A cheap and dirty way to convert a standard Twin Star II kit to Brushless.
Flights 20/3 and 20/4 Hidden Valley Overlook Late August 2016 clear winds of (8) mph to (10) It finally cooled down. For the first time in months I’ve felt comfortable outside and the first time in months I’ve had a field to myself.
Despite the slightly bent output shaft of the right motor, with the 7X6 APC propellers (for ground clearance, 8X6 flew better) this twin now draws (19) amps at (210) watts-in on a 3S LiPo (11volts under load). Which despite the wind is seldom needed, nearly all of both flights were at a third of maximum. At most full amps were use on the climb side of loops. As hoped the reduced up-thrust of the motors makes inverted flight smoother, no more the dip and fight for level. Use the rudder and it can turn into the hill to catch some slope updraft extending the flights. But more important making the wind and the hills sparing partners. No longer a common sight in SoCal, this airplane would be a delight to most sport flyers.
Although I’ve had Twin Star IIs with twice the watts-out, this is now a suitable sport airplane for either Mission Bay or the Corona RC fields. Not set up for climb, agile in a small space and yet able to cruise way out there. Even with the aileron throw cut back to the hole closest to the shaft of the aileron servos with the additional fiberglass bracing of the control horns that axis is still too “hot”. The stiffening of the middle third of the ailerons, together with the one piece with and the whole center section being fiber glassed this is now a model, at least flying wise, of an STOL cargo airplane. Like you’d see in Alaska, Siberia or the Alps. Those five layers of fiberglass over the motor-gondolas do as expected.
Oh I’ll continue to develop this airplane a little more, but it’s time to transfer this one to somebody else and build something higher performance. As is flown reasonable it has another fifty flights in it although the motors were already giving out new out of the box. Better motors and a couple of more hours of fiberglass reinforcement and it would withstand a couple of hundred my “fly the stuffing out of it” flights.
Flight 13 middle of August 2016 With the traded for it big 3S 5200 mAh LiPo (new old stock) the balance is just a little too far forward, the motor runs most of the time. In this any wind is a result of thermals moment a lighter battery would have the advantage, any thoughts of clipping the wings two inches a side for better handling have been put aside, at this weight it needs the lift. The additional layer of (2) ounce per square yard (fiberglass cloth only, more like (5) ounces per square yard with the resin) make for a sufficiently stiff wing. To my disappointment the newly applied spray paint swelled the previously flat Elapor up, the individual cells now bulge up. Landing here is beating the fuselage up, Those grass sticks dried in the sun poke through the very thin fiberglass. For rough field flying more fiberglass is going to be required. With the addition of now the entire bottom of the wing is fiberglassed the expected life of this airframe is now in the hundred additional flights range. Elevator and ailerons are now balanced, only lack of a longer servo arm restricts adding more rudder.
Above Hidden Valley is now the last big chunk of openness in all of SoCal, but that sixteen mile round trip drive adds to the costs of flying. As it turns out a Reinforced Fun Cub with it’s big wheels can be flown just blocks away, a Twin Star II would be ruined by the big chunks churned up by the cars. Still, that big wing is not rigid, and trimmed two inches a side with a light weight battery and higher performance motors…
The investigation has begun if I can get Hacker motors here in SoCal as I would like to run two (60) gram 1000 kV motors, one of which I have new already.
This original year 2016 article for the SEFSD covers the use of a Multiplex Twin Star II kit, converted over to a basic brushless configuration. Alternatives could be the Radio Ready (RR) version (recommended) from Multiplex or buy the kit and the equipment separately. If you stick with Multiplex equipment the RR version costs less than buying the parts individually from Multiplex. And there isn’t much, if any, savings from sourcing anything from other Multiplex, at most maybe making your own wiring harness. This used airplane will be picked apart (with more photos then previously articles) going over the advantages of as received (inexpensive and functional, pleasant to fly under ideal conditions) and shortcomings (wrong throw at the ailerons, the motor mounts aren’t rigid enough, develops a hinge at the wing servos, poor performance from inexpensive motors, using just one battery eliminator circuitis marginal capacity, no reinforcement at the elevator control horn and so on and so forth) before making it a more durable and better flying Reinforced Twin Star II. Economics will be detailed. The original layout was straight forward, so too the improvements to get all that airframe has out of it.
