Project Overview
Mini VTOL is a sub-10g coaxial helicopter, designed and built from scratch for Boston University's Mechanical Engineering Senior Design Capstone. The vehicle combines a custom 4-layer PCB, scavenged Syma S100 powertrain components, and an Arduino IDE control loop to achieve takeoff, hover, and landing within extreme mass and volume constraints.
Skills and Tools:
Conceptual Design Sketches
Competitive Landscape Analysis
Quality Function Deployment (QFD)
Onshape 3D CAD Models
3D Printing (FDM, PLA)
KiCAD PCB Schematic & Layout Design
Hand Soldering (SMD, 0603 components)
Arduino Code
Reverse Engineering (Syma S100)
DFX Analysis (Cost, Manufacture, Assembly, Reliability, Sustainability)
Pugh Matrix Concept Evaluation
Features and Specifications:
Total Mass: 9.80 g
Volume: 150 cm³
Propulsion: Coaxial counter-rotating rotors, 2× 6mm brushed DC motors
Control: ATSAMD21 microcontroller
Power: Tethered 6V / 1A external supply
Payload Capacity: 2.1 g
Flight Duration: Theoretically unlimited (tethered)
PCB: Custom 4-layer FR4, 50 × 50 mm, fabricated by JLCPCB
Yaw Control: Differential rotor torque via MOSFET PWM
Assembly Time: Under 1 day
Contents
Sep 2025
May 2026
Boston University
Senior Capstone
Problem and Task
The challenge was to build a fully self-contained, autonomously stabilized VTOL aircraft that fits inside a 150 cm³ bounding box, weighs under 10 g, and achieves controlled flight, all while designing a custom PCB, sourcing precision micro-components, and navigating international supply chain delays. Product competitors include the Syma S100, UPenn's Piccolissimo, and Harvard's Robobee.
My Tasks:
Designed full PCB schematic and layout in KiCAD, including component selection, trace routing, copper pours, and 4-layer stack-up configuration.
Hand soldered all SMD components onto the custom PCB, including 0603 passives, QFP microcontroller, and SSOP motor driver.
Flashed Arduino bootloader onto the ATSAMD21 microcontroller via SWD interface to enable Arduino IDE firmware uploads.
Wrote Arduino control code implementing gradual PWM ramp logic for smooth rotor velocity transitions and stable takeoff.
Designed all 3D printed components in Onshape including the airframe top plate, motor mount, landing legs, and PCB gaskets.
Originated the initial coaxial VTOL concept and led early-stage background research establishing the project direction.
Served as team leader, coordinating subteam responsibilities, setting milestones, and driving design decisions across mechanical and electrical tracks.
Organized and ran all team meetings, maintaining agenda structure and ensuring progress against deliverable deadlines.
Managed team finances, tracking component purchases, PCB fabrication costs, and overall budget against the $400 project limit.
Constraints
Total vehicle mass could not exceed 10 g (threshold), with a target of 9 g, requiring milligram-level accounting of every component including solder, wire insulation, and adhesive.
The vehicle had to fit within a 250 cm³ bounding box (threshold), with a target of 150 cm³, tightly constraining PCB geometry, rotor span, and landing leg placement.
The vehicle needed a minimum of 1 autonomous factor (threshold) with a target of 2, while also achieving stable hover within 5 rotor diameters for at least 10 seconds (threshold) and up to 30 seconds (target).
Full 3-axis flight control was required (threshold and target), covering independent pitch, roll, and yaw moments.
Total unit cost was capped at $50 (threshold), with assembly and manufacturing each targeted at 2 days or fewer, demanding low-cost sourcing and rapid fabrication strategies from the start.
Engineering Solutions
Mechanical
Airframe
PCB doubles as the structural base plate, eliminating a dedicated lower frame member and saving significant mass.
Carbon fiber rods press-fit into 3D-printed gaskets bridge the PCB to the top plate, forming a protective cage around the drivetrain.
Powertrain
Coaxial shafts, gears, rotors, and flybar scavenged from the Syma S100 for precision-fit, low-vibration operation beyond the team's in-house manufacturing capability.
Flybar provides passive gyroscopic stabilization, critical in the final vaneless configuration.
3D-printed motor mount holds two 6mm brushed DC motors at the exact gear mesh distance.
