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

A Tale of Two Multiplex Geminis, pt.1


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.

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

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.

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