Dedicated to the Promotion of Electric Propulsion in all types of Aeromodeling

Direct or Geared?


The original article appears in the German magazine FMT,  issue Eight of 2011 by author Andreas Maurer on pages 28 through 31.  It is accompanied by many color photos.  FMT is the largest of the German Language magazines covering Radio Controlled Model Airplanes (extending from Germany into Austria and Switzerland),  it covers just about every aspect of RC flight.  As such,  some articles such as this one go into more background details then they would in a more specialized publication.

This is not an authorized translation,  the constraints of a precise legally defendable conversion were set aside to make it as readable as a comparable write up in American English,  without masking it’s Germanic origins.  It’s just a hobby to make it accessible to the members of the Silent Electric Fliers of San Diego.  The original Author made some glaring material ruining mistakes and errors by admission!  We have some of the most knowledgeable Electric Powered RC pilots and manufacturers in the world among the San Diego Silent Electric Fliers,  some completely new to RC flight members,  and some converts from reciprocating power too.  In a big change from previous translations,  only  portions were translated,  to cut translation time.  The translator, a former and future Shame of the Club,  has addressed some glaring errors making this a new article.

 While researching how to make the best of my new Multiplex Fun Cub I was disgusted at what even the not dependent on manufacturer’s advertising Internet authors left out.  And at the disparaging stuff (like foam so soft it can’t be expected to last more then a weekend) routinely left out of some FMT model tests.  I am incensed that the Editor at FMT did not weed out some of the content of the article.  Usually I do  a formal,  legally correct,  language conversion,  not this time!

Begin the Article
Motivation for the conversion was the noise from the engine of the ten foot wingspan airplane.  It was either convert to something quieter,  or quit flying it at the field on Sundays.

Since the Su-26 from Hanger 9 is relatively light the conversion seemed plausible.  There were already LiPo Batteries in the size 6S 5000 mAh on hand.  Four of them together results in 12S 10,000 mAh.  After researching things it was decided on Scorpion Motors,  as distributed in Germany by Ringel Modelbau.  The manufacturer makes a suitable program for their motors available.  It kicked out the HK555-180.  It would deliver the same performance as a DA85 oil spewing,  vibrating,  howling engine.  A simple calculation indicated that the additional weight over a engine would be about 2.2 pounds.  Right here you need to question the understanding of the person doing the conversion.  To quote the German F5B World Champion Rafl Fickensicherer as previously published in an FMT article translated and forwarded;  Not including the fuel that the engine fraction so willingly omits.

Just for the fun of it I did the calculations for combination using a transmission.  To my surprise a HK4035-800 with a 5:1 transmission provides the same performance as direct drive at 15% less weight.  I take that that here the weight difference is just the direct drive motor verses the geared motor.  The whole package has to be evaluated to be valid,  something the started with combustion types still don’t seem to have in grip.  If that is really so,  would be determined.

From Theory to Practice
The following candidates were thrown in the ring:
Direct drive Scorpion S5545-180,  20 poles,  live weight with mounting cross-plate 1.120 gram,  certified for operation on 12S LiPo,  maximum 4,500 Watts continuous.
Transmission Scorpion HK4035-800 with Reisenauer-Motor Chief 5:1,  8 poles,  overall weight with transmission and motor cross-mount 620 gram,  certified for operation on 12S LiPo,  the transmission maximum is 4000 Watts continuos,  the motor 4,200 Watts. 
That’s watts- in folks,  not what is delivered to the output!  We wouldn’t rate a fuel engine’s output by measuring the fuel consumed,  but,  at least because it’s easy to measure and standard,  that is typical for electric motors.

The score is already 1 to nothing for the transmission combination.  500 grams less weight by almost the same performance.  This is misleading,  no motor vehicle publication would omit/forget that output through a transmission must include the additional losses in the gear train.  See the term horse power at the rear wheels.

