How these gliders keep level flight is different from the stability of airplanes:
Drop a flat piece of paper and it will flutter and tumble through the air as it falls, but a well-fashioned paper airplane will glide smoothly. Yet these seemingly simple structures involve surprisingly complex aerodynamics. Researchers at New York University's Courant Institute of Mathematical Sciences conducted a series of experiments involving paper airplanes to explore this transition and develop a mathematical model to predict flight stability, according to a March paper published in the Journal of Fluid Mechanics.
"The study started with simple curiosity about what makes a good paper airplane and specifically what is needed for smooth gliding," said co-author Leif Ristroph. "Answering such basic questions ended up being far from child's play. We discovered that the aerodynamics of how paper airplanes keep level flight is really very different from the stability of conventional airplanes."
[...] An amusing "scientist playing with paper planes" anecdote comes from physicist Theodore von Kármán. In his 1967 memoir The Wind and Beyond, he recalled a formal 1924 banquet in Delft, The Netherlands, where fellow physicist Ludwig Prandtl constructed a paper airplane out of a menu to demonstrate the mechanics of flight to von Kármán's sister, who was seated next to him. When he threw the paper plane, "It landed on the shirtfront of the French minister of education, much to the embarrassment of my sister and others at the banquet," von Kármán wrote.
While scientists have clearly made great strides in aerodynamics—particularly about aircraft—Ristroph et al. noted that there was not a good mathematical model for predicting the simpler, subtler gliding flight of paper airplanes. It was already well known that displacing the center of mass results in various flight trajectories, some more stable than others. "The key criterion of a successful glider is that the center of mass must be in the 'just right' place," said Ristroph. "Good paper airplanes achieve this with the front edge folded over several times or by an added paper clip, which requires a little trial and error."
[...] This differs substantially from conventional aircraft, which rely on airfoils—structures designed to generate lift. "The effect we found in paper airplanes does not happen for the traditional airfoils used as aircraft wings, whose center of pressure stays fixed in place across the angles that occur in flight," said Ristroph. "The shifting of the center of pressure thus seems to be a unique property of thin, flat wings, and this ends up being the secret to the stable flight of paper airplanes. This is why airplanes need a separate tail wing as a stabilizer while a paper plane can get away with just a main wing that gives both lift and stability."
The team also developed a mathematical model as a "flight simulator" to reproduce those motions. Ristroph et al. think their findings will prove useful in small-scale flight applications like drones or flying robots, which often require a more minimal design with no need for many extra flight surfaces, sensors, and controllers. The authors also note that the same strategy might be at work in winged plant seeds, some of which also exhibit stable gliding, with the seed serving as the payload to displace the center of mass. In fact, a 1987 study of the flying seeds of the gourd Alsomitra macrocarpa showed a center of mass and glide ratios consistent with the Ristroph group's optimal gliding requirements.
Journal Reference:
Huilin Li et al., Centre of mass location, flight modes, stability and dynamic modelling of gliders [open], J Fluid Mech, 937, 2022. DOI: https://doi.org/10.1017/jfm.2022.89
(Score: 1, Insightful) by Anonymous Coward on Sunday January 08, @06:01AM (2 children)
Before you get all excited, read the comments to tfa. Several of the comments point out that the aerodynamics of flat plates and similar thin sections have been researched and well understood for many years.
(Score: 5, Interesting) by JoeMerchant on Sunday January 08, @01:41PM (1 child)
What we determined at University (in the 12 story dorm building) was:
1) smooth rising air is the strongest factor for long flight times. Slight imbalance of aileron set angles could effect a large circular course keeping the plane in the updraft of the building longer.
2) when the paper size is restricted to a standard letter or legal sheet, maximal flat wing area maximizes flight time. Legal sheet planes fly significantly longer than letter sheet planes of similar design.
3) optimal design is essentially a small (1/4 to 3/8") central rib and upturned wing edges with a near blunt front, say 85 degrees from the central rib to the outside edges. Small upturned ailerons on the trailing edges as much for structure as for aerodynamic effect.
Basically just enough structure to hold the flat surfaces flat. During high humidity the paper structure would soften enough during flight to significantly negatively impact flying time, best to store the paper and build in a controlled lower humidity environment and bring the plane to the balcony immediately before launch.
4) US penny in the nose for stable forward flight, solid copper better than zinc filled (one zinc filled was too light, two zinc filled too heavy, if you must use zinc filled they work better in letter sized paper)
5) Most flights were launched from the 8th floor of the 12 story building (as much because that is where most of the plane builders lived as any optimizations of the updrafts). The best flight ever had taller wing edges at the front, shorter at the back and opposite taper on the central rib. It flew about three circles near the building in an updraft nearly balanced with it's downward motion through the air, rising to somewhere just over 9th floor level. As the flight went on the rear of the wing softened in the humid air and the downward motion slowly decreased as it flew further from the updraft area near the building, sustaining near level flight for almost 5 minutes, eventually suffering structural collapse of the central rib while around 6th floor altitude and rapidly descending approximately 750' away from the launch point.
