Hobby Lobby Taxi Cup

An ARF Glow to Electric Conversion Aircraft

The Graupner Taxi Cup-II was designed as an ARF model aircraft with a wing area of 597 square inches, a wing span of 63 inches and an all up weight of 96 ounces (6 pounds). As originally conceived, the TAXI was powered by a .40 size glow fueled engine and intended as an intermediate RC aerobatic trainer and general sport model.

Photo 1

But recent technology improvements in electric-powered flight (efficient brushless motors and high capacity/low weight Li-Poly batteries) have made both power sources now essentially equal. So it shouldn’t come as a surprise that many popular ARF RC model aircraft are being offered for conversion to clean and quiet electric power.

 
Photo 2   Photo 3

The main thrust of this article is to tell you how easy it was to perform this “electric conversion”. Interestingly, after converting this Taxi Cup-II aircraft to electric power, the total all up weight came out to the same 96 ounces. This isn’t hard to imagine when you consider that the battery pack, which is capable of flying this aircraft for over 15 minutes, weighs only 10 ounces. A 10-ounce (filled) fuel tank weighs about the same. I hate to mention this fact, but after a 15 minute flight and with all that fuel consumed, the glow powered model has 10 ounce less weight in the nose which tends to shift the CG aft just as you are coming in for a landing. Electrical energy inside your battery weighs the same at full or minimum charge. Something to think about!

 
Photo 4    Photo 5

A few words are in order concerning this high quality Graupner-manufactured ARF model. The structure is basically all wood, covered with an iron-on covering material. The wing comes as two separate panels that must be joined. The ailerons are pre-hinged at the factory. The tail surfaces likewise have the control surfaces in place and are pre-hinged.  

 
Photo 6    Photo 7

The fuselage structure is all wood, framed and covered with the control rod sleeves already installed. Even the servo-mounting tray is in place. A custom molded fiberglass cowl comes already finished and ready for installation. Note that a glow engine radial mount is attached to the firewall (photo 7). This mount, of course, is removed prior to installing the electric motor. As already explained, the Taxi, as originally conceived, was intended for glow engines of .40-46 cu. in. displacement.

 
Photo 8    Photo 9

Every conceivable piece of small hardware is supplied with the TAXI kit. Even the 10-ounce fuel tank is provided with the kit (photo 8). Both this tank and the radial engine mount are not used when you go to electric power. About the only extra thing to buy if building the glow-powered version is some 5 or 30 minute epoxy cement (photo 9).

 
Photo 10     Photo 11

A well made, 2 1/4 inch diameter, nylon spinner is supplied and it certainly adds to the overall appearance. Also adding to the appearance is a large sheet of decorative decals. They are easy to cut out and apply. By looking at the color photos on the shipping box you will know the recommended locations of these decals.

Photo 12

The Graupner hobby company is located in Germany. So it shouldn’t come as any surprise that most of the instructions are in the German language. Actually, two instruction books are supplied. In one book you will find a sequence of assembly photos that are quite good. In the other book (and you will have to look for it!) is a set of English translation instructions that closely follow the photos (photo 12). So be prepared to have both books opened to the correct place as you proceed with the assembly. The claimed assembly job is supposed to take something like 6-8 hours. Since I was developing my own electric motor installation scheme, and taking all the photos, it took me about three full days to totally complete the assembly process. Still, that is a brief time for such a large model aircraft.

Photo 13

You will need to have available (or must purchase) a full four-channel capable RC system. All of the radio system components I employed on the Taxi can also be purchased from Hobby Lobby International. I personally used a Hitec OPTIC-6 transmitter with a Spectra synthesized module (photo 13). The receiver is a Hitec Electron-6 dual conversion type. The servos are the standard size Hitec HS-422. Only three of these are required, one for the ailerons, one for elevator and one for rudder. Hobby Lobby suggested a four-cell receiver/servo battery pack with an 1800 mAh rating. This was necessary because of the increased flying surface loads and resulting higher than normal current drain of these particular components. A Hitec heavy-duty switch harness was selected because of the higher capacity battery pack. A final item that is most helpful is a 12-inch aileron extension cable. This equipment worked out perfectly in this application. The OPTIC transmitter also has eight model memory positions that will allow you to fly seven other aircraft (besides the Taxi) from this one transmitter.

