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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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).

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

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.

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.

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.

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,

Gary

"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.



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.



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.

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.   

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