Picture yourself on a boat on a river, with tangerine trees and marmalade skies....
No, that was the song that jumped my brain when I wrote, oh so innocently, "picture yourself". It should have gone on to say that you're in a glider, not suspended below it. And when you make an effort to turn to the left, the airframe can contort in such a manner that the entire right wing noses up slightly, the left wing noses down a bit, the tail twists in anticipation of a yawing moment being needed. So how does one make an airframe that does this? Inquiring minds want to know!
I don't have a structure designed yet, but I'm making progress. I'm not really qualified to design an aircraft, I have only an airframe and powerplant mechanics certificate. Yes I can change out a part of a Hg but I don't consider myself a mechanic, I did work on planes after getting licensed, but it didn't take long to recognize that I wasn't cut out for it, as a profession anyway. I've been building model planes and gliders all my life, and flew Class 1 Hg's for 40 years, was an instructor/observer in the early '80's.
My glider might have a form of adjustable CofG and billow, but how you define those things will determine if it does.
I think the best thing is to be able to alter camber, thickness, and reflex while flying, and I imagine the CofG will need to be located differently for one configuration over another. The birds alter CofG by sweeping their wings fore'n'aft, perhaps we can vary the planform by a little bit, spread the wings wider and forward while at the same time spreading and cambering the tail.
That out of the way, I will say that the glider I'm trying to create, in model form, has what you would think of as two keels. These longitudinal beams support the pilot between them, and hopefully will provide some protection for the pilot in some low speed accidents, such as getting blown over on launch, blowing a launch (catching a tip on a bush and ground looping, failure to attain/maintain adequate airspeed, bad luck).
By use of effort amplifying levers or other mechanisms, or perhaps just direct action, the beams can be moved into or out of alignment with each other. The right beam is a component of the right side of the glider, the left one is a part of the left side of the glider. The two halves of the glider are connected by a horizontal component at the nose and tail. They are made so that the pilot can "tilt" them, resulting in the beams changing their pitch attitude relative to each other.
The way I'm trying it now has a wing spar, fully cantilever, that is also hinged in three places, the center hinge is also made so that the spar halves can rotate a bit opposite each other, so that each wing can have it's own angle of incidence. This is how I hope to control roll, by twisting the airframe and making each wing change it's incidence.
The other two hinged points of the spar allow the spar to effectively shorten, so when you fold the glider the center hinge and the other two work together and the spars will move inboard as they also sweep aft. This results in the ribs, both the nose ribs and the aft ribs, folding along the sides of the spar. One option I'm considering, if this project gains too much weight, is to make the glider separate into a right and left half for easier loading and carrying around, you'd need two trips to the truck but for most that's not an issue. What might be an issue is bulk, I suspect that if I'm not clever this thing could have some frontal area going down the road on a car, and not as many gliders would fit on the rack. Gotta get creative with the folding stuff.
The tail I'm envisioning is much like the one found on the red tail hawk. It will be able to tilt, or "bank" so that it can provide some yawing moment, and it will also be enlisted to damp and/or assist in control of pitching motions. But just as our present and past flex wings are allowed to yaw away from the direction of flight to some degree, so will my glider, maybe. Ya never know, the darn thing could end up with a big rudder on it. But I think that the system we use today, of having the dihedral or anhedral "just right" so that a skewed slipstream resulting from adverse yaw doesn't create much, if any, rolling moment. But like I said, ya just never know how you're gonna think next month or year or whatever. We learn by making mistakes, that's a reason for flying to be hazardous. It will be awhile yet before I have an Rc model ready to fly, and a lot of how this thing works will be determined by my experiences there. For economic and practical reasons my first models will be small, in the 1 to 3 meter span size. Before a full scale version is built a very large scale model will be used to try to determine if a full scale glider can be flown safely. But ya never know, maybe I'll just whip something up and go to the sand dunes, like it was done going on 50 years ago. A manned version will have to be vehicle tested before flights of any significance are made.
The area between the wings has to be able to flex fairly easily, allowing each wing to set it's AoA by itself, but the use of servo tabs and trim tabs will give the pilot control of that. The wings but only when commanded. Have you ever heard of George Spratt's "Control Wing"? I'm hoping to be able to use some of his ideas, coupled with my own. The airplane he designed wasn't all that efficient, but it was able to be flown by inexperienced pilots as safely as a plane can be flown. His wings could rotate in pitch, separately or collectively. Supposedly this gave a smoother ride in turbulence, with each wing being pitch stable and free to find it's designed AoA. My hope is that my glider will be pretty much pitch neutral without any work being done by the tail, except for compensating the CofG location. Of course it would be cool if I could vary the sweep of the wing slightly, from a little aft sweep to a little forward sweep, with forward sweep it would be like moving the CofG aft, so the tail could be employed to provide lift, essentially decreasing wing loading. We've all watched in wonder as the Hawk, poised motionless in a wind equal to or greater than the bird's minimum flying speed, so he could see movement on the ground. It's fun to watch them do this when the conditions are marginal, and the bird spreads his wings forward and droops his tail, trying with all his might to get the lowest possible flying airspeed. The trick is to design an efficient tail, and it will be a real trip trying to actually fly tail heavy like that. I think that with a glider that's got a lot of overall length, and is damped sufficiently in pitch, a skilled flyer could keep his balance in that mode, after all we're gonna be goin' really slow, and at low speeds things don't happen too awful fast. A highly skilled flyer, with his attention sharply focused could pull it off. He'll just have to be spring-loaded to move the glider back to a stable condition real fast, get rid of that positive incidence in the tail and get the wings back to a position that provides a bit of nose-heavy before she pitches down too much. Or noses up way too sharply when the tail stalled...
