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The Twin Star II as Brushless III New Revelations


With the exception that you must not run two brushless motor controllers with their Battery Eliminator Circuits simultaneously (as supplied by Multiplexes wiring harness) the Twin Star II as modern brushless Version III from Multiplex is a great airplane for not only Mission Bay. This is the RC version of an all weather airplane,  the Twin Star II can be successfully landed at places where only a fool would fly..  It just needs a separate BEC to be perfect.  A Twin Star II flies just fine with a simple four channel radio (I often flew with just three channels as I wandered for miles through the Back County) although it benefits from a computer radio.  Six hundred fifty flights later I have yet to find any need for servos any better then the recommended plastic gear (18) gram ones although the supplied Multiplex motors can easily,  at a higher purchase price,  be improved on.

As I’ve noted before these Internet reports need an internal date as things keep getting better,  it just won’t do to evaluate an account that may be way out of date thinking it might be representative of today.  There could be as many as thirteen years of Twin Star II reports circulating,  two,  of now three,  less then successful Multiplex brushless setups included although the airframe itself seems to be the same. This article centers on our experience with the third Multiplex brushless system,  for some old times note the previously available in the SEFSD Newsletter Part 1,  Old Testaments.

He later noted that the factory gluing of the control horns was weak,  some of them tore right out.  See the first part of this report,  Old Testaments,  for useful improvements to the Twin Star II,  in particular the elevator control horn needs about two square inches of area to stay glued in.  Although at first we thought that flying at twenty degrees Fahrenheit the LiPo battery went so dead that it didn’t respond because it ran out of thrust in the irregularly swirling air currents at our favorite flying spot,  we later decided that loss of radio contact was also an increasingly common issue.  He was flying at the far end of our usual field because the one we usually use was recently covered with shaving cream.  Manure isn’t the correct description as that material has been left to age until any seeds have germinated and died,  this stuff was fresh.  At least we don’t sit on a crowded four lane freeway to get to where we go flying here.

Bent up from wear and tear,  no other way to fix an aileron servo that sometimes either didn’t respond,  or stuck in position (due to a faulty extension in the factory built airplane,  which required cutting up the wing up to get at)  that,  despite a report by an electronics technician who publishes in Germanys most popular RC flight magazine that it was ok to run the motor controllers in parallel,  that it wasn’t.  The radio kept cutting out for a while more and more often.

Something missing
from all other available magazine and Internet stories,  the life experiences that are part of flying RC.  Six months before the friend that sent the above SMS had lost orientation flying a Merlin.  Using boiling water and CA glue we had it flying again.  After which he realized he needed a bigger,  slower,  flying machine and he found flying simple aerobatics great fun.  As the former underwater recovery service of the SEFSD,  getting Charlie White’s plane back from eight feet under water at Mission Bay is probably unique to our club’s flying field.  A Tau of Poo moment,  after spending two hours looking at the featureless bottom (out in the middle,  it was near the sidewalk) at least  I now know not to look for lobster or hope to spear fish for lunch there.

I for one do not believe any report made about any airplane system after the pilot only made a couple of flights as representing the truth,  the whole truth and nothing but the truth.  My reports are based on at least fifty flights,  in the case of the Twin Star II,  hundreds.  That running two motor controllers in parallel (both of them having their BECs active) doesn’t adequately function,  the hinge in the wings that all Twin Star IIs develop out past the spar (after fifty flights or so) and the tearing out of the elevator control arm are omissions from other reports that render them invalid!  Why they other authors don’t mention the inadequately dimensioned wing joiners that pop out of the wing profile right from the start is incomprehensible.

Take a look at the videos on the Internet
With even modestly capable hands on the controls and modern power a Twin Star II can perform most of the dynamic aerobatics,  the biggest omission is sustained knife edge (not enough power or rudder,  the dihedral interferes),  in a size (kind of big for aerobatics,  so it’s easy to see) and speed (slow) that allows you to study what’s happening and can fly near yourself.  Twin Star IIs with brushless power are the GREATEST for sport fliers who spend a too much time at a desk,  and/or middle aged on up reflexes,  or fading eyesight,  or for pilots who also must be able to put their RC airplane down on irregular fields.

