Quadrotor configuration I think is the most common configuration for drones. Why has it not been made into a full-size production aircraft? I think that it could be better than helicopters as it does not need a tail.
In: Engineering
We have issues already with human-rated dual rotor aircraft. A quad copter has 4 potentially fatal points of failure.
This is acceptable in an unmanned aircraft. It is less so with passengers onboard.
That said, there are some proposed sky taxi services that would use automated quad-copters to transfer passengers between office towers or to and from airports.
Looks like there is one you can buy now
https://www.jetsonaero.com/jetson-one
Granted, it is kind of limited at this time
Pilot: 210lbs no cargo
Flight time: 20 min
The drone quad rotor configuration hits a bit of a mechanical design sweet spot. Each rotor is fixed pitch (simple), and driven by a directly connected electric motor (no mechanical transmission). Changing the lift generated by each rotor is achieved by changing the RPM of the electric motor. Everything is small enough that these changes in RPM can happen sufficiently quickly to maintain control and manoeuvre.
The trouble with trying to scale this stuff up is that some things don’t scale in the same way. As the rotors and motors get bigger (larger diameter) proportionally to account for the heavier vehicle, the moment of inertia of these things increases faster than the resulting increase in rotor area and motor torque. The end result is a system that can’t respond sufficiently quickly (ie change motor/rotor rpm) to maintain control or manoeuvre safely.
Solutions can include variable pitch control on the rotors (but then you’re back to half the complexity of a conventional helicopter), or using lots and lots of small motor/rotors to carry the required payload (but you’re moving away from the elegance of a simple quad rotor).
Better question, why are we still putting people in the air in small numbers?
One engine fails and it drops like a stone. Probably all dead. Single blade helicopter can have an engine failure and still land with a skilled pilot using auto-rotation(?). Fixed wing aircraft can glide. For unmanned flight you can get away with whatever you like providing you are okay with the cost of it getting destroyed. You add humans into that mix and your fail-safe measures need to be REALLY high!
Here is a list of recent VTOL’s. There’s a few quads in there.
[https://www.electricbike.com/12-electric-vtols-that-are-actually-flying/](https://www.electricbike.com/12-electric-vtols-that-are-actually-flying/)
manned vehicles require failsafes for control in case of mechanical failure, unmanned drones do not (unless they are big and/or fly over populated areas).
electric dronecopters don’t have swashplates, which are far more complicated than adding additional independently controlled propellers. to completely replace the swash plate mechanism, you need at least three propellers because you need three points to establish a plane (tri-copter). these three points draw a triangle within which the center of thrust can be placed anywhere by the control system to achieve pitch and roll control. when the COT is over the center of gravity, the drone is stable.
in case of motor failure however your control of the COT is now limited to a line that connects the remaining two propellers. your cg is outside the line which will cause the copter to roll and crash. this is just a pick up, repair and replace scenario for an unmanned copter but impossible to sell as a manned one.
if you had four propellers, and lose one propeller, the control area turns into a triangle. if your cg was exactly in the center of the original square, it is now right on the line of the remaining triangle and you are on the edge of losing control. additionally, the motor #3 (the motor diagonally opposite the failed one) must also turn off to prevent a roll/pitch crash. so you are down to two motors reduced thrust and also either no roll or no pitch control. crash.
five is probably the minimum configuration where loss of a single will still maintain COT control vs the CG.
you can add arbitrarily many more to improve redundancy, but you also run into a limiting case eventually since the efficiency of thrust is proportional to the swept area of the propellers; adding more propellers greatly increases the footprint of the craft while losing efficiency. i.e. if you have a given allowed footprint, putting one big propeller is more energy efficient than fitting many small ones in the same footprint. today’s manned craft can (actual working ones), i have seen up to eight (octocopter).
tldr; drone copters don’t have swash plates to control pitch and roll and replace this mechanism by having at least three propellers. the loss of one is a confirmed crash with likely fatalities. in quads, the loss of one requires another to voluntarily shut down to prevent total loss of control, leaving two and still the loss of either pitch or roll control and also halved thrust leading to likely crash and fatalities.
Here is some perspective from a helicopter pilot. In a drone sized vehicle you have four rotors which are essentially just molded plastic props that are individually powered by electric motors. The motors are relatively powerful and the props are very lightweight so they can be throttled up and down to change their thrust quickly and easily.
In a full size helicopter you cannot change rotor rpm fast enough to use that reliably for control. The engines and transmission have limits and there is a lot more mass involved. So in a helicopter the rpm is held at a constant speed and all control is made by angling or “pitching” the blades to vary how much lift they are producing. To pitch the blades requires a complicated mechanism called a swashplate and in a traditional helicopter you only need one for the main rotor. A quadcopter would require at least 4 which adds tremendously to the complexity and cost.
Another consideration is the disc loading. Take the ratio of the helicopter’s weight and the area of the main rotor disc and you will have its disc loading. This can be used as a measure of how concentrated the lifting force and the rotor downwash is. A traditional helicopter has a relatively low disc loading whereas something like a V-22 Osprey has an enormous weight and relatively small rotors so the noise and downwash is crazy. To bring the disc loading of a quadcopter to a reasonable level you would have to increase the size of the rotors which brings me to the next point.
Footprint. This is the area that the aircraft takes up on the ground. A quadcopter needs rotors big enough to provide lift and those rotors need to be adequately spaced apart to avoid colliding. And this drastically increases their footprint which takes up a lot of space in hangers, on the ramp, and limits the size of potential landing areas.
To support the four rotors you need to have arms large enough to separate them and keep rigidity. This adds weight which needs a more powerful engine whichs leads to diminishing returns. Those arms also add drag to the airframe which limits the aircraft’s speed. You would also need a transmission system that could run all four rotors off of a single engine and I dont even want to guess how a quadcopter autorotates.
All in all a quadcopter drone is a good idea for the applications that are currently using them. Mostly as a cost effective and stable aerial platform to film from. But when it comes to scaling them up to the kind of work that actual helicopters do they are vastly inferior.
Different operating principles, helos are driven by 1 to 2 engines and they increase lift primarily by increasing the angle of attack of the blades, quads primarily increase lift by increasing the thrust of each motor. So we would need a super complicated fly by wire, 4 collective, 4 cyclic system to drive a quad powered by an engine, we can’t drive the blades directly because they wont be able to change thrust fast enough.
>Why has it not been made into a full-size production aircraft?
It has. The problem is the physics don’t scale well. It is far more aerodynamically efficient and mechanically robust to use the standard single rotor with a tail helicopter layout.
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