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Mr.RC-Cam

Multiplex Twinstar II Build Blog

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It's time to replace my Magpie AP model with something more exciting. After searching for a stable model that was bigger than the Easystar, TerryS made a strong pitch for the Multiplex Twinstar II. This is a 56-inch wingspan foamy twin electric that has a bigger payload capacity than the Easystar.

Terry privately shared some photos and tips on his Twinstar and I soon ordered one from the local hobby store. Cost was about $100 for the brushed motor ARF kit.

When it arrived my first impressions was that it was smaller than I wanted. But, with very little fuselage carving, it certainly will hold a lot of stuff. Kit quality is great, but even so I didn't use the brushed motors and I deviated from the printed instructions.

Before I maiden it I thought I would post some photos of the build. Please keep in mind that my modifications were just to satisfy my craving to hack things. So, there is no real need to do anything I propose here.

So on with some photos! First, here is a Multiplex catalog image:

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Carbon fiber rods were added to the fuselage shells. The local R/C Hobby Shop stocks popular sizes of the rods.

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I remotely mounted the 900MHz's antenna on the tail with a short coax extension. With 900MHz equipment, short coax extensions do not adversely affect range. Moving the video Tx's antenna to the tail gets the strong RF away from the other electronic goodies mounted up front. I don't want EMI/RFI headaches.

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A wooden saddle for the TX905-PRO 900Mhz transmitter was added too.

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From the outside you can see the video transmitter has access to air cooling. It runs hot, so this is necessary.

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Rudder and Elevator servos are suppose to be installed in the cockpit with long pushrods. To free up the space up front (I have a lot of stuff to put there) I moved these servos to the tail.

Elevator uses a Hitec HS-81. The rudder has a E-flite EFLRS75 micro servo (a bit wimpy, but should be fine). All pushrods are lightweight carbon fiber rods ($5 from the hobby shop).

Toroid donut shaped filters (dpcav.com #TOR-001) with 10 tight wraps of the servo cable were used on all servos. The Toroid filters needed to be close to the servo and the servo cables needed to be longer to reach the R/C Rx. So, I un-soldered the factory installed cables and reworked them with new custom cables.

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I didn't trust the CA adhesive to hold the horns. So I drilled and added 2-56 screws (with backplate).

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I dumped the kit's 400-size brushed motors and went with brushless instead. I installed a pair of HobbyCity's Turnigy 2209 26-turn 1130kv 15A Outrunners. These are nearly a drop in replacement (require very little rework to install them in the kit's motor mounts).

Props are 8x4.5, counter rotating. Bench tests measured 11 Amps @ 10V (per motor). These numbers seem to be in the ball park (maybe a bit on the low end, watts/pound wise?).

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For ESC's, I used two of HobbyCity's Mystery 20A Brushless Speed Controller (Blue Series). I tore them apart and rewired them. Now there is only one throttle cable and a single Deans Ultra-T connector for the main battery. The ESC's BEC feature was not used. Instead, a dpcav.com 5VDC/3AMP (p/n #BEC5V3A-023) was installed for R/C Rx and servo power.

After some soldering iron magic, the results looked like this:

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I repackaged that mess into a removable module. It slides into the fuselage (under the wing). Takes a few seconds to put it in or take it out.

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The two Brushless ESC's plug into the motors via a pair of 3-pin 3mm bullet connectors.

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I also installed Gyros on pitch and roll. They are remote gain servos. That is to say, they require a spare R/C channel to adjust their gain. Although I will be using them in the standard "rate" mode, they can be remotely switched to heading hold mode (which is not a mode I expect would work in this application).

Both Gyros are in this photo, but they are small so you have to look carefully to see them:

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Access to the Gyros is via a hatch cut in the bottom of the fuselage. A toroid core was installed on this end of the servo cable too (10-wraps). Shown here is the Pitch Axis Gyro and its Toroid filter (Roll Axis Gyro is there too, but barely visible):

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Two Hitec HS-81's were used on the Ailerons. Due to the single Gyro on the Roll axis, I had to Y-cable them. If I hadn't used the Gyro I would have set them up on two R/C channels configured as Flaperons.

Because a Toroid filter was mounted near the servos, and longer cables were required, I reworked the servos and installed long one-piece cables to them. The Toroid filter is seen in the photo.

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I prefer to use Li-FePo4 (A123 / M1) cells so I built a custom pack to try in this model. If I find I don't like the performance I will revert back to a 3S LiPO.

So, I created a 3S A123 pack (cells were harvested from a new Dewalt power tool battery). The rough build looked like this:

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After Kapton tape and heatshrink:

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It fits perfectly in the Twinstar's nose. It is such a nice fit that you would think the model was designed for it. :)

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So now I find I have a problem. The ESC's I bought do not support A123 cells. Their LV cutoff voltages will work with LiPO, but not A123.

After reverse-engineering the ESC's circuit board, the low voltage circuit is deciphered. All I need to do is swap a resistor. The resistor value has been ordered; hopefully the value I calculated is the right one!