Although nothing that flies can survive all that a beginner can do to it,  the Twin Star II comes as close to the perfect trainer as it gets,  other then the expense of the double power system.  You can start with careening the thing around,  then chase slope lift and catch a thermal to fly way further up and out then the law allows,  all in a potentially half an hour long flight on a single 3S 2200 mAh LiPo.

When checking the Internet,  if you can follow the reports in German,  apparent is how popular the Twin Star II is for remotely piloted (First Person View) flight.  There is a dedicated community in Germany willing to spend a couple of grand above the price of a standard airplane to fit it out with an airborne camera,  GPS and telemetry so they can fly it with a pair of glasses in a virtual reality like experience.  There is even a well made custom mount that replaces the canopy available.  Which brings up a disadvantage of foam,  there is a limited amount of strength available by just increasing the thickness,  it’s tight in any foam airplane.  Since it’s glued together,  some of the equipment must be positioned beforehand,  and stays in there.

If you can afford it,  put the receiver in back and leave it there for the life of the fuselage.

Like every other Multiplex airplane except the Gemini and Acromaster,  if just put together stock,  the Twin Star II develops a “hinge” just past the wing spar after about fifty flights.  Even the latest lighter versions with LiPo batteries and lighter brushless motors.  The first and easiest prevention,  a strip of fiberglass reinforced packing tape from about six inches before the end of the spar out to wrapped about 2 inches up and over the wing tip.  My personal Twin Star IIs,  the whole wing is one piece fiberglassed.  That keeps the wing joiners from poping out of the wing plane too.

Elapor foam is great stuff,  but if you don’t cover the leading edge of the wing with packing tape,  it quickly erodes.  With the high wing that isn’t an issue down at Mission Bay,  until you land off field.  I use fiberglass on the nose and either fiberglass or fiberglass reinforced tape down the belly.  My last (of three) Twin Star IIs went (421) flights before a transmitter quit,  I must be doing something right.

There are far more RC air shows in Germany then the USA,  the German technician who wrote the latest review of the Twin Star II with the brushless package used a pair of Twin Star IIs for his airborne monitoring of frequency control at shows,  until a couple of years ago when roomier airframes made out of EPP became available.  There have been conventional airframes with loads of cargo space all along,  he wanted reliability,  the airplane had to be able to loiter up there,  his expensive electronics went into a foamie Twin Star II.  I lucked out and met him once,  and thanked him for the many informative articles he’d written.  Physically small stature,  he looks like the mad scientist in the Back to the Future movie trilogy.  Well,  I look like a beach bum that got lost at the flying field…

I sometimes slope soar Point Loma,  but,  the bigger size of the Twin Star II hangs up too much on things there,  and the speed range is a little to limited too.  That no matter what you do low maximum speed is one of the charming aspects of the Twin Star II that make it so pleasant to fly.  When roaming the back country (just forget going shooting out there from now on though) the Twin Star II can be set down on rough dirt roads and the simplest of grass patches.  It’s big enough to send it way out there investigating updrafts,  and tough enough to withstand frequent rough landings.  RC pilots are notoriously well behaved,  now the boarder patrol leaves me alone.  Sad fact is though,  as you leave the metropolitan San Diego area you had better know who you are dealing with.  Even in Jamul the two of us moved to the back side of the hill at the sounds of a gun battle in the near distance.

Of all “slow” aerobatic airplanes the Twin Star II bucks wind better then any other,  right up to where you make a fool out of the other pilots at Oestrich-Winkel by going above the trees,  right there where the wind is strongest,  to demonstrate a ground speed that has the airplane flying backwards,  back down slope,  at minimal amps.  The real problem with wind is landing.  A gust of wind that would send a “normal” RC airplane cart-wheeling into the garbage,  just kind of shudders a Twin Star II.  The German pilots would be just as convinced that I’m a fool as their American counterparts,  except that I keep landing my RC airplanes right next to me and going  right back up.  After you accept the physically medium size and twin expense,  a Twin Star II comes as close to a one airplane does everything as any airplane I ever saw fly.  From a somebody that made six hundred flights with Twin Star IIs,  they are a great,  fun,  durable airplane for our flying conditions here at Unter Igstadt bei Wiesbaden am Rhein (Germany) and out and about in San Diego County,  California (USA).

