there are two motor each propeller could be operated cooling pumps. independently through the The motors and motor cooling control system and static and pumps are virtually silent at full mobile tests were completed power. Noise is limited to that using single propellers to drive from the propellers which was the test vehicle forward and in subjectively identified as lower reverse to record single engine than that emitted by a single, performance and simulate ‘same power’ propeller. engine failures. The CRPS unit runs vibration- Next steps free at all power settings. The CEP Ltd, has chosen a unit was bolted directly to the Falcomposite Furio aircraft as the test frame with no anti-vibration best airframe to install and flight Top to bottom: mechanical simplicity – the two splined collars mountings or provisions. There test the CRPS, as it is supplied (right in photo) bolt directly to the rotor inside each motor. The are no exhaust emissions and in kit form. Built without any longer of the two shafts fits inside the short outer shaft, which has an integral propeller hub, to form the coaxial contra rotating very little heat output. combustion engine components assembly (the second propeller hub is not shown); CRPS drive No maintenance or fluid and instruments, the airframe can unit, showing the front bearing support and coaxial shaft with level checks were required accommodate an 850kg payload twin propeller hubs; and a rear view of the same, showing the between tests. This is a function (including occupants), sufficient rear bearing support for the coaxial shaft assembly of the extreme simplicity of to install the complete CRPS
38 | Pilot January 2019 | pilotweb.aero Tech Log: electric contra-props Tech Log: electric contra-props
The test crew preparing for mobile runs on the runway at Gloucestershire Airport
Above: Falcomposite Furio with 180hp piston engine installed. Below: CG image of the Furio with the planned 300hp electric CRPS. The elimination of ‘swirl’, deletion of radiator intakes and improved aerodynamic profile will elevate performance significantly. The aircraft would also qualify as a ‘twin engine’ type and have enhanced access to airspace over built-up areas etc
Above: the CRPS fitted to the test frame complete with is therefore undertaking a equipment, at their cost. course no mixture, manifold cooling system and equipment, power electronics instrumentation. The and twin 200kg, 43kWhr battery development that is within its A 65kWh battery pack and pressure or prop pitch controls. assembly weighs packs, giving the aircraft up to a investment capability, that of all power electronics operating Reversing the motors in contra- 88kg one hour flight time. an ‘entry level’ single-motor, at 600V DC will be delivered rotation provides yaw-free The elimination of the normal geared, contra-rotating system. with the system. A single motor thrust and facilitates extremely Right: the control box swirling propeller slipstream, This system will provide yaw- delivers power to a simple 1:1 rapid deceleration on water, ice, and data- gathering loss of cooling air intakes and free forward and reverse thrust, ratio gearbox where the inner snow and slippery surfaces and computer used to improved aerodynamic profile a continuous power of 125kW shaft of a coaxial pair passes in emergencies. operate the CRPS will elevate performance (155hp) and, importantly a straight through the gearbox Finally, electric CR has huge system. significantly. The aircraft maximum power for takeoff and to power the forward propeller implications for the design of For static tests climb of 250kW (310hp)−far and a set of gears and shafts the next generation of military the control box would also qualify as a ‘twin interfaced remotely engine’ aeroplane and have greater than the present piston take power from the central trainers. Present day single- with the vehicle considerably enhanced access to powerplant with which the shaft and deliver it to an outer, engine turboprop military through a 25m controlled airspace. Piper Super Cub floatplane is hollow coaxial shaft to drive the trainers simulate the experience umbilical. During To undertake the Furio equipped. An Alaskan company rear propeller. of pure jet flight using complex mobile tests the project requires an investment has offered the aircraft stripped Power control will be a computer driven actuators control box was of £450k. This level of funding of all internal combusion single proportional ‘thrust and trims to counteract yaw. operated from within is not presently available in engine components plus the lever’−forward for more power, An aircraft with electric CR the vehicle by the CEP Ltd, and external finance engineering skills and facilities backwards through a gate for propulsion would simply not co-pilot is being sought. The company to install and flight test the reverse power. There are of need these systems.
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