Variations of the traditional narrow dart design were the most tried, they generally flew 100' at most before turning downward, either directly or after a stall, then heading straight at the ground. Wider wing designs would fly longer in stall / recover horizontal flight patterns, with the penny weighted near full sheet wing designs doing the best at minimizing stalls while maximizing horizontal flight distance and time.
Україна досі не є частиною Росії Слава Україні🌻 https://news.stanford.edu/2023/02/17/will-russia-ukraine-war-end
(Score: 1, Interesting) by Anonymous Coward on Sunday January 08, @09:24PM
One pleasant day near the end of the spring term I was in a common study area with some friends. The 5th floor window was open and nearby someone had left a pile of scrap paper. Naturally I made an airplane and idly tossed it out the window.
As I was starting to fold up the next aerial experiment, from across the room came an anguished cry, "My Thesis, what the hell are you doing??!!"
Turned out that wasn't scrap paper after all...and this was in the days before wordprocessing, each page had been manually typed.
The story had a happy ending, all was found and flattened out enough to be acceptable. And when I get together with that friend we have a good laugh.
(Score: 2) by Mojibake Tengu on Sunday January 08, @07:25AM (4 children)
China’s annual kite festival takes off with 280-metre-long ‘dragon’ in the sky [SCMP]
https://www.youtube.com/watch?v=Yt3zCsE6b2Y [youtube.com]
It seems they don't need so much surprisingly complex aerodynamics stability science to get things done... for dozens of centuries.
The edge of 太玄 cannot be defined, for it is beyond every aspect of design
(Score: 0) by Anonymous Coward on Sunday January 08, @06:26PM (3 children)
Back to the topic, I've always wondered about the claims about the Bernoulli effect producing lift for the "classic wing" with curved top flat bottom. If the angle of attack is zero (flat surface completely parallel to airflow) does it really produce lift assuming horizontal wind across the zero attack wing?
What I can agree on is that if there's an angle of attack a curved wing could produce lower turbulence than a flat wing.
(Score: 3, Insightful) by JoeMerchant on Sunday January 08, @09:14PM (2 children)
>If the angle of attack is zero (flat surface completely parallel to airflow) does it really produce lift assuming horizontal wind across the zero attack wing?
No. If an infinitely thin flat wing is perfectly parallel to a flow of air, no lift is produced. However, if the wing is attached to a plane that has weight, that weight will cause the plane to start to fall, causing the wing's travel through the airflow to be at a more positive angle of attack, creating a vacuum on the top side and a high pressure area on the low side, creating lift. If there is a propulsive force in the plane, it will maintain this pressure differential. If there is any kind of control surface and a pilot who does not wish to descend closer to the ground, they will maintain such angle of attack as is necessary to maintain level flight.
We were sailing yesterday, explaining the combined forces of wind on the sail plus the vacuum on the back side pulling the boat forward usually takes people by surprise - including our 69 year old passenger yesterday. Watching the crab trap floats pass by at a different angle than where the boat is pointed helps, a little.
Україна досі не є частиною Росії Слава Україні🌻 https://news.stanford.edu/2023/02/17/will-russia-ukraine-war-end
(Score: 0) by Anonymous Coward on Sunday January 08, @09:35PM (1 child)
> explaining the combined forces of wind ...
You missed a piece. The ability to go other than downwind also depends on the ability of the hull/keel/rudder to make side forces when traveling at an angle in the water. It's the reaction of the wind forces against the water that lets you pick a range of path directions other than straight downwind.
Ice boats and land-sailers (on wheels) can sail much closer to the wind (eg, nearly-directly-upwind) than typical boats, because the force reactions take less "slippage" (or slip angle for the tires) to make the necessary reaction force.
(Score: 2) by JoeMerchant on Monday January 09, @02:08AM
Yesterday was a lazy sail, only the roller furling jib was out, can't really get much upwind without also raising the main (sloop rig) so we just bobbed along at 1-2 knots SOG on broad reaches. It's not like we were going anywhere anyway, and raising/stowing the main is about 5x as much work as unfurling/refurling the job. Have to use the iron sail to get out of and back into the marina in any event.
Україна досі не є частиною Росії Слава Україні🌻 https://news.stanford.edu/2023/02/17/will-russia-ukraine-war-end
(Score: 3, Informative) by agr on Sunday January 08, @07:32PM
A great quote from a lecture, their first in public, that Wilbur Wright gave in Chicago in 1901 about their early manned glider experiments:
"If I take this piece of paper, and after placing it parallel with the ground, quickly let it fall, it will not settle steadily down as a staid, sensible piece of paper ought to do, but it insists on contravening every recognized rule of decorum, turning over and darting hither and thither in the most erratic manner, much after the style of an untrained horse. Yet this is the style of steed that men must learn to manage before flying can become an everyday sport."
https://invention.psychology.msstate.edu/i/Wrights/library/Aeronautical.html [msstate.edu]