After getting the Radio Ready airframe,  used new in box,  for half the original retail of Euro200/$270,  the friend scored the latest Multiplex (nearly) complete Twin Star II brushless setup new for Euro120/$165-,  a little discount over the Hobby Store retail.  It’s sad,  but the Internet is killing of traditional hobby shops in Germany,  and the USA,  and probably world wide,  it’s just too easy to mouse click and the package arrives in just a couple of days.  See the demise of the San Diego Sure Flight.  After flying his Gemini and my Fun Jet Lite that sunny,  windless December Sunday midday,  we went back to his place to examine the new Twin Star II parts.

During the quarter of a mile walk from where we can park (fifteen minutes from my apartment,  five from his) to the grassy field,  we skirted frozen over puddles.  I finally realized,  that when I leave my airplane on the ground to let the other pilots fly,  I also have attention available to admire the women passing by,  I’m still broad minded.  My Fun Jet Lite jumped six inched in the air at nearly the end of it’s fifteen foot landing skid on frozen grass,  because it ski jumped after hitting a hidden small mole hill.  That four inch high pile of dirt wasn’t even frozen solid and didn’t contain any rocks,  but the frozen grass has laid down and lost both it’s braking effect (on even wet grass the landing skid is about a meter) and it’s cushioning.  It will soon be time for something that can withstand winter flying,  and landing,  in this warmest corner of all Germany,  Wiesbaden am Rhein.  And April before we get a show of skin tight clothing on attractive women parading by again.  We have a great,  affordable,  thermal swimming complex right down the hill from my place though.  I’m broad minded,  but they won’t let me wear my mask and schnorkel in the swimming pool like I used to at SDSU.  I wish I could afford the sauna more often as Germans don’t normally wear cloths in them.

Although Multiplex directions,  both drawings and text,  to date,  have been as good as it gets,  the current issue block diagram (early 2012) of how to hook up the motors/controllers had a serious couple of confusions in it.  The drawing shows the separate leads for the ailerons,  which are included in an unusual combination with the connectors for the motors,  as going to separate receiver inputs.  Worse,  the two paired separate motor controllers servo leads are ALSO shown as going to separate receiver inputs.  Only the Spektrum 5 channel receiver includes paired aileron servo inputs (they are exactly the same channel as they are internally connected),  no receiver ever sold overtly included two outputs for the motor.  Even if the modern radios allow creating a second motor channel,  it’s hard to imagine why,  and this is intended to be a simple airplane which should be flyable with the simplest of four channel equipment.  In general you must not connect two separate motor controllers in parallel on the same channel,  the BECs can conflict to burn each other out.  Only after reviewing the article in FMT from November 2011,  written by an electronic technician RC professional,  who stated that these specific Multiplex motor controllers may be used in parallel,  was it clear the drawing is misleading,  two “Y” cables (or a five,  six or seven channel radio) are required.  After using the latest Twin Star factory wiring for fifty flights we concluded that running both motor controllers with their BECs both active was causing lost of radio contact.

A requirement was that the wing be able to be separated from the fuselage by just unplugging connectors,  and that the wing halves also be separated from each other just unplugging them.  Multiplex will quickly note the confused calls from the customers,  no doubt the drawing will be improved shortly.

The power package included new aluminum motor mounts to the motors replacing the black plastic ones included with the brushed motors.  The motors them selves are the Multiplex Permax brand.  After the typical length leads are 2,5 mm “bullet” connectors (check them,  not all of them always fit just right  Tape the leads to prevent them shorting against each other),  the leads back to the paired controllers,  where there is another set of connectors, might seem a little thin.  The cable system takes full advantage of the green six channel connectors.  Still common in Germany,  although unusual for the American market,  they have been demonstrated to function flawlessly for this type of application.  As a comment on the quantify of wires,  you bought a twin,  with a wing that splits in half and can be removed this is the minimum number of wires and connections that will function.

As I’ve noted so many times,  there is loads of competent help within the SEFSD.  If you are alone,  and bought the stuff through the internet instead of a hobby store,  good luck,  find a club and get some help.  Take your time,  get it tucked in right,  that is a first rate cable harness included with the system.

