1 Design of a Subscale Reconfigurable eVTOL Aircraft for Transition Corridor Flight Testing Frank K. Kozel, 1 Jacqueline Q. Tu, 1 and Eddie Q. Li 1 Georgia Institute of Technology, Atlanta, Georgia, 30332 Matthew M. Warren 2 Georgia Institute of Technology, Atlanta, Georgia, 30332 Urban Air Mobility (UAM) is an emerging class of transportation that is envisioned as a low-cost, on-demand, point-to-point passenger air service with flights between rooftop “vertiports” situated throughout cities. The types of aircraft being considered for UAM are electric VTOL (eVTOL) aircraft that typically include a wing for forward flight efficiency and distributed rotors for vertical flight and hover. Many of these aircraft also include tiltrotor or tiltwing mechanisms to achieve "conversion" or "transition" from horizontal to vertical flight and vice versa. The complex aerodynamics, flight dynamics, and control of transition are among the most difficult aspects of the design of tiltwing and tiltrotor VTOL aircraft. In this paper, we describe a scale aircraft that was designed and built to investigate the flight performance of new configurations of eVTOL aircraft that have difficult transition aeromechanics and flight performance. The aircraft is a modular tiltrotor that uses four rotors for vertical lift and tilts the two rear rotors for forward thrust. The aircraft was designed and constructed using rapid prototyping techniques with a focus on robustness and reconfigurability. Material selection was primarily light balsa and plywood for structural shape with metal and composite components used to transfer large structural loads. Custom control systems based on the open source PX4 software were implemented to control the aircraft in vertical flight and transition to forward flight. A flight test plan to measure transition performance was developed in which the flight envelope is incrementally expanded to manage risk. The aircraft will be used to test a variety of eVTOL configurations and to collect representative subscale flight test data. Nomenclature α = angle of attack cd = section drag coefficient CD0 = parasite drag coefficient cl = section lift coefficient Clβ = roll damping coefficient Cmα = pitch damping coefficient Cnβ = yaw damping coefficient CL = airplane lift coefficient CLmax = maximum airplane lift coefficient CD = airplane drag coefficient Re = Reynold’s number Wtotal = total weight 1 Aerospace Engineering Undergraduate Student, Daniel Guggenheim School of Aerospace Engineering, AIAA Student Member. 2 Aerospace Engineering Graduate Student, Daniel Guggenheim School of Aerospace Engineering, AIAA Student Member.
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Design of a Subscale Reconfigurable eVTOL Aircraft for
Transition Corridor Flight Testing
Frank K. Kozel,1 Jacqueline Q. Tu,1 and Eddie Q. Li1
Georgia Institute of Technology, Atlanta, Georgia, 30332
Matthew M. Warren2
Georgia Institute of Technology, Atlanta, Georgia, 30332
Urban Air Mobility (UAM) is an emerging class of transportation that is envisioned as a
low-cost, on-demand, point-to-point passenger air service with flights between rooftop
“vertiports” situated throughout cities. The types of aircraft being considered for UAM are
electric VTOL (eVTOL) aircraft that typically include a wing for forward flight efficiency and
distributed rotors for vertical flight and hover. Many of these aircraft also include tiltrotor
or tiltwing mechanisms to achieve "conversion" or "transition" from horizontal to vertical
flight and vice versa. The complex aerodynamics, flight dynamics, and control of transition
are among the most difficult aspects of the design of tiltwing and tiltrotor VTOL aircraft. In
this paper, we describe a scale aircraft that was designed and built to investigate the flight
performance of new configurations of eVTOL aircraft that have difficult transition
aeromechanics and flight performance. The aircraft is a modular tiltrotor that uses four rotors
for vertical lift and tilts the two rear rotors for forward thrust. The aircraft was designed and
constructed using rapid prototyping techniques with a focus on robustness and
reconfigurability. Material selection was primarily light balsa and plywood for structural
shape with metal and composite components used to transfer large structural loads. Custom
control systems based on the open source PX4 software were implemented to control the
aircraft in vertical flight and transition to forward flight. A flight test plan to measure
transition performance was developed in which the flight envelope is incrementally expanded
to manage risk. The aircraft will be used to test a variety of eVTOL configurations and to
collect representative subscale flight test data.
Nomenclature
α = angle of attack
cd = section drag coefficient
CD0 = parasite drag coefficient
cl = section lift coefficient
Clβ = roll damping coefficient
Cmα = pitch damping coefficient
Cnβ = yaw damping coefficient
CL = airplane lift coefficient
CLmax = maximum airplane lift coefficient
CD = airplane drag coefficient
Re = Reynold’s number
Wtotal = total weight
1 Aerospace Engineering Undergraduate Student, Daniel Guggenheim School of Aerospace Engineering, AIAA
Student Member. 2 Aerospace Engineering Graduate Student, Daniel Guggenheim School of Aerospace Engineering, AIAA Student
Member.