 
Photo 14    Photo 15

I followed the suggested assembly procedure reasonably closely. What follows is exactly the sequence I used to assemble my Taxi. The wing panels (photo 14) were joined first. You will find a square opening at the center section of both wing panels that accepts a large plywood (or composite material) wing spar or joiner (photo 15). There is also an opening provided for the aileron servo which sits in the bottom of the wing, at the center section. Although it is recommended to use the slower curing 30-minute epoxy cement, I was able to use my regular 5-minute variety. (Ed. Note: You may want to consider using the 30-minute epoxy for the spars and 5 or 12-minute epoxy for the center section. This provides extra working time and maximum strength, but still allows the wing halves to be held in position until the faster epoxy sets. Not everyone has Bob’s quick building skills.)

Photo 16

I first applied the epoxy to half of the wing joiner and inserted half of it into the hole in one wing panel (photo 16). As soon as that cured I mixed up a larger batch of 5 minute epoxy and then, working fast, coated the center section of both wing panels as well as the protruding other half of the joiner. The panels were slid together and held in place temporarily with masking tape until the epoxy cement cured.

 
Photo 17     Photo 18

To mount the aileron servo, first mark off the area of the plywood tray and remove the covering material from that area (photo 17). The plywood aileron servo tray is then epoxied to the bottom of the wing. Final step is to install the two aileron control rods. At the servo arm I made 90-degree bends and used the supplied keepers to hold these wires in place. At the aileron end I used the supplied nylon clevises (photo 18). You can screw these clevises in or out to obtain the proper neutral position for both ailerons. Later, with the radio turned on, you can more accurately set this neutral position.

Photo 19

The electric power system selected by Hobby Lobby for this conversion involved the use of an AXI 2826/12 brushless outrunner motor. This motor runs the APC 13 X 10 or 13 X 8 prop via direct drive. There is no need for a belt or gear drive. The battery pack recommended is the new Thunder Power (TP) 3S2P Li-Poly rated at 2100 mAh. But since two sets of three cells each are placed in parallel, the total capacity available is 4200 mAh. This particular TP pack has the extra connector that accepts the new Thunder Power 2-5 Cell balancer (HL catalog No. TPB205). The last item in the power system is the Jeti Advance “PLUS” brushless ESC (speed controller). This is the 40 P “Opto” unit that can be programmed for optimum performance using a special programming card. The “Opto” type controller does replace the throttle servo but does not have a Battery Eliminator Circuit (BEC). That is the reason why I already mentioned the separate four cell 1800 NiMH receiver/servo battery pack.

 
Photo 20     Photo 21

The AXI 2826/12 brushless, “outrunner” motor (an electric motor where the outside case rotates while the armature remains stationary) is capable of running at 350 to 450 watts input power on a three cell (nominal 11.1 volt) Li-Poly battery pack. Keep that figure in mind because it is telling you that to replace a .40 glow engine, in a 6 pound model aircraft, will take an electric motor capable of 350 to 450 watts input power. There are many variables to this comparison, but it is still a good guide (photo 20). The easiest way to mount this particular version of the AXI motor is with the AXI prescribed radial motor mount. This mount (photo 21) is Hobby Lobby catalog number PM282002 known as the AXI radial motor mount for the AXI 2820 series. An excellent prop adapter is supplied with this mount.

The most important aspect of any glow to electric power conversion involves the substitute mounting of the electric motor. First of all, any glow engine mount supplied with your kit must be removed. The Taxi came with a pair of heavy-duty nylon beam to radial mounts. These were both removed leaving a flat firewall. If you were to simply radial mount the AXI motor to the firewall with its short flat mount (photo 21), the propeller would never reach out of the cowl.

So, the most important item is to fashion a spacer that moves the motor forward from the firewall enough so that the prop adapter (and prop) project outside of the cowl. There are several ways this can be done. Hobby Lobby has a scheme that appears on their website at this specific location: http://www.hobby-lobby.com/details_Taxicup.htm. It involves the use of a set of Hillman nylon spacers, several long screws and lock nuts. These Hillman spacers can now be purchased from Hobby Lobby or you can find them at most major hardware stores. The photos on this website tell the complete story.

Photo 21A

Another interesting electric motor adapter was developed by Greg Covey (photo 21A). It employs a PVC adapter that is sold at any home improvement store, like Home Depot and Lowe’s. The adapter is identified as a PVC ¾ inch adapter (Sch. 40) D-2466 (C435-007). Four long screws will be required. I think photo 21A tells the entire story.

