Quadcopters and other multicopters are getting popular in RC hobby and scientific research. One of the main reasons might be the simple mechanical structure.
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Regular helicopters have 1 big motor and a small tail motor, while quadcopters have four motors. Many small size quadcopters seem pretty agile and easy to control, in fact they are generally less stable than an helicopters. Quadcopters and other multicopters are also LESS efficient.
Can quadcopter be scaled up to carry out tasks like full size helicopters, for transportation for example?
Table of Contents
Mechanical Simplicity
On a traditional helicopter the propeller blade spins at a constant speed. Changing thrust is achieved by adjusting the propeller pitch mechanically. This pitch mechanism is complex and expensive. Other movement (e.g. Yaw) is achieved by the small tail rotor.
Quadcopters has four motors which uses four fixed pitch propellers. Mechanically this is very simple compared to what the helicopter has. Along with the compact electric motors powered directly by battery, the simplicity makes small size model quadcopters more popular in RC hobby. But when it comes to full size aircraft, things gets more complicated in the such as whether to have four engines in quadcopter.
Here is a picture of the helicopter blade pitch mechanism.
A small quadcopter motor with its fixed-pitch propeller.
Stability
Quadcopters are less stable than helicopters. That’s right! They might seem pretty stable in many small hobby models, but that’s the Flight controller and speed controller doing all the stabilization for the pilot. It would be extremely hard to fly a quadcopter without the asistance.
Flight controllers are getting better and better it’s amazing how well it’s working. It leads to the misunderstanding that quadcopters are more stable than helicopters by nature.
For the quadcopter to maintain balance, each individual motors must adjust speed to change thrust. If motors on one side are spinning faster, it will generate more thrusts and the quad will tilt to the other side. Because quadcopter relies on changing the speed of the propellers to make yaw, roll and pitch movements (3 dimension movements), how fast it can change the motor RPM (rotation per minute) determines how agile the quadcopter is.
Apart from the electronic speed controller, this is also very much depends on the inertia of the propellers. As you can imagine, generally the bigger and heavier the props, the harder it is to slow down or speed up. Therefore that contributes to the less agile and unresponsive characteristics of a large quadcopter. Just like stopping a bicycle is easier than stopping a train. :)
Efficiency
As mentioned, when quadcopter is making movements, the rotation speed of the propellers and motors need to change. Helicopters’s varying-pitch propeller mechanism is a more efficient solution, because it takes more energy to slow down / speed up propellers than keeping the motor and propeller at a constant speed.
Basically, to be more controllable and stable, the more frequent the motors change speed, and therefore the less efficient it will get. This is why you don’t get much flight time out of a quadcopter generally.
Finally The larger your rotor disk is, the less energy wasted in moving the mass of air. You also don’t have air flow interference from adjacent rotors / propellers in helicopters. This is why long flight time quads tend to use large propellers. (But again, they are less agile)
Safety
When disaster happens such as loss of power or motor failure, quadcopters might spin around or flip sideway, because the quadcopter can only be balanced effectively when all four motors are working. On the other hand, helicopters tend to just glide down to the ground vertically.
Although quadcopters are less stable and efficient than helicopters, they are more suitable to be made in small form factor. they are more mechanically simple and also cheaper to make. When they are scaled up, the disadvantage becomes more significant too.
