One thing that the new Dune film captures extremely well is the idea that the primary small-capacity air transportation mode on Arrakis is travel by ornithopter. The choice of flapping wings as a lift/propulsion mechanism can be in-fictional-universe justified by the idea that jet turbines probably won't do well in an atmosphere with lots of suspended dust and sand, especially on take-off and landing. Still, I think Frank Herbert decided on ornithopters because it just sounded cool.
The actual physics and engineering of flight via flapping wings is complicated. This site is a good place to do some reading. The basic idea is not hard to explain. To get net lift, in the cyclical flapping motion of a wing, somehow the drag force pushing downward on the wing during the upstroke has to be more than balanced by the flux of momentum pushed downward on the wing's downstroke. To do this, the wing's geometry can't be unchanging during the flapping. The asymmetry between up and down strokes is achieved through the tilting (at the wing base and along the wing) and flexing of the wing during the flapping motion.
The ornithopters in the new movie have wings on the order of 10 m long, and wing motions that look like those of a dragonfly, and the wings are able to flap up and down and an apparent frequency of a couple of hundred hertz (!). If you try to run some numbers on the torque, power, and material strength/weight that would be required to do this, you can see pretty quickly why this has not worked too well yet as a strategy on earth. (As batteries, motor technology, and light materials continue to improve, perhaps ornithopters will become more than a fun hobby.)
This issue - that cool gadgets in sci-fi or superhero movies would need apparently unachievable power densities at low masses - is common (see, e.g., Tony Stark's 3 GW arc reactor that fits in your hand, weighs a few pounds, and somehow doesn't have to radiate GW of waste heat), and that's ok; the stories are not meant to be too realistic. Still, the ornithopter fulfills its most important purpose in the movie: It looks awesome.
OK, this is something I actually know a little about.
ReplyDeletePart of the reason I applied to Princeton was because I spent a week with a dear family friend, James Fitz Patrick, who rented hangar space out at the Forrestal campus to work on his pet invention: an ornithopter. By the mid-80s, he'd developed the Mk IV model, which was a square contraption with a wingspan around 25 feet, and was able to hover at 6-ft using a tethered compressed air fuel supply. When I was there in summer '88 we were working on the MK VII, and it was really fascinating working with a grouchy old New Yorker.
Jim worked out his model of a bird's wing from watching seagulls, and looking at bird bone structure. The basic motions are relatively simple, and can be replicated with your arm.
- Starting at the bottom of the stroke, your 'wing' would be a straight arm angled down from your shoulder, with your fingers splayed and rotated 'back' at the wrist.
- Contract your bicep, and translate your wrist toward your shoulder, while pulling your fingers together; raise your shoulder slightly later. You'll end up with your hand even with your shoulder brace.
- At the start of the downstroke, your fingers expand (like an umbrella) as your arm extends out and continues to the bottom of the stroke.
If you replace your human arm with bird wing geometry, you get a powerful downstroke and a very efficient upstroke. The geometry of the 'elbow' and 'wrist' joints is the key. Jim had a ~3-foot model (in the video below) and would sit me on a rotating stool and let me flap it to spin around.
Here's an interview from that era: https://www.gettyimages.com/detail/video/james-fitzpatrick-explains-what-his-ornithopter-flying-news-footage/1271421986
Jim's papers are at CUNY:CSI https://archivesspace.library.csi.cuny.edu/repositories/2/resources/32
To the topic of the post, it's entirely credible that ornithopters can be made to fly at human scale, given enough power, but the periodic energy needs are not well aligned with current motor designs (ICE or electric). Also, the wing elements, which must be light and strong, but are critical failure points, are subject to great forces.
The images I've seen from the movie show wing elements that are simplistic at best, more similar to dragonflies than to birds, and I find them unconvincing.
I'm unsure how periodic energy needs are an issue.
DeleteJust use hydraulic radial piston motors connected to a gear and a flywheel. The flywheel takes care of downstroke. The gear applies constant power in a figure 8 shape. During the upstroke the flywheel is still powered.
Keep in mind a hydraulic radial piston motor/pump is a 90% efficient drivetrain
Same as an electric drivetrain when taking battery converter and motor into account. It's not just about motor efficiency.
Fuel cells would be even worse.
So long as the wing can withstand the stresses
The ornithopter would have higher efficiency then a helicopter in hover and much higher in forward thrust, while having greater maneuverability.
If quartz microfiber with a 7GPA tensile strength can't do it, carbon nanothreads will. Carbon nanotheeads have 4 times the strength of carbon nanotubes.
(Aerospace engineer here)
ReplyDeleteBeating the air into submission generally doesn’t provide much in the way of high efficiency :-)
Sure, the physics of ornithopters likely rules them out as realistic.
ReplyDeleteBut I expect that some fairly generic considerations of biomechanics also rules out mile-long worms, probably much more convincingly.
At least I hope so.
p.s. IMHO, the worms in the movie weren't big enough compared to the descriptions in the book. Maybe they are saving the big ones for Part 2.
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ReplyDeleteOne question I've had is just how much of the vehicle/load weight has to be borne by the wings? Given the (potential) availability of suspensor technology?
ReplyDeleteMy rationalization is in a few thousand years material science has taken us to the extremes. The wings in Ornithopter in Dune are as strong as they appear in the film but weigh like 5% of what they appear to our 2021 understanding of material science.
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ReplyDeleteThanks for sharing the article. This blog is full of useful information.
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