Included is an analysis of the servos of a dead Reinforced Fun Cub. Which, with (134) flights on it is a one to two experience with servo selection. In case you were wondering, I expect hundreds of flights out of a Reinforced Twin Star II. For most people something more like a hundred flights is reasonable, so that will be the basis for the economic analysis.
This will be my fourth Twin Star II article (see the SEFSD archives) and as such will be more to stay in contact with my friends back in Germany then for San Diego. As you read this keep in mind that in San Diego California’s Mission Bay we are flying at sea level in shirt sleeve temperatures with either no, or not much, wind. In Rhein-Main it’s about a thousand feet above sea level with winds varying from none to sometimes quite breezy with temperatures four months a year either near or below freezing. If you fly at higher elevations the propeller sizes need to be adjusted and speeds will be higher. The basic glide speed of a Twin Star II is around fifteen to twenty miles an hour, that set’s the upper limit on flying in the wind. A Twin Star II can be landed almost anywhere a RC fixed wing airplane can be landed at all. I’m prone to showing off, a few times I took advantage of the lower wind speeds at ground level in hilly, alternating clear and forest, Rhein-Main, and the higher under power flight speeds of my reinforced Twin Star IIs, to shame the rest of them. Ya have to remember to land before the batteries give out as otherwise it’s going to be way down wind…
The original Multiplex Twin Star (I) was about the first easily built, a sort of semi-ARF, at the Silent Electric Fliers of San Diego (SEFSD) Mission Bay flying field starting around 1998. In gray Styrofoam with twin Speed 400 motors (one hundred grams at thirty to forty watts-out each, for about a hundred flights) on the then C sized Nickel-Cadmium batteries (the six cell size intended for RC cars) and so easily available. Using glued in spruce spars it started the trend of not having to spend forty to sixty hours to assemble a single motor radio controlled flying machine (more like five hours for the first one) still using the then standard “RC car” sized components. Easily broken and subject to abrasion on the sandpaper like Mission Bay San Diego flying field, they were good for about twenty flights with anything past a hundred unlikely. It didn’t take much to break them, after a few snaps repeated repairs were pointless.
At a swap meet in San Diego the summer of 2016 there were three of them, original Twin Star (I)s in Styrofoam, intact, for sale. Or at least appearing intact, both Styrofoam and Elapor go brittle over time. According to people who have flown both the Original in gray Styrofoam and the II in white Elapor, with the same pair of Speed 400s ((40) watts-out per motor) the earlier version flew better. I wouldn’t accept an RC airplane, one expected to fly and land, made of Styrofoam, even if you gave it to me. Even less so after two decades of going hard and brittle. Well, maybe for nostalgia. The same for balsa and heat shrunk covering.
Figure four years after being molded that either the white Elapor of a modern Twin Star II giving out or rust inside the control rods starts setting an end. There’s a lot of care free RC flying satisfaction in that four years.
The Modern Twin Star II (Brushless)
Although I haven’t had an original Twin Star (I), I’ve built a dozen of the updated version (The Twin Star II) in impact absorbing (i.e. packing crate) white Elapor foam. About half with the brushed motors of the day, and half with modern brushless motors and Lithium-Polymer (LiPo) batteries. Figure eight hundred flights plus equal for friends. See three previous articles in the SEFSD archives.
