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

A Tale of Two Multiplex Geminis, Pt. 2

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

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