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Hobbico's Twinstar ARF Converted to Electric!

Mike McDougall walks us through the E-Conversion of a Hobbico Twinstar ARF. This article includes details for programming differential thrust (Rudder/Throttle) mixing. In the Flight Section there is even a side by side comparison with two glow powered versions.

Splash

Introduction


Wingspan:56"
Wing Area:560 sq. in.
Weight:5 lb. 6 oz.
Length:50"
Wing Loading:22.1 oz/sq. ft.
Motors:(2) AXI 2814/12
Props:(2) APC 9x6E
ESC's:(2) JETI 40 OPTO plus
Batteries:(2) Thunder Power Pro Lite 2100 3S1P 15C
Max Current:30 Amps @ 10 Volts
Max Power:300 Watts per Motor
Power Loading:112 Watts/lb.
Max RPM:11,120 rpm
Servos:(3) Futaba S3151 Digital
Transmitter:Futuba 9C
Receiver:Futaba R127DF
Rx Battery:1800 mah NiMh
Plane Made By:Hobbico
Available From:Tower Hobbies
AXI Motors, JETI ESC's, TP Batteries From:Hobby Lobby

TWIN ENGINES!! These words strike terror in the hearts of even the bravest and most accomplished RC’ers. Building a twin involves complicated nacelle structures, intricate plumbing, and difficult servo installations just to house, feed, and control the two power plants. Then you have endless hours on the bench. Or perhaps what scares us most is the endless hours at the field -- running and tuning the engines to get them to “reliably” perform at about the same power level from idle to full throttle. Even after all of our careful running and adjusting, the dreaded “engine out” can still occur. If Murphy hangs around your club field like he does around ours, that engine out will occur at the worst possible time – on takeoff, during a touch-and-go, or on the downwind leg as you turn on final.

Maybe that’s why all the twins I’ve ever seen fly usually make a few cautious circuits of the field and land. Over the last 30+ years of RC’ing I can’t remember any twin-engined model that made repeated low inverted passes over the runway, performed consecutive stall turns, or recovered from repeated intentional snap rolls and spins.

Well, all that is about to change! In this article, I've teamed up Hobbico and AXI to bring a one-two punch that will knockout all of these problems. The first punch is Hobbico’s excellent ARF model, the Twinstar. This model is designed for .25 to .40 glow power and incorporates a number of tricks to simplify the construction process and tame the engine out problem. However, the real knockout punch comes when you convert this plane to reliable e-power with the awesome AXI 2814 series outrunner brushless motors! Controlled by JETI ESC’s, these motors perform flawlessly time after time and are always in perfect sync from idle to full bore. Engine outs are now a thing of the past! Tuning and fiddling? Gone as well.

I know you’re convinced, so let’s get started building.

CONSTRUCTION

One of my goals in this e-conversion project was to employ the KISS (Keep It Simple ______) principal. Hopefully you will agree that this conversion makes very few changes to the glow model. And, since the Twinstar ARF has already been reviewed in its glow configuration, I’m only going to cover the modifications needed to accomplish the conversion to e-power.

All major parts are individually wrapped and carefully packed in a sturdy box with tantalizing graphics. The Twinstar comes with an excellent 24-page construction manual that includes clear instructions and numerous pictures of the various steps.

The E-Conversion starts on Page 8 of the manual under the heading of “Mount the Engines”. Since I couldn't find 2-1/4" long 4/40 bolts I had to use 6/32 bolts to mount the AXI motors. That meant I had to remove the 4/40 blind nuts in each nacelle. I ground off the epoxy around the kit nuts and using one of the 4/40 bolts, carefully tapped out each blind nut. Next, I enlarged each mounting hole with a 3/16” drill bit so I could install the new 6/32 blind nuts. I used a short 6/32 bolt and washer to completely seat each nut into the firewall.

I used Pacer Z71 red thread locking compound to lock the prop adapter screws and mounting plate in place on each motor.

Next I needed to cut some spacers to position the motors far enough forward in the nacelles to insure adequate prop clearance. For the APC 9x6E props, I needed 1-3/4 inch spacers. I like to use 1-1/2 inch diameter plastic drainpipe for the motor spacers. An ACE Hardware 12 inch Tailpiece is light in weight, not too brittle, and the proper outside diameter for the AXI Motor mount. I made two spacers 1-3/4 inch long and drilled a 3/8 inch hole in the middle of each piece for the motor wires. Now I could mount each motor using four 2-1/4 inch 6/32 bolts.

