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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Brad,

Have you posted any pictures of you machine?

thanks,

Mike

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.

@Gary McCray: This manned electric multicopter party is really just getting started. As far as I know, Draganflyer had the first electric VTOL flying machine in 1997.  With the advance rates of electric power technology, it was only a matter of time before bigger things began to emerge.

For me, it started on Christmas Day, 2003.  I had left EMC as a field engineer manager a year before, and was still in the data storage architecture business as a consultant.  I bought my son one of those flying saucer toys from Walmart for about $30, and was amazed that they could make a profit selling VTOL, wireless electric-powered toys for that price.  Also, the power-to-weight ratio seemed to be rather good, so I wondered what might be possible if you strapped two or more of them together.

The it hit me - why do helicopters cost so much?  Because they relied upon a basic design philosophy that was diametrically opposed to what I had been telling clients was a good approach for protection of their data - redundancy.  Conventional helicopter are flying examples of reliability through creation and maintenance of mechanical perfection - flying single-points-of-failure.  This is very expensive.  Reliability through redundancy was a much more cost-effective approach, and perplexing as it is, some areas of aviation DO in fact embrace this concept.  The only way to create a fault-tolerant lifting platform was through electric power plants which could be precisely controlled.

You've covered quite a bit of ground in your post, so for the sake of brevity I'm not going to try to address every point.  Yes, the lightest disk loading is good for efficiency, but with the same area and fewer rotors, the dilemma of rotational inertia starts to rear its exponentially ugly head.

I won't comment on the Germans, except to say that (in my opinion) their product development seems to have been limited to placing the largest order Hobby King has ever seen.  It was a highly successful publicity event.

The best FM I've seen achieved was Paul Pounds' effort at roughly 3 lbs/ft^2 disk loading of approximately .80.  Careful airfoil selection over the Re range and twist with taper geometry were largely responsible for this level of performance.  FM's in the 60's can be had with some of the better hobby props, as shown by the data from the UofI testing.  APCs seem to be better than most.

Short chord is actually a good thing (solidity ratio being the denominator of the FM equation), until you get to a disk loading that's so high you start to get localized compressibility effects.  That's why turbofan blades are short, squat little beasts with a zillion of them on the disk. 

Let the party continue.

I have been very interested in this idea , using rotomax motors that put out about 20hp a pc ,using a 6 motor setup ,doubling the electronics for safety ,also using mini gas rc engine rated about 11.5hp to turn a generator head instead of using batteries ,using an ac/dc converter of corse.it sounds plausible, but safety will always be concern , just as real airliners use flight paths , airspace must be laid out to accommodate thest types of vehicles.
Hi Brad, very interesting conversation. Just wondering how you got to that NACA 63-1211 foil and if you could point me to the coordinate file so I can do some real world testing on it. I'm at about 300.000 on the Re numbers so that should fit in well.

Please tell us more of your thoughts. Love your writing style.

Kind regards, joe

Thanks, Joe.  I contribute sporadically when the spirit moves me.  You can find a treasure trove of airfoil data at this site:

http://www.ae.illinois.edu/m-selig/ads/coord_database.html

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