Let’s take a better look at the transmission.  It’s a planetary transmission (standard for decades within automotive automatic transmissions) with plasma hardened steel wheels and a titanium shaft.  The planet gears have needle bearings.  Planetary gear transmissions have the highest energy transfer density per volume and the best efficiency for concentric output.  The output shaft has double angles bearings diameter 32 mm.  The transmission itself has a diameter of 38.5 mm,  examination of it leaves an impression of solidity.  According to the transmission manufacturer if the gearbox was correctly lubricated they has never been a failure,  not even by crashing or wear.  It’s almost unbelievable that 5,5 horse power can be set free here.

Assembling the transmission to the motor turned out to be simple.  The input shaft would be attached with High-Strength Loctite in the correct extension on the motor shaft.  The motor mount attached and assembly is complete.  Since the transmission is already on the upper limit of it’s capacity,  actually a little over,  that glued input shaft could be a weak point.  Help was provided by a jeweler who routinely makes ultra small laser welds.  That worked great here.  Like how many of us have access to that type of exotic welding?  That just blew any general purpose open to the ordinary public comparison.  Still,  if the transmission manufacturer knows it lasts with Loctite,  he must be right.  It’s important to point out that only a motor with a 6 mm output shaft may be used.  And,  the lousy manufacturing tolerances of cheap motors plus sloppy bearings won’t work with a precision piece of equipment like this transmission.

Into the Model
Conversion of the model was more difficult.  Due to the difference in weight of the motors getting the balance right required that the position of the batteries had to be variable.  And the construction of the model precluded installing the batteries in one piece.  The muffler of the combustion engine was underneath the landing gear shock absorber,  which placed it relatively high.  Therefore two batteries were under the cowl,  two more further aft.

In comparison the instillation of the motors was easier.  The direct drive motor went on a fiberglass reinforced plywood box.  Another construction was used for the transmission combination.

The motor controller went on top of the motor mount box.  Since it wouldn’t fit with the cooling fins in line with the incoming airflow,  a duct was formed to direct the airflow over it.  The huge opening in front was then nearly closed off,  only a ring around the motor and the inlet for the controler cooling was left open.  This is frequently required for effective electric motor use in models of airplanes originally equipped with radial engines. Otherwise that big cupped opening performs just like the references out of the aerodynamic fluid engineering as a maximum drag device.

So,  with that fully equipped the model weighs including the direct drive motor exactly 11,010 gram,  or 1,100 gram more then the engine combination.  What a glaring by omission misrepresentation of weight!  It leaves out the weight of the required fuel for a combustion power plant.  Not to mention the slow but unavoidable increase in weight as the thing gets soaked in oil…

A short trial run in the shop with full gas (make that full ampere) without a propeller made for questions.  After a few sections of floored the transmission developed a temperature of 60 C.  A phone call to the manufacturer brought the answer.  The warmth came from the motor.  The motor should not be run without the propeller.  First off that omits the cooling,  second at the high RPM the motor controller makes the mistake of double clocking,  the resulting waste energy turns into heat.  The only way to overheat the transmission is to over fill it with lubrication,  but 60 C is otherwise ok.  Was the builder stupid?  Nobody would test an automotive transmission by disconnecting the driveshaft and flooring the engine.  The manufacturer’s of electric motors always include the warning to not run an electric motor without load as the resulting high RPM may easily completely exceed the capacity of the bearings,  ruining them before they ever get a chance to show their stuff.  If this wasn’t a common mistake on the part of the fuel burner pilots I’d be convinced this guy had a screw lose.

Which Propeller
The next question was the correct propeller.  Mr. Reisenauer desperately recommended the new wood props from Fiala.  They have a clearly better efficiency then the usual propellers,  for E-motors very important. 