Next I cut away a portion of the rear former of each nacelle. This area was needed for battery mounting and for cooling airflow.

Using the nacelles as a pattern, I marked each wing shear web for a matching oval hole. I then cut a hole in each of the two wing shear webs on each wing. These holes were needed for additional airflow to cool the ESC’s and batteries. Next I coated the inside of the shear webs and the back of the rearmost shear web with slow curing (30 minute) epoxy. This would restore the web strength lost due to the cooling holes. The front surface of the front shear web would be coated with epoxy in a later step when I installed the nacelles.

Now I was back to the manual on Page 10 under the heading “Mount the Tail”. After Step 4, I choose to bend each aileron torque rod so that the threaded end was perpendicular to the wing centerline when the aileron was centered on the wing. The kit had the torque rods bent forward slightly to help clear the rear fuselage former. This forward rake would result in unwanted differential aileron throw. I then needed to relieve the rear fuselage former to allow the torque rods enough room to move freely after the wing was installed.

After Step 3 on Page 12, I programed and installed the JETI ESC’s in each nacelle. Using the JETI Advance plus Prog-Card, I set the following parameters: Battery Type – Li-XX, Cut Off Voltage – High, Cut Off Type – Slow Down, Brake – Off, Timing – High, and Throttle Curve – Linear. I then powered up each motor/controller combination (without propeller) to determine proper motor rotation direction. If the motor rotated the wrong direction, I just switched any two wires between the controller and the motor to reverse the rotation. When I was satisfied with the controller settings, I installed the ESC in the bottom of the nacelle. I choose the JETI OPTO ESC's because of their excellent radio isolation circuitry. However, these controllers did not provide a BEC function and required me to use a seperate receiver battery pack. This was not really a problem as I would need the weight of the receiver battery pack for balance adjustments. I next installed a 3/16” x ½” x 3-1/2” piece of Lite Ply in each nacelle to hold the battery. I installed Heavy Duty hook and loop fastener material to the wing to hold the rear end of the battery and a loop of fastener material to secure the front of the battery.

I was able to omit step 4 on page 12. I skipped steps 13 thru 25 on pages 14 and 15. I carefully cut the excess plastic from each of the nacelle covers and then cut a 1-3/4” x ¾” hole in the back of each cover for cooling air exits. I installed Heavy Duty hook and loop fastener material to the cover and the nacelle as shown.

On page 18, before I installed the nose cone in step 10, I checked the center of gravity with all of my equipment installed. I found that the AXI motors, Jeti ESC’s and Thunder Power batteries were lighter than the glow powered engines and fuel tanks. I needed to install the receiver battery pack in the nose cone to achieve the proper CG location. This required me to cut a 3/16” by ½” hole in the fuselage nose former for the battery lead and then install the receiver battery to the bottom of the fiberglass nose cone with heavy duty loop and hook fastener material.

RADIO PROGRAMMING

Using my Futaba 9C Transmitter, I was able to program dual rates and exponential as well as several interesting mixes for this model. I programmed Aileron/Rudder coupling, dual motors, and Rudder/Motor differential thrust.

Differential thrust allows you to speed up one motor and slow down the other motor in response to rudder input commands. This greatly aids in ground handling and adds a little something extra to snap rolls and spins.

To program differential thrust, begin by setting up one of your program mixes for dual throttle. Plug a spare servo into channel 3 of your receiver and a second spare servo into one of your AUX channels. Set your mix with the throttle as the master and the AUX as the slave. Set this mix as always “on” by choosing any switch and the “null” position. Use 100% for the mix amount. Verify that both servos move in the same direction and move the same amount from idle to full throttle positions.

Next set up a second program mix with the Rudder as master and Throttle as the slave. Choose a switch to activate this function and use a mix amount of 25%. Verify that the servo in channel 3 moves toward the up throttle position when left rudder is applied and toward down throttle when right rudder is applied. If not, change the mix amount to –25% and test the movement again. (Leave link OFF so that it doesn't try to command the aux2 servo to move also.)

Finally set a third program mix with the Rudder as master and AUX as the slave. Choose the same switch and position that you used for the second mix and again use 25% for the mix amount. Verify that the servo in the AUX position moves toward the up throttle position when right rudder is applied and toward the down throttle position when left rudder is applied. If not, change the mix amount to –25% and test the movement again.

Now we are ready to check the programming on the plane. Plug the right hand motor ESC into channel 3 and the left hand motor ESC into the AUX channel. Turn off the switch that controls the differential thrust mixing. Turn on your radio and plug in your batteries to the ESC’s.

