Electromechanical multirotors

Electromechanical multirotors change the mechanism of control from pure differential throttle to some combination of differential throttle, servo thrust-vectoring, and collective (or rarely cyclic) pitch.

Traditional helicopters use three variables to control their flight:

• The collective lever, which is when the pitch of the main rotor's blades are all increased by a fixed amount to increase overall lift
• The cyclic lever, which is where the pitch of the main rotor's blades are temporarily increased every cycle during a certain part in their rotation to generate more lift in one direction and control roll & pitch
• The anti-torque pedals, which vary the pitch of the tail rotor blades to produce yaw.

Helicopters are designed this way for efficiency and because internal combustion engines have economies of scale in their power to weight ratio, particularly driven from a single-gear transmission at an optimal RPM. Compared to electronically controlled multicopters, they are mechanically complex and difficult to fix, particularly cyclic control.

In the event that someone wants to make a multicopter using internal combustion engines, even aside from the gearing, they would likely be incapable of the rapid shifts in throttle that brushless motors are, necessitating a shift to one of these methods.

These designs are often built as experiments at the amateur hobbyist level, but with the exception of the tricopter have not yet attained the popularity of the pure electronic designs.

Variable Pitch

These models utilize the same type of variable pitch rotor and swashplate as a helicopter, but (usually) use it by applying collective differentially to non-coaxial propellers. There is reason to believe that as fixed-pitch propellers scale up and their angular momentum goes up, they become less responsive to throttle inputs and less able to control themselves, and the variable pitch option may not suffer from this control agility problem.

Servo Thrust Vectoring

These models, such as the tricopter, the Y4, and some VTOL gliding craft, utilize both differential thrust as well as at least one motor which is mounted on a servo, free to change its orientation.

Bicopters

Bicopters use a combination of variable differential thrust and servomotor control of two side-by-side rotors to achieve roll, yaw, and pitch, with gyroscopic force reactions playing a significant role.

Tricopters

Tricopters usually use two fixed motors on differential throttle, and a third motor mounted on a servo to accomplish yaw control by tilting and provide a pitch balance by differential throttle.