Yearly Archives: 2012

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reports need an internal date as things keep getting better,  it just won’t do to evaluate an account that may be way out of date thinking it might be representative of today.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

Continued from Part 1:

Although not as problematic as the Fun Cub,  cooling air through the motor compartment was an issue with the smaller motors being run hard.  We never did get to adding on the clear canopy for the Mark I,  it flew great,  we didn’t feel like fiddling with it.  If it made a difference on Mark II,  it was too small to find.

The stock landing gear,  when used as a “take off from the asphalt and catch the grass on landing” device,  tears out after twenty landings or so.  Gluing it back in gets another twenty landings or so.  I’m good with fiberglass,  but that little attachment plate just isn’t able to reasonably receive enough holding reinforcement to spread the load to the foam over repeated hits.  So,  switching to a fiberglassed on carbon fiber landing gear gets another thirty or so landings until it tears out.  Fix it with more layers of fiberglass and figure another thirty landings before it tears out again.  By then,  even though the whole rest of the airframe in fine shape,  maybe it’s time to just build another airframe.

With it’s compact size the Gemini doesn’t really need to have the wing removed,  Mark II we went straight to the after market landing gear fiberglassed from under the nose to half the width of the wing.  And it still won’t last forever,  that attachment at the landing gear sets the limit on durability.  If it was me,  I’d omit the landing gear to just belly it in on the grass with the prop folded all the way back,  but the friend to whom the airplanes belong,  likes the landing gear…  Maybe for Mark III or IV.

Construction Notes for the Mark II

2 hours    open up box,  contemplate,  check inventory (not once over a dozen Multiplex kits has anything been missing or unusable) weigh parts and sand
1.5    fiberglass inside motor trough and glue in a carbon fiber wing spar
1.5    tape hinges and leading edges,  glue the fuselage halves together,  glue in control horns
4.0 plus         Final assembly  Substitute different control rods when the available (used) servo arms had holes too big.  Fit and fiddle with a couple of different motors  clearance for the wires coming out of the motor
Optional

2.0    Initial fiberglassing

Getting it to Function Really Well

2.0    Additional fiberglass  try different motor combinations  tune servo throws mechanically and with software

1.0    Change propeller blades and motor controller  loosen up a tight control linkage pivot  clearance more for the larger diameter prop blades

1.0    remove the 35mm HiMax motor and Castle Creations motor controller  mount the (80) gram HiMax motor (recommended for Gemini and Fun Cub) and a 4S Robby motor controller
clearance for the 8X5 folding prop  paint tail red,  top of wing yellow

For the first flights,  omitting all landing gear,  paint and minimal fiberglass reinforcement the weights came in as:
410 grams the whole airplane ready to fly less power package
plastic “large diameter bolt circle” motor mount,  Multiplex
HiMax (135) gram motor and Castle Creations (36) amp 2-3 LiPo motor controller
average weight for a 3S 2200mAh LiPo battery
800        grams all up flying weight

From reading my reports it might seem I’m unconcerned about weight,  that’s not the case at all.  That armor and reinforcement is well thought out,  as is the minimal weight paint scheme.  What good is the lightest weight when the airframe quickly gets trashed by abrasion,  or you lost orientation?

When it came to the wing spar,  the supplied 6mm diameter fiberglass one from Multiplex weighs (23) grams,  it functions perfectly.  But,  for Euro4,90/$6-  a lighter carbon fiber one that weighs (8) grams was substituted,  some was left over from the standard one meter length.  It doesn’t seem like much,  (15) grams less,  but,  it was easy to swap and it makes every single flight.  You could easily spray that much paint on without even recognizing how much weight paint adds.

My airplanes aren’t near as ratty as they look.

My current production (i.e. new) standard Gemini kit included a plastic motor mount,  with the small bolt circle.  That restricts you to smaller diameter motors.  The plastic isn’t dimensionally stable over time,  even new if you run the flat side over sandpaper,  it’s shrunken in.  It takes five or ten minutes to sand it flat,  that is tough plastic.  Since the motor will still bolt up,  why bother?  Because if you don’t get a good solid contact surface to the motor,  it vibrates.  Even if you tighten the motor mount screws until the threads strip out,  it won’t be enough to warp the plastic motor mount flat into the required full face contact.  Lots of Internet Forums comments where confused owners had problems.

Multiplex itself offers a stamped aluminum motor mount with the large (for their 35mm diameter motors) bolt circle,  it appears that it will become standard.  In Germany there is a carbon fiber replacement motor mount with some down thrust built in that,  if the budget allows it,  is worthwhile.  And,  it fit’s a whole bunch of different Multiplex airplanes.  I use a Fun Cub as a test bed,  if a combination works in it,  it might be a good starting place for the Gemini,  Park Master and others.  See the above comment about carbon fiber landing gear and spar,  improvements will cost you.

If you buy the Acromaster/Tuning power package,  get the motor mount for the larger diameter bolt circle.  My Dog Fighter with a (135) gram motor came with an aluminum mount:  It was available for the Gemini Mark II after the twitchy Dog Fighter (being flown with a 60 gram,  lighter then standard,  motor on 5S LiPos) hit the ground.

What would I do different
for Mark III?  Cut out the bottom wing mount assembly so a 4S 2200 mAh LiPo would fit far enough back to make balance.  I glued the wings on,  the bond of CA to foam is more then the bond between the individual cells of Elapor,  the screws weren’t needed for the Mark II.

Unlike the magazine articles,  sometimes it takes some fiddling and experimentation to get things just right.  Take note that I write my flight logs with the most recent entry at the top.

I’m going to Italy for three weeks,  our editor would like this before I get back.  By then as I come over the Alps,  and have to dress warm,  I’ll be thinking of San Diego California.

Although of what I have I like that smaller,  more expensive (105) gram motor on 4S LiPo the best,  most of the SEFSD would be well served with the (135) gram one from Multiplex on a 3S LiPo in the 1500 mAh size.

We can skip some of my combinations like a mediocre quality (60) gram motor on 4S and 5S LiPos as being too far off standard for most of you, and too,  well,  mediocre.  Or that a (45) gram motor just didn’t have the required oomph no mater how many cells were used,  laid out by the manufacturer for 3S LiPos,  it was never going to be able to convert that much more magnetic flux then designed for into thrust.  That and expectations have changed,  I expect even affordable combinations to take an aerobatics RC airplane way up there right now,  it isn’t that long ago since a 30% up climb would have been more then enough.  Anybody else remember finding out that using eight NiCads or ten,  a six volt Speed 400 won’t really put out any more power?

