Greg Covey

Hi,

Although many new electric Almost-Ready-to-Fly Airplanes (or ARFs) are entering the R/C market almost daily, the widest range of choices for larger size planes, .40-size and up, are still the models intended for glow flight. Today’s ARFs use lighter materials and have much less over-building than their predecessors. Many electric power systems offer a similar mounting footprint as a glow engine making for easy glow-to-electric conversions that perform with unparalleled reliability.

The Graupner [link=http://www.hobby-lobby.com/extra300.htm]Extra 300S[/link] was designed for a .40-size glow engine but easily converts to electric flight with very few changes. The beautifully appointed Extra 300S has a large 63” wingspan, 46” overall length, 639 sq. in. wing area, and is meant to fly at around 6.5lbs with glow fuel.

My goal for this project was to use a maintenance-free, direct drive AXI motor and demonstrate that it is not only easy to convert the plane but that it actually flies better with clean and quiet electric power!

Greg Covey

Before removing the parts from the box, I immediately noticed that each piece was wrapped in plastic and carefully placed in custom-sized areas. A tri-lingual manual with photos and several decal sheets were also supplied wrapped.

The pre-built Graupner ARF model has amazing quality! The fiberglass cowl is pre-painted in three colors. The light yet strong built up balsa and ply wings has the ailerons pre-installed and each hinge is pinned for strength. The covering is quality heat shrink film with each color a separate piece (not preprinted film). The fuselage is conventional wood construction with the motor mount and pushrods pre-installed. The landing gear hardware and the wing mounting hardware are pre-installed, and the entire fuselage is already covered. The pilot and canopy are also pre-installed.

Although the box shows a male pilot, my ship had a Patty Wagstaff pilot figurehead.

Greg Covey

The fuselage is built light yet strong and pushrods pre-installed including clevis and keeper.

The landing gear hardware is pre-installed and designed to take a real hit on landing without suffering any damage other than bending the gear mains.

The wing mounting hardware and motor mount are also pre-installed. I'll be removing the stock motor mount brackets that are held in place with screws and T-nuts.

Greg Covey

The fiberglass cowl is pre-painted in three colors. It fits perfectly onto the fuselage nose!

All the control surfaces are pre-installed and each hinge is pinned for strength.

Greg Covey

Since there was little work left to complete the ARF model, the key design choices for my conversion would be to select light radio components and in the power system design itself.

Required for completion are a 4-Channel radio, 4 standard-size servos (2-Aileron, 1-Elevator, 1-Rudder). Note that for my electric conversion, the throttle servo was replaced with a Jeti Advance 70amp Electronic Speed Control (ESC).

To keep my [link=http://www.hobby-lobby.com/extra300.htm]Extra 300S[/link] light, I used some Hitec components from Hobby Lobby. My four HS-85 "Mighty Micro" metal gear, ball-bearing servos provide 49oz/in torque and weigh only .77oz or 1/2 the weight of a standard size Futaba S148 servo. My Hitec "Electron" 6 channel receiver is a "full-strength" dual-conversion design that weighs only .66oz compared to my Futaba R127DF that weighs 1.5oz.

That's a 4oz or 1/4 pound drop in weight which will help offset the heavier electric power system.

Here is a list of my components that I selected for the conversion:
[ul][*] 4 of the HS-85MG Hitec HS-85 "Mighty Micro" Plus Metal Gear[*] Hitec "Electron" 6 Ch. Micro FM Receiver[*] AXI 4120/14 External Rotor Brushless Motor[*] Radial Mount Set for AXI 41 Series Brushless Motors[*] Jeti "Advance" 70 Amp Opto Brushless Controller, 6-16 Cells[*] Ultimate BEC[*] APC 14x7 e-Prop[*] Graupner (GR605360) Prop Shaft Adapter for 6mm Motor
Shaft
[/ul]

Greg Covey

I haven't decided which mount will work best yet so I actually ordered both mounts below.
[ul][*] PM41002 Radial Mount Set for AXI 41 Series Brushless Motors[*] PM41001 Aluminum Motor Mount for AXI 41 Series Brushless Motors
[/ul]

Since mounting the motor is one of my favorite parts on a glow-to-electric conversion, I will start with this task.

