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By DMarley
Controllable surfaces on a flexwing, if that's possible. Reduce the need for all the washout for better speed envelope. Perhaps include an aft horizontal stabilizer to also reduce the requirement for washout.

After study of Prandtl's, Kroo's, Panknin's, Schrenk's, Morris', Culver's, and others' theories and ideas, and that of Al Bowers (ha!), I am swayed to believe that without a bunch of computer-controlled surfaces, flying wings will always be second best to conventional planform aircraft. So, what I would do is to work towards the conventional planform rather than strictly adhere to the flying wing ideal.
Even with Horten's aircraft, one of the main problems in real-world conditions was the lack of yaw control, and the speed envelope was very limited, even though the propaganda from media reports indicated something different.

Some would say that aircraft design should emulate bird design. I believe that we, in conventional planform aircraft design, have far surpassed the abilities of birds and their design to enable us to safely fly in rough weather conditions in which birds would not dare. I believe that the large volume moment arm afforded by the horizontal and vertical components of the empenage have surpassed anything that a flying wing planform could possibly accomplish in control and efficiency. In that end, look at a small empenage while retaining a manageable static balance.
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By raquo
Sorry to be blunt but I've seen this posted too many times on facebook already as if it's something groundbreaking, but I don't see what the fuss is about.

- That design is completely arbitrary and unsuitable for hang gliders. What is the wing made of? Why does the strut attach in the middle, way behind the center of lift? What is this funky wing shape even good for? Why is the rear wire a tube now? The A-frame is so wide you wouldn't be able to put it on your shoulders. Why? A lot of whys with no answers.

- Or if this is supposed to be about the CFD simulations, you didn't address any reasons why CFD would seem inappropriate for flex wing design – both the airframe and the wing flexes with varying load drastically changing the shape of the whole thing. Does the software allow for that? I'm guessing not, so it's only for rigid wings. Or for some very limited preliminary design stage?

- Or is this about the prototype you mention? You're quoting NASA on flying wing designs that reduce drag and improve maneuvrability, but those things come at a cost of expensive manufacturing methods and sailplane-level pain in logistics, if not worse. They are in no way optimized for hang gliding that has its own distinct set of constraints, not just optimal aerodynamics.

I'm all for celebrating initiatives in hang glider design but this whole story reads very incoherent to me. If you want to be taken seriously, you have to address at least a couple of the points above.
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By Watson
Hi DMarley,

I understand what you are asking for, but whenever I'm out in the field, I never see anyone complaining about the lack of maneuverability on their gliders. Therefore, my goal here is to get to a wing that is easier to assemble from short pack, lighter to carry around, while having the performance of a topless or, even a rigid.

The flying wing is not far from the conventional airplane design if you consider that they end up having a vertical stabilizer either along the trailing edge (with reflex airfoils) or, at the wingtips (with washout). Also, I believe that if enough computer effort is put beforehand, on the design of a fling machine, than active control systems, like on the B-2, won't be necessary.

I agree with you that for high speed flying mankind has already surpassed birds, but for the flight envelope of our gliders, we still have much to learn from nature's millions of years of trial and error.

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By atmosphereship
I like the idea of flight control surfaces out at the wing tip trailing edge. Imagine those new sexy raked tips that are controllable. They may be independently controlled with left and right control bar triggers. Squeeze one or both and they deflect down only. It might offer quick roll corrections while in ground effect. Squeezing both might be like having instant flaps. Wingtip flaperons.

How about a V tail for pitch and yaw stability?
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By Watson
Hi Nikita,

I'm sorry if I didn't explain to your expectations, but bear in mind that I'm just one person.

1. There are so many changes from a regular hang glider that I intend to implement that indeed the design looks strange:
- My idea is to use ram air to inflate the airfoil, just like a paraglider, but with an internal structure plus struts to keep it from collapsing.
- The center of lift isn't at 1/4 chord, therefore everything needs to be shifted back to keep it balanced.
- The shape comes mostly from trying to keep the bell lift distribution without much twist of the airfoil.
- I admit that I didn't give much thought for the exposed structure yet, since I just wanted something to create a reasonable drag for the pilot plus the structure, in order to evaluate the wing.

