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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  • Developer

    The RMAX has been operating commercially since 1997, and there are currently more then 2500 of them in operation around the world. And in all that time there has been a single fatal accident. The alternative is manned crop dusting where pilots die every year.

  • 3d helis aren't the only dangerous ones around, the Yamaha RMAX has already killed someone...

  • The idea that a single or coax copter does not tilt to move laterally is incorrect. Coax and single copters both use the exact same method to translate that quad or other multicopters do, the vanes create a moment about the center of gravity of the craft the same way a multicopter does with differential thrust and they translate identically.

    The real issue is that simple PID controls are generally not suited for the heavily non linear inverted pendulum that is a single or coax copter, which has a lot to do with their perceived poor performance.   

  • Rob_Lefebvre said:

    You have to compare apples to apples.  People are not intentionally flying large, powerful multirotors directly at themselves, backwards and upside-down, at 100mph.

    Indeed.  RC flying for fun is one thing.  Flying like you left your brain back in the hanger is insanity, and it's no different than any other activity.  I don't think there's any inherent safety advantages with a multi and they are inherently less stable than a single rotor with collective and cyclic pitch.  You remove the autopilot and IMU from the picture and a competent pilot can fly a single rotor with four channel control - it has been done for years.  Flying a multi without the electronics is pretty much a no-go.  If you built a multi equivalent in power and size to most of the single rotors, I'd say it is every bit as dangerous.  But most of the multi's are toys, in comparison, of up to maybe 1.2m wheelbase.  I've seen things in RC single rotors like one local guy here that has a Sikorsky CH-53 model with a 12 foot rotor and twin Wren turboshaft engines.  That turbine chopper can easily lift a 175 lb payload out of ground effect.  So it's really a apples to oranges comparison in most cases simply because of the size and power of a lot of single rotor models.

    Multi's are more popular with hobbyists because of the simplicity and availability of parts to build them after decent IMU's became readily available.  But one thing about a multi is that you have a snowball's chance of controlling it if the IMU goes haywire.  So consider a scenario where someone would build say a 8-16 rotor multi capable of lifting 175 lbs.  And something goes awry with the IMU and it takes off out of control and flies either into you or some other people.  Somebody will get killed by such a machine.  The people that fly single rotors are simply in a different class because most of them don't fly little plastic toys.

  • Hugues said:

    You are confirming what I said about the mass, speed and thus lethal energy in the blade of an helicopter. 

    I am not in a debate against helicopter but its danger should be acknowledged.

    About deaths, just Google " RC helicopter death" and you will find way too many cases. I find amazing you think 4-5 deaths is not a lot! 

    No one ever died of (mid)using a multicopters. It is a fact.

    You have to compare apples to apples.  People are not intentionally flying large, powerful multirotors directly at themselves, backwards and upside-down, at 100mph.

  • 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. 

    • Hi Sun,

      The videos of single and coax copters that I have seen have all been very low performance.

      IE, they could not respond quickly or strongly to either flight inputs or to external conditions such as gusts.

      That said, most of the existing examples have been perhaps sub-optimal designs.

      The fact, however, is that a multirotor maneuvers by tilting it's entire airframe in the desired direction of motion, literally to whatever angle and amount of force it is necessary to generate to provide the required motion and speed in the desired direction.

      A coaxcopter or single copter however, does not tilt and simply relies on vanes beneath it to direct a portion of the air to one side or another.

      Barring some more advanced operating method, not yet in evidence, it is quite literally impossible for a single or coaxcopter to achieve anything remotely like the response rate or  force that can be achieved by a multi-rotor.

      Possibly a more advanced system that could both direct the exiting air and provide beneficial tilt to the aircraft could achieve this, but I have not yet seen such a thing.

      You can tilt a multirotor beyond 45 degrees and accelerate very briskly in any direction, not true at all for a coaxcopter or single copter.

      Of Course, a helicopter can do all that and more.

      Best regards,


      • "The fact, however, is that a multirotor maneuvers by tilting it's entire airframe in the desired direction of motion, literally to whatever angle and amount of force it is necessary to generate to provide the required motion and speed in the desired direction."

        Yes, and they accomplish this because of the fact that electric propulsion motors can change speed rapidly to make it work.  That's why multi-rotor manned aircraft with fixed-pitch rotors have not been too popular or practical.  At least until somebody comes up with an efficient powerplant for one.  The only reason they fly in the model world is because of the fact that you can make a brick fly if you strap enough horsepower to it.

        A hybrid multi-rotor/flying wing will probably eventually be the answer to long flight times with VTOL capability combined with relative simplicity.

  • 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.

    • 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.

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