Eli5: what are the benefits of a normal helicopter design vs a quadcopter we see on drones?
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Simplicity, making a normal helicopter a quad copter would be really really really hard and complex.
That’s because helicopters use combustions engines running on aviation fuel, while quadcopter a uses electric motors.
so for a quad computer, the only moving parts are the spinning motor out on each corner. All that runs back to the center is wires, no moving parts.
A helicopter has a spinning shaft attached to the engine like how a car does. So if you wanted to doing 4 sets of blades with 1 engine, you’d need to figure out how to run spinning shafts through all for corners of the helicopter.
Or you’d had to have 4 different engines, one for each set of blades, and figure out how to easily control the thrust each individual engine is producing.
Is that possible? Yea I’m sure a good engineering team with enough money could figure it out.
But when a helicopter with 1 set of blades works just fine 99% of the time, why make one that would be much more expensive to make and require a lot of maintenance to keep running?
Helicopters with a single rotor will tend to spin on their axis because of the momentum of the rotor, so a tail rotor is required to counter that force and keep them flying straight.
Quad rotors don’t have this problem because the rotors spin in opposite directions and cancel each other out. This makes them far more stable.
So why do drones use this configuration but helicopters don’t?
Cost mostly, having 4 rotors and the associated engines would make a full sized helicopter far too expensive to build an maintain as well as adding a lot of weight. It just isn’t a practical design.
While in a drone it’s more practical because the tiny electrical engines are very light, cheap and provide more than enough power for those rotors. This also makes them easier to fly for amateur pilots.
With a helicopter, you generally have less moving parts & better stability with mechanical controls. The fact that they require combustion engines doesn’t help. That being said, they’re still very difficult to fly, especially without computer assistance.
With a quad(+/-)copter, you’ll generally have electric motors and computer stability systems to help keep you level in the air. These would be nearly impossible with a combustion engine, unless you had a really efficient drive system and pitch controls on every corner. Even then, you would still need computer controls to help keep things in check. The power to weight ratio in hobby grade toys is ridiculous. You would need a stupid powerful fuel engine and would likely use way more fuel than required on even the thirstiest helicopters. Unless they start building them with generators and electric motors, I don’t see the future for large scale quadcopters.
Even if you got the physics figured out and the power sources for efficient flight, you would still have the noise. The little ones are annoying enough, I couldn’t imagine a huge one flying overhead.
Stability, power, and control from what I’m remembering. More props means more lift so it could support more weight. That being said, you would also decrease the battery life of the drone because its using more juice to power the additional motors. You can add more batteries but that then increases the overall weight so it’s a balancing act to get to the optimal design. I designed an 8 prop drone a few years ago for a school project and this was one of the things that we ran into. Basically, you need to define the use case for the drone…do you need more lift without as much flight time or more flight time sacrificing the amout of weight it can carry.
For control and stability, the 2 additional props give you 2 more axis(I think) for stabilizing the drone and movement. Essentially it’s more like using a joystick in place of wasd on a keyboard.
The main difference between them is one of technology.
A drone has multiples fans, which are (almost always) fixed fans, and its controlled by varying the speed of those fans. So to go forward, the fans on the frontend slows slightly, while the back speeds up slightly, tipping it forward, and moving it on a now vectored thrust. And to keep stable, the fans are run in paired directions, clockwise and anticlockwise, so to rotate, you adjust the speed based on the fan direction, to keep overall thrust up, but rotate by gyroscopic procession.
A helicopter works very very differently. instead of ‘fans’ just blowing air down, it has rotors, which are wing-shaped, and on a movable mounting. A helicopter moves by the helicopter wing generating lift like an aircraft. The aircraft is steered by altering the angle of the rotor, generating more or less lift at certain points of the sweep. [this link](https://www.explainthatstuff.com/helicopter.html) explains the method much better than I can.
