FCU multicopter hybrid

A Flight Control Unit, or FCU, is the flight control computer of a hybrid multicopter. In this type of aircraft, energy comes from a piston engine feeding a distributed propulsion architecture with fixed-pitch vertical-thrust propellers. The FCU translates pilot commands into safe, stable, and efficient flight behavior. It distributes power between the rotors, keeps the aircraft balanced, continuously monitors onboard sensors, corrects deviations caused by wind or system anomalies, and manages warnings as well as emergency procedures. Unlike a conventional helicopter, where the pilot directly manages much of the aircraft response, or a fully electric VTOL focused mainly on battery management, the FCU of a hybrid multicopter must simultaneously stabilize flight and coordinate a more complex propulsion system.

10 Operating Cases, from the Simplest to the Most Complex

For clarity, vertical climb and vertical descent are presented here as two separate cases, even though they are grouped together in the original FCU requirements document.

1. Progressive takeoff When the pilot increases throttle, the FCU distributes power across the rotors so the aircraft lifts vertically while remaining level, limiting climb rate and preventing motor saturation.

2. Stabilized hover When hover mode is requested, the FCU uses sensor data to maintain position and altitude, even in the presence of wind or drift.

3. Vertical climb The FCU adjusts thrust so the aircraft climbs along its vertical axis while preserving trajectory control, stability, and motor availability.

4. Vertical descent The same principle applies during descent: the FCU regulates power to keep the aircraft stable and on a controlled vertical path down to the target altitude.

5. Yaw rotation on the spot When the pilot commands rotation about the vertical axis, the FCU modulates rotor power to produce accurate yaw motion without excessive drift or loss of control.

6. Straight forward flight When a forward command is given, the FCU generates a pitch command and adapts rotor power so the aircraft moves ahead in a stable, controlled way.

7. Turning while moving forward The FCU combines roll, yaw, and power management to perform a coordinated right or left turn during forward flight.

8. Turning while climbing In a more complex maneuver, the FCU must manage the turn, altitude increase, and reinforced sensor monitoring at the same time to maintain safety and stability.

9. In-flight emergency stop with parachute deployment In a critical failure, the FCU detects the emergency, confirms parachute deployment logic, stops or manages the rotors as required, continues monitoring critical systems, and issues the alerts needed to secure descent.

10. Final shutdown on the ground After landing, when the pilot commands shutdown, the FCU confirms the instruction, prepares the aircraft for a safe stop, and cuts the motors according to a controlled sequence.

Why it Matters

The FCU is a core technology for the missions targeted by a hybrid multicopter, including light air ambulance operations, tourism, and pilot training. Without it, distributed propulsion would be difficult to operate safely and reliably. With it, the aircraft can take off, hover, move forward, turn, handle emergencies, and shut down in a controlled way while maintaining a high level of operational safety.

1. Flight Control Unit (FCU) for a Hybrid Multicopter — Specification Summary

• Source: Cahier des charges — Flight Control Unit of a Hybrid Multicopter (Author: Chérif Hidoussi, Reviewer: Xavier Dutertre, 2024-11-08)*

1. 1. Overview

This document specifies the functional and technical requirements for a **Flight Control Unit (FCU)** that controls a **hybrid multicopter** — an aircraft powered by a single piston engine driving electric generation, with fixed-pitch vertical-thrust propellers (no tilt rotor). The reference design uses around 60 rotors.

The goal is a flight control system that is **safe, reliable, and efficient**, suitable for the multicopter's operational needs across takeoff, stable flight, maneuvers, and emergency procedures.

1. 1. 1. Target applications

- **Light air ambulance** — transport of a stabilized patient (no winching). The pilot may be the doctor/stretcher-bearer, seated on the left; the patient is a passenger and does not fly the aircraft. - **Tourism** — one pilot (left seat); the second person is a passenger with controls disabled. - **Pilot training** — both left and right control sets are active.

1. 1. 1. Certification target

CS 27 (small helicopter) or EASA VTOL (a relatively new certification path as of 2024).

1. 1. What makes this FCU different

Unlike a traditional helicopter (single variable-pitch rotor, often pilot-controlled directly or via fly-by-wire) or a fully electric VTOL (Volocopter, Lilium, Joby — where battery management is central), this aircraft relies on **a single, less powerful piston engine**. Because there is no redundant second engine, the aircraft carries a **whole-aircraft rescue parachute** to handle total engine loss, allowing it to land without any flight control. Stabilization and control methods must therefore be adapted to this single-engine hybrid configuration.

1. 1. 1. Pilot controls and flight axes

The pilot uses a **right-hand joystick**. The three flight axes are pitch (tangage), roll (roulis), and yaw (lacet).

1. 1. Functional requirements by use case

The specification breaks FCU behavior down into distinct flight scenarios, each with defined inputs, outputs, and rules.

