RCU Review: Graupner AXI-fied Extra 300S

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    Contributed by: Greg Covey | Published: August 2004 | Views: 110880 | email icon Email this Article | PDFpdf icon

    5614 Franklin Pike Cr.
    Brentwood, TN 37027
    (615) 373-1444

    Watch videos of the
    Graupner Extra 300S.

    First Flight
    A WINDY Day
    Showing Off

    • Easy Electric Conversion
    • AXI Motor is Clean and Maintenance-free
    • Graupner Extra 300S is Excellent Quality
    • All Hardware Included AND Pre-Sorted

    • No Pull-Pull Rudder setup

    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 Extra 300S 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!

    Product: Extra 300S ARF
    Manufacturer: Graupner
    Available from: Hobby Lobby
    Street Price: $179.00

    Wingspan: 63 in
    Length: 46 in
    Wing Area: 639 sq. in
    Weight: 90oz (5.6lb)
    Radio: 4 channels w/ 6 servos (glow), 5 servo (electric)
    Recommended Engine: Thunder Tiger Pro .61 2-stroke

    Electric Motor: AXI 4120 (excellent aerobatics) or 4130 (extreme aerobatics)

    Kit Contents:

    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 in their own plastic wrapping.

    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.

    My ship had a Patty Wagstaff pilot figurehead

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

    The landing gear hardware is pre-installed and designed to take a real hit

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

    All the control surfaces are pre-installed

    The wing mounting hardware
    and motor mount are also

    Each hinge is pinned for strength.

    Power System:

    To keep my Extra 300S light, I used Hitec parts from
    Hobby Lobby

    Hitec's Electron 6 receiver

    The Ultimate Battery Eliminator Circuit replaces the receiver battery

    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 Opto Electronic Speed Control (ESC). The "Opto" version provides maximum isolation between the noisy driver outputs to the motor and the input lines that connect to the sensitive receiver.

    An AXI 4120 External Rotor Brushless Motor

    Aluminum Motor Mount easily replace the stock .60-size glow engine

    The ESC and UBEC inputs are wired together to connect to the flight pack

    Here is a list of my components that I selected for the conversion:

    • 4 of the HS-85MG Hitec HS-85 "Mighty Micro" Plus Metal Gear Servos
    • Hitec "Electron" 6 Ch. Micro FM Receiver
    • AXI 4120/14 External Rotor Brushless Motor
    • PM41001 Aluminum Motor Mount for AXI 41 Series Brushless Motors
    • Jeti "Advance" 70 Amp Opto Brushless Controller, 6-16 Cells
    • Ultimate BEC (UBEC)
    • APC 15x10 e-Prop
    • Graupner (GR605360) Prop Shaft Adapter for 6mm Motor
    • Shaft or MP Jet (4706) Prop Adapter

    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.

    Mounting the AXI Motor:

    Only two new holes are needed to mount
    the AXI power system
    I used the stock T-nuts and then supplied four M4 screws to mount the motor

    Mounting the AXI motor onto the Extra 300S firewall was an easy task. First, I removed the stock glow mount bars. Using the PM41001 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.

    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

    I had pre-measured the distance needed from the firewall

    The cowl fit perfectly.

    I used an MP Jet MJ4706 collet prop adapter for 6mm shafts

    an APC 15x8 e-prop and the stock red spinner supplied

    Wing and Tail Assembly:

    I added a plywood block to mount the smaller, lighter HS-85MG servos

    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 linkage "keepers" are
    supplied with the kit

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

    The tail assembly was simple and quick

    The tailwheel has a built-in loop in the metal rod for shock absorption

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

    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.

    Graupner supplies all the hardware pre-sorted for easy and quicker assembly

    The stock wheel pants were left unchanged

    Leave it to Graupner to find a way to make things even easier through good organization. I had 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 Extra wheel pants had a great finished look and they were
    mounted very solid to the gear mains.


    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 separate 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.

    The original design was changed by Graupner for a second elevator servo

    All three servos are installed per the manual

    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.

    Finishing Up:

    The antenna wire is routed
    through the fuselage

    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.

    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.

    The fiberglass cowl was a perfect fit, but 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 15x8 prop and supplied spinner fit perfectly for a great scale look!

    Battery Options:

    Here are some options for battery power on the Graupner Extra 300S:

    • 16-cells CP2400 NiCd (yellow packs) = 34.7oz
    • 16-cells CP1700 NiCd (black packs) = 27.0oz
    • 4s3p Kokam 1500HD pack (4.5AH, not shown) = 16oz
    • 4s2p Kokam (20C) 4AH pack (red packs) = 22.0oz

    As shown, all these packs fit perfectly in the Extra 300S. The packs vary in weight and cost. The NiCd packs can be moved aft, if needed to fine tune the desired flying performance.


    My AXI-powered Extra 300S is ready-to-fly at only 96oz using the 22oz., 4s2p Kokam 4AH pack. That's only 6oz over the glow-powered weight! The setup listed below is basically the same setup used in my Hobby Lobby Skylark except that the 14x7 prop is replaced with a 15x10 because I have more ground clearance.
    • AXI 4120/14
    • Jeti Advance 70amp Opto ESC
    • APC 15x10 e-prop
    • Kokam 4s2p 2AH (20C) pack
    • 50amps measured (640w) static full throttle

    Before test flying the Extra 300S, I re-programmed the Jeti ESC to "Brake OFF" and "Hard Timing" modes since these are not the default settings and greatly enhance the flying performance.

    We test flew the Graupner Extra 300S this weekend and the performance was excellent! It took off with authority and needed no trimming once airborn. The power system didn't provide unlimited vertical climb but it was strong enough for most maneuvers. I found no glitches with the plane or power system. Take-offs and landing were smooth. The power system can handle sustained full-throttle when needed.

    Charging was made easy by first unplugging the battery cable using my opening cut into the bottom of the cowl.

    One of my favorite parts about glow to electric conversions is that the plane looks just as good at the end of the flight as it did when I first finished it. No wipe-down needed!

    We flew the AXI-powered Extra 300S a few more times on the following weekend. The 4120 motor does provide a decent power level for all maneuvers except for sustained knife edge and long vertical runs. To obtain extreme power with the Extra 300S, I would recommend an AXI 4130 motor.

    Landings were smooth and controlled without any bad tendancies. Sometimes the elevator can get too sensitive on Extras when the CG is too far aft.

    The Extra 300S conversion to clean and quiet electric power is now one of my favorites. Using an AXI motor makes it simple and maintenance-free. The AXI motors are also a cost-effective choice for high-performance electric conversions. The flight times will range from about 5 minutes when using the CP1700 NiCd cells to about 10 minutes with the 4AH Lithium pack.

    Lastly, the Graupner Extra 300S is a great .40-size plane that simply flies fantastic!

    AXI Power system and Graupner 300S
    5614 Franklin Pike Cr.
    Brentwood, TN 37027
    (615) 373-1444

    FMA Direct
    Kokam 2AH Packs at FMA Direct

    5716A Industry Lane
    Frederick, MD 21704
    (800) 343-2934
    Tech/Service: (301) 668-4280

    Kool Flight Systems
    355 Sunderland Circle
    Fayetteville, GA 30215
    (770) 716-7578

    Comments on RCU Review: Graupner AXI-fied Extra 300S

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    The comments, observations and conclusions made in this review are solely with respect to the particular item the editor reviewed and may not apply generally to similar products by the manufacturer. We cannot be responsible for any manufacturer defects in workmanship or other deficiencies in products like the one featured in the review.

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