AIAA Design, Build, Fly — The Silver Bullet

Acknowledgements

Project Advisor: James Beseda

Technical Advisor: Dr. Angel Abbud-Madrid

Donations Made by: AIAA Club at Mines

Video

Elevator Pitch

Design Approach

Design Solution

Laser Cut Structure 

Creating a lightweight but strong fuselage was a critical part of the manufacturing process. The first prototype was made with low-grade thin plywood and hot glued together. This prototype frame was used for load testing and to confirm other hardware would work on the designed frame. Light weight aircraft balsa wood was then laser cut into a new frame based on the plywood prototype with improvements made on the previous prototype. The laser cutter used was the Epilog Legend 36ext, which made rapid prototyping and creating a fuselage/wing struts quick and easy. 

3D Printed Parts 

3D printing was used to create custom detailed parts necessary for the assembly, aerodynamics, and function of the aircraft systems. Custom parts such as the motor mounts bridged the gap between the motors and the wing structure. The prototype nose cone with its unique geometry was 3D printed and functional parts such as the spool for the sensor wire were also 3D printed. Later some of these parts would be replaced with light-weight products that are less dense than PLA, but the 3D printed versions were necessary to refine the design before investing time and material in more complicated manufacturing processes.

Due to the large time constraints associated both with light weight manufacturing methods, such as fiberglass or carbon fiber casting or waiting for a manufacturer to produce them, the tight schedule led to 3D printing a majority of required parts. There was also a concern of cost, as the small-number production runs of a part would cost far too much.

Test Wings 

For the initial prototype, it was decided to use a CNC foam cutter that was available to fabricate a pair of foam wings for quick, effective testing of our airfoil design. Ultimately, this would be detrimental, as the CNC foam cutter was far from working order and the time necessary to fix it extended into the second prototyping sprint.

The other airfoil option, and the one utilized in the prototype, is a carbon fiber central spar with balsa wood spacers. This covered with a layer of Monokote, would give a wing sturdy enough to pass the wing tip test with a full cargo load, as well as an airfoil that is aerodynamic enough to get the plane off the ground. While Monokote is heavier than Econokote or some of its competitors, its excellent puncture and damage resistance made it desirable. Its ease of attachment was also a big factor in the decision, as time was still a large concern. 

Performance Results 

Subsystem Results vs Expectations 

The sensor LED was not as bright as it was expected to be when it was tested. It should have been bright enough to see in the sunlight, but it could not be seen from more than 20 meters away. To be seen from the ground while the plane is in flight, it needed to be able to be seen from much farther than 20 meters away.

The winch system was also tested to make sure that it had enough torque to pull in the sensor during flight. This was tested by calculating the estimated drag at the airplane’s expected max speed, and having the servo pull equivalent weight vertically. The servo was able to lift the whole mass, meaning it should be able to retract the sensor in flight. 

Aircraft Performance 

As of writing, the motor, power, and overall assembly were tested. Thrust testing yielded an actual thrust about 25% less than manufacturer specifications listed. This is caused by a difference in atmospheric pressure; the specification sheet uses data collected at sea level while lab testing was carried out at an atmospheric pressure of .84 atm. This pressure difference would have likely not been noticeable in Arizona had the in-person fly-off been held. There also could be extra resistance in the wires that feed the motor. The maiden flight of the aircraft proved it was airworthy and faced the challenge of strong winds. The aircraft was able to fly for approximately one minute before the wind conditions made it too unstable to handle. This test also showed that the handling of the aircraft needed to be refined. Post-crash landing damage assessment also showed that the majority of the airframe sustained minor damage and the components that were more seriously damaged were simple to replace and rebuild. This was an important aspect of the design as it would allow for quicker repair and modification.

Next Steps

The next steps for the team are preparatory measures to best ensure the success of the next year of Senior Design’s success. The lab space will be organized and cleaned so that the next team is able to find every supply and component with ease. This will be extremely advantageous as valuable time will be saved by avoiding the need to sift through tools, components, and various supplies. Additionally, the Silver Bullet will have its electronic components removed and sorted so that they may be reused for an upcoming team’s competition. The airframe will be stowed for reference and parts in the future.

The team hopes that by taking these preparatory measures, the lab will be an element of success for upcoming Senior Design students. This year’s experience has shown how valuable the lab has been as a resource and making the lab optimized for future use is an important aspect to being successful in long term design project.

Meet the Team