Design Concepts
General Design
Initial design studies resulted in two configuration choices: a flying wing with planform dictated by the container, or a multi-paneled conventional wing configuration with a thin fuselage. The flying wing model would result in the quickest assembly time; however, the conventional aircraft had a higher payload capacity.
Flying Wing Design
Multi-panel Design
Airfoil Design
The aircraft’s purpose is to lift maximum payload which leads to selection of a high-lift airfoils that can operate at low Reynolds numbers. A survey of suitable airfoils identified the S1210, S1223, and E423 (UIUC database). The lift curves of each airfoil show that S1223 possesses the highest lift coefficient in the predicted Reynolds number regime, and it was most compatible with the airfoil volumetric constraints posed by the competition box; however, its very thin shape introduced manufacturing challenges. Primarily, the thin trailing edge is difficult to accurately replicate while sustaining structural integrity.
Airfoil Selection: Lift vs Angle of Attack
Airfoil Design
In order to combat this drawback, several iterations of wing panels were constructed utilizing 3D printed, highly-relieved, leading/trailing edges and balsa ribs. This approach allowed the team to build a wing that accurately held the S1223 shape while providing enough strength to sustain flight. The material used for each 3D printed component was ABS plastic, with a density of 1.07 g/cm^3. Permeable design techniques such as introducing holes into the plastic components and minimizing the infill density were not enough to overcome the order of magnitude difference in material densities between ABS plastic and balsa wood. This extreme weight difference pushed for alternative methods of balsa wood manufacturing to optimize the aircraft’s weight.
3D Printed Wing
Balsa Wing
Wing Design
The wing panels connect together in two pairs of three, utilizing a rectangular aluminum spar insert, a pin on the leading edge, and two neodymium magnets at the leading and trailing edges. The rectangular aluminum piece inserts into each of the carbon fiber spars of the mating wing panels. The neodymium magnets that are flush with the outer surface of the end ribs mate with the end rib of the neighboring panel. The pin comes out of the leading edge of one panel, and inserts into the next panel.
Fuselage Design
The tubular fuselage separates into four sections, each able to connect with a friction-fit wooden rectangular insert. A wing mount was designed to screw permanently into the fuselage, and contains two arms that provide the 12 degree dihedral. These arms have a tight clearance fit in the root spar. The wing mount features magnets to prevent lateral motion.
Tail Design
The tail is designed to provide the maximum surface area, and to fit into the box. The horizontal section is mounted to a section of the fuselage. The vertical section is a separate piece that is fitted onto the fuselage using a pin design. To ensure the vertical stabilizer stayed connected to the fuselage, neodymium magnets were used. The servos are also mounted. The servo for the vertical section is mounted on the frame of the stabilizer, while the horizontal is mounted to the fuselage. The section will then be connected to the main fuselage, and then the words for the servos could connect to the wire harness.
Payload
According to section 9.8 in 2018 SAE Aero Design Rules, final flight score is mostly dependent on payload fraction. Payload fraction is calculated by dividing the weight of the payload by the takeoff weight of aircraft. If the airplane lifts more weight, it gets a higher score.
Final Design
The biggest challenge of designing this plane was having it separate into multiple sections. Each section has to be made to fit together snugly without flexing or causing the assembly time to increase significantly. Minimizing the number of connections to be made was paramount to keeping the assembly score as high as possible. To ensure all components would fit together, Autodesk Inventor was used to envision every connection and joint. A complete physical model of the plane was constructed and each assembly and subassembly was tested for fitting.
