Why aren't there more manned quad copter designs?

[AnzacJack]

... Btw, do five rotors (instead of four) not present the more obvious choice? In the picture you can see that there's a more even distribution of lifting power with a 'pentacopter' if one rotor fails. Even when two rotors show failure (as long as they are not adjacent). Hence, one can imagine rotating the assembly itself, so that left and right are balanced...

A lot of inherent problems related to torque, are avoided with an even number of motors

 

[voyager]

You're just thinking of lifting force balance. You also have to consider torque balance.

A lot of inherent problems related to torque, are avoided with an even number of motors

Dammit, completely forgot about that... Thx guys.

 

[voyager]

Btw, is it safe to say that it's not an issue of one-rotor failure? Meaning that if there's a failure (in the electric drive or transfer box), all four rotors won't spin...

If there's a failure, are the rotors supposed to autorotate to slow down the fall?..

 

[Kilrah]

Nope, that would require variable pitch.

 

[voyager]

Nope, that would require variable pitch.

You mean tilting the rotors backward? Why not? I assume that during flight the rotors are tilted slightly forward...

 

[quick_dry]

You mean tilting the rotors backward? Why not? I assume that during flight the rotors are tilted slightly forward...

no, a helicopter is designed to change the angle of its blades. Whereas generally, consumer multirotors have a fixed prop and to change the lift they spin faster or slower. A helicopter will keep the same RPM, but change the angle on the blades. (There are some multirotors that have variable pitch, one of them was a weird looking 3D acrobatics quad, it changed prop pitch to do upside down flight, there have also been experimental multi-rotors including a single motor quad that used one motor to 4 variable pitch props to fly like a normal quad.. but I digress)

On a normal heli, if the engine dies you can take all the pitch out of the blades and let it freespin - if you have forward speed it it's a bit like an autogyro. Basically, you keep the blades turning then to land you flare them to give more lift, this takes the kinetic energy out of them so they'll stop spinning. but done right, you've also not plowed a chopper sized hole in the ground.

(Hi Mavic folks, first post, coming over to straddle the Pixhawk/Solo and Mavic worlds)

 

[voyager]

Forgot about the blades too... I know it sounds complicated, but why not make the rotor blades of the quadcopter variable too?... Do the nacelles influence autorotation btw?

 

[Kilrah]

A helicopter can autorotate because
- Its single huge rotor can store quite a bit of energy
- Said rotor gives slow acceleration to a lot of air, which means the speed the air needs to have when it goes through to keep it running is compatible with flight speeds. On a small fan inertia is tiny, and you take little air and accelerate it a lot, so even if you could reverse the blades you'd have to be falling down like a rock to keep them running.

Also if you make variable pitch then you've basically got the mechanical complexity of a helicopter so there isn't much point in making a multicopter in the first place.

 

[voyager]

Hm, in case of a dramatic failure the Airbus Pop.Up can of course deploy a parachute. I guess that the box on top of the cabin contains such a chute.

 

[Kilrah]

That's certainly going to be the way out all multirotor based projects choose. There are some pretty good parachute systtems out there, even some that self-inflate.

 

[voyager]

That's certainly going to be the way out all multirotor based projects choose. There are some pretty good parachute systtems out there, even some that self-inflate.

A sort of airbag or cushion could be deployed too underneath the cabin to soften the impact of touchdown.

 

[voyager]

With the quad rotors churning out the equivalent of 180 hp, Airbus thinks it can airlift two passengers. Most designs I have seen so far, only one person gets airborne. What's the maximum of a quadcopter, with the sort of 'wingspan' (16x14 ft) and lifting power that the Airbus quad is exhibiting?

 

[Kilrah]

Well the common 2-seater helicopters fly with around 150hp, so it doesn't look off. The fact it's a quad won't change much, you save some on things like no energy-wasting tail rotor but lose some on efficiency, probably averages out...

 

[voyager]

You're just thinking of lifting force balance. You also have to consider torque balance.

Found out that the Airbus quad has counter-rotating rotors within each nacelle. That means that using five rotors (ten actually) would not change torque balance I guess...

 

[Wombat55]

Found out that the Airbus quad has counter-rotating rotors within each nacelle. That means that using five rotors (ten actually) would not change torque balance I guess...

As you noted, in that case it would really be a 10-rotor aircraft although from its appearance it may look like a 5-rotor design. If one of the rotors were to fail, it would have 9 working rotors which would mean a torque imbalance so it would have to turn off one of the remaining rotors and fly with 8 rotors.

 

[rightstuffed]

You can bet there are many many very good engineers working all aspects of this as its going to be huge. Electric vertical takeoff and land. Monstrous capabilities.

 

[Wombat55]

You can bet there are many many very good engineers working all aspects of this as its going to be huge. Electric vertical takeoff and land. Monstrous capabilities.

Not so sure about that. Read the following article by an engineer who appears to know quite a bit about quadcopters and drones. He doesn't seem to be very keen about even Amazon drone deliveries and if you read his article you'll see why large quadcopters big enough to carry people make even less in terms of efficiency compared to traditional helicopters:

--------------------------

by Yuan Gao, Engineer, Builds drones and co-founded Mavrx,


There are many misconceptions surrounding quadcopters and multirotors. The single main reason that they are popular for small drones is mechanical simplicity. It's worth pointing out that a quadcopter is LESS stable than an equivalent regular helicopter (and I'll explain why in a sec), and is also LESS efficient than an equivalent regular helicopter. I will cover the advantage of more mechanical simplicity and the disadvantages of lower efficiency and stability of quadcopters versus conventional helicopters.

. . . . . . . .

Perhaps one other small benefit of a quadcopter in terms of operation, is that it's slightly safer due to having smaller blades (which are less lethal when hitting something, or falling off mid-air). And therefore they're operationally a bit more convenient to deploy. But I really doubt that there is any serious research going on involving long-distance delivery and quadcopters, everything you have seen will be almost entirely PR and publicity stunts, ESPECIALLY Amazon.

What Makes The Quadcopter Design So Great For Small Drones?

 

[rightstuffed]

Efficiency has little to do with this subject at this time. Early designs of the Modern Helo did not fare so well and we humans have hardly peaked in fields such as battery design, etc. Patience.

 

[voyager]

Efficiency has little to do with this subject at this time. Early designs of the Modern Helo did not fare so well and we humans have hardly peaked in fields such as battery design, etc. Patience.

Battery technology (weight : yield ratio) and lightweighting are paramount I assume...

Btw, would it help if you make the cabin and the hub container on top a bit more aerofoil-shaped? Obviously not the complete weight can be compensated by aerodynamic lift, but everything helps... Say the weight of a passenger (80 kg). Airbus says that max. airspeed is 100 km/h (60 mph).

 

[voyager]

Not so sure about that. Read the following article by an engineer who appears to know quite a bit about quadcopters and drones. He doesn't seem to be very keen about even Amazon drone deliveries and if you read his article you'll see why large quadcopters big enough to carry people make even less in terms of efficiency compared to traditional helicopters:

--------------------------

by Yuan Gao, Engineer, Builds drones and co-founded Mavrx,
What Makes The Quadcopter Design So Great For Small Drones?

Interestingly enough Mavrx inc is a 100% about crop culture management. Has he given up on copters then?