It's said that nose-heavy airplanes fly sluggishly.
But tail-heavy airplanes fly once...
And the above brings up another thing. I'm old school analog, and so I think in terms of basic mechanics to solve problems. In this co-op-to-be most of the engineers are likely to be younger folks, they don't usually fly the 2 channel, rudder and elevator Rc gliders like I do, no no no, they've got "multi-copter" drones that are artificially stabilised with computers. They don't actually "fly" the drone, they tell it where to go and it does it autonomously. They can turn on or off some of the automatic functions and fly them a bit.
But what I'm getting at is if a drone can be artificially stabilized, why couldn't the BNG be done similarly? With computerized stability the trim drag could be reduced to zero, and pilots could focus more of their attention on centering lift or deciding the configuration of camber and reflex to get best glide to goal. Imagine entering a thermal and telling your glider to bank left 37 degrees and hold the bank and airspeed steady while you eat your peanut butter sandwich.
That wouldn't be my style, but who knows what these whippersnappers might get into in Future Games. Control their gliders by tilting a Smartphone this way and that? If you can imagine it they'll do it someday. Your self-driving car will load up your diver by itself and take it to the hill, where your self-assembling glider will fly away and record the flight so you can stay home and nail down a piece, then you'll plug yourself in to a thingy that will blow wind in your face and feed sensation directly to your nerve endings, even artificial G forces. You'll experience the flight just as you would have if you had been there. Now that's getting a handle on the safety thing! And all on your own schedule...
Ok let's have some simple fun and get imaginative.
Grab a sheet of thin foam, or cardboard or stiff paper. Draw a planform shape on it that is a mixture of bird (Hawk?) Manta Ray (low aspect yeah, but they look cool) and an F-22 Raptor fighter plane and a butterfly. This may take awhile, but it will keep you out of as much trouble as it will get you into later on.
This BNG that I'm thinking of has gobs of area, we want to go slow and while small gliders with hard-working airfoils can slow up decently, they have a more abrupt stall and a longer recovery arc. We want a glider that, if stalled, practically immediately is flying again. And we want to be able to use a relatively thin section with really mellow characteristics. Little or no pitching moment, a modest lift and as low drag as we can get. We want a very modest wingspan, so we can turn in really tight radius turns with a minimal bank angle. We want to sink slowly and circle in small thermals. We want a stall speed as low as it is humanly possible to attain.
Now, gobs of area and a short wingspan add up to a low aspect ratio. To get as much glide as we can from this thing we'll literally have to bend over backwards to reduce parasitic drag wherever it can be found. To keep it light we're tempted to use wire bracing, but I believe that although a fully cantilever construction may exact a weight penalty it may be necessary because of the drag thing. This is one of several reasons why the pilot can't be hanging down below the glider, we've got to get up into the thing and not be exposed to the airflow. Once you get used to it, you'll like it, especially in cold weather. But I'm not saying that you can't have great visibility---I'm trying to make one where my head is very close to the center of the leading edge of the wing when reclined, I'll have the option of going prone at any time should I desire.
So I can look straight out to the side, and by varying my posture a bit I can look either over or under the wing, which may be forward swept a bit. To help with the weight problem I'm experimenting with the "joined wing" configuration, or maybe just some of it. So a form of canard design, mixed in with the previous suggestions, might have some very real advantages. The models are flying well, so we'll see. Just for fun, cut out a delta wing from foamboard, de-papered.
Use a planform reminiscent of the Standard Rogallo/Dickenson design, with the centerline being the same length as the L.E.'s. Add a small vertical fin, I have at least some of the fin on the bottom to use as a handle.
After you find the CofG and the reflex needed to get it to glide, remove the nose weight and reflex, and cut two big holes in the glider, one for each wing. The shape of these holes should somewhat sorta kinda mimic the existing planform. You'll be removing about 1/3rd or so of the area.
Add some decalage, it won't need much, by adding some camber to the aft portion of what is now the foreplane. Before you add too much of that you can add a tad of reflex to the inboard area of the aftplane. Oh yeah you'll need to balance it, but what you're going to find is that: This "joined wing" glider has a much better glide than what you got with the delta glider you made it from. And, it requires considerably less nose weight, so even though you removed a lot of area your wing loading went down from what the delta wing had.
You may want to cut out the holes a bit small to start, and gradually make them bigger and the fore and aft planes smaller as a result. It's a fun hobby, try it. The foam board I like best comes from the Dollar Tree store. While there grab a $1 hot glue gun and a $1 package of hot glue. Don't bother working with foam if you don't have the sharpest of blades.
A few sheets of foam board will keep me happy for days. I go to a park, make gliders of all styles, get 'em trimmed out then give 'em to the kids (and adults!) that take some interest. It's a lot of fun. Now that pot is legal...