Why aren't there more single rotor drones out there?

Hello all,

I have just recently joined this forum and find it fantastic what you guys are doing, very nice job.

There is one question going around in my mind for quite a while now: Why aren't there more single rotor drones out there? Though some posts here touch on that topic, I could not find any post discussing this matter in more detail. There is probably no better place to get some informed opinions on this than this forum here, so I would love to hear your take from any possible perspective you might offer.

In my mind there are many points speaking for a single rotor design. Though its implementation might be more difficult. A single rotor drone could be designed to be a lot more energy efficient than a multi-rotor design, resulting in longer battery life. It could also be designed for higher payload and high-altitude capabilities (e.g. high-payload in mountains etc.). In addition to that they could offer higher flight stability, increasing their application potential in strong winds (e.g. inspection of wind turbines). So, what do you think is the reason why there aren't more single rotors out there?

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Setting all the arguments about safety, speed/torgue/blade size aside, the one thing that multirotor (especially those with more than four rotors) vehicles have that single rotors do not is redundancy during motor failure.  If a motor fails on a single rotor craft, it comes down unless it has a backup (parachute, which adds weight).  I have a hexacopter and HAVE myself had three different times when a motor has failed me.  Two of those times I was able to land the rig safely.

I also understand that the argument saying if you have more motors then you have more chance of failure being true.  But like I said, on a single, it just takes one failure and you're likely rebuilding from scratch.

This was already brought up, a single rotor helicopter can autorotate to get down safely, however, if a swash servo fails it's generally more critical. But for a motor failure a helicopter is much more likely to make it down safely if autorotation is setup in the autopilot than a multirotor. Even loosing the tail servo you can get the heli down without issue with an autopilot or stabilization system. A quadcopter is almost impossible to salvage if there is a single failure.

Hi Jason,

Over a year ago ETH (the people who brought us the PX4 and Pixhawk) actually designed an emergency hover and landing routine that would work pretty well with 3 motors working on a quadcopter.

It was none to neat, but it did work and could permit emergency landings.

Unfortunately I haven't seen anybody pursue this further.

Autorotation can and frequently does save helis these days and there is even an autorotation to landing challenge in helicopter contests these days.

And yes if the rotor or it's main control mechanism fails, not much short of a parachute will help.

That said, quality helicopters and their assorted hardware have become very reliable due to constant development driven by 3D flight contests and their extreme requirements.

The efficiency gain of the single rotor heli over a multi's numerous propellers is pretty much at least 20 percent for similar capabilities and for larger ones can even be more.

Helis are also more efficient traveling horizontally at speed due to superior aerodynamic profile both of the chassis and of the continuously variable pitch rotor versus fixed pitch prop.

Bigh helis are scary, but so are big multis and safety considerations are similar for both.

For a lot of uses, multis will be cheaper and easier to fly and deploy, but helis are going to find a lot of uses especially at larger sizes.

This is kind of an aged thread at this point, but I'll add my 2 cents.

I fly real (manned) airplanes and as pilots we use autopilots in both fixed-wing and rotary-wing aircraft all the time.  But they have not arrived at the point where they can replace a human pilot yet when something goes awry.  Consider a human pilot flying a single rotor aircraft, where upon landing it is discovered that that some obstruction on the ground must be avoided or the aircraft will tip over if a skid hits it.  The human pilot compensates and there is not an issue.  But an autopilot cannot do this without a bunch of expensive sensors.  And GPS is not always 100% reliable or accurate enough.  So if you do something like autonomously land a multi-rotor and one skid lands on a log or big rock, or it lands on a hillside, it can tip over with the motors running and you don't get much damage.  Do that with a single rotor and you have a bunch of expensive parts flying due to the fact that instantly stopping a rotary wing that contains a lot of kinetic energy is somewhat catastrophic for the driveline components.  And usually quite significant damage caused to whatever stopped it.

I've seen more than a couple Bell Jet Rangers wadded up due to a wind gust causing the aircraft to drift sideways into a tree or power pole during hover.  Few things invented by man that fly apart, with the possible exception of the denotation of a nuclear device, are as impressive as a single-rotor helicopter when it hits something.  As impressive as robotic autopilots are, they are still not as good as a human pilot. So it might be just me, but I would not trust one to fly a single-rotor machine in full autonomous mode.

I will have to disagree with some points on single copters here. As someone who has built a great number of single and coax copters with vane stabilization I can say that they are in fact perfectly stable in even gusty conditions. The real issue is that they are very difficult to properly design. 

Hugues, the only people that have been killed by helicopters, are stunt flyers who intentionally put their lives at risk in the search of thrills. The rotors are spinning at extremely high RPM, much higher than UAV applications require.  And they fly close and fast.  For a good example, check the flying here starting around 1:30:

Do you see anybody doing that with a quadcopter of equivalent size?  No, actually, answer is a quadcopter of equivalent size could not move like that in the first place.  But even 250 racer quad pilots, aren't stupid enough to fly like this.  So the statement that helicopters are more dangerous than multirotors, is completely skewed because of the stupid way some helicopter pilots fly.

And I even question the claim about "lots".  I'm aware of 4-5.  In all cases, it was the pilot who hit himself.  I'm only aware of one case where a little girl was hit because they stupidly flew one of these things in a crowded stadium.