• • Case 1 — Takeoff (throttle increased by hand):** As the pilot raises the throttle, engine speed and available electrical power increase, spinning the rotors faster until the aircraft lifts off. The FCU distributes power evenly across all rotors to keep the aircraft level and lifting vertically. It manages progressive power increase, automatic tilt correction (e.g. wind gusts or a failed rotor), continuous sensor monitoring, climb-rate limiting, motor-saturation prevention, altitude and vertical-speed control, and alarm generation for abnormal conditions.

• • Case 2 — Hover via the stop button:** Pressing the red push/pull stop button on the joystick holds the aircraft stationary in the air with a fixed heading. The FCU uses positioning and altitude sensors to maintain position and altitude, correcting for wind drift. It handles fallback cases such as GPS dropout (switching to inertial mode), rotor performance loss (redistributing power), logs the stop event, manages subsequent pilot commands (restart/resume), and can disable automatic functions (e.g. auto-follow, return-to-home) temporarily or permanently.

• • Case 3 — Climb and descent around a vertical axis:** With the stop button engaged and joystick centered, the pilot presses climb or descent. The FCU detects the command, adjusts motor power proportionally (reducing for descent, increasing for climb), maintains stability against wind, monitors altitude, and handles emergencies (e.g. obstacle detection triggering emergency climb, motor overheat triggering controlled descent, motor jam during climb).

• • Case 4 — Hovering yaw rotation:** With the stop button pressed, turning the joystick left or right rotates the aircraft about its vertical axis. The FCU modulates individual rotor speeds to produce the required yaw torque while keeping pitch = 0, roll = 0, and altitude constant. It maintains stability against wind, respects the aircraft's operational limits, and handles faults (gyro failure, loss of yaw authority). Returning the joystick to center reverts to the Case 2 hover.

• • Case 5 — Straight-line forward flight:** Pressing the forward button with the joystick centered, the pilot adjusts throttle to hold altitude. The FCU generates pitch setpoints to tilt the aircraft forward for horizontal thrust, controls attitude in real time, compensates for disturbances (gusts, front-rotor loss, speed-sensor failure), manages propulsion and thrust distribution to hold the target speed, and respects pitch-angle limits to prevent stall or instability.

• • Case 6 — Turning while moving forward (left/right):** With forward already engaged, turning the joystick makes the FCU coordinate pitch and roll for a balanced turn at constant speed and altitude. It differentiates motor power side-to-side to bank the aircraft, caps the maximum bank angle (with pilot alerts and automatic intervention if exceeded), compensates altitude loss from banking, and returns the aircraft smoothly to stable flight after the turn.

• • Case 7 — Turning while climbing or descending:** Combining throttle (climb/descent) with a joystick turn, the FCU analyzes the trajectory, calculates the required bank angle while accounting for changing gravitational load, redistributes motor power, manages attitude and vertical speed, compensates for wind, and continuously monitors sensors and safety systems.

• • Case 8 — In-flight emergency stop and parachute deployment:** For critical failures (piston engine, generator/Emrax, flight computers, or many rotors), the pilot presses the red emergency button. The FCU's logic: if the emergency button is pressed and the parachute is not yet deployed, it triggers the parachute. Once deployed, it stops all rotors and shuts down systems — though it may keep a few rotors running at adjusted power to keep the aircraft level during the parachute descent. It continues monitoring critical systems (sensors, batteries, controls) and transmits the emergency situation, location, and flight conditions to ground operators to aid recovery.

• • Case 10 — Final shutdown (power-off button):** On the ground after landing, pressing the "shut down" button makes the FCU confirm the order, progressively deactivate onboard systems (non-essential first, then flight-critical), cut electrical power to the motors and components, and confirm safe shutdown to the pilot interface. Backup procedures cover faults such as a motor not responding to shutdown, overheating during shutdown, or a non-functional shutdown button.

1. 1. Cross-cutting concerns

• • Sensor and system monitoring:** The FCU continuously collects and validates data from gyroscopes, accelerometers, and barometers (plus GPS) to compute motor commands, orientation adjustments, and telemetry. It detects sensor anomalies, falls back to redundant or backup data, and logs critical flight data for post-flight analysis.

• • Power management and safety:** Dynamic power balancing prevents motor overload, especially during power transitions in climb and descent. Maximum power limits protect against overheating and damage, climb-rate limits prevent excessive reactions, and saturation prevention preserves control margin.

• • Alarms and warnings:** Abnormal conditions trigger graded visual or audible alerts on the radio-control display, designed with ergonomics and pilot workload in mind, and built with redundancy for reliability.

1. 1. Document sections not yet detailed

The specification reserves (but does not yet fully populate) sections on: - **Testing and validation** — test methodology and scenarios covering standard flight and emergencies. - **Maintenance and support** — preventive/corrective maintenance, spare parts, manuals, and manufacturer technical support. - **Schedule and budget** — development/delivery timeline and cost estimates for design, manufacturing, and certification.

1. 1. Reference projects

- **Mini-Bee** (mini-bee.com) — Rotax 915is/916is piston engine (~140 hp). - **Workhorse SureFly** — Honda engine (~200 hp). - **Zephyr helicopter** — cited as the first helicopter with a parachute rescue system.

• Note: the original document states it was partially drafted with the help of ChatGPT.*