2018 - 2020
In 2018, I embarked on an ambitious project to develop a hybrid aircraft that would combine the vertical takeoff and landing (VTOL) capabilities of helicopters with the efficiency and endurance of fixed-wing aircraft. This research was inspired by the V-22 Osprey, though aimed to explore simpler technical solutions at a small scale. The project, while ultimately not achieving all its goals before its conclusion in 2020, provided valuable insights into hybrid aircraft design and control systems.
After examining various VTOL configurations, I opted for an unconventional approach: a wing bicopter with fixed motors and aerodynamic control surfaces. This decision stemmed from studying previous attempts at tiltrotor designs, which often suffered from motor bearing failures due to gyroscopic loads. The delta wing configuration seemed promising for its structural simplicity and potential ease of transition to forward flight.
The aircraft utilized a modified Pixhawk flight controller running customized Ardupilot firmware. Initial testing began with the aircraft suspended on tethers, allowing for safe PID controller tuning. These early hover tests showed promise, performing better than initially expected, though they also revealed the first hints of challenges to come.
The most immediate challenge emerged in the form of extreme sensitivity to center of gravity (CG) positioning. The aircraft required precise CG alignment with the wing axis for stable flight. While this could be managed through careful design, a more significant issue arose with wind sensitivity. The large wing surface area, necessarily perpendicular to the wind during hover, made the aircraft highly susceptible to wind disturbances, effectively limiting testing to calm conditions.
During high-altitude descent testing, we encountered a phenomenon later identified as vortex ring state, resulting in limited cycle oscillations that proved unrecoverable beyond certain descent rates. This discovery led to implementing strict descent rate limitations and requiring forward velocity during faster descents.
Ground effect posed another significant challenge, particularly during takeoff and landing phases. The aircraft required rapid ascent to escape ground effect and enable effective use of aerodynamic surfaces. Landing proved especially treacherous, as any forward velocity at touchdown, combined with the high CG and low pitch inertia, often resulted in the aircraft flipping and damaging its propellers.
Attempts to address these issues through design modifications, such as adding protective arms for the propellers, introduced their own complications. The additional mass necessitated larger batteries, but the changed mass distribution rendered previously tuned control gains unstable. This cascading effect of design changes highlighted the delicate balance in aircraft design.
A breakthrough in understanding came through studying NASA's GL-10 project, which demonstrated that with electric propulsion, using multiple smaller motors could be as efficient as fewer larger ones. This insight suggested that future iterations could benefit from distributed electric propulsion, offering both redundancy and simplified control schemes similar to multicopter designs, while potentially avoiding many of the aerodynamic complications we encountered.
While the project concluded in 2020 without achieving transition to forward flight, it provided invaluable experience in aircraft design, control systems, and real-time software development. Future work in this area would benefit from starting with a conventional aircraft platform, implementing distributed propulsion, and focusing more heavily on transition aerodynamics in the initial design phase. The lessons learned continue to influence my approach to aerospace engineering challenges today.
Link to GitHub repository