The Twin Star II is still the best selling of the Multiplex line up and worth every bit of its purchase price. Although a good deal as a kit, and easily converted to brushless, may I recommend going straight to the Radio Ready (RR) version for most of you.As previously covered in a previous SEFSD article (The Twin Star II, Fly It Like It Was A Crow) the supplied RR wiring harness is complex, but does allow for complete disassembly. As in remove the wing and split it in two for transportation. It has the disadvantage of coming with both motor controller battery eliminatorcircuits, the motors pulse back and forth. And for here in The USA the green connectors have gone out of style.The RR version of the Twin Star II comes with well-matched brushless motors, propellers, speed controls (with settable lead, a twenty percent plus for outrunners) and aluminum motor plates to the plastic gondola mounts. They come with perfectly match propellers, which are more durable then the APCs. The difference in pitch between eight inch APC at 8X4 and 8X6 verses the black Multiplex 8X5 can throw fine tuning. Although the Multiplex aluminum motor plates are an improvement, put any real power to it and the motor gondolas vibrate, they are only just equal to the Multiplex supplied motors.An easy improvement is multiple layers of thin fiberglass over the motor gondolas. Multiple layers because to get the material to follow the small radius curves you have to use thin fiberglass.You may expect that if, as I and my friends do, you fly it hard, that our around fifty flights a “hinge” develops at the end of the wing spar right at the servos. A simple patch of fiberglass over the wing servos, or fiber tape,can way extend the life of the wing. As for abrading away the belly (and wing leading edge) see as follows, tape or fiberglass.
Personally I glue and fiberglass the wing into a single piece and make my own wiring harness. The trough down the middle is a relic of bygone days, for use with typical C sized car NiCad battery packs. With the from Multiplex wiring harness down the middle using brushed motors everything fits and balances, but with two sets of motor controllers and modern, even with modern smaller and lighter LiPo batteries, things in there get crowded.
To Decide on Servos; An Autopsy of a Dead Fun Cub
Wide Open Spaces to Fly with sometimes Rough Landing Strips
I’ve had three Fun Cubs to date (also detailed in an archived SEFSD article) and assembled half a dozen more for friends. Even delivered with modest propulsion and ten hours of reinforcement above the minimum four hours to slap a kit together the latest astounded club members at the other end of my long commute in Germany’s Rhein-Main. Set up right both the Twin Star II and Fun Cub can be much better performers then generally observed. A Twin Star II uses four servos in the (18) gram class, the Fun Cub two of the same and two in the (8) gram size. The current Fun Cub offering, a “heavy” version as RR, is a good deal. When that huge diameter (13X4) APC propeller it comes with breaks put a 9X6 or 10X5 on it as a better choice for see level Mission Bay or slightly higher altitude with wind more often Rhein-Main in Germany.
At this end of that long flight (SoCal) I confirmed my long standing status as a “Bad Example” last year out in Hidden Valley. The wind was blowing a base speed of twenty miles an hour with gusts to twenty-eight. Convention has it that no Fun Cub should take to the air under such conditions. Since I wasn’t a member, just there to enjoy the last piece of unbuilt metropolitan SoCal, I wasn’t planning on flying. The club members are a pleasant lot, including some giant scale pilots who fly competition. But in general they haven’t made it past using electric power to substitute for fuel.For five harrowing minutes a fuel pilot kept his airplane in the air to landed mostly intact, then he sneered at the rest of us.
My personal Fun Cub has an internal beam of fiberglass down the inside from the (front) motor mount back to the air outlet and layers over the outside from the nose to the rear servos, the rear wheel assembly has been replaced, there is a patch over the wing servos and the originally two piece wing is a single piece. For this flight the main landing gear was removed, done in a minute with just a screwdriver. It also had a (135) gram Scorpion (same weight and exterior dimensions as the RR version but more effective and more expensive) motor with a folding three (3) bladed 12X6 Aero-Naut propeller drawing forty amps on (14) volts under load on a 4S 2200 mAh LiPo for about (400) watts-out. It took off straight up, sloped soared for a half an hour to land at my feet after gliding back a third of a mile. While I was in the air two other pilots demolished their airplanes. The 4S 2200 mAh LiPo was two thirds discharged.
And yet that same combination, “detuned” on a 3S LiPo (eleven volts under load) has been used to encourage beginners at Mission Bay. It was also used to test how “look the same and a lot cheaper” components function.
Originally assembled with house brand Hobby People servos one of the (8) gram aileron servos failed between flights (25) and (26) from transport impacts. The second made it the whole (134) flights although by then there were a couple of bands where it didn’t follow control inputs, it was worn out and almost to failure.
