Quadrotor configuration I think is the most common configuration for drones. Why has it not been made into a full-size production aircraft? I think that it could be better than helicopters as it does not need a tail.
In: Engineering
It works on drones because those propellers can be driven directly by a DC motor. That’s a very simple configuration.
One helicopter prop is already very complicated but at least it’s the only real load on the engine, so it can be easily throttled. Splitting off the tail rotor isn’t a big deal because it’s relatively a really small load. To have 4 big rotors, you’d either need 4 complicated engines with their own throttles or some kind of crazy transmission and differential gearing system to drive them all with one engine. Regardless, if any one of those complicated things fails, you’re in big trouble; whereas a normal helicopter can actually glide safely down during an engine failure.
A quad rotor will immediately lose stability if one of the rotors fails. If a helicopter engine fails, it’s actually possible for the pilot to still land it relatively safely and gently. Safety tends to be the top priority for human-piloted vehicles, especially flying ones.
If you look at the developments in electrical powered aircraft, they’re all using multiple rotors, but with only one or two conventional engines, given the complexity of the gearboxes and rotor heads etc, the “2 or 4 engines on a wing” or the current helicopter designs fit the airframes better (drones don’t need to have space to carry passengers so their airframe has different design constraints anyway)
And multiple conventional engines lead to their own complexities (harmonics of vibrations etc).
In the first place, a tail is not exactly a big deal so removing it is not exactly a design priority.
4 engines are going to be more expensive than 1, even if the engines are smaller. The fuel efficiency is probably going to be pretty bad, meaning either the craft has to carry a lot of fuel (reducing payload) or have very limited range. Also, the operator has to maintain 4 engines rather than 1, expensive. This makes it a not very good aircraft for many purposes.
Because it is extreme complex to make new mancarrying aircraft.
As a matter of fact, people started to make the design the moment it appeared feasible.
The boeing dudes could´t make the undercarriage of a stupid bus longer in 30 years, so you kind need to allow some time for a total new concept. (On top of that the helicopter-market is an? order of magnitude smaller than said Busbussines).
Let me introduce you to EVTOL (electric vertical take off and landing). The eve-100 has 8 rotors.
The energy density of Jet fuel is massive vs energy density vs batteries.
Batteries can drive four simple low power motors. Jet fuel can drive a complex but powerful engine.
The fuel and weight drives the design decisions. Four small turbine engines would be a massive amount of complexity but also a lot of mechanical complexity .
There are a whole bunch of companies working on eVTOL (Electric Vertical Take Off and Landing) aircraft. Billions upon billions of venture capital is being spent. Joby, Archer, Lillium, Wisk, Vertical, Volocopter…..
They all have more than 4 propulsive units because you have to be able to survive the loss of one to be certified to carry people.
Helicopters can auto-rotate, so they can still be controlled after loss of thrust, so even one engine can work there. This does not work on multirotors, so you have to maintain thrust to maintain control, and thus you need 6 or more stations.
It’s coming though.
When it comes to anything mechanical, scaling is always an issue due to the nature of physics. Think about the earliest electric cars: they were small and never went that fast. That has now changed, but they still suffer other unique issues like battery life, charging, and torque.
Now let’s explore aircraft. Airplanes need massive amounts of power and fuel as they scale in size and capability. For helicopters, they have to work harder as they cannot rely on pure speed to keep them airborne like a plane with wings. The bigger the helicopter, the bigger and more powerful engines are needed. As you add rotors, like a Chinook or Osprey, there needs to be mechanical linkages to keep them spinning under the power of one engine for safety (adding weight) and now those engines need to be even more powerful. There is no electric motor powerful enough to accomplish that which would also fit into an economical aircraft design. Plus, imagine the charging times! It’s not entirely impossible for something big enough to carry a single pilot, but adding weight adds power requirements and complexity.
Now, why not 4 rotors using ICEs? Simple answer is you don’t need that many for what we use traditional VTOL for. Small drones are omni directional as they are largely remote controlled from the ground or via a camera interface. While helicopters are capable of moving in any direction, it’s only during positioning movements close to the ground. An omni-directional helicopter would require a cockpit that also moved for the sake of the pilot. Lastly, the tails of helicopters aren’t just for mounting the tailrotor, its for aerodynamic stability, just like all other aircraft capable of flying at significant speeds.
Source: am pilot
The way that quadcopters and helicopters are controlled is completely different. There are 4 basic movements that it has to be able to do. Collective (go up/down), pitch (go forward/back), roll (strafe left/right), yaw (turn left/right)
The quadcopters have simple rotors that are one fixed piece. The get more or less lift by having the motors spin faster or slower. That’s something that electric motors are really good at. With four of them, you can easily do all the basic movements. If you want to go forward, you increase the speed of the rotors in the rear, and decrease the ones in the front.
With helicopters, gas turbine engines are not capable of throttling quickly enough to provide controls and it’s more fuel efficient to just have them operate at essentially max speed all the time. The rotor system is significantly more complicated because it’s what does all the controls (except for yaw which the tail does). The angle of the blade actually changes depending on where it is in the rotation.
[Here is an animation showing how the helicopter’s blades change angle](https://www.youtube.com/watch?v=kKkpC_0-wXg)
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