A clue,  a (70) gram top quality (38) mm diameter motor (think .92% real efficiency,  to get it took a variable timing,  it costs more then the Gemini kit does controller) on high performance 4S LiPos with those lightweight Aero-Naut blades can turn a Gemini into a whirling,  twirling,  then straighten it out and glide RC airplane the likes of which even the sponsored pilots have difficulty believing.  I had a full box of blades with me,  the other pilot knew how to select what he wanted as the Gemini needed a slightly different combination then his airplane.  But,  they loaned the motor and it’s controler and the batteries for that flying session,  which I had to give back…  At least they clued me in about exponential.

That short nose and short battery compartment sets a very narrow range of weights that will make balance,  which should be further back then the original (75) mm recommendation in the Multiplex instructions.  I liked (85) to (90) mm back from the top wings leading edge.

That needing more down thrust was also a problem with the Fun Cub,  so I spent the money for an after-market mount with the needed down thrust built in.  I glide a lot,  if you set the down thrust for it goes level under full power,  on application of thrust it pulls the whole airplane forward and down about four feet first.  Keep the power mostly on and it’s sort of ok.  There is no setting that is perfectly neutral.  Once again,  this is a semi scale,  easy handling biplane that is a blast to fly,  they sacrificed some precision.  The mount fits both Gemini and Acromaster and the up next Mentor.

I like to get the control throws balanced without exponential,  for purposes of this introduction to the Gemini report you don’t need the details of that part of the adventure,  except that it took a lot more then three flights.

Flights 10 and 11  Under Igstadt  Although I enjoy some wind as a fascinating,  unpredictable opponent,  this utter stillness is what’s needed to sort out an airplane.  I’ll have to remember this short pants and shirtless day for the long winter to come.

What an agile airplane!  I thought I was getting better at flying aerobatics with the Acromaster,  with this Gemini I now know I have a long way to go.  It needs more rudder throw and exponential to do decent knife edging,  thrust is already enough.

After two back to back flights the motor is just warm,  the motor controller and battery even less so.  Considering I left that chin scoop blocked off,  next time I’m doing fiberglass it will be sealed off.  Less of an air-scoop then a dirt,  snow and manure scoop is better.

The pull up on application of thrust is still there.

This motor combination can easily put out far more thrust,  which I realize I can’t use yet.  It can climb at 80% of vertical on up out of sight.  A count of ten climb results in a altitude gain on the order of a hundred and fifty yards followed by a count of sixty glide.  The best combinations in Blizzards,  Reinforced Mini Mags and Fun Cubs go to a (1:18) ratio.  This one would go to at least (1:10) by just putting on a higher pitch prop,  but that’s not the whole adds up to fun equation.

Even with the prop folded in inverted flight it takes maximum up at the elevator to just stay in a flat glide.  All that up thrust,  which becomes down thrust for outside loops,  is the only reason it can do outside loops at all.  And why,  inside loops are less then one third the diameter of outside loops.  So far it has more maximum up elevator then dynamic aerobatics can make use of.  The elevator throw is just slightly asymmetrical with a little more up then down,  well,  this isn’t an F3A airplane…

The “little” 1100 mAh 4S batteries are all the way forward for correct balance,  the “big” 2200 mAh 4S batteries would make it nose heavy.  The flights were a little short,  more like what most people quote at around twelve minutes.  At least the batteries charge quick.

Lets compare the two motors,  the (105) gram Plettenburg and the (80) gram Multiplex,  if both were run at (15) amps.  That is (15) amps X (14) volts for (210) watts-in.  However,  the Multiplex Park Master (80) gram motor is running at more like (.72)% efficiency (140) watts-out,  the Plettenburg is on more like (82)% efficiency for (172) watts-out.  It works out to about one fifth more for the more efficient motor,  however,  in reality the larger diameter propeller,  ten squared is a hundred,  eight squared is sixty four,  makes for a better couple (less of the thrust gets lost to slip) resulting in about a third more thrust for the same watts-in.  Ten inches is a comfortable size in the Gemini,  eleven is still ok,  out around twelve  hanging up in the weeds and the propeller’s gyroscope properties start showing up.

But,  that’s if the (80) gram Multiplex motor could sustain (210) watts-in,  my only one available to test couldn’t.  I will note though that,  when run on a 3S LiPo with a ten inch prop that,  until it overheated;  It was grand.  And about a seventy bucks less investment.  But,  that’s for a high drag moderate flying speed Gemini or Fun Cub,  in a pylon racer,  or something more sensitive to weight,  it might work out different.  Talk to the Editor,  he’s the one with a degree in Aerospace,  it’s more complicated then I care to numerically understand.

Since the 4mm propeller carrier was already fitted out with Aero-Naut 11X4 folding blades they were tried first on a 3S 2200 mAh LiPo.  At just over ten amps,  not enough.  Then Aero-Naut 10X6 were tried,  about the same.  When fitted with a (120) gram 4S LiPo it drew (14) amps.  Balance is just about where it was with the precious combination.  All up flying weight (still no landing gear) is now (730) grams.

The (105) gram Plettenburg general purpose (good stuff) motor and Robbi 6S motor controller were fitted,  weight of propulsion complete is (200) grams.

The nose was treated with another layer of (4) ounce per square foot fiberglass held on with two component resin.  Including sanding it was about an hour.  Unlike the first one,  where looks was important,  this is my personal airplane,  I can deal with it being just a little ragged.  The resulting weight increase is on the order of (6) to (7) grams.  Weight of the ready to fly Gemini Mark II is now (410) grams less propulsion complete and battery.  To keep that (7) grams in perspective,  removing the motor controler extension (makes motor swaps more convenient) back to the receiver would be half the weight of the fiberglass just added.

All together the nose was trimmed back about 4mm,  both the inside and the outside of the motor circle were beveled back in as a square lip is very restrictive to hoped for increase in air flow around the motor.  Before adding another couple of layers of fiberglass to the nose the airframe,  ready to fly without the propulsion complete,  with half the expected paint already applied,  still weighed (404) grams.

