View Full Version : Graupner Taxi Ii

08-13-2005, 09:32 PM
Anyone build a Taxi with any suggestions?

Mike Parsons
08-14-2005, 12:08 AM
I have never built one, but I have fondled it enough in my LHS to know it is a very nice kit. If you build it, please post details!


08-14-2005, 02:52 PM
I plan on using 2 Kokam 2100 20c 3s batteries in paralel with a 13 / 10 prop.

Greg Covey
08-14-2005, 08:11 PM
Yes, I did the original conversion to electric power for Hobby Lobby.


PLANE: Taxi Cup II
TYPE: Covered-balsa ARF
FOR: Intermediate Flyers
WING AREA: 597 sq. in.
WING LOADING: 23oz./sq. ft.
LENGTH: 47.5 in.
RADIO: 4 channels required; flown with a Futaba Super-8 transmitter, Hitec Electron6 receiver, 3-HS-422 servos,
POWER SYSTEM: AXI 2826/12 direct drive brushless motor, APC 13x10 e-prop, Jeti 40-amp speed control, 10-cell 3300mAh NiMH or 3-cell Li-Poly battery
FULL THROTTLE POWER: 37 amps, 385 watts; 65 W/lb.
TOP RPM: 7500
DURATION: 15 minutes mixed flying
PRICE: $199. [http://www.hobby-lobby.com/taxicup.htm]
COMPONENTS NEEDED TO COMPLETE: power system, radio system

The Taxi Cup II was designed for a .40-size glow engine but can easily be converted to clean and quiet electric power. The model is pre-built balsa and ply construction and beautifully covered with Oracover. The kit includes a pre-painted fiberglass cowl, landing gear, wheel pants, and all needed hardware. Every control surface is pre-hinged and pinned. The 63” wingspan has a semi-symmetrical airfoil and uses a single servo to control both ailerons.

The manual includes photo-illustrated instructions for a quick and easy build, which takes about 5-7 hours to complete. I built my Taxi Cup II per the manual and only changed areas related to the motor and fuel tank installation. The conversion to electric power is relatively simple. I will detail the changes in converting to electric power and offer several alternative suggestions that vary in cost and performance. All the options will provide great performance and the electric power system will keep your model looking new for a long time!

Greg Covey
08-14-2005, 08:13 PM

The Graupner Taxi Cup II has a beautiful look right out of the box and is a great flying balsa-covered model for either glow or electric power. Although the clean airframe is a bit fast to be considered a trainer, it has predictable flight performance with no bad habits at any speed. In this review, my conversion to clean and quiet electric power is relatively simple, which is also complimented by a direct drive maintenance-free brushless motor that truly demonstrates the evolution in electric conversions.

The conversion of the .40-size glow engine to electric power offers several advantages. Although the initial cost is a bit more for electric power, the overall cost drops every time the plane is flown. Over just a single season of flying, the cost of the electric conversion actually becomes less than the original glow engine and fuel cost. Another advantage is that there is no oil residue to wipe off after flying. The model stays clean and keeps looking brand new!

The AXI 2826/12 motor is a direct drive brushless design that is virtually maintenance-free. Since there are no brushes to wear out and no gears to lubricate or strip, the motor needs no maintenance other than perhaps a yearly lubrication of the ball bearings supporting the drive shaft. The motor is powered by readily available 10-cell NiCd/NiMH packs or 3-cell Lithium (LiPo) packs.

To mount the motor I used easily obtainable parts from my local home improvement store (like Home Depot) that were inexpensive. The unmodified PVC ¾” adapter provided the perfect 4” offset needed between the firewall and the cowl for spinner clearance. Alternatively, you can also use the Hillman Nylon Spacers to create a 4” offset from the firewall.

-PVC ¾” Adapter (Sch. 40) D-2466 (C435-007)
-#10-24 x 5/16” Tee Nuts
-#10-24 x 2-1/2” Round Head Slotted Screws

My initial step is to install the Radial Mount Set on the AXI motor. I then wire up the Electronic Speed Control (ESC) to my motor and other parts depending upon which option used below.

The Jeti 40-3P ESC comes with a Battery Eliminator Circuit (BEC) which eliminates the need for a receiver battery. The BEC provides voltage to the receiver and the servos through the ESC control cable which eliminates the need for a second battery for the receiver. This convenience means that you’ll never need to worry about the charge on the receiver battery again! When using a folded 10-cell CP2400mAh NiCd pack or a 10-cell GP3300mAh NiMH pack, the model will balance perfectly without adding and lead weight to the nose.

