# The Case for Large-Scale Electric Multicopters

This discussion thread is a follow-on to several conversations I've had with people in the forums who are particularly interested in the aerodynamics of vertical take off and landing (VTOL) aircraft.  Much of the dialog in these forums appropriately surrounds the mechanisms for robotic automation of VTOL aircraft, and in those contexts, I am much more a listener than a contributor.  There are some brilliant engineers, code smiths, and experimenters who frequent these hallowed pages.  The group effort to yield such a marvel as the APM platform is nothing short of astounding.

However, I think we can all agree that the primary functionality of anything that flies is related to how it generates forces to oppose gravity.  Much of the focus here has been on the control system, for a myriad of reasons.  Seemingly ignored is the aerodynamics of propeller thrust, but fairly speaking, it is unromantic as having been largely figured out 90 years ago.  In fact, here's a link to the NACA (forerunner to NASA) original paper entitled "The Problem of the Helicopter", dated 1920.  It is of interest to note that we widely applaud Sikorsky for inventing the modern helicopter, but his contribution was one of a control scheme; he gave us cyclic pitch variation for thrust vectoring coupled with a variable pitch tail rotor to counterbalance torque.

http://naca.central.cranfield.ac.uk/reports/1920/naca-tn-4.pdf

If technical papers like that make your eyes glaze over, perhaps an essential basic treatise is in order.

We go back to Newton's basic laws here, and one in particular: Force=Mass X Acceleration, or F=MA.  In order for our craft to fly, we need it to generate a force equal to and directly opposing the force of gravity.  To produce this force, we normally take the air around our craft as our readily available mass, (except in the case of the rocket and to some degree, the jet engine, where the mass is a product of combustion), and accelerate it (add to its velocity) toward the ground.  Yes, rotors, wings, and propellers all do this, and they all rely on the same principles.

However, there is another factor to consider.  While this particular law is not attributable to Newton, it is still a primary expression: energy is equal to half the mass times the velocity squared, or E= 1/2M X V^2.  So while the lifting force is linearly proportional to mass and acceleration, the energy required to perform the acceleration increases exponentially with the change in velocity.  It naturally follows, then, that taking a lot of air and accelerating it a little takes a lot less energy than taking a little air and accelerating it a lot.  This is why heavy-lift helicopters have such large rotor spans, and their technically analogous cousins, sailplanes, have long wings.  (I drive some people in the pseudo religion of ducted fan technology crazy by pointing out that all their purported efficiency gains can be had by merely making the propeller blade longer...ah, but I digress...)

In the final analysis we must be concerned about lifting efficiency.  The basic expression for us in comparing efficiencies of different designs can be simplified to merely the number of watts (power) it takes to produce a pound of thrust (mass).  Of course, we cannot simply make our rotors infinitely large and fly with no power expended at all.  There are therefore some engineering compromises which must be made in a VTOL aircraft design.  I hope you can see now why aerodynamic designers first examine the ratio of lifting surface area to the weight lifted as an indicator of potential efficiency.  In the rotary wing world, this ratio is called disk loading, and it is expressed as so many pounds per square foot of total rotor swept area.

Disk loading is a basic predictor of hovering efficiency, but it is by no means the only one.  In my next message, I'll get into evaluating basic rotor (or propeller) blade design criteria.

I hope you've enjoyed this little introduction, and yes, I do plan to eventually show that electric multicopters can be a very viable solution for large payloads compared with conventional helicopters.  However, we need to "level set" on the concepts.  Let the discussions begin.

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### Replies to This Discussion

Here's another in the news recently:

Google's Larry Page claims this will be in production within the year (and is already 'legal to fly')

I do not have the same experience with the quite efficient  hexas (11,5 gr/watt ) with 16" props.

I notice that in no wind conditions the energy used to move at 5 m/s was only 5-7% more that the energy used for hovering , and increase the speed to 10 m/s request only  another 5% - 8 % .

I think that the reason is that the largest amount of used energy is to keep the multirotor in the air without any aerodynamic help while when it flies or more correctly it translate  , there is just the friction of the air and the  propellers might play a role.