I’m not sure what caused the crash. A descending slope at the site with a chain link fence has caused radio problems (even with modern digital GHz radios) and there is some nasty turbulence too. I doubt that worn servos were the cause, they all responded to inputs on dissection. The (16) gram house brand rudder servo, as could be expected since it isn’t connected to the rear wheel and otherwise gets very little use, made it the distance. The house brand elevator servo was replaced after flight fifty when it failed on takeoff resulting in an impact that tore whole nose off. That crash ruined two-hundred dollar’s worth of motor and folding propeller, plus having to replace the elevator servo and rebuild the front end (any modern foamie would have been a total write off) so any savings on inexpensive servos were way lost. Conclusion: If you do some nominal reinforcement a Fun Cub can be made to really fly, just don’t use cheap anything.
Well, maybe if you aren’t going to make to fifty flights anyway (for example the due to pilot error by one of the beginner pilots who did fine with my Fun Cub who had ruined five airplanes in a row long before servos or worn motor bearings became issues) inexpensive servos are ok. But maybe the Multiplex recommended servos or, since they aren’t usually available in SoCal, HiTek servos should be used. If you buy the radio recommended version it comes with Multiplex brand servos, which seem to be the same as HiTecs (Hitek owns Multiplex) except the Multiplex servos have longer leads. Keep that in mind when buying servo lead extensions.
My second Fun Cub lasted longer than expected, even if the crash hadn’t ended it I’d soon have given it to a beginner. If you don’t put a patch over the weak area of not enough foam at the end of the wing spar and wing servo, figure maybe fifty flights out of the wings, with it they were still hanging in there for up to three times that long. Not bad for a simplest change taking an hour which, after you own the materials, costs two dollars. Half of that for the paint brush to apply the epoxy. Well, maybe four hours as the wing tips and leading edge of the wing had been hardened too, without which the wings wouldn’t have lasted long enough for the “hing” at the servos to develop.
The Twin Star II weighs more than a heavy Fun Cub, the bigger aileron servos even that out. I now know that a Reinforced Fun Cub has an expected life of two hundred flights (nobody flies a Fun Cub more violently then me) with a Twin Star II about double that. Inexpensive servos will start to fail long before then, or be so worn as to degrade the performance. The inexpensive motors won’t last near that long either, the bearings give out.
The moral of the dissection, these modern RC airplanes in impact absorbing foam, maybe with some select reinforcement, are durable enough to justify the use of better then minimal servos! If all you expect is thirty to fifty flights out of an RC airplane, and you fly gentle, go ahead and use cheap servos, but not in my airplanes and not for anybody I know.
The standard size for all four servos of a Twin Star II is in the (18) gram range. Although I personally use a metal gear servo for the elevator, most of you will do just fine with ordinary servos. About the HiTek HS-85 or so. With it’s slow responses to control inputs faster and more precise servos are likely a waste for this airframe. Somewhere out past six hundred flights, you read that right, (600) flights, the HS-81s at about (18) grams the internal electronics start giving out, by then the plastic gear train is worn out too.
Even the definition of a “Flight” has been changing. With the original Speed 400 motors on a 1800 mAh(6) cell NiCad battery the Twin Star only just flew with about four minutes of thrust. With skill and the increase of capacity of the batteries flights lasted up to twelve minutes without finding some lift. The little brushed motors were good for not more than a hundred flights on 7 cell NiMh batteries, so replacing them was routine. I now expect my friends to get eight to twelve minutes out of a brushless Radio Ready Twin Star II on an ordinary 3S 2200 mAh LiPo with a far more energetic flight profile. With better motors and fine tuning, together with a “burst and glide” flights profile I use I expect double that.
The Used Twin Star II, As Bought the Summer of 2016; What Was Right and Wrong
See the pictures for what at first looked like a simple and effective way to mount brushless motors to a Twin Star II. It appears a soldering iron was used to melt a trough and a piece of plywood using high volume glue secured to the foam. This rear mount system has the advantage of being easy to do, it is also currently “conventional”. In a crash, even though the motors are well protected in a Twin Star II, rear mounted motors become a long throw lever which prys the motor out. Which is why I’ve stuck with front mount motors even though that has gone out of fashion. But that is seldom an issue with the wing mounted motors of a Twin Star as the motors almost can’t hit the ground. That makes one of the big advantages of this RC version of an “all weather” one airplane does everything plane. The nose and wings hit first saving the motors.
