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

The Swap Meet Gemini Biplane



And another warning, this project failed utterly. The foam was just too old, too much of the rest was damaged beyond recovery or so cheap it should not have been used in the first place. I’ve had a hundred foam airplanes, the tail surfaces warped like nothing before. I grafted the tail of a Fun Cub on, it wasn’t worth the effort.



Warped Tail from Behind


11 May 2017 (We.) I’ve lost track of how many times I’ve done fiberglass on this airframe, in about the ninth and tenth time the upper ailerons were fixed in position and some more minor dents spachteled and sealed.


It might have been warped up the whole time, on application of fiberglass hardening of the horizontal stabilizer it was evident the right side was up. It’s starting to feel like too much fiddling with this airframe.


02 May 2017 (Tu.) It was time to do it. Although not run up with them, two high discharge 3S 850 mAh LiPos were ganged together. I’ve forgotten the balance point.


Flight 1/20 Hidden Valley Flying field 02 May 2017 (Tu.)


It may be tail heavy and it pulls up too much on application of power. The top wing flutters at some speeds and attitudes. So bad that the upper wing went blurry. The left aileron was not secured, the corrosion made the nut feel tight, it wasn’t. The flight was stopped way early.


30 April 2017 (So.) The airframe is now built out to an RC airplane. All that is left is to fit batteries and go fly.


The white plastic motor mount it came with was not going to function by just redoing the back side with fiberglass. The hack job done by the previous owner and the small bolt circle for the three screw mount transmission were just too on the not enough. So, a plywood plug was fitted (to be drilled out later) and another bigger plug, a single layer of fiberglass in between, two over the top, were done. Sanded even, not much fiberglasss left on the front, but it should have enough strength to secure the transmission(s).


Well, after fooling with it some, it wasn’t worth doing. Just too much clearancing for the folding propeller. I’d be better off just making a new mount out of aluminum.


After reviewing the situation, all of my RC fleet is over old, the foam tired, it’s time to start over with fresh airframes. There is only one thing holding me back, money, or rather, the lack of. As it turns out one of the last places to get Multiplex stuff anymore is the best venue anyway, Discount Hobbies in San Diego CA. In the “musical chairs” of finding a summer project I wound up in Rialto CA, so, the hobby shop in Corona (Hobby City) is the venue of choice. They can’t remember who to order a Twin Star II or Fun Cub from. So, I bought parts to build out an Horizon Hobbies Apprentice at Hobby City (Corona CA) and since the only easy source for a Multiplex kits is at Discount Hobbies (San Diego CA).


About the Multiplex Gemini


Gem 5



This will be my third personal Gemini and the fifth I have reinforced. Now obsolete (no longer in production) they are great sport biplanes as in semi-scale, and semi-aerobatic. Actually, it is about as aerobatic as the airplanes it is a model of.


Between a friend, and, in Germany’s Rhein-Main with a couple of dozen RC airplanes at our disposal, when we just want to go over to our flying place (a farm field with all-weather access roads) to relax and play with RC airplanes the Gemini is THE favorite with the Twin Star II in brushless for use on days with a little more wind or mud. For the Gemini it takes reinforcing the nose, for the third and fourth ones we deleted the landing gear, modify the nose (clearance and fiberglass reinforcement) for folding propellers and reinforcing the wings. We liked them so much that dismayed when our first onew wore out (two hundred and one hundred and sixty flights respectively) we found new, old stock kits. Furthermore, they fit in my camper. A close alternative, and still available, is a Reinforced Fun Cub, also covered in an archived SDFSD article.


This is an original year 2017 article for the Silent Electric Flyers of San Diego. In the SEFSD archives is a previous article about building up a Reinforced-Gemini from new kits and the experience of twenty different propulsion combinations.

This new article goes over the common mistakes made by a first owner and trying out a couple of new propulsion components including two that should be the “ultimate” choices. That correcting mistakes, and the advantages of correct propulsion, are what transform my notes to a general interest article.


