Friday, August 28, 2015
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Building a Competitive Electric IMAC Plane on a Budget – Terry Ferentinos  

IMG_2410 I have been flying an Extreme Flight 70” Electric for about 250 flights now and have come up with a “formula” that will work for Basic and Sportsman class allowing plenty of time for 2 sequences, landing as well as a safety margin should a go around be needed. Considering that the winner of last years Nats for IMAC Sportsman was flying a 75” plane, this proves to me that a 30-40% airframe (and the associated expense) isn’t really needed in the first two classes to succeed. It’s practice, practice, practice and a good setup on a good airframe.

My goal here is to show that getting into IMAC initially doesn’t have to be expensive and you can do it with an airframe that is easy to transport and is fun to fly and 3D as well. This plane also presents well considering the size so if judges can overcome what some might consider the “need” for larger size planes to compete in IMAC successfully, it should score well. Plus since it’s electric, you should get maximum sound scores. This way you can try IMAC and if you like it move up to a larger plane when you move up in classes.

The airframe I chose is the Extreme Flight 70” Extra which at $389.95 + shipping and a bargain considering what you get and how good this plane trims and IMG_2409flies. We all know Chris builds a great product by his airframes success in IMAC.

On the engine, I have gone a more economical route than some of you may choose to go. Certainly AXI, Hacker and Hyperion make some great motors but they will add $100 - $200 to the total cost of this project.

I choose the Turnigy AerodriveXp 90 SK Series 50-65 270Kv / 2100W  because of some experience I had in the past with smaller SK series engines and how well they were built and performed for the money. I have done quite a bit of testing with these motors, flown them pretty hard to try and break them and found them to be solid performers with the exception of three areas. That is, the Xmount (it’s cast from HK and won’t hold up), the bearings (also won’t hold up over time and hard running so just replace them up front) and the magnets which have to be inspected to make sure there is glue part way in the spaces between then (there was a problem with the early SK series and magnets coming loose, that was eventually overcome at the factory but some old stock is still leaking out) But even with the few problems at $39.99 these motors are a real bargain!!IMG_2408

The bearings are two front 12x8x3.5mm and 1 rear 16x8x5mm. You can just get some quality ABEC3 or better bearings to replace them. I changed mine to semi ceramic, but only because I was able to find some for about $5 each. Fixing the magnets is pretty easy by just adding some glue to the area between the magnets if these isn’t some there already. Some recommend Loctite 410 for this.

The XMount can be replaced with a Hyperion 30mm XMount which is higher quality or a plate of aluminum that was set up for 30mm mounting holes on electric engines (which is the route I choose because I tend to stress the engine mount pretty hard). My local hobby shop had these mounts and they are available on the net if you look.

I make 2100W on an 8S setup (which is perfect for this plane and what Chris intended when he designed it) on this motor day in and day out and my timer is set for 9 min with a APCe 19x10 prop (actually I run a 20x10 that I cut down to a 19x10 because it seemed to draw a few less amps that the straight 19x10) Vertical performance is unlimited.

On batteries I choose the Turnigy 5000mAh 20C Lipo  which is capable for 100A IMG_2411continuous discharge. This is more than enough for the engine and after 150 flights or so, my flight packs are still rock solid and show no signs of puffing. I generally put 4000mAh back in the packs after flying which is good as I believe that in order to keep Lipos in good condition you shouldn’t discharge them more that 80-85%

I choose to use a Castle Phoenix 100A ESC because I wanted data logging capabilities as I was bringing the engine/prop up to speed, but any quality 80-100 EAc will do. I spent about $100 here. (If you look around on the net you can find that deal)

I choose a BEC (8-15A)  and separate 1500mAh 2S Lipo for receiver and servos because I wanted a separate system to power this up and I wanted 6V servo operation.

On servos I went with the recommended digital Hitecs for the model and probably spent $300 or so with metal MPI arms. But choose what you like for this.  Some misc stuff cost like wire extensions, series lipo connector, arming plug, spinner, etc.

So just to review what we have spent to far on a competitive IMAC setup:

  • $389.95 Plane
  • $49.99 motor
  • $15.00 high quality bearings
  • $100.00 ESC
  • $15.00 Prop
  • $60.00 Lipo 4S 5000 two batteries
  • $25.00 BEC and Turnigy Lipo 2S 1500 20C
  • $300.00 Servos
  • $50.00 misc stuff

So for just over $1000, you will have a great practice plane, a fun sport flyer and 3D’er and one that you can compete in IMAC with and be competitive. You will need to add to this a few more sets of batteries and a good charging system if you don’t already have one.  This is a substantial cost savings over what a 40% or even a 35% plane will cost you to put together.

Since writing this I have put together a 78” EF Extra EXP for a friend and he fly’s that on a Torque Series Electric motor and 10S 5000 mA 20C batteries. Once again the timer is set at 9 min, there is plenty of reserve power at the end he never uses more than 80% of his batteries, so this size is also worth considering. I believe this plane would be very competitive in Basic and Sportsman.

By: Terry Ferentinos, Georgia

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