Greg Covey

Mounting the AXI motor onto the Extra 300S firewall was an easy task.

First, I removed the stock glow mount bars. Using the [link=http://www.hobby-lobby.com/motormt.htm#AXI1]PM41001[/link] Aluminum Motor Mount, I only needed to move two of the four pre-intalled T-nuts. This required drilling two new holes in the firewall as shown in the photo.

I removed the unused throttle linkage and screwed the motor mount onto the firewall T-nuts using the four stock screws. I then applied some blue Locktite to the threads behind the firewall.

Greg Covey

The AXI motor doesn't come with any mounting hardware so you need to supply four M4 metric screws and washers. My screws were about 3/4" long but 1/2" long would still be plenty.

I added a little Locktite to the screw threads to keep the motor securely fastened.

The resultant mount was rock-solid!

Greg Covey

I had pre-measured the distance needed from the firewall so I already knew that the cowl would fit perfectly.

I used an MP Jet [link=http://www.hobby-lobby.com/propadap.htm]MJ4706[/link] collet prop adapter for 6mm shafts along with an APC 14x7 e-prop and the stock red spinner supplied in the kit.

Greg Covey

Here are a couple more views of my mounted AXI motor in the Extra 300S.

Greg Covey

The aileron servos and linkage installed with ease. To compensate for my smaller HS-85MG servos, I added a plywood block on one side of the servo and glued it to the existing post.

All the parts were supplied including snap keepers and ring keepers. Great attention to detail is a trademark of Graupner models.

Greg Covey

The tail assembly was fairly simple and I saw no problems. First you need to mark the area and then cut away the covering so the glued surfaces make a stronger bond.

After gluing the horizontal stabilizer with epoxy, then glued the vertical stabilizer and measured for correct incedences.

My wing, tail, and motor mount assembly were finished in only a few hours.

Greg Covey

Leave it to the Germans to find a way to make things even easier through good organization. I hadn't noticed when mounting my aileron control horns that all the supplied hardware was already organized by attaching parts to their respective screws or threading. Even the set screws were already installed into the quick links. The photo here shows how the hardware comes in the bag.

This may be my first glow-to-electric conversion where I used the stock wheel pant mounting scheme and hardware. Needless to say, I was pleasantly surprised?

My only deviation from the instructions was to not cut off the far end of the wheel axle and allow it to go through the pant on a second hole.

The supplied wheels are Ultra-Lite foam wheels that weigh practically nothing!

Greg Covey

My Extra wheel pants had a great finished look and they were mounted on solid to the gear mains.

Greg Covey

Graupner changed the design of the elevator linkage since their initial release of the Extra 300S last year. My first model used a metal joiner to connect the wing halves together. The new models use seperate control rods which require two servos instead of one.

Ideally, I wish they had added a fourth control rod for a pull-pull setup on the rudder.

After I realized that my mechanical setup on the two elevator servos would not allow for a "Y" adapter cable, I utilized the power of my Futaba Super-8 transmitter by plugging the second elevator servo into channel 6 and programmed a mixer to follow channel 2.

Alternatively, you can reverse one of the servo arms or even use a single servo by running both control rods to the same side of the servo arm.

Greg Covey

I try to route my receiver antenna wire through an existing hole or exit in the fuselage whenever possible.

On my Extra 300S, I used the existing elevator control rod slot in the fuselage for my antenna exit hole. I have a long piece of stiff copper wire that I first run through the control rod slot into the fuselage and then solder the end to the receiver antenna wire. The copper wire is then pulled back out the slot and cut away from the antenna wire.

Greg Covey

The UBEC input is wired in parallel with the ESC battery connector. Since the ESC is an "Opto" isolated type, it needs power into the plug from the receiver which it gets from the UBEC device output. Simply plug the ESC into the throttle channel and the UBEC into any free channel. If no free channel exists, then a "Y" adapter is needed for the throttle channel.

Non-Opto ESCs usually need the on-board BEC disabled which requires disconnecting the red "+" wire from the receiver connector. Opto-isolated ESCs have no on-board BEC and require power from the receiver via an Rx. battery or UBEC.