So the BIG difference is that drones work by blowing air down and using that as thrust, while a copter generates lift on the rotor. This makes a HUGE difference in one area – loss of power. If your drone runs out of power, the fans are no longer being spun, and no thrust is being generated, and as it drops the wind force will back-spin the props. When a helicopter runs out of power, the wind force also spins the blades, but because they’re wings, that produces ‘some’ lift, this is called ‘autorotate’, and is a way to not have everyone immediately die. It’s also how autogyro/gyrocopters ‘fly’. The world record for helicopter altitude is 40,000ft, and its record is when the engine flamed out and died, and it only survived because of autorotation. No autorotation, no passenger safety certification.
And then we come to design. a drone needs an even number of rotors, all of equal size, and for controlability they need to be spread around it. That means you’ve now got 4 (or more) large rotors spread around a large aircraft which means a lot of shafts to drive it, and then a lot of really complex gearing to adjust the speed on each of them. Its pretty much impractical to do it with ICE, so it has to be electric. And because motors get less efficient in a power/size ratio the larger they get – the motors needed for an equivilent of a light utility helicopter (a Robinson R22 for instance) would be 50lb each, plus expensive power controllers, thick cables for power plus all the batteries, and you’re getting to the same problem as the rocket equation, more motor power means more weight, which means more battery for the same flight time, etc. ICE doesn’t scale the same way. For instance the R22 has a [Lycoming O-320](https://en.wikipedia.org/wiki/Lycoming_O-320#Specifications_(O-320-A1A))engine, which develops 160hp, and its 244lb. Its big brother, the R66 uses a [Rolls Royce RR300](https://en.wikipedia.org/wiki/Rolls-Royce_RR300#Specifications_(RR300)) turbine engine, which develops 300hp from 200lb of engine, but being a turbine it drinks a bit more fuel, and has a more complex transmission. The medium size Augusta AW109 (best known from the best TV show ever, 1990s Interceptor featuring the black Interceptor copter G-MEAN started with a 300hp Allison model250 turboshaft, but now has a PW206 turbine making 500hp, for a similar weight and size (they’re retrofitable). Plus, if you upgrade the engine in a copter, you just need to adjust your throttle, controls the same, while an upgrade to drone motors needs everything redone because a 1 % difference in the motors is now far greater.
Basically, quadrotors are ok for small, short flights not carrying people. for anything people-sized they’d be huge, heavy, and poorly performing, with no safety. For small light stuff it’s cheaper to do than the complex mechanical controls of a helicopter, and more physically robust too.
The landing footprint of a drone = 4x rotor area.
The landing footprint of a Helicopter = 1x rotor area + 10% for tail.
Needing 4 times as much space to land is a serious negative.
Lots of good answers here, but I’m missing one of the main points: efficiency.
On one hand, rotor efficiency scales with disk loading (power per unit area of the rotor disk) because it’s much easier to accelerate a lot of air a little bit, than to accelerate a little air very much.
On the other hand is the control system. A quadcopter is controlled by varying the rotation speed of its rotors. Like in a car, constantly accelerating and decelerating is less efficient than cruising at constant speed. A normal helicopter does so by tilting the rotor disk and varying the blade pitch angle while keeping the rotor speed almost constant.
All in all, independent of the challenges of scaling a quadrotor system to something liquid-fuel based, a system with multiple smaller rotors is inherently much less efficient than one large rotor.
Source: MSc Aerospace Engineering
I’m sure this is most likely a stupid question since I know nothing about any of this but if 1 of the 4 engines,(or propellers) failed, wouldn’t it be impossible to control it back to the ground? And yes, I realized that if the one propeller on a normal helicopter failed, you’d be fucked also, so maybe that wouldn’t be any more of a consideration than it usually is. Like I said, I’m completely ignorant of the engineering of any flying machine, the thought just occurred to me.
You can safely land a helicopter with autorotation without power on the main engine through carefully changing the pitch of the rotorblades.
Quadcopters have fixed-pitch rotors and do their steering by changing the rotor speeds.
If power fails, it’ll drop like a stone.
If even one motor fails, it will be uncontrollable. (I think hexacopters and octacopters have more tolarence.)
Manufacturing cost, most likely. Simpler machines are cheaper and easier to produce, and in business, as in most things, if the question begins with “why don’t they”, the answer is almost always “money”.
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