Arms, base, motors, onboard modules: the multirotor project computes the whole assembly, and shows what tilting the frame costs in thrust.

See also: the contra-rotative pair

A drone propeller can be computed perfectly well on its own — Heliciel does that in its main interface. But a multirotor is not a propeller: it is four, six or eight rotors carrying a structure that has its own mass and its own drag, and that tilts in order to move forward. The multirotor project deals with that assembly.

• Prerequisite: an air propulsion propeller project open — it is the one that supplies the blade of the rotors;
• What you get out of it: a mass budget, an available thrust, a maximum horizontal speed and the pitch angle that gives it.

• Tutorial outline:

1. Opening the multirotor project
2. Project type and number of arms
3. The geometry of the propellers
4. The frame: base, arms, motors, loads
5. The onboard modules
6. Pitch angle and maximum speed
7. Reading the forces on the 3D view

1: Opening the multirotor project

Files menu > Multi Rotors project. The window takes over the propeller of the current project: everything you have designed on the blade side — sections, chords, twist — becomes the blade of the rotors.

The menu entry

2: Project type and number of arms

The Multi Rotor Project tab carries the choice of the project family and the number of arms (Nb Arms). Quadcopter, hexacopter, octocopter: the number of rotors changes the available thrust, the structural mass and the redundancy, not the way the blade is computed.

The project type and the number of arms

3: The geometry of the propellers

Propellers Geometry tab: diameter, number of blades, rotation speed. This is where the classical multirotor compromise is settled — a large slow rotor lifts better than a small fast one, but it takes more room and reacts more slowly.

The geometry of the rotors

4: The frame — base, arms, motors, loads

Frame settings tab. Four groups, and each one weighs:

• Frame Base: the central plate;
• Arms geometry: length and section — they fix the spacing of the rotors and bring their own drag;
• Motors: KV constant, number of battery cells, weight of the motors;
• Frame loads: the arms + frame weight, kept up to date as you build.

The frame is described, and its mass is counted

5: The onboard modules

A module is a mass the craft has to carry: battery, flight computer, gimbal, parcel. You add it with Add module, name it, give it its mass and a colour to spot it on the 3D view. The sum of the modules and of the structure is what the rotors have to lift: that is the real specification of the sizing.

The onboard modules and the mass budget

6: Pitch angle and maximum speed

A multirotor moves forward by tilting: part of the thrust then goes to overcoming drag, the other part to holding the mass up. The Frame Pitch angle tests group deals with exactly that split. Two commands:

• Max horizontal speed at pitch 0°: the reference case, frame level;
• Find Max flyable Horizontal speed pitch: Heliciel sweeps the angles and keeps the one giving the greatest horizontal speed, displaying the horizontal and vertical projections of the speed.

The angle sweep: what tilting brings in, and what it costs

7: Reading the forces on the 3D view

The View tab serves for checking as much as for illustration: force vectors at an adjustable scale, induced velocities, upstream and downstream flow particles, rotation speed of the animation. It is the picture to put in a file, and it is also the quickest way to see that one rotor is working in the wake of another.

Forces and flow on the complete craft

The result: masses carried, thrust, maximum speed

Going further: the drone propeller tutorial (the blade alone, in French), the contra-rotative project, and the presentation page of VTOL drone propellers.