Projeti on Steroids - ARC 28-47-1.5 Turn BL Inrunner - RC Groups
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Nov 12, 2007, 08:08 PM
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Build Log

Projeti on Steroids - ARC 28-47-1.5 Turn BL Inrunner

I was inspired to build a screaming fast Projeti after reading through a build log entry by numb_thumb, his build log can be found here

Some background on the Projeti - it's a "delta wing", 1-piece molded EPS (expanded polystyrene foam) airframe, and can be purchased at for around $110. There are a wide variety of power systems that would work well in this wing - it could easily be built as a slow, docile park flyer, or a screaming ballistic rocket (what I'm attempting to do ). The Projeti kit includes servo connection hardware, a thin plastic canopy, a plastic motor mount (not suitable for powerful builds), and a large set of decals for sprucing up its looks.

I'll start with my motor choice. I wanted something that could feasibly top the Mega 16/15/3 power set up on 5S, built by numb_thumb. I initially jumped to conclusion and assumed that a hotter Mega wind (i.e. 1 or 2 turn) might get me there, but the increased power consumption just wouldn't be worth it. After researching the power systems forum ( ), I decided on an ARC inrunner 1.5 turn motor (the 28-47-1.5 turn ARC), sold by The link to the specific motor page is here . LightFlightRC also has their own RCGroups thread which is VERY useful for reading up on their ARC power systems. I highly recommend reading through the following thread in order to become acquainted with ARC motors...

Here are the complete specs of the Projeti that I will building to push the 168mph speed barrier (theoretical prop speed calculations by Dr. Kiwi within LightFlightRC thread of 190mph on the set up below):

* Projeti Airframe: Projeti Reno Racer
* Motor: ARC 28-47-1.5 Turn
* ESC: Castle Creations Phoenix 60-Amp
* Servos: Hitec HS-65HB Karbonite Servos
* Battery: 20C/30C 3S, 4S, and 5S Lithium Polymer batteries
* Prop: Graupner 4.75x4.75 CAM Speed
* Motor mount shaft: 10mm carbon fiber arrow shaft
* Carbon fiber spars on wing edges and through center of Projeti

Theoretical calculations by a number of knowledgeable RCGroup'ers are as follows ( ):

~43,000RPM, 55 Amps, 91% efficiency, ~90 oz. of static thrust on 5S and 4.75x4.75 Graupner CAM Speed - a 193mph theoretical prop pitch speed. Factoring in the 10-15% drag of the airframe, this plane should be able to push 170-180mph at full throttle.

The numbers above were obtained from post #606 on the following page:

Now, for the build process, read below...
Last edited by ChrisWNY; Dec 05, 2007 at 07:18 PM.
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Nov 12, 2007, 08:12 PM
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Reinforcement of the wings

The Projeti comes out of the box as a 1-piece foam airframe, no gluing of wings necessary. Rear stabilizers (tail fins) need to be glued on at some point during the build, but the first step is to prep this Projeti to withstand a lot of force on the wings due to extreme airspeed. I also ran a carbon fiber spar through the middle instead of using the flimsy plastic straw included with the Projeti.

Decals go on easy with a little bit of detergent/water mixture sprayed on the decals for placement. They are somewhat tricky to cut out and line up, but patience and care will give you a perfect fit. You can install the decals AFTER reinforcing the wings...

numb_thumb covers this in his thread quite well, I'll post what I did below...
Nov 12, 2007, 08:23 PM
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Reinforcing the Tail

Knowing that the motor is somewhat heavy and that it will put a tremendous amount of force on the tail section, the stock parts won't cut it. I'll start by prepping the tail for the powerful ARC 28-47-1.5 turn motor. I had some spare 0.015" thick carbon fiber sheets available, which actually look aesthetically decent. I used 30-minute slow-cure epoxy to secure them to the foam as strongly as possible, clamping things with masking tape during the cure.