Flight 9  Wiesbaden Waldacker  some clouds have come out,  it’s gusty swirling wind coming down over a mile of forested hill and then up and around the trees right at this long narrow field  It’s still very comfortable temperatures,  but only the most wind experienced pilots would dare it.
Just not really enough thrust.  It still pulls up at full amps.  Climb looks to be at about 30%,  speed against the wind is now an issue.  Although Multiplex recommends a maximum of (12) amps,  and I have 10X4 blades in my pocket,  the (80) gram motor still ran on the verge of too warm and less thrust just wouldn’t be what I want out of this airplane,  or safe to fly under these conditions.  It’s that effortless throw it around of the bigger motor combination that makes Gemini I such a blast to fly.  I want an all weather airplane,  if I need something that can’t be flow in the wind,  I’ll build a Park Master.  For all the gusting the airplane is still very controllable,  another perfect landing.

The Aero-Naut 10X6 blades were replaced with Aero-Naut 11X4 blades,  past peak it draws (13) amps.  Eleven inches is more prop diameter then I want,  and,  this unusually flat pitch has to be ordered even here in Germany.  But they don’t make 10X5 blades.

The two 3X10 stock upper motor mount screws were replaced with 3X12s turned all the way in.   Six of them cost a buck.

Flight 8  Under Igstadt  Still warm and sunny,  but too windy for a Park Master
Between fixing the binding aileron and using the individual trim to create some differential rolls are now a lot closer to axial.  It still pulls up way to much at full amps.  Although the climb is off quite a bit,  it’s still on up at 70%.  But,  after five minutes although the motor isn’t as hot as before,  it’s still too warm.  Any concerns about having the CG further aft were quickly gone,  it still just walked into another perfect landing.

The center of gravity is now at 85mm from the leading edge or a couple of mm behind the upper wings hold down screw access hole.

As the more valuable 6S (35) amp Robby motor controler isn’t needed for this 3S setup,  a older Multiplex 17 amp 2-3S motor controller (fixed timing,  the latest ones can be set with the correct lead for outrunners) was installed.  Weight for the propulsion complete is now ten grams less at (170) grams.

On replacing the Graupner 10X6 blades with a 3S 2200 mAh LiPos and 10X4 Aero-Naut blades it drew (10.5) past peak amps at (11.5) volts for (120) watts-in.  More drop then expected,  and not as much power as wanted,  so Aero-Naut 10X6 folding blades were tried which resulted in (13) amps draw for (150) watts-in.

That’s two amps difference between the different manufacturers for propeller blades marked 10X6,  which is why you must use a meter every time you change anything!  Higher performance LiPos would push the power right back up,  which in this case,  would bring overheating right back.

I’m starting to wonder about Multiplex rating this combination at (200) watts-in as this individual motor,  bought used from a pilot who destroyed the Park Master it was originally in (at a swap meet,  for Euro20/$26- including a serviceable motor controller) overheats.  Maybe this motor has worn bearings that drag.  But,  while it lasts  (as the motor overheats performance falls off) that (200) watts-in was a really satisfying combination.  Not as good as the bigger motor,  but fun all the same.

Overheat a cheap motor just once and the magnets may quit being magnets,  the cheap lubrication in the bearings gives out,  so the bearings quickly turn into gravel filled sleeves,  and you find out where the difference between inexpensive and cheap is.  Note that quality manufactures list the temperature the motor can withstand,  and the rpm rating of the bearings.  A single hard landing can bend that economical to manufacture soft shaft reducing cheap to,  junk.  As bearings wear out,  they not only increase the load on the motor,  they heat up.  Usually though you can feel it by pulling forward on the propeller and turning the motor over,  the bearings feel “gritty”.  If you insist on using cheap stuff,  at least check the motor’s current draw now and then with a wattmeter as if the current draw goes up,  it’s a warning of impending doom.

To secure the motor mount all four adjustment screws are all the way in,  longer screws for the top will be required to get the needed down thrust at the motor.

CA dissolver was used to free the brass nut securing the upper left aileron to the lower left aileron.  If you just try to turn the glued on control arm nut out,  the aluminum shaft breaks!  The hole had to be clearance a little and then all set back.

While I was at it the “chin” was sanded down some more to get the longer blades to fold flat.  Although a thin layer of fiberglass was already on the nose,  once the contours are decided on it gets a lot more fiberglass as although the weather is delightful now,  winter,  including rock filled frozen molehills,  is already scheduled.

Flights 6 and 7  Under Igstadt  although wind varied from twice walking speed to zero (depending on if a thermal pulled or pushed) still just great flying conditions.  Another pilot,  concerned about that the (Mission Bay ten o’clock wind tunnel) speed wind might be too much,  was encouraged to test fly something new from Horizon/Spektrum,  a stabilized twin motor trainer.  Contrary to most people,  I like some wind for test flights as the airplane is already nearer flying speed.  Once a week I look for work,  otherwise,  what a great moment to just be there and fly.  The trainer was just what it should be.

Well,  Gemini II with the “medium” motor flew and rather well at that.  Way better then expected,  evidently the greater efficiency of the larger propeller more then made up in the reduction from (200) watts-in on 4S to (165) watts-in on 3S.  Not as much acceleration as the bigger motor,  if you curve into vertical it keeps going up about sixty yards,  instead of until the battery goes dead.  Since a hundred yards away is about the useful limit for a Gemini that’s not much of a loss.  So not as powerful,  but almost as much fun.  The trim at the elevator had to be taken back,  on landing it was only slightly up.  As the battery was as far forward as it fit the balance can still be shifted rewards a little.  Handling improved although with the motor still needing down thrust it still took a lot of down elevator to fly inverted.

However,  in the way that too drunk to fish is too drunk to swim,  too hot to touch is too hot to fly.  On landing after twenty minutes or so of flight 6 the motor was too hot.  It was impressive while it lasted.

The aileron servos were advanced with individual trim to 50%,  the overall aileron throw reduced to 90%.  While re-centering the ailerons it was determined that the rotating nut connecting the left ailerons was too tight,  so the connecting rod was removed.  The second flight was managed for minimal output,  but this isn’t a kite.  Out past a hundred yards,  contrasting paint or not,  the orientation blurs out.  After the second flight the motor was still too hot.

Want something to just sail around,  sometimes close in right past you and then way far away;  Get a Fun Cub or Twin Star II instead.