First, verify that the throttle control operates both motors in normal fashion. Hold the airplane securely and move the throttle stick from idle to full power. Both motors should track each other from idle to full bore. Now set the throttle stick to the idle position and switch on the differential thrust-mixing switch. Slowly advance the throttle until both motors barely begin to turn. Now move the rudder stick to give full right rudder command. The left motor should speed up and the right motor should stop. Left rudder command should cause the right motor to speed up and the left motor to stop. Taxi test your model, switching the differential thrust “off” and “on” to test the effectiveness of the 25% setting. If you have a paved runway, you may want to reduce the mix amount to 15%.

FLIGHT TESTING

Pre-Flight static testing of the motor/ESC/battery combinations showed 300 watts of power per motor turning the APC 9x6E props at an amazing 11,120 rpm. That translated into a 1:1 thrust ratio at about 2/3 throttle! With 112 watts of power per pound, this was going to be fun!

Initial taxi tests showed that the differential thrust was very effective in helping turn the plane without the tipping over normally associated with sensitive nose gear steering. The weather finally cooperated and we were able to roll out the Twinstar for its maiden flight. All of the Club “Pros” gave their somewhat negative opinions about electric power before the flight. The tone of their comments changed dramatically when I advanced the throttle to 2/3, pointed the Twinstar straight up and briefly released the fuselage and the plane just hovered in space.

For the first flight I set all the rates to low and switched the differential thrust to the “off” position. The first flight takeoff occurred quickly and included a 2/3-throttle climb out at about 80 degrees. The Twinstar was rock solid and just wanted to keep climbing.

Two clicks of down trim and two clicks of left aileron trim produced straight and level flight. I went through my usual 1st flight routine of testing control authority, stall characteristics, and some mild manuevers. Before I knew it, my flight timer said it was time to land (6 Minutes), but the Twinstar was begging to keep flying. I reluctantly set up a landing pattern and the Twinstar settled in smooth as silk. It must be the Twinstar, because my landings are not usually that smooth. Post-flight inspection showed the AXI motors cool to the touch and the TP batteries and JETI ESC’s barely warm. There was nothing but positive comments up and down the flight line after that first flight. It wasn’t until later in the morning that I realized how relaxed I had been during the first flight. I'm sure that not having to worry about engine outs, like on the glow twins I had flown previously, made a big difference. I was able to fly lower, more aggressively, and with greater confidence because of the reliability of the electric power system on this Twinstar. AXI brushless motors, JETI ESC's and Thunder Power LiPoly's are truly the power source of choice for Hobbico's excellent Twinstar ARF.

STANDARD FLIGHT PERFORMANCE

  • Rolls were axial but the roll rate was a bit slow for my liking. I switched the ailerons to high rate.
  • Stalls were straight forward without a tendency to drop a wing.
  • Stall Turns required high rates on the rudder, but they looked pretty good.
  • Spins were difficult with rudder set at low rates. High rate rudder resulted in fine spins that stopped as soon as the sticks were released.
  • Knife Edge fell off badly even with high rate rudder. (Read on to differential thrust!)
  • Point Rolls fell off due to lack of rudder. (Read on to differential thrust!)
  • Inverted flight required just a touch of down elevator for level flight.
  • Inside and Outside Loops were straight and true. With this much power, I could make them as large as I wanted and the Twinstar would pull all the way through them without a trace of falling off.
  • Touch-N-Goes: The Twinstar looked really great shooting repeated touch-n-goes. Very predictable sink rate and excellent control all the way through touchdown.

DIFFERENTIAL THRUST FLIGHT PERFORMANCE

  • Stall Turns were "Pattern Perfect". The yaw performance of the Twinstar was really affected by the different motor speeds when the differential thrust was enabled.
  • Spins were spectacular! Upright and inverted spins were very impressive. They still stopped as soon as the sticks were released.
  • Knife Edge flight was now possible.
  • Point Rolls were a piece of cake.
  • One Motor Off - I turned off the left motor with the TX and the Twinstar flew just fine. Flew a little slower, but very stable even turning to the left. The only thing more impressive than the plane's single motor performance was being able to turn the motor back on in the air. E-Power is truly wonderful!
  • Taxing back to the pits was a real treat as the differential thrust would speed up one motor and stop the other to help with the turns off the runway and into the Twinstar’s pit space.