Included is adding enough fiberglass and redrilling from the original four hole to three holes of the first series of plastic motor mount to mount a modern NeuMotors 1105 series with a 4.4:1 transmission. It will be to be run at about half of the available (600) watts-in and an (80) gram sport Hacker to be run at the limit of (300) watts-in. In particular, having once modified a motor mount for the NeuMotors transmission (the modern style with three mounting screws) the same combination can be used in a Multiplex Fun Cub or Multiplex Mentor or Fun Cub, by just undoing a couple of screws and moving it over. That, and not the airframe, proved to be the most valuable to me portion of the original purchase. Or so I though…


Think This Is Expensive, Try Full Size


Gem 6

Rusted Out (new reciprocating) Motor(s)


While we are at it lets laugh, and cry, about an 60s era Aero-Commander (a sort of full scale Twin Star II, what the author flew by RC while building back up a Gemini) derelict, on Corona CA Airport. Somebody fitted it with new engines over twenty years ago as the start of a general build back up, however, the owner fell out, the motors remained open while the airframe sat for twenty years (which ruined them) the interior and propellers lay about in a garage, until the $1500- it is worth as scrap pales in comparison to the mechanics lien, unpaid tie down and back taxes.


And seeing the remains of a Piper Apache, another just after WWII general aviation twin, the type of which the propulsion (engine, propeller and all the stuff attached to them under the cowl) had been retrofitted to my father and I’s last airplane, a 1956 Piper Pacer PA-20, started out in. My father bought one like it new, his second one was older than I am. As preparation for ours, I helped overhaul two other ones, at which the conclusion was, ours wasn’t worth it. Full size or RC there is a place on the curve where writing it off is the best option. For our Piper Pacer (the basic engine was in ok shape, the propeller already recently overhauled) replacing the, some of them obsolete, entire instruments alone would run an estimated two hundred hours and fifteen thousand dollars plus recovering it and… The twin Apache with it’s retractable landing gear wasn’t just two motors more complicated, more like four times, no wonder it’s remains sit at the Oakdale CA airport.


That puts the thirty hours of sanding, fiberglass and writing this up plus a couple of hundred dollars outlay for an expected sixty RC flights (at twenty minutes to half an hour a flight) in perspective.


In Search Of a General Purpose RC Airplane Which Fits In My Camper


Maybe a good direction for me, looking for something which could be transported in my camper and used wherever I happened to be, would have been Multiplex Park Master Pro. Available complete ready to fly (less battery and receiver plus the usual RC stuff) over the counter for $230- plus tax at Discount Hobbies in San Diego Ca and needing only a couple of batteries in addition to the stuff I already have. That would put the investment at about $275-. But the friend with one wasn’t that enthused about his, what we flew most were Twin Star IIs and the out of production Gemini.


But, the Park Master Pro is for not more than walking speed winds and I like to fly in the wind. A former slope soarer, to me the wind is a fascinating ally and opponent both. Thermals, yawn, even if I recently put a Multiplex Reinforced-Merlin out in Hidden Valley way over the Mission Bay altitude limit riding one on up to the limit of my eyesight. As a club safety officer once put it; “Please tell me that dot up there isn’t your airplane and quit laying on your back to fly”. I have since reformed myself, but out where there is room (way outside San Diego) have an occasional relapse. My stuff must be able to withstand rough field landings, with it’s almost zero forward landing speed and long landing gear a Park Master (Pro) will. With other airframes rough field landings usually require folding propellers and some reinforcement.


But the worst about a Park Master Pro was not having three hundred dollars for a new toy.


Alternative Apprentice


I priced out refitting my beat up, bought last year for the new motor and computer radio it contained (full size) Apprentice. That figured in at around $110- for replacing all the airframe parts. The new motor (with an epoxy stirrer for a propeller) and a radio and receiver I wanted were the real deal. The three dead 3S LiPos went in proper disposal. The airframe was beat up and patched as I bought it, in a moderate dive the tail went into flutter, the wing should not be further repaired. In short with the exception of the new cowl the airframe was done, I didn’t trust the (possibly) beat up servos and motor controller either.


Now with more experience on others Apprentices I consider them an excellent choice for out of the box trainers. A careful balance between decent quality, average time between crashes and durability. Who, other than the author, expects over a hundred flights out of such an RC airplane anyway?


Except that the Apprentice as it comes with the motor mounted to the fuselage firewall with a canopy, if you hit the nose, you break it. And only a specific motor with extension fits. What’s the fun in that, if you don’t have a runway? I had a pile of motors, correct for the Apprentice size, I wanted to try, which won’t fit. I wanted a front mounted motor (gone out of fashion) so I could play with different combinations. I find the search for optimum interesting. For the Apprentice I’d have had to designing and build a whole new nose. Easy enough in a shop, but not in a camper.