Photo 22

Being a model builder, first and foremost, I decided to construct a simple ¼ inch plywood box to act as a spacer (photo 23). The dimensions I’m about to give you are important and were not supplied in the instructions, because they would vary considerable based on your choice of an electric motor. First thing to do is locate just how far the molded cowl overlaps the forward fuselage sides. I picked a point 11/16 inch back from the front of the firewall. Placing masking tape in the area, I drew a line that represents the rear edge of the cowl (photo 22).

Next I slipped the cowl over the front end of the fuselage until it rested on that line that I just drew. You can temporarily tape the cowl in this position. I marked five locations for the cowl attachment screws. Two screws are placed on each side and the fifth is on top. Each screw clearance hole is back 3/8 inch from the rear edge of the cowl. I selected five servo mounting screws (#2 X 7/16, catalog No. STW0207, Micro Fasteners Inc., 1-800-892-6917). These work out nicely because they have hex heads making it easy to remove the cowl when necessary.

 
Photo 23    Photo 24

The ¼-inch plywood box has one critical dimension. The front (new) firewall must be exactly 2 1/4 inches from the existing Taxi firewall. It consists of two sides, a top and a front firewall (photo 23). The bottom is left open and that space will be used later for the Thunder Power Li-Poly battery pack. A single clearance hole must be drilled in the center of the new firewall to clear the protruding shaft from the AXI motor (photo 24). The entire plywood box assembly is carefully located in place so that the motor shaft is roughly centered on the front opening in the cowl.

 
Photo 25    Photo 26

When you have that location marked, attach the plywood box to the Taxi’s firewall with 5 minute epoxy cement and some triangular shaped pieces of balsa (photo 25). The AXI motor’s radial mounting plate is attached to the new firewall using four 4-40 X 1/2 inch hex head screws and “T’ nuts (also called blind nuts). As a final check, mount the cowl to the fuselage to make sure the motor shaft and prop are clear to rotate (photo 26).

 
Photo 27     Photo 28

At this point it is a good idea to program your Jeti ESC (speed controller). To do this you need the motor, the Thunder Power Li-Poly battery, the Jeti ESC with program card and the 4 cell 1800 Rx battery pack (photo 27). First you must set all six of the shorting plugs on the program card to your personal choices. For reference purposes I set my ESC to: Li (Li-Poly) battery operation, low voltage cut-off (which is 2.7 volts per cell), slow down for cut-off type, brake-off, motor timing (high for an outrunner – and this is most important!) and linear throttle response (photo 28).

Photo 29

After you set all six shorting plugs, plug in the 4 cell battery pack and wait for a single beep. After that, your program choices are stored in the Jeti ESC speed controller. For information, I mounted the Jeti ESC with the help of hook and fastener tape to the left side of the plywood box motor adapter (photo 29). The two power cables that go to the battery hang out the bottom. Note in the photo the black battery pack sticking out the open bottom of the plywood box. The servo type cable exiting from the Jeti ESC is passed through a hole in the Taxi’s firewall and eventually gets plugged into the throttle port (#3) on the Hitec Electron-6 receiver.

 
Photo 30     Photo 31

With the motor now in place, I went back to installing the horizontal and vertical tail pieces. First thing you must do is cut the covering away from the three precut control wire slots in the fuselage. There are two slots for the elevator control rods and one for the rudder (photo 30). Next, determine exactly where the stab and fin will be located and then cut away the covering on the portion that will be inside the fuselage (photo 31). This will allow the epoxy cement to adhere a lot better. (Ed. Note. Be careful not to cut into the wood and weaken the stab. Bob has 40 years of building skills. If you don’t, try cutting away the covering using a small soldering iron instead of a knife.)

 
Photo 32     Photo 33


Photo 34

Next, install both elevator control horns and the rudder control horn. Make sure that on the side where one elevator horn and the rudder control horn are located in close proximity, that they don’t touch or interfere with one another as the control surfaces are moved (photo 33). Since the three control rods are heavy gage wire, you may find it hard to bend them (when reaching into the radio compartment) to get them inside the sleeves that are factory installed in the aft fuselage interior. I found an easier way was to pass these control rods through the holes in both firewalls and on back to the sleeves (photo 34). This way, the control rod wires did not have to be bent.

 
Photo 35    Photo 36

The complete, steerable tail wheel assembly is provided (photo 35). It comes with the wheel and a wheel collar. The small spring acts as a shock absorber. Attaching the nylon tail wheel bracket involves just two screws. The last thing you must do is attach the tiller or steering arm to the bottom of the rudder (photo 36). By doing this, the tail wheel strut and the rudder move in unison. Any excess tiller arm wire can be cut off.

