Hi guys,

            Rather then re-invent the wheel, i thought i would ask questions here, as this is the place for real answers.

Im working on a UAV which will have 2 sets of coaxial rotors, the blades are off the shelf types with a diameter of 425mm. I want to use brushless motors and my power source, both voltage and current wise is practically unlimited, so power worries are not an issue.

I am looking basically at the end of this, the amount of thrust i can expect from a certain set up. I realise there are many factors, such as rotor RPM (my limited googling reveals a rotor speed of between 1800 and 2000 RPM is common, but with available power, i can go higher?)

Both sets of rotors in one unit can have 2 blades or more, i am familiar with wind generation and the benefits of adding an extra blade to a turbine, does the same rule apply here in reverse.

So if you started with a clean drawing board.....!

Whats the ideal amount of blades per rotor, 2, 3 or 4 etc?

What type and size of brushless motor would be ideal for the highest available thrust?

What sort of thrust can be expected in Kg's?

I know i can use the brute force approach, a 1 HP motor, 10 blades and 5000 RPM and see how it goes, but i would like to be a little more scientific then that!

Many thanks,


Views: 1447

Reply to This

Replies to This Discussion

I can't answer most of your questions, but I can tell you that 2 blades will absolutely be more efficient than 3+.  The reason is that as the blades slice through the air, and each one travels in the wake of the one in front.  The more blades there are, the closer they are to the wake of the one in front.

If you consider the viscosity of air, etc, you'll see that there's a huge difference between a gigantic wind turbine moving at... 2 RPM?  vs. a tiny propeller moving at 4000 rpm.

Furthermore, I would suggest for simplicity sake, buy motors designed for 425mm blades.  What size heli is that for, a 500 class?

One aspect you will run into is the fact that the vast majority of heli blades on the market are symmetrical profile for aerobatics, and do not provide efficient lift.  You'll want to look for flat bottom or "semi-symmetrical" blades.  Let me know if you find any. ;)

Thank you Robert, that answered some of my questions, and with the answers you gave, i googled for more info that answered others. As to the size, its a custom build with 2 sets of 2 blades in 2 seperate units, so i cannot strictly give you a size based on conventional sizing, except to say the unit will be between 1,2 and 1.3 metres long.

As to the number of blades, i was rightly or wrongly applying theories applicable to wind turbine generating and the % difference between 2, 3 and 4 blades, but again i find that the increase in efficiency is only 3% between 2 and 3 blades, not really worth the effort!

Do you have any rough idea what the rotor speeds should or could be?


Sorry, I misread when you said 425mm, that's diameter, not blade length.  425mm blade length is for a 500 class, but it sounds like you're building something "250 class" as they use blade length of about 200mm.

Yeah, these fast turning blades don't have much in common with turbine blades.

Sorry, I don't have any idea on rotor speed.

You'll have to ask around.  Or, you could buy one of these and reverse engineer it to a certain extent.


Looks like this is a common motor people use.


The whole project started a year ago with me buying a small heli and fitting a walkera basic system to it. I played with this at the Uni in Jakarta, and got lots of interest, thats where this story starts.

We went his route and played with bits from the local model shop and went from there, but now the project is reaching a more serious level.

The motors though i feel would be underpowered.

As an engineer, but with NO RC experience, i tend to crack a nut with a sledgehammer, its the finesse of people like you, that allow me to refine, im confident i can put more back into the forum when im up to 'speed'.

It was the first heli that got me enthused, i played with that, and after mastering the art of crashing it, we took the bits, to play with just a simple hovering co-axial but with gyro feed back, the aim was to get a UAV totally stable in loiter mode, no more


Dr. John, your questions beg a slew of other ones.  First off, Robert is quite correct; a 2 blade configuration is always going to be more efficient than >2.  While I have not researched wind power generation specifically, it seems intuitive that the same rules would apply (at some level it's all fluid dynamics).  The reason I understood the vast majority of windmills are 3-bladed has everything to do with preventing mounting pylon stall (one blade perpendicular to the ground would have its flow occluded by the pylon, leaving only one blade fully exposed to the wind). 

Please do tell why your power source is unlimited?  What is the application for this?

