$\begingroup$

There are three challenges for making a market-competitive full scale quadrocopter.

1) Energy efficiency

The quadrocopter is not an energy-efficient design. Helicopters increase their range by using relatively under-powered engines that take a long time to spool up throttle and try to keep it at relatively constant RPM while changing the pitch of the blades to control the helicopter. While this can be used to control pitch and bank, the quadrocopter controls yaw by changing the RPMs of the motors, which I would think would be difficult, and inefficient to do on a full scale helicopter without some sort of CVT to adjust the RPM of the blades without changing the RPM of the engine.

2) Structural integrity and weight

Tail rotor helicopters essentially have a fuselage and a tail boom. The tail boom doesn't even need to be that beefy, because the load on the boom is rather small compared to the load of the main rotor pulling up on the frame of the fuselage. Coaxial helicopters can probably get away with just a fuselage, and no tail boom, and probably have less forces acting on the entire structure in normal operating conditions than conventional helicopters. Quadrocopters have four main rotors. Granted, each of the rotors is approximately 1/4 of the power of the main rotor of a conventional helicopter, but the booms would still need to be able to withstand more than 1/2 of the power of a conventional main rotor - think of the extreme case: you're full yaw - that means two rotors diagonally across from one another are either off, or spinning in the opposite direction, creating either loss of lift, or even down force that the other two rotors have to compensate for by producing two times the thrust/lift. It's a rather common phenomenon among miniature R/C planes and helicopters to be completely overpowered, and much beefier than their full-sized counterparts.

3) Cost of manufacturing

It's worth mentioning that a larger rotor is easier, and cheaper to manufacture than three additional engines/motors. I think (I could be wrong) the only reason military helicopters have two engines instead of one big one, is for redundancy. If one gets shot, you still have the second one that you can use to limp back to base, or a friendly location. So when you're looking at a whopping four overpowered engines (see point #1) compared to 1-2 underpowered engines, I would imagine you would be looking at something that costs more.

Those are the reasons why a quadrocopter has not been scaled up, and become commercially available by a large manufacturer. I'm sure each manufacturer is trying to be the first in that market, but I don't think QUADROcopters will ever become human-piloted. If you were talking about making a MULTIcopter (more than 4 rotors) however, there are a few DIY prototypes out there, and I think they have potential.

4) Addressing the proposals for hybrid systems:

As things stand now, conventional helicopters run 1-2 turbines for power. They spin in their optimal RPM ranges, and the pilot uses the collective to change the pitch of the rotor blades to control flight. A hybrid system is appealing for cars because it allows the ICE to run in its optimal RPM range rather than constantly changing RPMs. In rotorcraft this isn't an issue - since they already spin at optimal RPM ranges, a hybrid system would introduce a generator, and batteries to generate and store power for the electric motors to consume. This would add weight, without any particular added benefits.

The big question is - why bother? What makes quadrocopters better than conventional helicopters? For drones the biggest benefit of quadrocopters is their ease and cost of production. It's cheaper to produce four small electric motors than all the mechanisms for a collective. It's also more robust and easier to service. However, we're talking about different scales and missions. Rotorcraft are already inherently inefficient. Their niche - close air support, and transport to locations where it's difficult to land an airplane. The tendency is to create a hybrid, or make a plane with VTOL capabilities. (Osprey, Yak-141, Harrier, F-35 aka US Yak-141). Quadrocopters may find their niche as cheap utility drones, or surveillance drones, but I think a much more likely design, is the likes of the one shown in Avatar - two counter-rotating shrouded rotors on either side of the fuselage, which can be vectored to control flight.

Technology is ever-changing. Scientists in Russia have miniaturized nuclear reactors and fit them inside missiles. This negates the need for batteries and hybrid systems. If anything, this is the powerplant of choice for large quadrocopter systems, but only time can tell how technology will evolve. Right now, I don't see the benefit of quadrocopter configurations for large rotorcraft.

Amendment, based on new tech in the market (23.03.2018):

A Chinese company is actively marketing their EHang 184 autopilot drone air taxi, that uses the quadrocopter design. Specs:

100kg payload

25min cruising time

1hr charging time

500km/h cruising speed (very curious to see this)

500m cruising altitude

Basically, one can compare it to conventionally designed Mosquito helicopters. They fly slower, but duration of flight,range, and useful load are significantly higher. Also note that while fueling a helicopter these sizes takes five minutes, the air taxi will spend 2 times longer charging, than it will flying. But this is more of a EM vs IC type issue. Time will tell if quadrocopters can prove they're worth up-scaling.

Adding from recent professional experience (09/19/2018) I was recently asked to develop a quadrocopter concept. While attempting to use electric power, I found that payload weight, and mission flight time drastically increased the weight of the batteries required to fulfill mission parameters, and you ended up with a 3-4 ton four-seat quadrocopter (yikes!). Converting to turboprop solved the weight issue, but dramatically increased cost and complexity. As a result, the decision was made to drastically reduce mission parameters, making for a very short range vehicle, which under normal circumstances would be quite useless. You would need to charge the batteries for several hours to get 15-30min flight time. Either your quadrocopter would be sitting on the tarmac charging, or you would need to swap batteries after every hop, requiring expensive infrastructure at every landing pad. Ultimately the idea was proposed with infrastructure and battery swap, and is awaiting its fate at the hands of upper management, but everyone on our team has already scrapped the project as unfeasible.

There ARE a number of upcoming battery technologies that would drastically change the potential of such products, but they're not market-ready yet, mostly in theoretical, conceptual, and testing phases at the moment. Can't say much more on the subject without an NDA, sorry.