The 8X5 Aero-Naut folding propeller blades were replaced with 10X6 Graupner folding blades.  On 3S 2200 mAh LiPos it drew (15) past peak amps.  This combination was ho hum in the Fun Star,  I wasn’t expecting much so if the blades didn’t fold flat,  so what,  I wasn’t expecting to make many flights with this combination.  It would take a wider connection bar to get them to fold,  or a lot more material removal.  The reduction in weight moved the CG from on the upper wing spar back about a finger width to about the access hole in the top wing.

Flight 5  Wiesbaden Waldacker  Best possible flying weather
Well,  this combination was “ho hum” in the Fun Cub,  other then not overheating as much in Gemini II it wasn’t any better.

The “big” used HiMax was removed and replaced with a used “medium” HiMax originally specified for the Park Master and in the USA,  the Gemini.  Odd,  when you read the description of the Gemini on the Multiplex USA web site the propulsion is specified as (200) to (300) watts-in,  which is the Acromaster motor.  But,  the Acromaster motor/controller/prop aren’t listed as being for the Gemini in the motor combination section.

The 8X5 Aero-Naut propeller on 4S LiPo (80) gram motor combination overheated in the Fun Cub if run at full amps more then a quarter of the time,  although the Fun Cub always did have problems getting enough air over the motor (even the bigger 35mm motors) despite additional clearancing.  On my “small” 4S 1100 mAh LiPos it drew past peak (14) amps at (14) volts.  Unfortunately,  it took the a 4S 2200 (220) gram mAh LiPo to make balance,  so the net weight of the propulsion stayed just the same.

(135) gram HiMax 1130 35mm motor with mount and controller    (220) grams
(105) Plettenburg (good stuff) sport motor  complete            (195)
(80)    HiMax Park Master  complete                        (180)

Although possibly less then ideal,  an old stock (new stock ones are even better) Aero-Naut hub with the blade support bar at 47mm pivot center to pivot center was used.  It was time to make a choice,  I knew it worked and it was sitting right there on the bench.

The nose was clearanced using a sharp knife and 240 grit sandpaper.  You don’t get a nice smooth finish shaping Elapor by dry sanding,  even after coating with fiberglass.  Getting the nose to fit a folding propeller is specific to which propeller blades and carrier.  Changing folding blades or bar width can require a different “rake” to the front end.

Having nearly crashed the night before when I lost orientation,  the vertical stabilizator and top of the horizontal stabilizator were painted red,  the outside edge of the top wing yellow.  More color to be added later.

Flights 3 and 4  Oestrich-Winkel high above the Rhine River   
In that really still air I could conclude that since roll and yaw axis are neutral with everything straight that the airframe is in alignment.  I can’t in words explain just how much building skill and fiddling that would take with a biplane built up out of individual pieces.  The required measurement equipment alone would cost more then the Gemini airframe kit!  I couldn’t decide on the center of gravity where Multiplex recommends it at three inches back from the upper wings leading edge (right on the spar) though.  It takes more down elevator for inverted flight then what seems right,  might move the CG back some next time.  Throw on the elevator is about right,  the rudder might be able to use a little more,  maybe the ailerons should have less then the recommended travel.  It will take tuning at the computer radio (Spektrum Ds6i) to get the 20% differential of more up on the Ailerons then down.

The Editor has wondered what flying here is like.  When it’s good,  like then, it’s really good.  Clear sunny sky with that always a pleasant view of the green Rhine River valley from Mainz to Bingen.  The down hill in front of us is vineyards for about a mile,  forest behind and across,  little villages here and there,  zero wind,  not much city noise except the jets out of Frankfurt,  perfect short sleeve barefoot temperatures (winter is on the way),  six all very experienced pilots bantering with each other the way a society of men does when they aren’t in competition with each other.  One of the pilots is old enough to have had to have built his own servos!  Germans stay in one place longer then we Americans,  these guys have known each other,  and flown in the same club,  for from twenty to forty years.  I’m just fifteen miles from my apartment and yet this area has a dialect that I can only with difficulty and practice  understand.  Present were from an inexpensive (one square foot) quadracopter (his biggest RC airplane is a ten foot wingspan Wilga set up to tow gliders) on up to two expensive electric helicopters and back down to my simple Gemini.

It was flying with a club at it’s best. I just played with the Gemini Mark II as “flying is only learned by flying”.  As is typical for me I didn’t try to make it perfect for the first flights,  the tuning begins after establishing a beginning,  often there are unexpected things to modify,  or not,  depending on how it actually flies. 
I’ve been a guest often enough that they know I can get more out of a Multiplex airframe then anybody else.  Any questions to my flying skill were answered six years ago.  As usual,  how do you get it to stay up so long? From the very experienced pilot with the Wilga,  it flies great,  why do you want to change anything?  While not as obsessed as the helicopter pilots,  I too do not accept the first combination as ideal.

However,  that club doesn’t have all that many members,  and they only fly under really good conditions.  From November through May two thirds of the time I make the drive out I’m all alone there,  sometimes either rain or,  ugh,  snow precluded even getting to the flying field,  any wind faster then walking pace keeps the rest of them at home.  From November through March I wish I could make a living back in San Diego and move the wife there.

Not all clubs are that pleasant,  a nearer one,  with four times as many members,  where there is always something going on (even if they have to park half a mile away and carry everything in their hands as they trudge through a foot of snow),  the club members don’t all get along with each other all the time.  Despite the biggest airport in the country being almost in sight,  and between two freeways,  they fly everything you ever thought of or read about,  even the big kerosene jet turbine stuff.  Maybe they are justified in being paranoid.

As close as we are to a busy two lane highway at Mission Bay (seconds away at even the flight speed of a floater),  maybe more of us at the SEFSD ought to be more safety conscious too.

As I observed last March 2012 at Mission Bay there currently is insufficient room for the “intense,  their whole world is that physics deifying aerobatic helicopter” flight and simultaneously “I need not just the whole sky to fly,  it takes not only the entire length of the runway but a quarter of a mile straight out past it to land” fixed wing flight.  One side or the other is either going to have to yield some time…

If it’s me,  I’d leave my airplane on the ground and enjoy watching the helicopters,  they only have

This is an original
for the Silent Electric Fliers of San Diego (SEFSD) year 2012 article.  I’m anything but satisfied with the shilling magazine write ups,  or the stuff in the Internet.  This is the truth,  the whole truth,  and nothing but the truth. At which,  after just a dozen flights on the second Gemini (plus a hundred on the first one I’m going to end this article as it should be evident that reports made after just three flights haven’t proven they have it right,  or had a chance for crummy quality and durability issues to become evident,  or explore other combinations.  I hope this answers some questions,  and provoked some interest,  in things that censorship by omission,  well,  omits.

