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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  • 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)

    What I imagined would be the big problem would be the central rotors amplifying the corrective thrust changes of the outer thrusters too massively, forcing the thrusters to provide even stronger corrections in the opposite direction, and so on, in an unstable feedback loop, followed by disaster.

    Analogous to the potential in current multi-rotors, I guess, just much more so. 

    George

    Rob_Lefebvre said:

    Unfortunately, no, I don't think so.

    You should get a copy of Gordon Leishman's Principles of Helicopter Aerodynamics, even if only to read the first chapter, which is a primer on the development of helicopter technology.  Many concepts like this were tried.  Actually, multirotors were tried before the standard helicopter design of today was settled on.  And many variations, some not unlike what you are proposing.

    The problem is, that you are ignoring the effect of disymmetry of lift on the large main rotor during forward flight.  This would need to be countered by the stabilizing thrusters.  And if the thrusters were large enough to counter the main rotor torque during forward flight, they would also be much larger than your theoretical design would want for efficiency.  Pretty soon, you would find you had surprisingly large thrusters, and surprisingly small main rotor.

    Another way you could solve the disymmetry of lift problem, would be to use articulated main rotors.  But then... since you're articulating them anyway to solve DoL, why not just go ahead and make them to cyclic control while you're at it.

    This parallels the development of VTOL flight.  There were no successful designs until they instituted cyclic control

    I actually designed a coaxial helicopter sort of like this years ago, and it didn't work out.  It used a fixed pitch top rotor, and a variable pitch bottom rotor with cyclic control.  In order to get good flight control, I had to increase the size of the lower rotor, while decreasing the size of the top rotor, so that the bottom rotor had sufficient cyclic authority to overcome the moment being generated by the top rotor.  But then, in order that the top rotor could still counter the torque of the larger bottom rotor, I had to add more and more pitch, until it was at the point where the top rotor had a silly amount of angle of attack and it was killing efficiency.


    It started out as an interesting design, mechanically simple, but eventually evolved into a disaster as we tried to actually make it fly.  

    There's a lot of good reasons why the conventional helicopter design was converged on from several different directions, and then has remained largely unchanged for 100 years.  It works.



    George Kelly said:

    Given the severity of the trade-offs - I have wondered if a hybrid approach might work.

    A large, central rotor similar to traditional heli, (or perhaps a co-axial pair for torque neutrality). But fixed-pitch, for mechanical simplicity and the possibility of optimizing airfoil efficiency.

    The primary, or sole, purpose of which is to do most of the sheer 'grunt work' of offsetting gravity, ie providing almost enough lift , or just enough, to get, and stay airborne, but no requirement to play a role in stabilization or lateral movement, thus allowing larger rotor diameters and higher efficiency, without control penalty. (though, of course, they would inevitably affect lateral movement greatly by default)

    Combined with a ring of smaller, electric rotors around the perimeter of the craft. Also fixed-pitch for simplicity.

    Freed of the bulk of the 'grunt work' of lifting, they could be smaller in diameter, thus conferring the benefit of responsiveness. Their primary responsibility is to stabilize, and move laterally.

    The central, lifting, rotors could be made gas-powered, to capture the (still) much higher energy density of gas, for doing most of the lifting.

    Instead of 'big (props) vs small', 'electric vs gas', 'fixed pitch vs variable', could we not have the best of all worlds in the same craft?

    George

     

  • Unfortunately, no, I don't think so.

    You should get a copy of Gordon Leishman's Principles of Helicopter Aerodynamics, even if only to read the first chapter, which is a primer on the development of helicopter technology.  Many concepts like this were tried.  Actually, multirotors were tried before the standard helicopter design of today was settled on.  And many variations, some not unlike what you are proposing.

    The problem is, that you are ignoring the effect of disymmetry of lift on the large main rotor during forward flight.  This would need to be countered by the stabilizing thrusters.  And if the thrusters were large enough to counter the main rotor torque during forward flight, they would also be much larger than your theoretical design would want for efficiency.  Pretty soon, you would find you had surprisingly large thrusters, and surprisingly small main rotor.

    Another way you could solve the disymmetry of lift problem, would be to use articulated main rotors.  But then... since you're articulating them anyway to solve DoL, why not just go ahead and make them to cyclic control while you're at it.

    This parallels the development of VTOL flight.  There were no successful designs until they instituted cyclic control

    I actually designed a coaxial helicopter sort of like this years ago, and it didn't work out.  It used a fixed pitch top rotor, and a variable pitch bottom rotor with cyclic control.  In order to get good flight control, I had to increase the size of the lower rotor, while decreasing the size of the top rotor, so that the bottom rotor had sufficient cyclic authority to overcome the moment being generated by the top rotor.  But then, in order that the top rotor could still counter the torque of the larger bottom rotor, I had to add more and more pitch, until it was at the point where the top rotor had a silly amount of angle of attack and it was killing efficiency.


    It started out as an interesting design, mechanically simple, but eventually evolved into a disaster as we tried to actually make it fly.  

