|Today's Subject:||Goldberg Eagle II ARF|
|Wing Area:||715 sq. in.|
|Wing Loading:||22 oz/sq. ft.|
|Flight Battery:||Pair of Six Packs|
|Motor:||Jeti Phasor 45/3|
|ESC:||Jeti JES 40-3P|
These days there is literally no ARF that can’t be flown electric! Forties (.40) are very common and sixties (.60) aren’t rare either; I’ve done 5 forties and a .60 which can be seen in my RCGroups Gallery. And, 1.4 (plus) are being done by more and more folks. Not just giant Cubs but 3-D and Warbirds.
The leap from struggling to convert .40 kits just a few years ago (built with extensive attention to making them light and excessively expensive equipment) to converting any ARF is the result of: a wide selection of brushless motors, a growing assortment of “E” accessories, and rapid developments in battery technology (increased capacity & reduced weight).
A quick look at the links posted above will likely scare some folks away; looks like it takes a lot of dough, technology and math! And, the bigger they are, the truer that is.
But for a typical .40 ARF (trainer specifically), all you need are 2 Six Packs, a motor, and a few other items -- all of which are commonly available at most local hobby shops (LHS) these days (or obtained via catalog or the Internet). Bolt the motor up front and put the Six Packs where the tank was supposed to go and you’re ready to fly “quietly” using electrons rather than fuel.
Last fall a friend told me he wanted to get into RC, but didn’t want to mess with engines and fuel, or to fly something "small and twitchy". After several conversations he decided he wanted his 1st plane to be a standard .40 trainer.
With $100 in his pocket he cruised through a couple of our local hobby shops and found his plane.
“I really like the Carl Goldberg Eagle II ARF, so I got one,” he told me a week or two later. “Great!” I responded, adding, “Start building it and get a radio next. We’ll figure out the power system later”.
A few days later the phone rang again. “Servos are installed, Jim --the standard ones that came with my new 6 channel JR. It's time to get a motor!”
Now I had to do some work; but its the kind of “calcs” I like, must be the engineer in me. My goal for the power system was for it to be “Plug and Play”, and available from a LHS. I also wanted it to be expandable if at all possible; that is reuse as much as possible in a future application. Even though personally I always keep the old and just add new/more to my fleet, my friend may want to move beyond his trainer without another complete outlay.
Out-runners are new & popular, geared systems do a lot with less watts; direct drive would make for an easy installation, etc. Lots of questions to consider!
"Twelve cells...," I thought, "Could we do this larger (& heavier) .40 Trainer on 12 cells?"
I’ve been flying a smallish forty advanced trainer for years (4+) on 12 cells, using a geared solution. It is elegant, efficient, but not “Plug and Play”. I’ve also flown my American Flyer with a brushed system; economical, but needs maintenance, is more parts than a direct drive solution, and not as efficient as a brushless motor.
Why 12 cells? Its Two Six Packs! And, some speed controls will let you use their Battery Elimination Circuit (BEC) on 12 cells with 3 standard servos (rudder, elevator, & ailerons).
Two Six Packs? Not sure what I mean? Well go to the RC Car section of any hobby shop and look at the packs they sell; they’re 6-cell flat packs. A wide variety, capabilities, and prices are easily obtained. Several of the packs I’ve been flying in the good old American Flyer are RC Car 6 packs; propped (& geared) for the mid 30s (amps), they last 3 or more flying seasons. Skip a few “real” six packs, or lunches, and you have enough funds to get another set!
RC Car six packs are a good value for a fully assembled pack. Individual cells can be gotten at a lower per cell cost, and a soldered pack can fully use the potential of the cell; but, the convenience and value of the Six Pack is hard to beat, especially for the beginner. Or, for a guy who now wants to fly after years of driving RC Cars, he can even use his existing charger (charge both packs!).