 
Photo 37     Photo 38

The main landing gear, wheels and pants are provided along with all hardware. The pants are molded, painted fiberglass, similar to the cowl (photo 37). Each leg of the main landing gear will require this exact hardware shown in photo 38. Note that both a hole must be drilled and an elongated slot cut out of each wheel pant (photo 39).

 
Photo 39     Photo 40

The slot goes on the inside (the side attaching to the aluminum landing gear strut. The hole goes to the outside of the pant (photo 40). Any excess shaft material sticking out of the pant can be cut off with a high-speed rotary tool and an abrasive cut-off wheel.

Photo 41

Inside the radio compartment the elevator and rudder servos are mounted to the factory installed plywood tray (photo 41). You may have to do a little trimming to get these Hitec HS-422 servos to fit. It’s kind of unusual, but Graupner chose to employ two separate elevator control rods that run from each elevator up towards the servo. A special joiner bracket (supplied) accepts both elevator control rods, while a single third (short) rod goes from this bracket to the servo output arm. By doing this it isn’t necessary to join the elevator halves at the rear.

Photo 42

Just as was done with the ailerons, the wire connection at the servo output arm is a 90-degree bend and a keeper (supplied). On the control surface end, the connection to the control horn is by an adjustable clevis. The Hitec Electron-6 receiver was mounted to the radio compartment floor, directly in front of the servos with the help of hook and fastener tape. The heavy duty switch harness was installed with the switch just above the receiver location on the left fuselage side. This switch harness also has a charging jack for the receiver battery. A 12 inch aileron extension cable was plugged into the aileron port on the receiver. This makes it easier when attaching the wing to the fuselage. You might, at this point, want to turn on your radio, center the servos and then adjust all the control surfaces for a neutral position (photo 42). See the Sport Aviator article, “Ready To Fly?…Maybe” for details on this procedure.

 
Photo 43    Photo 44

I had been advised by several friends that my Taxi could possibly come out tail heavy. Knowing that possibility I took several measures ahead of time to make sure that didn’t happen. First of all, I decided to locate the 4 cell 1800 mAh NiMH receiver/servo battery pack in the cut-out just in front of the main landing gear. This battery was held in place with hook and fastener tape (photo 43). I also placed the thunder Power Li-Poly battery pack inside my plywood motor mount adapter box (I made sure ahead of time that this battery pack would fit inside that box!). This battery was also held in place with hook and fastener tape (photo 44).

 
Photo 45     Photo 46

Allow the two main battery wires (red and black) to exit out the bottom of the pack where they can conveniently plug into the mating connectors on the Jeti ESC. A portion of the cowl has to be cut away to allow access to the main battery, without having to remove the cowl each time you want to recharge. I fashioned what looks like a “tongue depressor” out of 1/32 plywood and slipped this plywood strip between the two pieces of hook and fastener tape so that the tape separates and the battery can be easily withdrawn out of the fuselage (photo 45). With the main battery in position, I was concerned that the protruding AXI rear motor shaft might damage (puncture) the battery case. So I added a small piece of plastic sheet material (1/32 inch thick), held in place with double sided tape, to protect the battery (photo 46).

 
Photo 47     Photo 48

When the Taxi was completed and ready to fly, I found that it was perfectly balanced (Center of Gravity [CG] is located 3 1/2 inches back from the wing leading edge). That being the case, it would be possible to place the four cell 1800 mAh NiMH receiver/servo battery pack inside the RC compartment (directly behind the main firewall).

 
Photo 49    Photo 50

In photo 49 you see my suggested location for this pack, but keep in mind it would go inside the fuselage. If you do leave the 4 X 1800 battery pack outside (as I did) I suggest you add a few rubber bands (photo 50) to make sure that the battery doesn’t drop out of the Taxi on a tight loop.

The recommended prop for the Taxi is the APC 13 X 10E. That prop, along with the three-cell Li-Poly battery prompted the following motor parameters: 39 amps motor current, 10.9 volts (under load), 433 watts (input power), 6300 rpm and at 96 ounce (6 pounds) total weight, a power loading of 72 watts/lbs. I felt that 39 amps was just too much motor current for this aircraft. As such, I switched to a lower pitched prop, namely the APC 13 X 8E. That choice of propeller gave me these more reasonable parameters: 32 amps motor current, 11.15 volts, 357 watts, 6800 rpm and 60 watts/lbs.