Special thanks goes to Hobby Lobby for their technical assistance and excellent advice on E-conversions. They provide this service for any modeler wishing to convert from glow to E-Power. Check out their helpful website. Hobby Lobby E-Conversions It was Hobby Lobby's recommendations that were the basis for the selection of the motors and ESC's for this project.

SIDE BY SIDE BY SIDE TESTING

With the help of a fellow club member I was able to fly and compare three versions of the Twinstar in one morning. It was my personal version of a “Triple Double”. Twinstar #2 was an older version of the ARF powered by OS .40 LA engines. It weighed 7 lbs 4 oz. Ready-to-fly (RTF). Twinstar #3 was a recent version powered by Thunder Tiger .25 engines. It weighed an even 6 lbs RTF.

The .40 powered Twinstar flew “heavy”. Even though it had more power than its counterparts, it had to fly faster to keep it’s extra weight airborne. Maneuvers were not nearly as crisp and the plane took a lot more sky to turn around and get back in the pattern. It also took a lot more elevator to flare on landings. Five minutes was maximum flight time due to those little fuel tanks in the nacelles. Even though it may not have flown as good as the other two Twinstars, it flew better than most other twins I have flown.

The .25 powered Twinstar flew almost identically to the e-powered version. It was not quite as fast at top speed as the e-twin, but the .25 glow version flew great. At 2/3 throttle setting, the .25 Twinstar flew maneuvers with ease and there was no problem keeping it in the pattern. Landings were a thing of beauty. Six minutes was maximum .25 glow powered flight time. Glow power or E-Power, Hobbico really has a winner with this ARF!

Photo and Video Gallery

Downloads

Conclusion

Plusses:

  • Lighter RTF Weight = Better Flight Performance
  • Better Vertical Performance
  • 6 to 10 Minute Flight times depending on flying style
  • No Engine Fiddling
  • No Engine-Outs
  • Perfect Motor Sync
  • Differential Throttle Control Simple and Reliable
  • Less Field Support Equipment
  • No messy cleanup after each flying session.

Minuses:

  • Cost - about $300 more than 2C glow or $100 more than 4C glow
  • Waiting for LiPolys to recharge between flights

Discussion

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Old Mar 08, 2006, 01:42 PM
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jonnyjetprop's Avatar
USA, FL, Apopka
Joined Sep 2003
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Neat article. I have just converted one as well. I used 2 Jeti Phasor 30/3 motors, 2 Jeti 40 esc and a single TP 3s4p gen 1 pack in the fuse. I'm pulling 30 amp a side with 9x7 apc eprops.

John
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Old Mar 08, 2006, 03:05 PM
We shall serve the Lord
kingsflyer's Avatar
United States, TX, Kingsland
Joined Sep 2005
4,970 Posts
Glad you liked the article. What was your RTF weight with that battery system? How does yours fly? Sounds like it should fly great. Have you tached the motors for max rpm under static load? Try the differential thrust I described in the text. It will really make a big difference in manuverablity.
Mike McD
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Old Mar 09, 2006, 05:26 AM
know it all
Miami, Fl
Joined Nov 2003
2,949 Posts
Great article!.. I have been flying mine with two axi 2826-10 and 9*6 props.. but with one central battery, and two BP 60Amp esc. Lots of heavy wiring in the wing.. using a 4s2p 4200 tronics pack (17 oz) total weight is 6lbs 6oz, each motor is seeing around 22amps, total power is around 650watts I believe.

Flies great as you describe but no spins or snaps.. I have not setup differential throttle with my JR622.. what cg position are you using?.. it can move back quite a bit from stock position.

I cut a huge hatch in the fuselage for battery replacement.. makes it a snap to change them. My average 8 minute flights consumes around 3500mAh. You can see pictures and movies in this thread:

http://www.rcgroups.com/forums/showthread.php?t=439619

Had vibration problems until I replaced the apc 9*6 e props with a pair of Graupner 9*6 gray props, the one on the right is a reverse pitch pusher so now I have counter rotating props. Everybody at the field is amazed and the stability and docility of this plane, plus it does great fast touch and goes very gentle stall characteristics. Pretty much a fast trainer with no bad behaviors.