The connectors on RC Car packs don’t stand up to 30 amps and must be changed to either Deans Ultras or Sermos/Anderson Power Poles. Sermos connectors allow you to easily connect the two packs in series (12 cells), or individually if needed for charging. Dean’s have lower resistance, but a series harness is needed.
Looking at brushless direct drive motors that can do 12 cells & 30 amps narrowed the choices to AXI’s 4100 series and the Jeti/Mega 45/3. The local hobby shop regularly stocks the Jeti, and gets AXI’s in on occasion.
Beyond Hobby Lobby’s info, I turned to RC Groups Discussion forums for input on 45/3 experience for prop selection. This exchange helped with the final configuration/prop selection, a 12x8 APC“E”. The 12x8 pulls 31 amps and provides plenty of power, with some room to get more power if wanted; use a 12x10 for more speed or a 13x8 for more thrust.
Use the mount as a guide (just like the manuals show with an engine) to drill the mounting holes. Four 4-40 bolts are needed for the mount, by using 3 lock nuts with nylon inserts on each bolt you clamp the motor and also let it stand off the mount by about ½ inch. With the motor raised this small amount you get a bit more clearance for the larger diameter prop.
With everything mounted in the Eagle, all bought from our LHS, it was ready for the maiden flight. Success! The Two Six Pack set-up flies the Eagle with glow performance, with 8 to 12 minutes duration -- same duration as glow! The all up weight was 6 lb. 13 oz. Plenty of training time is available to the student when using 3000+ mah RC Car 6 packs.
Now for some more details about the Six Packs. If you can’t see the cell label you don’t know what you’re getting! Sanyo NiCds and GP NiMhs work best, and can be obtained from your local hobby shop. Two Six Packs of GP3300s come boxed together from SMC, they’re excellent. 3 years ago I bought a set of RC2000s (still in use), in an SMC box; they’re packaged that way for use in the Traxxas E-Maxx Monster Truck. Also, DuraTrax does a 6 pack of Sanyo RC2400s; got two of their RC2000s nearly 4 years ago.
The Two Six Pack solution is an excellent set-up for any of the .40 ARF Trainers; its performance has been proved in the Eagle II which is a bit larger (& heavier) than the others from Hobbico, Horizon, or Tower. The Eagle also comes with a hatch, allowing quick and easy Six Pack changes – six Six Packs and you can fly all day long (1 set flying, 2nd charging, & 3rd cooling).
A “Sport” 40 could also be flown with this set-up (CG Tiger 2, Tower Voyager, or Sticks); but its just a bit shy on watts for the 3-D ARFs. Use higher quality packs (matched, soldered, zapped, etc.) and pull the full 40 amps for added performance.
Going beyond 12 cells can get any 40 ARF off the ground with better than glow performance. 16 round cells (NiCd/NiMh) or a set of 5S LiPos can provide the Watts to hover/3-D, or fly a warbird at blistering speed. The 45/3 can go to 16 cells (5S) use the 40-OP, opto versus BEC; direct drive for a fast model or with a low ratio gearbox for more prop. Other popular motors in the range come from Aveox, AXI, Hacker, and Mega.
All You Need for this Exact Conversion on the typical .40 size Glow Model:
|Aug 15, 2004, 05:35 PM|
Central Lake, Mi.
Joined Nov 2002
Goldberg Eagle 2
Very good article. I've been flying mine since last year, first with an Astro C25G and 16 cells and then last week tried it with Model Electronics new BL 36-40 and super box at 4.7:1 ratio, 14X12 APCE prop and 16 GP 3300 cells. I converted it to a tail dragger and used a UBEC for radio power. Takeoff was very smooth with plenty of power and duration although at first I used an old Panasonic 3000Nimh was around 6 or 7 minutes. With the GP 3300 power was up as was the duration.
I then switched the motor system to a Mega 22/30/3 with innerdriven box at 3:1 ratio 14X12 prop and again 16 GP 3300 cells. I also used the UBEC for radio power. Now it leaps into the air and can climbstraight up! I generally takeoff at half throttle. It flies as though it has a .60 on it. I cruise around below half throttle and haven't timed it but it is better than previous system.