We Under Igstadt are using Deans connectors (increasingly common in conservative RC Germany),  the input from the battery connector was switched over.  Converting to Deans is just for our convenience,  the supplied green connectors will reliably carry the intended power and signals.

Although not the best available propeller carriers/collets,  two decent ones are included.  If you buy King Hobby stuff,  the cheap propeller collets are just that,  cheap,  they are often the source of material ruining,  energy wasting vibration.  The black plastic propellers are something new from Multiplex rated as the 8X5 size.  Similar in layout to the Speed 400 ones,  pointed tips and slight flexibility included,  the propellers are clearly laid out for the available power of rated as (14) amps on 3S LiPos of the motors.

Some bad news about electric motors in general,  the price of Neodynium has been raised to six times what it was by the monopoly of Communist China.  They are reactivating a mine out in the California desert,  but for now many manufacturers will be faced with either raising the price of the motors to include the higher cost of materials,  or cheapening up the motors.  These Permax motors flip right through indicating not all that good magnets.  I’ve made over a hundred flights with a similar Graupner motor (now at Euro42/$55-) where the magnets aren’t any better,  every single flight was entertaining.  I also made (23) flights where the magnets were even worse,  it cut the thrust by a third and the duration in half.  You get what you pay for,  but the next step up from the included Multiplex is,  financially,  thirty dollars per motor more.  Check with our Scorpion dealer for details,  figure seventy five dollars per motor.  Sell the Multiplex ones to recover part of your costs?  Find somebody about to buy the $18- ones and do him a favor,  sell him yours for the same price.

Even if you anticipate wanting more motor(s) I’d still buy the complete power package as a starting place.  Among other things the diameter is correct,  even though we had to clearance one motor mount for the motor wires.  At first we thought those  could be paired,  something ordinary controllers shouldn’t be trusted to,  each BEC can put out (3) amps and WOW,  not only can they administer to 4S LiPos,  they have lead settings!  That,  low,  medium and high timing ought to make up some for the weaker magnets.  Once we jet this thing to work on 3S,  I already have 4S NeuMotors batteries waiting on the bench and APC 7X4 props to go with them.  Comparable individual controllers cost Euro45/$62-,  each.  That’s half the price for the whole power package (battery(ies) and two Y cables not included)  without having to solder things up.  Make your own wiring harness,  screw up just one pair of leads without knowing it,  smoke a controler,  and you will have to replace more then the price of the cable set over individual parts.

Since this was a Radio Ready airframe package,  the servos are already glued in place.  Multiplex installs the aileron servos with the control arms straight out from the wing.  For most pilots some aileron differential is beneficial (otherwise it corkscrews in the opposite direction from aileron inputs),  the ailerons need about half again more up travel then down.  It can be programmed in on a six channel system,  or,  use a thin screwdriver to just poke through the foam to remove the screws,  pull the arms,  and reinstall one notch forward for a good approximation of balanced ailerons.  Personally,  I want the controls as close to balanced as mechanically possible,  then I use the computer transmitter to get everything just right.

For reasons I can’t fathom,  and it’s left out of all the other reports,  most people neglect to take five minutes with 220 grit sandpaper to remove the casting flash along leading edges.  I use lightweight spachtel to fill in the quarter sized casting indentations.  See Part 1 for more details on improving the airframe although this may be the most carefree out of the box airplane you can buy.  I’d treat the leading edges of the wing and horizontal stabilizator to heat shrunk on packing tape though,  do the belly while you’re at it too.  In case you were wondering,  my personal Twin Star IIs had a lot of thin fiberglass on them,  the wing halves wouldn’t separate anymore.  I use “circus” colors that even I don’t like on the ground,  but then,  when away from the Mission Bay SEFSD field almost nobody flies further out,  closer in,  higher up,  or lower,  I can identify the attitude of my airplanes…