The ideal number of blades is 2 until you start trying to produce so much thrust in such a small area that you begin to have to worry about compressibility effects (i.e. the pressure differential above and below the rotor place becomes too great).  This is why you see lots of blades in compressors, such as at the intake of jet engines.  Full scale copters go to more blades to raise the vibration fundamental frequencies (easier to attenuate) and for more agile control responses (there's always a blade nearly at perpendicular to the pitch or roll axis).  However, the designers are fully aware that this makes the copter less efficient (it's rolled into a factor called rotor disk solidity ratio which is a variable in the denominator of the figure of merit equation). 

So given no constraints of mission, payload, or airframe design (and not to be a smartass), the type and size of brushless motor for highest thrust would be the biggest, baddest torque producer you could possibly afford.  I am sure that's not the answer you were looking for.  :-)

That said, in this art space, one horsepower doesn't strike me as anything approaching "brute force"; most of the midrange brushless motors (746 watts or about 40 amps with a 6C LiPoly) can easily surpass this at the shaft.

If you want good advice, we're going to need more context.

Good stuff Brad.

Your point about multi-blades and fundamental frequency... would this mean that for Aerial Photography situations, if vibration is a problem, that potentially using a 3-4-5 blade rotor head might help in some way, albeit at the expense of efficiency.

And how come extra blades change the fundamental frequency?  Isn't that frequency related to the head rpm?  Or is it more strongly related to how often a blade passes over a given point?

@Robert: There are people who make careers in the analysis of rotorcraft noise and vibration; to be sure, I do not consider myself an expert by any means.  You're on the right track- it's all about perturbations of the air column per second, which is directly related to both RPM and the number of blades.  As a practical observation, those 2-bladed Hueys shake the ground for miles, whilst the MD-500s (OH-6s) with the 5-bladed disks seem to just buzz loudly at comparatively shorter distances.

Of course, that's with one main rotor.  If you start mixing in 3 or more other rotors, the complexity of the acoustic signature analysis (and vibration) rises proportionally.  The best thing to do is make the airframe as rigid as possible (low frequency frame resonances will drive you nuts) and isolate the camera.  I can conceive that moving to a three-bladed prop might help in some circumstances, sure.

@Brad: I have been reading some papers on coaxials as I run an X8 multi rotor. I have started to consider prop efficiency. currently I use Graupner electro 13" top and bottom. My reading suggests the slipstream of the upper rotor disc concentrates loads more to the inner half of the bottom disc. This makes me think I need more of a 'paddle' style prop on the bottom as these have a bigger surface area in the inner half. Although I don't see any commercial helicopters using different rotors on the bottom (possibly countered in pitch control)


@Troy: I think you're on to something.  The induced power factor (efficiency penalty) varies from 1.4 when the coaxial rotors are spaced very closely together to 1.2 when the distance is enough for the upper rotor to establish a full slipstream to feed the lower rotor.  Flow contraction could very easily explain this phenomena.

It would be worthwhile to experiment with higher-pitched bottom props, hypothetically using a 13 X 5 on the top and a 13 X 8 on the bottom for example. Otherwise, you're left with the prospects of making your own props.


                My apologies for my late replies.

After much hunting around, for various bits of data, most i find, are confusing, so the good old fashioned 'gut feeling' is playing a major part in this!

So far ive semi decided on a 3 bladed sytem with a H/D of around 0.09 and 0.12

The power source is a small nitro or diesel engine, when i return to the UK next week, i will play with a small Kyosho engine i have that delivers 1 HP at 29,000RPM There will be a Li ion battery to give approx 5 mins of flight in the event of engine failure. MAXIM the ic manufacturer are working on the power management solution, and Sullivan in the states who specialise in DC generators for UAV's and small RC aircraft are looking at a custom generator, theoretically, the Sullivan generators, give max output at 5000RPM, so a 5:1 reduction will be needed, offering a theoretical 5 HP or 3.7Kw

Rotor speeds, again i have received conflicting information, ranging from 2000RPM to over 6000RPM

I am curious about Troys comments, and will try and do a little reading on this to learn a little more.

At the end of the day, if you had to create a wish list for a unit as described above, and wanted to get the maximum thrust and efficiency out of it, would you go for a 2 bladed design, or a 3 bladed design, and what commercial blades would you go for?

Some idea of thrust would be useful, otherwise its going to have to be the 'brute force' method and measure what i get.


That's a really interesting idea Brad.  More pitch on the bottom.  It makes some sense that the bottom prop would need more pitch because the airflow is already moving downward.  Sort of how multi-stage turbines work.

Reply to Discussion


© 2019   Created by Chris Anderson.   Powered by

Badges  |  Report an Issue  |  Terms of Service