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I. Introduction
HE primary objective for conducting this research is to advance the understanding, technology, and operating
efficiency of electric transition aircraft. A team from the Georgia Institute of Technology, consisting of one
graduate and three undergraduate aerospace researchers, worked together with the intent to develop a small prototype
aircraft with the ability to transition from vertical flight to horizontal flight. Three of the members possess expertise
in the techniques used to rapidly prototype unmanned aerial vehicles for the AIAA Design Build Fly (DBF)
competition. Using these techniques, the team fully developed the requirements for this aircraft, conducted analysis
on a design, iterated on this design, manufactured the final aircraft, and flight tested in just over four months. This
paper describes the process used by the design team to create a fully realized flying model that will provide data to
understand the dynamics of flight during transition. With this information, the team will characterize the transition
states of the model to show that rapid prototyping can yield insightful flight test data about complex and non-linear
flight mechanics for a specific eVTOL configuration before a full-scale version is ever built.
II. Project Overview
Instead of operating on a linear design process, the team used an interconnected, rigorous aircraft design cycle to
explore the design space. For example, reconfigurability of the aircraft was implemented early in the conceptual design
phase to ensure a multipurpose aircraft was produced for future research endeavors. Additionally, flight testing is
shown to not only validate the design but expose solvable defects in the aircraft conceptual design such as the stability
and control of the vehicle.
Figure 1: Aircraft Design Cycle
Each team member contributed to the manufacturing
of the vehicle which took place over several weeks.
Previous rapid prototyping experience from the team’s
senior members guided the scheduling, while knowledge
of supply chains, familiarity with construction, and
effective management techniques enabled on-time
completion. As a benefit, the modularity of design meant
no time was lost to component integration.
Material and component costs were documented
thouroughly to demonstrate the affordability of a small
scale research vehicle. Figure 2 shows a visualization of
the main component groupings and their contributions to
cost. As the team expected, the largest contributor to cost
were the electronics. The flight control system and the
propulsion system electronics made up 83% of the budget.
III. Design Process
A. Requirements
The success of the research depends upon the aircraft’s ability to satisfy the desired mission requirements in a
reliable and effective manner, namely providing flight test data during transition between horizontal and vertical flight
for a given configuration. Stability in both vertical and horizontal flight modes and robustness for many flight tests
factored heavily into the design. Additionally, electric propulsion shall be used for its ease of use in model remote
control (RC) flying, reliability as a system, and in the spirit of a future full scale eVTOL aircraft. Multiple power
T
Adhesives
4% Structural
Materials
13%
Propulsion
System
37%
Flight
Control
Electronics
46%
Adhesives
Structural
Materials
Propulsion
System
Flight Control
ElectronicsTotal Cost: $2,838.16
Figure 2: Cost Analysis
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systems shall be used in a multirotor fashion due to the availability and low cost of hobby grade electric motors. The
aircraft shall also be a Group 2 UAV, a vehicle with a maximum takeoff weight (MTOW) between 20 and 55 lbs, that
will provide flight test data with the intent to pass or fail a potentially feasible urban air mobility (UAM) eVTOL
configuration. Lastly, modular components shall be used for ease of transportation, replacing damaged pieces, and for
the ability to test multiple aircraft configurations.
B. Risk Assessment
The primary risk is complete loss of control of the aircraft, leading to a crash. This risk is mitigated by reducing
both the probability of occurrence and severity of consequence. Temporary loss of control is probable by nature of
this research vehicle for transition dynamics. They key is to ensure a high probability of control recovery. This is
achieved by creating a simple and stable fixed-wing platform with predictable gliding characteristics, and testing
transition maneuvers at sufficient altitude for a power-off recovery. In the event of temporary loss of control, the
vehicle is designed to allow immediate manual override by the pilot into “airplane mode” or, if appropriate, reversion
to the onboard flight controller for “multirotor mode”. The severity of consequence is measured in injury first and cost
second. The potential for injury to any people, including flight test crew and pilot, is low due to the small size of the
vehicle and remote operation over open fields. Small size also reduces the cost of the project and the financial burden
incurred for any crashes or failed components.
Secondary risks include structural failure, especially when landing. To reduce this potential, the aircraft is made
of components that can be easily replaced. Furthermore, additional plywood structure was added around areas known
to experience high stresses such as the attachment points for the landing gear and motors. The same modular system
used for testing different aircraft configurations is also used to replace broken or failed subsystems.
Other risks from a broader, systems level include susceptibly to manufacturing errors by engaged engineering
students and engineering complexity. Strategies used to combat this were jigging the wing structures for more precise
assembly, instituting a team hierarchy to enforce standardized construction procedures, and routinely performing
inspections and tests leading up to final assembly.
C. Configuration Selection
Figures of Merit (FOMs) were used to analyze different airplane configurations, as seen in Table 1, with higher
weights corresponding to more important characteristics. Considerations for the merit values were determined from
prior DBF experience with similar unmanned aerial vehicles (UAV) of the same general size. The vertical flight mode
would be performed with four motors in a quadrotor arrangement around the center of gravity, and transition would
occur by rotating motors to a new position. The quadrotor approach is a simple solution that is well understood and
that allows for a simple transition mode. This design also allows for modifications to utilize an octo-rotor configuration
or other combinations of tilting and stationary motors. Another consideration was each configuration’s modularity to
enable new rotor arrangements and transition strategies.