Graupner Taxi 2400: A Deep Dive

The world of model aviation is rife with enthusiasts who push the boundaries of what's possible, transforming traditional models into cutting-edge electric powerhouses. One such ambitious project is the conversion of the Graupner Taxi 2400, a model that, in its original form, might have been powered by a glow engine. This particular build showcases a meticulous approach to electric conversion, focusing on power, duration, and the practicalities of integrating modern brushless motors and high-capacity lithium polymer batteries. The user's experience highlights key considerations, from initial unboxing and instruction interpretation to the nitty-gritty of motor mounting and flight performance. Let's delve into the details of this impressive undertaking.

Unboxing and Initial Impressions

The arrival of the Graupner Taxi 2400 was met with a sense of scale; the box was described as 'massive'. The service from Motors and Rotors (UK) was lauded, with excellent packaging and timely delivery. Upon opening, the model presented itself as a true ARF (Almost Ready to Fly), with pre-hinged and pinned surfaces. The only significant setup required was for the stabiliser. The hardware provided was extensive, perhaps even 'too extensive', a point that would become clearer during the build process. This initial impression set the stage for a project that, while straightforward in many respects, would present its own unique challenges.

Deciphering the Instructions

A common hurdle in model aviation, especially with international kits, is the instruction manual. In the case of the Graupner Taxi 2400, the instructions were primarily in German, with English translations appearing later in the manual but notably lacking accompanying pictures. This necessitated cross-referencing, particularly for crucial dimensions. The builder ingeniously solved this by separating the manual and placing the German and English pages side-by-side, a practical solution for navigating the translated, yet visually sparse, guidance.

Construction and Hardware Considerations

The construction itself was described as 'pretty simple'. However, the hardware packages presented a few noteworthy points. The supplied control horns were unusually large, making it difficult to set up the pivot point on the hinge line. The discovery of a more suitable set of horns later in the build, specifically when fixing the close loop to the rudder, led to a minor setback, as the pushrod had already been soldered. The three-point control horns came with 3/4” screws, requiring significant trimming. A template, fashioned from a margarine tub, proved invaluable for ensuring a neat finish after using a 'dremel' on the covering.

The elevators were designed in two sections (left and right), each controlled by a standard servo. Connection to the main body was facilitated by an aluminium tube secured with a robust plastic clamp. The rudder and tailwheel were managed through separate closed loops. To prevent undue stress on the rudder servo, particularly given the model's intended use as a glider tug, a powerful Hitec 645mg was chosen for the rudder. This decision was based on prior experience, recognising the rudder's critical role during take-off.

The Electric Power System: Motor and Battery Choice

The builder's objective was to achieve both ample power and extended flight durations, a common but often challenging goal in electric flight. Understanding the relationship between amps, watts, and volts, a 12S High Voltage (HV) setup was selected to reduce amp draw and increase duration. The process of selecting a brushless motor involved extensive research, including stumbling upon a valuable formula from Bob Boucher for determining the correct propeller size for desired power or amp draw. The builder even developed a spreadsheet based on this formula, offering to share it via email.

After careful deliberation, the chosen motor was an AXI 5345/16, paired with a Jeti Spin 99 ESC, both sourced from Puffin Models. The battery selection involved navigating the vast array of lithium polymer (LiPo) batteries available. Considering the wing loading and deeming weight less critical, the decision was made to opt for two 6S 10,000 mAh LiPos from Maxamps in the USA. These batteries were competitively priced, boasted a good reputation, and arrived surprisingly quickly. Upon inspection, they reminded the builder of older 12V batteries, but with the significantly higher output of 22V and a continuous current capability of 200 Amps.

Motor Boxing: A Critical Detail

A significant aspect of converting many large models to electric power involves accommodating the motor within the airframe, particularly the cowl. The cowl on the Taxi 2400 is described as 'massive' and 'dalotel style'. To achieve the correct motor protrusion, 70 mm of boxing was required. While many electric conversions utilise aluminium spacers to mount motors away from the firewall, this builder opted for a more traditional approach using readily available plywood and 4mm threaded rod bolted through. Since large AXI motors typically do not come with a motor mount, one was fabricated from 1/16” epoxy board sourced from Macgregor Industries.

Building and Preparation for Flight

The construction phase was completed over a week of evenings, proceeding at a 'sedate' pace. The quality of the parts was exceptional, with precise cutting, and aside from the hardware, no unexpected issues arose. The anticipation for the maiden flight was high, but the weather proved to be an impediment, delaying the event for a couple of weeks.

The Maiden Flight: Challenges and Triumphs

Finally, a break in the clouds presented the opportunity for the maiden flight. The model was assembled quickly, and a range check was performed. This initial check revealed an issue: the right flap and aileron appeared erratic. The cause baffled those present, with various theories proposed but none proving correct. The solution came from a younger member, Connor, who suggested that the metal spinner on the ground might be causing interference. When the model was repositioned with its tail on the ground, the range check passed, much to everyone's relief.

With the range check successful, the motor was armed, and the model taxied to the runway. After a fail-safe check, power was applied. The model accelerated and took off with 'authority' after only a few feet. Minor adjustments with left aileron trimmed the flight path, and the model settled into a stable, 'innocuous' first flight. The landing was uneventful, with flaps not being necessary due to the presence of a strong wind.

Post-Maiden Flights and Future Prospects

Since the successful maiden, the builder has logged several flights, consistently achieving 25 minutes of flight time from the batteries. The weather has not yet been conducive to testing the model's capabilities as a glider tug, but the builder intends to provide updates on this aspect of its performance. The conversion of the Graupner Taxi 2400 stands as a testament to the dedication and problem-solving skills within the model aviation community, demonstrating how a classic airframe can be reborn with modern electric technology.

Frequently Asked Questions

Q1: What was the primary challenge during the construction of the Graupner Taxi 2400 conversion?
A1: The main challenges involved deciphering the German-heavy instruction manual and dealing with the size and suitability of the supplied hardware, particularly the control horns.

Q2: How much motor boxing was required for the electric conversion?
A2: Approximately 70 mm of boxing was needed to accommodate the motor within the massive cowl.

Q3: What was the chosen power system for the Graupner Taxi 2400?
A3: The builder opted for a 12S HV setup, specifically an AXI 5345/16 motor and a Jeti Spin 99 ESC, powered by two 6S 10,000 mAh LiPo batteries.

Q4: What caused the erratic control surface behaviour during the initial range check?
A4: The interference was caused by the metal spinner being on the ground. Moving the model so the tail was on the ground resolved the issue.

Q5: What is the typical flight duration achieved with the new setup?
A5: The builder is comfortably achieving 25 minutes of flight time with the current battery configuration.

Key Specifications Summary

This detailed account of the Graupner Taxi 2400 conversion offers valuable insights for anyone considering a similar project. The emphasis on careful planning, component selection, and practical problem-solving is evident throughout the build, resulting in a powerful and long-flying model.