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
Peter,
This idea that "helicopters are hard to fly" is just completely untrue. It's a very common misconception that permeates the drone industry. But commonality doesn't make it correct.
Helicopters are not hard to fly. Multirotors, are hard to fly. Ever fly one without gyro stabilization? Nope. No human can. That's why we created these autopilot systems. But they quickly went beyond simple gyro stabilization, and went all the way to GPS positioning.
Helicopters, by comparison, can be flown by humans with no gyro stabilization. However, add the same electronic system as the multirotor has, and it is just as easy to fly as any multirotor.
Here is a pilot operating one of my Procyon helicopters. His only previous experience is with a Phantom. After a couple of hours of training, he's not just flying a helicopter, he's doing a complicated sling-load operation. Think you could do that?
Now, the mechanical stuff, there's some truth to that. But even that is not always true. Look at the Colibri G2 helicopter. Composite rotorhead design has NO fatigue limit. It's fully composite with no moving parts. It requires virtually no service.
Conventional manned aircraft can typically go 2000hrs between major service.
I think you're underestimating how long the electric multirotors will fly without needing motor bearing replacements, ESC failures, etc.
How much experience to you have operating *large* commercial multirotors to understand what you're getting into?
Rob and Gary,
It's a nice little chopper and an impressive video. Issue with all conventional helicopters is that they are difficult to pilot and very expensive to maintain.
Flying choppers like this will be a manual task for the foreseeable future. It takes many hours of flight training to master.
Combustion engines or in this case a turbine are expensive to maintain. Also the mechanical parts of the rotor system require lots of maintenance by specially skilled technicians.
On the hand electric multicopters like the Kitty Hawk or my home build Sky-Hopper are extremely easy to fly. I have zero flight training but could easily control the machine you see flying in my video's. As for the Kitty Hawk, the lady flying it in the video is a reporter who got a few hours of training in a flight simulator before they made their video. You try that with a mosquito or any other conventional helicopter.
As for cost, the only mechanical parts in a electric multicopters are the motor bearings. Off cause there will be maintenance of battery packs and other electronics, but this can to a large extend be automated. In fact, wear and tear of motor bearings can also electronically measured further reducing maintenance cost.
So for a small group of enthusiasts with deep pockets conventional helicopters will remain the ultimate but most other consumers will be more appreciative of electric multicopters. At least, after they become available :-)
Regards,
Peter
Gary McCray said:
In my purely amateur opinion, Peter - 'hexa' is not a good long-term approach to manned electric multi-rotors.
Even unmanned, its apparent redundancy seems to be largely illusory. Manned, I doubt I would seriously consider flying in one any time soon, personally.
If you're not gaining meaningful safety-via-redundancy vs, say, a quad (I don't think you are), but you are gaining points of failure, each of which is potentially catastrophic, then why bother?
I was initially surprised the Kitty Hawk was relying on only 8 rotors (I'm skeptical that even a single rotor could be lost, under such a high load, without coming down fairly fast). Then again, they don't have far to fall, it seems - they're taking baby steps by limiting it to very low heights, over water only. (Methinks they're after 'Minimal Viable Product' here to begin with).
Efficiency and agility, imho, come second and third to safety/redundancy, if you plan to market your craft for civilian manned use.
I think Brad's designs are using way (way) more than 8 rotors.
I'll dissent from the consensus that there's no likely market for this. The market for recreational watercraft is large, and this one, even with only say 10 minutes flight time, will be a hit with the large majority of casual watersporters/boaters/cottagers who don't have any inclination to get a pilot's licence, and who are instintively nervous about large-bladed helis.
Of course, time will tell
George
Peter Dobber said:
Hi Rob,
That is one really cool little copter alright, and $50,000.00 is actually an amazing deal with that lovely little turbine.
Frankly I seriously doubt the Kitty Hawk can manage 5 minutes, really quickly hit the weight design limits of that system and extra battery weight would make it not be able to get off the ground at all.
I think they went to a lot of trouble making the video they did (5 miles from my last house by the way) and I think it has very little potential to be a financially sound venture.
I think a practical (at least somewhat practical) manned multi can be built, but it isn't this one and it may just be a battery with significantly more energy is required first.
There are some in development but it's going to be at least 5 to 10 years before really significant new battery tech becomes actually available.
I also do not believe true fixed pitch props stand a chance of being genuinely viable or practical on a manned multi.
We will see, I've stuck my foot out there, will it end up in my mouth, I'm guessing not.
If I were younger, lighter and richer, I'd seriously be looking at that little copter.
Best,
Gary
Gary,
Thing is, I don't really considering anything shown in that video to be dynamic flight. It's just really easy hover manoevering. Compare that the performance shown here, skip ahead to 1:50 and hang on to your hat.
Also note that the KittyHawk is using very powerful motors compared to the disk size. This thing probably only flies for 5, maybe 10 minutes tops. You have to think of this on a 3D graph, not 2D. You have size on the X-axis. And "dynamic performance" on Y-axis. The larger the size, the less dynamic performance you can have. But there's a third axis, and that is relative power or disk-loading. The lower the disk loading, the lower the dynamic performance potential. So as you combine size, and low disk loading, dynamic performance really falls off. And it's the disk loading/relative power that you need to achieve long flight times.
There are people using Multirotors for Hollywood shoots, that are large and can lift a pair of Red Dragon cameras. They are fairly dynamic. But only for about 10 minutes at a time.
It is interesting to me that the Kitty Hawk works as well as the video seems to indicate it does.
The relatively few large fixed props would seem to indicate that control should be marginal at best.
I imagine the video was achieved over several attempts in totally calm air, even light wind or gusts should be able to easily over power this thing.
Optimal computer flight control is probably an absolute necessity to make it even barely flyable under even optimal conditions.
A lot of the flights seemed seemed like they might have been in the region of compression ground effect also meaning possibly working more like a hovercraft than a multicopter - significantly changes flight dynamics.
My personal view is that with standard fixed pitch props this will never be a commercially salable product, far too twitchy for normal people to use even under ideal conditions.
And my guess is achieving even a 5 minute flight time will prove daunting to say the least.
We will see, One thing that is definitely true is that my predictions do not always come true.
Frankly the 1 person helicopter is infinitely more useful and practical.
And the really cheap build it in your own garage Bensen Gyrocopter actually worked remarkably well.
Of course what goes up eventually comes down and there were always a few idiots who insisted on doing it in the non-prescribed fashion, killing themselves and eventually the Bensen in the process.
Best,
Gary
Yes, in a multirotor, the rolling moment in lateral flight cancels out across the sum of the disks. However, there is still the pitching up moment that remains. This ends up being resisted by the autopilot system. You can see it as the forward motors are throttled back as speed builds in order to maintain the target pitch angle.
This is something that is also terrible about large/efficient multirotors. The larger, slower turning propellers, have even more pitching back moment. The large Octo I referred to, was limited to a flight speed where the forward motors were nearly stopped.
Actually, it could fly above 5 m/s OK. The problem was stopping. When you pitch up to decelerate the machine, the pitching up moment seems to suddenly increase. I'm sure this could be explained aerodynamically but I haven't done it yet.
When pitching up to decelerate, or even just coming level to coast to a stop, the forward propellers would nearly stop. It tell you, it's mind-bending, to see this really large machine flying, holding level, with the forward motors nearly stopped. You can see the sum of the thrust vector is nowhere near the CG of the vehicle, and by rights, it should be falling nose-down. But it doesn't. It just hangs there. That's the pitching up moment from the propellers.
I wish I could show some of these videos, it's fascinating to see, but it's not my machine and I can't.
You can buy a single seat helicopter, that will do 80mph for over an hour, today, for about $50,000.
The only advantage to the KittyHawk, is that it has an autopilot so anybody can fly it.
So put an autopilot on the kit helicopter. <shrug>
Because those motors would be so much heavier, and operating in a less efficient range, that the flight time would greatly suffer.
This is one of the big design conundrums with fixed pitch multirotors. For maximum efficiency, you want the smallest motors, turning the largest propellers. But then you want those motors to have enough power to be able to speed up and slow down those propellers for stability control.
Totally seat of the pants engineering. That's just my gut feel and what I've observed to be true.
Don't get me wrong. There are large multirotors with 30" propellers that fly well. It's just that they only fly well for 10 minutes. The entire design is optimized the other way, for dynamic performance.
It's difficult, or nearly impossible, to achieve both dynamic flight performance, and long duration, on very large multirotors. The only way it can be done, is what Brad was suggesting. Instead of a few very large propellers, you use many small propellers. But then you have tradeoffs for cost, complexity, etc.
That would take care of the roll torque from the main motors, but not the flap-back (pitching up moment) torque in forward flight.
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