The c/f sheet was easy to cut with sharp, durable kitchen shears. I measured the tail, then cut out a piece accordingly that would fit over the tail and deck of the plane. You'll also notice that I used a good amount of strapping tape (3M) on the rear. I love strapping tape, it's an easy and quick way to reinforce foam. I used it on both of my fast FunJets for reinforcing the elevons and wing tips. The stuff is virtually abrasion proof, and good for overall ruggedness. The photos show the reinforcement better than I can describe...
Last edited by ChrisWNY; Nov 12, 2007 at 09:28 PM.
Nov 12, 2007, 08:29 PM
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Tail now reinforced with carbon fiber

The photos below show how the tail turned out, I was pleased with it. At this point I'm waiting on parts to arrive by mail, I don't have my c/f 10mm arrow shaft (which will be used for mounting the motor to the fuse), nor do I have the prop or the 28mm inrunner mount. Hoping it'll arrive tomorrow.

I wanted to add that I have glassed the Projeti with 4 coats of Minwax Polycrylic, painted on EXPOSED areas of foam (that weren't covered by decals), for abrasion resistance and a tad more reinforcement. It also improves appearance a bit.

I've decided on a c/f 10mm arrow shaft for actually mounting the motor. The pre-shaped rounded area will need to be enlarged with a soldering iron so that I can drop the arrow shaft into the airframe for a snug fit. I will also run bolts and a bracket through the c/f reinforced tail to bolt the arrow shaft to the c/f reinforced tail, as well as using epoxy to bond the arrow shaft to the foam airframe. Once I get around to that portion, you'll know what I'm talking about when I post the photos. For now, it might not be clear to others what I intend on doing, but my solution has to be light weight (as well as super strong) to avoid c/g balance issues in the future.
Last edited by ChrisWNY; Nov 12, 2007 at 09:07 PM.
Nov 12, 2007, 08:33 PM
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Power system

Here's a layout of the power system I will be using. Photos show the motor, heatsink, ESC, and the soldered connectors. I used Arctic Silver heat sink compound (sold at Radio Shack) on the motor to improve heat conductivity to the heat sink. The motor will need plenty of cooling at 55 Amps (over 1100 watts!!). Thermal compound improves the conductivity so that heat transfers more efficiently to the heat sink...
Nov 12, 2007, 08:36 PM
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Very cool! Cant wait to see it go!!
Nov 12, 2007, 08:41 PM
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Keith -

You're telling me! I am really anxious, had it not been for the holiday today I might have received the rest of the parts I've been waiting on. Hopefully everything arrives tomorrow...
Nov 13, 2007, 07:34 PM
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My compliments on your excellent documentation. I agree with Keith, THIS IS GONNA BE SOMETHING TO WATCH!! Good going!
Nov 13, 2007, 07:43 PM
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NorCal - thanks for the compliments! I'll do my best to keep up with documenting this build at every step.

I ran some more bench tests tonight on 3S with the ARC 28-47-1.5 turn, now that my 4.75x4.75 Graupner CAM Speed props finally arrived. I'm a bit disappointed that the cone on the Graupner Precision spinner didn't quite fit over the prop blade, so I'll have to widen the prop gaps in the spinner cone with a Dremel or something.

4.75x4.75 Graupner Cam Speed on 3S (static bench test):
High Amperage: 34 Amps
High Watts: 428 watts

Even on 3S, the Projeti should rocket near 100mph.

5x5 APC Prop on 5S (tests were run last weekend)
High Amperage: 53 Amps (sustained for 4-5 seconds)
High Wattage: 1070 Watts

I'm waiting on a new 4S to arrive, the 2 4S batts that I currently have use sermos connectors instead of ultra deans, so unfortunately I can't test the 4S right now without clipping wires. Once I mount the motor to the Projeti, I'll install the RPM sensor to get some readings on how fast that Graupner prop is rotating.
Nov 14, 2007, 07:41 AM
Annoying people since 1970 :)
Wow, that's going to rip! Good luck with the maiden
Nov 17, 2007, 11:29 PM
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HS-65HB Servos Installed

Just finished installing the HS-65HB servos tonight. Would've had them in MUCH sooner, but some mail forwarding issues with USPS delayed the arrival of my parts. My general rule of thumb is to use HS-65 (or metal gear) servos for fast electrics that approach or surpass 100mph; HS-55's are simply too weak, not enough torque at the faster speeds. I've used HS-65's in almost all my fast electric builds, including 2 FunJets. They have been very reliable for me, with plenty of torque.

