VTOL Aircraft: Perfecting the Art of Flight

Srikar Gouru Thomas Jefferson High School for Science and Technology

This article was originally published in the 2021 print edition of Teknos Science Journal.

As part of my high school’s Unmanned Aerial Vehicle team, I participated in its annual competition. Captivated, I watched as our plane gracefully soared through the skies, weaving through virtual obstacles and autonomously identifying targets scattered across the ground. However, the process of launching the plane by hand was hazardous, especially for our launcher, Jack, who had to wear thick gloves and a hard hat. Since we only had access to a grass field without a runway, we could not test with a wheel-based landing gear. As I pondered the dangers of manual launching, I wondered how planes could operate in rural areas where roads are unpaved and infrastructure is lacking. Thus, as part of the Energy Systems senior research laboratory at Thomas Jefferson High School for Science and Technology, I began investigating vertical take-off and landing planes.

Vertical take-off and landing (VTOL) aircraft can both take-off and land vertically without the use of a runway. The most well-known example of a VTOL aircraft is the helicopter, but VTOL technology has also been applied to conventional airplanes, with the first such plane being the British Royal Air Force Harrier in 1969. VTOL planes combine the fuel efficiency and speed of conventional planes with the hovering and vertical-takeoff capabilities of a helicopter. There are six main types of VTOL planes that have been designed and built in the last 50 years: compound aircraft, tiltrotors, tiltwings, tailsitters, lift fans, and vectored thrust jets [5]. Each variation has its own advantages, with tailsitters allowing for fast take-offs and vectored thrust enabling for quick maneuverability [5]. For my senior research project, I decided to develop a tiltwing aircraft due to their fuel-efficient thrust production, allowing them to travel longer distances at high speeds.

The primary consideration for any aircraft is its propulsion system. Since my main objective was to produce a plane which could travel long distances at high speeds, I searched for efficient mechanisms of generating thrust that have been used in VTOL aircraft. Conventional planes often use gas-powered thrust since its mechanical actuation mechanism enables high cruising efficiency. However, altering the exhaust angle, an important step in performing VTOL maneuvers, was calculated to result in a major loss of thrust [6]. Yang Zhou (personal communication, Feb. 18, 2021) was able to conclude that the engine’s energy coefficient,, should be less than 0.2 to ensure a viable propulsion system. Since a gas-powered vehicle requires a large engine, and the proposed small-scale tiltwing has minimal space, developing such an engine is impractical. While gas-powered engines are popular in large-scale aircraft, many Remote Control (RC) hobbyists prefer using rechargeable batteries, as they are energy-efficient and allow for faster testing. Specifically, lithium-polymer batteries are preferred due to their compactness and durability.

Another crucial aspect of VTOL aircraft is the complex system used to control and balance them midflight. Conventional airplanes implement a Proportional, Integral, Derivative (PID) feedback loop. An example of a PID loop in real life is balancing on a tightrope. When you’re starting to fall towards the right, you tilt yourself leftwards to compensate. The output of the PID algorithm tells you how much to push yourself in a given direction. While flying, planes control themselves in three degrees of freedom: pitch (up/down), roll, and yaw (left/right). In horizontal flight, conventional planes and tiltwings, can actuate their control surfaces accordingly to stabilize themselves. For vertical flight, tiltwing aircraft also implement a variety of techniques such as differential thrust to maintain control of the plane. However, since the PID mechanisms of a tiltwing plane are different in vertical and horizontal flight, the transition periods between vertical and horizontal flight cannot be controlled with conventional PID. In their 2020 study, Bauersfeld and Ducard developed a new control mechanism for VTOL aircraft: Fused-PID (FPID), which combines the outputs of both the vertical PID and horizontal PID in such a way that allows for complete control of the aircraft during the transition period [1]. Using this simple approach, they were able to precisely control tiltwing and tiltrotor aircraft during the transition stage.

Most VTOL research and development has been conducted by militaries and has focused on stealth and maneuverability (which enhance combat) instead of long-range sustainability and ease of development. This led to many tiltwing aircraft projects being recycled, since they were not as agile as their vectored-thrust VTOL counterparts. Additionally, since the military focuses on large-scale applications such as fighter jets, minimal research has been conducted on small-scale VTOLs. In their 2020 study, Dündar et al. analyzed the design process and development of a small-scale battery-powered tiltwing aircraft [2]. They listed out and calculated various constants, such as the power required for take-off (PTO), induced drag coefficient (CD), and cruising speed (Vcruise), and balanced them to design a tiltwing aircraft that maximized flight distance and speed and minimizing agility. They also simulated the model in Simulink before developing it in real life, allowing them to tune their PID controllers without damaging equipment.

In my VTOL aircraft, I plan on developing a system similar to FPID that transitions from vertical flight to horizontal flight after take-off (and vice versa) before landing. Additionally, I plan to use cheaper materials and a rapid development system. This would allow for the planes to be quickly reconstructed/replaced if broken and make them more accessible in poverty-stricken areas. In addition, my VTOL aircraft will be capable of carrying a payload that can be autonomously and precisely dropped at a given location, which is useful in time-sensitive cases such as delivering blood to a hospital patient or helping survivors in search and rescue operations.

While VTOL planes have mesmerized researchers for half a century, significant research in the field has stalled in recent years. This is largely due to the development of larger ships and aircraft carriers. However, advanced projects such as DARPA’s X-Plane, which introduces a novel technique called active flow control, have reignited life into the field of VTOL aircraft [4]. Additionally, VTOL technology could lead to a future of flying cars, as a Japanese company is currently working on a civilian electrical VTOL vehicle [3]. Whether it's helping the common good or creating the unfathomable, VTOL technology is essential in various tasks. Through my research, I hope to take part in this aerial revolution.

References

[1] Bauersfeld, L., & Ducard, G. (2020). Fused-PID Control for Tilt-Rotor VTOL Aircraft. The Institute of Electrical and Electronics Engineers, Inc. (IEEE) Conference Proceedings. http://dx.doi.org/10.1109/MED48518.2020.9183031

[2] Dündar, Ö., Bilici, M., & Ünler, T. (2020). Design and performance analyses of a fixed wing battery VTOL UAV. Engineering Science and Technology, an International Journal, 23(5), 1182-1193. https://doi.org/10.1016/j.jestch.2020.02.002

[3] Taylor, D. B. (2020, August 29). Humans Take a Step Closer to 'Flying Cars'. New York Times. https://www.nytimes.com/2020/08/29/world/asia/japan-flying-car.html

[4] Tegler, E. (2020, January 9). The Next DARPA X-Plane Won't Maneuver like Any Plane Before It. Scientific American. https://www.scientificamerican.com/article/the-next-darpa-x-plane-wont-maneuver-like-any-plane-before-it/

[5] Yaoming, Z., Haoran, Z., & Yaolong, L. (2020). An evaluative review of the VTOL technologies for unmanned and manned aerial vehicles. Computer Communications, 149, 356-369. https://doi.org/10.1016/j.comcom.2019.10.016

[6] Zhou, Y., Huang, G., & Xia, C. (2020). Analysis of fixed-wing VTOL aircraft with gas-driven fan propulsion system. Aerospace Science and Technology, 103. https://doi.org/10.1016/j.ast.2020.105984