That getting things really right, along with basic quality and good flying skills are why I am considered a bad example. The club members out in Corona CA were slightly dismayed at what I could do with their worn out Apprentice, using new batteries, propeller, a motor controller set up outrunner correct and my flying skills. “If I knew it still flew that well I wouldn’t have sold it”. As in half an hour flights including a dozen touch and goes plus inverted. Blast off the runway, do a roll, half split then glide for a while. I had to keep the speed in a dive down though as the damaged and repaired tail went into flutter and it won’t due to tempt fate with worn out equipment too many times. When it comes to fixed wing they just haven’t gotten a grip on substituting electric power for fuel out there, as is the case for many clubs. The form fitting batteries it came with were shot, so I built up some packs out of “get these out of here” low C discharge (5,000) mAh cells I’d been given and cut up the belly to make them fit. Run at (17) amps on 3S 3200 mAh (that original capacity is down a bit as they age a few percent a year) and use of the amps stick flights are in the half an hour range, even with the beat up wing. They also made for worthwhile performance in a Fun Cub and Twin Star II.


But at (400) grams those battery packs are too heavy for a Gemini. In case you were wondering; My airplanes are infamous for looking like expletive deleted and flying very well.


I’d have overhauled my SoCal Apprentice (they are also very popular in Rhein-Main) except the last day before returning from Rhein-Main I found a complete, unbuilt, Multiplex Mentor kit (same motor mount as the Gemini and Fun Cub) for use testing motors. So, I left most of my prospective medium power stuff back in Germany.


And I needed a modern production motor controller suitable for outrunners in that the timing can be set.

Half of my pile of average (150) gram motors are geared inliners, the other half outrunners.


In case you were wondering, in year 2017 with Multiplex stuff disappearing from the USA Market, The Apprentice is the best available trainer for roomy airfields such as the Silent Electric Flyers of San Diego at Mission Bay, San Diego CA.


The Merlin didn’t Do It


A friend had two of them, the second built, but unused. His first was in the learning to fly and try everything phase. He reported the Merlin as too fast and too small for him. The Multiplex Merlin, no longer in production, was an in-between, smaller and with ailerons more agile than the rudder and elevator Twin Star I with it’s “can” motor barely enough for flight, the Merlin as introduced was at the leading edge of the brushless motor plus LiPo batteries, it later lost out when the Easy Star II with its brushless motor (and a lot less howl changing from six inch to eight inch pusher propeller) and ailerons became available. The Merlin is small for outside, it is fidgety in flight, neither fast nor really acrobatic and easily damaged. As received (flown as the 3S tuning version) the elevator throw was just right (factory preset) the rudder fine, but the ailerons a little too quick. Flights were in the ten to twenty-five minute range. Which might seem reasonable, but they somehow the Merlin didn’t connect with me. I’m not as pleased with mine as the people who wrote them up back when they first came out around 2008.


In truth if you know what you are doing, or even more so, if you don’t, an up motored Easy Star II is a better choice than the Merlin. Part of that was that neither airframe is acrobatic, but it would take a computer radio to deploy the ailerons as either flaps or spoilers to make the Merlin land as well as the Easy Star II. Since they both take the same servos and receiver, the bigger diameter propeller of the Easy Star II cuts the pusher prop howl back to acceptable, and the difference in the price of the batteries makes little difference, for which you get longer, relaxing flights with no concern about scratching up the rear mounted folding propeller or dinging up the wing on landings…


After twenty five flights I put the Merlin on the shelf. Un-reinforced it needed a golf course to land on. Mine had some light fiberglass “hardening”. Hidden Valley with a wind out of the east, the grass still long, was ideal for that airframe, and yet other modern airframes did everything better, the small size was a disadvantage.


A Third Gemini


I haven’t been carrying as much back and forth between Rhein-Main (West Germany) and SoCal as previous years. Still, with me, was a new, great little (35) mm Hacker outrunner in the (80) gram size which swings a 9X6 folding propeller on 3S Lipos. Two of them would make a “killer” Radial Motored Twin Star II (and great for a light winds Fun Cub) but I’m bored with the Twin Star II (and bored with the Fun Cub) plus they take up too much space in my camper.


For the Gemini we liked the (105) gram motor size, run at well under maximum input, best for Geminis although I flew ours with motors from (40) grams through (135) grams on 3S and 4S. Somewhere out around (400) watts-in and you are forcing a low to medium speed (by the standards of outside) faster than is fun at the cost of weight taking the fine handling away. Gemini’s (set up right) fly fine in still air, yet they will buck some wind, up to two or three times walking speed (as much as San Diego’s Mission Bay ever gets) too. My personal Modified Gemini(s) stay up for twenty minutes or more per flight. The expected life of a Multiplex Gemini, if I build it new from the kit and reinforced it before it takes a beating, is one hundred to two hundred flights by which time the flight loads will have fatigued the wing foam to replacement. Flown gentle way longer. Cost new (no longer in production) fully fitted out, four hundred to six hundred dollars plus batteries and the usual RC stuff.