 

FLYING

Photo 51

The Taxi maneuvers well on the ground. I did add some exponential rate control to the rudder to smooth out the ground steering. Being an old single stick flyer I still suffer when using my left hand to operate the rudder. Once in the air, the Taxi is fast! The 13 X 8 prop proved just perfect! In fact, it can be flown at 1/2 to 3/4 throttle settings. The Thunder Power 3S2P 4200 Li-Poly battery provides almost 15 minute flying times.

Photo 52

Just about any standard aerobatic maneuver can be performed. When landing, you have to go out a ways and bleed off some speed before making your approach. I ended up at dead idle at about 100 feet from the runway threshold and still had trouble not floating past the runway’s far end.

 
Photo 53     Photo 54

So my recommendation is to really slow it down for landings. At 23 oz. /sq. ft. wing loading and a slightly forward CG location, this airplane does not stall easily when you slow it down. Try that at altitude some time and you will see. Final control throws were close to the Graupner recommendations: ailerons +/- 5/16 inch, elevators +/- 1/2 inch and rudder +/- 1 1/8 inches.

THUNDER POWER CELL BALANCER

Just as I was completing this review project Hobby Lobby provided me with one of the new Thunder Power Li-Poly Battery Cell Balancers. The Hobby Lobby catalog number is TPB205 and it sells for $59.90. Basically you can use any existing Li-Poly charger. This new balancer plugs into a special multi-pin connector that is now placed on many of the Thunder Power Li-Poly battery packs. During the charging process, this add-on device is able to “even out” the individual cell voltages so that at the end of the charge cycle, they are all equal or as we say, “balanced”. A balanced Li-Poly battery pack will provide more capacity (longer flight times) and will last a lot longer. It may even allow for the charging at a higher than 1C rate. I expect to have a detailed review of this product in the near future.

For those who would like more aggressive aerobatics and extreme vertical performance by all means go up to the originally recommended APC 13 X 10E prop. The AXI motor can certainly take it – no problem! But you will end up with a shorter flight time. It’s your choice!

Flying the Taxi proved to be one of the best hobby experiences I’ve had in a long time. I’ve been flying mostly small size, parking lot and indoor RC models for the past couple of years. They are certainly fun and can be flown at local fields. But there is still nothing more pleasing than flying a 600-square inch, six pound aircraft, similar to .40 glow power, except that in this case it is “clean and quiet”! This is what glow to electric conversions are all about. It is now made possible by our new highly efficient brushless motors and our high capacity, lightweight Li-Poly batteries.

 

 

SPECIFICATIONS:

Aircraft: “Graupner TAXI CUP-II”

Type: An ARF RC aircraft originally intended for glow engine power that is now being converted to electric power. Model is considered an intermediate aerobatic trainer and/or advanced sport flyer.

Distributor:

Hobby Lobby International

5614 Franklin Pike Circle

Brentwood, TN 37027

Phone: (615)-373-1444

FAX: (615)-377-6948

Website: www.hobby-lobby.com

E-Mail: sales@hobby-lobby.com                        

Wing Span – 63 inches

Wing Area – 597 square inches

Length – 45 inches

Average all up weight: 96 ounces (6 pounds even!)

Wing Loading – 23 oz/sq.ft.

Street price of the ARF kit alone (as of October 2005) – $199.00

Items you must still purchase:

Four Channel RC system with three servos (one for ailerons, one for elevator and one rudder— motor control is provided by the ESC)

RC Equipment on Review Model: Hitec OPTIC-6 transmitter with Spectra synthesized module, Hitec Electron-6 dual conversion 6 channel receiver, three Hitec HS-422 standard servos and a 4 cell 1800 mAh NiMH battery pack supplied by Hobby Lobby (catalog number HT00925).

Motor – AXI 2826/12 outrunner type brushless motor

Speed Controller (ESC) – Jeti Advance “PLUS” 40 amp brushless “Opto” (Hobby Lobby catalog no. JESAP40P) and the Jeti Advance Plus Program Card (JESAP02)

Battery – Three cell Thunder Power 3S2P Li-Poly (total rated capacity because of parallel hook-up is 4200 mAh) (15C load capable) (weight 10.1 ounces) (Note: This battery pack has separately wired cells to accept the TP cell balancer)

Best Prop Choice– APC 13 X 8E

Motor Current – 32 amps (at start with a fresh charged battery)

Voltage (under load) – 11.15 volts

Watts – 357

RPM – 6800

Power Loading- 60 watts/pound

 

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