Again, there are multiengine helicopters.
I don’t know if I mentioned it upthread, but I don’t think a quadcopter helicopter would be in the Rotorcraft-Helicopter category. They’re closer to the Powered Lift category, like the Osprey… except they don’t fly like an airplane like the Osprey does. The FAA might create a new category, ‘Rotorwing-Quadcopter’, only what if the aircraft has six or eight rotors?
Eh, if we can have three-wheeled bikes, then we can have six-rotor quadcoptors.
I think it was mentioned, but the point with a quad-copter (hex?) and modern tech is that they’d be electric motors with all the simplicity that involves - but then the issue is power supply reliability. (Ignoring the weight-vs-flight-time issue). If something caused the battery output to die, then you’re toast. The next logical thing would be - separate battery for each rotor. 4 or 6 completely separate systems. After all, if one rotor fails, time to land, and presumably the other 3 or 5 rotors - and their batteries - had enough juice to make it to the destination anyway, so they should have enough to manage a soft landing ASAP. Redundacy would suggest that the hex-copter tolerate 2 engines out (preferably not on one side?) meaning the engines should normally operate at 66% power except in an emergency. An octo-copter in this configuration seems even safer… tolerate 2 or 3 out, run at 62% power. After all, even fixed-wing aircraft don’t operate at 100% power all the time. Even so, interconnecting the batteries to allow them to share power is not difficult. (and turn the connection off when that one battery has a problem)
For larger configurations, I would think variable-pitch props are a better tech - flxed-wing have used them for years, and are simpler than a cyclic swash-plate on a helicopter? That would solve the inertia problem with large propellers, and presumably an electric motor has rapid power response.
The other point too is that a hex-copter configuration is a limited range short distance craft. It’s not a configuration for high speed horizontal travel and that’s not very efficient, unless it’s one of those tilt-rotor devices where it has a aerodynamic lift wing or body, and the rotors tilt to become propellers. So short-duration batteries are less of an issue. I assume its biggest application would be short hops within cities, or to the airport from downtown.
We just await lighter batteries.
(And an octocopter does not have to be a circular design - two rows of props on beams from the body could work too. Just takes more computation to balance)
A tandem approach seems the most common, like this one:
They’re actually potentially more efficient than single props.
An octocopter like this could lose up to 3 props–2 on one boom and another on the opposite boom, or 2 on any boom (well, if you made it fantastically overpowered, you could lose up to 5, but that seems like overkill).
It’s hard to tell that has 4 booms…? but it must.
What’s the efficiency result of stacked rotors? Does it make it better or worse, or is it that the weight saved with less booms makes up for it?
My only concern is that the design should allow for whatever catastrophe happens to one engine in a pod does not hurt the other or its power supply. (I.e. fire, meltdown… whatever can go wrong with an electric motor)
You can see the tip of a prop on the fourth boom just poking out from behind the cabin. The perspective just makes it look funny.
The Wiki page on contra-rotating props cites a paper giving a 6-16% efficiency increase. That might be optimistic, but even a few percent is better than nothing. Basically, a single prop leaves the air rotating, and there’s loss from the angular momentum. A second prop partially cancels that out.
Should be straightforward to handle a bunch of failure modes, and distribute the controllers so that they can’t take each other out if one does fail.
We’re reevaluating those wacky old designs after all.
Yeah, with a lot of those wacky old designs, the problem was stability, and with computer controllers that can react in microseconds, you can get away with a lot of instability.
I’ve never flown a drone, so I don’t know how they work. I’m guessing that the motors, deriving power from a single source, are individually controlled by the computer. Power failure? The drone falls out of the sky. But what happens if a motor fails? What happens to compensate for asymmetric lift?
In multi-rotor helicopters, the rotors are connected so that both engines (I think most multi-rotor helicopters have two engines) power both rotors. If one engine fails, the rotors are still powered by the other one. If both engines fail (or if you’re in a single-engine Kaman K-MAX), then the freewheeling unit disconnects both rotors from the engine(s) and the pilot enters autorotation. So the rotors are either powered by both engines, powered by one engine, or not powered and disconnected from the engines.
How would this work in a hypothetical n-copter?
I think the first key is that electric motors are so much more reliable than internal combustion engines that motor failure is negligible compared with other possible failure modes.