2. Any CFD simulation coupled with structural analysis needs to start from a CFD only simulation and, this is that initial one.

3. I believe that NASA's Prandl-D was proving the opposite of what you've mentioned. They managed to have a more simple structure at the tips, while gaining proverse yaw when turning.

I hope this address most of your concerns, but if you have more, please let me know.

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By raquo
Thanks, that makes more sense now!
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By dayhead
Thank you for inviting comment on this matter. I'm here to take you up on that invitation.

I was learning how to fly airplanes in 1976, having been an avid aeronautical enthusiast since I was the proverbial knee-high. I was troubled by the fact that I was having to drag myself to the airport to take more lessons or practice the ones I'd been taught, after all, I had been in love with the whole idea of flying for a long time, so where had my enthusiasm gone?

Instead of practicing the cross-country work I would instead spend my flying time going to the "Practice Area" to do spins and loops and rolls, or spend the whole hour shooting touch and goes while using the "abbreviated pattern" that, while somewhat less safe, allowed a lot more landings and take-offs per hour.

I was slowly beginning to realize that airplane flying wasn't where it was at for me. I wanted to maneuver, to fly like the birds; droning along in a straight line to another airport just didn't do it for me.

Salvation appeared in the form of a T-shirt a fellow aircraft mechanic was wearing. It had a photo of a standard Rogallo/Dickenson hang glider, and the words "Flint Hills Flyers" written around the illustration. This was at the Cessna Aircraft Company in Wichita, Kansas, where I was employed doing final assembly of the 404 Titan airplane.

The occupant of the shirt gave me directions to a 150 foot high hill located 150 miles from Wichita, and I loaded my RC gliders in the van and went to the Wilson Reservoir near Lucas.

That guy may have saved my life, or he may have contributed to it's lack of productivity, depending on your viewpoint. Whatever the case, I ended that Saturday of Memorial Day weekend with no model gliders, instead I was the proud new owner of an 18 foot Pliable Moose hang glider and harness.

I never went back to the airport. After a couple weeks had passed my flight instructor called to see if I was ready for my private pilot check ride, and I told him the whole deal was off, as I was now an aircraft owner. A hang glider pilot.

He said "Oh no" and hung up. Good riddance.

So that's how it all started. While I may not have accomplished much, I did survive 40 years of hang gliding, much of it practiced with wild and reckless abandon. I'm so lucky to have made it this far, I just can't find the words to describe my gratitude.

I write all this as a way of introducing myself and giving some credibility to the fact that some of my ideas just may have some merit.

I don't fly much anymore, but I'd like to do more. While it seems that about 99% of the current hang glider pilots are satisfied with the state of the art of hang glider technology, I am in the 1% that whole-heartedly believes we can do much better.

The problem we face here is in defining just what "better" means.

While "better" would normally be defined as a flatter glide angle at higher speeds, I personally have a different view.

I believe that "better" means a glider that has a lower stall speed, hopefully as slow as a paraglider. The stall speed should be so low that a pilot of average athletic ability will have no fear of making a zero-wind launch, and be confident of making a safe and easily controlled landing, each time and everytime, again in zero-wind conditions.

The glider should be able to make small radius circles at low angles of bank.

The glider should be controllable even when stalled. The pilot should be able to override the glider's wanting to nose down sharply when stalled, allowing "parachuting" landings in unfamiliar or rough terrain.

IMO, the best way to achieve the above is to have a whole bunch of wing area. While a high aspect ratio flexie or rigid can have a relatively low stall speed due to having an efficient airfoil and large span, It's almost a sure bet that it's sink rate, when stalled, will be high. My experience has me convinced that if I want a glider with mellow stall characteristics, low sink rate when stalled, and some semblance of control in the stall, then I want a gob of area, and only enough span to make it work.

That's right, a low aspect ratio. It's almost an unforgivable sin to suggest it, I know. But the degree of control that I want in a glider is doubly or triply hard to get with long skinny wings.

