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

  •  As something related to T-Motor, I bought six of their 80A ESC's for $70 each. They left the factory receipt in the box with each ESC costing $6 each to manufacture. Funny how amazingly cheap these electronics are to make. 

  • Interesting points Lu and Stmp,

    I have used quite a few KDE motors on 400 and smaller sized quadcopters with up to 18" props but have not had occasion or need to operate them near maximum.

    I have found them to be excellently reliable and they have good design with quality proper ball bearings that are also easy to replace.

    KDE claims to be able to have a high max power and current because of the use of high temperature epoxy and windings, but I have never had the slightest urge to operate at that end of the spectrum, I operate at the high efficiency low power end of the spectrum and KDE performs as claimed there at least in the various motors I have tried.

    They have also operated within suggested Xcopter calc determined efficiency ratings, so I have been happy with them.

    But max performance is a different story 4 or 5 years ago there were some people on DIYD who set up static rigs and definitely they found exaggerated claims in the widely available cheap Chinese motors of the time.

    But nothing recent.

    It would really be nice to see what some static thrust tests showed on some of these "quality" motors versus manufacturers claims.

    Just a thought.

    Stmp, What U8s and props do you have and what do you want for them (it really is OK to post that here).

    Best Regards

    Gary

  • Gary McCray said:

    It is a market they are trying to create, not one that actually exists.

    I can attest to that. Have been trying to unload 6 new U8 with props and can not give them away.  Would rather rewind them or come up with a risky test scenario with guaranteed entertainment value for them before accepting the $500 low ball offers I've received.

  • About T-Motors and other hardware companies I would like to underline that they  often lie about the performances of their products , I had  a long discussion with T-Motor because of 20% less performance than their data sheets even with the same propeller and esc for one of their motor, the only final argument from them was that "air is different in China" .

    I saw also motors manufacturer that in their Cartesian chart had a X axis with variable interval to "domesticate" the performance curve .

    I do not like much KDE Direct too, they claim unreal performances.

    The result of this marketing rampant dishonesty is that you have to buy and test everything because you cannot rely on RC manufacturers even the more famous as T-Motor.

    Gary McCray said:


    Besides, T-motor is and was always a bit over priced but they make good stuff, better value and truly great performance is the KDE Direct stuff you recommended and I have been using ever since.

  • Hi Rob,

    Sorry if I've given the wrong impression, I totally agree, that the Phantom and Mavic (and Solo) class quad copters and the newer smaller Spark and even smaller ones have a fantastic future and because of their simplicity, reasonable flight times and improvingly intrinsic safety will proliferate at thousands to one over helis and they should.

    Its in bigger heavier stuff with longer endurance and larger payload capacity that helis edge makes them a truly superior solution.

    Arguably some of the small current multis can even handle pretty significant wind and gust conditions adequately.

    As for manned ones, until battery technology gets a lot better neither electric helis nor multis have a practical future.

    As faddish toys maybe but not actually good for anything.

    I cant wait for Amazon to show up at my door with that anvil I ordered in a multi (or heli) for that matter. :)

    Best,

    Gary

    Rob_Lefebvre said:

    Gary, I actually think that for basic hovering operations, or flight at low speeds, or in light wind conditions, multirotors actually make more sense.  You basic consumer aerial video, or even professional stationary aerial video, a multirotor makes more sense than a helicopter.  Efficiencies can be comparable.  They are simpler, more robust, etc. 

    It's really only once you start wanting to fly at speed, or in heavy winds, where there is no competition. And that's where I don't really understand the push for these manned electric multirotors.  They can be used for fun, cruising around, experience of flight, etc.

    But there are a lot of companies developing them as a means of transportation.  And it just doesn't make sense.  Same deal with drone deliveries.

  • Hi Rob,

    Just to respond to parts cost differential: I think the market for 30" T-motor propellers is vanishingly small, they are virtually a custom item, same thing for their largest $600.00 motors.

    It is a market they are trying to create, not one that actually exists.

    Reality is at the large end, Helis are more mass produced than really large multis.

    An interesting turn of events.

    Besides, T-motor is and was always a bit over priced but they make good stuff, better value and truly great performance is the KDE Direct stuff you recommended and I have been using ever since.

    Best,

    Gary

    Rob_Lefebvre said:

    Yes.  Again, my intent here is not to say that electric multirotors have no place, or that helicopters are just better. It's just to have an honest discussion about the issues. We can't advance the technology if we don't tackle the issues and problems head-on.

    As Brad points out, the cost of large electric multirotors is currently much too high, and there's no need for that.  As one example, the price that T-motor charge for their large propellers is unfounded.  Why does it cost about $300 for a 30" propeller, when you can buy 70" helicopter propellers for $150?  Why are large multirotor motors more expensive than similarly sized helicopter motors?  Etc.etc. 

    I think a big part of the problem is just lack of competition in the market.  T-motor has a commanding position in this market, and nobody is competing.  I'm not sure if it's just that demand is not high enough to justify multiple companies.  Because the helicopter market isn't that big either, just a lot of boutique manufacturers. 

    The KittyHawk appears to be using Hacker brushless motors.  Hacker is an old manufacturer that used to target the RC Airplane field normally.  But they produce some very powerful motors in the KV range necessary for these very large multirotors. The price on these is not unreasonable.

    Oh, and just to reinforce the point about easy to fly, here's a Mosquito with a full autopilot onboard.

    http://targetsystems.qinetiq.com/en-ca/products-and-services/rotary...

