Eli5 why do drones have four blades and helicopters only have one?
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It boils down to drones having electric motors while helicopters have an internal combustion engine. The helicopter’s engine can only go at very high RPMs (revolutions per minute), so you need a transmission system to reduce those RPMs for the blade. You can’t really afford having four independent transmissions in a helicopter. A quadcopter has four independent electric motors which can turn at whatever speed required and so are connected directly to the rotors without any transmission. This makes for relatively easy control of flight by changing the speeds of different motors to move in the desired direction. A helicopter has another complicated system for adjusting the angle of the blade instead. Helicopter-size vehicles can’t yet be powered by electric motors because of battery size and weight limitations, but once we get there, helicopters will likely have four rotors too.
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Well, some problems are that it would need to be massive and it would require more space to land, and it would be much more complicated
It would require more parts, and additional transmissions or driveshafts or engines or something, which would add mechanical complexity, weight, and maintenance time and upkeep cost. More parts means more parts that can break. Drones are plastic and cheap in comparison, so it doesn’t matter.
There was a soviet concept called the Mil Mi-32 that where they planned to make it a giant triangle with 3 rotors, it’s hilarious, type ‘Mil Mi-32’ into google images
I think the soviets figured the extra rotors would also have resonance problems and vibrate weirdly
There are multiple reasons:
* Helicopters like to have the biggest possible rotors because it’s more efficient to push lots of air down slowly compared to a little air down very fast. Quadcopters are small so they can better tolerate the loss of efficiency.
* It’s difficult to make a quadcopter that can tolerate having a rotor or its engine fail. That gives you four single points of failure which is a safety issue. Quadcopters avoid spinning by having their rotors turn two each in opposite directions. Losing one rotor means you have to shut down the opposite rotor to avoid spinning, and staying up on the two remaining rotors requires a large safety margin. I’ve seen a man-size quadcopter use two motors with two separate rotors on all four corners, presumably in an attempt to work around the safety problems.
* It’s the square-cube law. The various properties of things scale up differently so you can’t just scale up a design drastically and expect it to still work. If you scaled up a quadcopter by a factor of 10 then it would weigh 1000 times more (cube law) but only be 100 times as strong (square law). Rotor area would also be only 100 times greater but be expected to generate 1000 times more lift.
Small rotors can change speed rapidly.
A drone can be controlled by changing each rotor speed independently. This is very simple to do, the rotor is just a fan in front of the motor, the computer changes the motor power, and you control the craft with a decent response speed. This would work with any powerplant, except slow response powerplant such as turbines. Electric, or piston engine would do the job.
Big rotors have massive inertia, don’t change speed easily. So, when you scale it up, drone or not, quad rotor or single rotor, you can’t control the craft by rotor speed changes. What is done then is to use a mechanical system to allow some actuators to change each blade angle. The rotor rotates always the same speed, the blades angles determine the thrust and direction of thrust. In this condition, any powerplant is good for the job, including rockets or pulse jets at blade tip, turbine engines connected to the rotor by a gearbox, piston and electric engines. What you must have is a way to change the blade pitch to achieve fast response to the flight controls. Rotor speed may or may not vary, but won’t let you control the craft quick enough. So in case you have variable rotor speed, that’s just there for efficiency purposes, not control.
Consequently, on big rotorcraft, it can be easier and cheaper to use one single big rotor with individual blade pitch control, compared to many small rotors with individual blade pitch control. As the more rotors you add the more things get expensive and complicated, more parts means also more points of failure.
This is the great scheme of things. Small: multi rotor and control by rotor speed. Big: as few as possible rotors with pitch controls.
In the middle, there you can find bizzarre things that are a mish-mash of the two. The middle is crafts around 100-500kg. In this area none of the classic configurations is beneficial, so you can find drones with tilting rotors, pitch change, or single rotors controlled in speed (the latter flies like a drunk monkey, the bigger the drunker).
It’s square cube law, same law that makes impossible to fly a parrot scaled down to 1 cm long or fly a bee scaled up to one meter big. Mass increase by 8 times every time you double the dimensions, so things that move decenti well in small scale may become incredibly clumsy incredibly quickly when scaled up.
Drones have four blades because it is easy to design/control. You can change the pitch and motion of the craft in any direction simply by changing the speed of the individual rotors, which can be done by changing the voltage. This can be done quickly and easily by modern computer chips, and it makes for a very stable craft with low technical complexity. Essentially, these require higher technology but are much simpler, and thus are easier to build and maintain.
Helicopters, on the other hand, were developed before high speed switching with computer chips was viable, so they were built primarily around single or dual rotor designs. This design alters the pitch of the blades instead, changing how much lift they generate, and modifies their angle, allowing the helicopter to lean forward or backward to move in that direction. This requires a bunch of specialized parts, which are expensive. However, the designs are both mature and efficient, so modern helicopters have continued to use them.
Helicopters are more efficient than quad copters because of the larger blade area and less edge area. Quad copters exist because they are simple to make and more agile due to the ability to control each individual motor.
Helicopters have at LEAST two blades. Many have four or more. So your ELI5 is kind of squashed. Sorry.
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