The idea then is to try to minimize the damage done by the low aspect ratio by obsessive-compulsive attention to detail when looking for places to reduce drag. The glider must be fully cantilever in construction, no bracing wires or structure exposed to airflow.

We have high aspect gliders now, but in many places the local distance records are held by paragliders. These aircraft have practically no pitch control and also the smallest speed range you can get. The distance record is a moot point for me anyway, I couldn't care less about flying 4 or 5 hundred miles in a hang glider. In fact, I'm so lazy and laid back that going 15 miles is a feat.

The majority of the pilots I know fly little or no cross-country flights, they're mostly "fishbowl" or "site" pilots. I think the general attitude is that hang gliding is dangerous enough even when the LZ is a well known factor. Why tempt fate by landing in "LZ's" you've never seen before, or at least have never walked, and, oh yeah, you haven't prior permission to use anyway. We have an old saying, "It's easier to get forgiveness than permission", so we'll land there anyways.

In days of old pilots loved flying in the pre-frontal strong ridge lift. The sky would be crowded even though there's an overcast, and thermals were rare and weak.

If we had a fully aerobatic glider those strong ridge lift days would be back in fashion.

Forty years ago I "just knew" that it wouldn't be long before we'd get real gliders to fly, none of this 60 degrees of bank and 30 degrees of pitch bullshit. Gliders that could do what my RC gliders could do, including whip-stalls.

It never happened. Somehow we all decided that only pure weight-shift would be used, and we would stay with the sailboat construction scheme.

The builders of our gliders claim that competition results drive the sales of new equipment. And since the rules don't allow for a system that augments our roll control, they have little incentive to try new ways of doing things.

I wonder, how many T2's or Litespeeds actually end up being used in a comp. 10%? 15? You get the idea. Most purchasers of these gliders never compete with them, and because a few do then all the rest of us have to put up with the truly insane idea that we get to pull on a rope to make a choice between getting a good glide angle, or being able to turn the thing. In 2017, no less.

About 50 some years ago an aeronautical engineer built himself a swept-aft with tip devices flying wing glider that he flew in airshows. He didn't have even so much as a calculator, let alone a computer capable of computational fluid dynamics. And he would routinely tumble the glider, and nonchalantly just fly out of it, with no damage to himself or the glider. He would also place the glider in a very deep stall, and demonstrate a survivable rate of descent while also having complete control of the glider.

Nowadays, you can buy a $12,000 hang glider and a $5,000 harness, and while heading out to play in the sky, you get rolled over to inverted, and fall on the glider which promptly snaps half in two. The HGMA wants to see the kite withstand a negative load, and they certify the glider as airworthy if it passes the test on a truck tearing down the runway. But there's no test of it's ability to withstand having the pilot fall on it from five feet away, which happens practically every time a glider gets inverted,a good possibility whenever the atmosphere is such that cross-country is a good bet.

People say we're crazy, but what do they know? I do believe we're wearing collective blinders though. Hang glider pilots will criticize paraglider pilots for sacrificing safety to get convenience, but that's just another case of the pot calling the kettle black. We don't want the inconvenience of a pilot restraint system that would prevent him from ending up on top of the sail, or reduce the risk of severe injury in those bad landings or blown launches. We don't want a better way to control our bank angle, which in turn necessitates that four different sizes of the same glider be produced. At the airport, I was never faced with having to choose a different sized Cessna 150. One size fits all.

We now have a lot of high powered tools at our fingertips. But in 2017 we don't have half of what Witold Kasper had in I guess about 1960. Gliders are expensive and last only a few years. I'm just hoping for the day when the gliders being built today will be considered obsolete, and we'll have gliders that, while somewhat less convenient, will be safer and capable of aerobatic flight. And their technology won't be just in the hands of an experienced sailmaker, it will be such that almost anyone can build it in a garage over the winter months, for far less money. No tricky luff curves, just good old fashioned aeronautical engineering like that which built the DC-3 or a Long-Eze.

When the sport shrinks to the level that won't support glider factories anymore, then it will fall back to the homebuilder to make his own glider, and true innovation will return to the scene. Truck testing will reduce the risk to test pilots. These fancy new materials and techniques will allow us to make gliders that we can barely imagine today.