Could not tell any difference in roll rate with the counter-rotating props

Herm
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Old Mar 09, 2006, 07:05 AM
We shall serve the Lord
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United States, TX, Kingsland
Joined Sep 2005
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Herm,
Thanks. The Twinstar is really a great flying twin. My CG came out at the rear of the recommended CG range. Mine would not spin with low rate rudder. With max rudder throw, it spins pretty nice. With max rudder and Differential thrust, it almost turns inside out. Its hard to do only one snap roll they happen so fast! I highly recommend trying the DIfferential thrust. It's worth the programming hassels. I had to balance the APC's before they smoothed out and the stock spinners were unusable. Thanks for the tip on the Graupner reverse pitch props. I was wanting to try counter rotating props. I like the sound of the AXI electrics, most folks comment that my Twinstar sounds like a real turboprop jet.
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Old Mar 09, 2006, 10:25 AM
know it all
Miami, Fl
Joined Nov 2003
2,949 Posts
one problem with the Graupner props is that they are a bit quieter than the apcs.. probably means more efficient but I like hearing the props

My next goal is to get it to spin..

Herm

Quote:
Originally Posted by kingsflyer
Thanks for the tip on the Graupner reverse pitch props. I was wanting to try counter rotating props. I like the sound of the AXI electrics, most folks comment that my Twinstar sounds like a real turboprop jet.
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Old Mar 26, 2006, 09:39 PM
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United States, TX, Kingsland
Joined Sep 2005
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Prop Update. Tried Graupner 9x6 counter-rotating props. Reverse pitch prop placed on the right side motor. Current draw was too high so I trimmed 1/8" off each blade tip of both props and rebalanced them. Now they pull the same current as the APC 9x6E. Made a big difference in flight performance. Takeoffs and loops are straighter, right and left spins and rolls are the same rate and snap rolls are the same to the right or left.
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Old Apr 16, 2006, 01:40 PM
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USA, FL, Apopka
Joined Sep 2003
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Here's some pics of mine. Jeti Phasor 30/3, jeti 40 amps esc, a TP 3s4p gen 1 pack and apc 9x7.5 eprops. It draws 30 amps a side.

John
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Old Apr 17, 2006, 02:43 AM
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United States, TX, Kingsland
Joined Sep 2005
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Nice looking conversion, John. What was your RTF weight with this set up? Your hatch arrangement should make battery replacement a cinch! Have you tried Differential Thrust and Counter-rotating props? They really makes a great flying plane even better.
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Old Aug 30, 2006, 10:57 PM
know it all
Miami, Fl
Joined Nov 2003
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reconverting mine to batteries in the nacelles, had to move motors forward and fool around with balance.. could boost power to over 1kw, but I doubt I will do that.

Herm
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Old Aug 31, 2006, 11:41 AM
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Sounds interesting. I take it that you are still running with the AXI's on 4s.

John
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Old Aug 31, 2006, 12:35 PM
know it all
Miami, Fl
Joined Nov 2003
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Yes..

Herm

Quote:
Originally Posted by jonnyjetprop
Sounds interesting. I take it that you are still running with the AXI's on 4s.

John
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Old Aug 31, 2006, 05:28 PM
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I'm away from home for a couple of weeks, but around 6 pounds is close. I haven't tried diff trust or counter rotating props. To be honest, I have been looking at changing my setup. Nothing wrong, but I'm looking for extra power and my older TP gen 1 packs have started to show their age. They owe me nothing. They have given flawless service for 3 years.

I too am looking at mounting batteries in the nacelles. I was thinking of going 4s, but the phasors are not the motor for that.

John


Quote:
Originally Posted by kingsflyer
Nice looking conversion, John. What was your RTF weight with this set up? Your hatch arrangement should make battery replacement a cinch! Have you tried Differential Thrust and Counter-rotating props? They really makes a great flying plane even better.
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Old Sep 03, 2006, 07:18 AM
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United States, TX, Kingsland
Joined Sep 2005
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John,

I'm using the GenIII TP's - 3S1P 2100 packs in each nacelle. I've still got some GenI Packs that are still working, but on their best day their performance pales in comparison to the new packs. I've run them side by side and the GenI's will fly the plane and the GenIII's make it perform . It must be the internal resistance reduction of the new cells that allow the motors to really put out their max power. Might consider some 3S GenIII Packs with your existing motors for a considerable performance inprovement. Also, the 4S packs might be a tight fit in the Nacelles.

Looking forward to your update. Include some video if you can.

Mike McD
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Old Mar 11, 2009, 08:11 PM
JR > Futaba
pvtzemerak's Avatar
Green Lane, PA
Joined Mar 2008
340 Posts
Mike,
That seems like a powerfull little plane, and i loved the review.
I'm not sure if I would go with glow or electric because of the costs, but it looks like an "Everyday" model.
--PvT
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