There hasn’t been anything interesting,  or unusual,  turn up in the German language magazines I read worth translating for the club back in San Diego California USA lately.

Our Editor has asked for some coverage of events,  but,  due to my limited financial range,  the three RC Flight Weekends I spectated were more of a “you had to have been there” kind of thing.  The competition events were too far away.

Had I found work the spring of 2012 back in San Diego,  see you the next “hope things turn up someday” trip back.

Conclusion
As we built our way through the Multiples series,  the Gemini proved to be the biggest flying thrill in the whole bunch.   It is a consistent performer,  always a blast for both the pilot and spectators.

The Gemini has proved to be a great RC sport aerobatics airplane for average fliers.  If you were looking for an F3A substitute,  this isn’t it,  the flight envelope is not symmetrical,  for which however it is stable.  The friend with our first one thinks it’s the greatest.  Flying his a hundred flights with indifferent tuning,  a very ordinary propulsion combination,  ground off props,  nose heavy,  aileron differential in the wrong direction,   he has average middle age reflexes and eyesight,  with which he is representative of at least half of the SEFSD members,  and yet the Gemini is what he enjoys flying the most.  Oh,  I tuned my personal Gemini,  but truth is for a breezy day when you just want to effortlessly throw an airplane around,  it didn’t make that much difference.

The Gemini has the highest ratio of enjoyment to outlay in cash and build/repair time of any of our RC airplanes.  It also ground off more folding propeller ends (more cash then the kit cost over the first hundred flights) then all the others combined.  Our second one,  I did the omit the landing gear and use a folding prop thing.  As much as I’d have enjoyed reporting on some seldom mentioned great propulsion combination,  the from Multiplex Tuning propulsion (that’s the Acromaster set to you Americans) run at about (270) watts-in at (25) past peak amps on a 3S LiPo proved to be the most satisfactory of the combinations we tried.

If I were flying back at San Diego’s Mission Bay,  and didn’t want to spend more then the minimum assembly time on a Gemini,  the only changes I’d make is to tape the leading edges,  reinforce the attachment of the control horns to the foam,  make sure the paint or decals contrast top and bottom and,  if you are up to it,  use a folding prop.

Although it likes still air too,  the Gemini flies particularly well on days with some wind,  such as the ten o’clock Mission Bay wind tunnel.  We Unter Igstadt bei Wiesbaden am Rhein (Germany) glide a lot,  our flights on 3S 2200 mAh LiPos with the specified “Tuning” HiMax (135) gram motors go on for about twelve minutes.  Some 4S combinations and better efficiency motors go on even longer.

The flying and glide speeds are a little higher as compared to a “floater”   Reasonable flying distances are from about ten yards away out to a hundred or so,  after a couple of hundreds the orientation fades away.  This one must have color which clearly demonstrated which way is up.  My personal paint scheme has a distinctive red tail color.

Let’s do a keep things in perspective check
The Multiplex Gemini is a SPORT aerobatics airplane made out of foam,  for relaxed fun flying,  that can be tossed in the back seat.  If our competition F3A pilots could score well in contests using a five hundred dollar foam airplane with inexpensive servos,  instead of their thousands of dollars combinations,  that’s what they would use.  But it still wouldn’t be the Gemini,  the design sacrificed some neutral acrobatics capacity for being stable and easy to fly.  I’d still have our aerobatics aces check my airplane for me though.

But then those F3A pilots either have left over skills from an earlier era when you had to build your own airplanes,  or access to people who do.  They likely spent more on their transmitter then a whole fitted out Gemini costs.  Do you really have the required skill to program a motor controller’s rate of rpm adjustment for constant speed loops like the experts use,  or the budget as they cost more them the whole propulsion from Multiplex?  I balk at Euro24/$30- for a servo,  they spend three times that,  for which a matching high frame rate transmitter is required.

The Multiplex Gemini is as close to self jigging (zu Deutsch  selbst Heiling) as any foam airplane ever is,  but you have to build it yourself,  they don’t offer an RTF one.   I can’t put to words how important it is  that by just gluing it together a Gemini is straight!  Figure about six to eight hours of assembly.  If I were doing it assembly line style,  I could slam a Gemini together in four hours,  if every piece of equipment worked perfectly on the first try and I’d built one recently.  I expended about a dozen hours on mine,  that included four hours of fiberglass and sanding.

If just getting in the air quick is a goal,  and you can make the eventually necessary repairs,  an ARF might be a different choice.  First crash though of that lazered out every last bit of not strictly in flight required ready to fly machine,  and any time spared in assembly will be long gone.  Ask around the field,  nearly every single ARF has to have some of it improved to sustain use over multiple flights.  That and figure in that no RC airplane so easily allows using “anything that will fit”  for propulsion as the Multiplex stuff.

Related Multiplex Airplanes
In case you were wondering,  on nearly windless days,  the Park Master can be great flying too,  the flight profiles overlap with the Gemini with the Acromaster being about in between.  Although the Park Master isn’t as durable,  at least shortening the propeller ends with ground strikes wasn’t an issue.  The Acromaster is closer to F3A,  but I wasn’t thrilled with mine.  If your reflexes are falling off (or were never all that good to begin with),  a Park Master might be a better choice as long as you don’t fly when the wind blows.  Although somehow average pilots destroy a lot of Park Masters,  overconfidence being the reason,  not any problem with the airframe.  It might come down to eyesight,  you can keep a Park Master right up close,  hover it even,  the Gemini is going to need some swing through.

The Park Master and Acromaster can hover and do the latest balance it on the prop stunts,  the Gemini is for traditional fluid aerobatics.  I’d pass on the Acromaster,  I cut mine up after fifty ho hum (zu Deutsch so so la la) flights,  the newer stuff optimized for modern equipment is just so much better it wasn’t worth bothering with.