It's a Brave New World, so I'm calling it the Brave New Glider. And it's on it's way, though I don't want to predict when it gets here.

P.S. No, I don't want an Alpha. I want some speed range. Performance roughly equivalent to a Sport 2 or U2 would suffice. I just don't want to sacrifice the low-end to get a little more high-end performance. Safety starts with a low stall speed. I know this because I have crashed, and I'm now a Stand Up Aeronautical Philosopher. Fire at will, I'm ready for ya.

I am looking forward, with great enthusiasm, to the fully aerobatic part. Imagine joining your buddy in a thermal--while flying inverted. Slow rolls, snap rolls and outside loops might be fun in the glass-off.
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By red

I know the aircraft that you described. The KasperWing design exists as a hang glider, a powered trike, and a built-up sailplane. 'Way back in the day, I flew my Fledgling as the "chase" plane on the HG version for hours. I watched as the KasperWing HG flew from 0-to-60mph-to-0 (0-to-100kph-to-0) without a break stall. It could fly and turn at any forward airspeed, which includes 360s at will in both directions, while at zero forward airspeed. A zero-airspeed landing for the HG version would have been fairly rough, but (I believe) reasonably survivable. This is the trike version:


A picture of the sailplane version (fully cantilevered) is attached.
Fn1853_2.JPG (126.54 KiB) Viewed 2858 times
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By Watson
Thanks for you input dayhead,

Even before I did my engineering course, I would daydream about a glider that would combine the simplicity of a paraglider with the performance and safety of a hang glider. This was probably because my dad was the first president of the Brazilian hang gliding association and, I ended up getting exposed to those things much earlier than other kids :)

Much later when I've bought my own Sport 2, I was outraged at the fact that my undersurface was completely flat! If you delve into RC world, you'll get to know that this is the worst kind of airfoil available. So, I knew that something better was definitely possible.

I've had the same doubt as you as to what better would mean, but I've decided to take a different approach. In the history of mankind, technological advancements were performed by a few, but were driven but the need of many, so I though of simply taking note of what people complain about when hang waiting.

What I came up with is the reason why paragliding is beating us in absolute numbers: assembly time and transport issues. Although current glider manufactures show off L/D plots or polars as a comparison between gliders, I've never heard a pilot complain that they thought that they didn't have enough glide.

Therefore, my objective is to cut a lot of the structure that is not needed and only makes gliders cumbersome, but try to keep the performance of a simple H3 glider. Also, by analyzing everything working together in the computer, I'll make sure to have complete functionality, but minimal structural waste even before the first part is made.

I'm planning on having a 10m wingspan, but a super massive wing area is not needed for a small stall speed, just a better airfoil will do it. In Addition, I know that all flexies look envious at control surfaces, but with a small wing span plus the proverse yaw from a bell lift distribution, I believe that a pure weight shift might be enough and, keep the structure simple to be transported.

Ps.: No flying machine is controllable when stalled, but you can push its design to have an onset of stall to be at an extremely low speed, with a good airfoil.
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By Karl_A
If this model is to be used for flexwing hang gliders I'm assuming it includes the elastic properties of the frame and sail because some aspects of the shape of a flexwing are determined by deformation of the wing caused by air loads which in turn determines the air loads.

I am interested in how the model will be validated, that is, how it is verified that the model captures how a flexwing actually behaves in flight regarding both performance and handling.

