Tech log: con­tra-ro­tat­ing pro­peller elec­tric mo­tors

With the push to de­velop more ef­fi­cient elec­tric propul­sion for GA, could con­tra-ro­tat­ing pro­pel­lers be the an­swer?

Pilot - - CONTENTS - By: Nick Sills*

Few would dis­agree that elec­tric propul­sion has a big part to play in the fu­ture of avi­a­tion−as it does in the car in­dus­try. Yet, while a num­ber of devel­op­ers have in­stalled and tested elec­tric mo­tors in place of pis­ton en­gines in sev­eral types of light air­craft, few have re­alised the im­mense ben­e­fits that the change from com­bus­tion en­gines to elec­tric mo­tors could bring to the de­sign and per­for­mance of light air­craft propul­sion sys­tems.

Sim­ply swap­ping a pis­ton en­gine for an elec­tric mo­tor in an aero­plane will not fun­da­men­tally change the per­for­mance: in a pro­peller-driven aero­plane it is the prop that dic­tates per­for­mance far more than the en­gine. Ex­chang­ing the tra­di­tional sin­gle pro­peller for a pair of con­tra-ro­tat­ing (CR) ones, how­ever, can sig­nif­i­cantly in­crease the ca­pa­bil­i­ties of the propul­sion sys­tem and an air­craft’s per­for­mance. This is be­cause the CR for­mat coun­ter­acts ‘swirl’, giv­ing yaw-free thrust (sim­i­lar to a jet en­gine), im­proves ac­cel­er­a­tion, and of­fers a higher top speed com­pared to a sin­glepro­peller driven by an elec­tric or com­bus­tion en­gine of the same power.

Driv­ing each pro­peller with a sep­a­rate mo­tor fur­ther im­proves per­for­mance and con­fers ‘twin en­gine’ safety. Cer­tain types of elec­tric mo­tor of­fer an ex­tra­or­di­nary op­por­tu­nity to con­struct a sim­ple, in­ex­pen­sive con­tra-ro­tat­ing propul­sion sys­tem for gen­eral avi­a­tion.

One of the com­pa­nies look­ing at this, Con­tra Elec­tric Propul­sion Lim­ited, has de­signed, built and ground-tested a com­plete twin mo­tor con­tra-ro­tat­ing pro­peller sys­tem. From the re­sults achieved, the com­pany be­lieves that the CR for­mat will be­come the new stan­dard for high per­for­mance sport and aer­o­batic air­craft. It will also be ideal for com­pa­nies op­er­at­ing air­craft from wa­ter, snow or ice, and in moun­tain­ous ter­rain, where per­for­mance is of­ten far more im­por­tant than range.

The com­pany plans to in­stall and flight-test a pre-pro­duc­tion 225kw (300hp) twin mo­tor sys­tem in a Fal­com­pos­ite Fu­rio kit­plane, and build an en­try level 125kw (155hp) sin­gle-mo­tor, geared sys­tem to be tested in a Su­per Cub float­plane. Op­tions up to 1,000kw are be­ing ex­plored.

Proven tech­nol­ogy

While con­tra-ro­tat­ing pro­pel­lers are com­monly used to in­crease the per­for­mance of leisure and com­mer­cial ma­rine craft, the con­cept has never been adopted in civil avi­a­tion, at least in the West.

In mil­i­tary avi­a­tion it has long been known that a pair of CR pro­pel­lers pro­vides sig­nif­i­cant per­for­mance ben­e­fits over a same­horse­power sin­gle pro­peller. In the late 1940s, ’50s and ’60s the for­mat was used in many mil­i­tary air­craft to in­crease per­for­mance and al­low ver­ti­cal take­off in some

types. How­ever, us­ing pis­ton and gas tur­bine power plants the ar­range­ment proved in­cred­i­bly com­plex and ex­pen­sive to build, as vari­able pitch pro­pel­lers and gear­boxes had to be used to man­age the en­gines’ torque curves and speed. Now, de­vel­op­ments in the automotive in­dus­try of ad­vanced high-torque, slow speed elec­tric mo­tors, and as­so­ci­ated power elec­tron­ics set the stage for a rev­o­lu­tion in light air­craft propul­sion sys­tems. For the first time fixed-pitch pro­pel­lers and di­rect drives can be used, re­duc­ing the com­plex­ity and cost of a CR pro­peller sys­tem from a mil­i­tary bud­get to a civil one.

