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
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.
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