Our January 2012 morning attempt was canceled when it snowed where I live most of the way up a hill,  for the friend lower down it kept raining.  Typical Wiesbaden though,  towards evening (2 pm at this time of year,  sundown at 4:30,  darkness at 5 pm!) the weather cleaned up and we had at it.  At the flying field we ran the motors up just as they came,  after ten seconds at full amps and then a little more the draw stabilized at (20) amps and (11.0) volts on a (190) gram 3S 2200 mAh LiPo.  If we take a efficiency of (.75) (higher at part amps then at maximum is typical for sport motors) that works out to (165) watts-out.  Under whelmed,  as they were listed as a combined (28) amps on 3S LiPos,  we tossed out the airplane anyway.  Ah,  what fun.  Yes,  with the ailerons installed with equal up and down travel it corkscrews left on application of maximum right aileron,  and when the amp stick is pushed forwards it pulls up and away,  but,  clumsy and all,  it flies.  Although flying conditions,  other then that our body thermostats are still not turned up so our fingers went cold,  are still rather good.  The months come though when a clumsy RC airplane that can be flown on a gusty day and landed on refrozen slush when a Twin Star II is like the only woman in town.  The only worrisome aspect,  at about one third amps the motors vibrate.

Back at the ranch,  the control input and power to one side were cut by just pulling the servo connector and separating a green connector.  Each motor controler was then,  separately,  reset from presumably low to medium timing.

I have a certificate in electronics and spent five years in CNC manufacturing ,  where among the five trades I had to keep in my head to get those expensive,  complicated machines to reliably function was electrician (that CNC technician/programmer occupation was a license for bizarre behavior,  preferential treatment and a earning decent living if you could withstand the solvents,  after fifteen years of working with my hands I couldn’t any more),  from that and my construction experience,  too many wires and an ordinary assembler/pilot gets overwhelmed.

One thing that was also clear,  the Dogfighter pylon racer was too fast for him.  As much as we enjoyed it’s first flights,  he was uncomfortable with it,  when the radio lost contact for a few seconds about flight 20,  before it was smashed up,  I became the new owner.  After just four flights I lost orientation and trashed it.  Before then he had acquired a Radio Ready Twin Star II and ordered the Multiplex complete (except for the battery,  we thought) motor and controler set.  Part of the reason for typing out Part I of my report on the modern Twin Star II was to clue him in on what to expect.

To the Twin Star II Flight Log,  We Get it to Work Well

Follow along as we optimize,  philosophize and have a great time with the Twin Star II.  Something missing from most airplane reports,  the adventure of getting things to work.  My log books are written in reverse order,  the newest entries are at the top.  The reds are reserved for warnings,  greens for costs,  blues for Old Testaments and new Revelations,  pink for being a fool.

Out past flight 50 we concluded that the left aileron servo often wasn’t working some of the time.  Having a wing servo stick in place is irritating,  but what was heading to going fatal was the increasingly often loss of radio contact.  The wing was bent up anyway so it was destroyed getting to the wiring.  It took too long for the spare wing ordered at the peak of the flying season in June to arrive,  so a new kit was bought.

One of the 30cm wing servo extensions was defective,  so both of them were junked to be replaced with new ones.  Past experience with HS-81s was that they could easily go three hundred flights,  so the HS-82s were reused.

The red leads for the motor controllers were disconnected and taped off,  an Euro8/$11- BEC was added.

It was decided to create a second wing wiring harness as we are going to end up with two complete airplanes.  Three thin wires for brushless motors fit where two thicker wires went to the brushed motors.  Since the motors already had bullet connectors the new harness has them too.  A couple of the original motor/wiring connectors were so lose fitting that they were replaced.

One of the motor mounts had to be clearanced for the motor’s wires.

This time the servos were angles forward one tooth on the servos,  even though that required making new linkage wires.  Although Multiplex pointedly in the assembly instructions calls for installing the servo arms straight out from the wing (and that is they way they are installed in the radio ready version),  they also specify 18mm up at the ailerons and 10mm down.  How that is suppose to happen without the servo arm being angled forward is unclear,  it exceeds the adjustments available by off centering the servos with a computer radio,  and that isn’t the way to get differential anyway.