The ben­e­fits

Sev­eral ben­e­fits are an­tic­i­pated from an elec­tric con­tra-ro­tat­ing, fixed pitch pro­peller propul­sion sys­tem. Thanks to its swirl-free thrust, the re­sult­ing de­crease in air­frame drag alone (the drag caused by bal­anc­ing yaw) makes the CR pro­peller set-up five to seven per cent more ef­fi­cient in pro­pelling an air­craft than a sin­gle pro­peller. Max­i­mum power can be ap­plied or re­duced al­most in­stantly with lit­tle ef­fect on the air­craft at­ti­tude or trim, con­sid­er­ably en­hanc­ing air­craft per­for­mance.

For the first time in civil avi­a­tion it al­lows a ‘twin en­gine’ sys­tem to be mounted on the nose of an air­craft. This is pos­si­ble be­cause the an­nu­lus drive ar­chi­tec­ture of some high power elec­tric mo­tors al­lows a coax­ial shaft ar­range­ment to be in­stalled cen­trally be­tween two (or more) stacked mo­tors and to di­rectly drive the pro­peller shafts−no gears are re­quired. And, al­though no leg­is­la­tion yet ex­ists, it is thought that the coax­ial ar­range­ment of CR pro­pel­lers will not re­quire a pi­lot to hold a twin rat­ing to op­er­ate the sys­tem: an en­gine fail­ure would cause al­most no asym­met­ric flight ef­fect−as op­posed to a wing-mounted en­gine fail­ure, for which train­ing has to be un­der­taken.

The num­ber of ben­e­fits gained by us­ing a twin elec­tric mo­tor­pow­ered fixed pitch CR propul­sion sys­tem over a same power sin­gle fixed or vari­able pitch pro­peller and pis­ton en­gine sys­tem could al­most be seen as too good to be true. They in­clude: Twin en­gine safety and ef­fec­tive clas­si­fi­ca­tion Sim­ple, two-lever thrust con­trols and in­stru­men­ta­tion Low im­pact on flight sym­me­try from an ‘en­gine’ fail­ure No air­craft yaw dur­ing power changes Al­most in­stant ‘throt­tle’ re­sponse Very pow­er­ful re­verse thrust Short­ened take­off and land­ing runs Im­proved climb per­for­mance and higher top speed Propul­sion sys­tem im­mune to ic­ing No weight change dur­ing flight Smaller over­all pro­peller disc di­am­e­ter Ex­tremely sim­ple me­chan­i­cal con­struc­tion Vir­tu­ally silent mo­tor op­er­a­tion, with no ex­haust pol­lu­tion Very high en­ergy ef­fi­ciency No en­gine warm­ing, shock cool­ing or spool up time Recharge us­ing ground power, wind or so­lar sources No change in power out­put at sea level or high al­ti­tude No liq­uid fuel or lu­bri­ca­tion sys­tem Of­fers aer­o­batic air­craft unique ma­noeu­vring ca­pa­bil­i­ties Neg­li­gi­ble vi­bra­tion Very low main­te­nance and op­er­at­ing costs TBO ex­tended to 5,000 hours Sys­tem can be retro­fit­ted in many air­craft.

What’s the catch?