  • Gary, I actually think that for basic hovering operations, or flight at low speeds, or in light wind conditions, multirotors actually make more sense.  You basic consumer aerial video, or even professional stationary aerial video, a multirotor makes more sense than a helicopter.  Efficiencies can be comparable.  They are simpler, more robust, etc. 

    It's really only once you start wanting to fly at speed, or in heavy winds, where there is no competition. And that's where I don't really understand the push for these manned electric multirotors.  They can be used for fun, cruising around, experience of flight, etc.

    But there are a lot of companies developing them as a means of transportation.  And it just doesn't make sense.  Same deal with drone deliveries.

  • Hi Peter,

    In the UAV area where we normally are there really isn't any difference in control of a heli or multi, they both use the same fly by wire control.

    Comparing ease of controllability of a manual manned helicopter to a fly by wire manned multicopter is not a reasonable comparison.

    Fly by wire heli systems are and would be no more difficult to fly than a fly by wire multicopter system.

    Even for a normal manned fixed wing pilot license a $20,000.00 plus investment is necessary for even the most rudimentary basic pilots license, at least it was 25 years ago when I got mine, and I doubt it is cheaper now.

    However, not really trying to promote manned helis here, just pointing out that in our little UAV arena, they are superior to existing multicopters in many ways and a lot of the current applications could be better filled by them.

    This is really, just a fact and has to do with the higher performance and efficiency capabilities versus equivalent multicopters.

    It is possible that exotic multicopters with ducted compression fan blades as shown in Brad's link may truly compete with helis but that remains to be seen.

    In fact what has made helis even a reasonable choice now is that they are actually making rugged and robust variable / cyclic rotor hubs.

    That was really the missing ingredient.

    Also, thanks to tireless work by Rob Lefebvre, the fly by wire control issues have been solved and they can out fly any multi.

    It is a simple fact that for UAVs helis are more efficient than multis and I doubt anything can change that.

    That having been said, what keeps small manned helis expensive is that they have a tiny market, if the market were larger, component costs would come down and redesign would reduce maintenance costs.

    One of the real advantages of a multi is that it uses a quantity of relatively simple ingredients, each of which which can be used on various platforms.

    So individual component costs can be kept lower.

    Electric multicopters in particular have very few moving parts (Motor armature and propeller) which can be cheaply manufactured very robustly.

    Helis may only have one motor, but their hubs and rotors are very complex and difficult to manufacture robustly and generally they are only good for one brand and size of helicopter.

    So cost of production for (reliable) helis tends to be relatively high.

    Eventually I have no doubt that the cheap reliable manufacturing possibilities for multis will make man powered ones at least somewhat reasonable for 1%r look what I got, very very short flight use.

    But for them to be practical at all is going to require a very significant leap in battery energy density over current lithium technologies, and I don't think even doubling it would do it, I think they really need to be 4 or 5 times higher energy density.

    Frankly a battery powered manned heli you could make right now would (and always will) outperform and out last any of the current manned multis by a considerable margin.

    The bottom line is you can't beat disk diameter - period.

    I learned that from Rob, and it is simply a physical fact.

    Best Regards,

    Gary

  • Yes.  Again, my intent here is not to say that electric multirotors have no place, or that helicopters are just better. It's just to have an honest discussion about the issues. We can't advance the technology if we don't tackle the issues and problems head-on.

    As Brad points out, the cost of large electric multirotors is currently much too high, and there's no need for that.  As one example, the price that T-motor charge for their large propellers is unfounded.  Why does it cost about $300 for a 30" propeller, when you can buy 70" helicopter propellers for $150?  Why are large multirotor motors more expensive than similarly sized helicopter motors?  Etc.etc. 

    I think a big part of the problem is just lack of competition in the market.  T-motor has a commanding position in this market, and nobody is competing.  I'm not sure if it's just that demand is not high enough to justify multiple companies.  Because the helicopter market isn't that big either, just a lot of boutique manufacturers. 

    The KittyHawk appears to be using Hacker brushless motors.  Hacker is an old manufacturer that used to target the RC Airplane field normally.  But they produce some very powerful motors in the KV range necessary for these very large multirotors. The price on these is not unreasonable.

    Oh, and just to reinforce the point about easy to fly, here's a Mosquito with a full autopilot onboard.

    http://targetsystems.qinetiq.com/en-ca/products-and-services/rotary...

    QinetiQ Target Systems | Products and services
  • @Peter Dobber - You've made an excellent point . The bottom line: how to achieve lifting platform reliability?

    Conventional single-rotor helicopters are reliable because they set a goal of attaining and maintaining mechanical perfection. Think of a flying Rolex watch with a multitude of single-points-of-failure. Most of "parochial" aviation has this basic tenet - nothing can be allowed to fail because such failure could result in human injury or death.

    This paradigm makes conventional aircraft - especially helicopters - expensive to make and expensive to keep, piloting skills notwithstanding. There's that old joke about helicopters - you couldn't afford one if they were free.

    I started the present thread with the hypothesis that manned electric multicopters could be produced which compete with conventional helicopters in hovering efficiency. The potential economic advantages of the multicopter are obvious, especially if FAR Part 103 compliance can be achieved. But alas, this too must temporarily remain a hypothesis, as the earliest commercial examples command retail prices north of $200,000. That's crazy. For that kind of money, a 1%-er person who needs a helicopter will just buy a Robinson R-22 and be done with it.

    There is still much work to do.

This reply was deleted.

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