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Graupner Taxi 2400 Electric tug Print
Written by James Hindle   
Saturday, 29 December 2007 10:07

Graupner Taxi 2400 Electric Conversion.

the Big Box !

A cryptic text message from my wife asking what was in the box as big as her signified the arrival of the graupner taxi, in short the box was massive ! The service from Motors and Rotors was excellent, packaging was first rate and delivery was as promised. Opening the box showed that this was a true ARF, all surfaces were pre hinged (and pinned) the only setup was required for the stab, the hardware was extensive, possibly too extensive as we will come onto.


The first job after checking all the components were present was to decipher the instructions, unfortunately this lets the kit down a bit. The instructions and photos are primarily in German, English is presented later in the manual but without pictures so you have to cross refer, especially where dimensions are present, in the end I pulled the instruction book apart and put the German / English pages side by side, perhaps Graupner will look at this for the future.


Construction was pretty simple the only things worth pointing out is the hardware packages, the control horns supplied were very large and setting up for the pivot to be on the hinge line was impossible, it wasn’t until I was fixing the close loop to the ruder that I found another set of horns that were more suitable, unfortunately I had soldered the pushrod before discovering this. The 3 point control horns supplied come with 3/4” screws so quite a lot of trimming is required, I found that making a template to fit over the horn is needed (I used a margarine tub) as covering does stand up to a freshly ‘dremeled’ end.


The elevators come in two sections (left and right) and each are controlled by standard servo, connection to the main body is via a aluminium tube kept in place by a robust plastic clamp.


The rudder and a tail wheel are connected through separate closed loops, the tail wheel is joined to the closed loop via 2 springs to prevent stressing of the servo, as I chose this model as a glider tug I opted for a mighty Hitec 645mg as rudder is (from bitter experience) your best friend during take off.


The choice of motor was made on the basis that I wanted lots of power and lots of durations, in electric flight terms this isn’t always easy to achieve, I knew from the distant days of school that amps watts and volts were all related in some fashion so decided to opt for a 12S HV setup so I could reduce the amps and increase duration.

Determined not to be beaten by the myriad of numbers associated with brushless motors I started ‘googling’ and stumbled across a great formula from Bob Boucher that assists with determining the correct prop size for desired power or amp draw, anyone interested can e-mail me for the spreadsheet I developed.


After several days cogitating different setup I opted for an AXI 5345/16 and a Jeti spin 99 all purchased from the ever reliable John Emms at Puffin Models.


Looking into the multitude of lithium polymer batteries available and determining from the wing loading that weight wasn’t important I opted for 2Nr 6S 10,000 maH lipos from Maxamps in the USA, these were competitively priced with a good reputation, they arrived faster than some UK mail order items, upon close inspection they reminded me of my ‘oily’ days and the 12v battery used for the power panel, with the exception that these were 22V and 200A continuous !


The cowl in the Taxi 2400 is massive and 70 mm of boxing was required to get the correct bit of the motor to poke out !, I’ve seen many electric conversions use aluminium spacers to mount electric motors away from the bulkhead but I’ve always opted for widely available plywood with 4mm threaded rod bolted through. As with all large AXI motors mo motor mount is supplied so one was knocked up using 1/16”” epoxy board from Macgregor industries.


Building was completed in a sedate week of evenings, all the parts were exceptionally well cut and apart from the hardware no surprises were found


Unfortunately the terrible weather delayed the maiden for a couple of weeks but a break in the clouds one Saturday evening gave me the opportunity I was looking for, the model was put together in a few minutes and after a few static photos a range check was carried out, the range check didn’t go well, the right flap and aileron appeared erratic, everyone present was baffled by this, several ‘theories’ were offered but all to no avail, eventually one of the younger member, Connor asked if it was because the metal spinner was on the ground causing interference, when the model was place with the tail on the ground it passed the range check thank goodness.



The motor was armed and ‘taxied’ out to the runway after a fail safe check was carried out power was applied and the model moved forward, after a few feet it took off with authority, a couple of clicks of left aileron saw the model flying straight and things settled for an innocuous first flight, the landing was uneventful, flaps were not required as the wind was quite strong.


Since completing the model I have had several flights and comfortably get 25 minutes from the batteries, the weather has not been good enough for towing yet but the setup has ample power to tow most 10-12lb gliders effortlessly.



 I’ve always admired the Taxi Cup and this model was a dream to build and fly. The performance spectrum from gentle pussycat to aero-monster is incredible!


The box that this plane arrived in was massive, almost as tall as my wife who did ask what I was up to !, the build was effortless and electric conversion straightforward only requiring 70mm of boxing for the electric motor, I prefer traditional materials for making the motor mount and so opt for ply for the boxing out and epoxy board for the motor mount itself, this provides plenty of support for very little weight.


Initial test flights of the model were delayed due to servo-glitching but this was found out to be (by our youngest member) interference from the metal spinner when it was touching the ground !


The first flight was fantastic, the 2 degrees of down and right thrust were spot on and the model only required one click of up trim. The 5345 setup provides too much power for the plane on its own but as I am using it for glider towing it is just about right for towing 4-5 kg gliders with ease. I chose a small prop for the motor 20x10 so as to preserve duration and can easily get 30 minutes of flying or 8 monster tows from the 10,000 mah packs, at full throttle the draw is 58A.


The simple colour scheme works well both on the ground and in the air, all in all this is a fantastic setup for a fantastic model.