Back when we flew with heavy, low powered can motors the RPM of electric motors was so much lower then the engines that,  due to the available power being so low,  special propellers were the only way to get our stuff to fly at all.  In a sense the special E-props we are today using for very slow flight still represent that,  propellers are them selves an airfoil,  which must be matched to the demands of the whole assembly.  What a propeller sees is the RPM it is turning a verses the airspeed,  how the torque arrived there is of no consequence.  If an electric motor under load turns the speed the propeller is constructed for,  it has exactly the same performance as if an engine were turning it.  There is still the difference that electric motors may still have flatter torque over changing RPM (such as  when airspeed during a maneuver changes) ratings,  with the result that a different propeller may perform differently as the rotational speed varies differently with the source.  Take note of a previously forwarded translation about developments within the highest Acrobatics class,  F4A,  where the electric motor manufacturers are designing their motors and controllers both to function at just the same speed as the current (now losing ground but by no means out) combustion engines. 

The combustion guys seem to very slow to catch on.  I routinely get up to a hundred flights out of my folding reinforced plastic propellers. If you are wondering why my seemingly ratty looking airplanes outperform yours,  it includes a few minutes with fine sanding paper tuning up my props.   At the flying field former “power” pilots often cannot be convinced to even debur propellers,  even when the flashing is obvious.  They broke their wood ones so often and the erosion from debris always did it before for them,  “why bother now” seems to be their opposition ignorance.

From theoretical considerations the sizes 24X14 and 23X16 should deliver good results. In addition I wanted to try the already on hand from the DA85 27X12 Elster and 24X12 Engle Super Silence propellers.  That DA stands for Desert Aircraft filthy, stinking,  lubricant slinging,  shakes so hard it blurs,  combustion things.  It’s a large size consummate with moving a ten foot wing span all out aerobatics RC airplane,  and a good quality one at that.  Those props,  described in inches,  which was once a common measurement in Old Germany and is still used there for pipe,  represent a large investment.  At that price and quality the props are jewels that require no deburing.

The Engel 24X10 had the best results on the ground,  particularly with the direct drive motor.  It was otherwise in the air.  It required the most energy in the air and had the worst dynamics.  When used on direct drive it drew down 4,100 mAh.  When used with the transmission combination it took about 3,500 mAh to fill up.  Due to the RPM it was also the loudest in the test.  I take it that he is flying a set routing,  common to among other things,  F4A.  It may very well be that he set out to match the energy requirements to the specific duration,  common among competition pilots to which,  despite his gaps in the electric power understanding,  Mr. Maurer no doubt belongs to.

It was different with the Fiala 24X14.  On the ground it took more power then the Engel,  in the air it was much more efficient and gave terrific dynamics.  The vertical climb was phenomenal,  apparently the model weighs nothing.  You notice that you need much less adjustment with the gas hand.  It’s amps! That was the case with both propulsion concepts although the transmission one drew less power.  After an eight minute flight the direct propulsion required about 3,600 mAh,  the transmission one 3,200 mAh.  It was analogues with the Fiala 23X16 although it drew more power.  At full voltage it drew (170) amps.  When hovering you could sometimes hear the prop stall.  With both motors seem annoyed,  the pitch is too high.

The Elster 27X12 was disappointing.  At maximum voltage the direct drive drew (170) amps,  the upper limit of the motor controller.  If it was desired to not draw the batteries down more then 80% an eight minute flight was not possible.  Due to a very wide flange the prop could not be tested on the transmission as only 4mm of the output shaft stuck out. 

If this section reads like a propeller test it’s because of the tremendous influence a propeller has on flight performance.  There is more experimentation required with electric then fuel burners.  As I note in my book “So You Want to Fly RC”  nobody flies dynos,  you have to try it.

The Winners Honor
So who won?  It’s clear cut,  if you want a high efficiency,  super light and problem less operation,  get the transmission combination.  Just be willing to accept the Euro150/$225 higher expense.  You hardly hear the transmission,  in the air even less.  But if money is more important,  get the direct drive.  Talk to Steve Neu about an Outrunner Killer combination he has.
Some comments to the rest of the program.  The Hacker motor controller (great stuff) made the maximum out of the motors.  May I quote Steve Neu “you get what you pay for”.  Don’t expect the same performance out of the cheap stuff.  I was amazed at the improved performance of even an inexpensive 45 gram outrunner when my finances finally allowed the use of a motor controller where I could set the lead from the general purpose works for anything to correct for the outrunner.