Things show up over fifty flights that most reports omit,  like eroded leading edges,  control horns that tear out,  hinges that rip,  tail wheels that don’t hold up and motors that wear out quickly.

If you are really still a beginner,  or recognize that you aren’t going to have the skills for a traditional aerobatic RC airplane,  and still want to “stunt” some;  A Twin Star II with the latest brushless stuff makes a great,   if only semi-acrobatic,  RC airplane.  Although the initial investment for the brushless version is higher then either the Gemini or Park Master,  the configuration precludes equipment ruining prop strikes.  All four,  Gemini,  Acromaster,  Fun Cub and Twin Star II like the 3S 2200 mAh LiPo size.  The first three can all swap motors and the motor mount.

What does it Cost?
For a step by step with pictures description of basic assembly of a Gemini,  see the Internet.  There are some worthwhile picture by picture assembly articles in there.

Did you really believe the “I just made three flights and everything is great” stories you read elsewhere are really the truth,  the whole truth and nothing but the truth?  Think about it,  did they have anything to compare to other then it functioned at all?  They didn’t fly them long enough to know where the weak stuff is!  I take longer then that just getting the adjustments right.

I quote my prices in Euros,  because I’m currently in Germany.  Check around for what you can get in the USA,  my favorite source just closed their doors.  Considering the ease with which the transmitter and receivers can be changed,  I don’t include them,  or the batteries,  in my price estimate.  Although now a little out of date,  see my book “So,  You Want to Fly RC” and ask around at the club for details.  My book also includes some assembly tips.

Before you buy the cheapest stuff, 
read some of the Internet forums where things,  unlike the magazine articles,  aren’t working right.  Most of it is due to cheap junk equipment that doesn’t perform as advertised.  See my previous article “Censorship by Omission” and the detailed “what a piece of shaving cream” inexpensive motor experience in my Acromaster and Fun Jet Lite articles.

Is throwing away a third of the available performance confusing watts-in as being the same as watts-out worth saving thirty or forty bucks on a motor that quickly gives out anyway really worth it to you?  What a piece of (expletative deleted),  that HP outrunner in the (28)mm diameter (35) gram size was.  Installed in Fun Jet Lit and Mini Mags,  it was so poorly manufactured that the output shaft broke after ten howling flights.  After the three dollar replacement shaft,  and fifteen bucks worth of collet and prop lost in the long grass out in Temecula (plus the loss of a flying session),  were replaced,  it ran quieter,  but overheated,  the efficiency was awful.  The otherwise identical airplanes were great performers with $80-,  and terrific performers with $120- motors.  The whole airplanes were just generally disappointing fliers using an $18- plus tax motor.  I went through a similar experience with a Brand Extreme motor in an Acromaster and Fun Cub.  A Tau of Poo experience,  at least I tried it,  how else was I going to find out as nobody else seems willing to commit to print that cheap isn’t as good as real.

I have used several Brand Jamara (28)mm diameter (35) gram motors in a couple of different airplanes on 3S LiPos.  For about thirty flights until the bearings start to give out,  bought for Euro12/$16- new,  that price at the twice a year RC show in Lampertheim Germany,  they have been worthwhile.  They would no doubt have held up a lot longer swinging a bigger prop at lower RPM on 2S LiPos.  I just used the included motor mounts in two balsa flying wings.  They can be great fun in a Mini Mag and Fun Jet Lite when almost,  but not quite,  completely broke.  They would be worthwhile in a Twin Star II.  Although the bearings are about the same,  recent ones have noticeably less effective magnets and lower real efficiency.  Not that the efficiency was as good as the stuff costing two or three times as much in the first place,  but they were good enough to be fun.

My first Brand X motor,  in the fifty gram size,  bought for Euro (27),  burned up on flight twenty seven as I didn’t realize the bearings had given out.  Maybe most people don’t put enough flights on their stuff to notice.

You can easily change the propulsion
of a Multiplex airplane quickly (under an hour,  two screws plus the electrical and electronic connections is all it takes) as often as you want,  it’s the glued in servos that are the biggest initial decision.

It’s about sixty bucks more for the much better resolution HiTek HS-65s over and above the cost of the usable HS-55s.

I just had to try them,  in my usual test bed of late,  a Fun Cub,  the $6- (plus tax),  (8) gram ones from Hobby People did function fine,  for forty flights to date.  A clue here is speed,  cheap servos that have to hunt for position are less of a nuisance when things are happening slower,  precision in a flexible flier Fun Cub that can fly at walking speed doesn’t matter as much as in a faster Gemini.

I wouldn’t want $6- servos in my Gemini when $15- ones work so well.  That’s four times (15-6) works out to a thirty dollar difference.  The reasons for putting $15- HS-55s in the wings of my Mark II Gemini were mostly being almost broke,  I had them already,  and wondering,  how much difference there really would be.  They cheapened up the whole airplane some,  although less then we expected.

There is no amount of fiddling at the transmitter or pilot skill that can replace quick,  accurate,  repeatable positioning of the control surfaces by quality servo(s).  See the above comment about our SEFSD F3A competition pilots and their airplanes.  And note all the Internet write ups of Geminis where,  disappointed at cheap servos,  they finally installed decent ones and the whole airplane flew better.

At a hundred flights the big,  (35)mm diameter (135) gram,  intended for the Acromaster,  HiMax motor in a friends Gemini run at (25) amps on 3S LiPos is just starting to show some bearing wear.  Heat was just never an issue for either the motor,  controler or battery.  I used an identical HiMax motor in my Fun Cub at (350-600) watts-in on 3S,  4S and 5S LiPos.  That combination can take a Fun Cub straight up to three times the Mission Bay height restrictions at a count of ten,  without any problems to date.  No doubt run continuously at that input overheating might/would have been a problem,  despite the modifications to the Fun Cub to get more cooling air over the motor,  but you can’t run a Fun Cub that hard for much more then a count of ten anyway!  It was actually too much for an Acromaster.  I don’t know how many flights that (80) gram Fun Cub/Gemini/Park Master motor had on it when I bought it used for Euro20/$25-,  because the former owner wiped out the Park Master airframe he originally had it in,  but at forty more flights on 4S LiPos at (200) watts-in the bearings are just starting to show some extra drag.  Heat becomes a limit above (150) watts-in for that (80) gram motor if you run it continuously at that power level.