I agree about propellers size, their weight is crucial and large propellers weights a lot  I think also because of a commercial choice.
Over 17" there are only few manufacturers and the propeller is a "quality" product with a high price so it must meet different needs as an high load and that means a robust design and a high weight to catch the maximum numbers of clients.

Some companies made multirotors with 22-35 kg AUW , for heavy load , IMO these multirotors are an hymn to inefficiency , intrinsically unsafe devices;  a heli is much better for heavy payloads I totally agree with Rob Lefevre.

My best selling  multirotors are a light quad with 45' fly time , 250 grams payload , 12,1 gr / watt  and a hexa with 30' fly time and 1100 grams of payload , 11,3 grams / watt, both with 16" props.

Rob_Lefebvre said:

CUT...

There's something else that I think is not talked about often.  I haven't figured it out the theoretical reason yet, just observed it in practice.  And that is the incredible amount of power increase required for multirotors to fly with lateral speed.  My helicopter hovers on about 50A, but that drops to 40A by 15 m/s due to the benefits of translational lift.

My Octocopter however, hovers on 35A, but it's pulling 80A if I hold it on the 30 degree pitch limit, and it's only doing 13 m/s.  Why?  I don't think it can be attributed purely to the increased airframe drag.  I think it is probably due to the airflow through the rotors.

We know that with helicopters at high flight speeds, some of the retreating blade is actually subject to reverse flow.  The airflow is going over the blade the wrong way, and it's destroying lift.  I suspect this effect is even greater for multirotors.  Particularly those designed for long duration, with very large, slow turning propellers.

This thread that Brad started in 2012 remains very interesting! I notice that over the years I have come back various times.

I have build a "large scale electric multicopter" for personal flight and together with a small group of people are in the process of building a much more  capable one. A link to our first manned flight can be found here: https://youtu.be/Omv_WdryGRc

For our next prototype we're using a modular setup that will enable us to try out various motor/propeller combinations. One of the things we haven't decided on is the exact motor/propeller combination. Our optional designs still include a hexacopter with 56" props to a more cluttered design with 24 drives in stacked formation.

Though the hexacopter should be the most efficient (at least in theory), I'm worried about the agility of such a set up. You can also read it in some of the posts in this thread but whith props over 50", inertia becomes a real topic.

I would be interested to hear what the people of this forum think what would be the best motor/prop combinations for a small person carrying multicopter with a take off weight of ±210 kg?

Regards,

Peter

Rob!

Thanks for your kind words. I've been focusing on my day job for a while to continue my self-funding. In this thread I have tried to stay in the theoretical realm. Talking about what I am (or anyone else is) planning to do would just spur a heated, unproductive debate.

You did make a number of points about which I'd like to offer an opinion.

You're right; Olaeris seems less real today than it was before. They did have some flight videos up but they're obviously gone now.

http://web.archive.org/web/20140503035958/http://www.olaeris.com/vi...

"...Anyway, the machine can hardly deal with 5 m/s lateral airspeed without flipping over.  I have a helicopter UAV, with similar specifications.  Only it's smaller, lighter, uses the same exact batteries, and it can fly for the same period of time.  But it can fly at 30 m/s in complete control..."

A fixed pitch angle optimized for hover will have an effective angle of attack with inflow of around 5 degrees or so (integrated across the disk, yada yada). The instant you start flying forward, that AoA will drop, and 5 degrees isn't a lot of room before the airfoil becomes useless. With these skinny foils common in model airplane propellers (for reasons previously discussed), you can't begin with a higher pitch or you'll start out with a partial blade stall in hover which will obviously ruin efficiency.

A single large blade copter - by necessity - will have a fatter ~12 t/c (like the ubiquitous NACA 0012) because the AoA and pitch will be changing constantly. At a head speed of 1500 RPM this translates into a torsional frequency of 25Hz, which is decidedly non-trivial considering flapping/teetering, unbalanced load mitigation, resultant vibration, and all the other mechanical variables with cyclic pitch. If you have a mechanical control system flexing a blade that fast with all the other stresses, the aerodynamic loads (like pitching moment) start to matter a great deal, making airfoil selection much more limited.

