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Normally, a dual-motor plane has two ESCs but could I connect two brushless motors to one ESC? What would happen to the torque and speed of each of the two motors connected to the ESC?

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Technically, you could connect 2 motors if they're exactly the same and it will probably work. However, there is a good possibility of various weird things happening due to this.

The problem with connecting more than one motor to a single ESC stems from how these motors are driven. An electric motor works by using electromagnetic coils to pull permanent magnets and create a force on the rotor. However if you just supply current to a coil and leave it as is, at first a closeby magnet will get attracted to the coil, rotating the motor bell, but then it will just sit there in the nearest position after oscillating back and forth a bit like a pendulum.

To create a continuous rotation, you need to stop the current as the magnet passes by and then reverse it to now repel the magnet, adding more rotational momentum to the system. In a brushed motor this switching is done mechanically using contacts (brushes) inside of the motor. A brushless motor, on the other hand, does not have this kind of mechanical switching mechanism inside, so the switching must be done externally, and this is the brushless ESC's job.

To determine when to switch the currents around, the ESC must detect when the magnets are passing the coils. To do that it constantly measures the voltages on its three terminals (as a magnet passes by an electromagnetic coil, it creates a voltage in it), and uses that information to switch the currents in the motor in perfect sync with the motor's rotation, speeding up the current pulses as the motor speeds up and slowing them down as it slows down. Real-world ESCs and motors are not perfect, though. sometimes the ESC is incorrect in its assumptions about what the motor is doing right now or how it's going to react, and sends pulses that don't correspond to the motor's actual position. This is called desynchronization, or, colloquially, desync, and results in the motor suddenly losing almost all of its power for a short amount of time (usually less than a second) while the ESC tries to determine the correct state of the motor and reapply power in sync with its rotation. With quadcopters this condition is most common when the motors are spinning at very low RPM, loaded by a reverse airflow (e.g. falling out of the sky bottom-down) and need to suddenly speed up, all of which mess with the ESC's expectations, and can lead to a violent uncontrolled spin.

Now, if you connect two motors, the ESC will still think it's one motor. If these motors are sufficiently different, the most likely thing is that one (or both) of them won't work. Different motors will both create voltages of different magnitudes when rotated, and react with different speed to currents flowing through their coils, so one of the motors will invariably get far ahead of the other in response to a current pulse, making synchronization of the two near-impossible, and the ESC very confused. Depending on the ESC and motor combination, it can either give up entirely, or try to drive this weird "motor" anyway, likely leading to one of the motors spinning at reduced efficiency and the other just jerking.

If the motors are identical, though, e.g. of the same brand and model, and with identical propellers, they are likely to want to rotate at about the same speed, in which case they will produce more or less the same induced voltages in their wires and react the same to what's supplied by the ESC. So this setup will probably work, at least on the ground. Even then, nothing is perfectly matched. One motor will likely be slightly ahead of the other, or otherwise slightly off, which is probably not enough for triggering a desync, but means a more confused ESC and less efficiency overall, as one of the motors (or both) is driven suboptimally.

Further, when you mount the motors on an actual airplane, conditions may be even more unequal. Suppose that one of these "identical" propellers is chipped, or the plane is flying in a crosswind, or even that it's just in a banked turn, so one wing moves through the air slightly faster than the other. Then the load on the motors can differ considerably even though they are the same. Because of that differing load, one of them might get just enough ahead of the other to confuse the ESC into either giving up entirely, or making erratic assumptions about this weird combined "motor", leading to wrong signals being sent, which might, for example, slow down one motor and speed up the other, possibly ending in one of them desyncing and stopping entirely. This, in turn, can potentially lead to very unpredictable behavior of the model in flight.

So, to conclude this long-winded explanation, the answer to your question is:

Don't. Just don't.

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From a technical standpoint it is possible, but there are a number of considerations:

  • Only need one ESC so lighter
  • ESC needs to have a higher current rating to fully power the motors
  • Motors must be identical (more of a problem if home-building with what's on the bench)
  • Loss of redundancy (if a prop jams or motor fails, you have no thrust)
  • Cannot use differential thrust

Which of these are pro's or con's will depend on what you're building. For example, if you are building something for the fun of the build, to see what you can make and you're happy with it not surviving the day, then doubling up the motors could get you in the air quicker. If you are building a much-anticipated scale model that you would like to fly for years to come, then the redundancy benefits probably make it worth picking up the extra ESC.

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You can, but it's not recommended. First of, you will need an ESC that can handle twice the current of what one motor consumes. The motors will also be seen as one single motor by the ESC, so of one of them has an error or becomes out of sync, both of them will stop.

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