Servos were fixed in with a dab of 30-minute slow cure epoxy, then covered and sealed in with strapping tape. I've had bad luck in the past in cases where I did not fix the servos with a strong glue like epoxy - just wrapping tape around the outside of the servo is usually not enough - the servos often work their way loose, which could mean loss of control during a fast flight. A small dab of epoxy means that those servos are not coming out.

I ditched the white plastic caps that are supposed to cover the servos. First of all, the servo arms on the HS-65's are too long to fit underneath that plastic cap. Secondly, all they do is add weight/drag. The FunJet also uses similar plastic caps to protect the servos, but I've never liked them. To best serve my intentions, I was better off leaving them out. Also, the instructions recommend bending the control rod at a 90-degree angle to connect the control rod to the horn, but there was no way I was going to chance bending at the wrong point, which would completely screw up the control rod installation. So, I used Mini EZ links on the control horns. I hate Mini EZ-links, but had to use them here as it was the easiest option for me. The control rods were pre-bent otherwise and installation was pretty easy.

[Photos on the way tomorrow...]
Last edited by ChrisWNY; Nov 20, 2007 at 06:48 PM.
Nov 20, 2007, 06:53 PM
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Motor Mount prep

The 10mm arrow shaft arrived (finally!). Here are some photos of what I will be using to mount the motor. Should be fairly simple. I'll fill the void in the circular arrow shaft with epoxy and will wrap with kevlar string for extra strength...
Nov 25, 2007, 07:51 PM
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More ARC 28-47-1.5 Power stats

Here are more power readings from my EagleTree MicroLogger on brand new 20C/32C 3S - 5S batteries, 4.75x4.75 Graupner CAM speed prop...

3S 2200mAh: 37 Amps WOT sustained
4S 2200mAh: 48 Amps WOT sustained
5S 2300mAh: 58 Amps WOT sustained (over 1100 WATTS!!!!)

On 5S, the spinning sound from the ARC 28-47-1.5 is literally deafening. I had ringing in my ears after running the motor for a few seconds at WOT on 5S. Additionally, the motor is pumping out an insane amount of thrust, even with a strong karate grip, that thing managed to push my arm around.
Nov 25, 2007, 11:15 PM
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What the heck are you holding that with your hands for???

I've been bench testing the 28-47-1.5 as well using a PHNX 80. APC 4.7 X 4.25

With 2300 20/30C 6s I was getting 18VDC 80a, 1440 watts

I just put some 3000 20/30C batteries in on 6S and getting 18.6VDC, 91a, 1701watts
This finally made it start to warm up in a 30second run static.

This is some insane amount of power. One thing thats really intriguing to me is to get 6s I'm just using 2, 3s 2300MAH 20/30C packs in series and I can't figure out how the heck it is outputing 80 amps? Its like I'm getting something for nothing? As a single 3S setup on this motor and a Graupner Speed (not CAM) 6.5 X 5.5 This same pack is down to 9.9VDC @ 55 amps, 568watts.
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Nov 27, 2007, 01:39 PM
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Good point - believe me, losing a finger did cross my mind a few times, but I've always been a risk-taker . 1701 watts is unbelievable...imagine the potential of this motor. Now, if we could only get that out of a smaller battery with less weight.

You're right about the Amp draw not making any sense. I always thought Amp draw was determined by the battery capacity and the discharge rating. So, in theory, a 2200mAh battery that is 20C/30C should be able to safely output 44 Amps continuous, and 66 Amps for short bursts (before the battery hits LVC). If anything, a higher-voltage battery should mean less Amps to produce the same wattage (total power output), right? The Amperage that the motor can pull is limited by its internal resistance, at some point that extra power is simply lost as heat, meaning a potentially burned up motor. I would think that a 6S 2200mAh battery, even considering that you have 2x3S batts wired in series, would not be able to produce 88 Amps. I would think you would have to hook those batts up in parallel, like 3S2P, for an 88 Amp output (at 4400mAh rated at 20C continuous discharge). I've seen this same behavior when hooking 2 3S 2200mAh batteries in series - I'd run the motor up to WOT and would get readings in the 60Amp range, although this is in line with the 30C burst discharge capability...
Last edited by ChrisWNY; Nov 27, 2007 at 03:11 PM.

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