My total budget for the next airplane was in the hundred dollar new outlay (use what I had as much as possible) range. And, if possible, fit it with an economical motor and sell it afterwards.


In case you were wondering; I’ve had over twenty propulsion combinations in various Multiplex Geminis. Probably the best is propellers (all four I assembled were fitted with either Aero-Naut or Graupner folding propellers) in the nine to ten inch range, 3S 2200 mAh LiPos and motors in the (35) mm diameter size weighing between (80) and (105) grams. I tried (40) to (60) gram motors in the (28) to (30) mm diameter, brands Hacker and Scorpion, on small 4S LiPos with fixed propellers. The currently available common little outrunners are rated at 2S and 3S, all 4S did was wear them out too quick. Bigger motors put out more power than the (250) watts-out that seems to be optimum, left too little room for the battery and usually were nose heavy. The original (80) gram motor is average efficiency, also with its equal diameter to length a little short on torque. The Hacker puts out more watts-out for the same watts-in and with it’s larger diameter swings a bigger prop better.


Found a Gemini Core To Overhaul


The drive over to the Lakeside for the swap meet on an oddly gray morning, 18 March 2017 (Sa.) was an easy twenty-five miles over mostly freeway. When I first arrived in San Diego County this section was kind of like being in the country, not any more. On arrival it turned magnificently sunny. The field is located in a valley between steep rocky hills, currently green like not in decades and even wild flowers!


Not a lot for sale, and not many buyers either, but a very nice place in a steep valley to fly RC. I decided on a (likely ten to twelve year) old Multiplex Gemini fitted out with a, current to then, sport electric motor and good servos. The main landing gear had been rereinforced, it was painted a simple yellow above and black below with sprayed on paint, nominal scrapes and dings. That included a short range Spektrum receiver of the period, a motor and speed control. Per the owner bind it to your Spektrum receiver, put a battery in it and go fly.


After which I hung out for a while, took a bike ride (first time in that area since year 2006) and watched just a few people fly on a great flying morning. Actually, if they weren’t so fragile (by today’s standards) that 90s era Ugly Stick refurbished by an 70s era pilot (his likey birth date too) did well. Until on a typically hot landing (required by the high wing loading, semisimetrical wing and no wind) the nose wheel snapped off.


That is a great asphalt runway they have, and our Silent Electric Fliers of San Diego Mission Bay hard packed sand is good too. But I needed something for everywhere.


First visual check


In general minimal wear and tear.


The front landing gear mount is gone (a chronic weak part, just not big enough to get a grip on the foam) replaced with a plywood and a decently brace to the always too weak main landing gear. Kind of small wheels though. Just glueing on a plywood mount for the landing gear, the same size as what tore out, isn’t a sufficient solution. That rear wheel was never strong enough either.

Fuselage servos are HiTek HS-65s, more expensive, stronger and most important, better responding than the same size case of HiTek HS-55s.


I can’t see the identification plate for the wing servos. However, one of them the servo arm with the ailerons centered is at right angles to the wing, the other back about (25)% degrees! Both wings needed more aileron up then down (positive differential) one was about equal each way, the other negative differential! It never flew as well as it could.


That motor was likely sold by the now gone for a decade Sureflite hobby store on Convoy, they were ok motors for back then.

It has an older Castle Creations Phoenix motor controller rated at (45) amps on 3S LiPos with the BEC and 4S LiPos without it. Deans type connectors. That sounds fine, except to my knowledge there was no way to advance the timing from the (5) degrees for inline motors to the (20) degrees a typical outrunner needs. The outrunner still turns, but used power inefficiently. As in shorter flights at less thrust then they could if the motor controller just fed the pulses correct for their type.


The rear wheel assembly is intact, but the thin wire loose in the mount. I doubt it was ever glued in place.


The whole airplane had been spray painted a high intensity yellow, then the bottom of the wing was spray painted black over the yellow, including some yellow overspray on the APC 8X6 propeller. I used to slope soar, I learned the hard way don’t make the underside of an RC airplane the same blue as the sky. From experience somehow the Gemini is hard to see at a distance.


Some scrapes on the lower wing tips, a leading edge ding and some abrasion, otherwise it appears to be straight.


