Monthly Archives: October 2012

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