However, the gluing in wasn’t sufficiently well done. There is only a little band where the stiff plywood connects to the flexible Elapor foam. Evidently not enough as the motors vibrated way too much. As for being done freehand, the motors weren’t angled the same. That probably wasn’t much of a disadvantage for just gently cruzing around.
All weather in Rhein-Main means some wind, mud and snow. For the SoCal readers mud is dirt mixed with water and snow is this freezer temperature crud that falls out of the sky (look east to the mountains in winter if it ever rains again), when it melts it turns dirt to mud, and frozen dirt clods and mole hills are rocks. They grow food around Rhein-Main, so we fly off of farm fields with grass as a crop being typical.
It should have been that shortening the effective length of the motor gondolas also deals effectively with that with front mounted motors the foam is either just enough, or with more power not stiff enough. But this combination shook way too much.
My personal Twin Star IIs have about a dozen layers of fiberglass over the black plastic stock motor mount (makes up for not having aluminum) layered back over the wing to an inch all around the top of the motor-gondolas. My wings don’t vibrate no matter how much power they get from the (28)mm diameter motors.After seeing what my Twin Star II can do a friend back in Rhein-Main would like me to reinforce his motor-gondolas too.
The Turnirgymotors are inexpensive. Surfing the Internet, Hobby Kind offers them for $10.62- plus the costs of ordering by Internet and mail. One of them already has problems with the bearings. If you can feel the bearings just flipping them over they are already a big drag at operational RPM, and you can feel these. My past experience is that run on 2S LiPos they go a good long while, on 3S they develop problems not later than fifty flights. Fact is I’ve had these only just function right out of the box, and go worse from then. As in the performance falls off as the power draw goes up, the increase going into heating the motors. Until they burn up. I’ve had that happen in (27) flights, even with good cooling.Actual efficiency, that is the power starting as electrical energy being converted to turning the output shaft, might go as high as two thirds of the power input makes it to the output shaft, but I doubt that.
Let’s clarify that as censorship by omission neglects that the amount of power going into a motor is never as much as delivered to the output shaft. There is always some loss at converting the electrical power to mechanical.The upper limit is our own SEFSD NeuMotors at about (95)%. Using even their simple Motor Calk indicates that even their maximum quality stuff if configured even a little wrong the actual efficiency quickly falls off. And there is only so much room in there, the (35) mm diameter motors evidently have an efficiency advantage over their (28)mm Twin Star II sized.
The non-competition manufacturers often purposefully overstate the efficiency of their motors by selecting just one winding of a series perfectly matched as what could be expected across the whole range. And don’t expect the published kV to be the actual either. All it takes is the anti-corrosion coating which looks nice to be just a little thick so the bearings press in at a slight angle to throw things. Or a supplier delivering bearings which look like bearings, or thinning out the material in the magnets which makes them magnets to way down the output/input ratio.
High effective energy conversion doesn’t come cheap. It takes better magnets, inductive spools and bearings. And some of that is hidden in quality control. These Turnigysfigure more like (65)% efficient maximum, probable even less. On 2S that would drop to more like (60)%. Don’t even try to run these inexpensive motors on 4S, the bearings won’t take it for even a few flights.
