Why are drones and helicopters shaped so differently? Is there a reason why drones have four or more horizontally placed propellors and helicopters one big one small one which a positioned perpendicular to eachother? Wouldn’t it be more logical for the ‘perfect’ design for a flying, hovering machine to look more alike?
In: Engineering
The ideal designs look alike if they serve the same purpose. In this case they do not.
Helicopters are designed to carry people, which means they need to lift high loads to air and in some cases take these high loads to distances with high speed. You need to apply force to the air to push air down and the vehicle up. For that reason you need the blades as long as possible as only some portion of the blades are actually effectively pushing the air down (for causes like tip vortices). But there is a limit to that as the blades should not break with the forces they apply. When designing the propeller of a helicopter, you end up with a single long (or two depending on how you look at it) blade turning in one direction. As the propeller turns in one direction, and as the helicopter is not attached to anything after take off, the body will try to turn in the opposite direction. So as to overcome this, you need another propeller to oppose this turning. You put it at the end of the tail, so the distance to the center of rotation is large, so you do not need to apply too much force, hence this propeller is smaller. This is good as you need the vertical propeller only for stability and maneuverability, and you do not need to pay money and increase the weight of the total vehicle too much.
Drones are designed for maneuverability. Their payloads are smaller, so the concern is not applying much force but being able to change direction and alignment very fast. First you put electric motors where you can change the force output very fast, then you attach four to six motors (and propellers) with some distance to each other, so you can easily balance the forces in any way you like. As you do not care about carrying high payloads you are not limited to how efficiently you apply forces to the air, so shorter propellers do. And as your propellers are symmetrical and hence balanced, you do not need a vertical propeller to balance the force trying to rotate the vehicle. As you do not need to attain high speeds with respect to the air you do not care about the fact that your body is not very streamlined and your area facing to the direction you are flying is very high during a flight in a straight line.
Both need rotating blades but some kind of counter-rotation to keep the body from spinning opposite the blade direction. Both also need some mechanism for heading in a specific direction. They achieve both of these 2 different ways.
Drones optimized for volume manufacturability. It’s really easy to make the same simple motor + fixed rotor assembly 4 times. They choose direction by varying how much power is sent to each of the 4 rotors. The only moving parts are the motors directly driving the blades.
Helicopters optimized for flight efficiency. Helicopter blades rotate, but they’re also mounted on a highly complex gimbal and usually have independent pitch to each blade. The direction the rotor assembly faces can pivot to spend energy driving the helicopter forward. Then a separate tail system fights counter rotation. This is super mechanically complex but much more fuel efficient for improved range.
Stability and economics.
For a helicopter, a single large engine is cheaper, more efficient and leaves more room for passengers / cargo. Because having this big heavy thing at the top causes the part below to want to spin in the opposite direction, the smaller blades at the back to fight against that rotation and stop it spinning out of control.
Drones, being electric, can have multiple motors and rotors from one battery withless space / weight cost. Many drones also have built-in software that does some of the flying for the operator, making it easier to fly.
You can get drone size helicopters (and smaller, I’ve seen down to matchbox size), just for some reason drones became super popular. Possibly because they look cooler.
Control vs. efficiency.
Early aircraft could barely fly. The amount of lift they produced was only slightly greater than the weight of an engine required to generate the thrust required to get that lift. For this reason, they had to be as aerodynamically efficient as possible, and even today efficiency is the driving economical factor when flying great distances. Also, while helicopters can hover, they are still mostly about going forward and are optimized to do so.
The drawback is helicopters are very difficult to fly, both mechanically and operationally. If you can drive a car, apart from landing and taking off, you can pretty much fly an airplane, a helicopter is much more involved. Model helicopters are notoriously difficult to control, it takes hours and hours of practice just to get them off ground.
The four (or more) rotor design is dead simple to control, especially for a computer. Speeding up or slowing down rotors translates into motion in a very predictable way. It is less efficient, but these sorts of drones are light enough cost of energy isn’t an issue, and they aren’t doing things that require them to be airborne for hours on end. And with the advent of cheap, miniaturized computer and GPS hardware, they are also simpler and cheaper to make.
Scaling doesn’t apply so well to inertia. And there aren’t really any materials that can deal with that either.
The difference is that quad-copters (drones) tend to use variable speed fixed-pitch rotors, full size helicopters used fixed speed variable pitch rotors.
Above a certain size you can’t easily or efficiently speed motors up and down quickly, and performance significantly drops as a result. (Although some large quad-rotors exist, they tend to maneuver rather poorly compared to their smaller counterparts. It’s like the difference between a full size bus and a Miata, and why people that try to stunt camera-trucks often crash them.) That puts you back to using a variable pitch system. Since it doesn’t make sense cost-wise to produce a variable pitch rotor 4X, then you’re back to the swash-plate control system in typical helicopters.
Economies of scale, pitch control, and safety.
Helicopter blades don’t just spin around the rotor, they also roll lengthwise, i.e [pitch control.](https://imgur.com/a/1iB5MWp). This adds a lot of maneuverability to the aircraft, but it’s complicated and expensive. When the motor is already complicated and expensive it’s worth the investment of pitch controls to get more out of a single engine.
Drones use very simple fixed blades on relatively cheap and simple motors, and they can get the maneuverability they need by using multiple motors, they can also scale up in size by just adding more motors.
But helicopters with their fancy pitched blades can do something drones can’t, they can be controlled and safely landed even when the motor dies, thanks to some aerodynamic properties that need their own ELI5. When a drone loses power it falls out of the sky at terminal velocity. When a helicopter fails it can be glided slowly down just like a plane. This is a make or break feature when you have living cargo.
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