For good brands (Graupner,  Robbe) in the Euro45/$60- price range I’m getting seventy five flights out of motors in the forty five to sixty gram size before the bearings give out.

I have yet to meet anybody that wore out a Neumotors,  or a Hacker. A great Neumotors,  with their best quality everything,  if they have one in a low enough kV for your application,  puts out more power from the same input for a longer duration at (70) grams then Hacker outrunners (heavier for the same output (about (85) grams)),  but outrunners are available in lower kVs that swing a more effective diameter propeller the direct drive inruners.  You get what you pay for.

A decent,  that is it has good magnets,  induction plates,  bearings,  quality control and so on,  in the (28)mm diameter (80) gram size as an inrunner now costs about seventy to ninety bucks.

A (105) gram Plettenburg outrunner in an Acromaster way beats the (135) gram HiMaxes,  but you have to watch heat build up,  they cost about twice as much,  and you have to ask around to find an equivalent in the USA.  Since either motor puts out more power then we had any requirement for,  I don’t see the (30) gram weight saving justifying the additional cost for most sport pilots.

Follow along
as we make minor improvements to survive a hundred tip over on the nose landings in long grass are detailed.   With the Mark II Gemini we began the puzzle of changing the propulsion to take advantage of affordable 4S LiPo systems.  We don’t get any chance to taxi where we fly around Wiesbaden,  for Mission Bay it might be worthwhile to put on a better tail wheel assembly,  like the one from the Mentor.

What did it cost
Euro (119)    airframe kit
Euro (7)    glue
Euro (24)    Karbonite better accuracy,  but still analog,  servos  HiTek HS-65(s),  four
Euro (130)    Tuning Motor/Controler/Prop from Multiplex  In the USA that’s the combination for the Acromaster
Euro (5)    paint,  instead of,  or in addition to,  the supplied decals
Euro (3)    Extension for the motor controler
Optional

Euro (40)    folding prop assembly  Maybe not necessary back at Mission Bay,  we couldn’t fly without them Under Igstadt bei Wiesbaden am Rhein.
Euro (110)    Gemini/Fun Cub propulsion  A delete option as it just doesn’t perform as well as the Acromaster combination.  If your reflexes are really slow though it might be a pleasant combination and it costs a little less.
Euro (15)     standard analog servos  HS-55s  This is a delete option as the some of the available precision gets lost,  if you hit the ground they are easier to break.
Euro (5)    carbon fiber wing spar  Cuts the weight (15) grams
Euro (24)    carbon fiber landing gear  Necessary Under Igstadt,  but probably not needed at Mission Bay.
Euro (10)    fiberglass  probably not needed at Mission Bay

If you can’t afford the best,
so that you have to chose between a computer radio, karbonite gear better positioning servos and the tuning motor and controler;  I’d go with standard servos,  the Gemini will still fly well.  The Gemini really benefits from getting the servos set for the balanced travel (separate adjustments before the trim at each servo so the throw is correct both directions for all servos,  the ailerons need tunable differential which cannot be achieved by purely mechanical adjustment,  and must match throws) only possible with a computer radio.  Our competition pilots all use exponential,  all the German reports include the authors using it,  I couldn’t quite match the pilots I flew with at my skill level until I started using exponential,  maybe you should too.   If that’s not enough,  get the smaller,  less expensive,  standard propulsion.  If you can only afford one really good servo,  put it in the elevator.

Beyond some throw a control surface turns into a spoiler.  I had for years tuned my maximum throw to just a little less so as to be able to save the airplane when a gust of wind hit it.  But,  that meant that near the middle I had minimal stick throw.  In particular coming over the top of a loop when I knew the wind was going to push it out I just couldn’t get it right.  Using exponential meant that instead of the tiny “minimum” input that I could now really move the stick.  It was after letting a front line competition somebody fly my Gemini using my transmitter (and his propulsion complete) who commented “I don’t understand how you can fly that well without expo” that I started taking full advantage of that Ds6i.  Then I realized that I need to separate holding the transmitter from actuating the control sticks,  next swap meet I buy a sender holder.

But what about an “average” I already have a perfectly good four channel radio,  everybody else is happy with cheap servos and this hundred gram Far East motor?  Even with a minimal radio,  $6- servos,  and a low efficiency motor a Gemini will still be the greatest aerobatic airplane possible at the lowest price.  Just make sure you either have the skill to assemble and tune it,  or get some help.  Components that cheap often don’t function as advertised.

When in San Diego I used to buy at ShurFlite.  Sad,  from reading the club newsletter down the Internet (thanks guys,  it’s nice to be virtually back with you) that option,  partly due to the Internet,  is now gone.  My last trip back I haven’t seen the roads so empty in my adult life.  I estimate the actual idleness at about a quarter of the available population that would otherwise be working (me included),  and shopping.  The owner was a club member,  they were willing to secure replacement Multiplex parts,  now we will have to order them.  The prices in Euros are just to give you a relative amount,  to convert to dollars multiply Euros by 1.3 although the rate changes constantly.

Building/Assembly
My reports are restricted to above and beyond basic assembly,  there’s just no need to reinvent the wheel for what’s already in manufacturer’s instructions and on the Internet.

With the exception of the admonishment to take five minutes and clean up the leading edges with sandpaper!  How can the Internet Authors leave out that critical step?  If the designer himself could sit there with you;  He’d likely remind you that for a staggeringly low price Multiplex delivered a great,  detailed airplane,  you have to clean up the casting flash yourself.  That blunt edge/lip at the wings leading/trailing edges is all the casting process allows.

My airplanes fly better and last longer then anybody else’s,  part of it is those clean rounded edges from sandpaper and heat shrunk on tape.

After that,  why mine flies so much better then yours is things like spending (8)% of the cost of the kit,  per pair,  trying out different folding propeller blades,  and not being satisfied with,  the servos are glued in and don’t fall out so they must be right,   to get just the right combination.

I don’t always punch a clock,  but my Neumotors battery charger keeps track of time,  my basic clean up of the airframe foam with a piece of sandpaper took an hour.  I don’t expect that the casting “pips” really have an effect on flying,  but smoothing them over makes taping,  fiberglass,  decals and paint go easier.  In this case perfectionism is knowing when to quit,  the wings are lightweight,  low density Elapor that is inherently flexible and compressible,  stopping before going too far is important.  Try wet sanding.  I’m proficient at applying Brand Tessa packing tape number 4124 (See the Editor and Steve Neu for how it works as I gave them both a roll last trip back) that took about an hour.  The paint I use sticks to the tape,  and Elapor without a primer,  just fine,  but then it costs three times as much per can as the cheapest stuff at Home Depot.