As Jason says, if you compare apples to apples, you would need 29" props on a quad to compare to a 700-800 size helicopter.  Do you know what would happen to a person hit by a 29" props?  I doubt they will survive either.  You would end up with very severe lacerations.  I crashed a quadcopter with just 10" props into a house, and the props cut into the aluminum siding.

Chris, I've been flying autonomous helicopter UAV's for years, and it can be done very safely.  You are correct that multirotors are more tolerant of ground mishaps.  This is actually one point that I concede, and regularly do.  If you tip a multirotor over on landing, usually you just dust it off and flip it back over.  Worst case, you change a prop.  Helicopters suffer much more damage, and most likely won't be flying again that day.

But this just comes back to piloting skills.  Only takeoff/land in locations that have level ground and a reasonably clear area.  This is really basic stuff.  If somebody crashes a helicopter into something, it is the pilot's fault, not the machine's fault for being fragile.

Gary, I have yet to see that quad-copter spinning trick work on any machine of a size large enough to actually serve a useful role.  This is something that I see a lot, where somebody does something in a lab environment, on a test machine, and then it because zeitgeist.  Well, let's see this trick done on a quad with 15" props, carrying a decent camera, and then we'll talk. ;)

You are right that, while multirotor proponents usually point to redundancy to prove their reliability, it must not be missed that, as you say, helicopters are very mature technology.  They have been made for a long time, and most of the failure modes have been sorted out.

Jason is just one of many that I have heard struggling to make reliable large copters.

Rob, I use my UAV (currently a DJI product flown with Litchi software) to fly fully autonomous agricultural aerial survey flights.  On a few occasions in 1,000+ flights I've had it auto-land due to usually wind issues that cause it to run out battery before it can complete its flight.  When it lands off-site, I've yet to have it land in what I would consider an ideal landing zone.  In every instance, so far, it has never ended up on the landing skids in a forced landing.  Only once did it get any damage due to wind shear where the autopilot couldn't compensate fast enough, it descended out of sight behind a treeline so I couldn't control it manually, and I got two busted props.

Occasional forced landings are unfortunately a reality in the type of flying I do where I try to cover as many acres in a flight as I can.  But can't always predict the winds aloft at 328 foot altitude.  Like you say, it's pilot error because as the flight progresses I should cancel the flight and bring the aircraft home when I calculate that it's possibly not going to make it.  But it ends up being, "I think I MIGHT be able to make it and bring 'er in with 15% battery left".  It hits the hard limit at 10% and lands itself, and the cell voltages are still above 3.3, so it's never hurt the batteries.  But it's still exciting sometimes when you get low on fuel in the wind.

I'd love to fly a single rotor due to the inherent efficiency advantage, and have considered building one.  But the idea of the repairs when I have a forced landing has kept me from doing it.  I can currently get 20 minutes of flight time, safely, and fly 30,000 - 33,000 feet flying distance in one flight, covering 250-275 acres.  I need to at least double that in one flight but the inherent inefficiency of a quad-rotor sort of limits what is possible to do with it carrying a high-definition camera payload.  That's what caught my eye on this thread.  If someone is flying a single rotor in full autonomous flight mode on long distance flights, I'd love to hear more about it.

Well, it's funny you ask, I have been developing a helicopter for mapping that I will be launching very soon. (actually look for a blog post in the next couple days).

I agree with you, a forced landing a heli would not be good.  I actually had one of those last December, when I was doing almost exactly what you say.  Though in my case I was specifically pushing it to see how long it could fly.  It came down in a rough (plowed) field with tall weeds.  Damaged tail boom and one landing gear.  That's it.  Didn't even damage the blades.  One of the big advantages of a properly engineered UAV heli, is there is much less energy in the blades.

So if you were using my UAV heli, you wouldn't need to worry about forced landings. If my math is right, you're flying about 5 miles, or 9km.  My heli can fly at least 40 km with a 1kg payload.  Probably capable of more than 50.  Flight time is 45 minutes hover, up to an hour at 15 m/s  About 50-55 minutes at 20 m/s.

That was Phase 1 prototype, here's Phase 2:

Rob, very nice.  That's exactly what I had in mind when I was considering building one.  There is no doubt a single rotor can far exceed the capabilities of a multi.

For the present I'm building an all aluminum quad-rotor to keep me in the air until I come up with a better solution.  My Phantom 3 is worn out, with stress cracks around the motor mounts, needs new motor bearings, and even the screws in the upper shell are starting to come loose and won't stay tight anymore.  Those things were made for casual flying, and definitely not what I've been doing with it.

My all-aluminum quad is 550mm wheelbase with 14 x 5 props, and although slightly heavier takeoff weight than the plastic one, I expect better flight time from it, somewhere around 24-25 minutes in hover with a 10A battery and pushing 28-30 minutes @ 9 m/s flight speed carrying a 300g payload.

I also have a fixed wing I've been working on for quite awhile.  But haven't progressed much with that yet due to the fact that there's limited landing areas for a fixed wing where I fly most of the time.  I usually end up taking off from a field driveway or something next to a corn or soybean field and crash landing a fixed wing in 6 foot tall corn is not really ideal either.

+1 A heli would be much better for your case offering greater flight times and payloads along with increased reliability. 

For the airplane application, have you looked at alternative landing methods? With ardupilot there is now support for reverse thrust landing which can greatly reduce the area required to land, there is parachute support, and quadplane support with tilt rotor coming soon.

There are so many options now for excellent UAS platforms that far exceed the capabilities of multirotors in both the fixed wing and traditional heli platforms. 

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