The UBEC is a state of the art switching regulator designed to convert an input voltage from 5.5v to 35v DC into a regulated output voltage of 5v to power your receiver and servos. The UBEC can deliver a continuous current of 3amps and a peak short term output up to 5amps. This is meant to handle power for up to 8 servos.

A typical 4-cell receiver battery pack weighs 3.2oz so I saved 2.2oz along with the added convienience of not having to worry about re-charging another battery pack.

Greg Covey

Here are some photos of the tail linkage, tail wheel, and tail brace on my Extra 300S.

The steerable tailwheel mounted easily and has a loop in the metal rod for shock absorption. I found an extra aluminum plate in the tailwheel bag that already had holes drilled into it so I used that one instead of the aluminum strip that came in the tail brace bag.

Greg Covey

To mount the cowl, I cut slots in the bottom for the gear mains to slide through. After positioning the cowl next to the firewall, I marked the spots for the cut. I found that after first tracing my markings with a sharp razor knife, I could easily cut away the slots with sizzors.

I also added a large opening onto the bottom of the cowl to act as both an air exit for motor cooling and as an access point to connect the battery for flight or disconnect it for charging.

Greg Covey

I didn't see any screws supplied in the kit so I mounted the cowl using 6 small SIG (SIGSH711) sheet metal screws.

You can see the battery cable access is simple and very convenient. The 14x7 prop and supplied spinner fit perfectly for a great scale look!

Greg Covey

Here are some options for battery power on the Graupner Extra 300S.
[ul][*] 16-cells CP2400 NiCd (in yellow) = 34.7oz[*] 16-cells CP1700 NiCd (in black) = 27.0oz[*] 4s3p Kokam 1500HD pack (4.5AH, not shown) = 16oz[*] Kokam 4-cell 2AH pack (in red) = 11.0oz
[/ul]

As shown, all these packs fit perfectly in the Extra 300S. The packs vary in weight and cost. The NiCd packs are placed in positions that will likely make the plane nose heavy so they can be moved aft when I can test the balance after my rudder servo arrives.

We are still waiting for pricing on the new Kokam 2AH packs but it will be an interesting test to fly a .60-size Extra on only 4 cells of Lithium.

Greg Covey

My AXI-powered Extra 300S is ready-to-fly at only 85oz using the 11oz., 4-cell Kokam 2AH pack. That's 5oz under the glow-powered weight!

I switched from a 14x7 APC e-prop to a 15x10 e-prop to compensate for the voltage drop on the single pack and from less voltage than the initial 16-cell, CP2400 NiCd pack measurements. The prop change provided a 40amp draw.

I am thinking about re-configuring my elevator control rods to use a single servo like the original Graupner Extra 300S that I first converted to electric power. This would give me over a 1/2lb total drop in weight versus the glow-powered version.

My original Extra 300S weighs more without any battery pack installed. I'll have to investigate why there is such a weight difference. Perhaps back then I was using standard size Futaba components.

Greg Covey

I replaced the 14# or 16# wires supplied with my test pack with 12# wire and my current draw jumped up 3amps to 43amps using the 15x10 APC e-prop.

My biggest problem now is balance. Even after moving my pack forward on top of the motor mount (as shown), and, removing my 0.4oz tailbrace on the horizontal stabilizer, my CG is about 1/2" aft from the "expert" setting suggested in the manual. The CG is also 1" aft from my setting on my original Extra 300S.

I may decide to combine the two elevator control rods that were pre-installed into a single "Y" bar using wood and short rods.

After my successfull test flights on the Skylark using the AXI 4120/14 motor, 14x7 APC e-prop, and 16-cells, I know that I have plenty of power for the Extra 300S and that it already balances with either CP1700 or CP2400 cells.

Hannu Vuorinen

Hi, Greg,

Did I got understood right, you are now going to fly with new Kokam 2000SHD 20C cells?
Thats really interesting to see, how they perform ! I have those same cells coming up with 3p and 6p configuration,
but havent got time to make them up yet.

Greg Covey

Hi Hannu,

Yes, these are the new 20C, 2AH cells. Did you mean to say that you have 3s and 6s packs to make instead of 3p and 6p?

I flew my Skylark yesterday on a 4s2p pack and it worked great! I made up a new 3-cell pack this weekend using the compression technique that Kokam USA sent me. I'll be testing it out on several 30-40amp applications.