Comments about Propulsion
I tried all kinds of different setups,  in the end the Multiplex Tuning combination with the (135) gram HiMax motor on 3S LiPos was the best value.  I’d be delighted to report some other combination that worked wonders,  as was the case with the Mini Mag and Fun Cub,  or even just a striking improvement,  as was the case with the Acromaster,  but that just wasn’t the case for the Gemini.

With their weak (but affordable) magnets,  durable bearings,  and set the timing right for outrunner motor controllers;  If you don’t know where else to start,  as noted in my book “So,  You Want to Fly RC”  just buy the Multiplex propulsion setup,  switch to a folding prop,  and be delighted with it.  We just didn’t want the maximum thrust available,  Aero-Naut 10X6 folding propellers were our favorite,  even if 11X4 through 12X6 functioned.  Graupner’s 9X6 are different then Aero-Nauts 9X6 (Graupner at the low end of the RPM scale,  Aero-Naut when you are going to turn it up higher,  carbon fiber for if you can afford the best),  something you have to try with each combination it as that varies with drag and rpm.  I have a three blade Aero-Naut hub that although expensive (sixty bucks for the hub and spinner,  to get three blades you must buy two pairs of folding propeller blades for each size),  and heavy,  it functioned very well too.

Follow the results section of the Gemini Mark II as I try different propulsion components.  A clue,  low power wasn’t worth while,  neither was high power,  or cutting the propulsion weight down.  Unexpectedly,  unlike the Mini Mag,  Fun Cub and even Acromaster,  where there were some unexpected propulsion combinations so good that even Multiplex representative were surprised,  this time Multiplex hit the Tuning combination just about right.

Want a better performing propulsion combination then the one from Multiplex?

Use their price as a starting point,  and go up in price from there.  In our experience (to date a hundred Gemini Flights on the Mark I,  every single landing the motor took a hit and the performance has hardly changed,  three other HiMax motors all working fine after being flown hard) anything less expensive then the Multiplex package,  is just cheaper that either won’t last all that long,  or perform as well.  A big omission from almost anything in print,  it’s watts-out we are interested in,  not watts-in.

A geared Neumotors with a Castle Creations fixed timing motor controler would be the ultimate,  a Hacker outrunner with a motor controler where the timing can be tuned to the motor as a slightly more economical big step up too.  We liked about (150) to (250) watts-out.  See (25-30) amps in at (11.0) volts before the motor controler and (0.72)% efficiency.   Most people have a grip on those first two parts of the equation,  it’s censorship by omission of published articles that seems to blind them of that net efficiency part.

And then don’t go and throw it away using a cheap or mismatched prop!  That motor delivers efficiency into a load,  be it a great carbon fiber prop,  or a paint stirrer.  You don’t get to thrust until the propeller finishes the equation.

However,  the improvements from different motors wound up being mostly longer runs from the same battery and better amps response as the net weight is just about pre-set by the airframe.  To reaffirm that;  If you don’t really know what you are doing with motor/controler/batteries,  and want to keep the net price per flight down,  just buy the Multiplex Tuning setup.  Even if at first it seems expensive.

That cheap Far East stuff,  be it motors,  controllers,  servos ect,  is just that,  cheap.  Something that becomes evident around twenty to thirty flights.

With the Gemini’s short nose cutting back the weight of the motor helps some,  as does a smaller battery,  there isn’t much room in there.  If you have the skills,  might as well put some reinforcement fiberglass inside the fuselage.  I cut that 3mm X 3mm  foam beam out to make the inside forward fiberglassing practical and make more needed room in there.

Our Geminis have to be arrested wire into the grass,  the whole fuselage tends to bend between the leading edge of the wing and the fuselage,  but there is a strict upper limit to what you can do to improve it though.  See the above comment about aren’t a hundred or two flights enough.  The slight additional weight of the fiberglass doesn’t seem to mater.

I’d have liked to have created a fiberglass beam down the middle of the inside of the fuselage,  but that would have required cutting out the wing hold downs top and bottom.  The weight of the foam is negligible,  the hard plastic parts and nylon screws weigh (10) grams.  Since they also lock the fuselage in alignment for this Mark II it’s not worth the risk.  Maybe for Mark III…

Our first Gemini
Was assembled mostly stock with the much better resolution (and stronger too) HS-65 (11) gram servos,  which made for great precision flying.  With it’s stiffer wings,  the Gemini doesn’t flex/wallow around like many foam airplanes.  It and the Acromaster are the only ones of our foam airplanes to date that,  without reinforcement,  didn’t develop a “hinge” just outboard of the wing spar(s).  Right from the start it received taped leading edges and hinges.  The erosion to the leading edges,  so common with foam airplanes,  still hasn’t happened a hundred flights later.  This airplane needs contrasting top and bottom colors,  which the oh so good from Multiplex decals don’t provide.  If you still remember how to use a covering iron,  the heat shrink packing tape requires a covering iron,  the decals benefit from it too.

After getting the first Gemini flying we quickly realized:

What an improvement the more expensive,  better resolution servos are.

When landing we can’t reliably hit the twelve foot wide asphalt farm roads available,  so the grassy fields get used as a landing place.  At which every single landing results on hitting the nose,  sometimes it tips all the way over on it’s back.   Since the leading edges had already been hardened with packing tape,  the First modification from field experience with this,  to us,  then new airframe,  was a folding prop.   Then the nose was “armored”.  Ultimately,  aren’t a couple of hundred flights from an airframe that can be assembled as quickly as a single evening,  or beefed up over two days,  for a cash outlay of a hundred and fifty bucks enough?  The Gemini kit is so economical that the better carbon fiberglass landing gear (which outlasts the foam and can be reused) cost a quarter as much at the rest of the airframe.

That slight improvement at a significant cost isn’t unique to the Gemini,  if I ever built another Fun Cub with landing gear,  it would get the Mentor tail wheel,  which,  as a replacement assembly,  costs about a quarter of what the whole Fun Cub kit costs.  Our Geminis tail wheels didn’t ge