"I agree with Gary, that there's a limit beyond which multirotors using fixed pitch propellers, begin to have serious difficulties with control.  Only, IMO, its around 18" where it starts happening."

I agree with Gary too. Rotational inertia is 1/2 mass times radius squared, adding commensurate delay to your control system. That's why Dr. Pounds went to exotic ducts; if he made the blades any longer, control purchase would be compromised. One of the major points from his first paper 15 years ago was that placing the CG under the lifting plane reduces stability even more. That's why you see so many "people mower" designs lately; they're struggling to maintain control, with an obvious compromise to safety and ergonomics.

"There's something else that I think is not talked about often.  I haven't figured it out the theoretical reason yet, just observed it in practice.  And that is the incredible amount of power increase required for multirotors to fly with lateral speed.  My helicopter hovers on about 50A, but that drops to 40A by 15 m/s due to the benefits of translational lift."

Transitional lift is definitely a thing, and in forward speed the induced power at the disk should be about 30% LESS according to Leishman (and Stepniewski and every pilot I've ever talked to). Why do fixed-pitch multicopters fail to show this effect? See "fixed-pitch skinny foils" above.

"My Octocopter however, hovers on 35A, but it's pulling 80A if I hold it on the 30 degree pitch limit, and it's only doing 13 m/s.  Why?  I don't think it can be attributed purely to the increased airframe drag.  I think it is probably due to the airflow through the rotors."

Absolutely. See "fixed-pitch skinny".

"We know that with helicopters at high flight speeds, some of the retreating blade is actually subject to reverse flow.  The airflow is going over the blade the wrong way, and it's destroying lift.  I suspect this effect is even greater for multirotors.  Particularly those designed for long duration, with very large, slow turning propellers."

Retreating blade stall is an issue everywhere. With a single rotor helicopter, it is usually akin to having your left wing fall off in a fixed-wing ship. Theoretically, I believe it would be less of an issue in a multicopter because of some maintenance of thrust symmetry across all the rotors. In the metaphor, it would be like losing only part of your wing.

It's nice to see that people are still interested in this stuff.

@ George Kelly - Part 103 is a beautiful thing indeed. Keep it under 254 pounds empty, don't fly it like an idiot, and all is OK with the g-men regulators. That's subject to change, of course, at the first incident caused by a sufficiently talented idiot.

George Kelly said:

"Google's Larry Page claims this will be in production within the year (and is already 'legal to fly')"

Yes, I hadn't considered Dissymmetry of Lift.

Though, would a pair of large, fixed pitch co-axial rotors in the centre remove that problem? (albeit with attendant co-axial issues)

That would take care of the roll torque from the main motors, but not the flap-back (pitching up moment) torque in forward flight.

Why did you use motors that do not have higher capacity for work that would allow the octocopter to be more controllable?

Because those motors would be so much heavier, and operating in a less efficient range, that the flight time would greatly suffer.

This is one of the big design conundrums with fixed pitch multirotors.  For maximum efficiency, you want the smallest motors, turning the largest propellers.  But then you want those motors to have enough power to be able to speed up and slow down those propellers for stability control.

What made you choose 18" rotors over any other size?

Totally seat of the pants engineering.  That's just my gut feel and what I've observed to be true.

Don't get me wrong.  There are large multirotors with 30" propellers that fly well.  It's just that they only fly well for 10 minutes.  The entire design is optimized the other way, for dynamic performance.

It's difficult, or nearly impossible, to achieve both dynamic flight performance, and long duration, on very large multirotors.  The only way it can be done, is what Brad was suggesting.  Instead of a few very large propellers, you use many small propellers.  But then you have tradeoffs for cost, complexity, etc.

Google's Larry Page claims this will be in production within the year (and is already 'legal to fly')

You can buy a single seat helicopter, that will do 80mph for over an hour, today, for about \$50,000.

The only advantage to the KittyHawk, is that it has an autopilot so anybody can fly it.

So put an autopilot on the kit helicopter. <shrug>

Retreating blade stall is an issue everywhere. With a single rotor helicopter, it is usually akin to having your left wing fall off in a fixed-wing ship. Theoretically, I believe it would be less of an issue in a multicopter because of some maintenance of thrust symmetry across all the rotors. In the metaphor, it would be like losing only part of your wing.

