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.
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.
Also - I like 12 - a 12 would have the benefit that you could have side-by-side fully redundant systems powering 2 interleaved hexes which would mean you could lose half the system and still be able to do a controlled landing.
Also if each system had redundant controllers/sensors you could have 4 parallel controllers doing parity/sanity of each other so if one controller went down or began doing things "wrong" the other 3 could vote it out of the loop and take over.
Another note - IBM's recently publicized Lithium Air battery project could make a fully electric large scale multirotor practical as well although that is probably around 10 years out. But they are saying the potential power to weight ratio for lithium air (in the long run after solving a lot of problems) is up to 15x of lithium ion. In the next 10 years the target is about 4X for electric cars. They plan to have a prototype to show the world in the next year or so. Lots of stuff is trending in the right direction for large scale multirotors to be a reality within the next decade in my opinion.
I'm planning on one in 12-18 months, actually. The initial production unit will be a FAA Part 103-compliant 12-thruster with a M/G powered by a Rotax-582-style-on-steroids 2-stroke as the primary power source (200H MTBOs are acceptable in the ATV market). On the other hand, I'm not opposed to a 4-stroke if we can keep the weight and the cost down. It will be a bit of a challenge to keep the dry weight under 254 pounds so the Feds are happy. I'm figuring a range based of the FAA-allowed maximum of 63 MPH at 42 miles on the requisite 5 gallons of fuel.
hey ive only just got here!
been meaning to read it for a week or two now, and have finally done so now.
right at this moment im not really ready to comment, but ive certainly got two or three themes that havent shown up much so far and definitely should. need to digest it all and see if can put some thoughts together in a comprehensible reply :)
Have you posted any pictures of you machine?
Does introducing tilting rotor system (using rotating servo mechanism) in the design give better stability?
If by "stability" you mean an aerodynamic tendency to self-level, then pointing the thrust axes inward to a common point above the craft would be one simple method of accomplishing this. The analogy would be fixed-wing dihedral, except in both pitch and roll. The no-free-lunch cost would be a reduction in efficiency and less control headroom on the APM (slower response).
Articulating thrust units would just add mechanical complexity and points of failure.
To be honest, I can't see any possible stability advantage to this compared to a simple well built frame, good motors, fast ESC's, and good PID tuning.
Not disagreeing with the theory. You're right. But I'm just saying to anybody reading that I think this is a lot of effort with little reward over what is currently possible with a standard setup.
Well, then, amigo, let's just say we're both right. The best thing in the vast majority of use-cases is to point the rotors straight up and rely on the APM for stability.
Also, servo-tilting anything is a bad idea.
Yep. Because then you might as well be using them to tilt a swashplate controlling one really big rotor. ;)
well, since i'm a kid in this regard... and have a very less idea on APM,
i thought controlling all the four rotors in a single function input would be less complex, with consequences like heavy battery-drain and overall design weight increase..
and, since we are controlling all the four rotors with a single input, what i felt is, distinct thrust values may not be created
I was actually dreaming of automated ambulance sort of multicopters, which might be able to carry payload with stability.. So I started thinking of different possibilities. :)
Boy I'm really late to this party but I've got a few observations and a few questions.
Thank you Brian for starting and continuing this thread.
You are covering several important issues and propeller efficiency in particular is way under addressed especially as it relates to multicopters.
Your generator powered man carrying multicopter is an amazing undertaking.
While the Rotax can provide adequate power I am sure, you are going to need one really state of the art ultralight and ultra-efficient generator to make it work.
And clearly brushless motor efficiency at hover is going to be really important as well. AC or DC?
However, as you have illustrated the most important thing is going to be propeller efficiency at hover, with a little compromise for actually doing anything else. Pretty much the normal multicopter (or any copter for that matter) conundrum.
The most basic tenet is that for a given set of conditions the biggest propeller diameter you can manage is going to have the highest efficiency within practical structural and weight boundaries.
I am curious that your man carrying copter is going to have a high number of propellers, obviously redundancy can increase safety in the event of failure, but efficiency says that the fewest blades (4) with the largest diameter would be best.
Thomas Shenkel made a short (and very brave) flight in Germany with a whole pile of motors and props, but this was clearly neither efficient enough or safe enough to consider for actual practical use.
Your unit will get around flight longevity by using the comparative high energy density of gasoline versus LiPo batteries, but it still seems to me that 4 rotors would be a lot less complicated and require less net horsepower than more rotors.
I am sure the rotors would need to be custom made and I don't know if appropriate and appropriately efficient brushless motors are available, but the brushless motor used by the Swiss conventional electric aircraft was at least 15hp and looked like it might work.
There was an extremely complex mechanically balanced man carrying quadcopter made back in the 60's or 70's which certainly had plenty of power, but failed for now obvious reasons of mechanical complexity and inadequate control capability. Of course there was the Piasecki AirJeep dual rotor ducted fan also with plenty of power but very unstable and not really controllable.
Basically just a question why lots of rotors and not a Quad?
Then there is the main issue, propeller efficiency for our multicopter hobby.
Propellers with top efficiency at static thrust in a hover yet still usable above and below that threshhold.
Clearly a multicopter propeller has far more complex requirements that a straight fixed airfoil like a straight wing for a whole bunch of reasons.
What are the best propellers commercially available to us right now for various multicopter sizes?
Ones that actually incorporate the best and most appropriate designs for multicopter use in various sizes, power, weight and motor speeds.
I'm sure you or some of the other followers of this thread have some thoughts on that.
Way too many of the props I see do not seem designed at all for maximum static thrust, have way to short a cord, are too symmetrical, too tapered and while pretty, seem like theyd work much better on a high speed fixed wing aircraft than a multicopter.
Also, possibly, any thoughts on appropriateness of multi-blade or more turbine like designs as in some of the newer full sized unshrouded fan jet designs would be appreciated.