So – clearly quadcopters are more stable (see all the drones), so why aren’t actual helicopters all quad copters?
In: 5
1) Helicopters have variable pitch. That’s how change direction, speed up, and basically fly. Drone quadcopters don’t. Variable pitch allows the helicopter to autorotate in the event of an engine failure, instead of crash like a drone.
2) Four rotors isn’t four times the redundancy, it’s four times the chance for failure. Quadcopters don’t fly well on 3 rotors.
3) The ‘heavy lifting’ is done by the outer parts of the rotor, because they’re moving faster than the inner part of the rotor. This favors one (CH-53) or two (CH-47) large rotors over four smaller rotors.
4) Likewise, to compensate for the smaller rotor diameter, quadcopter blades spin faster. This has the advantage of helping with retreating blade stall (the retreating blade is also going forward into the relative wind as the aircraft goes faster, until it’s not going fast enough to work). But it also means the advancing blade is going faster and is limited by the speed of sound (blade tips going supersonic causes a host of problems). And the blade has to be stronger (and hence heavier) to compensate for the higher forces that come with higher RPM.
Drones are not built that way because the are more stable. The design is because it is a cheaper way to build them if you power them with batteries.
Quadcopters drones work because they have electric motors that quickly can control the rotational speed of a fixed rotor.
Regular helicopters have one or more internal combustion engines usually gas turbines. They do not change the rotational speed fast. Helicopters usually operate them at a constant rotation speed for the flight
So the control system of a helicopter is a https://en.wikipedia.org/wiki/Swashplate_(aeronautics) that controls the angle of attack of all blades. There are both changes for them all if you want more or less lift to go up and down. It also has directional control so you then have a higher angle of attack on now side to provide more lift there and to move around. To that add a tail rotor.
A quadcopter also needs a structural part that keeps the rotors apart and the weight compared to strange do not scale in a way that is advantageous for a large helicopter. The square cube law is in effect
there are helicopters with multiple rotors that provide lif like a https://en.wikipedia.org/wiki/Boeing_Vertol_CH-46_Sea_Knight , https://en.wikipedia.org/wiki/Bell_Boeing_V-22_Osprey etc. They have a mechanical connection between the rotors so they rotate at the same. A V-22 has one engine just below each rotor, they are still connected because you do not what a single engine failure to result in a crash.
The reason they have multiple rotors is to provide enough lift for a CH-46 and similarly designed. A single-rotor would be very large. A V-22 have two so you can fly like an airplane with them tilted forward and the lift is provided by the wing. A single-rotor ontop is cheaper for most helicopter requirements.
For drones, it is ok if a single engine failure results in a crash but that would not be ok for a crewed helicopter.
So what is a cheap and good design for a small electrically powered front that does not carry humans is not a cheap or good design for an internal combustion engine power helicopter with humans onboard.
Drones are stabilized by changing each rotor rpm. Tiny, lightweight rotors that are spun faster or slower by a computer controlled electric motor for each one.
Helicopters are bigger and sudden rpm changes is not a good way to control it.
1 It takes too much to spool up or down due to rotor mass.
2 engines are turbine type, which won’t spool up or down easily. For that scale of things, electric power plant is too heavy and batteries are more than too heavy at that scale. Fuel burning and turbine is the only practical powerplant there.
So helicopters use fix speed rotors, controlled by changing each blade’s angle of attack, by cyclic and collective control. This give the pilot instant control on the thing.
At this point a single, or two counter rotating rotors is the best way to keep it simple.
You have to transmit the power from engines to rotors, and for safety purposes you need all rotors and all engine to be mechanically connected, so a single engine failure won’t stop a single rotor. All rotors are always spinning the same speed and engine shortages are spread equally on all rotors. Quad rotor would require so many shafts and gearboxes, and each will subtract power due to gears and bearings creating some friction. Plus adding multiple points of failures, of the lethal type.
Drones are stabilized by changing each rotor rpm. Tiny, lightweight rotors that are spun faster or slower by a computer controlled electric motor for each one.