But would you bet your life on that reliability? Aircraft need to be designed to be recoverable from failure modes such as power failures. (And of course they must be operated within the envelope so as to prevent things like wings falling off or rotors hitting the tail boom.) How would an n-copter allow a safe landing in the event of the failure of one or all motors?
Not exactly… but I would be comfortable with a trained, qualified team of engineers betting my life on my behalf. Which we all do, frequently.
As for “what happens when all motors fail”, the answer is “You die”, of course. No matter how safely you ever design anything, there’s always some “You die” outcome in there somewhere. But all motors dying at once is a sufficiently unlikely circumstance that it’s acceptable.
I went to a lecture on early flight at Bell Labs once, and they played a film taken from the pilot view on a Wright biplane being demonstrated in France in the late 00’s. the lecturer mentions “notice how the horizon goes up and down. The Wright biplane with the leading ‘stabilizer’ was dynamically unstable and the pilot was constantly adjusting. Which is why tails are at the back since then.”
Does a Chinook have a shaft running down th spine of the copter? Or is autorotation the default for single engine failure?
Or you plunk down in the Hudson…
But yes… for example, total engine failure in the Rockies or over Greenland is not an easily survivable scenario in fixed wing aircraft. You need a special permit to take a single-engine aircraft across the Atlantic. (Usually via Labrador, Greenalnd, Iceland, which is how small planes get shuttled to Europe and beyond)
The question becomes - what are the odds. Single engine failure in a large mult-engine aircraft is rare, but it happens. Multiple engine failure is even rarer by a long shot, because it’s a failure of something in common - large bird flock, fuel exhaustion, etc… Similarly, between construction standards and inspections, wings don’t even fall off. (They do, but very very very rarely and for preventable reasons) So you fly with the knowledge that there are scenarios that kill, but as much as possible the foreseeable scenarios have been accounted for.
For example, if I can come up with the idea of independent batteries for each engine, but the option to share power if needed, I suspect engineers who build this stuff have thought of it already. I would suspect if bearing failure is a scenario, then redundant bearings in the electric engine are a thing. Temperature sensors to detect wiring or battery failure. etc. Anticipate and plan for obvious sources of failure.
Both engines feed into the combining transmission, which sends the power to the fore- and aft rotor transmissions. In case one engine fails, the other one is still providing power to both rotors.
Yup, different devices have different “you die” scenarios. Now compare what happens if the Jesus nut fails in a single-rotor helicopter, versus what happens in a vehicle with six or eight rotors.
We still fly helicopters, even though the failure of the Jesus nut means certain death. We do this because Jesus nuts are made to be extremely reliable, such that the chance of one failing is small enough to be tolerated. The critical systems on a quadcoptor-based vehicle could also be made reliable enough to be acceptable.
This thread sat dormant a long time and has suddenly burst back to life. I’m just now catching up on the last ~week’s many contributions.
Returning to the “If engine fails you must land ASAP” idea as a basis for design criteria …
That is current regulation for airline ops. But not for lesser regulated segments of aviation. The relevant cite is eCFR :: 14 CFR Part 121.565. The meat is in the very first sentence which, in pertinent part, reads:
… whenever an airplane engine fails or whenever an engine is shutdown to prevent possible damage, the pilot in command must land the airplane at the nearest suitable airport, in point of time, at which a safe landing can be made.
“In point of time” is awkward legalese for “as measured by time, not by distance”.
Now the word “suitable” is doing all the heavy lifting there and the definition of suitable is pretty flexible. It definitely extends far beyond just “Is the runway long enough you have a decent chance of stopping before the end?”
ETOPS / EROPS further amplifies on this by permitting flight up to ~3-1/2 hours from any “suitable” airport for certain airplanes and operators. So the engineers & authorities & operators are all confident enough that you can suffer an engine failure, drone along for another 3-1/2 hours with no powerplant redundancy, then land, and still be safe enough.
As mentioned upthread, the definition of “suitable” for a quadcopter-style vehicle is going to be a lot easier to fulfill. Unless over open water.
Moving beyond that narrow issue …
IMO these vehicles will succeed or fail as engineering projects on the basis of distributed redundant lifting power and distributed redundant control power. Which will be achieved by electrical / computer control, not by mechanical moving parts.
There is no reason in principle that a quadcopter couldn’t be designed to fly adequately with one dead rotor. The problem only gets easier with more rotors.