History of Quadcopters and other Multirotors

Quadcoptors are VTOL rotorcraft with 4 propellers/rotors for propulsion. Multirotor is a more general term encompassing not only quadcopters but also tricopters, hexcopters, octocopters and all other form of rotorcraft with more than 2 rotors.

The early pioneers actually first attempted rotor flight using multicopters, because using more than one rotor seemed to be the natural solution to the problem of VTOL flight.

The very first experimental attempts of taking off with a rotorcraft were mostly done with multirotors. Around 1907 Jacques and Louis Breguet, French brothers, built and tested Gyroplane No 1, a quadcopter. They managed take-off, although the design proved to be very unstable and hence impractical.

In 1924 French engineer Étienne Oehmichen flew his quadcopter a distance of 360m (1,181ft) setting a world record. In the same year he flew a 1km (0.62miles) circle in 7m and 40s.

Around the same time George de Bothezat built and tested his quadcopter for the US army, completing a number of test flights before the program was scrapped.

Early designers experimented with quadcopters, because the alternative, using a single main rotor with a tail rotor to counterbalance the torque created by the single main rotor seemed wasteful, complex and inefficient. The tail rotor on a single rotor helicopter design consumes between 10 and 15% of the engine power yet it creates no lift or forward thrust. Part of the main rotor rotates over the fuselage, pushing down washed air against it, reducing effective lift. Making large rotor blades, 4 or 5 m long or even longer was a huge problem and larger rotors to this day are proportionally much heavier than smaller ones.

However, at a time when computers and good electric motors did not yet exist, the single main rotor helicopter design had two huge advantages:

1. It was naturally stable because a weight hanging below a single attachment point naturally 'wants' to hang straight down and so naturally corrects unwanted tilting
2. Its engine only had to be mated to only one rotor, and could be mated to it more or less directly over a short distance rather than with a belt over the several meter/yard long extensions/arms connecting the rotors of a multicopter with its fuselage.

Early quadcopters would typically have the engine sitting somewhere centrally in the fuselage of the copter, driving the 4 rotors via belts or shafts. Belts and shafts however are heavy and importantly, subject to breakage. As the 4 rotors of a quadcopter are all slightly different from each other, a quadcopter is not naturally stable, simply running 4 rotors at the same speed, while producing enough lift to hover the copter, does NOT produce stable flight. On the contrary, quadcopters have to be constantly stabilized. In the absence of computers, this meant a monumental workload for the pilot.

As a result, multicopter designs were abandoned in favor of single, or on rare occasions for very large transport helicopters, double rotor designs.

With the advent of electric motors and especially microelectronics and micromechanical devices, a few years ago it became possible to build reliable and efficient multirotors. Modern multicopters have an electric motor mated to each rotor, sitting directly below or above it. A flight computer constantly monitors the orientation of the copter and corrects for instability by changing not the pitch of the rotors but simply the rpm of the individual motors/rotors. This fixed pitch design is much simpler than the complex swashplate mechanics that are required for single rotor helicopters.

This design has proven to be hugely successful and most modern VTOL drones and hobby aircraft are now multicopters rather than single copters. The scaling up of this to aircraft that are able to carry people has only just begun and Krossblade is part of this development.