eVTOL

Introduction

An eVTOL is an eight-motor electric aircraft that start-ups are using to take advantage of the FAA Part 103 to fly without a pilot's license. The FAA allows a 115 kg aircraft with a max speed of 55 knots, a power-off stall speed no greater than 24 knots, and a fuel capacity of no more than 5 U.S. gallons, to take to the skies.

Buying one is possible. Several companies in America and abroad sell them. However they are either expensive from Western companies or sketch from Chinese companies. If you were to make it yourself, it would be expensive and dangerous, but it might also be cheaper than the American ones and more trustworthy than the foreign alternatives with limited documentation.

I attempted to design one myself at lower cost while accounting for the propulsion, battery pack, structure, controls, and safety systems required to put a person in the air. Many in production now have short battery lives and risk involved with the open propellers. I wanted to see if I could build one in my own backyard.

Propulsion

I considered the cost, physical footprint, rated thrusts for continuous hover and max power, and general power consumption when selecting the motors, propellers, and ESCs for my design.

The candidate motors were the V13L, V10L, and U15 series motors by T-Motor and the Hobbywing H13 coaxial motors. While there were appealing aspects to each of them, the V10L and U15 motors were not powerful enough for a 215 kg aircraft. Choosing between the V13L and H13 meant weighing major tradeoffs. They had roughly equivalent capabilities in regard to continuous hover, but they differed greatly in cost and physical footprint. The differences in max thrust were considered where if hover was 50% of max thrust or 70% of max thrust, 50% was considered better for various heat and control reasons.

Between the H13 and V13L, the H13 is designed for agricultural drones so its footprint with 52 inch propellers was very large. However, it consumed less power, was significantly cheaper, and had the ESC and both coaxial motors baked in as one product. The V13L was much more expensive, required a dedicated mounting system, and required buying the ESCs and the two V13Ls separately. However, the V13L was significantly smaller with 42 inch propellors, which brought the footprint of the vehicle down nearly 2 feet. When considering each arm as a whole, including the propellers, the cost of the two systems came out closer than originally thought. I ultimately decided to go with the V13L, with the ESC and propellers that T-Motor recommends for it: the Thunder 12/24s ESCs and VZ42x16.5 props.

The eight V13L motors, Thunder 12/24s ESCs, and VZ42x16.5 props cost about $18,000 and would be a significant part of the budget. The collective mass of 20.5 kg of the total 115 kg budget.

With these motors, there will be 110 kgf of max force on each arm. They also determine the size of battery, 100 V, and around 50 to 60 A per motor. I want the aircraft to be hovering around 50% max throttle so the motors don't burn out, are less subject to wear, and have more upward and downward thrust margin for stabilization. There needs to be some room for voltage sag and the ability to lift the aircraft without overheating. This is because when voltage drops, amps have to compensate to match the same power output, which increases the heat by I²R and can push the ESC and batteries closer to limits quicker. Testing the motors to see how they hold up to the manufacturer test data would provide accurate thrust and power assessments, but absent of that I will trust the manufacturer data.

Further, due to the coaxial design, there is some efficiency penalty from the lower motor and prop pair being in the disturbed airflow from the top. This penalty wasn't calculated, but I acknowledged that it could be a later consideration, and I chose a motor that would allow prototyping in that regard so that the lower motors would be operating at a higher power for equivalent thrust relative to the upper motors without overheating or reaching their limit. The smaller propellers would allow the aircraft be more responsive in comparison to the 52 inch propellers, which may have made it feel more sluggish despite providing more g/W efficiency.

Battery

The battery requirements come directly from the motor selection and the goal I set for the aircraft to fly for 15 minutes. The motors require 100 V and roughly 50 A each. The 100 V translates to a 24s battery, which gives 100.8 V full charge and a nominal charge of 86.4 V. I chose lithium-ion class batteries for energy density and manned endurance. However, for amps, the issue of how to split up the parallel batteries came up.

I considered various battery architectures — an all-one battery system, eight batteries in series internally but parallel to each other around the aircraft, or two systems with four batteries for the top motors and four for the bottom motors in the coaxial configuration.

By all-one battery system, the battery would be at the bottom of the aircraft, similar to an electric car, with the motors and high current ESCs routed from there. This would be simpler and a more streamlined manufacturing process, since it is one part. The major drawback is redundancy and cooling. With it all concentrated under your seat, there would be 400 to 500 amps continuously flowing out with no good airflow solution with you sitting on top of it.

I considered spreading 8 battery modules across the aircraft for redundancy in the system so you could replace one easily and charge them individually. There could be better heat distribution and cooling solutions with them spread out. and it would distribute the center of mass better, as these will be nearly 50% of the weight budget.

Finally, I considered what would happen if the battery died or malfunctioned midair. I also considered the wire size moving 400 A would need. Splitting the system in two could help with both issues. Battery system A supports the top motors while system B supports the bottom. If one fails you could run the other at a much higher power and maybe get a semi-controlled descent, at the expense of hurting the battery or the motors. With this, there would only be 200 A going through each system. While a lot, it is more manageable. Some consideration was given to whether to go further with this and split them up more, but I that could be tested and simulated more effectively later in the process.

The top choices of the various battery cells and pack options were the Molicel P42A, the Samsung 30Q, the Molicel 45B, the Samsung 50S, the Molicel P50B, and then some other full packs such as resold Tesla Model S packs, Alibaba options, and the Tattu Pro LiPo. I looked at energy density, cost, and different layouts. With the prebuilt Tesla packs, the battery wasn't optimized for flight and was too heavy. The Tattu Pro LiPo was also in consideration for ease of manufacturing, but the LiPo battery type made it unappealing for long-term use. The various battery cell options meant that I could make and design the battery pack myself.

