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.
Replies
Hi Brad,
Very interesting to see Dr. Pounds involved in this project and a ducted fan with considerably different prop blade design than his University Quadcopter.
I would have thought that normally a bit larger diameter and no duct would have been more efficient, but I do understand both the safety advantages and the thrust increase for a given diameter that can be achieved with proper ducting.
And that the prop has to be specifically designed for duct use to be optimal.
@ John Hansen, fixed pitch props for hovering use always are more efficient the larger the diameter you can make them (Higher FM). (discounting ducting applications).
The problem with fixed pitch props is that the bigger you make them the less you can use change in velocity for control (or simply as they get bigger they become less able to control the attitude and motion of the multicopter.)
Experience has been as you get above 24" in diameter for a multicopter, control starts becoming more and more marginal.
This simply isn't something that can be fixed with a fixed pitch prop which is why larger props / rotors for hovering use are more successful with variable and cyclic pitch control. (EG Helicopters or variable pitch quadcopters: see Curtis Youngblood - Manta Ray)
This problem does not show up on conventional aircraft because they do not use the prop for control, only forward thrust to which corrections can be applied slowly.
There are clearly a few manned multicopters (like the octo that flew over in Lakeport, California earlier this month) that use bigger propellers and have flown successfully, but they are dogs with marginal control, something they don't talk about.
Brad Hughey above, is truly an expert in this field and has himself attempted construction of a manned quadcopter.
He truly knows about the mechanical and physical limitations and what is necessary to operate in this difficult heavy weight end of the spectrum.
Best Regards,
Gary
PS Brad how is your current project going?
@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.
I do not believe at all that the Olaeris Aeva is a usefull design.
What is safer in a propeller with something around ?
Are the airplanes propellers shrouded ?
There is nothing practical in that design , try to land in high grass , where do you mount the camera gimbal for a DSLR ?
Remember that a multirotor is a flying brick so the safest multirotor will be the one with the highest efficiency not the one with shrouded props.
I would like to know what is the real efficency of the Olaeris Aeva , it is the first technical information a company should wrote , how many Grams per Watt their flying brick needs.
The less rotors and the largest propeller gives you the best efficiency , this is why people in real world flies in helicopters ans not in multirotors.
The Aeva is a typical example of a commercial product that finally might work even worse that a DJI Matrice but cost 20 times more because "certfied suppliers"
I saw in my 9 years experience , some drones made for military purposes with highest quality certified components but poor performances and capabilities compared to recent "toy" drones.
@Brad Hughey
Thank you for that link. I am happy to see the design of a hexacopter as shown in the photos. It is a design I believe in strongly, but I am puzzled by why more "hobbyists" are not pursuing it. Six ducted fans or shrouded propellers as I call them seems so much safer and even practicle. And why do they deny that this is a drone when they call it a VTOL? Do drones have such a sinister military stigma attached to them? If it is a useful tool, give it its own new name like hovering data acquisition platform. I bet they do not like the word hovering because the users will expect them to fly.
So, now I ask: How do I make one that I can afford? With all the references to uber-high specs and credentials, I can imagine that the Olaeris Aeva has an untouchable price tag for my pocketbook.
@John C Hansen
Nobody hijacked anything; it was a technical discussion from the start, not a marketing one. My assertion was (and still is) that for a given disk loading, an electric multicopter can have a higher FM than a single-rotor helicopter due to the design compromises inherent in an oscillating cyclic pitch control system. A higher FM means a longer flight time with all other things being constant - which they never are, hence my next point...
A cogent marketing discussion would have to be in the context of comparing actual, tangible embodiment things. Here's one that IS marketed on its FM as being one of the highest (the site used to talk about it). I would also suspect this represents a single thrust unit maximum diameter fairly near the rotational inertia control limits of a fixed-pitch blade system (kudos Dr. Pound):
Olaeris Aeva
If I remember correctly, Paul was claiming an FM somewhere in the low 90's. Apparently, the Olaeris folks believe the benefits of that optimization will help them sell a few more.
Oh boy, you ask some questions. In my case definition of "Large Scale" is the next ship bigger than the last one I've built :)
I recognize that this is an old discussion, but I would like to interject my thoughts on this topic. The discussion title drew me in because of the term "large scale" added to the word multicopter. Then I see in the first paragraph the use of the term VTOL. I frequently see this in discussion threads where the initial discussion gets hijacked by references to facts that are not central to the initial topic.
I would enjoy seeing a definition of what "Large Scale" means and then correlate that definition to the responses and additional input from around the globe. At some point, we should agree that rotors large enough to be efficient must be as large as a helicopter. And before long we are no longer talking about a multicopter.
If a small multicopter is about 4" or 6" (100mm to 150mm) then a Large Scale might be 20" to 30" (500mm to 750mm). And, what will we want to do with this size multicopter? Fly it? Or put a payload on it? Is there a medium sized multicopter? Something in between the small and large? And what if the design parameters are written before we analyze the flight characteristics of the craft? What will we want to do with this large scale craft? What size is a good size for our needs? What figure of merit should we be aiming for?
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. What are the Figure of Merit for the products on the market no mater what their size is? Are the larger multicopters that are on the market today marketed based on their Figure of Merit? No, they are not. They are marketed with an appeal to the buyers desire to do something specific with their purchase.
So, my questions are: What size multicopter do we want and what do we wish to do with it? Related to our intended use, is what are we willing to pay for the pleasure of owning that dream machine? How long do we want it to fly? And finally, what is the Figure of Merit for that machine?
Hi Brad,
Sorry I didn't get back to you sooner, major problems completely unrelated to "drones" (rebuilding a previously sold house after mortgagee abandoned it).
Please feel free to email anytime direct to grmccray@yahoo.com
I believe you actually pretty much confirmed my observations and anecdotal considerations in a far more technically competent way.
I think the bottom line is that for multicopters of all sorts, sizes and weights, current propeller design seems way under engineered.
Current designs are all sort of bottom up simple adaptations of designs for completely different applications with minor tweaks to make them work a little better.
And the gross effects on the vehicle of widely different propeller diameters are also not at all well understood.
I think there are going to be really major applications for tiny little 200 and 250 class quad copters with 6" and under props as well as significant applications for big multicopters using 24" and larger propellers and both of these have very significant propeller design requirements (and, it seems, current major deficiencies).
This should be an interesting year.
Best regards,
Gary
In the model airplane community, it is accepted that 1/3 of the lift of a fixed airfoil comes from the bottom, and 2/3 from the top. Propellers are treated as a type of airfoil. It has been interesting to read your comments on the design of helicopter rotors.
Also, larger propellers are accepted as more efficient than smaller propellers (moving at higher rpms). One of the reasons that I don't like multicopter designs (and electric ducted fans) is that they use multiple smaller propellers, and I can't see that this is efficient for energy use (although it may lead to much simpler control logic).
The Germans, long ago, demonstrated that the air stream on the top of a wing (regardless of the longer path due to airfoil shape) moves faster than the air stream on the bottom. The top air stream does not join up with the bottom air stream at the trailing edge, at the same place that it split at the leading edge. This does produce a relative vacuum on top of the wing. But I don't believe "Clark Y explanations".
I'm curious to find the rationale behind the design of the Osprey's rotors.
Hi Brad,
I see you recently responded to another blog and I was just wondering if you had been doing any more development on your (large) multicopter.
One of the things I have observed is that as our large fixed pitch multicopter propellers get bigger and slower, it appears that we have increasing instability.
Possibly this is due to asymmetric relative blade position actually producing angular torques on the motor shafts causing occasional wobble or large amplitude low frequency oscillation or vibration.
The bigger (and slower) the props the bigger the effect (also the fewer the props).
Presumably helis do not have as pronounced a problem because they actually correct during each rotation of the prop blades.
My suspicion is that this is actually based on the fact that air is a fixed physical medium and you can't simply scale things up or down willy - nilly and expect the results to remain the same. (Lilliputian effect, possibly).
I have noticed that really large Quadcopters with 26" - 29" props have not taken off so to speak as it earlier might have been predicted that they might have.
I have noticed on some of my own quadcopters with only 14" blades that accelerating rapidly up or rapidly stopping a descent can result in high vibration amplitudes, loud noise and even wobble and instability whereas reducing just 1" in prop diameter greatly improves the situation.
I am wondering if the effect that I have observed and taken an intuitive shot at describing above, might be partly the reason why.
Basically I am wondering if you think this idea has any validity at all, or is what I am observing due to other factors.
From the other direction I have definitely observed that although tiny little quadcopters suck at efficiency their vibrations are all really high frequency and low amplitude meaning that they can often be effective camera platforms whereas large ones can need considerably more vibration isolation.
I'd really appreciate your feedback on this, when it comes to props and basic multicopter design you really do know more than the rest of us.
Best Regards and I hope all is well with you,
Gary
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