One down­side is the lim­ited en­durance and range us­ing ex­ist­ing bat­tery en­ergy ca­pac­i­ties. In­ef­fi­cient as they are, hy­dro­car­bon-fu­elled en­gine sys­tems have a range some six-to-eight times greater than an equiv­a­lent power and range elec­tric sys­tem with the same to­tal sys­tem weight. And it is likely to be up to ten years un­til elec­tric range ex­ceeds com­bus­tion range, sys­tem weight-for-sys­tem weight, by which time bat­tery sci­en­tists say they will be able to con­struct bat­ter­ies with up to three times the en­ergy den­sity of hy­dro­car­bon fu­els.

Ini­tially, there­fore, adop­tion of elec­tric propul­sion is most likely in air­craft where a huge im­prove­ment in per­for­mance is most ben­e­fi­cial and short en­durance of 45 to 90 min­utes be­tween charges can be tol­er­ated. In some types, elec­tro-com­bus­tion hy­brid sys­tems may be an ap­pro­pri­ate in­terim so­lu­tion for longer range.

An­other is­sue is the pro­peller. To re­alise the po­ten­tial of elec­tric mo­tors re­quires the de­sign and man­u­fac­ture of a com­pletely new type of fixed-pitch pro­peller, ca­pa­ble of pro­vid­ing high thrust both for­ward and in re­verse, and able to ab­sorb the much wider range of power avail­able from elec­tric mo­tors. Whilst the max­i­mum power avail­able from a pis­ton en­gine is rarely more than thirty per cent greater than its con­tin­u­ous rat­ing, an elec­tric mo­tor is of­ten ca­pa­ble of 100200% over its con­tin­u­ous rat­ing for short pe­ri­ods, suf­fi­cient to of­fer huge ad­di­tional per­for­mance at take­off and climb out. The 125kw (155hp) con­tin­u­ous rated CRPS des­tined for in­stal­la­tion in the Su­per Cub float/ski plane will be ca­pa­ble of pro­vid­ing well over 250kw (310hp) for take­off and ini­tial climb, and re­duc­ing dras­ti­cally the land­ing run on wa­ter, ice and snow.

Pro­peller de­vel­op­ment

The pro­pel­lers to be used were de­vel­oped with Her­cules Pro­pel­lers dur­ing an in­ten­sive eigh­teen month R&D pro­gramme, as­sisted by UK Govern­ment and NATEP fund­ing. The re­search gen­er­ated com­puter al­go­rithms ca­pa­ble of cal­cu­lat­ing the shape, di­am­e­ter, pitch, blade area, chord and other pa­ram­e­ters from data in­puts such as mo­tor per­for­mance, air­craft flight en­ve­lope re­quire­ments and drag pro­file. The soft­ware gen­er­ated

‘3-D’ mod­els that could be di­rectly in­put to a pur­pose-de­signed CNC ma­chine to man­u­fac­ture pro­peller sets. A ‘loss’ process of pro­peller man­u­fac­ture was cho­sen over other meth­ods used for com­pos­ites or met­als, where ex­pen­sive tool­ing is gen­er­ally re­quired. Lam­i­nated beech wood blocks (the ‘orig­i­nal car­bon fi­bre’ as Ru­pert Wasey from Her­cules puts it) were used as the base ma­te­rial from which the twobladed pro­peller pairs were ma­chined and fin­ished by hand.

The two pro­pel­lers were de­signed to be driven at the same speed and to ab­sorb the same power, but as a con­tra-ro­tat­ing pair they dif­fer sig­nif­i­cantly in pitch, di­am­e­ter, blade area and shape. The outer por­tion of each blade was given a sym­met­ri­cal aero­foil al­low­ing the pro­pel­lers to gen­er­ate both for­ward and re­verse thrust.

Par­tic­u­lar at­ten­tion was paid to the man­ner in which two fixed­pitch con­tra-ro­tat­ing pro­pel­lers work to­gether to gen­er­ate thrust and in es­tab­lish­ing their ideal spac­ing. The pro­pel­lers are de­signed to be ef­fi­cient over a wide range of speed from 1,800 (cruise) to 2,700rpm (max power) in or­der to ab­sorb the mo­tor’s power de­liv­ery curve and of­fer a wide air­speed range.