R-SENSE is 220K ohms. I need to change it to 150K, or so my crystal ball has advised me:

UPDATE: The Resistor mod does not work as expected. Information posted further below.

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Edited by Mr.RC-Cam
Resistor mod problem

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The video camera is setup for pan only. A mounting plate for the pan servo was installed in the nose. A Hitec HS-81 will go there.

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The biggest challenge for this build has been finding time to work on it. But I'm nearly done. :) Once I reconfigure the ESC's to accept the A123 pack I should be ready for some test flights (without video gear). Hopefully I can get to that within a couple weeks.

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Looks good, I will be interested to hear how it compares to my Twinstars. Your plane is very different to any of my planes in many ways but I'm sure it will fly great, I look forward to the maiden.

No need for a 6ft wing Ron, mine will lift a G9 plus all the usual FPV stuff with ease, just out of interest I did convert one to a biplane to see how lift improved but it's not realy needed.

Terry

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Terry, pardon if I have asked before, but what typical wattage have you prop'd your Twinstars to use? What is the AUW for them?

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At full power about 120W per motor and level flight at about 25W per motor. You could get away with less as your not lifting a G9. I never use full power as take off is nice at about 75% power.

The bi-twinstar has about 150W per motor with gearboxes driving 12x10 props. Fantastic thrust and will go vertical if pushed. 14.4v and 3300mAh for flight times over 30min, never runthe battery flat as the glide is not so good.

Terry

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At full power about 120W per motor and level flight at about 25W per motor.

Thanks for the watt info. Sorry for asking again -- I think you've told me those numbers before.

... never run the battery flat as the glide is not so good.

That's the one thing I don't like about Li-Fe/A123's. When they are done, they go flat very quickly. I've been caught motor-less many times. So, maybe I need to think about battery chemistry choice a bit more. I'll see how things go as-is and if I need to I will revert to LiPO.

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Nice build Ron. I really like the flush vidTX mount but with it on the side, aren't you worried about stuff getting in through the holes like sand, snow, or grass while landing? The gyros are interesting. We've used IR sensors for stabilization on this model but are ugly the way they have to be mounted. The gyro's are a clean install. If you want or need more room inside the fuse, try moving the ESC's out to the nacelles, behind the motors. That'll get the noise away from the RX as well. Just my $.02.

The maiden flight should be a pleasure, especially if you haven't flown a BLS TS before. It's a very positive airplane. Enjoy!

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aren't you worried about stuff getting in through the holes like sand, snow, or grass while landing?

Good point, I'll keep an eye on that. I fly from nice and tidy landing zone areas, so I don't expect too much trouble.

Here's another photo (forgot to post it earlier). It shows how the Berg 7P Rx is in a foam pocket carved in the fuselage. The aerial wire lead pokes out the bottom.

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That´s a really nice construction, lots of details on the building. The only thing I don´t really like it´s that berg receiver, I don´t think it´s a nice receiver but that´s my personal taste.

Which fpv cam are you planning to use? any OSD+ GPS equipment on it too?

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I installed the resistor on a ESC and was surprised that the effect was not entirely what I expected. It changed the ESC's cell counting feature, but not the actual low voltage detection threshold. I put the original resistor back in and set the ESC's battery type to NiCD instead of LiPO. This effectively disables the low voltage cutoff when A123's are installed. After thinking about it, I think this is best for a twin powered model.

But if it were only so easy! While testing things out I noticed that one of the ESC's has a problem at lower voltages. Below 9V, the motor will not start up. It cogs back and forth and has spastic episodes. The other ESC does not do this at all.

So, I'm about to order a pair of Castle Phoenix 25A ESC (never had a problem with them). But I noticed that they have a low cost version called the Thunderbird 18A. If anyone here has used the Thunderbird series, can you offer any comments on it? Does it allow me to set the Low Voltage threshold voltage values with the Castle-link, like the Phoenix?

The only thing I don´t really like it´s that berg receiver, I don´t think it´s a nice receiver but that´s my personal taste.

Well, I have mixed feelings about it too. The last one I used worked fine so I thought I would use another one. But, I have certainly heard the stories about range problems. I am hoping that Castle has fixed the bugs when they took over the Rx products from Peter Berg. We will see.

Which fpv cam are you planning to use? any OSD+ GPS equipment on it too?

I plan to use the DPC-480A camera with EagleTree OSD. But, these initial choices may change (I've got a lot of stuff to pick from :) ). Plus, I will use this model to test new products out, so the video gear will change from time to time.

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Looks great! You shouldn't have much trouble spotting that one in the tall grass should it ever go down... ;) That's brings me to my question, what paint to you use to paint Multiplex foam? I can't find anything that sticks well and doesn't flake off. I'd like to find a nice flat or satin black for the nose.

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I too had problems with one of my controllers, it was a Turnigy 18A and it always started way before the other one. I gave up trying to sort it out and just swapped it with one I had in a pico jet.

I also disable the LVCO, every last drop of juice can count ;)

Terry

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