Helicopters are bigger and sudden rpm changes is not a good way to control it.
1 It takes too much to spool up or down due to rotor mass.
2 engines are turbine type, which won’t spool up or down easily. For that scale of things, electric power plant is too heavy and batteries are more than too heavy at that scale. Fuel burning and turbine is the only practical powerplant there.
So helicopters use fix speed rotors, controlled by changing each blade’s angle of attack, by cyclic and collective control. This give the pilot instant control on the thing.
At this point a single, or two counter rotating rotors is the best way to keep it simple.
You have to transmit the power from engines to rotors, and for safety purposes you need all rotors and all engine to be mechanically connected, so a single engine failure won’t stop a single rotor. All rotors are always spinning the same speed and engine shortages are spread equally on all rotors. Quad rotor would require so many shafts and gearboxes, and each will subtract power due to gears and bearings creating some friction. Plus adding multiple points of failures, of the lethal type.
An early experimental VTOL aircraft, the Bell X-22
https://en.wikipedia.org/wiki/Bell_X-22
was a “quad copter” I suppose.
It had many problems, starting with the fact that it predates anything you’d call computers, hence had no sophisticated automatic controls.
But the biggest problem was a phenomenon called “recirculation”, where airflow near the ground was deflected back around into the ducts, seriously reducing lift.
An early experimental VTOL aircraft, the Bell X-22
https://en.wikipedia.org/wiki/Bell_X-22
was a “quad copter” I suppose.
It had many problems, starting with the fact that it predates anything you’d call computers, hence had no sophisticated automatic controls.
But the biggest problem was a phenomenon called “recirculation”, where airflow near the ground was deflected back around into the ducts, seriously reducing lift.
So the main basis has been missed somehow so:
Quadcopter drones exist due to the advent of ultra cheap micro-computers.
It is a terrible design for any normal aircraft (highly unstable among other issues) but it is an ideal way to control an aircraft with a *computer.*
Computers can perform simple operations at lightning speed, but struggle with complex multi-variable decision-making (think of the challenges in getting a self-driving car).
The Quadcopter design requires constant inputs, but the actual calculations and actions are very simple, as a result it is much easier for a computer to precisely control a quad-copter than a traditional helicopter. When for a human it would be the opposite, trying to make a quadcopter work without advanced computers would be extremely difficult
So the main basis has been missed somehow so:
Quadcopter drones exist due to the advent of ultra cheap micro-computers.
It is a terrible design for any normal aircraft (highly unstable among other issues) but it is an ideal way to control an aircraft with a *computer.*
Computers can perform simple operations at lightning speed, but struggle with complex multi-variable decision-making (think of the challenges in getting a self-driving car).
The Quadcopter design requires constant inputs, but the actual calculations and actions are very simple, as a result it is much easier for a computer to precisely control a quad-copter than a traditional helicopter. When for a human it would be the opposite, trying to make a quadcopter work without advanced computers would be extremely difficult
Quads are LESS stable than conventional helos, they are only stable because they are computer stabilized.
Quads only makes sense at toy size because it’s mechanically cheap to manufacture and it’s inferior power efficiently isn’t that big of an issue with lighter toys.
As soon as the size comes up, quads efficiency problem quickly becomes apparent and at this size, it is possible to build complex rotor articulation systems to effectively control a single rotor that is not only more efficient, but also safer to operate. Quads have no mechanical redundancy, so it’s going to be either a hexa or octo before anyone is comfortable flying on it.
Quads are LESS stable than conventional helos, they are only stable because they are computer stabilized.
Quads only makes sense at toy size because it’s mechanically cheap to manufacture and it’s inferior power efficiently isn’t that big of an issue with lighter toys.
As soon as the size comes up, quads efficiency problem quickly becomes apparent and at this size, it is possible to build complex rotor articulation systems to effectively control a single rotor that is not only more efficient, but also safer to operate. Quads have no mechanical redundancy, so it’s going to be either a hexa or octo before anyone is comfortable flying on it.
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