The motor mount has had the four adjustment screws removed, it is held flat to the motor mounts. That made for too much up thrust. A weak point of the original, the four adjustment screws gouged into the side rail mounting plates until there was no adjustment although there is no indication the screws were ever used on this airframe. It has the original white plastic motor mount, which are prone to warping, modern production is aluminum.


I had carbon fiber motor plates of this type, but they (along with a motor plus propeller plus motor controller) went missing after a flying session in the vineyards of Rhein-Main. I can’t clearly remember, but that would explain the case of local wine and the empty bottles on the floor of my camper one morning.


Three of my geared motors, now a decade old (two from Multiplex, another an old stock NeuMotor) the transmission used four bolts, my one modern production NeuMotor’s gear box uses three mounting bolts. For which there is no “standard” Multiplex motor mounting plate. With four they bolt straight to the white plastic motor mount as do smaller motors. But, they always warp. Sanding and fiberglass take a couple of hours to correct that. A couple of hours as that plastic is surprisingly tough, and after thinning it out it needs some additional reinforcement. Still better, if you have several airframes that take the same mount, then whittling one out of plywood, which still needs fiberglass to hold up in service. You can’t get carbon fiber ones anymore.


The nose has been cut flat with the motor mount. As good a way as any to easily repair the damage from a nose over.


Disassemble and Sand


As so often, be it a new kit or a used airplane, it was taken apart (the wing came off easily, just unscrew the two screws) and then sanded. About an hour and a half, which used up one of three sanding blocks. Just going over things deciding what did I have and how much was left.


The Gemini kit comes with sharply pointed wing leading edges. Too sharp for the intended speeds. The reason is to get them out of the moulds with a minimal casting flange. That is a chronic problem, casting flanges at the wing leading edge, with foam ARFs! And I can’t convince the owners to take five minutes with a block of sandpaper to make them right. These wings had taken nominal use, the leading edges were a little worn (plus a dent and a tear) so, I sanded until they were an even, if blunter, round. The dent was patched with lightweight spachtel and a layer of thinnest fiberglass. A critical issue as if the wings were not really usable, with no replacements available, I could just strip the hard parts and junk the airframe.


The aileron control horns are from then production, the attachments to the “barrel” connectors are loose in the holes! Well, it no doubt flew, but with that slop, it flew uneven.


The Motor is a Dualsky XM3548CA 1080/rpm/V weight 165 grams. That information from the tag on the motor. On research in the Internet they are still available although, current production is listed as 1000/rmp/V so they might have made some changes. From Dualsky the best efficiency is at (14) amps, the maximum amps (fifteen seconds) is (24) The maximum power is (250) watts. I take that that is (250) watts-in at whatever efficiency. To keep that in perspective that is one third of a horsepower. If it weren’t for the excessive weight and length that would actually be ok for a few flights. Ho hum flights that would make you wonder why you bothered, except nobody else’s did any better. Well, if they all keep making the same cheap choices…

The motor was run up with a 3S 5200 mAh LiPo, old stock that hung around too long. That battery is good for hour and a half long flights (you read that right, (1.5) hours) in a Twin Star II with cheap motors. So, the voltage under load seems a little low.


With the as purchased (used) Phenix (old stock, circa ten years) (45) amp controller


10.4   Volts under full load It that seems kind of low, that’s what happens when a LiPo sits on the shelf too long.

22       Amps   In case you were wondering that is the same load as two smaller motors when used in a Twin Star II.

230     watts-in A guess at watts-out, two thirds of that or about (150) watts to the propeller. At that weight of motor it would have only flow on, and landed fast. The previous pilots likely wound up flying way nose heavy too.


Next up was an out of box traded for (35) amp motor controller

10.8   Volts

14.6    Amps

156     Watts-in   WtExpletive Deleted That friends and readers is why you use a power meter and check things. Something was wrong, might be the motor controller or it’s settings.


Then a latest series (new) Castle Creations (25) amp Talon motor controller with no changes to timing or brake. Later the brake was set, there are no timing adjustments other than the frequency.

10.6    Volt

21       Amp

220     Watts-in


So, the motor was being operated at maximum input. Except we want that power to turn the propeller, not heat up the already going shot bearings of the Daulsky motor in addition to eddy losses of the inexpensive materials and ineffective transfer of inexpensived watered down magnets. The assembly (motor, motor mount, propeller collet) was disassembled. Somebody did an ok job of drilling out a motor mount intended for the original bolt circle for a bigger one. I’ll likely unsolder the connectors and throw the motor away. From the gouges pliers were used on the propeller collet, that goes in the garbage, too. After sanding it approximately flat (the white plastic always war