Background on Ornithopters
Kyelok, 2023-02-21, Rev. D [ Draft ]
An ornithopter is a machine that flies by flapping its wings. I focus my attention on large ornithopters (big enough to carry at least one person); if I don't specify, I am probably talking about large ornithopters.
The target audience for this piece is people who are interested in serious technical development of large ornithopters. I want people to have solid ideas about ornithopters so that they can avoid delusions and time-wasters and focus their efforts on things which are likely to bear fruit. (And I'd feel a little less alone and strange if I wasn't the only one.)
Motive
As far as I know, there's not an accepted serious reason to pursue developing large, man-carrying ornithopters—or at least not one that's serious enough for an investor to buy in to. Modern fixed wing aircraft work well, and it isn't clear that ornithopters have any advantages. It's also evident that an ornithopter will be very different and expensive to develop. We might also reasonably anticipate that if we solved all of the problems ornithopters are likely to have, the resulting machine would be very complex. We simply don't have a compelling reason to believe that an investment is worthwhile.
At least for today, large ornithopter development is something you do because something about the idea of flying like a bird intrigues you. Since I am interested in flying like a bird, I work on it. I've personally come to believe that ornithopters have a serious future, and so I believe this will change, but I can't easily show that, and until I can do that (or until you can), you should operate as though ornithopters are a curiosity. (I am working on other articles and on a very ambitious group of ideas that are all ultimately meant to facilitate ornithopters and make them make sense, but it's way too much for this page at this time.)
There's plenty of room for hobby work; ornithopters aren't banned or anything, but I wouldn't expect anyone to pay.
Where in Aerospace Would An Ornithopter Belong?
It might help us to visualize things if we can compare ornithopters to more familiar machines.
The most common aerospace products are airplanes (of various sub-types), helicopters, blimps, tilt-rotors, and rockets. There are some more exotic aircraft configurations, like cyclogyros, boundary effect machines, or various hybrids. Each machine has a different role, or a different mission profile it tends to be best suited for.
It'd be a good idea to figure out what kind of role an ornithopter would have. My guess is that it would be closer to a helicopter or light aircraft. Right now I think an ornithopter could be designed for either horizontal (airplane-like) or vertical (helicopter-like) takeoff and landing. DeLaurier's machine (and every other large ornithopter I'm aware of) took off horizontally, while small ornithopters are typically hand-launched or VTOL. Dune 2021's large ornithopters were VTOL, and while those weren't real I don't think a large VTOL ornithopter is out of the question.
Prior Art, Small Ornithopters
Small ornithopters are pretty common to see, and there's a pretty substantial body of work around them. I haven't personally been impressed by their performance, but they do work, and people seem to think they have some advantages at very small (e.g. dragonfly) scales over quadcopters or other miniature aircraft.
There's an entire branch of mainstream-ish aerospace engineering that deals with what's called "micro aerial vehicles", or MAVs. You'll often come across ornithopters designed at these small scales (see this paper or this one, for instance). AIAA holds contests for the best; see the SAE Aero Competition page (if the link doesn't break) and look for the Micro Class in the rules. I recall seeing ornithopters entered into the MAV competition reasonably often.
Aerodynamics has a certain level of scale sensitivity. Small insects may "feel like" they're swimming in peanut butter or molasses when what they're really swimming through is water. Engineers use something they call the Reynolds number to put a number on this "feeling" and how it changes when you get bigger or smaller.
Because aerospace problems tend to be scale sensitive (via the Reynolds number, among other things), we have to assume that the information we have available from work with small ornithopters is unlikely to be of great use when developing large ornithopters. To the extent that it could be of use, it still needs to be verified at a larger scale. Thus, for our purposes, we can consider time spent studying small ornithopters as having a low return on investment; we can probably mine more and better information in other ways (though I'm open to being proved wrong.)
Prior Art, Large Ornithopters
There's various mentions of Otto Lilenthal having worked on ornithopters. I've never seen any specific details.