    There's a lot of good reasons why the conventional helicopter design was converged on from several different directions, and then has remained largely unchanged for 100 years.  It works.



    George Kelly said:

    Given the severity of the trade-offs - I have wondered if a hybrid approach might work.

    A large, central rotor similar to traditional heli, (or perhaps a co-axial pair for torque neutrality). But fixed-pitch, for mechanical simplicity and the possibility of optimizing airfoil efficiency.

    The primary, or sole, purpose of which is to do most of the sheer 'grunt work' of offsetting gravity, ie providing almost enough lift , or just enough, to get, and stay airborne, but no requirement to play a role in stabilization or lateral movement, thus allowing larger rotor diameters and higher efficiency, without control penalty. (though, of course, they would inevitably affect lateral movement greatly by default)

    Combined with a ring of smaller, electric rotors around the perimeter of the craft. Also fixed-pitch for simplicity.

    Freed of the bulk of the 'grunt work' of lifting, they could be smaller in diameter, thus conferring the benefit of responsiveness. Their primary responsibility is to stabilize, and move laterally.

    The central, lifting, rotors could be made gas-powered, to capture the (still) much higher energy density of gas, for doing most of the lifting.

    Instead of 'big (props) vs small', 'electric vs gas', 'fixed pitch vs variable', could we not have the best of all worlds in the same craft?

    George

     

  • Brad!

    The best thread in the history of DIYDrones lives.

    That Olaeris AEVA machine.... count me as being skeptical.  Looks like nothing but a bunch of renderings to me.  Coupled with half an order of magnitude improvement over anything coming before, and not a single flight test video.  Throw in some cheezy looking marketing materials and...  I've seen this story told too many times in the UAV industry.  If it didn't claim Dr. Pound's involvement, I'd be sure it was fake news.

    I see so many small craft being marketed that I wonder if a larger rotor that is so much more efficient must bring with it new  technical issues not discussed in this thread.

    Yes, absolutely.

    It's been a few years since I read this thread.  But IIRC, it only touches on the theoretical hovering performance of multirotors, and does not delve into the complexities of the control dynamics.  Something I've become quite acquainted with over the years.

    I actually have in house, a quite high performance large multirotor. It can hover for up to an hour with no payload, or lift up to 13lbs of payload for 20 minutes.  It accomplishes this with T-motor U8 motors and 28" propellers in an X8 configuration.  However, it's not a successful design, because while the flight time numbers are impressive, it doesn't fly well.  

    The compromises necessary to achieve that flight time, kill the dynamic performance.  28" propellers are large, heavy, and with very high inertia.  The motors, are the minimum size possible to carry the load.  They have little power overhead needed to change the speed of the propeller.  The situation is not helped, that the only propellers available, are designed to support 27.5 kg, but in this application, each are only carrying about 1.7kg.  They are therefore much heavier than they need to be.  But such is the compromise one has to make when using COTS components.

    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. 

    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.  

    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.

  • Given the severity of the trade-offs - I have wondered if a hybrid approach might work.

    A large, central rotor similar to traditional heli, (or perhaps a co-axial pair for torque neutrality). But fixed-pitch, for mechanical simplicity and the possibility of optimizing airfoil efficiency.

    The primary, or sole, purpose of which is to do most of the sheer 'grunt work' of offsetting gravity, ie providing almost enough lift , or just enough, to get, and stay airborne, but no requirement to play a role in stabilization or lateral movement, thus allowing larger rotor diameters and higher efficiency, without control penalty. (though, of course, they would inevitably affect lateral movement greatly by default)

    Combined with a ring of smaller, electric rotors around the perimeter of the craft. Also fixed-pitch for simplicity.

    Freed of the bulk of the 'grunt work' of lifting, they could be smaller in diameter, thus conferring the benefit of responsiveness. Their primary responsibility is to stabilize, and initiate and end lateral movement.

    The central, lifting, rotors could be made gas-powered, to capture the (still) much higher energy density of gas, for doing most of the lifting.

    Instead of 'big (props) vs small', 'electric vs gas', 'fixed pitch vs variable', could we not have the best of all worlds in the same craft?

    George

     

  • @John C Hansen - You're asking a lot of good questions, but at the heart of the matter is the universal engineering axiom: compromises must be made. The two essential attributes of an airfoil shape are the coefficients of lift and drag. The Cl expresses how good the shape is at diverting the airflow down, and the Cd tells you proportionally how much work that diversion is going to take. It follows that for optimum efficiency you want to use the highest Cl to Cd ratio airfoil you can get. The Cd is roughly correlated to how thick the airfoil is compared with it's width (or chord). Because of the properties of air, the Cd also goes UP - all other things being equal - as the Re goes down. That's why low Re wings (and naturally, propellers) are made very thin. However, thin airfoils have the unfortunate attribute of having proportionally lower stall angles. Generally speaking, fat, well-rounded leading edges are best for stall resistance, but the worst for efficiency. 