I won't be using this system next year as it is going into a Goldberg Tiger 2 with an MEC box.
Maybe I'll try the Pasor 45-3 next year. Right now I'm having too much funn with this system. Cheers. John z
|Aug 15, 2004, 05:47 PM|
Jim, great article! Definitely took the mystery out of gas conversion and made it plug and play.
Looks like a higher initial investment. How does the total operating costs compare to a stinker over a couple of flying seasons? How did your friend justify the overall expense and experience?
Seems like the plane flew really well even on a windy day.
|Aug 15, 2004, 08:32 PM|
Joined Jun 2002
I have a plane I flew on a Phasor 30-3 and 15-16 cells and a Modelairetech beltdrive. I threw a magnet on the rotor so be careful. The 45-3 would probably fly an even bigger plane than a 40 sized trainer but the bigger can is probably more efficient.
When your flying such a heavy plane, the few ounces you have saved by a bec esc is pretty insignificant.
The AXI 2820 10 is another option for a 5 lb plane as it has plenty of oomph. It will sewing a big prop too.
Those who would say, geez thats an expensive motor and ESC I would tell that the quality is excellent and they don't wear out.
Hats off to the Czech Republic for manufacturing such excellent products.
|Aug 16, 2004, 01:09 AM|
Thanks for the article...real helpful. I have a couple of old Tower .40 trainers gathering dust in the attic...hummm....got me thinking now!!!
I am not that familiar with electric motors larger than the standard parkflyer stuff, so I will have to do some more research into those beasts.
Again, thanks for the info.
|Aug 16, 2004, 11:15 AM|
One thing for sure about electric flight is that there is no one way to do it! As referenced in the article I’ve flown my American Flyer with several different set-ups and others have flown theirs differently than mine.
One parameter that didn’t get listed in detail in the article that influenced the motor selection was to maintain a current in the low 30s.
If you look at the AXIs, Mega, or even the Jeti 30/3 smallish props can only be used – 10” max. The 30/3 on 12cells gets near glow rpms on roughly the same prop as a typical .40.
But that’s not the best “prop” for the mission profile of a trainer.
The 2826/12 (available after 8/20) would provide about the same performance as the 45/3; the 2820/10 is too hot like the 30/3. Outrunners aren’t as easy to mount either.
The amp draw of the configuration described is gentle on both the motor and ESC; barely warm upon landing. This provides headroom if more power is wanted; but more importantly doesn’t overload things to the point where you might have an exciting motor/ESC issue with a student at the controls (i.e. throwing a magnet).
Prices have been moving in the right direction; but its true this cost more than the glow. The ESC is a few bucks more than a .40, then add the motor and batteries – buying several years of fuel in advance. A recent article showed that a detailed analysis of a .40 conversion with LiPos has a lower per flight cost.
But here’s a real advantage; between Rick (the owner/student) and I we have four 3000+ packs and can get 4 training flights in by abusing our lunch hour by an extra 20-30 minutes! No goop to clean or cranky engine to deal with.
Peak a pack while the plane is readied, and peak the others during the other flights; the packs can be swapped faster than a glow can be re-fueled! And when we leave no one knows that we’ve been – i.e. a puddle of fuel on the ground.
|Aug 16, 2004, 09:00 PM|
Nice article Jim.
Very well written and easily understood.
My floats for the Cub are still in the box!
Maybe next wekend I'll get some time to get them glassed.
|Aug 18, 2004, 09:16 AM|
Joined Jul 2004
Thank you for this article i've been wanting to get in to r/c but did not want the trouble of glow power. I was looking at smaller planes i.e. the mini blue max and others. I thought a body could convert a .40 trainer kit and now I know how easy it can be. Thanx again for giving me even more to think about.
|Aug 18, 2004, 12:27 PM|
Based on the great response and interest I’ve pulled together some additional information.
