Extra 3.25


Extra 3.25 "Endo" Powered

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This time I'm joining forces with my IC buddy (Lynn B.) to create a "foamie" version of the Rich Uravitch Extra 3.25 pattern ship (shown above).

bulletWingspan 47"
bulletLength 35"
bulletWing area 364 sq. in.
bulletRTF @ 59oz with 10 cells of CP2400SCR

These 50", Speed 600 (or .20 IC) size pattern ships seem hard to come by. My 47" Kyosho Flash EP has proven to be an awesome flyer but is no longer in production. The gap right now seems to be in Speed 600 pattern ships. Even the hardcore IC flyers seem to really light up when they see a .20-sized Extra that doesn't need to be wiped clean after every flight.

Our plan is to build our own lighter foam version of the 4 lb (64oz) glow engine $62 Lanier kit.

Lynn has both plane design experience and a foam cutting shop. I would provide the electric flight knowledge and design the power system with help from my fellow E-Zone members. The Canopy, Cowl, and Wheel pants can be purchased for $22 ($32-$10 since we have the plans) from Rich's Home page.

The thinner foam wings, fuselage, and "lifting" tail should be lighter than even the 4lb Lanier kit meant for glow engines. We are targeting around 3.5lbs (or 56oz). It should also still be around the same Lanier $63 kit price but including the power system!

Instead of an IC gas engine or expensive Cobalt (which is already installed in my Hacker Arrow 3D) or brushless motor, I'm going to try a geared Kyosho Endoplasma ("Endo") car motor for the power system.

I had great success with the "Endo" in my 48oz Kyosho Flash pattern ship. I replaced the stock AP36 motor with the superior "Endo" motor for 1/2 the price and went from 9000 RPMs to 10,000 RPMs at the 9x6 prop with a 2.5:1 gearbox. The "Endo" has dual ball-bearings, large 5x5 brushes, 4.9 wet magnets, and dual braids to the terminal. It seems happy to run in the 30-40 amp range.

Here is our low-cost approach to the power system:

bullet$20 Kyosho Endoplasma car motor
bullet$15 Great Planes GD-600 gearbox
bullet$4 for the 4.6:1 10T pinion gear
bullet$4 for APC 12x8 E-prop
bullet$7 for a very light Sonic-Tronic (ST-160) Speed 600 motor mount
bullet10 sub-C cells 1900SCR, CP2400SCR, or 3000RC Hydrides

With a 12x8 prop, it should draw 35-40 amps, spin the prop at xxxx RPMs and give about >50oz of thrust. With an almost 1:1 thrust to weight ratio, it should have plenty of vertical flight and top speeds of 50mph.

Much of the pioneering work has been borrowed from the E-Zone's geared-Endo enthusiast Gary Wright. Gary has created many of these systems using even higher ratios on a $50 MEC Superbox. His latest E3D project is very impressive!

We have 3 goals in our design philosophy:

bulletImprove the flight design
bulletBuild it cheap but good
bulletBuild it light but strong

At first glance, you may think this is every persons goal, but with just a few compromises and tests, it may workout very well.

Our improved flight design will come from a thinner wing than is used on the designer's plans, a lifting stabilizer, and lighter weight.

Our low cost will come from using an Endoplasma motor, Great Planes gearbox, and only 10 cells.  The incredible power is obtained from the combination of the "beefy" Endoplasma car motor and a high ratio gearbox that utilizes a 12" prop.

Our light weight will come from the foam fuselage, wing, and tail section. Lynn has been cutting foam wings (and things) for over a decade so he has convinced me that it can be done cheaply and properly. The strength will be from a combination of proper foam assembly and covering techniques.

Cutting the Foam Parts:

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Foam fuselage parts

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Foam wing halves using thinner airfoil

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Tail parts, Ailerons and Stabilizer side view

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Assembled fuselage top and side view

The ailerons were very delicate and retained a curve memory so we decided to replace the foam version with balsa aileron stock.

Choosing the Covering Technique:

Our first choice was to try using Litespan applied with a white glue and then shrink it. Initially, we used an iron to shrink the covering but all the foam imperfections showed through. A second attempt using a heat gave a much better appearance.

We thought about using Silkspan but would have been forced to either paint the colors on or use a secondary film covering scheme.

Balsa Products sells a lighter version of Litespan called Airspan (or LM Tissue) that replaces Japanese tissue. It comes in various colors and can be applied with either Elmer's white Glue-All or Aliphatic resin (wood glue) or water-based Balsaloc. This is what my buddy, Lynn, will likely choose for his covering technique.

I finally decided to use some of the 10 rolls of Oracover (a.k.a. UltraCote) that I purchased from a recent Hobby Lobby sale. It turned out to be a good decision because the Oracover is a designed plastic model covering material that works differently from other plastic covering materials. Oracover won't trap a single air bubble when applied to a sheeted surface. It won't ever sag or loosen and covers corners perfectly!

I was very happy with the final look and strength of the foam wing when finished. I used one coat of Goldberg Jet Model Mate and then after some sanding, simply covered it with the Oracover. This was the lightest 47" wing that I had ever seen and it had excellent strength!


It's Starting to Look like an Extra:

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Extra 3.25 with mounted wing, cowl, canopy, and stabilizer

Most of my planes were ARFs (Almost Ready to Fly) so this scratch built foam Extra was much more work than I was used to. There is a good deal of satisfaction gained in building and designing a plane from parts...it just takes longer. After mounting the wing, cowl, canopy, and stabilizer, it really started to look like an Extra! This helped inspire me to continue.

I mounted the Kyosho Endoplasma motor to the G.P. GD-600 gearbox and Sonic Tronic Speed 600 motor mount. This assembly was then mounted to the plastic cowl first and then mounted (after measuring) to the balsa block firewall. I then applied glue around all 4 sides of the firewall and inserted the whole assembly (cowl, gearbox, motor, balsa block) into the fuselage. I taped the cowl in place and let it dry overnight.

My motor was now held in two places. It was held by the balsa block firewall via the Sonic Tronic motor mount and it was held by the plastic cowl via 2 screws and then taped around the fuselage. I felt that this technique would best distribute the load onto the foam fuselage.

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Mounted Endoplasma motor, GD-600 gearbox, and cowl

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Painted canopy made from plastic, foam, and balsa

The canopy took some patience to build also. It started out as a few weak pieces of foam that I attached a balsa backboard to and then spray painted it with Testors Enamel paint. The plastic canopy cover was glued with clear ProBond.

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HS-55 aileron servos (2) were mounted under the wing

  Each aileron has its own HS-55 servo. I used 2 channels to mix them together on my Futaba Super-8 transmitter.

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Components fit in the fuselage under the wing

I used a Great Planes C-50 ESC, Futaba R127 receiver, and 10 cells of CP2400SCR. All these components fit inside the fuselage compartment just under the wing. Due to the weight of the flight pack, I reinforced the foam compartment with a balsa lining. To keep the receiver and 8-cell CP2400SCR pack from shifting during flight, I used some foam blocks cut from the container pack of a GWS R4-P receiver. This white foam is a high density foam that is extremely durable yet light. I also used an entire block of it to hold the wing into place (shown below).

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Wing is held in place with foam block from GWS R4-P receiver

My "foamie" Extra 3.25 is RTF at 59oz with the 10-cell CP2400SCR pack. The pack itself weighs 22oz and the empty plane weighs 37oz. My CG may be a bit tailheavy but I will see how it flies before changing it. Once the plane has proven itself an excellent flyer, I will then finish the covering and make it look much better.

Here are some pictures of the plane ready-to-fly on its maiden voyage...wish me luck!

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