There is a small amount of old German work, from around the WW2 timeframe. I am pretty sure I saw it in some of Ornithopter.org's references; it's been a while since I went through their materials thoroughly.
Dr. DeLaurier in Canada probably has the most notoriety for ornithopter work. His work eventually led to a machine actually being built. The Project Ornithopter did result in a man-carrying ornithopter capable of taking off and landing in such a fashion that you could walk away from the crash site. Observing the video, we can infer that control is a serious problem with this design. That said, it's still the best ornithopter I know of at this time.
I tried to get in touch with Dr. DeLaurier via email at one point, but I'm pretty sure the email came off as strange, and in any case I never received a reply.
University of Toronto had their "Snowbird", which was a human-powered ornithopter. I generally question the decision to use human power, and for the Snowbird specifically I tend to wonder if they achieved steady level flight and didn't just sink very slowly.
Human-powered Ornithopters
The tale of Daedalus and Icarus is very old. It tells of when Daedalus and his son Icarus escaped from an island by building great wings and flying away, followed by Icarus flying too close to the sun, making his wings' wax melt, and causing him to plummet to his death. It's an ancient Greek myth that most people have probably heard of, and its longevity shows the power that the idea of human flight has. Many people have dreamed of being able to attach some kind of device to themselves, flap their arms / wings, and fly just like so. What I myself want is superficially similar to this.
But it's important to avoid getting married to the idea of flapping under your own power. The human body barely produces enough power to fly a fixed wing aircraft, and it took over 30 years for someone to fly a human-powered helicopter for a single minute. Perhaps in the far future, once ornithopter design is perfected to a point we can engineer one efficient enough for a human to be able to fly it, it will be reasonable to work on the idea, but for now it's almost certainly a waste of effort.
Yes, I know the University of Toronto thinks they flew one for about 20 seconds in steady level flight, but I'm not convinced they didn't lose a touch of altitude in a machine with a huge, highly efficient wing that wouldn't have lost much altitude in that time. You can see the video if you want. In no case does this have a desirable level of maneuverability.
Other
Fake Ornithopter Video
Around 2012, there were a number of blogs about ornithopters that popped up. I believe one was called Daedalus or Daedalus' Blog or something; there were two or three others. At least one of them posted progress on a machine they were working on that (if I remember correctly) had a typical up-and-down style flapping mechanism. The machine was powered by one or two outrunner engines, if I recall. After about ten progress updates, they posted a video where they tried the machine and the pilot took off in flight.
The catch? It was a fake. They did put effort into it, but the motion wasn't particularly natural, so it wasn't too hard to tell. A better sign that it was fake was that the blogs all were taken down at about the same time without explanation. I don't know for sure if anyone ever noticed this particular bit of internet stagecraft besides me.
I tried to contact the owner of one of the blogs (Daedalus if I recall) and see what kinds of research material they could point me at, but they never got back to me.
Dune
The ornithopters in Dune (2021) look alright. I get the impression that someone with aerospace knowledge was consulted for their design. It's interesting to note that Dune's ornithopers are VTOL / capable of hovering; machines that can hover have a very different kind of usefulness compared to machines that need a full runway.
The ornithopters in the the movie also do not seem to sweep their blades as much as I think they ought to. There's a simple model of propellors called momentum theory that tells us a lot of useful things about airflow, and it models propellors as magic circles that cause thrust. The smaller the circle, the less thrust, essentially. (It's also less efficient.) And to my thinking, if you sweep an ornithopter's blades more, you'll get a bigger area, and thus more thrust for the same blade size. I think it's likely better for the blades to sweep through a larger angle.
Resources
I've consistently seen Ornithopter.org in search results. They may be the only long-term internet effort around, outside of forums.
Ornithopter.ca points to the two Canadian ornithopter efforts discussed above (Project Ornithopter and the Snowbird).
The Biokinetics of Flying and Swimming is the best book I know of that covers flapping wing flight from an aerospace perspective in a coherent way.
Acknowledgements
Much thanks to Pico for reviewing and providing feedback.