I am very interested in how well the model will be able predict the behavior of a flexwing in highly dynamic situations like a tumble or G induced pitch divergence.
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By Charlie Romeo
First off good luck with your computer designing Guilherme ,we need all the innovations to keep our shrinking sport alive.Though always remember ya have to land em :roll: Low aspect ratio and low wing loading are the best friends for the average pilot :) I see more and more video"s of pilot"s landing on wheels,i dont believe thats the way to go.A flying mate decades ago was so convinced the wing tip feathers of soaring raptors were the answer ,he made some fiberglass "feathers" the size of our tip battens. Alas he couldnt work out how to articulate them to spread out for circling and soaring and also close them in for fast,penetrating flight.I believe without a steady influx of young people into h.g its all academic :( The age demographic for our sport is a sign.....For me personally i"ve done my fourty years too like Dayhead {who"s post for me hits the nail squarely on the head} and i still love my sky carving so its soon bye to the topless and bring on a wide speed range intermediate :thumbsup:
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By icaroaccordi
Hi Guilherme,
There is a reason why the shape of hang-glider airfoils. The camber and thickness is more pronounced near the leading edge to help the longitudinal stability. No one likes to tumble. Before I have gotten a proper knowledge, my father and I have design and build a low intermediate hang glider with battens with a sailplane like camber and improved wing tips. How this wing glide! It impressed every one that has seen it. Probably near a modern Sport 2 or Sting 3. However, we did not know how much life risk we have taken.
Nowadays Aeros is using this kind of shape in the Combats because they are using a little tail.
The lower surface is not flat. It looks flat. When the hang glider gets pressure the lower surface is completely deformed, included the flat battens. To be more complicated it can be convex or concave because they can be supported by the cross bar. It is hard to predict the real shape of a hang glider airfoil. It is a hard FSI task.
It is good idea to have air intake to get a predictable airfoil shape. In a usual flight, it is easy to calculate the aerodynamics of a paraglider than a hang glider (if you do not think that collapses are part of the usual flight of a paraglider). However, these air intakes can make the longitudinal stability unpredictable in low attack angles. Ramair from Wills Wing uses air intakes and some units have hard longitudinal stability. I am not sure if it was about the air intakes, some frame tuning or sail version.
Even an optimum paraglider airfoil has worse drag than low-tech hang glider airfoil due to the smooth leading edge. I have run some 2D hang glider airfoils and they are not bad in cl_max. To be honest, they are pretty good because the trailing edge works as a plain flap. When inflated they look like (if you have a good imagination) a low-Re high-lift airfoils as Wortman’s and Selig’s.
Avian has already used CFD to decrease some parasite drag of the keel pocket.
I think it is better using a good panel method with an expliticty structural solver to get the real shape of a hang glider than use only CFD.
Prof. Karl Nickel has worked with the Horten Brothers when they are in the research of the bell shape lift distribution where Prandt-D shape came from. Prof. Nickle and Prof. Wohlfahrt explain a lot about the tradeoffs of hang glider aerodynamics and show some interesting math models to calculate stability in the book “Tailless Aircraftsâ€￾. I cannot remember why they not tried bell shape in hang gliders. They came with other wing shape that could improve hang glider performance. They called butterfly shape and Moyes used something like that in the first generation of the Litespeed S. This pronounced S shape did not work as expect by Moyes and a more usual shape was used in the next generation. The legend says that the soul of the S shape still living in the modern hang glider shape.
The weight shift is only a part of hang glider control. The sail needs to twist asymmetrically due to weight shift distorting the geometry. In the end, the structure of a flexible hang glider is made to be flexible (of course). Any structural change needs to maintain all flexible features, even putting some proverse yaw in the equation.
Decrease the sail weight is important as frame weight. Wills Wing is working hard in it. I would prefer the simple, efficient and durable solution of Finsterwalder Dacron.
..and comes stability. That topic I am afraid of talk about owing to my lack of knowledge. There are people here with a much better knowledge in this topic and it is the hardest one.
In resume, sadly, the order of importance in a hang glider design is stability, structural and then my beloved aerodynamics (everything strong coupled).
It is good to have someone to talk about nerd hang glider things.
By mr_guy99493
icaroaccordi wrote: The weight shift is only a part of hang glider control. The sail needs to twist asymmetrically due to weight shift distorting the geometry.
I don't believe it 'needs' to twist, although it does. Perfectly rigid sails would fly and maneuver. It may feel strange though.
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By icaroaccordi
Hi Mr_guy,
There is no perfect rigid sail in flexible wings. It can look like rigid but when it is loaded with the pilot weight, even the flexibility of the structure will help to deform it. All hang glider junctions and tubes diameters are designed thinking in flexibility vs stiffness. The float crossbar junction and sail cut is made to maximize the asymmetric torsion while it is submitted to weight shift. It let the hang glider designers to use high aspect ratios (for hang gliders) without aerodynamic controls since the seventies. The design of sails with high-tension and the correct flexibility is a masterpiece of technology. That is why few companies can build competitive comp wings (blade wings). In VG full, the wing can lose the majority this flexibility and became stiff while the vg off gives all flexibility necessary to aileron like behavior. I think the float crossbar is easy explained in the Denis Pagen book but there is a lot of literature about that even in hang glider school materials.
A perfect rigid sail could work perfectly only with weight shift in two occasions:
Very low aspect ratio;
Moderate aspect ratio with massive washout;
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By aeroexperiments
icaroaccordi wrote: I think the float crossbar is easy explained in the Denis Pagen book but there is a lot of literature about that even in hang glider school materials.
A perfect rigid sail could work perfectly only with weight shift in two occasions:
Very low aspect ratio;
Moderate aspect ratio with massive washout;