The Molicel P42A had the best combination of energy density, cost, and current per cell. The Molicel 50B and 45B were better in many ways, but they were harder to obtain and more expensive. The Samsung 30Q and 50S were very energy dense, but required many more cells due to their low current capability.

I tried 30p, 35p, 40p, and 46p battery pack configurations with the Molicel P42 A in 24s. The 30p configuration was the best on weight and cost. 48 kg for about $1,980 in cell cost alone, a nominal energy of 10.9 kWh, and a theoretical max current of 1350 A. Nominal energy assumes you're taking the battery to 0%, which is probably not wise. With a usable capacity of 80%, usable energy is 8.7 kWh. Since hovering won't be constant and the VTOL will be climbing, descending, changing speed, adjusting, and so on, the required power of the motors will be changing. With that knowledge, I estimated flight times at 40 kW, 45 kW, and 50 kW of about 13.1 minutes, 11.6 minutes, and 10.5 minutes respectively.

This would be split into two systems of 24s15p to power the upper and the lower motors, each split into four 6s15p modules. Not quite achieving the 15 minute flight time goal, but achieves the weight and cost considerations. The cells alone would weigh 48.24 kg, 42% of the 115 kg weight budget. That is not counting the busbars, BMS, wires, insulation, and the other electrical components connecting and making this battery pack, which could push it closer to 50 to 60% of the 115 kg weight budget.

With that last part, I realized that it would be hard to manufacture this battery for a first pass design. While the above architecture with the Molicel P42A could be a future project, a first design would be better using a premade battery or set of 8 batteries, so that more effort could be put into making everything else work.

Chassis

The chassis was designed in SolidWorks. It was developed similar to a racecar spaceframe since it has to be strong and stable enough to contain and distribute the forces from the motors without bending or destroying itself It would have to distribute roughly 60 kgf from the two motors on each arm during hover, and 110 kgf at max thrust, through the primary members of the frame. I decided to use SolidWorks weldments to design this because of the ability to rapidly prototype and prior knowledge with it.

Aluminum 7020-T6 was used for the weight requirement of the Part 103, and since the battery and motors took up so much of the weight budget. For the primary chassis tubes, 1.5 inch diameter tubes with 0.063 inch thick walls were used, tested through FEA. Diameter increases were needed more than thickness, since tube bending stiffness scales with radius more than thickness. However, depending on the crushing safety and requirements of welding the members together, there were versions made with thicker tubing, as much as 1/8 of an inch.

The chassis was designed with a high factor of safety. The design uses triangulation, shear transfer, and load paths through primary elements to ensure a secure ride in the eVTOL. This led to using arm trusses. The arms would connect at a node cluster and distribute the force across the aircraft through the upper and lower rails, diagonals, and side triangulation. This allowed for better spaceframe behavior and ensured the arm wouldn't be dependent on one mount. The arm would behave similar to a cantilever, so a rectangular 2 inch by 3 inch tubing was chosen for stiffness benefits.

The arms would be about 1 meter long, with the propellers adding additional size, so hinges were considered to allow the arms to close and make it easier to transport the VTOL. The most promising design was one with inserts that could be secured into the arm tubes with a closing and locking mechanism with full positive stops so most of the force goes along the desired load path while minimizing force in the hinge.

After a few iterations of the model and parts, I assembled a 1:5 scale version out of wooden dowels and glue. This was to see ease of manufacturing with similarly round and rectangular pieces, while replicating how welding geometry and sequencing would work with glue substituted.

The major realization was the nonstandard cut lengths that came from using the trim and extend features in SolidWorks weldments. More care would be required so the lengths would be rationalized to better numbers. The design needed to be standardized so that two members with near equal member lengths would be the same length. The model largely validated the geometry and helped me understand the assembly sequence of the parts, along with ideas about how to fix awkward joints.

Safety Issues

I tested some drone software and put together a regular drone to see how it could be extrapolated to larger versions. Trying Betaflight and ArduPilot, I found ArduPilot was better matched for this safety-first and autonomous prototype of the VTOL, and safer for if a person were to ride it. The drone made me realize that there would need to be serious safety efforts and better methods for testing and calibrating a full sized VTOL, so that it would be safe before anyone got in it, and so everyone was safe while calibrating it.

Even done perfectly, the safety issues with VTOLs in their current state seem large. Having four one-meter-long coaxial propeller pairs around you is a serious safety risk no matter how careful you are. Accidents happen, and crashing with one of those around is a significant obstacle to overcome, along with the possibility of rotor or battery failure. There may be a way, but it may take a different form from drone-style flight.

Conclusion and Next Steps

The motors, ESCs, and propellers come to 20.5 kg. The battery cells come to 48.24 kg. The best version of the chassis was about 20 kg. That is 88.7 kg of the 115 kg budget from three subsystems, and the battery figure is only the cells. Allowing for the rest of the battery pack, seats, avionics, wires and other necessary elements, this design most likely wouldn't get under the 115 kg requirement in its current form. Getting under it would require a shorter flight time or a denser cell chemistry.

There were also limitations with the spreadsheet calculations done to evaluate batteries, energy, and heat output. A more robust solution would be to use MATLAB Simscape and Simulink to more accurately simulate the VTOL as a whole and systematically test different options to arrive at proper parameters. The motors, batteries, and thermal modeling could work as a whole through this instead of as estimates. That would also settle the weight question with stronger than a spreadsheet.

No prototype got welded due to the mounting costs the project would require and the general safety issues. While this is unfortunate, it serves as a strong feasibility study on how far these devices have come, and it is reasonable to hope that as materials and batteries continue to improve, a serious effort to bring about new modes of transport will become more realistic for the future.