The second option uses an Opto-isolated ESC which requires either a 7.0 ounce Hitec receiver battery or a 0.7 ounce Universal BEC (UBEC) from Kool Flight Products. When using an Opto-isolated ESC like the Jeti JESA40OP, there is no BEC built into the device so you need to supply external power to the receiver and servos. Optically isolated ESCs are often used to reduce the electric motor RFI “noise” from getting into the receiver. They are also used for cell counts higher than 10 cells. The power system recommended on the Hobby Lobby site uses a 4-cell, 1800mAh NiMH receiver battery that weighs about 7 ounces. This extra weight is needed to balance the plane when using the 3s2p 4200mAh ThunderPower Lithium pack that only weighs 10.5 ounces. However, the extra weight of the receiver battery is not needed when using a 10-cell NiCd or NiMH pack with a capacity of 2400mAh to 3300mAh. The weight of the 10-cell (sub-C size) NiCd pack is about 24 ounces so you can optionally use a much lighter UBEC to eliminate both the weight and the need to keep a second battery charged. All these options are cost trade-offs that offer various amounts of performance and convenience. My personal preference is to use Option 1 with a folded 10-cell NiCd pack. This can also be made from connecting two 5-cell packs in series and placing one on top of the other under the windshield area of the fuselage.

One final option that I’ll mention is to use two of the ThunderPower 3s2p 4200mAh Lithium packs in parallel with Option 1. While this is the most costly, the additional weight of the second Lithium pack not only balances the plane but also provides incredible 30-minute flight duration! Note that the combined capacity of these two paralleled packs is 8.4AH.

-Jeti (JESA40) 40-3P ESC with BEC
-Use folded 10-cell NiCd/NiMH packs for a perfect balance

-Jeti (JESA40OP) 40-3P Opto ESC (no BEC)
-Receiver battery with Lithium pack or UBEC with NiCd/NiMH pack

I removed the stock glow engine mount and positioned the AXI Radial Mount centered over the hole in the firewall. The holes were marked and then drilled for the #10 tee nuts. The three motor wires were connected to the ESC via a 1” hole that I drilled into the firewall below the PVC adapter. Once mounted, the motor can pick up the entire fuselage!

When first testing the motor without a prop attached, if the direction of rotation is not counter-clockwise when viewed from the front, you can swap any two of the three motor wires to reverse the direction. This is a good time to re-program the Jeti ESC for better flying performance. The procedure is simple and it is well documented in the two page manual that comes with the ESC. I recommend re-programming the ESC for “Brake OFF” and “Hard Timing”. The hard timing feature is best suited for the AXI outrunner motors and can provide up to 20% more power. The brake feature is not desired for most models other than gliders that fold back the prop blades to reduce drag.

The battery(s) is mounted under the windshield area using industrial strength Velcro. All battery options previously mentioned fit well. When using a 10-cell NiCd/NiMH pack, it is best to use a folded pack to keep most of the weight forward.

I mounted a Tower Hobbies System 3000 (TOWM6060) On/Off switch in the side of the fuselage so I can connect the flight battery and attach the wing before enabling the power system. For added convenience, you can route the ESC and battery connectors out the bottom of the fuselage to re-charge the NiCd pack while it is still in the plane. For safety reasons, charging a Lithium pack inside the plane is not recommended. However, charging a NiCd or NiMH pack inside the plane can be a great convenience!

On the aileron linkage, I used the outer most holes on the servo arm. For all other linkages, I used the second hole from the control surface. The rudder and left elevator linkages are very close to each other which requires the top portion of the elevator control horn to be cut off.

My final assembly step was to route the antenna wire through a hole in the fuselage bottom and tape it along the way to the tailwheel. Lastly, I applied the decals which enhanced the look even more!

My Taxi Cup II was ready to fly at 100 ounces or 6-1/4 pounds.

Greg Covey
08-14-2005, 08:23 PM
Flight Testing
The first test flight was from an outfield-sized lot of mowed grass. We had plenty of power for take-off and had the Taxi Cup II trimmed out before finishing the first circuit. The model is very clean and retains energy well. We were surprised at how fast it could fly! With the recommended AXI motor, the plane responds to your touch on the throttle and most aerobatic maneuvers are possible like rolls, loops, and inverted flight. Although I could start a knife-edge pass, the plane would descend at a noticeable rate.

My CG was right on the manufacturers recommended setting and we saw no bad habits at any speed. Most of the flight was flown at only half throttle. When landing, I throttled back a bit and let the Taxi Cup II slowly descend into the grass. Note that the plane lands a bit hot so it is best to slow it up first and obtain a proper approach before reaching the field.