Throwing out a random note--

In the course of some experiments, I wanted to see what would happen if the crossbar was NOT free to float. I lashed the crossbar to the keel with a cord that I was able to release in mid-flight. I aerotowed several gliders to altitude in this configuration (WW Spectrum, WW Sport 2, WW Falcon 1.) The gliders were stiffer but still controllable. Some degree of flexing was still quite evident in keel-cam videos. Specifically, whenever the glider was rolling, you still saw the trailing edge move higher on the descending wing and move lower on the rising wing. This will counteract the effect of roll damping-- i.e. there will be less roll damping than if the wing were completely rigid. But the observed flexing was less than when the crossbar was free to shift-- naturally.

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By TjW
mr_guy99493 wrote:
icaroaccordi wrote: The weight shift is only a part of hang glider control. The sail needs to twist asymmetrically due to weight shift distorting the geometry.
I don't believe it 'needs' to twist, although it does. Perfectly rigid sails would fly and maneuver. It may feel strange though.
If it doesn't, it won't turn very well. One of the big breakthroughs in hang glider controllability was the keel pocket, which allowed the sail to shift asymmetrically under load.
The next iteration used the floating crossbar, which accomplishes the same thing.

Yeah, you can get some roll capability strictly from shifting your weight, but without the aerodynamic "boost" of the sail twisting, you aren't going to like it.
By mr_guy99493
TjW wrote: Yeah, you can get some roll capability strictly from shifting your weight, but without the aerodynamic "boost" of the sail twisting, you aren't going to like it.
Yes, agreed. I just wanted to add the note because some people think/teach/are taught that it's impossible, and it's useful to know the full story.
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By jheissjr
On the discussion of paraglider tradeoffs with collapses vs logistic benefits, would it be possible to add a structural member to a paraglider to keep the wing rigid during a collapse? Can something similar to flexible tent poles be designed into a paraglider to prevent collapses?

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By Watson

In order to start a FSI (fluid-structure interaction), you need to have a CFD only first. I've done the CFD now and, now I'm adding the structural part.

I'm using a established commercial code (as in used by major aircraft companies) and I'm not a research center, so I don't see the point of doing any validation of the code itself.


Take your lower battens out for inspection, if it's not a straight cylinder shape, you need to fix it. I agree that this shape will deform during flight, but I'm absolutely sure that no glider manufacturer takes this into consideration when design the glider structure.

I work with jet engines and, even we don't take deflections of stationary parts in to consideration of the aerodynamics. The current glider manufacturers just pick a design and test it out to see if the deflections are too bad for a normal flight, but they have no idea beforehand of what their airfoil is going to look like under normal load.

Like you, I believe that these inflight deflections are important and, I want to use simulations in order to achieve a smarter design (lightweight, easy to assemble and intermediate performance).

But differently than you, I believe that a "perfect rigid wing" (some small deflections will always be there) might work with only weight shift by having moderate aspect ratio and twist, with a different lift distribution.


Paraglider collapses are a big jolt to the structure, you are going to need something stronger than that to prevent it.

Thanks for all your comments,

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