The Marathon Project

Hello,

in the following letter I will give you a brief introduction into the Marathon Project, into its goals, its achievements and the potential it inherits.

The Marathon project is an Open source research project based in Germany, that aims to innovate small solar-powered UAV (Unmanned Aerial Vehicle) that weigh less than 20 kg (ca. 45 lbs).

The Project participated 2022 at the annual science competition "Jugend forscht", where it won the second place with the Marathon V-4.1 A.

The Marathon V-4.1 A was the first officially build Prototype for the Project. It is a Flying Wing with a Wingspan of 6 m (20 ft) a gross weight of 7 kg (15 lbs) and a payload capacity of 1 kg (2.2 lbs). The 200 low weight, high efficiency solar cells supply the 12,4 V 4500mAh battery and the Three 360kv High-Torque motors with energy. The test conducted on the Marathon V-4.1 A shown that even with 60% throttle, the battery was able to recharge to its full State.

The development of the Marathon V-4.1 A prototype was made possible through the support of key sponsors. Two businesses played a vital role by providing the carbon rods for the wing spar and other structural components.

In addition to the business sponsors, a foundation played a pivotal role by financing the flight electronics. Flight electronics encompass a wide range of components, including the flight control system, sensors, communication devices, and other electronic systems that facilitate the UAV's autonomous flight capabilities.

Thanks to an Autopilot based flight computer, the Marathon V-4.1 A is capable of fully autonomous flight, making it very easy to handle.

Although the Marathon V-4.1 A was a success for gathering data and developing building techniques for UAVs of this size, it still had some limitations. One prominent limitation was its inability to conduct nighttime operations. The design of the wing shape, which aimed to strike a balance between accommodating a substantial area for solar cells and ensuring aerodynamic efficiency, resulted in reduced efficiency during low-light conditions. Consequently, the Marathon V-4.1 A was not optimized for sustained flight through the night.

Pic.1: The Marathon V-4.1A

The Marathon LE (Long Endurance) represents a significant advancement towards achieving perpetual flight capability in a solar-powered UAV. With a wingspan of 7.2 meters and an impressive aspect ratio of 37, the Marathon LE benefits from reduced induced drag, allowing for enhanced performance and extended flight times. However, such a long wing with a high aspect ratio presents its own set of challenges.

One of the notable difficulties associated with a wing of this configuration is the reduced flutter speed, which poses a significant threat to the UAV's structural integrity. Flutter refers to a self-excited aerodynamic vibration that can occur when certain speed and airflow conditions align with the natural frequencies of the aircraft's wings.

Left unchecked, flutter can lead to structural failures.

To mitigate the risks associated with flutter, the Marathon LE incorporates a well-established and thoroughly tested Active Flutter Suppression (AFS) system based on a PID (Proportional-Integral-Derivative) controller that was flown on a smaller rc aircraft. This advanced system actively monitors the wing's behavior through strain and gyroscopic sensors and applies corrective control inputs to counteract any potential flutter.

Pic.2: A rendering of the Marathon LE (Design for the Rotors are still pending and thus not included in the render)

Designing a robust wing spar for a wide wingspan like that of the MRT LE presents a significant challenge. The key lies in creating a structure that can support thin, high-aspect ratio wings while maintaining strength and durability.

To tackle this challenge, the MRT LE incorporates a square profile carbon fiber spar with rounded edges, ensuring a balance between structural integrity and weight.

The manufacturing process for the wing spar involves a wet layup technique. Carbon fiber sheets are carefully layered and impregnated with resin, forming a strong composite material. These layers are then placed on a positive mold, meticulously shaping the spar to the desired dimensions. Once the carbon fiber layers are in position, the entire assembly is enclosed in a vacuum bag to eliminate any air pockets and ensure proper consolidation of the materials.

With the vacuum bag in place, the spar undergoes a curing process.

Pic.3: A render of the Carbon wing spar

Designing a mold of such length poses unique challenges that need to be addressed.

Currently, 3D printing has shown to be the most cost-effective solution for manufacturing such molds. In addition to the mold, the fuselage and rotor blades of the Marathon LE can also benefit from 3D printing technology.

3D printing offers advantages in terms of design flexibility, customization, and rapid prototyping. By utilizing 3D printing for the production of these components, it becomes possible to achieve intricate and optimized designs that meet the specific requirements of the UAV.

The fuselage, as the central body of the Marathon LE, plays a crucial role in housing various subsystems and providing structural integrity. 3D printing allows for the creation of complex geometries and lightweight structures, enabling the design of a robust yet lightweight fuselage. Similarly, 3D printing the rotor blades offers advantages in terms of weight optimization, and material reputable manufacturing companies like PCBWay are essential to facilitate the production of these crucial components.

Best regards

Joel M.