Yes, in a multirotor, the rolling moment in lateral flight cancels out across the sum of the disks.  However, there is still the pitching up moment that remains.  This ends up being resisted by the autopilot system.  You can see it as the forward motors are throttled back as speed builds in order to maintain the target pitch angle.

This is something that is also terrible about large/efficient multirotors.  The larger, slower turning propellers, have even more pitching back moment.  The large Octo I referred to, was limited to a flight speed where the forward motors were nearly stopped.

Actually, it could fly above 5 m/s OK.  The problem was stopping.  When you pitch up to decelerate the machine, the pitching up moment seems to suddenly increase.  I'm sure this could be explained aerodynamically but I haven't done it yet.

When pitching up to decelerate, or even just coming level to coast to a stop, the forward propellers would nearly stop.  It tell you, it's mind-bending, to see this really large machine flying, holding level, with the forward motors nearly stopped.  You can see the sum of the thrust vector is nowhere near the CG of the vehicle, and by rights, it should be falling nose-down.  But it doesn't.  It just hangs there.  That's the pitching up moment from the propellers.

I wish I could show some of these videos, it's fascinating to see, but it's not my machine and I can't.

It is interesting to me that the Kitty Hawk works as well as the video seems to indicate it does.

The relatively few large fixed props would seem to indicate that control should be marginal at best.

I imagine the video was achieved over several attempts in totally calm air, even light wind or gusts should be able to easily over power this thing.

Optimal computer flight control is probably an absolute necessity to make it even barely flyable under even optimal conditions.

A lot of the flights seemed seemed like they might have been in the region of compression ground effect also meaning possibly working more like a hovercraft than a multicopter - significantly changes flight dynamics.

My personal view is that with standard fixed pitch props this will never be a commercially salable product, far too twitchy for normal people to use even under ideal conditions.

And my guess is achieving even a 5 minute flight time will prove daunting to say the least.

We will see, One thing that is definitely true is that my predictions do not always come true.

Frankly the 1 person helicopter is infinitely more useful and practical.

And the really cheap build it in your own garage Bensen Gyrocopter actually worked remarkably well.

Of course what goes up eventually comes down and there were always a few idiots who insisted on doing it in the non-prescribed fashion, killing themselves and eventually the Bensen in the process.

Best,

Gary

Gary,

Thing is, I don't really considering anything shown in that video to be dynamic flight.  It's just really easy hover manoevering.  Compare that the performance shown here, skip ahead to 1:50 and hang on to your hat.

Also note that the KittyHawk is using very powerful motors compared to the disk size.  This thing probably only flies for 5, maybe 10 minutes tops.  You have to think of this on a 3D graph, not 2D.  You have size on the X-axis.  And "dynamic performance" on Y-axis.  The larger the size, the less dynamic performance you can have.  But there's a third axis, and that is relative power or disk-loading.  The lower the disk loading, the lower the dynamic performance potential.  So as you combine size, and low disk loading, dynamic performance really falls off.  And it's the disk loading/relative power that you need to achieve long flight times.

There are people using Multirotors for Hollywood shoots, that are large and can lift a pair of Red Dragon cameras.  They are fairly dynamic.  But only for about 10 minutes at a time.

Hi Rob,

That is one really cool little copter alright, and \$50,000.00 is actually an amazing deal with that lovely little turbine.

Frankly I seriously doubt the Kitty Hawk can manage 5 minutes, really quickly hit the weight design limits of that system and extra battery weight would make it not be able to get off the ground at all.

I think they went to a lot of trouble making the video they did (5 miles from my last house by the way) and I think it has very little potential to be a financially sound venture.

I think a practical (at least somewhat practical) manned multi can be built, but it isn't this one and it may just be a battery with significantly more energy is required first.

There are some in development but it's going to be at least 5 to 10 years before really significant new battery tech becomes actually available.

I also do not believe true fixed pitch props stand a chance of being genuinely viable or practical on a manned multi.

We will see, I've stuck my foot out there, will it end up in my mouth, I'm guessing not.

If I were younger, lighter and richer, I'd seriously be looking at that little copter.

Best,

Gary

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