The wing was treated with the usual taped leading edges and hinges.  We don’t ever pull our wings apart,  so they were glued and fiberglassed together.  That also take care of the two white plastic wing joiners (that the two wing hold down screws go through) warping up out of the wing and screwing up the airflow.  The foam pieces that cover the wing spars of Twin Star II’s have never ever fit right,  they always stick out from the wing.  I use a razor knife,  sand paper and heat shrunk on packing tape to make them even with the bottom of the wing.

The nose and belly were fiberglassed.

Those white plastic control horns don’t have enough area for Twin Star II size control surfaces,  and the control surfaces of the ailerons will bend at the control horns,  so not only were the control horns treated with fiberglass,  the fiberglass was extended to a third the width along the wing of the ailerons.  If you don’t due fiberglass at least epoxy a one inch by two inch piece of thin plywood over the elevator control horn.  I use a little drop of CA to hold the little brass nuts on to the control horns.  Don’t forget that that those little aluminum connectors at the control surfaces must be free to rotate!

It is noted that the motors bearings are showing wear at just fifty flights where “only” 3S 2200 mAh LiPos were used.  The intended 4S LiPos won’t fit in a stock Twin Star II,  so that was left out of the flying,  for now.  The props weren’t much different after fifty flights then new.

Assembling the wing to the fuselage is a problem in that there just isn’t enough room in there for everything.  The “tung” of the canopy was cut off and replaced with a dowel to gain some clearance.

Flights 50 through 56  Under Igstadt  13 October 2012 (Sa.)  If cool (60s Fahrenheit) otherwise just perfect flying conditions.

The airplane was delivered to the owner using my receiver on the bench before leaving for a tour of Northern Italy three weeks ago.  He was confused by the new BEC,  so he waited until my return to re-bind his receiver.  Friends and fellow club members,  if nothing in your life’s experience prepared you for wiring,  or radios,  get yourself some help!  At his place I was shown what fantastic little radio controlled cars you can get now demonstrated by his two young daughters.

What a lot of stuff to cram into that little room in the fuselage.  The,  control wires exit at right angles,  Spektrum receiver doesn’t really fit. 
Since the Ds6i’s separate travel adjustment seems to have quit,  we made new,  longer,  aileron control wires.  As it turns out we still need to offset the servos at the computer transmitter to get really axial rolls.
As we moved the 3S 2200 mAh LiPo back the handling improved. 
Although I made it as far as painting the vertical stabilizator red,  even under these perfect clear blue skies we quickly loose orientation on this otherwise white airplane,  the owner will add some paint.  The owner liked the addition of the window decals. 
It could use some more differential,  but flies great the way it is,  better then any Twin Star II I have ever had.
It still pulls up on full amps,  so what. 
That odd pulsing between the motors is gone. 
Not once did the radio lose contact.
Neither BEC,  motor controllers or battery heats up no matter how hard we fly it. 
For such a seemingly simple big flat winged airplane the agility of a Twin Star II is still a surprise. 
It was decided to increase the throw on the rudder.

Life was good.

Flights 10 through 15  December 21,  2011 (We.)  Under Igstadt
Despite the seven,  only through the Beep and move power lever,  stages to effect settings having nearly brought the owner to frustration,  we’re at the snow melting temperatures in the 40s (Fahrenheit) at the field on a no wind afternoon.  For Euro13/$18- plus the owner has some new 3S 2200 mAh LiPos,  Brand,  don’t buy their servos,  but we are doing fine with their LiPo batteries,  that are rated for 35C instead of 25C.  Between the moving the timing from low to medium,  the airplane picked up ten percent thrust.  That alone put the Twin Star II into the relaxing toss around it around airplane we always knew it could be.  The lower internal resistance 35C batteries brought another ten percent.  On the bench it now pulls (23) amps at (11.2) volts.  But,  the duration went down about ten percent.  Over time that will be recovered,  the additional power no longer requires full amps the whole flight.

One thing to watch for if you are reading this back in warm sunny San Diego California,  we were flying at near freezing,  watch for the controllers and motors overheating.  When it gets even colder we are going to have to start heating the batteries for flight,  and accepting a shorter battery life as over an twelve to thirty minute flight,  even if they start out warm,  the batteries will quickly cool down in freezing air.  They don’t cost any near as much as they used to,  we will accept the shorter battery life.

In the air,  with the motor controler timing advanced,  that annoyin