RC helicopter flies light with few electronics

With few electronics, both cost and mass of Revell's RC helicopter are kept down. (Click image to view teardown.)

Heading into the holiday season, toys come to mind and so we examine an interesting remote-control (RC) device, in the form of Revell's RC helicopter. Measuring over 30 cm long, the device is quite a lot of fun—we put it through some parking lot shenanigans before teardown analysis.

The helicopter is powered by a rechargeable Li-Polymer battery pack, which is tucked between the two underside runners and is responsible for controlling all helicopter electromechanics. To receive the 27MHz controller signal, the helicopter's antenna uses a simple stranded wire, compacted and hidden by spiraling around both runners and snaking over the main helicopter body area. Because an "open" 27MHz communication band is used between remote and helicopter, electrical length of the antenna is significant. But wire is cheap, as is the labyrinthine implementation of the Chinese manufacturing.

Counter-rotating helicopter rotors in the Revell counter the natural tendency of helicopters to spin from torque reaction in a single main-rotor design. The vertical tail rotor you see on real helicopters is the means by which torque-spin is normally controlled; co-axial (torque-canceling) designs have been commercially implemented in the real thing, though without much success.

The design is based around three DC motors, the two largest of which drive the two main rotors by way of a set of co-axial shafts, each serving a pair of blades. The outer shaft "sleeve" controls a lower rotor while the inner shaft drives the upper, counter-rotating blades. Pitch and yaw are supported as the control axes, because the blades are at a fixed incline. Steering is accomplished by setting up a differential rate of spin for each of the two rotor blades to control directional torque and thus pointing. A third motor spins a horizontally oriented tail rotor to affect pitch—a nose-up or nose-down orientation—by implementing lift or sink at the helicopter tail.

Keeping mass, BOM down
That sounds pretty fancy but the product must be implemented on the cheap in light of the $100 retail price, including the handheld controller. Unlike the conveniently rechargeable helicopter itself, the controller uses a rather mind-boggling (and bank-breaking) eight AA cells. The resulting 12V rail is perhaps necessary to implement a simple radio with adequate output to cover a useful range.

Electronics in the controller are simple enough visually, with just a single IC and a handful of other passive components. The circuit implements what is almost certainly an FM radio transmitter, though the circuit used remains an enigma. Seemingly well-known to the hundreds of China-based parts brokers, the HS153SP turned up only one place with a description, translated from Chinese in Google as "Police Motorcycle Alarm," likely an artifact of its use in other remote-controlled toys and the like. Its function would seem pretty straightforward though, first taking input from the two control-stick potentiometers—one a single axis, the other a dual-axis—to determine desired control. One trim potentiometer also provides input to keep the two main rotors spinning at equal speeds for a zero-input stick, and a second trim knob is used to null offset to pitch in the tail. Essentially, three wireless control links in the remote dictate motor speeds—two for the main rotors and one for the tail.

RF output from the extendable mast antenna is assumed to use a coded narrowband signal, a notion fortified by the Youwang Technology UTC3361 Narrow-band FM IF IC receiver in the helicopter itself. After demodulation of the FM signal, the signal chain goes to the Elan Microelectronics EM78P258N 8bit microprocessor, whose on-board ADC translates control signals to the driver MOSFETS that power the motors. A two-sided PCB for the receiver/driver electronics is joined with quite a bit of hand-wiring to finish things out.

With relatively little in electronics, both cost and mass are kept down, allowing more of the weight and BOM budget to be used for the helicopter's battery, motors and rotors, which dominate the "flight package." What electronics are to be found come from mostly smaller vendors in China, all a bit off the radar screen for chip supply.

- David Carey
President, Portelligent