With a helicopter, even if you lose the motor (lift) you still have control by adjusting the pitch, angle and ratio of the blades.

A quadcopter depends on the motors for any control. While it is possible to regain control after a single motor failure on a quadcopter, it's extremely difficult and not something a human pilot could do, so you'd be relying on software. Also, if you don't detect the failure instantly, the craft will flip over before the software can react because of the inherent instability.

Any solution where you have some kind of passive lift and inherent stability (eg. parachute, wing, etc.) is always going to be much safer.

Would a proper parachute on a quadcopter be fine in case of motor failure / etc. be okay, then?

Multirotor designs are not limited to 4 motors. You can add more for redundancy. Four is common because it’s the smallest number that has good control characteristics.

Mind explaining that one? To me it seems they have an unnecessary degree of freedom: the "diagonal" one.

    ox
    xo

What issues do you get with three motors in an equilateral triangle?

EDIT: I managed to find it, with even number of motors you can prevent unwanted yawing, by letting half spin clockwise and other half spin counterclockwise.

Yep. More generally, you can induce the yaw rate you want (up to a limit) in either direction, including zero.

I’m not an expert but IIRC with three rotors you need to be able to control a bit of roll of just one of them to have stable flight control.

As they say "If you're in a helicopter and the fan goes off, you'll start sweating" :)

If you lose power on a quad they become unstable, if you lose power on a heli you can pitch the blades slightly downwards which keeps rotation in them and level out close to the ground landing perfectly smooth. IIRC this is something you practice to get your license. Can be done with both RC helicopters and real ones.

But brushless electric motors are so much more reliable than combustion engines (internal or turbine), the comparison is not even funny. They are basically solid state devices. "Losing power" is simply not much of an error state outside of complete battery failure. And if you still want to be safe from motor failures (or from propeller failures), octocopters exist. I share the "multicopters are boring" attitude, but they are boring for a reason.

Absolutely, as long as they're operating within parameters there's no failing them. I've been running brushless systems in RC cars for the last 7 odd years. The only motor failures has been when It's been subject to extreme abuse, as in the rotor reaching 100+° (internally), going from ~55-60K RPM to 0 in fraction of a second or the rotor being forced through the bearings.

For unmanned flight i do agree on multicopters being the best option. See this[1] video for an example of a multicopter flying with disabled props.

However, as you see this flightmode isn't appropriate for human beings, humans should keep flying helis for now. The swash plate design even though more complex than a prop mounted on a shaft is still a rather simple design, litterally a bearing mounted on a 2 axis system and rods attached to rotate the blades. Multicopters with swash plates are interesting too as they can be simplified not to tilt the plate but only offset it up and down to alter pitch the same way all around the rotation. If. This exerts less force on the bearing, but considering there's still 4 of them rather than one might increse failure rate. With swash plates on multicopters we could have two motors driving one system that drives the props, and do many amazing things with computer aided controls in case of failure.

Idk why i keep on going, TL;DR: Being able to control pitch is key, multirotor or not.

I'm not really with Elon Musk on alot of things, but personal human flight is one thing where i agree with him, It's not going to happen.

[1]: https://www.youtube.com/watch?v=w2itwFJCgFQ

Well, simple propeller pitch control is a lot easier than a swash plate because it's idle when the pitch is kept the same and it's no problem if making it slower simplifies it. A swash plate is active on each rotation and needs to be fast enough to work the blade through a certain range of pitch values within have a rotation interval.

I don't think a quadcopter is controllable with 3 engines.

However, a drone designed to carry pax can be designed with redundancy - the Volocopter, for example, has 18 rotors and engines, and can withstand at least 3 failing.