To my knowledge there are no sport motors which easily fit a Twin Star II suitable for use on 4S LiPos. I tried the (28) mm diameter outrunner Hackers and Scorpions on 4S (both rate their (28) mm motors as 2S and 3S)the bearings quickly wore out (sixty flights) for only a slight performance gain. It has to do with as these sport motors are built they can only use so much magnetic flux. The complexity is on a par with E=MC squared, see the full parameters in NeuMotors motor calk for a better idea. And then let somebody else design your motors. If you need that kind of efficiency and power of 4S maybe a different airframe is a better choice. But then you lose the ability to fly a Twin Star II carefree. Until I found some old stock Hacker (80) gram Heli motors wound for (2500) kV and high rpm both…
Then too the manufacturer’s used to rate their motors for sustainable output. Although the manufacturer Robbe has left the field, their (62) gram (30) mm motors were a good choice for a typical Twin Star II. Robbe rated them at six to seven amps. I always ran mine at double that. So if out past sixty flights the bearings gave out… At six or seven amps in (per motor, twice that for two motors) a Twin Star II would fly gently. At twelve amps in it can chase it’s own tail. All it took were motor controllers with sufficient capacity (set the lead on automatic) and changing propellers.
NeuMotors may not have anything suitable for a Twin Star II. Their 1105 series with a gear box could be made to fit, but weigh too much at (120) grams each. Although the original Speed 400s weighed (100) grams each, we now expect a pair of outrunners to weigh, combined, about (130) grams.To get geared motors to fit you’d have to rework the motor mounts quite a bit. I’d put a dozen layers of fiberglass over the stock black plastic motor mount(s) extending back to half the width of the wing and add a rear “ring” mount to the motor(s). But what are what are we going to do with potentially (350) watts-in to (600) watts-in (per motor) in a Twin Star II? My Twin Star II wings have improved wing spars, the wings are fiberglassed together at the center, there are patches over the wing servos and current production Multiplex wings come with fiberglass tape on the underside of the wings. Attempts with a single nose mounted motor demonstrate that with the speeds available above (300) watts-out total (more than double the best you should expect out of the as bought Turnigy motors, even if the motor gondolas are improved) the whole airplane vibrates. The wings go into flutter too. Who cares if your Twin Star II needs three seconds to exceed the height restrictions of Mission Bay, or seven, as the glide back down takes the same time? A pair of hundred and fifty dollar Hacker (28) mm motors and a Twin Star II can do loops until the battery goes empty, there just isn’t much to more. Even back in Rhein-Main we just don’t need that kind of climb and like a school bus, no matter how much power we put to it no Twin Star II will ever be fast.
Even using smaller propellers to down- rate the NeuMotors 1105 series of motors to (250) watts-in (on 3S around eighty eight percent efficiency) just one motor then still puts out enough. And there is the question of economics, do you really want to spend four times the price of the airframe kit on motors and transmissions plus twelve hours improving the motor mounts and gondolas? Probably not. Let’s set the comfortable limit for a pair of motors as between fifty and a hundred fifty dollars for a Twin Star II, motor controllers additional. Since the transmission alone exceeds that price limit, for now forget geared motors. I’ve never heard of a Twin Star II using them anyway.
Personally I use sport Hacker or Scorpion brands of the (28)mm diameter at about sixty to seventy dollars a motor. They claim an actual efficiency in the (80)% range, if perfectly matched to the load and then only on 3S LiPo voltages. That (80) % verses (65)% claimed isn’t just one tenth more for the same in, it is one third more and in truth the resulting flights are even more of an improvement that a third.
You can wear out the (28) mm diameter size motor bearings no matter who makes the outrunner before a Twin Star II airframe gives out.
It is reasonable for about fifteen hours of changes to mount (35) mm motors in the (80) gram class in a Twin Star II. At which you have something which looks more like an old radial motor model and have way more torque then needed for anything other than towing another airplane aloft. See upcoming article Five for details. A clue, the conversion isn’t as finically as expensive as you might think. The materials only cost twenty bucks, the motors, if you were already paying Euro55/$62- only go to Euro79/$89- the net difference is only $54- and it really impresses the rest of the club!
The Twin Star II though should use at least 3S LiPos.Yes, you could run a big 2S (like the car people do) but why bother. Um, unless you happened into a deal on helicopter motors that still hit efficiency in the (80)%s on 2S…
Regarding one of two Turnigy motors having rough bearings, it’s a quality control issue. Two isn’t a good enough sampling, but one of them is already nearing failure. It could have been two good motors, or two bad ones. Replacement bearings are available, they cost more than the mot