3.1. Helicopter platform

A number of inexpensive microhelicopters are now available to RC model hobbyists: Ikarus Piccolo, MS Hornet, Carboon, Dragonfly, Honeybee, Hummingbird, Tiny, Aerohawk, Blade CP, Sky Lark... These are essentially scaled-down versions of regular model helicopters, made possible by advances in battery technology. Some models have a rotor head with fixed collective pitch (FP), while others have both cyclic and collective variable pitch (CP). Most have a dedicated tail motor rather than a variable-pitch tail rotor.

A recent radical innovation is the "ProxFlyer" self-stabilizing deformable rotor design. Unfortunately, current commercial implementations are too small to carry mainstream sensors and embedded computers. However, due to its passive stability, this design will probably turn out to be the preferred choice for hovering robots which do not require high maneuverability.

Another alternative is the quad-rotor helicopter which is more silent, mechanically more robust, safer (with ducted rotors), and probably easier to control than a single-rotor/swashplate design. Potential weaknesses include: the overall size of the aircraft for a given payload, the energy efficiency of four small motors versus a larger one providing the same lift, and the impact of (usually fixed-pitch) rotors with high inertia on maneuverability.

For this project we use a microhelicopter kit containing:

A pre-assembled helicopter with variable-collective-pitch rotor, two brushed motors and three miniature servos

An "all-in-one" controller package with BEC, 6-channel RC receiver, yaw gyro, and ESCs

A 41 MHz 6-channel RC transmitter with hardwired CCPM mixing

A 11.1 V lithium-polymer battery

A battery charger.

The aircraft has a mass of 270 g and can lift about 50 g of payload.

Figure 1. Contents of the integrated controller in a typical commercial microhelicopter kit



An "all-in-one" controller package connects all the components together. This is in contrast with larger model helicopters, where the connections between the receiver, gyro, BEC and ESCs are exposed and documented. Integrating all these functions reduces size, weight and cost, but makes modifications harder.

Fortunately, in some commercial microhelicopters, the "all-in-one" controller can be tinkered with fairly easily. It actually consists of two boards connected back-to-back with a 2x3-pin connector (see Figure 1, “ Contents of the integrated controller in a typical commercial microhelicopter kit ”):

A generic RC receiver board with seven 3-pin PWM servo outputs.

A power/gyro board with BEC, gyro, fail-safe, and ESCs.

Table 1, “ RC receiver PWM outputs ” lists the PWM outputs of the receiver board, two of which are routed internally to the power/gyro board.

Table 1. RC receiver PWM outputs Channel Usage 1 Right servo 2 Front servo 3 Main motor (internally connected to the power/gyro board) 4 Tail rotor (internally connected to the power/gyro board) 5 Unused 6 Left servo B Unused (12 ms sync pulses)

Any similar helicopter can be used. The main requirements are: