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Retractable Wing UAV
How can we create a UAV with the flight time of a fixed wing drone but the versatility of a quadcopter? In this project, I inspired three other people to help me investigate this question.
Inspiration
When flying small quadcopters behind my school, I noticed that their flight times were really low. I wondered how commercial drones were practical if they were plagued by the same flight time issues, so I decided to investigate. Curious, I inspired three others to work with me and we started to research the problems inherent with commercial drones.
Research
During our research, we found that fixed wing drones had significantly longer flight times than quadcopters, but that they lacked the versatility of quadcopters. In particular, we noted that it was difficult for fixed wing drones to efficiently generate the same amount of lift at different speeds due to the nature of how they generate lift.
Analysis
In the equation used to calculate lift from a body moving through the air, we noticed that lift is proportional to the square of velocity, so changes in the flight speed of a fixed wing aircraft will by definition create large changes in the lift generated if no other variable is changed.
Solution
I took on the design, analysis, and prototyping roles for our team, starting by brainstorming methods through which we could change the speed of flight while generating the same amount of lift. According to the lift equation above, I needed to change either the air density, the coefficient of lift, or the surface area of the wing in order to compensate for the change in velocity. The air density cannot be changed without drastically changing the altitude, and while the coefficient of lift could be changed by altering the angle of attack, the difference was not significant enough for true speed versatility. With this in mind, we decided that the most feasible solution would be to change the surface area of the wing. Changing the chord of the wing was too mechanically complex, so we decided on a telescoping wing design that hid one section of the wing under another to reduce wing surface area. We chose to use a pulley to accomplish this retraction as it was lightweight, robust, and mechanically simple.
Further Considerations
I also decided to pursue a tandem wing design due to its stability, lifting power, and lack of external control surfaces. VTOL was accomplished by using a tiltrotor design where four propellers originally point upwards to achieve quadcopter-like takeoff, and tilt forward to accomplish forward flight.
CFD Simulation
Using velocity inlet and pressure outlet boundary conditions, I extracted the lift to drag ratio at cruise speed, continually improving and retesting it. By reducing the cross-sectional area and slashing the thickness of the airfoil, I reached a l/d ratio of 2.4. With a 20,000 mAh 8S battery and 300 KV motors, we could reach two and half hours of flight time with no payload. I chose the motors and electronics by adding 30% to the thrust needed for vertical take off and searching for motors that accomplished this with low KV ratings, as they are typically more efficient.
Ask me for anything; I'll always deliver more
Prototyping
After finishing the design, we ordered the materials we had chosen and began the prototyping process. Using a hand saw and power drill in my basement, I cut and drilled aluminum struts to length, 3d printed parts that I couldn't make by hand, and used nuts and bolts to attach everything together. It took around 2 weeks for our first functional prototype to take off from the ground.
Improving
We noticed that our first prototype had barely enough power to get off the ground. Upon further investigation, we found that we had wired our batteries in parallel instead of in series, creating a 4S battery instead of an 8S battery. A second issue was that the tolerances on our aluminum frame were too large, which caused flexibility in the frame, and therefore an inability of the flight controller to understand how to control the drone. We fixed all of these problems in our second prototype. I was much more careful with the manufacturing and added an extra aluminum brace to the motor mounts in order to eliminate flexibility in the frame. Rewiring the battery gave us double the power to our motors. Finally, we reprogrammed our flight controller in betaflight to enable tiltrotor and retractable wing functionality.
Results
In addition to the completion of technical objectives like designing and prototyping a drone with retractable wings, this project also gained a fair amount of external recognition. It won the international Pete Conrad Spirit of Innovation Challenge, helped me become the AIAA scholar of 2020, inducted me into the Sigma Xi Research Honors Society, and became my first patent application.
Unable to continue testing, we realized after our second prototype that there was a lot of optimization to be done in order to fully reap the benefits of retractable wings. First of all, the wings needed to retract more than 50%, perhaps through multistage telescoping. This would allow us to change the surface area enough to make a real difference in the lift generated. Second, the aluminum frame of the drone was far too large and heavy for the size of the wings, generating far more drag than was necessary. An optimization of this frame could easily save thirty to forty percent in weight. Finally, upgrading to a more professional flight controller would have been helpful, as it would have allowed us to program more advanced functionality with greater control stability.
Next Steps
The 2nd Prototype
Our second prototype was much more stable and had more than enough power to get off of the ground. Unfortunately, we landed harder than we expected after our first hover test, which caused one of our motor mounts to break. Combined with the failure of our one 3d printer, we were prevented from testing the transition to horizontal flight. It was later identified that the infill percentage of the motor mounts had been reduced to speed up printing, making it unable to withstand the stresses of the hard landing.
Learnings
This project exposed me to the world of conventional drones, including flight controller programming and electrical component choice and integration, helping me with every future project. In addition, I learned valuable CAD and CFD skills while refining my design and prototyping abilities. This was one of my first experiences creating a complex project from scratch, illuminating the challenges of making educated design choices and validating each component. As the leader of my team, I learned invaluable leadership and marketing skills that I've used to motivate teams ever since. I also learned how to create schedules, stick to them, and update them when needed. While I've stopped at prototype 2 for now, I hope to someday completely redesign our implementation of retractable wings to truly enable innovative drones that are both efficient and versatile.
Extra Resources
Looking for more? The video below was our submission to the Conrad Challenge; a description of the technology starts at 2 minutes and 11 seconds. Feel free to download the executive summary below as well.