Weight Management
Thrust vanes removed to remain under 10g, disabling lateral control for weigh saving.
All 3D-printed parts oriented for support-free printing to eliminate waste and minimize mass.
Tethered power option removes the 2 g battery entirely for competition weight savings.
Electrical
Custom PCB
Custom 4-layer FR4 PCB (35 × 35 mm, 0.8 mm thick) designed in KiCAD and fabricated by JLCPCB.
TPS73733 LDO voltage regulator provided a stable 3.3 V supply to sensitive ICs.
Decoupling capacitors, flyback diodes, and a hardware reset button improved reliability and supported rapid firmware iteration.
Control & Sensing
ATSAMD21G18A microcontroller executed an Arduino-based PID control loop.
BMI323 6-axis IMU provided linear acceleration and angular velocity measurements across all three axes via I2C.
Real-time sensor feedback was used for attitude estimation and stabilization control.
Motor Drive System
Two IRLML2502 MOSFETs independently controlled upper and lower rotor RPMs.
Differential rotor speed control generated yaw torque for directional control.
PWM signals from the microcontroller regulated motor output and system response.
Software
Arduino IDE (Flight Control)
Implemented gradual PWM ramp logic for smooth rotor velocity transitions, preventing abrupt torque spikes that would destabilize the vehicle at takeoff.
Programmed flight sequences including timed takeoff, hover, and landing routines executed without real-time pilot input.
Used I2C library to interface with the BMI323 IMU, reading linear acceleration and rotational velocity across all three axes at high loop frequency.
KiCAD (PCB Design)
Designed full 4-layer PCB schematic including all component symbols, net connections, and power rail assignments across battery voltage and 3.3V domains.
Performed PCB layout with manual trace routing, copper pour ground planes, decoupling capacitor placement, and design rule checks against JLCPCB fabrication tolerances.
Generated final Gerber files for submission to JLCPCB and maintained version control of all design files through GitHub.
Onshape (CAD)
Modeled all 3D-printed structural components including the airframe top plate, motor mount, landing legs, and PCB interface gaskets.
Generated detail drawings with three-view and isometric projections, fully dimensioned for in-house FDM reproduction.
Testing
Conducted precision mass analysis confirming 9.80 g takeoff weight using a calibrated 0.01 g scale.
Verified tether power delivery at 6V / 1A under maximum motor load.
Ran 10 consecutive autonomous landing cycles to evaluate landing reliability (10% success rate due to absent pitch/roll control).
Confirmed 2.1 g payload capacity through incremental mass-added lift tests.
Verified full vehicle assembly time under 1 day with components on hand.
Validated single-axis yaw control through variable rotor RPM differential.
Results
The vehicle successfully achieved autonomous takeoff, yaw-controlled hover, and pre-programmed landing at 9.80 g, under the 10 g competition threshold, while carrying a 2.1 g payload and fitting within the 150 cm³ target volume. Of 16 engineering specifications, 6 were fully met or exceeded, with flight duration theoretically unlimited under tethered power. The project was limited primarily by the loss of pitch and roll actuators during assembly.
Mass: 9.80 g ✓
Volume: 150 cm³ ✓
Payload: 2.1 g (exceeded 2 g target) ✓
Flight Duration: 10+ min (tethered) ✓
Assembly Time: < 1 day ✓
Unit Cost: $128.69 (exceeded $50 target)
Hover Stability: 5 s (target: 30 s)
Flight Control Axes: 1 (yaw only; target: 3)
Lessons Learned
At sub-10g scale, solder blobs, wire insulation, and minor adhesive shift center of gravity in measurable ways; a milligram-level mass budget must be enforced at every step.
The magnetic actuators were the highest single point of failure in the design; future teams should order 3-4x the required quantity and avoid handling them until final assembly.
PCB design should start on day one; FDM parts iterate in hours, but a 4-layer PCB requires weeks of lead time and should be ordered before the mechanical design is finalized.
A tethered power supply saves ~2 g instantly, but the tether itself becomes a mechanical load on a 10 g aircraft; use the thinnest available insulated wire.
This project is overwhelmingly an electrical and software engineering challenge despite its mechanical framing; future teams should cross-train in embedded systems or recruit cross-disciplinary members early.