A sig­nif­i­cant ben­e­fit of con­tra ro­ta­tion is that as a two stage sys­tem it can raise the ve­loc­ity of air sig­nif­i­cantly higher than is pos­si­ble with a ‘sin­gle-stage pro­peller’. Thus a sin­gle pro­peller driven air­craft de­signed for high speed ‘maxes-out’ at about 0.5 Mach whilst a sim­i­lar air­craft with same power con­tra-ro­tat­ing pro­pel­lers maxes-out at about 0.6 Mach, some 0.1 Mach higher (0.1 Mach = 75mph). This is the rea­son the Tupolev 95 ‘Bear’ air­craft is one of the world’s fastest pro­peller driven aero­planes.

Prov­ing the de­sign

In or­der to prove the pro­peller de­sign and es­tab­lish how they per­formed in a real world air­field en­vi­ron­ment, in­clud­ing ground op­er­a­tions, taxy­ing and sim­u­lated take­off and

land­ing run per­for­mance, it was nec­es­sary to de­sign and build a com­plete elec­tric propul­sion sys­tem and in­stall it on a mo­bile test rig.

The unit has a coax­ial pro­peller shaft assem­bly and mo­tor cool­ing sys­tem. Con­sid­er­able care had to be taken in de­sign­ing the com­po­nents to en­able a work­able assem­bly se­quence, as the coax­ial shaft assem­bly and bear­ings had to be de­signed to be in­stalled through the mo­tors and take ax­ial load­ing in both direc­tions dur­ing for­ward and re­verse thrust tests.

The com­pleted unit was mounted to a pur­pose-de­signed, fully in­stru­mented test frame. 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 power from the rear mo­tor to the front pro­peller and from the front mo­tor to the rear pro­peller. An an­nu­lar splined cou­pling within each mo­tor de­liv­ers power di­rectly to a spline on each shaft. There are no gears.

The ve­hi­cle and propul­sion sys­tem were ex­ten­sively in­stru­mented and real time data recorded for thrust, torque (yaw), rpm, tem­per­a­ture and power con­sump­tion dur­ing static tests and ad­di­tion­ally ac­cel­er­a­tion and speed dur­ing mo­bile test se­ries on the run­way.

Video record­ings were made to show the in­de­pen­dent con­trol of the pro­pel­lers for­ward and back­wards, in con­tra-ro­ta­tion and each pro­peller sep­a­rately and also to show the brak­ing ef­fect of re­verse thrust to stop the ve­hi­cle and then back it down the run­way−much to the amaze­ment of pass­ing avi­a­tors.

The static and mo­bile test pro­gram was un­der­taken at Glouces­ter­shire Air­port.

Test Re­sults

In tests the 1,200kg ve­hi­cle assem­bly (in­clud­ing oc­cu­pants) was ac­cel­er­ated from stand­still to 63kt IAS in 200m us­ing 138kw (180hp) to­tal power or 69kw (90hp) from each of the mo­tors. This test was un­der­taken to com­pare the per­for­mance of a Fu­rio air­craft fit­ted with a sin­gle Ly­coming IO-360 en­gine rated at 180hp and three-bladed VP pro­peller with a take­off weight of 1,200kg to that with an elec­tric CR sys­tem of the same power. (The Fu­rio’s ro­ta­tion speed, Vr is un­der 63kt.)

Static thrust tests from the pis­ton pow­ered Fu­rio showed a 310kg for­ward thrust at max­i­mum power. With this thrust, take­off run at 1,200kg (MTOW) to Vr is above 300m. Static per­for­mance tests from the Crps-equipped ve­hi­cle showed 470kg for­ward thrust and, in­ter­est­ingly, 360kg re­verse thrust. (The dif­fer­ence in for­ward and re­verse thrust is due to the bias in pro­peller de­sign to main­tain max­i­mum for­ward propul­sive ef­fi­ciency.)