I managed to cartwheel the plane on a take-off into a cross wind without sustaining much damage. It took off successfully on the next attempt but it is best to let the plane gain some ground speed before lifting up into a strong cross wind. We had about 10 minutes of mixed flying with the 10-cell GP3300mAh NiMH pack and 13 minutes with the 3s2p 4200mAh ThunderPower Lithium pack.

Greg Covey
08-14-2005, 08:24 PM
When setting up the control surfaces, make certain that the two elevator halves are perfectly aligned with each other. This will prevent having to offset a slight roll in flight with the ailerons. I typically mount my wheel pants with the axle passing through both sides to help support it on grass take-offs and landings. One side of the pant is tightened directly against the gear mains, which looks great and allows it to still turn when bumped hard. A 12 servo extension plugged into the aileron channel of the receiver makes connecting the wing much easier.

The Taxi Cup II is a great looking plane that flies superbly! Since the clean airframe is a bit fast to be considered a trainer, I recommend it for intermediate to advanced flyers.

The plane can perform most aerobatic maneuvers while remaining incredibly stabile in flight. The design is quite rugged and can handle those less than perfect landings without sustaining any damage. The Graupner Taxi Cup II is an excellent next step after mastering a trainer and will also offer seasoned pilots great aerobatic performance with increased speed.

Greg Covey
08-14-2005, 08:34 PM

Your two 3-cell Kokam 2.1AH packs should work fine.

Good luck!

08-15-2005, 12:40 PM
Thanks for the info. Will ine lipo work or go with the 2 in parallel

Greg Covey
08-20-2005, 01:51 PM
A single 3-cell pack of 2.1AH cells may work if you limit the full throttle to bursts of 5-10 seconds but it would likely stress the pack, reduce its lifespan, provide short flights, and give CG balancing problems.

I would recommend using a 3s2p configuration of the 2.1AH cells.

08-21-2005, 09:07 PM
according to your test, even 3S2P is not enough for finding CG, except if you add more lead or rx battery,(battery which is not really needed, even on 3S.....)

10-18-2005, 05:33 PM
I have purchased a Taxi Cup II but have not started assembly. I will be installing the electrics as recommended by Hobby-Lobby. Your solution for mounting the motor is very clever, but raises a question. You mention that the PVC coupler provides the needed 4" offset from the firewall. The instructions however, say 120mm or 4.72" is necessary. Which is correct?

Greg Covey
10-20-2005, 12:28 AM
luc, You are correct. That is the recommended setup from Hobby Lobby. They use a single 3s2p Thunderpower 4200mAh pack and a heavy 4-cell NiMH receiver pack. My preference for first time conversion enthusiasts is to use a folded 10-cell 3000-3300mAh NiMH pack and a Jeti 40-amp ESC with BEC. (no Rx. battery needed). My personal preference is to "load up" on Lithium. Using two of the packs above in parallel works great as does my 4s5p configuration of Kokam 1500mAh cells for a 7500mAh capacity that flies for 30 minutes. simibill, I agree that there is a discrepancy but cannot explain it. Perhaps the measurement is different for the stock glow engine. Simply position the cowl as you see correct for mounting and run a ruler through the center until it hits the firewall.

Greg Covey
10-20-2005, 12:29 AM
simibill, I agree that there is a discrepancy but cannot explain it. Perhaps the measurement is different for the stock glow engine. Simply position the cowl as you see correct for mounting and run a ruler through the center until it hits the firewall.

Mike Wizynajtys
10-20-2005, 01:32 PM

Whenever I see these nice conversion threads we always get a nice shot of the motor mount, but many times we don't get to see how the batteries are installed. Any chance of getting a photo of that? Also, are the batteries easy to change out between flights, or do you have to remove the wing?



Greg Covey
10-22-2005, 09:48 PM

I run the connector out to the air scoop exit under the fuselage and recharge the pack in the plane during the day. The four wing screws are much too cumbersome to remove for every recharge.

There are no pack photos because the review was originally done for a hardcopy magazine, Fly RC, and they have limited space for photos. Additionally, the pack is literally buried up under the dash of the plane. If you have a Taxi Cup II, it is obvious where the pack goes as it has a solid shelf under the dash that is perfect for laying down some Velcro and holding a heavy battery pack.

Greg Covey
10-22-2005, 09:54 PM
I found some helpful photos of the Taxi Cup conversion at Hobby Lobby. You can see where the pack (or packs) go as well as the ESC mount in the Cowl air exit. I run my battery cable out the hole and connect it to the ESC externally. It makes for easy field recharges from my car.


Mike Wizynajtys
10-24-2005, 12:19 PM
I see. That's disapointing, but it's also what I figured. I'd like a plane like the Taxi Cup, but I don't want a plane where I can't get the batteries out easily. I don't care for charging my Lipos inside the airframe.