The picture that follows lists the specs for some 40 ARF Trainers (a couple Advanced Trainers as well) that I have collected and analyzed; back when I originally did this the Eagle wasn’t on the list.
I used the American Flyer (listed 1st) as the baseline since it flies very well with excellent aerobatics, handling and slows very well for landing. The remainder are sorted by increasing: wing span (ws), wing area (wa), and weight (wt); in inches, square inches, & pounds respectively. The Eagle is nearer the bottom; while the other easily obtained/typical Trainers are near the top. Any plane on the list will successfully fly in “trainer like” fashion with the combo listed in the article.
The last 3 columns of data calculate “cell count” based on some long standing Rules of Thumb (RoT). Time has proved them to be worthy; however technology has improved such that they don’t need to be blindly applied. Most E-Flight RoT were developed when cells wouldn't give you much more than 25amps, and duration wasn’t worth darn at anything more. Also, RoTs assume the motor efficiency of a typical “Can” motor; today BL, Cobalt, and several Can motors do much better.
“wa/50” – Ormes Law (thank you Mathew) states that for Trainer+ performance use 1 cell for every 50sq” of wing area. For 500sq” use 10cells. “wa/35” for aerobatic performance.
I believe “wt*50” was 1st presented by Dr. Keith Shaw and is more commonly known as 50watts per pound for ROG performance. My variation “wt*50/30” calculates cell count based on 50 watts per pound @ 30amps with 1volt per cell.
The last column is an average of the two previous; note “spec” weight is used. For the Eagle the numbers are respectively: 14, 8, & 11; so 12 as you’ve seen is good to go!
No doubt a quick check around here will show that some have flown these planes on less; while other pour the watts on/in.
The one thing I would suggest to change from the kits is to use Dubro’s molded composite main gear ( http://www2.towerhobbies.com/cgi-bin...&I=LXNM65&P=ML ) rather than the stock wire gear; its far more robust. You might need to get a new nose gear wire ( http://www2.towerhobbies.com/cgi-bin...?&I=LXD844&P=7 ) if the Dubro main gear is taller than the stock wire gear; it’s a good thing as it helps improve the ground clearance so you can easily swing the 12’ prop.
Again thanks for the interest and kind words.
|Sep 04, 2004, 05:40 AM|
Joined Feb 2004
Hello just to add an extra option I have used a 60" Moonraker trainer for well over a year with outrunners. first with a AXI 2820/12 and 12 cells then a Electronic Model Twister 40 again with 12 cells 11/7 prop weight 5.5 lbs flew fine with the axi but better with the Twister 40 nice round loops etc, but I would say that as I am now the UK importer of Twister motors.
|Sep 08, 2004, 10:33 PM|
I have an AXI 2820/10 sitting around with a 60amp CC ESC. I would LOVE to do a trainer style ARF and the 6 pack thing. I have a 90* motor mount that will make motor installation a snap. I'm thinking the CG Eagle or a Sig Kadet 40, or Avistar would be a good, cost effective ARF. It sounds like the planes are over the 5lb limit that HL states. Do they mean airframe wt or AUW?
|Sep 09, 2004, 05:51 AM|
Joined Jun 2002
This is all up weight, including batteries. If you are running 12 cells, try a 10 or 11-6. With 35-40 amps this puts the output in the 300-400 range. This is pretty near the limit for this motor and you may not want to run wot all the time, and lots of cooling air.
I run folding props so the motor doesn't risk damage on landings. It sounds like the motor mount isn't gonna have any give. I don't see how you mount this using a gas plane mount. I would firewall mount the motor, or else behind a 1/8 plywood epoxied to the front of the plane behind the spinner. Be sure to keep the wires away from the spinning case of the motor, which is the rotor in an outrunner motor.
|Sep 09, 2004, 07:41 AM|
I'm using this mount with the AXI on a failed experiment. It is very sturdy, but I guess it could be mounted on a break away plate. What plane are you flying and how do you like performance of the AXI? Are you running 12 cells?
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