The first two graphs above show that torque/yaw (or­ange curve), in­duced by the CRPS pro­pel­lers run­ning sep­a­rately, in­creases with power and thrust (blue curve). This would cause an air­craft to yaw left or right de­pend­ing on the di­rec­tion of pro­peller ro­ta­tion. The third graph shows that the torque, in­duced by the same two pro­pel­lers run­ning in con­tra ro­ta­tion, is ef­fec­tively zero at any power set­ting. The air­craft would there­fore ex­pe­ri­ence no yaw.

The test pro­gram re­vealed many other im­por­tant char­ac­ter­is­tics of the CRPS de­sign. No ini­tial warm­ing cy­cle was re­quired prior to op­er­at­ing the power/thrust level to max­i­mum power or other

set­tings. No cool­ing or idling pe­riod was re­quired to avoid shock cool­ing. Be­tween tests the CPRS was sim­ply turned off and on, and switched to for­ward or re­verse as re­quired. As a true ‘twin en­gine’ sys­tem each pro­peller could be op­er­ated in­de­pen­dently through the con­trol sys­tem and static and mo­bile tests were com­pleted us­ing sin­gle pro­pel­lers to drive the test ve­hi­cle for­ward and in re­verse to record sin­gle en­gine per­for­mance and sim­u­late en­gine fail­ures.

The CRPS unit runs vi­bra­tionfree at all power set­tings. The unit was bolted di­rectly to the test frame with no anti-vi­bra­tion mount­ings or pro­vi­sions. There are no ex­haust emis­sions and very lit­tle heat out­put.

No main­te­nance or fluid level checks were re­quired be­tween tests. This is a func­tion of the ex­treme sim­plic­ity of the de­sign. There are only two ro­tat­ing com­po­nents (two shaft/ pro­peller/drive ring as­sem­blies) other than the bear­ing as­sem­blies, which are sealed self-lu­bri­cat­ing com­po­nents. In ad­di­tion there are two mo­tor cool­ing pumps.

The mo­tors and mo­tor cool­ing pumps are vir­tu­ally silent at full power. Noise is lim­ited to that from the pro­pel­lers which was sub­jec­tively iden­ti­fied as lower than that emit­ted by a sin­gle, ‘same power’ pro­peller.

Next steps

CEP Ltd, has cho­sen a Fal­com­pos­ite Fu­rio air­craft as the best air­frame to in­stall and flight test the CRPS, as it is sup­plied in kit form. Built with­out any com­bus­tion en­gine com­po­nents and in­stru­ments, the air­frame can ac­com­mo­date an 850kg pay­load (in­clud­ing oc­cu­pants), suf­fi­cient to in­stall the com­plete CRPS

equip­ment, power elec­tron­ics and twin 200kg, 43kwhr bat­tery packs, giv­ing the air­craft up to a one hour flight time.

The elim­i­na­tion of the nor­mal swirling pro­peller slip­stream, loss of cool­ing air in­takes and im­proved aero­dy­namic pro­file will el­e­vate per­for­mance sig­nif­i­cantly. The air­craft would also qual­ify as a ‘twin en­gine’ aero­plane and have con­sid­er­ably en­hanced ac­cess to con­trolled airspace.