    5 (!) years ago when I started this message thread, the vast majority of electric multicopter companies were using model airplane propellers because they were easy and cheap. That may no longer be the case, especially in higher-end units, but I have not been paying attention. Perhaps all is as it should be for the < = 50 LBS class.

    If you're curious enough about how all the design attributes of a propeller interrelate, you should visit Dr. Martin Hepperle's excellent AeroTools site. Just download JavaProp and JavaFoil (or use it in the cloud) and play around.

    mh-aerotools

    John C Hansen said:

    @Brad Hughey, I enjoy your input on this subject. As I read your last comment in this thread: "...the angle of attack range can be very limited.  This means that there's a very narrow range of speeds (advance ratios) in which such blades can operate." My limited understanding of the terms used here brings me to ask for clarifications. Are you saying that because "the angle of attack range can be very limited" this diminished the usefulness of a given propeller at a specific Reynolds number? Or, does the propeller airfoil stall and stop providing lift at a higher angle of attack?

    (clipped for brevity)
    JavaFoil
  • @Brad Hughey, I enjoy your input on this subject. As I read your last comment in this thread: "...the angle of attack range can be very limited.  This means that there's a very narrow range of speeds (advance ratios) in which such blades can operate." My limited understanding of the terms used here brings me to ask for clarifications. Are you saying that because "the angle of attack range can be very limited" this diminished the usefulness of a given propeller at a specific Reynolds number? Or, does the propeller airfoil stall and stop providing lift at a higher angle of attack?

    Perhaps if we are to limit the design parameters of the multicopter that uses this propeller to fit within the narrow range of advance ratios that allow the propeller to remain reasonably "high-performance", we will have a pretty good design? When we stop expecting high flying speed (angle of attach) and focus more on hovering and modest lifting, can we expect to get a propeller that efficiently fits that criteria?

    And let's be more specific about this.... let's say we need an efficient system that hovers and are willing to trade off some efficiency as the angle of attack increases, how narrow is that narrow range? Are we dealing with a craft that sinks badly at an angle of attach that propels the craft at perhaps less than 1 meter per second? Or is the issue one of just using a bit more power to maintain altitude at a rather slow speed? What if we are willing to pay the price of inefficiency when flying so that we can have high efficiency while hovering? What if 80% of the time in flight we are expecting the multicopter to perform only minor movements and primarily are asking the craft to hover? At what angle of attack does the craft stop flying altogether?

    And, why are these "high efficiency" propellers very difficult to find? Is it because prop manufacturers CANNOT make them or that they have not yet been asked to make them? Is this an issue of engineering impossibility, or one of low market demand for the propellers in question? Why don't we simply ask them to make the propellers that we need?

  • @Gary McCray - Thank you for your kind words, Gary. When it comes to control, it's poles and zeros, baby, poles and zeros. For a time I was working on a passive variable pitch prop, but the IP protection ecosystem is truly crowded with players. When I see grossly overlapping claims between competing patents, all I can think of is the lawyers ending up with all the money. But that was just a sidebar project to the ultimate goal: manned electric VTOL fight with an entry price of < $30K.


  • @Luc Maximilien

    I see the note on page 13 that you refer to related to Open Source, and I believe it refers to the network that would integrate the different state-wide agencies in North Carolina. With various patents for the actual AVEA, I presume the aircraft itself is closely protected technology.

    This project really is a bit of a mystery. I found some promotional videos on YouTube and I now see the scale of the machine is much larger than I had originally presumed it to be. At the size that I see in those videos, I now understand more of what it is intended to do, but I still am puzzled why it does not seem to get media coverage. I have not seen any media coverage myself. It really is an unmanned "large scale" multi-helicopter. I still like the design, but it might not scale down well to the size that I would be able to use.

  • I totally agree with Gary.

    The funny thing about Aeva is that if I have well understood  it is Open Source

    Look at page 13 of this presentation (with no technical datas but a lot of slides and talk ): http://www.ncleg.net/documentsites/committees/BCCI-6624/2014-3-17%2...

    Another thing, they are locate also in Vicenza, Italy, I'm from Milan and I work with several Italian research centers, never heard about their project.

  • @ John C Hansen

    I did not found anywhere the specs of the Aeva .

    All I found was 60 minutes of fly time and 4,2 Kg AUW , somewhere else I read about 5kg of payload but I doubt that even with  28" propellers it can be possible because it  would mean an efficiency near 23 grams per Watt , possible only if the motor/prop efficiency was  50% greater than the actual  systems have.

    Just tell me how big are the props of the Aeva and his AUW and with some simple maths I can tell you his  fly time upon the batteries used.

    John C Hansen said:

    @Luc Maximilien

    From their website, The AEVA can carry 5 Kg./ 11 pound camera gimbal for ONE HOUR between quick recharges.

    While I cannot determine the Figure of Merit for this aircraft, these specs far surpass what is currently available in the market. And if you have designed and built a hexacopter that can do as well as that, please send details.

    I am convinced that the Olaeris Aeva has a superior design and a ready market.

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