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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      • 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.

            • Your issues with take-off and landing on fixed wing are exactly why I have been pushing helicopter development.  Because it solves those problems completely, while also being able to perform a similar role as a fixed wing in many applications. (ie: Simple, rugged airplanes, don't fly very long).

              • Rob, this is a bit off subject, however, do you by chance know if the internal vibration dampening in the pixhawk 2 will be effective for the lower frequency vibrations of traditional helicopters? Or is it basically designed to filter out high frequency vibrations of multirotors?

                • I suspect it will not work helicopters.  I already have some evidence that it does not work with very large (29" prop) multirotors, and helis will probably have even more trouble.  The standard setup is designed for higher frequency on smaller multirotors as you expect.  I have one on a quad with 15" props, and it is fine, other than the 3rd IMU which is not-isolated is causing vibration warnings, it should just be ignored, this is a code problem however that I'm sure will get fixed.  The PH2 could probably be made to work with some external isolation.  Even if the internal isolation did work, I would still prefer some external isolation anyway, especially in the case of a gas motor.

                  AUAV also is working on another product that I would consider for this application. 

                  I personally won't give my money to any of the multitude of Chinese vendors who are milking the Ardupilot ecosystem without contributing anything.  Whether it's a clone, or a derivative design.  These guys are taking advantage of this project and not putting anything back in.

                  • Where else, at present, can a person get quality hardware running APM?  I believe 3DR even went to China to build the Solo.

                    One of the considerations in what platform I use is that I do not own a Windows or Mac computer.  I've used Linux on my personal computers since 1998 - Debian all that time.  Over the years I've never seen an open-source project that doesn't fork off into multiple directions, and commercial vendors that take advantage of it, like Libranet which was re-packaged Debian sold for 90 bucks back in the day.  They all end up having their heyday and then disappear, and Debian is still here with one of the more popular distributions (Ubuntu) being based on it.

                    So I don't see the Chinese vendors as taking advantage of the platform being a bad thing.  It puts the platform in the hands of more people, which is after all, what the purpose of any open-source project is.  Not all people are developers.  Most are end users.  The more end users you got, the better the feedback is to develop the project.

            • +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. 

              • I was not aware that ArduPilot offers that option.  But I still cringe at the idea of using reverse thrust, having the wing stall and the aircraft landing anything but gently with an expensive NVDI camera onboard.  Or using a chute and having it get caught by the wind, causing an adventure to retrieve the aircraft a 1/4 mile into a corn field.

                I can't complain on the reliability of multi-rotor machines.  The biggest danger, of course, is a motor failure in flight with a quad-rotor.  However, in my experience the motors give ample warning before failure, and the usual problem is bearings. So I don't consider that a real issue.  The biggest issue is flight time.

                There is no doubt a single rotor aircraft would be better, and I may have to give that more serious consideration.  Not now in the peak of my flying season.  But a nice winter project here.

                Since the Pixhawk is not available from 3DR any longer I've looking for a source for them.  I found a kit from Banggood and not sure if it's the genuine article.  But I ordered one and will find out when it gets here.

                • www.pixhawk2.com 

                  Should be available soon. Also, I like the gearbest version of the pixhawk currently for alternatives to 3D Robotics.

                  The reverse thrust does not stall the plane, it simply slows the plane down to maintain landing speed at high angles of decent. So you could do a 60 degree landing and maintain your landing speed which may traditionally require a 10 degree landing pattern. 

                  Check out the Event 38 E386... www.event38.com

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