What happens if...

[rbhamilton]

I was reading on here about a guy who was starting up his Phantom. Suddenly one motor stopped and the prop flew off. I think it was a bad esc and the sudden stop on the engine caused the prop to un-self-tighten. I believe it was on the ground and nothing serious happened.

But that got me thinking. What if your bird is up at 400' and suddenly one motor fails? Will it be able to remain airborne on just 3 engines like a 747 and limp home? Or is that pretty much the end?

 

[Meta4]

The 747 gets lift from airflow over the wings and has sufficient thrust to keep flying and provide the airflow that gives lift.
They just have to adjust trim to balance things.
The Phantom gets all its lift from the motors and because it's heavy it needs all the lift it generates just to stay in the air.
Add the serious imbalance of two motors clockwise, one anti-clockwise and no thrust in one corner and no question - with 3 motors no Phantom is staying in the air.
The good news is it's pretty rare to lose a prop or engine in flight.

 

[aartsf]

I haven't luckely any experience with such situation, but I think it is save to say it will crash because it seem impossible the bird can stabalize itself with only 3 motors.

 

[Chuck26287]

A quadcopter cannot maintain flight on three motors. You loose one for any reason during flight, and it tumbles to the ground. It has the glide ratio of a rock. That's where the real benefit of a hex or octo come in. From what I understand, once you have six motors, you have enough redundency to maintain flight to land if one is lost. I believe you have to deal with yaw issues becuse the torque is out of balance to the point that it can't be entirely compensated for, but flight can continue, and it doesn't necessarily fall out of the sky.

To me, this is a MAJOR flight safety factor. When it comes to quads, no matter how much regulation is applied in the name of safety, no matter how thorough a pre-flight you perform, no matter how safety conscious a pilot is, and no matter how perfectly a pilot controls his aircraft, there are going to be mid-air failures, and quads will occassionally fall from the sky onto whatever is below them. And the pilot may have done absolutely nothing wrong. Electronics can simply die. ESCs are electronic. Props are stressed and fatigued by flight forces, and can/do break now and then, having shown no signs of a problem during pre-flight. Bolts looses from vibration, and structures separate. It will happen, eventually. This is why is is so important that quad pilots in particular never fly in a state where the quad can't fall at any given time. I don't know about you guys, but I've met Murphy... and he can be one mean dude.

 

[dbfletcher]

Actually there are youtube videos showing quads can still fly if they lose a prop. It would have to be programmed in the to flight controller however.. so currently phantoms do fall like a rock.

https://www.youtube.com/watch?v=bsHryqnvyYA

 

[Meta4]

Even with complex programming, they would have to have enough thrust to carry their weight.
The Phantom is probably on the wrong side of the equation to still fly with three.

 

[Chuck26287]

Actually there are youtube videos showing quads can still fly if they lose a prop. It would have to be programmed in the to flight controller however.. so currently phantoms do fall like a rock.

https://www.youtube.com/watch?v=bsHryqnvyYA

I didn't know about that video. That will be a very good thing once it makes it to the end users. It appears to be some sort of development/prototype? I was actually speaking about today's quads that we can actually acquire and operate now. Of course, the future of sUAS systems, quads in particular, is in the process of blowing wide open I think, as partially illustrated by that very video. I would suspect there are collision-avoidance systems being developed to integrate into the FC just like that algorythm for staying aloft with only three props. I think developments like these will eventually make a quad just as safe as the hex or octo, but for now, not so much.

It will be amazing to watch what the next few years bring us in quads. Improvements will actually feed off each other. For instance, as Meta4 said, the thrust to weight ratio has to be there with only three props, or the programming still can't keep the quad aloft. Right now, there may be some, but probably very few quads with the thrust to weight ration required for three prop flight. However, as soon as that programming is available in the FC, the standard design of quads will incorporate a higher minimum thrust level to take advantage of the FC feature, making all quads much more robust in the process. This is a really exciting time to be involved in sUAS aviation.