To un­der­take the Fu­rio project re­quires an in­vest­ment of £450k. This level of fund­ing is not presently avail­able in CEP Ltd, and ex­ter­nal fi­nance is be­ing sought. The com­pany

is there­fore un­der­tak­ing a de­vel­op­ment that is within its in­vest­ment ca­pa­bil­ity, that of an ‘en­try level’ sin­gle-mo­tor, geared, con­tra-ro­tat­ing sys­tem. This sys­tem will pro­vide yawfree for­ward and re­verse thrust, a con­tin­u­ous power of 125kw (155hp) and, im­por­tantly a max­i­mum power for take­off and climb of 250kw (310hp)−far greater than the present pis­ton pow­er­plant with which the Piper Su­per Cub float­plane is equipped. An Alaskan com­pany has of­fered the air­craft stripped of all in­ter­nal com­bu­sion en­gine com­po­nents plus the en­gi­neer­ing skills and fa­cil­i­ties to in­stall and flight test the equip­ment, at their cost.

A 65kwh bat­tery pack and all power elec­tron­ics op­er­at­ing at 600V DC will be de­liv­ered with the sys­tem. A sin­gle mo­tor de­liv­ers power to a sim­ple 1:1 ra­tio gear­box where the in­ner shaft of a coax­ial pair passes straight through the gear­box to power the for­ward pro­peller and a set of gears and shafts take power from the cen­tral shaft and de­liver it to an outer, hol­low coax­ial shaft to drive the rear pro­peller.

Power con­trol will be a sin­gle pro­por­tional ‘thrust lever’−for­ward for more power, back­wards through a gate for re­verse power. There are of course no mix­ture, man­i­fold pres­sure or prop pitch con­trols. Re­vers­ing the mo­tors in con­traro­ta­tion pro­vides yaw-free thrust and fa­cil­i­tates ex­tremely rapid de­cel­er­a­tion on wa­ter, ice, snow and slip­pery sur­faces and in emer­gen­cies.

Fi­nally, elec­tric CR has huge im­pli­ca­tions for the de­sign of the next gen­er­a­tion of mil­i­tary train­ers. Present day sin­gleengine tur­bo­prop mil­i­tary train­ers sim­u­late the ex­pe­ri­ence of pure jet flight us­ing com­plex com­puter driven ac­tu­a­tors and trims to coun­ter­act yaw. An air­craft with elec­tric CR propul­sion would sim­ply not need these sys­tems.

De­vel­oped too late to see RAF ser­vice, Mar­tin-baker’s bril­liant MB5 was one of the first pis­ton-en­gine fight­ers to be fit­ted with con­tra-ro­tat­ing pro­pel­lers

BE­LOW: first pass CNC ma­chin­ing of the wooden pro­pel­lers

BE­LOW: test ve­hi­cle with full CRPS sys­tem in­stalled. The tech­ni­cian is cal­i­brat­ing sys­tem sen­sors ar­ray prior to con­duct­ing static tests

Com­bined mo­tors - net torque close to zero

Thrust & torque, rear mo­tor

Thrust (blue) & torque (or­ange), front mo­tor

TOP TO BOT­TOM: me­chan­i­cal sim­plic­ity – the two splined col­lars (right in photo) bolt di­rectly to the ro­tor in­side each mo­tor. The longer of the two shafts fits in­side the short outer shaft, which has an in­te­gral pro­peller hub, to form the coax­ial con­tra ro­tat­ing assem­bly (the sec­ond pro­peller hub is not shown); CRPS drive unit, show­ing the front bear­ing sup­port and coax­ial shaft with twin pro­peller hubs; and a rear view of the same, show­ing the rear bear­ing sup­port for the coax­ial shaft assem­bly

ABOVE: the CRPS fit­ted to the test frame com­plete with cool­ing sys­tem and in­stru­men­ta­tion. The assem­bly weighs 88kg

RIGHT: the con­trol box and data- gath­er­ing com­puter used to op­er­ate the CRPS sys­tem. For static tests the con­trol box in­ter­faced re­motely with the ve­hi­cle through a 25m um­bil­i­cal. Dur­ing mo­bile tests the con­trol box was op­er­ated from within the ve­hi­cle by the co-pi­lot

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: Fal­com­pos­ite Fu­rio with 180hp pis­ton en­gine in­stalled.

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