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.

        • 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.

  • Hi Jason and Hugues,

    Arguably a blade strike from a Single rotor helicopter may cause more damage than a similarly sized quadcopters propeller blade.

    But not necessarily.

    Although the vast majority of multicopter blades are one piece and also tightly affixed to the mass of the motor armature with resulting full transfer of all that mass into any blade strike, most helis actually have free pivoting blades thus only transferring the mass and inertia of a single rotor blade into the object being struck.

    This can easily be less than the equivalent of one multicopter propeller armature mass.

    Also, many multicopters have razor sharp carbon fiber blades, often heli blades are not nearly so sharp.

    As for performance, as you scale up the aerodynamics of multicopters worsen rapidly, a quadcopter with 25" propellers is near the upper limit of what can be controlled with a fixed pitch propeller, especially under windy or gusty conditions.

    Also, large multis handle very ponderously and do not respond quickly.

    This is not the case for helis, at any size, that variable pitch rotor lets them maneuver like lighting.

    Yes some really big ultra high performance helis have managed to kill a few people, generally being flown at their maximum 3D limits and without adequate safety precautions.

    And if similarly sized high performance multis were being flown that way they would pretty much represent the same threat.

    And Chris, you miss the point, the control issues on a big fixed pitch single propeller craft are not soluble, as a matter of fact the bigger you make a fixed pitch propeller for vertical lift the harder to control it it becomes.

    This  whole community has already been there and done that and the single copter or even coaxcopter have very marginal control under the best circumstances and they have very low performance and lack of control under all but the most optimal conditions.

    This is a simple result of this design, they don't work well at any size and work worse the larger you make them.

    It is possible some sort of surface effect design like Coanada can be made to work - sort of, but I greatly suspect it would be very low performance and severely lacking in controlability under sub optimal conditions as well.

    Best Regards,


  • MR60

    Another argument explaining the low use of helicopter or single rotor drones is their dangerousity. The long and strong blade that is required is deadly (many RC hobbyist actually got killed by the blade of their RC helicopter). It is due to their high rotation speed , mass and sturdiness (they can't brake). On the opposite multicopters are turning much lighter smaller blades that break easily upon impact.

    • I have to disagree with Hugues,

      If you're comparing a 700 class heli with a DJI Phantom then ok, fair point, however, if you're comparing a similarly capable quadcopter (Capible of 10kg of payload) the risk is actually considerably higher for the multirotor. For a quadcopter to handle this you would need 25+ inch blades and would weigh 20+ kg, if there is a failure of a power-system in a quadcopter it instantly falls out of the sky at free-fall and will crush anything below it. With a Helicopter, you have the chance of auto-rotating down safely and so long as you still have servo control, even without servo control, due to the disk still spinning, many times it comes down much slower than free fall like a large multirotor. 

      After working with large quadcopters and helicopters for UAS for the last 2 years, I have seen much much more failures and issues with large quads than I have with heli's (ESC sync issues, lack of stability, lack of response time when turning large props, etc). For small applications like a small camera in the sky, multi's are great, for any real work that needs efficient long flight times and heavy payloads, a helicopter is far superior, including for safety.

      • MR60

        @Jason, you are mixing the functionality argument with which I agree helicopters are more efficient , and the safety argument for which helicopter blade contains much more energy (due to: engine torque, blade mass, rotation speed, blade material made to resist impacts) than a single blade in a multi rotor. 

        Further, history shows nobody ever died from a multi rotor blade impact while lots of death cases happened with helicopters. 

        These are facts, not an opinion.

        • 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.

This reply was deleted.


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